design of high voltage ac/ac electrochemical capacitors in...

Poznan University of Technology

Faculty of Chemical Technology

Institute of Chemistry and Technical Electrochemistry

Field of study: Chemical Technology

Paula Ratajczak

DESIGN OF HIGH VOLTAGE

AC/AC ELECTROCHEMICAL CAPACITORS

IN AQUEOUS ELECTROLYTE

Pr o jek t o wan ie w yso ko na p ię c io w yc h ko nd e nsa t o r ó w

e lek t r o che mic z nyc h ,

pr acu ją c yc h w e lek t r o l it a c h wo d nyc h

DOCTORAL DISSERTATION

Promoter:

prof. François Béguin

Poznań 2015

D e s i g n o f h i g h v o l t a g e A C / A C e l e c t r o c h e m i c a l c a p a c i t o r s i n a q u e o u s e l e c t r o l y t e

Paula Ratajczak P a g e 2

Badania do niniejszej pracy prowadzone były przy wsparciu przez projekt ECOLCAP

realizowany w ramach Programu Welcome, finansowanego przez Fundację Nauki Polskiej

(FNP)zgodnie z Działaniem 1.2. „Wzmocnienie potencjału kadrowego nauki”, Programu

Operacyjnego Innowacyjna Gospodarka wspieranego przez Unię Europejską

Kierownik projektu: Profesor François Béguin

This thesis’ research was supported by ECOLCAP project funded in the frame of the

Welcome Programme implemented by the Foundation for Polish Science (FNP) within

the Measure 1.2. ‘Strengthening the human resources potential of science’, of the

Innovative Economy Operational Programme supported by European Union.

Project leader: Professor François Béguin



Część praca badawczej została wsparta przez projekt LIDER finansowany przez

Narodowe Centrum Badań i Rozwoju LIDER/018/513/L-4/12/NCBR/201„Kondensator

elektrochemiczny o wysokiej gęstości energii i mocy operujący w roztworach

sprzężonych par redoks:

Kierownik projektu: dr inż. Krzysztof Fic

A port of the research work was supported by the LIDER project funded by the National

Centre for Research and Development (NCBiR) LIDER/018/513/L-4/12/NCBR/201

"Electrochemical capacitor with high energy density and power operating in coupled

redox couples solutions”.

Project leader: Dr Eng. Krzysztof Fic



I am sincerely grateful to my supervisor,

Prof. François Béguin,

for his guidance and all the efforts he put in my PhD work

I am also greatly thankful to Dr hab Eng Krzysztof Jurewicz,

for our collaborative work on carbon materials and supercapacitors

It is also a great pleasure to thank

Prof. Dr hab Elżbieta Frąckowiak,

and Dr Eng Krzysztof Fic for helping me to develop the skills and knowledge

in electrochemistry and carbon materials

My sincere gratitude is also dedicated

to all the ECOLCAP group members, especially, Dr Qamar Abbas,

M.Sc Eng Piotr Skowron

and M.Sc Eng. Paweł Jeżowski for their experimental support



TABLE OF CONTENTS

INTRODUCTION _____________________________________________________ 9

CHAPTER I

LITERATURE REVIEW ON ELECTROCHEMICAL CAPACITORS __________ 16

I.1. General properties of electrochemical capacitors______________________ 17

1.1. The electrical double-layer models ______________________________________________ 17

1.2. Operation principle of an EDLC _______________________________________________ 19

1.3. Energy and power of electrochemical capacitors ___________________________________ 21

1.4. Pseudo-capacitive contribution _________________________________________________ 23

I.2. Electrode materials for electrochemical capacitors ____________________ 25

2.1. Commonly used carbon materials_______________________________________________ 25

2.2. Redox-active electrode materials _______________________________________________ 31

I.3. Structural and textural properties of activated carbons__________________ 31

3.1. Manufacturing of porous carbons _______________________________________________ 31

3.2. Surface functional groups on carbons ____________________________________________ 33

3.3. Effect of porous texture of activated carbons on the capacitive performance______________35

I.4. Electrolytes for electrochemical capacitors___________________________ 39

4.1. Aqueous electrolytes_________________________________________________________ 40

4.2. Organic electrolytes _________________________________________________________ 48

4.3. Ionic liquids _______________________________________________________________ 49

I.5. Conclusion ___________________________________________________ 51

CHAPTER II

ELECTROCHEMICAL TECHNIQUES

FOR ELECTROCHEMICAL CAPACITORS INVESTIGATION _______________ 53

II.1. Cyclic voltammetry ____________________________________________ 54



II.2. Constant current charging/discharging ______________________________ 56

II.3. Impedance spectroscopy _________________________________________ 58

II.4. Accelerated ageing test __________________________________________ 60

CHAPTER III

STATE OF HEALTH OF AQUEOUS ELECTROCHEMICAL CAPACITORS

WITH STAINLESS STEEL CURRENT COLLECTORS

UNDER ACCELERATED AGEING _____________________________________ 63

III.1. High voltage ageing assessment of AC/AC electrochemical capacitors

in lithium sulfate electrolyte ______________________________________ 65

1.1. Exploring the high operating voltage of AC/AC electrochemical capacitors

in lithium sulfate electrolyte ___________________________________________________ 65

1.2. Degradation of ECs electrochemical performance under accelerated ageing ______________ 67

III.2. Factors contributing to ageing in aqueous electrolyte __________________ 74

2. 1. Oxidation of carbon electrodes and corrosion of stainless steel current collectors __________ 74 2.1.1. Post-floating analysis of ECs by electrochemical techniques __________________________ 74 2.1.2. Post-floating analyses on carbon electrodes _______________________________________ 78 2.1.3. Effect of temperature on ageing ________________________________________________ 82

2.2. Gas evolution during floating __________________________________________________ 83

III.3. Conclusion ___________________________________________________ 87

CHAPTER IV

STRATEGIES FOR IMPROVING THE LONG TIME PERFORMANCE

OF HIGH VOLTAGE CAPACITORS IN AQUEOUS ELECTROLYTES ________ 89

IV.1. Corrosion reduction of positive current collector ______________________ 90

1.1. Alternative nickel current collectors _____________________________________________ 91

1.2. Improvement of the current collector/electrode interface _____________________________ 95 1.2.1. Carbon electrodes glued to stainless steel current collectors __________________________ 95 1.2.2. Nickel foil substrate _________________________________________________________ 97 1.2.3. Carbon conductive sub-layer _________________________________________________ 100

1.3. Addition of corrosion inhibitor ________________________________________________ 103



IV.2. Shifting of electrodes operating potentials __________________________ 109

2.1. Asymmetric configuration ___________________________________________________ 109

2.2. Current collectors coupling___________________________________________________ 117

IV.3. Conclusion __________________________________________________ 122

CHAPTER V

TOWARDS A NEW CONCEPT

OF HIGH VOLTAGE AC/AC CAPACITOR IN AQUEOUS ELECTROLYTES__ 124

III.1. The new concept of high voltage cell in aqueous electrolytes ___________ 125

III.2. Extension of voltage range by electrodes asymmetry _________________ 134

2.1 Adjustment of electrodes potential window by increasing m+/m- ______________________ 134

2.2. Voltage extension by use of different carbon electrodes ____________________________ 136

III.3. Conclusion __________________________________________________ 138

GENERAL CONCLUSION ____________________________________________ 138

EXPERIMENTAL ANNEX____________________________________________ 142

A.1. Cell construction _________________________________________________ 143

1.1. Materials and chemicals _____________________________________________________ 143

1.2. Preparation of electrodes ____________________________________________________ 145

1.3. Cells configurations ________________________________________________________ 146

A.2. Electrochemical characterization ____________________________________ 147

A.3. Physico-chemical and surface characterization _________________________ 147



REFERENCES ______________________________________________________ 149

SCIENTIFIC ACHIEVEMENTS________________________________________ 165

ABSTRACT ________________________________________________________ 172

STRESZCZENIE ____________________________________________________ 175



INTRODUCTION



Energy management has a deep influence in the humans’ everyday life,

considering social, economic, ecological and political aspects. During the last 50 years,

the world energy consumption, mainly based on petroleum-based fuels (oil, coal and

natural gas), has considerably increased (Figure 1), due to industrial development of the

western countries after the 2nd

World War, accompanied by improving wealth in

emerging markets and growth of the human population, especially in China and India.

Although renewable energy and nuclear power are the world fastest-growing energy

sources in the recent years (each increasing around by 2.5% per year), fossil fuels still

share more than 80% of the global energy consumption [1].

Figure 1 World energy consumption (based on [2]).

Over the past decade, a general awareness appeared that fossil fuel consumption

presents severe drawbacks, such as an important depletion of reserves and the emission

of noxious gases leading in particular to the greenhouse effect and to associated

temperature increase of the planet. The industry is partly able to handle with some of

these problems, by introducing modern solutions, such as reducing emissions by placing

catalysts in the exhaust systems of vehicles and in the chimneys of power plants.

Notwithstanding, if fossil fuels would remain the only power source for the future, the

forthcoming crunch of their availability would lead to economic dislocations and

serious political problems. Therefore, the incoming environmental and economic crisis

predictions have suggested to develop strategies for improving energy efficiency (e.g.,



by improving buildings thermal insulation, by introducing hybridization in

transportation systems, etc.) and for introducing renewables (sun and wind) in the

energy mix. Due to the intermittent character of the ‘clean’ resources, to ensure a real-

time balance of electricity supply to the demand over various time scales, the renewable

technologies require energy-storage devices in order to adapt the energy delivery to the

demand.

Figure 2 shows that energy can be stored via physical and chemical processes

and further delivered in the form of electricity. The main systems are based on gravity

(pumped hydro power storage), Compressed Air Energy Storage (CAES), kinetic

energy (fly wheel), magnetic (Superconducting Magnetic Energy Storage (SMES),

electric field (Electrical Double-Layer Capacitors (EDLCs)) and electrochemical

reactions (batteries). “Pumped-hydro” is the most traditional way of storing energy on a

large scale, by utilizing the excess of electric power to pump water from a lower to a

higher-level reservoir. During the periods of high electricity demand, water is released

to the lower elevation inducing the rotation of turbines and electricity generation.

Notwithstanding, this technology is geographically constrained and requires specific

locations with a sufficient elevation difference between the two reservoirs, which makes

the pumped-hydro plants non-transferable. A second interesting technology for large-

scale storage uses underground air compression (CAES) and requires specific geologic

characteristics. However, the required equipment to store and extract the energy,

including compressors and turbine-generators, generates high cost of the CAES plants.

Moreover, CAES generates heat in excess during compression, which reduces the yield

of the process.

A technology which tends to be well-suited to ensure a real-time balance of

electricity supply to the demand over various time scales is based on flywheels, which

feature in a rapid response time. However, due to the high rotation speed of the rotor,

for long-term performance, they require maintenance, and for this reason, are still

considered to be not completely safe.

Since capital cost and environmental impact are a major barrier to deployment of

energy storage, magnetic energy storage (SMES) seems to be a more economic

technology than, e.g., pumped hydro and CAES. However, a typical SMES system

includes a coil of superconducting material, a power conditioning system and



cryogenically cooled refrigerators, determining the final price of the equipment.

Moreover, SMES is not yet available on a large scale, but only for power application on

a micro scale.

Figure 2 Energy storage systems which rely on physical and chemical processes.

At present, electrochemical systems (secondary batteries, electrochemical

capacitors) appear as the most suited and flexible devices to adapt the electricity

delivery to the demand, provided that the amount of energy involved is not extremely

high. The storage batteries can convert the electrical work generated by, e.g., solar cells,

into chemical free energy needed to force the reaction in a non-spontaneous direction.

Since rechargeable batteries (lead–acid, Ni-Cd, Ni-MH, Li-ion) appear in many

different shapes and sizes, besides the grid energy storage applications, they are also



designed for individual customers to be used in automobile starters, portable devices,

light vehicles and power supplies. Due to the chemical character of the operation, the

discharge rate of batteries is limited and energy is lost due to the internal resistance of

the cell components. Moreover, the concentration of a relatively large amount of

chemical energy into a small package may result in hazardous events, such as numerous

cases of fire and explosion in case of Li-ion batteries.

Electrochemical capacitors, due to their simple construction and the electrostatic

character of energy storage (Figure 2), are characterized by a fast response time, as

compared to the other available devices. As they apply high surface area porous carbon

electrodes immersed in an electrolytic solution, they store several orders of magnitude

higher energy than conventional dielectric capacitors. The most commonly developed

systems at the industrial scale are electrical-double layer capacitors (EDLCs), which

store the electrical charge in the Helmholtz double-layer. Due to the specific principle of

operation, where a nanoscale layer of ions from the electrolyte is attracted to the surface

of a polarized electrode material, ECs display high power density of 15 kW kg-1

when

compared to 2 kW kg-1

offered by, e.g., Li-ion batteries which store the charge through

electrochemical redox reactions. Therefore, ECs are adapted for high power applications

in automotive industry, opening emergency doors of aircrafts, regenerative braking and

stop-start technology in vehicles or power buffer in electric drive train. Moreover, they

have a high cycle life of more than 1,000,000 charge/discharge cycles. However, due to

the electrostatic charge storage mechanism, ECs store lower amounts of energy (5–8

Wh kg-1

) than, e.g., Li-ion batteries (up to 180 Wh kg-1

). Therefore, an important

research attention is focused on enhancing their energy density, while realizing safe,

environmentally friendly and cheap systems.

Since the energy density of ECs strongly depends on the applied maximum

voltage, most of the industrial devices are based on organic electrolytes, although

environment unfriendly and unsafe, which allow reaching 2.7 – 2.8 V. Aqueous

electrolytes such as H2SO4 and KOH have been also investigated for high power

systems, but unfortunately voltage must be limited to less than 1 V in order to avoid

electrolyte decomposition. Lately, it has been demonstrated by our research team that,

by employing aqueous alkali sulfate and gold current collectors, voltage up to 2 V can

be reached, due to a high over-potential of hydrogen evolution at the negative electrode.



Taking into account the numerous advantages of water-based media over the

organic ones, such as high conductivity, low cost, safety in operation and environmental

friendliness, the ultimate aim of this doctoral dissertation is to develop a carbon-based,

environmentally friendly and low-cost electrochemical capacitor (EC) operating in an

aqueous electrolyte with cheap current collectors. To pursue this objective, the

undertaken research requires considering and facing some obstacles which cause the

cell performance to fade and reliability of the EC to decline. The perturbation

phenomena occurring during long time operation of the capacitor are essentially related

to aqueous electrolyte decomposition under high voltage operation, which can lead to

oxidation of AC electrodes and/or internal pressure evolution and corrosion of metallic

current collectors. Overall, the dissertation is divided into five chapters.

Chapter I is a literature review presenting the state-of-art on AC-based

electrochemical capacitors. The operation principle and general properties of electrical

double-layer capacitors (EDLCs) are described, and the common electrode materials

employed for these devices are briefly introduced. The influence of structural and

textural properties of carbons on the performance of electrochemical capacitors is

summarized, with a special attention to the effect of porous texture on the capacitive.

ECs based on organic electrolytes, ionic liquids and aqueous media are critically

compared, with a special emphasis placed on neutral aqueous solutions. Finally, in order

to outline the pathway for the performed investigations, the drawn conclusions contain

issues which still require to be resolved for improving high-voltage operation of carbon

based electrochemical capacitors, while utilizing cheap stainless steel or nickel

collectors and aqueous electrolytes.

To attain information about the performance of electrochemical capacitors,

chapter II presents a survey of the electrochemical techniques used in our investigations.

In order to accelerate ageing of the analyzed devices, a test (so-called ‘floating’),

initially developed by industry for systems with organic electrolyte, has been

implemented and validated during our research on ECs in aqueous media.

The further parts of the dissertation are dedicated to attempts for extending the

operating voltage of carbon-based ECs. The properties and performance of

environmentally friendly AC/AC electrochemical capacitors using neutral salt aqueous

electrolytes, e.g., essentially lithium sulfate, with cheap current collectors are presented



in chapter III. Since all the previous works with promising neutral sulfate electrolytes

were conducted with expensive gold collectors, chapter III identifies the possible

perturbation phenomena which occur during long-term operation in aqueous solution.

The actual effect of operating voltage on the state-of-health (SOH) of the device,

evaluated by measuring cell capacitance and resistance evolution together with internal

pressure evolution, is presented. The changes of physicochemical and surface properties

of the cells’ constituents after long time operation, such as modifications of surface

functionality and porosity of the carbon-based electrodes and corrosion of stainless steel

current collectors are disclosed.

The strategies proposed in chapter IV to improve the long time performance of

AC/AC electrochemical capacitors in the neutral salt aqueous electrolyte are particularly

intended to reduce the corrosion of stainless steel collectors and decrease its destructive

effect on ECs operation. The undertaken tactics involve the replacement of the

corrodible steel current collectors, the protection of the active material/collector

interface and the addition of sodium molybdate corrosion inhibitor to lithium sulfate

electrolyte. Cells with asymmetric configuration of electrodes and coupled kinds of

current collectors are presented in the second part of chapter IV to avoid the

decomposition of aqueous electrolyte by shifting the operating electrodes potentials to

lower values.

Chapter V introduces a new concept of AC / AC cell using potassium hydroxide

and sodium sulfate as catholyte and anolyte, respectively, and a cationic exchange

membrane. Due to the pH difference between the two electrolytes, the cell can operate

at higher voltage than the thermodynamic stability limit of water, e.g., 1.23 V. The

effect of cell asymmetry, either by electrodes mass balancing or by use of different ACs,

is critically discussed with regard to fit the electrodes potential extrema within the

thermodynamic limits of water oxidation and hydrogen evolution. Besides, the proof-of-

concept allows a better understanding of the over-potential origin at the negative

electrode of AC/AC capacitors in neutral aqueous electrolytes.

Finally, the manuscript ends with a general conclusion and perspectives for

future research in the directions investigated and presented in this dissertation.



CHAPTER I

LITERATURE REVIEW

ON ELECTROCHEMICAL CAPACITORS



This chapter presents an overview on electrochemical capacitors literature

appeared during the last decades. After a short introduction about the operation

principle and general properties of electrical double-layer capacitors (EDLCs), the

review will be focused on the fundamental role played by porous carbons and

electrolytes on the electrochemical performance of EDLCs. The effect of pore size on

the electrical double-layer capacitance (Cdl) and the strategies to adjust the pore size to

the size of electrolyte ions will be emphasized. Particular attention will be paid to the

ways by which the researchers exploit the potentialities of electrolytic solutions and

carbons to increase the energy density by capacitance and voltage enhancement.

Electrolytes with extended stability window which are designed and customized for ECs

will be presented, with a special emphasis on aqueous media. The sources of

capacitance enhancement through faradaic contributions arising from oxygenated

functional groups on the surface of carbons, redox-active electrode materials,

electrochemical hydrogen storage and finally redox-active electrolytes will be also

discussed.

On the basis of this literature review, the chapter finishes with a conclusion

introducing the consecutive parts of the thesis, and emphasizing issues required to be

improved for designing a high voltage ecologically friendly capacitor in salt aqueous

electrolyte.

I.1. General properties of electrochemical capacitors

Electrochemical capacitors store energy in an electrical double-layer by

electrostatic interaction at the interface created between the conductive solid material

and the electrolyte [3, 4]. Contrary to conventional capacitors (such as aluminum

electrolytic capacitors) which contain a dielectric material sandwiched between two

electrodes facing each other, EDLCs use the electrical double-layer in their function.

1.1. The electrical double-layer models

Over the last two centuries, scientists have developed various models of the EDL

defining how ions from the electrolyte aggregate at the surface of polarized electrodes

and in their vicinity. Helmholtz was the first to describe the phenomena which occur at

the solid conductor-electrolyte boundary, and suggested that the interface consists of



two electrical layers which are: (i) electrons at the surface of the electrode, (ii) and a

monolayer of ions in the electrolytic solution [5].

One of the shortcomings of the Helmholtz model was the assumption of

stationary conditions where ions accumulate on the electrode surface. It did not take into

account that, due to their motion, ions are not only compacted at the surface of the

electrode, but form a diffuse space charge. Therefore, in the 1900’s Gouy and Chapman

formulated a model according to which the capacitance depends also on the applied

potential and ions concentration n [6], and is expressed by the equation (1):

𝑪𝑮𝑪 =𝜺𝜿

𝟒𝝅𝒄𝒐𝒔𝒉

𝒛

𝟐 (1)

where 𝜅 is the Debye-Hückel length [m] described in equation (2):

𝜿 = √𝟖𝝅𝒏𝒆𝟐𝒛𝟐

𝜺𝒌𝑻 (2)

z - the valency of ions, n - the number of ions per cm3, T- the absolute temperature [K],

and k – the Boltzmann constant (1.3806488 10-23

J K-1

).

More than twenty years later, Stern included in his model both a compact and a

diffuse layer [7], while Grahame divided this combined Stern layer into two regions [8]:

(i) a layer of adsorbed ions at the surface of the electrode, referred to as the inner

Helmholtz plane (IHP) (ii) and an outer Helmholtz plane (OHP) formed by the diffuse

ions in the vicinity of the electrode surface. From the Grahame model, the capacitance C

of the double-layer is described by equation (3):

𝟏

𝑪𝑮=

𝟏

𝑪𝑯+

𝟏

𝑪𝑮𝑪 (3)

with 𝐶𝐻, which corresponds to the specific capacitance of the Helmholtz’ compact

double-layer, and 𝐶𝐺𝐶 which results from the diffuse layer described by Gouy and

Chapman.

The currently used model (BMD model) of the electrical double-layer was

described by Bockris, Devanathan and Muller [9], who proposed that a water layer is



present at the surface of the electrode and some other water molecules are displaced by

specifically adsorbed ions (e.g., redox ions) which contribute to the pseudocapacitance.

The BMD model may be extended to charge-transfer reactions occurring in organic

electrolytes with polar solvents, e.g., acetonitrile (AN), contributing to the potential

drop across the electrode/electrolyte plane. As presented on the example of a negatively

polarized electrode (Figure 3), the inner Helmholtz plane (IHP) passes through the

centers of the specifically adsorbed ions and solvent molecules, which are oriented

parallel to the electric field. Then, the outer Helmholtz plane (OHP) passes through the

solvated ions centers, which are outside the IHP. Behind the outer Helmholtz plane,

there is a diffuse layer region.

Figure 3 Schematic representation of the BMD double-layer model on a negatively

polarized electrode (based on [9]).

1.2. Operation principle of an EDLC

In general, EDLCs are made from two identical electrodes made from a porous

material (the most commonly carbon) coated on a current collector and separated by a

porous membrane soaked with the electrolyte. When a device is connected to a power



supply the ions from the electrolyte aggregate on the surface of positively and

negatively polarized electrodes (Figure 4). As energy accumulation proceeds during

charging, the device is equivalent to two capacitors in series of capacitance C+ and C-

and resistance Rf+ and Rf-. The electrical double-layer capacitance of each electrode Cdl

is given by formula (4) [3]:

𝑪𝒅𝒍 =𝜺𝒓𝜺𝟎𝑺

𝒅 (4)

where S is the surface area of the electrode/electrolyte interface, εr - the relative

permittivity of the electrolyte, ε0 - vacuum permittivity (ε0= 8.854·10−12

F m-1

), d - the

EDL thickness.

.

Figure 4 Schematic representation of the charged state of a symmetric electrical double-

layer capacitor using porous carbon electrodes and its simplified equivalent circuit [10].

Even in a symmetric capacitor, due to the different size of cations and anions in

the electrolyte, the two electrodes display different capacitance values. Due to the series

equivalent circuit, the capacitance C of the total system is given by equation (5):



𝟏

𝑪=

𝟏

𝑪++

𝟏

𝑪− (5)

According to this relationship, the electrode with the smallest capacitance determines

the capacitance of the system.

1.3. Energy and power of electrochemical capacitors

The stored energy is directly related to ECs’ capacitance C and operating

voltage window U, according to equation (6):

𝑬 =𝟏

𝟐𝑪𝑼𝟐

(6)

Likewise, the maximum power density also depends on the applied voltage and is given

by formula (7):

𝑷 =𝑼𝟐

𝟒𝑹𝒔 (7)

with Rs which states for the equivalent series resistance (ESR) of the device. During the

charging and discharging processes, as the charges pass, the EDL flows to and from the

electrolyte/electrode interface, and electrical losses take place. The main contributions

to ESR come from [11]:

• electrolyte resistance;

• electrode material resistance;

• electrode/current-collector interfacial resistance;

• ionic (diffusion) resistance of: (i) ions reaching small pores; (ii) ions moving

through the separator.

In order to customize energy storage devices for a wide range of applications,

energy and power are plotted versus each other in a so-called Ragone plot. Figure 5

shows the significantly large area covered by the ECs, which can deliver more power

(up to 15 kW kg-1

) than redox systems such as Li-ion batteries (up to 2 kW kg-1

) [12].

However, the specific energy reached by ECs is much lower than for Li-ion batteries,

(5–8 Wh kg-1

compared to up to 180 Wh kg-1

, respectively) [13].



Figure 5 Ragone plot of various electrochemical energy storage systems (adapted from

[14]).

The diagonal dashed lines in Figure 5 are obtained by dividing the energy

density by power, and inform how fast the energy can be distributed. This time constant

of the device τ reveals the electrical losses during the charge storage, and is related to

the equivalent series resistance Rs and capacitance of the system C according to formula

(8):

𝝉 = 𝑹𝒔𝑪 (8)

As seen in Figure 5, the charging/discharging process of EDLCs is very fast; this is due

to the purely physical character of the storage mechanism in the electrical double-layer.



Since EDLCs are able to deliver all the stored energy within few seconds, they are

particularly adapted for applications which require energy pulses during short periods of

time, e.g., electric and hybrid vehicles, cranking of diesel engines and renewable energy

harvesting, tramways, buses, cranes, forklifts, wind turbines, electricity load leveling in

stationary and transportation systems, in opening emergency doors of aircrafts, etc. [12,

15].

Notwithstanding, the charge/discharge mechanism in EDLCs is fully reversible,

with efficiency close to 100%. Therefore, the commercially available devices display a

high cycle life of more than 1,000,000 charge/discharge cycles [16].

1.4. Pseudo-capacitive contributions

Whilst the main mode of energy storage in EDLCs originates from electrostatic

charging, there are also pseudo-capacitive contributions associated with fast faradic

reactions at the electrode-electrolyte interface (Figure 6). In this case, the relation

between the charge exchanged dq and the change of potential dE is given by the

formula (9) [3, 17] as in a capacitors:

𝑪 =𝒅𝒒

𝒅𝑬 (9)

Figure 6 Schemes of EDL and faradaic energy storage in electrochemical capacitors

[18].



The pseudo-capacitive contributions are mainly associated with, e.g., redox

reactions of electroactive species and electrosorption of nascent hydrogen or metal

atoms (underpotential deposition). The contribution to capacitance from redox reactions

comes from faradaic electron transfer involving an electrochemically active material

and/or electrolyte species at the surface of an electrode. In the equilibrium state, the

value of potential E is described by the Nernst equation (10) [19]:

𝑬 = 𝑬𝟎 −𝑹𝑻

𝒛𝑭 𝒍𝒏

𝒂𝒐𝒙

𝒂𝒓𝒆𝒅 (10)

where E0

is the standard electrode potential, R- gas constant (8.314472 J K-1

mol-1

); T -

absolute temperature, z – number of moles of electrons transferred in the half-reaction,

F- Faraday constant (9.648 533 . 10

4 C mol

-1), a - chemical activity of reducer (ared) and

oxidant (aox). When an electric current is applied, the equilibrium is disrupted and the

electrode potential is changed to a value which depends on the amount of charge

transferred q, where q is the product of the moles number z and Faraday constant F. The

change of potential value is influenced by several factors: (i) the ionic conductivity of

the electrolyte, (ii) the transport of species which participate in the reaction; (iii) and

phase transition phenomena.

Another source of pseudocapacitance includes the reversible adsorption of

atomic species at the surface of an electrode, accompanied by a partial transfer of

charge, depending on the charge of the adsorbed atomic species A and the charge

density at the electrode surface area S, as described by equation (11) [20]:

𝑨±𝒄 + 𝑺𝟏−𝜽𝑨

± 𝒆−𝑬 ↔ 𝑺𝜽𝑨

𝑨𝒂𝒅𝒔 (11)

where, c - concentration of adsorbable ions, 1-θA is the fractional free surface area

available for adsorption, θA - coverage, E - potential. This specific process occurs when

the adsorption of, e.g., anions is not only electrostatic in origin but also depends on

electronic interactions between the valence electrons of the adsorbed anions and the

surface orbitals of the electrode.



Since the dissertation is focused on aqueous electrolytes, the pseudocapacitive

effects which are likely to appear in these electrolytic solutions are presented in

paragraph 2.2.

I.2. Electrode materials for electrochemical capacitors

Since the electrodes are the key part of electrochemical capacitors (ECs), the

kind of selected electrode materials is very essential to determine the properties of ECs.

In this section, the storage principles and characteristics of electrode materials,

including carbonaceous materials for EDLCs and redox-active electrodes for ECs are

briefly depicted. Since the objective of this dissertation is related to the design of a low

cost and environment friendly capacitor operating in aqueous electrolyte, special

attention will be paid in the next section (I.3.) to the influence of surface properties of

activated carbons (AC) for achieving high power and energy density.

2.1. Commonly used carbon materials

In order to obtain a system characterized by high energy and power and

excellent cycle life, materials with good physical properties and chemical inertness

should be applied. Therefore, porous carbons are the most widely used electrode

materials for EDLCs, due to their [11]:

• high electrical conductivity,

• high specific surface-area (from around 1 to around 2600 m2 g

−1),

• good corrosion resistance,

• relatively easily controlled porous texture,

• processability and compatibility in composite materials,

• low cost of production

• various forms (powders, fibers, nanotubes, graphene, foams, fabrics, composites,

etc.).

Figure 7 presents the most commonly used carbons as electrodes for EDLCs,

which include: activated carbons (ACs) [4, 21], carbon nanotubes (CNTs) [22], onion-

like carbons (OLCs) [23], graphene [24] and carbide-derived carbons (CDCs) [25].

Nonetheless, low cost and high specific capacitance are the essential criteria which

determine the choice of activated carbon as material for EDLCs.



Figure 7 Electron microscopy images of high surface area carbon materials: (a)

scanning electron microscopy (SEM) of AC particles [26]; (b) SEM of AC fabrics [27];

(c) SEM of AC fibers [27]; (d) SEM of vertically aligned CNT forest [28]; (e) SEM of

CNT fabric [28]; (f) SEM of randomly oriented CNTs within CNT paper mats [29]; (g)

transmission electron microscopy (TEM) of carbon onions [30]; (h) SEM of multilayer

graphene flakes [31]; (i) SEM of carbide derived carbons (CDC) [32].

Activated carbon

Activated carbon (AC) is a very complex and highly disordered material made

of nano-scale units. In the early model of non-graphitizable carbon proposed by

Franklin (Figure 8a) [33], the units constituted of few graphene layers [34] are oriented

randomly and connected with each other. The cross-links are sufficiently strong to

impede the movement of the layers to a more parallel arrangement. However, after the

model proposed by Stoeckli [35], it is believed that ACs sometimes involve single

fragments of graphene curved layers connected with each other, as presented in Figure

8b. It was found by high-resolution electron microscopy that high temperature treatment

of non-graphitizable carbon entails the production of faceted particles made of

misoriented stacks of parallel graphene layers [36].



Compared to some other forms of carbons (e.g., CNTs, OLCs), ACs are

characterized by a lower conductivity, which for supercapacitor electrodes is usually

compensated by using a percolator (carbon black or CNTs addition) and by appropriate

electrodes manufacturing process [37, 38, 39].

Figure 8 (a) 2D model of a non-graphitizable carbonaceous material [33]; (b) 3D model

of carbonaceous material [40].

Carbon nanotubes

Carbon nanotubes (CNTs) form a cylindrical 1D structure which contains either

one rolled-up graphene layer (single-wall CNT - SWCNT) or several ones (multiwalled

CNT - MWCNT) (Figure 9). Generally, they are produced either by catalyst assisted

chemical vapor deposition (CCVD) using a hydrocarbon feedstock, such as methane,

acetylene and propylene [41] or by CVD deposition in the nano-channels of an anodic

alumina template [42].

In contrast to ACs and CDCs, CNTs have relatively low SSA and low density,

which limit the volumetric capacitance and energy density of CNT-based EDLCs.

However, high electrical conductivity and open porosity of CNTs allow fast transport of

ions, and thus the system to reach high power.



Figure 9 (a) Structure of a single-wall carbon nanotube (SWCNT) and (b) multi-walled

carbon nanotube (MWCNT) [43].

Carbon onions

Carbon onions, also called carbon nano-onions (CNOs) or onion-like carbons

(OLCs) owe their name to the layered structure reminiscent to an onion, which contains

spherical closed carbon shells of fullerene or polyhedral nanostructure (Figure 10). They

offer a specific surface area up to 500-600 m2 g

-1 which is fully accessible to ions [30].

They are produced via several techniques, such as electron beam irradiation,

condensation of carbon vapor and vacuum precursor. Due to their 0D structure, small

diameter (<10 nm), high electrical conductivity, relatively easy dispersion as compared

to 1D nanotubes and 2D graphene, OLCs appear as a promising electrode material [44].

However, due to their high cost and low capacitance (about 30 F g-1

), they are more

preferably used as conductive agent to carbon based electrodes for high-power EDLCs.

Figure 10 3D structure of onion-like carbon (OLC) [45].



Carbide-derived carbons (CDCs)

Carbide-derived carbons (CDCs), also known as tunable nanoporous carbons,

are a class of highly porous carbon materials derived from binary (e.g. SiC, TiC) or

ternary carbides (e.g., Ti2AlC, Ti3SiC2), polymer-derived ceramics (e.g., Si-O-C or Ti-

C) or carbonitrides (Si-N-C) by selective etching of the metal atoms [46]. The most

commonly used preparation method of CDCs is a reactive extraction of the metal from

carbides with chlorine, where carbon grows from the outside to the core of particles

(Figure 11). To avoid sintering and aggregation of the material, generally, the synthesis

temperature does not exceed 1200 °C. In the last few years, CDCs attracted a lot of

attention as electrode materials for ECs and hydrogen storage applications, due to their

high specific surface area (up to 3100 m2 g

−1 for CDCs synthesized by electrospinning

of polycarbosilane with subsequent pyrolysis and chlorination) and broad range of pore

sizes (0.3 – 30 nm) [47]. Owing to the highly tunable porosity, SiC-CDC enables to

reach gravimetric capacitance of 75 F g-1

in 1.5 mol L-1

TEABF4/AN [48]. For the

further developments of this manuscript, structural/textural properties of CDCs and

activated carbons (ACs) will be considered as comparable.

Figure 11 Scheme of the carbide conversion to carbide-derived carbon (CDC)

depending on the reaction time [49].

Graphene

Graphene is a 2D structured carbon material with fully accessible surface area

(reaching in theory 2670 m2 g

-1) and high conductivity. However, due to the strong π-π

interactions, the graphene sheets tend to restack (Figure 12), which is a critical issue

entailing a decrease of accessible surface area and reduction of ions diffusion rates.

Therefore, techniques such as exfoliation and reduction of graphene oxide (GO), e.g.,



via microwave irradiation or heating of GO in propylene carbonate (PC), are applied to

increase the gravimetric capacitance of graphene-based electrodes (190 F g-1

in aqueous

and 120 F g-1

in organic electrolytes) [50]. Recently free-standing holey graphene

frameworks (HGF) with efficient ion transport pathways were reported [51]. The HGF

were prepared through hydrothermal reduction graphite oxide (GO) with simultaneous

low temperature etching of graphene, owing to the presence of H2O2 molecules. Due to

the formation of nanopores in the graphene sheets, this 3D self-assembled structure

enables to reach high and stable capacitance values (298 F g-1

) in 1-ethyl-3-

methylimidazolium tetra-fluoroborate/acetonitrile (EMIMBF4/AN) during 25,000

galvanostatic cycles with current density of 25 A g-1

.

Figure 12 Model of a layered microscopic segment of graphene sheets. [52]

2.2. Redox-active electrode materials

In the past decades, many redox-active materials have been studied to gain

additional charge from electrochemical reactions, such as conducting polymers [53] or

transition metal oxides (RuO2, MnO2, Fe3O4) [54, 55, 56]. However, due to the faradaic

charge storage mechanism, ECs with redox active electrodes do not exhibit long time

operation with a high efficiency.

Over the years, one of the most studied materials with pseudocapacitive

behavior has been conductive ruthenium oxide (RuO2) in acidic electrolytes. During the

transitions from the Ru+II

oxidation state to Ru+IV

, a fast and reversible electron transfer

with simultaneous electrosorption of protons on the surface of RuO2 particles takes

place, according to reaction (12) [14]:

)(22 OHRuOeHRuO

(12)



where 0 ≤ ≤ 2. The three distinct oxidation states of ruthenium during insertion or de-

insertion of protons (Ru+II

, Ru+III

and Ru+II

) occur within 1.2 V, and allow ECs with

amorphous RuO2 reaching specific capacitance values of more than 600 F g-1

[57].

Although, capacitance enhancement in Ru-based aqueous electrochemical capacitors is

very attractive, their applications are limited due to the very high price and voltage

window of only 1 V.

Therefore, less expensive oxides have been studied, such as iron, vanadium,

and cobalt oxides, with particular emphasis on manganese oxide. In capacitors with

MnO2 electrodes, the charge storage mechanism is based on the adsorption of cations

from the electrolyte (C+ = K

+, Na

+…) and incorporation of protons. Therefore, these

reversible surface redox reactions are fast and close to those in pure EDLC, according to

the reaction (13):

zHMnOOCezzHCMnO )(2 (13)

In neutral aqueous electrolytes, MnO2 micro-powders or micrometer-thick films exhibit

specific capacitance of ~150 F g–1

within a voltage window of less than 1 V. Therefore,

MnO2 electrodes are frequently used in asymmetric configuration with an AC negative

electrode, as an attractive alternative to conventional pseudocapacitors or EDLCs.

I.3. Structural and textural properties of activated carbons

To improve the performance of electrodes, researchers try to optimize the

properties of carbons, focusing essentially on conductivity and specific surface area.

However, to better understand the role of carbon materials in ECs, it is also important to

consider their structural/nanotextural diversity and surface functionality in more details.

3.1. Manufacturing of porous carbons

The vast majority of carbon based electrode materials is derived from organic

precursors by so-called carbonization process which involves heat treatment of a sample

in inert atmosphere. Therefore, the structural and textural properties of carbons are

dependent on the precursor, its state (e.g., solid material, gel) and conditions of

processing [58]. The common natural organic precursors for activated carbon synthesis



include: coal, peat, fruit stones, nut shells, wood, petroleum coke, pitch, lignite, starch,

sucrose, corn grain, leaves, coffee grounds, straw etc. [59, 60, 61, 62, 63, 64, 65, 66]. In

general, carbonized samples from natural organic precursors have a relatively low

porosity with a large number of interstices which block the pore entrances. Therefore,

the pre-carbonized product must be further physically or chemically activated in order

to open the porosity and to create new pores. The physical activation is conducted by

gasification of the pre-carbonized char at temperatures ranging from 700 to 1000 °C, in

the presence of an oxidizing agent (such as CO2, steam, air or mixture of these gases),

which increases the pore volume and surface area of the material by a controlled carbon

burn-off, according to equations (14) to (17) [67, 68]:

22 HCOOHC (14)

COCOC 22 (15)

22 COOC (16)

COOC 22 2 (17)

The production of ACs by chemical activation is carried out at slightly lower

temperatures (∼400–700 °C) and generally results in smaller pores and more uniform

pore size distribution [11]. The process involves the reaction of a precursor or a char

with a chemical reagent (such as KOH [69, 70], ZnCl2 [71, 72] or H3PO4 [73, 74]). As

reported, by activation with potassium hydroxide, it is possible to obtain ACs with

specific surface area above 2500 m2 g

−1 [75, 76]. Nonetheless, to remove residual

reactants as well as any inorganic residues (e.g., ash) which originate from the carbon

precursor or are introduced during preparation, post-activation washing is always

required.

Although it is generally believed that the activation process is required to open

the pores of carbonized precursors, carbons with well-developed porosity and good

capacitance values, as well as reproducible properties can be obtained by simple one-

step carbonization of synthetic polymers, e.g., through a rapid microwave heating of

polypyrrole (PPy) [77]. Recently, it has been also presented that self-activation proceeds

during carbonization of appropriate biomass precursors, e.g., tobacco [78] or seaweeds



[79, 80], where the second stage of chemical or physical activation is unnecessary. Due

to the presence of naturally embedded group I and II elements (such as potassium,

calcium, magnesium, sodium), during the thermal treatment, carbonization and self-

activation of the precursor occur simultaneously. For the Burley tobacco, the optimal

self-activation temperature is considered as 800 °C. At higher temperatures, annealing

of the materials dominates and provokes a decrease of specific surface area and average

pore size [78].

3.2. Surface functional groups on carbons

As presented in Figure 8b, carbon materials are constituted of fragments of

graphene layers connected with each other, each fragment containing edges and defect

like vacancies, leading to the development of surface functional groups [68]. As a result

of incomplete carbonization of the porous material, a part of the chemical structure is

associated with heteroatoms which are in the vast majority oxygen and hydrogen, and in

a lesser degree nitrogen and sulfur (Figure 13). Therefore, in addition to electrical

double-layer charging, faradic electron transfer reactions involving the surface

functional groups may be involved in energy storage [81, 82, 83]. In order to enhance

this contribution, the surface functionality of ACs is generally developed through: (i)

electrochemical polarization [84], (ii) chemical treatment [85], (iii) and plasma

treatment [86].

There are three types of surface oxides present on the carbon material, namely,

acidic, basic and neutral (Figure 13) [11]. Surface oxides with acidic nature are formed

when carbons are exposed to di-oxygen at 200-750 °C or by reactions with oxidizing

agents at room temperature. These surface groups include carboxylic, lactonic and

phenolic functionalities. The basic and neutral groups are formed after heat treatment of

AC to eliminate the surface functionalities, and further exposition of AC to di-oxygen at

low temperature. The basic oxygen-containing groups include ethers, carbonyls and

pyrone structures. Although, the acidic or basic nature of quinone/hydroquinone

functionalities is not strongly marked, their contribution to capacitance and creation of

catalytic active sites for, e.g., oxidative dehydrogenation reactions cannot be neglected

[87]. The contribution of quinone/hydroquinone pairs to capacitance can be observed in

cyclic voltammograms by cathodic and anodic waves at ~0 V vs Hg/Hg2SO4.



Nonetheless, the recent trend is to introduce the quinone/hydroquinone redox pair into

the electrolyte, which is simpler than, e.g., grafting of quinone derivatives on the surface

of carbon [88, 89].

Figure 13 Possible functional groups on the surface of carbons related to the presence

of heteroatoms: (a) oxygen, (b) nitrogen, and (c) sulfur. Acidic and basic functionalities

are indicated in red and blue, respectively (adapted from [90]).

Different techniques are available to analyze the surface functional groups on

carbons, such as Temperature-Programmed Desorption (TPD), X-ray Photoelectron

Spectroscopy (XPS), Fourier Transform Infrared spectroscopy (FTIR), and chemical or

electrochemical titration methods (i.e., Boëhm titration) [91]. Nowadays, the most

popular method for characterization of surface oxides starts to be TPD. In this



technique, the functionalities present on the carbon surface are thermally decomposed

releasing primarily CO2, CO and/or H2O at different temperatures [92]. The nature of

the groups is evaluated from the type of released gas and the decomposition temperature

[93]. The TPD patterns of CO and CO2 evolution are a sum of peaks, therefore, to

estimate the amount of each type of oxygenated surface group, the spectra can be

deconvoluted by using, e.g. a multiple Gaussian function (Figure 14) [92].

Figure 14 Deconvolution of TPD patterns for a carbon sample oxidized with 5 mol L-1

nitric acid for 6 hours at boiling temperature: (a) CO2 pattern; (b) CO pattern; TPD

experimental data /; individual peaks ---; sum of the individual peaks -) (adapted from

[92]).

Apart from the capacitive contribution, the presence of functional groups on

the surface of AC influence the double-layer properties of carbon, such as wettability,

rest potential, ESR, leakage current and self-discharge characteristics [3, 11]. As the

amount of oxygen associated with the carbon surface increases, the hydrophilicity of

carbon increases. Therefore, ACs with high oxygen content can be easier wetted by

water than pure carbons without oxygenated surface functionalities.

3.3. Effect of porous texture of activated carbons on the

capacitive performance

The nature of the organic precursor and the conditions of AC synthesis, such as

carbonization/activation temperatures and kind of used activating agent, influence the

pore size distribution of carbon materials. Due to the complex interconnected network

of internal pores, the BET specific surface area of AC ranges between 500-3000 m2 g

-1.



The parameters closely connected with the specific surface area (pore volume of

carbons, size and shape of pores, tortuosity) also play an important role in charge

storage. According to the IUPAC classification, there are three main kinds of pores: (i)

micropores (with diameters <2 nm), mesopores (diameters from 2 to 50 nm) and

macropores (diameters >50 nm) [94]. Since the macropores do not take part in the

actual adsorption processes, their contribution to the total surface area is negligible. Ions

are the most efficiently adsorbed in the micropores providing the high surface area,

while the mesopores are intended to allow the ions to be transported to the micropores

[95, 96, 97]. To enhance capacitance and to lower the ESR values, it is important to

keep an appropriate volume ratio of meso/micropores, while selecting carbons for

EDLCs [98, 99]. For AC/AC electrochemical capacitors in sulfuric acid, the optimum

mesopore volume ratio is in the range of 20 to 50% [100]. The role of micropores is

seen during slow charging (2 mVs−1

scan rate), whilst the beneficial effect of

mesoporous transportation channels on capacitance is pronounced at higher rates [100].

The adsorption of a gaseous medium at a fixed temperature (generally nitrogen

at -192°C) is the most common method used to investigate the porosity of carbons. The

characteristics of activated carbons are estimated by commercial sorption equipment,

generally using in-built software based on the adsorption isotherm of a given

adsorbate/adsorbent system and a model of the adsorption process [101, 102, 103].

Nevertheless, in highly porous materials, the adsorption may occur via a pore filling

mechanism, rather than by surface coverage only (as it is assumed by the Langmuir and

Brunauer–Emmett–Teller theory (BET) [104]). Therefore, in the narrow pores, the

application of the BET equation can lead to unrealistic surface-area (SBET) estimations

[105, 106]. More and more often, the regularized density functional theory (DFT) is

taken into consideration as a more accurate way to correlate capacitance with SSA. In

the model, slit-shaped pore geometry is assumed, and it concerns the adsorption and

capillary condensation in pores of different geometry and surface chemistry [107].

Figure 15a shows that the gravimetric capacitance of ACs and carbon blacks

increases almost linearly with SSA up to SBET ≈ 1500 m2 g

-1, and then for carbons with

higher activation degree a plateau is visible [108]. For the same carbons, the

proportionality region of capacitance with SDFT is more extended than when using SBET,

but still for SDFT higher than 1200 m2 g

-1 a capacitance saturation phenomenon can be

observed (Figure 15b). For carbons materials with SDFT around 1200 m2 g

-1, due to the



increase in pore volume, the carbon pore walls become too thin to accommodate

additional charges, which results in capacitance saturation [108].

Figure 15 Gravimetric capacitance vs (a) BET specific surface area; (b) DFT specific

surface area (adapted from [108]).

To overcome the over- or under-estimation of SSA derived from the BET

equation, it is more accurate to combine gas adsorption and immersion calorimetry for

porous carbons of different origins, as proposed by Stoeckli et al [109, 110]. Contrary to

the anomalous increase of C/SBET (F m-2

) for TiC-based carbons in pores of less than 1

nm when using TEABF4 in AN electrolyte [111], the C/Sav values are constant in pores

between 0.7 and 1.8 nm [112, 113]. Furthermore, in this pore size range, the volumetric

capacitance (C/Wo) increases with decreasing pore width (Figure 16). Interestingly, the

linearization of volumetric capacitance vs L0 led to similar trend in 1 mol L-1

TEABF4

in AN and 6 mol L-1

KOH electrolyte for two series of activated carbons, while

assuming slit-shaped pores [114].

According to equation (4), capacitance might be also overestimated when Lo

decreases, if assuming constant electrolyte dielectric permittivity εr. In fact, since slit-

shape micropores contain a constant amount of ions which are surrounded by a variable

amount of solvent molecules, the relative electrolyte permittivity in micropores

decreases with the solvent to ion ratio, i.e. with the decrease of L0. Therefore, the Feng

model [115] which suggests a gradual decrease of relative permittivity of TEABF4/AN,

explains the almost constant value of C/S in pores below 1 nm. However, the studies on

microporous carbons cannot longer rely on models, which still assume that solvated



ions occupy a central position in micropores, which in turn feature in well-defined shape

and rigidity, and are not interconnected.

Figure 16 Volumetric capacitance of various microporous carbons in TEABF4/AN

electrolyte vs average pore width (Lo) accessible to CCl4; Wo represents the volume of

micropores deduced from the carbon tetrachloride (CCl4) isotherm, assuming that the

diameters of TEA+ (0.68 nm) and CCl4 (0.63 nm) are comparable [113].

From the foregoing, and considering the diameter of solvated TEA+ (1.3 nm)

and BF4- (1.16 nm) and desolvated TEA

+ (0.67 nm) and BF4

- (0.48 nm) [116], it

suggests that ions need to be at least partly desolvated to penetrate into the micropores

[117]. Desolvation of TEA+ and BF4

- was confirmed by nuclear magnetic resonance

(NMR) on AC electrodes extracted from capacitors charged up to different voltage

values in the TEABF4/AN electrolyte. Figure 17 shows the molar proportions of TEA+

and BF4- and the relative amount of AN vs the total amount of electrolyte species after

polarization at various voltages [118]. Predictably, due to charging, large TEA+

cations

in the positive electrode are replaced by smaller BF4-

anions, leaving the place for

solvent molecules, which amount remains nearly constant up to 4.0 V. Simultaneously,

in the negative electrode, small anions are replaced by larger cations, and consequently

the AN concentration decreases rapidly and becomes negligible at 2.7 V (no AN

molecules are left in the micropores of the AC-based electrode). The AN solvent is

expelled by incoming TEA+

and is further stored in the mesopores.



Figure 17 Molar proportions of TEA+ and BF4

- in the positive and negative electrodes

of an AC/AC electrochemical capacitor calculated from NMR spectra, and relative

amount of AN versus the total amount of electrolyte species, after polarization at

various cell potentials for 30 min (adapted from [118]).

I.4. Electrolytes for electrochemical capacitors

In order to extend the range of ECs applications, the current researches seek for

strategies which improve their energy density. According to equation (6), the value of

stored energy can be enhanced either by increasing the capacitance C or by extending

the operating voltage U. Since the latter is closely determined by the stability window of

the applied electrolyte, this paragraph is focused on pros and cons of electrolytes which

are designed and customized for different ECs applications. Beside the electrochemical

stability window, which is a key factor affecting the electrolyte selection, the physical

properties of the electrolytic solution, such as, mobility and molar conductivity of ions,

are found to be also important in terms of energy storage efficiency. It is commonly

known that the charge storage capacitance and resistance of the electrode material are

affected by the nature of the electrolyte, i.e. the ionic radii of unsolvated and solvated

ions, the molar conductivity of ions and their mobility in the pores of electrodes [119].

Calvo et al. showed that it is possible to predict the capacitance for each electrolyte

based on the information about molar conductivity of ions and surface functionality of

the electrode material [120].



The most commonly used electrolytes for ECs are aqueous media (sulfuric acid

and potassium hydroxide), organic electrolytes and ionic liquids (ILs) [121]. According

to formula (4), the capacitance values of ECs with carbon electrodes of same SSA are

significantly higher in aqueous electrolytes than in non-aqueous solutions, due to high

dielectric constant of the aqueous media [122, 123]. The aqueous electrolytes display

values of ionic conductivity up to ∼1 S cm−1

for 30% H2SO4 [11], while for the

commonly used organic electrolytes (e.g., TEABF4 in propylene carbonate) it is only

∼0.02 S cm-1

[124], and ~0.01 S cm-1

for typical room temperature ionic liquids

(RTILs) [125]. The electrolytic solution should be also thermally stable, have low

viscosity, low toxicity and low cost [126]. But yet, none of the available electrolytes

fully meet all the mentioned desires.

4.1. Aqueous electrolytes

On the point of view of production, the main motive for the choice of aqueous

electrolyte is the low cost. While implementing non-aqueous media, all components

(carbon material, separator, electrolyte itself) need to be well-dried in order to ensure a

long cycle-life of the system, whereas drying is not required in case of aqueous

electrolytes, which dramatically decreases the production cost of the final device.

Moreover, water-based solvents provide strong solvation and tendency for complete

dissociation or minimum ion pairing, feature in large dipolar moments (through

hydrogen bonded structures) and high dielectric constants, leading to lower ESR values

than organic solvents [83].

When comparing the most commonly used aqueous electrolytes (sulfuric acid,

potassium hydroxide) in electrochemical power sources, the highest capacitance values

and best electrochemical performance are achieved with H2SO4 due to its greater ionic

conductivity, faster mobility of H+ than K

+ and greater activity of the basic oxygenated

groups on the surface of the electrode material.

Unfortunately, a major disadvantage of water-based electrolytes, when

considering formulae (6) and (7), is their low thermodynamic stability and consequently

the low reachable voltage of 1.23 V [120]. Practically, in symmetric AC/AC

electrochemical capacitors with H2SO4 and KOH aqueous electrolytes it is even less

than 1 V [127, 128, 129, 130, 131], whereas 2.7 V-2.8 V can be reached with ECs in



organic medium [132]. Therefore, researchers still seek for media which assure high

energy storage by extended operating voltage range, while lowering the costs of the

EC’s assembling process and enabling to apply various current collectors due to less

corrosive properties than, i.e., sulfuric acid.

Promising neutral salt aqueous electrolytes

Lately, voltage values as high as 1.6 V were found for AC/AC electrochemical

capacitors in 0.5 mol L−1

Na2SO4 [131, 79] and even 2 V when using 1 mol L−1

Li2SO4

[133]. As presented on the Ragone plots of AC/AC electrochemical capacitors in

Li2SO4 and KOH aqueous electrolytes (Figure 18), the energy density in Li2SO4 is

enhanced by 80% as compared with KOH [128]. The energy and power density reached

at the time constant of 25 s are 12.3 Wh kg−1

and 1.6 kW g−1

in Li2SO4 against 7.2 Wh

kg−1

and 1.0 kW g−1

in KOH, respectively. Furthermore, due to much less corrosive

properties than sulfuric acid, and possibility to extend the operating voltage by

appropriate combination of electrode materials, these neutral electrolytes are by far

much preferable for further scaling-up to an industrial production [130, 134, 135].

Figure 18 Ragone plots of AC/AC capacitors in 1 mol L-1

Li2SO4 and 6 mol L-1

KOH

aqueous solutions with cell operating potential windows 0−1.6 V and 0−1.0 V,

respectively. Values calculated for the total mass of active materials [128].



The different performance of AC in neutral, acidic and basic electrolytes is

presented by the three-electrode cyclic voltammograms in Figure 19. The potential

window in Na2SO4 is roughly twice larger than in the traditional KOH and H2SO4

electrolytes [127, 128, 129, 131]. Such enhancement of the operating potential window

has been attributed either to the strong solvation of cations and anions [133] or to the

high over-potential for di-hydrogen evolution at the negative electrode [136]. SO42-

is

one of the biggest and strongest solvated inorganic anions, having up to 40 water

molecules in the solvation shell, with desolvation energy of about 108 kJ mol-1

per one

bond between SO42-

and water [133].

Figure 19 Potential stability window of activated carbon in 6 mol L-1

KOH, 1 mol L-1

H2SO4 and 0.5 mol L-1

Na2SO4 determined by three-electrode cyclic voltammograms

(2 mV s-1

) [131].

Due to the full reversibility of the chemisorption process, hydrogen storage is

an interesting option enabling a potential faradaic contribution in addition to the EDL

capacitance and extension of the electrochemical stability window. Since ACs are

characterized by highly developed porosity and easily tunable ultramicroporosity, they



appear as the most interesting materials for this purpose. Activated carbons can store up

to 2 wt% of hydrogen formed by electrochemical reduction of water under ambient

pressure and temperature conditions [137, 138, 139, 140, 141].

Under negative polarization, the electrons are supplied to carbon and,

depending on pH, they lead to the formation of nascent hydrogen, accordingly to

equations (18) or (19) [142]:

in acidic solution: H3O+ + e

- → H + H2O (18)

in alkaline solution: H2O + e- → H + OH

- (19)

then, the in statu nascendi hydrogen is rapidly chemisorbed onto the carbon surface

[143, 144]:

C + H → CHad (20)

The increase of negative current below -0.8 V vs NHE, in case of 1 mol L-1

Li2SO4,

indicates the plausible limit for negative polarization beyond which evolution of

gaseous di-hydrogen takes place (as observed by the oscillations due to bubbling on the

CVs (Figure 20)), according to equation (21):

2H → H2 (21)

Di-hydrogen is also partly formed from the chemisorbed hydrogen, accordingly to

equations (22) and (23) [142]:

CHad + H2O + e- → H2 + OH- + C (22)

CHad + CHad → H2 + 2C (23)

The reversible hydrogen chemisorption is further evidenced in the cyclic

voltammograms (Figure 20) by an anodic desorption peak at around 0.4 V vs NHE

[136].



Figure 20 Three-electrode cyclic voltammograms of activated carbon in 1 mol L-1

Li2SO4 obtained by stepwise shifting of the negative potential limit. The vertical dashed

line at -0.383 V vs. NHE corresponds to the thermodynamic potential for water

reduction [145].

The high over-potential for di-hydrogen evolution can be explained by a higher

pH in the porosity of the AC electrode than on its outer surface. With highly porous

electrodes, where the adsorbed species are unable to leave the pores rapidly via

diffusion or electro-migration, the estimation of local pH changes is a difficult issue.

The in-situ pH variations on the carbon electrode surface, when cathodic charging at -

500 mA g-1

was applied, are presented in Figure 21 for electrolytes of different initial

pH [62]. The initial pH values were adjusted by addition of 1 mol L-1

H2SO4 or 1 mol L-

1 NaOH to 0.5 mol L

-1 Na2SO4. After 12 hours of charging, the pH value reached

approximately 11 for all the electrolytic solutions, except for the one with starting pH =

2, for which the value remained unchanged. The pH increase in the medium with initial

pH = 4, is associated with either formation of OH- or reduced amount of H3O

+.

Considering the electrolytic solution with pH = 2, the reduction of H3O+

results in a

negligible increase of pH, due to the excessive amount of hydronium ions. The

presented research highlights again the importance of electrolyte pH on the high voltage

performance of AC-based ECs in aqueous media.



Figure 21 Variations of pH values on the surface of AC electrodes after cathodic

charging for 12 hours (-500 mA g-1

) in 0.5 mol L-1

Na2SO4 (adapted from [62]).

Redox-active aqueous electrolytes

Besides hydrogen storage, redox-active materials and oxygen-rich carbons,

additional charge can also originate from redox reactions involving electrolyte species

at the surface of an electrode. A large part of reports about the redox reactions concern

the carbon/iodine interface formed in aqueous KI electrolyte [146, 147, 148]. The

electrochemical activity of this electrolyte is based on reactions appearing on the

positive electrode (24) to (27):

2𝐼−1 ↔ 𝐼2 + 2𝑒− (24)

3𝐼−1 ↔ 𝐼3−1 + 2𝑒− (25)

2𝐼3−1 ↔ 3𝐼2 + 2𝑒− (26)

𝐼2 + 6𝐻2𝑂 ↔ 2𝐼𝑂3−1 + 12𝐻+ + 10𝑒− (27)

However, the presented transitions occur in a very narrow potential window. Figure 22

shows galvanostatic charge/discharge at 500 mA g-1

and cyclic voltammetry performed

on an AC/AC capacitor with gold current collectors in 1 mol L-1

KI, using a reference



electrode to monitor the behavior of individual electrodes. Reversible redox peaks are

observed at the positive electrode (Figure 22b) between 0 to 0.14 V vs Hg/Hg2Cl2,

whereas the negative electrode has an EDL behavior with a typical rectangular shape of

CV [146]. Due to the series dependence of the electrochemical circuit, the gravimetric

capacitance of the cell (260 F g-1

) is limited by the electrode with the smallest

capacitance, i.e. the negative electrode [146]. The AC/AC cell with this electrolyte and

gold collectors has a good cycle life with more than 80% of the initial capacitance value

after 10,000 galvanostatic cycles at a current density of 1 A g-1

. It is worth to mention

that iodide salts allow using cheap stainless steel current collectors, which broadens the

possibilities of their applications as electrolytes for ECs. After 15,000 charge/discharge

cycles at 2 A g-1

as well as after 150 hours of floating at 1.2 V on AC/AC cell in 2 mol

L-1

NaI, no traces of corrosion of stainless steel collectors were observed [148].

Figure 22 (a) Two-electrode AC/AC cell in 1 mol L-1

KI solution with SCE reference

electrode: galvanostatic charge/discharge (500 mA g-1

); (b) cyclic voltammograms (5

mV s-1

) of the electrodes and of the cell [146].

As previously mentioned, the capacitance of the AC/AC capacitor is limited by

the low capacitance of the EDL electrode (see equation (5)). Therefore, to enhance the

capacitance of the negative electrode, an AC/AC capacitor using 1 mol L-1

KI as anolyte

and 1 mol L-1

VOSO4 as catholyte, and a Nafion membrane as separator has been

developed [149]. With the selected redox-active electrolytes, the galvanostatic (0.5 A g-

1) discharge capacitance of the system reaches 500 F g

-1. The capacitance of the

negative electrode is enhanced by multi-electron reactions as in equations (28) to (32):

𝑉𝑂𝐻2+ + 𝐻+ + 𝑒− ↔ 𝑉2+ + 𝐻2𝑂 (28)

[𝐻2𝑉10𝑂28]4− + 54𝐻+ + 30𝑒− ↔ 10𝑉2+ + 28𝐻2𝑂 (29)



[𝐻2𝑉10𝑂28]4− + 44𝐻+ + 20𝑒− ↔ 10𝑉𝑂𝐻2+ + 18𝐻2𝑂 (30)

𝐻𝑉2𝑂73− + 9𝐻+ + 6𝑒− ↔ 2𝑉𝑂 + 5𝐻2𝑂 (31)

𝐻𝑉2𝑂73− + 13𝐻+ + 10𝑒− ↔ 2𝑉 + 7𝐻2𝑂 (32)

Similarly, hydroquinone has been added to 1 mol L-1

H2SO4 electrolyte,

transforming a symmetric AC/AC electrochemical capacitor into a hybrid redox system

[150, 151]. The development of the quinone/hydroquinone redox reaction on the carbon

surface (Figure 23) [152] led the positive electrode to behave as a battery one, whereas

the negative electrode remains of the EDL type.

Figure 23 Redox reaction of the quinone/hydroquinone redox pair.

However, the battery-like behavior of the positive electrode limits the long-term

stability of the cell in HQ/H2SO4 electrolyte, which demonstrates a decrease in

capacitance to 65% of its initial value after 4,000 cycles up to 1 V at current density of

4.4 mA cm-2

. The initial capacitance decay after 1,000 cycles is probably attributed to

the non-completed quinone/hydroquinone redox reactions within the voltage window

from 0 V to 1 V [151]. Although the hybrid systems demonstrate potentialities for

gaining in capacitance, such cells operate almost always at the expense of the cycle-life.

Besides naturally occurring surface functionalities, active molecules can be

chemically/electrochemically grafted onto the carbon surface to enhance the capacitance

of AC/AC ECs in aqueous media through faradaic contribution. Grafting of quinone

derivatives is usually performed by electrochemical or chemical reduction of diazonium

cations [153, 154, 155]. Since, the possible redox mechanisms involve proton and

electron transfers, the pH of the applied electrolyte has a significant influence on the

capacitance properties of quinone-modified carbons. The attachment of anthraquinone



groups to the surface of mesoporous carbon black (BP2000) enhances the capacitance

from 100 F g-1

for the unmodified carbon to 195 F g-1

for the grafted carbon in 0.1 mol

L-1

H2SO4 [88] However, the highly corrosive character of H2SO4 would impose to use,

e.g., neutral electrolytes, especially in the presence of stainless steel current collectors.

4.2. Organic electrolytes

Most of the industrial EDLCs available in the market are based on organic

electrolytes, due to their high thermodynamic stability window. However, the practical

operating voltage of organic media in symmetric AC/AC electrochemical capacitors

depends strongly on the impurities of the components, such as water and functional

groups on the surface of carbons [156, 157].

The most commonly used organic salt is tetraethylammonium tetrafluoroborate

(TEABF4), due to its moderately good conductivity and good solubility in non-aqueous

solvents. Currently, the most widely used solvent is acetonitrile (AN); however,

propylene carbonate (PC) has also many adherents, especially in Japan. It was reported

that PC exhibits a slightly stronger polarity, a higher density, viscosity and dielectric

constant than AN [158]. However, solutions in AN exhibit lower electric resistance, and

the increase of power density is accompanied by nearly constant energy density values

[159]. The electrolytes based on AN are usually characterized by about four times

higher conductivity than the PC-based ones [160]. Regarding the safety issues, AN has

very low flash point (5 °C) and emits toxic combustion products [161, 162]. Therefore,

researchers have used different types of solvents for organic electrolytes, such as

sulfone, dimethylsulfone, and ethyl methyl carbonate [163]. Moreover, it has been

found recently, that nitrile- and dinitrile-based electrolytes, e.g., adiponitrile (ADN) and

sebaconitrile, due to their high electrochemical stability, are appropriate for i.e., high-

voltage Li-ion batteries [164, 165]. Therefore, ADN started to be also used for EDLCs

[166], and it was revealed that an EDLC in 0.7 mol L-1

TEABF4/ADN is stable up to

3.5-3.6 V with capacitance loss of less than 20% after 50,000 cycles [167]. Adiponitrile

has a very low vapor pressure and a moderated viscosity; however, poor solubility of

TEABF4 in ADN (the maximum concentration of TEABF4 in ADN at 25 °C is 0.8

mol L-1

) limits the physical properties of ADN-based electrolytes [157]. For instance, it

affects their ionic conductivity, which is about 11 times smaller than that of AN-based

electrolytes [166]. Figure 24 shows that an EDLC in TEABF4/ADN does not



demonstrate a rectangular CV, which is attributed to different contributions from

parallel resistances, due to diffusion of solvated ions in the pores of the electrode

material [168]. Thus, the studies are now focused on solutions of ionic liquids in ADN

which exhibit better electrochemical performance, such as those containing 2 mol L-1

1-

ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C2mIM][TFSI] room-

temperature molten salt) in ADN [168] . However, more detailed and comparative

electrochemical studies need to be conducted.

Figure 24 CVs (5 mV s −1

) of EDLCs using different electrolytes: 1 mol L-1

TEABF4 in

AN, 0.7 mol L-1

TEABF4 in ADN and 2 mol L-1

[C2mIM][TFSI] in ADN. The

gravimetric current is expressed by total mass of electrodes [168].

4.3. Ionic liquids

The recent trend focused on electrolytes for EDLCs with large stability window,

concerns AC/AC capacitors in ionic liquids (ILs). ILs are molten salts at room

temperature, entirely composed of cations and anions, which enable to operate at

temperatures as high as 300 °C with very low vapor pressure, featured in non-

flammability and electrochemical stability [126, 121, 169, 170]. Besides, solvent-free

ILs do not possess any solvation shell, and thus can offer a well-defined ion size,

enabling better understanding of the behavior of ions in the porosity of carbons and the

design of proper electrode materials. The most commonly used ILs for EDLCs are



imidazolium, pyrrolidinium, and ammonium salts with anions such as tetrafluoroborate,

trifluoromethanesulfonate, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide

and hexafluorophosphate [126]. Galvanostatic cycling of an AC/AC electrochemical

capacitor in N-methyl-N-butyl-pyrrolidinium bis(trifluoromethylsulfonyl) imide

(PYR14TFSI) demonstrated 95% efficiency at 3.5 V and 60°C after 65,000 cycles [169].

A constant voltage hold (floating) at 3.4 V revealed a long time operation (500 hours) of

EDLCs based on 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) with

mesoporous carbon black (BP 2000) [171].

Since high viscosity, and thus high resistance and low conductivity at room

temperature (typically ~14 mS cm-1

[172]) influence the efficiency of the charging and

discharging processes, the conductivity of EDLC with ILs phosphonium salts could be

improved by adding 25 wt% of acetonitrile (Figure 25) [173]. For capacitors with the

same mass of KOH activated carbon in the electrodes, the operating voltage is

significantly increased to 3.4 V in the case of the ILs/AN 25% electrolyte in comparison

with the conventional organic one (TEABF4/AN) and the aqueous acidic solution (1 mol

L-1

H2SO4). The irregular shape of CV which is determined by different size of cation

and anion of IL, suggests a need of suitable matching of pores size of positive and

negative AC electrode with the ions size of the ILs.

Figure 25 Cyclic voltammograms (5mV s-1

) for AC-based ECs in IL phosphonium salt

/AN 25%, organic and acidic electrolytic solutions [173].



Notwithstanding, despite the wide voltage window of ILs, low conductivity at room

temperature, high price and complex purification processes restrict further scaling-up.

Therefore, recently, the Azepane compound, which is a cheap by-product of polyamide

industry, was used for the synthesis of Azp14TFSI and Azp16TFSI [174]. However, the

large cation size of this electrolyte results in greater viscosity and lower conductivity as

compared to, e.g., Pyr14TFSI. A further study on potentialities of this new class of

electrolytes for EDLCs is still required [175].

I.5. Conclusion

In this chapter, the state-of-the-art on electrochemical capacitors (ECs) has been

presented. The charge storage mechanism of electrical double-layer capacitors EDLCs

is based on electrostatic interactions between electrolyte ions and the charged surface of

carbon electrodes. Since the pure charging of the electrical double-layer does not

involve any electron exchange, the power in EDLCs is much higher than in lithium

batteries; however, in turn, the energy density is lower than in batteries. Therefore, most

research efforts are focused on the energy density enhancement. It can be done by

controlling voltage and capacitance.

The voltage range is essentially limited by the electrochemical stability of the

electrolyte. Organic electrolytes allow high potential window – around 2.7 – 2.8 V to be

reached, against around 1 V for conventional aqueous electrolytes applied in battery

systems (sulfuric acid and potassium hydroxide). Hence, organic solutions are preferred

in many industrial capacitors, despite their high cost, environment unfriendliness and

low conductivity, while compared to aqueous media. A lot of attention is recently paid

to ionic liquids, however, considering economical, safety and ecological aspects,

aqueous electrolytes exhibit numerous advantages, and excel in high power densities as

well. Furthermore, while considering the operating voltage window of ECs based on

aqueous electrolytes, neutral aqueous sulfates, due to high over-potential of hydrogen

evolution and strong solvation of ions, offer to reach 1.6 V in Na2SO4 and even 2 V in

Li2SO4.

Since all the previous works with promising neutral sulfate electrolytes were

conducted with expensive gold collectors, this thesis research will be focused on design

and development of an environmentally friendly AC/AC electrochemical capacitor



using 1 mol L-1

Li2SO4, with cheap stainless steel current collectors. Furthermore, the

analysis on possible perturbation phenomena which occur during long-term operation of

ECs has been never conducted in the domain of high voltage AC/AC capacitors using

aqueous electrolytes. Therefore, the actual effect of operating voltage on the state-of-

health (SOH) of the device under accelerated ageing needs to be evaluated. The

identification of factors contributing to ageing of ECs with cheap stainless steel or

nickel current collectors in aqueous electrolytes is a preliminary step to apply any

further improvement for the long time performance of these systems.



CHAPTER II

ELECTROCHEMICAL TECHNIQUES

FOR ELECTROCHEMICAL CAPACITORS

INVESTIGATION



This chapter presents the most frequently used electrochemical techniques for

supercapacitor investigations, which include Cyclic Voltammetry (CV), Galvanostatic

Cycling with Potential Limitation (GCPL) and Electrochemical Impedance

Spectroscopy (EIS), as well as a technique adapted from industry to evaluate their cycle

life (floating). EIS is a stationary technique which does not involve current and potential

variations, whereas the transient ones (CV and GC) enable to investigate the whole

supercapacitor and also each electrode separately, by measuring current or potential

response. These basic techniques have served to establish a model of ideal EC

comprising a capacitor with equivalent series resistance (ESR) and parallel leakage

resistance (Rf) which determines the charge loss also referred to as self-discharge [176].

Among the years, the scientists have worked on specifying equivalent models which

describe the influence of frequency, voltage and temperature on the entire cycle life of

an EC [177].

II.1. Cyclic voltammetry

Cyclic voltammetry (CV) is a widely used technique in electrochemistry to

acquire qualitative and pseudo-quantitative information about the interactions between

the electrolyte ions and the surface of an electrode, as well as about possible redox

reactions. Consecutively to a constant rate potential sweep, the current resulting from

the flow of ions to charge and discharge the double-layer is measured. CV offers rapid

information about the redox reactions and adsorption processes and, thanks to the ability

to use a large range of scan rates, allows a quantitative kinetic analysis to be carried out.

A CV test consists of repetitive potential sweeps between two limits while

measuring the resulting current. Therefore, CV is also an accurate technique to estimate

the potential window of a supercapacitor (or an electrode in 3-electrode cell

configuration) by the current leap which appears when irreversible faradaic reactions

(i.e., electrolyte decomposition, oxidation of electrode material) take place. The

capacitance C in farad (F) can be calculated from the voltammogram using equation

(33), where I is the current (A), U the voltage (V), and U1 and U2 the limits of the

voltage window [178]:



𝑪 =∫ 𝒊 𝒅𝒕

𝒕𝑼𝟐𝒕𝑼𝟏

∫ 𝑼 𝒅𝒕𝒕𝑼𝟐

𝒕𝑼𝟏

(33)

The calculation is usually made from the backward scan, while a cell (or an electrode) is

discharged.

For an ideal electrical double-layer capacitor, where the charge separation takes

place between the surface of the electrode material and the solvated and non-solvated

ions of the electrolyte, the cyclic voltammogram is represented by a rectangular profile

(Figure 26a). In the EDLC concept, during the potential sweep, the charges flow from

the external circuit and through the solution only to charge and discharge the double-

layer [8]. The influence of resistive components of the system (electrode, current

collectors and separator material and its thickness) on the charging and discharging

processes is presented in Figure 26b by a parallelogram. The presence of this equivalent

series resistance (Rs) in series with the double-layer capacitance (Cdl) in the electrical

circuit affects the power and energy and contributes to internal heating of the system

[21].

The other key factor affecting the supercapacitor performance is a leakage

resistance (Rf), in parallel with the capacitance, which determines the charge loss, also

referred to as self-discharge, causing the voltammetry characteristics to deviate from the

parallelogram due to a delay while reversing the potential, ultimately coming from

kinetic processes during charging (Figure 26c). A deviation from the perfect rectangular

or parallelogram shape takes place when some charge passes across the double-layer

interface through, e.g., Faradaic reactions from redox active species, giving a

pseudocapacitive increase of current (Cp) and a parallel resistance associated to the

leakage reaction (Rp) (Figure 26d). In summary, Figure 26c shows the classical RC

model of an EDL capacitor which includes the most important parameters affecting the

shape of the experimental CV curves.



Figure 26 Typical charge/discharge voltammetry characteristics and respective

equivalent circuits of (a) an ideal EDL capacitor, (b) EDL capacitor with series

resistance, (c) real EDL capacitor, (d) pseudo-capacitor.

II.2. Constant current charging/discharging

Galvanostatic Cycling with Potential Limitation (GCPL), also called

chronopotentiometry, is based on measuring the voltage as function of time at imposed

current. This transient technique is found as the most representative to determine

parameters as capacitance and resistance, and also to test the cycle life of a

supercapacitor.

The capacitance of an EC is calculated from the slope of the discharge curve,

while the resistance is usually deduced from the potential drop when the current sign

changes from charge to discharge. However, a better estimation of resistance is obtained



by measuring the IR drop when initiating a constant current discharge after a

potentiostatic period [179]. The method is specified by the IEC (International

Electrotechnical Commission) to determine the ESR (Equivalent Series Resistance) and

EDR (Equivalent Distributed Resistance) values [180]. The ESR corresponds to all the

resistive components within the supercapacitor, and the EDR includes the ESR and also

a contribution from the charge redistribution process in the pores, due to a non-

homogeneous electrode structure. Hence, the resistance values are calculated by using

the expressions (34) and (35):

𝑬𝑺𝑹 =∆𝑼𝟐

|𝑰𝒅𝒊𝒔𝒄𝒉| (34)

𝑬𝑫𝑹 =∆𝑼𝟏

|𝑰𝒅𝒊𝒔𝒄𝒉| (35)

where, Idisch is the galvanostatic discharge current; ΔU2 - the voltage drop when the

discharge current is switched on, and ΔU1 is obtained from the intersection of the

vertical line at the time of starting discharge and the auxiliary line extended from the

linear discharge (see Figure 27) [181].

Figure 27 Galvanostatic charge and discharge of a supercapacitor with a constant

voltage period [adapted from [181]].



The capacitance of the supercapacitor is determined from the galvanostatic

discharge current (Idisch) and the time of discharge (Δt) for a selected voltage window

(ΔU), according to formula (36):

𝑪 =𝑰𝒅𝒊𝒔𝒄𝒉 𝜟𝒕

𝜟𝑼 (36)

It is also possible to monitor the cycle life of a cell by repeating many times the

inversion of the charging and discharging processes for a given maximum voltage limit;

in this case, C, ESR and EDR are plotted vs the number of cycles.

Whereas cyclic voltammetry yields basic information about capacitors (stability

window, capacitance, etc.), the galvanostatic charge/discharge technique is needed to

compute the energetic response [182]. Therefore, among the available techniques, CV

and GCPL are considered to give qualitative and quantitative information on

supercapacitor performance, respectively.

II.3. Impedance spectroscopy

Electrochemical Impedance Spectroscopy (EIS) allows determining the

EC’s real and imaginary components of the impedance response as a function of

frequency. It requires special equipment for applying a small alternating current (AC).

The ESR and frequency-response behavior of a capacitor are dependent on the electrode

characteristics:

nature of substrate

pore-size distribution

engineering preparation parameters (e.g., thickness, quality of contact between

particles).

The EIS technique can be implemented by measuring either the current or

voltage response of the system, while the potential or current is controlled. However,

the most widely used method is to set a sinusoidal signal of required potential with

small amplitude at several frequencies (f). As shown in equation (37), the impedance (Z)

is a complex quantity of magnitude (|Z|) which represents the ratio of the voltage

difference amplitude, and the exponential function of the phase angle (ф) and the

imaginary unit (-j) [183].



𝒁 = ∆𝑼

∆𝑰 = |𝒁|𝒆−𝒋ф = 𝑹𝒆(𝒁) + 𝒋𝑰𝒎(𝒁) (37)

The most widely used representation of ECs impedance data is the so-called

Nyquist plot representing the imaginary part of impedance versus the real part.

However, the major shortcoming of this kind of representation is the lack of information

about frequency on the plot. Therefore, another popular representation is the Bode plot,

where the phase angle (ф) is represented vs frequency, usually in conjunction with the

magnitude plot (|Z| vs log f), to evaluate how much a signal is phase-shifted.

Electrochemical systems are often very complex, needing to be modeled with a

combination of many elements. The most often used components are the double-layer

capacitance (Cdl) and the equivalent series resistance (Rs). For an ideal electrochemical

capacitor, where the total amount of charge comes from ions of the electrolyte, the

Nyquist plot is represented by a vertical line starting from the origin (Figure 28a). The

presence of an equivalent series resistance (Rs), representing the electrical losses

(caused mainly by the electrolytic solution, but also by the separator and the electrodes

during the charging and discharging processes), provokes a shift of the first point of the

plot by Rs (recorded at the highest frequency) towards higher values on the Re(Z) axis

(Figure 28b). Figure 28c presents the creation of the semicircle with two intersect points

on the real axis: the Rs (equivalent series resistance) point and the Rs+Rf point which

contains the equivalent series resistance and the charge transfer resistance which is

developed by the charge-complexes close to the Helmholtz plane. The contribution of

diffusion in impedance is represented by the so-called “Warburg impedance element”

(W) which is presented in the Randles circuit (Figure 28d), which consists of the

equivalent series resistance (Rs) in series with the parallel combination of the double

layer capacitance (Cdl) and the charge transfer resistance (Rf) in series with the Warburg

element (W) [3]. The Warburg element represents the impedance of semi-infinite

diffusion, and can be observed as a transition from the semicircular Im(Z) vs Re(Z) plot

to a 45° tilted line (Figure 28d).

The capacitance value at each applied frequency is calculated from equation

(38):

𝑪 = −𝟏

𝟐𝝅𝒇 𝑰𝒎(𝒁) (38)

where C is capacitance, f- frequency and –Im(Z)- imaginary component.



Figure 28 Nyquist plots of the respective equivalent circuits of (a) an ideal EDL

capacitor, (b) capacitor with series resistance, (c) series resistance and capacitor in

parallel with leakage resistance, (d) Randles circuit with Warburg impedance.

EIS is also found a useful method to distinguish various electrode degradation

processes, e.g., current collector corrosion, increase of contact resistance, increase of

electrode resistance, appearance of some inhomogeneity, adsorption processes etc.,

which give a resistive response. For instance, a non-homogeneous electrode structure,

results in increase of interfacial charge transfer resistance (Rf), whereas inhomogeneities

or adsorption processes can be disclosed by the presence of a constant phase element

(CPE) visible by a deviation from the pure capacitive vertical impedance response at

low frequencies. Since EIS enables to propose an equivalent circuit for the studied

systems, combined with other physical analyses (Electrical Quartz Crystal Microbalance

for example), it helps to understand the kinetics of the occurring processes.

II.4. Accelerated ageing test

From the above techniques it is possible to get information about the

performance of a capacitor (including capacitance, resistance, columbic efficiency,

energy and power density) and to distinguish a pseudo-capacitive contribution from the

pure EDL charge storage mechanism. In most of the scientific literature, the

determination of operating limit conditions, possible perturbation phenomena, and



stability of an electrode material or EC system is based on galvanostatic

charge/discharge cycling. In order to prove accurately the life-time stability of a device,

manufacturers and end-users generally expect an accumulation of over several hundred

thousand cycles. Taking into account that such tests are highly time-consuming, the

length of investigation period is generally limited to few ten thousands galvanostatic

cycles. Therefore, a voltage hold test (so-called ‘floating’), and in some cases using

temperature higher than the ambient (to accelerate possible ageing), is considered as a

relevant method of cells’ state-of-health (SOH) examination within shorter investigation

time.

So far, these accelerated ageing tests have been generally applied on

commercially available ECs in organic electrolytes [184, 185, 186]. However, since the

presented research aims in particular to optimize the design of high voltage capacitors

with salt aqueous electrolytes, the accelerated ageing protocol by floating has been

applied to examine these systems, and to serve as groundwork for researchers analyzing

new materials for ECs operating in aqueous electrolytes.

The validated accelerated ageing protocol is based on a combination of five

galvanostatic charge/discharge cycles followed by high voltage 2-hours floating periods

(Figure 29). The few galvanostatic charges/discharges, in an amount of five cycles, are

employed for two reasons: (i) to evaluate the discharge capacitance and ESR values

needed for the SOH assessment; (ii) to restore the system to its initial state after a high

voltage floating period, which promotes packing of ions in hardly reachable pores (see

part III.1). Hereof, the capacitance C is computed from the galvanostatic discharge in

the range (ΔU2) and the time (Δt) taken for this process, whereas the ESR is calculated

from the voltage drop U1, when the current changes from I (charge) to -I (discharge)

(ESR=U1/2I). Then, C and ESR are plotted versus the cumulated floating time. An EC

is usually considered by manufacturers as out of service when the ESR is increased by

100% or the initial capacitance is reduced by 20% [187]. The floating and galvanostatic

sequences are repeated, until reaching at least one of the mentioned end-of-life criteria.

It is usually sufficient to perform 60 series for a total cumulated floating time of 120

hours to distinguish the main failures which can appear during operation of carbon

based ECs in aqueous electrolyte, such as: increase of equivalent series resistance,

capacitance loss, corrosion of the positive current collector, oxidation of carbon and

electrolyte decomposition.



Figure 29 (a) Scheme of the accelerated ageing protocol and (b) magnification of the

fifth galvanostatic cycle. The fifth cycle of each series is considered to estimate the

capacitance and ESR values.



CHAPTER III

STATE OF HEALTH

OF AQUEOUS ELECTROCHEMICAL CAPACITORS

WITH STAINLESS STEEL CURRENT COLLECTORS

UNDER ACCELERATED AGEING



As it was presented in the literature review, AC/AC electrochemical capacitors

(ECs) with gold current collectors in aqueous alkali sulfate electrolytes are able to

operate up to 1.6 V in Na2SO4 during 10,000 charge/discharge cycles [21, 14] and even

2 V in Li2SO4 at room temperature [19]. Such high voltage is possible due to the over-

potential for di-hydrogen evolution in these neutral electrolytes [136]. Since alkali

sulfates are less corrosive than traditional battery electrolytes, e.g., H2SO4, they give an

opportunity to realize high energy density ECs with non-noble metal current collectors,

being environmental friendly, cheap and safe.

The objective of this chapter is to determine the performance limits of AC/AC

electrochemical capacitors using stainless steel current collectors in 1 mol L-1

Li2SO4.

Accelerated ageing by floating has been performed in order to determine the possible

perturbation phenomena occurring in aqueous media, while using stainless steel current

collectors. Since the main symptoms during ageing of ECs are a loss of capacitance and

an increase of resistance, the SOH diagnosis of the ECs in Li2SO4 was realized by

monitoring these parameters at various periods of time during the operation of the

system.

Beside electrochemical measurements, the chapter also presents the

examination of gas evolution under galvanostatic cycling and floating, to disclose

electrolyte decomposition as a failure which appears when an EC operates above its

voltage stability limit. Post-floating measurements on carbon electrodes (specific

surface area, porosity analysis, and quantification of oxygenated surface groups by

Temperature Programmed Desorption (TPD)) have been also realized to reveal the

origins of performance decay during accelerated ageing of ECs in salt aqueous

electrolyte with stainless steel current collectors.

The identification of factors contributing to ageing of ECs with cheap current

collectors in aqueous electrolytes is the first step to allow proposing and verifying

strategies for improving the long time performance of these systems, and thereby

gaining the scope of the dissertation.



III.1. High voltage ageing assessment of AC/AC

electrochemical capacitors in lithium sulfate electrolyte

For the present study, a commercially available activated carbon powder (DLC

Super 30, Norit, further named S30), with a specific surface area of 1843 m2 g

-1, has

been chosen as active electrode material (see experimental annex A.1.1). The pore size

distribution (Figure A1b) indicates that the micropores of this carbon are essentially in

the region of 0.8-0.9 nm; the carbon exhibits also some mesoporosity needed for

enhanced charge propagation. The Temperature Programmed Desorption (TPD)

analysis (Table A2) reveals a small amount of oxygenated functionalities on the surface

of S30, with relatively low oxygen amount of 1.5 wt %.

1.1. Exploring the high operating voltage of AC/AC

electrochemical capacitors in lithium sulfate electrolyte

In order to estimate the maximum operating voltage of ECs with stainless steel

current collectors in 1 mol L-1

Li2SO4, the electrodes potential limits were determined

by galvanostatic (200 mA g-1

) cycling on a two-electrode assembly with reference

electrode, and were plotted vs voltage (Figure 30). The practical di-hydrogen evolution

potential represented by a horizontal line at around -0.8 V vs NHE on this figure was

determined by the oscillations due to bubbling on three-electrode CVs, as shown in

Figure 20 in the literature part. This potential is much lower than the thermodynamic at

pH = 6.5 (E- = -0.384 V vs NHE). This is due to the reduction of water and production

of OH-, which accordingly to the Nernst law results in an increase of local pH in the

pores of S30. As seen in Figure 30, whatever the value of voltage up to 1.6 V, the

lowest potential of the negative electrode is always higher than -0.8 V vs NHE, which

means that di-hydrogen evolution at this electrode might be considered as negligible. By

contrast, the positive S30 electrode operates below the thermodynamic water oxidation

limit (marked by the upper dashed line) only up to a voltage of 1.4 V. Above the latter

value, one might expect detrimental oxidation of the positive S30 electrode. In other

words, these measurements suggest an approximate maximum operating voltage of 1.4

V for the S30/S30 cell in 1 mol L-1

Li2SO4.



Figure 30 Electrodes potential extrema vs voltage measured during galvanostatic (200

mA g-1

) cycling on an S30/S30 capacitor with stainless steel collectors in 1 mol L-1

Li2SO4. EOCP- open circuit potential.

Figure 31 shows the cyclic voltammograms of the individual electrodes recorded

in the potential ranges determined during galvanostatic cycling of the S30/S30 cell with

reference electrode in 1 mol L-1

Li2SO4 (see Figure 30). The CVs in Figure 31 prove

that, even at voltage of 1.6 V, the lowest potential of the negative electrode is always

higher than the practical di-hydrogen evolution potential (-0.8 vs. NHE), where

oscillations on the curves would be visible. By contrast, an anodic current leap together

with a corresponding cathodic wave appears for the positive electrode, as the potential

for oxygen evolution (0.845 V vs. NHE) is exceeded. The anodic peak might be also

related to the electrochemical oxidation of the carbon electrode [188], the redox

reactions between the generated oxygenated surface groups and the electrolyte [131],

and the corrosion of the positive stainless steel current collector.



Figure 31 CVs (5 mV s-1

) of the individual electrodes for a S30/ S30 cell in 1 mol L-1

Li2SO4. Scans are realized up to voltages of 0.8 V, 1.0 V, 1.2 V, 1.5 V and 1.6 V.

Overvoltage is the most commonly suspected origin for capacitors ageing, which

leads to decomposition of electrolyte and final starvation of ions, corrosion of positive

current collector and damaging of the positive electrode material by surface oxidation

and/or looseness of the electrode materials due to gas evolution. Therefore, taking into

account the data presented in Figures 30 and 31, real operating limits, possible

perturbation phenomena, and stability of S30/S30 cells with stainless steel collectors in

1 mol L-1

Li2SO4 have been determined by accelerated ageing tests and are presented in

the section III.2.

1.2. Degradation of ECs electrochemical performance under

accelerated ageing

Taking into account that an EC can operate millions of cycles, potentiostatic

floating is much more efficient than galvanostatic cycling to determine quickly the EC

operation stability limit and the perturbation phenomena which affect its stability. In

case of batteries, beside detrimental effects of applying high voltage, charge transfer

reactions which occur in the bulk of the electrodes at intermediate voltages may be even

harmful. This is why galvanostatic cycling is usually performed to demonstrate the

stability of battery systems. By contrast, in the case of electrochemical capacitors, the

electrochemical degradation reactions do occur only at high voltage [131]. Hence, when



an EC is investigated by galvanostatic charge/discharge, its voltage is most of the time

below the stability limit (Figure 32).

Figure 32 Schematic representations of voltage profile for a capacitor during floating

and galvanostatic cycling with potential limitation.

The changes which appear during operation of capacitors, such as electrolyte

decomposition, corrosion of positive current collector and damaging of the positive

electrode material by oxidation of the carbon surface and/or looseness of the electrode

material due to gas evolution entail a deterioration of the electrochemical performance

generally revealed by a decrease of capacitance and an increase of resistance. Therefore,

S30/S30 cells with stainless steel collectors in 1 mol L-1

Li2SO4 have been floated at 1.6

V and 24°C (RT), and the capacitance and resistance values were determined after each

floating sequence. Each two-hour period was preceded and followed by five

galvanostatic (1 A g−1

referred to the average active mass of both electrodes)

charge/discharge cycles. If not mentioned otherwise, the capacitance and resistance

were estimated from the 5th

discharge (see chapter II.4). Each series consisting of

galvanostatic cycling and floating period was repeated 60 times for a total floating time

of 120 hours. To stabilize the wetting of fresh electrodes, cyclic voltammetry (100

cycles) up to 1 V at a scan rate of 10 mV s−1

was applied to all systems before starting

floating.

Figure 33 shows that, at any time of the test, the capacitance values calculated

from the 1st discharge (just after floating) are always higher than estimated from the 5

th

discharge. This is explained by the fact that a prolonged high voltage period promotes a



better packing of ions, especially in the smallest pores, where the ions penetrate more

slowly due to steric effects. These narrow pores filled with ions during floating are

liberated after the few galvanostatic discharges to 0 V, and cannot be then reached again

by the ionic species during galvanostatic charging; they are accessed only during the

next voltage hold. Five galvanostatic cycles have been found sufficient to restore the

system to its initial state after a high voltage floating period. Therefore, capacitance and

resistance determined from the 5th

discharge are further considered to be representative

of the system’s performance.

Figure 33 Capacitance of an S30/ S30 capacitor in 1 mol L-1

Li2SO4 vs floating time at

1.6 V and 24 ⁰C measured from the 5th () and 1

st () galvanostatic discharge.

As the floating voltage increases from 1.5 V to 1.7 V, the galvanostatic

discharge capacitance increases from 80 F g−1

to 123 F g−1

. This capacitance

enhancement with voltage is attributed to a better packing of ions leading to a decrease

of EDL thickness (d in equation (4)). Due to this effect, to better estimate the effect of

voltage on the SOH of the S30/S30 capacitor in Li2SO4 electrolyte, in Figure 34,

capacitance and resistance are referred to their initial values, C/C0 and R/R0,

respectively. The evolution of the two parameters has been further analyzed by taking

into account the end-of-life criteria generally accepted by manufacturers, e.g., an

increase in resistance by 100% or a decrease in capacitance by 20%, as compared to the

initial values [187]. During floating at 1.7 V, the capacitance decreases by 20% after 70

cumulated hours (Figure 34a) and the resistance increases by 100% after only 40 hours

(Figure 34b), while when the EC is aged at 1.6 V, the capacitance drops by 20% after



120 h and the resistance is doubled after 50 h. Consequently, the cells aged at 1.6 V and

1.7 V are no longer in good SOH after 50 and 40 hours, respectively. It is interesting to

notice that, in both cases, the lifetime is controlled essentially by the increase of

resistance. It means that one can suspect parasitic phenomena altering the contacts

between the active material and the current collectors or between the carbon particles

themselves in the electrodes, due for example to corrosion of current collectors or gas

evolution.

When the floating voltage is reduced to 1.5 V, the SOH of the cell is still

excellent after a long period of 120 hours. The progressive increase of capacitance with

floating time is attributed to the progressive penetration of ions in pores of small size.

As compared to 1.6 V or 1.7 V floating, the increase of resistance after floating at 1.5 V

is much smaller. However, its slight increase still reveals some detrimental effects,

which will be further presented in the next sections of this manuscript. Notwithstanding,

the stability voltage determined by floating fits quite well with the conclusions driven

from Figures 30 and 31, and regarding effect of oxidation on the positive electrode data.

Figure 34 Effect of the floating voltage at 24 ⁰C on (a) relative capacitance and (b)

relative resistance of an S30/ S30 EC in 1 mol L

-1 Li2SO4.

To perceive dissimilarities between the impacts of galvanostatic cycling and

floating on ageing, the capacitance and resistance of the S30/S30 electrochemical

capacitor (with stainless steel collectors in 1 mol L-1

Li2SO4) has been plotted during

galvanostatic cycling up to 1.6 V or floating at 1.6 V (Figure 35). The voltage of 1.6 V



has been selected in light of Figure 34 which revealed that detrimental reactions are

negligible during floating at lower voltage. Figure 35 shows that the capacitance

decreases by 6% after 285 hours (10,000 galvanostatic cycles) at a current density of 1

A g-1

(Figure 35a), while for the cell aged by floating, the capacitance loss of 6% is

evidenced after only 124 hours (55th floating series). Despite the capacitance packing

occurring at the beginning of floating and leading to a transient increase of capacitance,

the time for reducing the capacitance by 6% is shorter than in galvanostatic cycling. The

efficiency of floating for ageing the cell, as compared to galvanostatic cycling, would be

even amplified if the investigation time would be extended. The poor effectiveness of

galvanostatic cycling is well-illustrated by Figure 32, showing that the time at high

voltage to provoke degradation phenomena in the cell is extremely short. This time is

even shorter, because the imposed voltage is not reached due to the ohmic drop (in the

present case, the ohmic drop at current density of 1 A g-1

is 8 mV). The acceleration of

cells’ ageing can be even easier seen by comparison of resistance evolution of the two

cells (Figure 35b). For the system aged by galvanostatic cycling, the resistance is very

stable until the end of the test, whilst R/R0 increases just from the initial 2-hour floating

sequence at 1.6 V.

Figure 35 Comparison of stability performance of S30/ S30 cells in 1 mol L-1

Li2SO4

during floating at 1.6 V and galvanostatic (1 A g-1

) cycling up to 1.6 V: (a) capacitance

and (b) resistance evolution.

Notwithstanding, Figure 35 reveals differences in the profiles of capacitance

and resistance evolution depending on the ageing procedure, either by GCPL or by

floating. As observed previously for the cells floated at 1.5, 1.6 and 1.7 V (Figure 34),



the capacitance of the cells initially increases, due to a better wetting of the electrodes

by the electrolyte during prolonged polarization, allowing narrow pores to be accessed

by ions. The initial capacitance enhancement could be also the result of electrostatic

field modification on the electrode surface due to an increase in oxygen content,

allowing the micropores to be more easily wetted by the ions from the aqueous

electrolyte [189, 82, 83]. When all accessible pores are reached by floating at 1.6 V, the

capacitance remains almost constant for a period of time, until it starts to decay

continuously after 50 hours of floating (Figure 35), indicating a progressive ageing of

the system, which is the most probably due to the reduction of accessible surface area of

the S30 electrodes. Therefore, due to this kind of capacitance and resistance evolution

profile, depending on the ageing procedure, it is not easy to establish a correlation

between a 2-hour floating period and a number of corresponding galvanostatic cycles.

To appreciate the importance of side reactions contributing to ageing by floating,

it is interesting to compare the evolution of leakage current (LC) during 60 repeated

sequences at 1.5 and 1.6 V (Figure 36) [3]. As shown in Figure 36a, during

potentiostatic floating, the leakage current rapidly decreases within a few minutes and

then stabilizes at an equilibrium value. The LC drop is related to the structure of the

double-layer formed at the electrode/electrolyte interface during charging. As

mentioned before, according to the Grahame double-layer model, the EDL consists of:

(i) a diffusion layer, with ions weakly interacting with the carbon electrode, (ii) and a

compact layer where ions strongly interact with the electrode [8]. The dramatic current

decay originates from the loss of charge, when weakly interacting ions flow to the bulk

of the electrolyte. As the floating time proceeds, the ions of the diffusion layer are

pushed to the compact layer, and the structure of the EDL is ordered, until reaching

equilibrium [190]. However, in an electrochemical capacitor, the electrodes are made of

a porous network which hinders so simple charge exchange; moreover, in the confined

volume of micropore, the traditional models of the EDL are not applicable.

The initial values of leakage current measured at the beginning of each floating

sequence at 1.6 V increase with the number of floating periods, indicating a slower

transition from galvanostatic charging current to leakage current. The equilibrium

leakage current itself reveals the occurrence of side reactions: the higher its value, the

higher amount of charge contributes to side reactions [3]. While the profile of

equilibrium leakage current is almost constant during ageing at 1.5 V, it can be observed



that it increases from eight cumulated floating hours at 1.6 V, reaching its highest value

at ~50 hours. By comparing Figure 34a and Figure 36b, it is interesting to note that both

capacitance and the equilibrium leakage current increase up to fifty cumulated floating

hours at 1.6 V, revealing a modification of electrostatic field on the oxidized surface of

the carbon electrode, allowing the micropores to be more easily wetted by the ions from

aqueous electrolyte [189, 82]. When considering the capacitance, from 8 to 50 hours,

there is a balance between capacitance increase related to access to narrow pore and

capacitance decrease related to side reactions, and after 50 hours capacitance decays

because new pores are no longer accessed. The charge revealed by the leakage current is

utilized for, e.g., decomposition of electrolyte, corrosion of current collectors, resulting

in resistance increase by 100% after 50 cumulated floating hours at 1.6 V. The parallel

decrease of equilibrium leakage current and capacitance is attributed to the blockage of

pores entrances and hindrance of ions access, due to the formation of oxygenated

surface groups and gases together with the deposition of corrosion products.

Figure 36 (a) Leakage current profile on an S30/ S30 capacitor in 1 mol L

-1 Li2SO4

electrolyte during one two-hour floating period at 1.6 V; (b) Evolution of leakage

current during 60 two-hour floating sequences at 1.5 V and 1.6 V.

The foregoing demonstrates that floating in potentiostatic conditions is an

accurate method to simulate aging during the performance of S30/S30 ECs in aqueous

lithium sulfate electrolyte and to monitor their SOH. The obtained results also clearly



reveal that GCPL with a limited number of cycles (ca 10,000) is definitively not an

accurate test to determine the maximum operating voltage of ECs. Notwithstanding, the

differences in the profiles of capacitance and resistance evolution depending on the

ageing procedure at 1.6 V, either by GCPL or by floating, disclose that it would be

inaccurate to estimate the floating ageing time corresponding to a given number of

galvanostatic cycles.

Nevertheless, the performed experiments allowed to assess that S30/S30 ECs in

1 mol L-1

lithium sulfate can operate with a very long cycle life at voltage as high as 1.5

V. At the same time, they indicate that most of the claims in the literature have to be

considered with great care, meaning that the mentioned voltage values should be

reduced by around 0.3–0.4 V, especially when gold current collectors were used.

Notwithstanding, the voltage value of 1.5 V is still remarkably high for an aqueous

electrolyte compared to only 0.7–0.8 V generally possible for KOH or H2SO4

electrolytes. However, to move towards further optimization of the high voltage AC/AC

ECs with stainless steel collectors in aqueous 1 mol L-1

Li2SO4, the possible

perturbation phenomena under long time operation must be identified.

III.2. Factors contributing to ageing in aqueous electrolyte

As presented in section III.1, the long term operation of an EC in 1 mol L-1

Li2SO4 at voltages higher than 1.5 V leads to ageing of the components, revealed by a

drop of capacitance and an increase of resistance (Figure 34). The plot of leakage

current evolution during repeated floating sequences at 1.6 V (Figure 36b) indicates the

occurrence of side reactions which contribute to ageing.

2. 1. Oxidation of carbon electrodes and corrosion of stainless

steel current collectors

2.1.1. Post-floating analysis of ECs by electrochemical techniques

We have used electrochemical impedance spectroscopy (EIS) to distinguish the

origins of electrode degradation processes, such as electrolyte decomposition, and other

possible perturbation phenomena (i.e., oxidation of S30 electrode and/or corrosion of

stainless steel collectors, internal pressure evolution). EIS data at open circuit voltage

(OCV) have been compared for a freshly built cell with S30 electrodes in 1 mol L−1



Li2SO4 and after 120 hours of ageing at 1.6 V (Figure 37). Due to floating, the ESR of

the system increases from 0.7 to 1.1 Ω, suggesting a more resistive path of ions in the

active surface of S30 and probably worse contact between the grains of S30 due to gas

evolution. The increase of equivalent distributed resistance (EDR) causes a decrease of

the slope in the low frequency branch and is attributed to the limited penetration of ions

in the pores of electrodes due to side reactions. Moreover, the presence of a constant

phase element (CPE) visible by deviation from the pure capacitive vertical impedance

response at low frequencies (Figure 37a) indicates a non-uniform thickness of the

double-layer, inhomogeneity or adsorption processes [186]. The increase of interfacial

charge transfer resistance Rf from 1.2 to 24.9 Ω results probably from an uneven

distribution of current in the positive electrode, due to the appearance of corrosion

products. The shift in transition from the high frequency semicircle to the mid-

frequency distributed charge storage impedance region suggests time dependence in the

charging process, probably as a result of a low conducting layer formed by corrosion

products.

The capacitance vs frequency dependence (Figure 37b) reveals almost constant

capacitance at low frequency for the fresh cell, which exhibits a typical EDLC behavior.

The almost ideal performance of the system in the initial state is confirmed in the Bode

plots by phase angle values very close to -90° at low frequency (Figure 37c). After 120

floating hours, the capacitance is higher than before ageing (107 F g−1

compared to 73 F

g-1

) at the lowest frequencies, and it decreases more rapidly than of the fresh cell in the

frequency range 0.1 Hz–1 Hz (phase close to 0). The higher capacitance values

measured up to 0.1 Hz for the aged cell originate from faradaic contribution, probably

due to a conductive layer of corrosion products and oxygenated surface functionalities

on the S30 electrode. Since EIS does not involve current and potential variations, it

allows distinguishing the phenomena occurring in the porosity of electrodes (at high

frequencies) and at the electrode/current collector interface (at low frequencies).

Contrariwise, in the transient techniques by measuring current or potential response (CV

and GCPL), the changes in the electric field of each component of the EC cannot be

differentiated. Therefore, for the aged cell, the capacitance value of 107 F g−1

determined by EIS at the electrode/current collector interface is not disclosed by CV or

GCPL, where the capacitance decrease related to blocked porosity of S30 is dominant.



Figure 37 Impedance spectroscopy data on an AC/AC capacitor in 1 mol L

-1 Li2SO4

before floating () and after 60 two-hour periods of floating at 1.6 V (Δ): (a) Nyquist

plots; (b) Capacitance vs. frequency; (c) Bode phase angle.



The time constant τ for the fresh and aged cells, calculated at the frequency

where reactance and resistance are equal (-45 phase angle) (Figure 37c) [119], is 4 s

and 100 s, respectively, showing an important decay after ageing and confirming

deleterious changes of the S30/S30 system in lithium sulfate electrolyte after ageing at

1.6 V at room temperature.

In order to analyze the energy storage abilities of ECs after ageing at 1.5 V and

1.7 V, cyclic voltammetry (10 mV s-1

) was used to record the capacitive current

variations (Figure 38). The fresh cells display a typical rectangular shape of CV,

whereas after each 20 series of floating sequences at 1.5 and 1.7 V the ECs exhibit a

more resistive character by an inclined voltammogram; the increase of capacitive

current at low voltage values could be attributed to pseudo-capacitive contributions

related with the creation of surface oxygenated groups on the positive carbon electrode.

Considering the CV curves recorded up to the ageing voltage of 1.5 V (Figure 38c) and

1.7 V (Figure 38d), the diminishing of capacitive current at voltage higher than 1 V

negatively affects the deliverable energy and power density. The capacitance decrease

and resistance increase can be attributed to electrolyte starvation as a consequence of

reduced ion availability [191]. Indeed, due to electrolyte decomposition, the electrolyte

reservoir decreases, and is finally not sufficient to cover the working surface area of

electrodes at higher voltages. The second possible explanation for the narrowing of

CVs, as voltage increases, is the saturation of the electrode material porosity by the

stored ions. The deposition of decomposition/corrosion products in the porosity of the

positive electrode and/or the formation of surface oxygenated groups reduces the S30

accessible pore volume for ions, thus leading to a fade of capacitive current at voltages

higher than 1 V [192]. Furthermore, the formation of such products results in an

increase of the leakage resistance (Rf), due to worse charge propagation after 20 floating

series at 1.7 V (Figure 38b, d). The phenomenon is not pronounced for the cell aged at

1.5 V; however, the slight saturation of the carbon porosity by the stored ions during the

prolonged high voltage period discloses a detrimental effect of 1.5 V for the deliverable

energy and power density after even 20 series of accelerated ageing.



Figure 38 Cyclic voltammograms (10 mV s-1

scan rate) recorded after each 20 series of

two-hour floating sequences (a and c) at 1.5 V and (b and d) at 1.7 V on a S30/ S30

capacitor in 1 mol L-1

Li2SO4.

2.1.2. Post-floating analyses on carbon electrodes

To explain the above demonstrated changes in electrochemical performance of

the aged ECs, the oxygenated surface functionality of aged electrodes has been

characterized and quantified by Temperature Programmed Desorption (TPD). To avoid

the interference of the electrode binder during the post-floating analysis of electrodes by

TPD, self-standing electrodes from activated carbon cloth (ACC 507-20, Kynol,

SBET=2231 m2 g

-1 and L0=0.99 nm) were selected. The TPD analyses have been realized

in helium atmosphere at 20 °C min-1

up to 950 °C, on the pristine ACC, the positive and

negative aged ACC electrodes treated. Figure 39 presents the mass loss, and CO2 and

CO profiles obtained by TPD for the fresh ACC and the positive and negative electrodes

extracted from the EC after 120 floating hours at 1.7 V. The important weight loss at

950 °C for the positive carbon electrode is related to an important surface oxidation and

formation of functionalities evolving essentially as CO2 together with a lesser amount of



CO. The surface of the aged positive electrode is modified by new oxygenated groups,

while some other functionality disappeared (disappearance of CO2 peak at 850 °C) as

compared to the pristine ACC. The aged negative electrode is also slightly oxidized,

although one could expect that it should not occur under negative polarization of this

electrode. In fact, di-oxygen which is formed at the positive electrode is dissolved in the

electrolyte, being able to diffuse to the negative electrode to form peroxide ions which

oxidize the carbon material [188]. The cumulated amount of CO2 evolved from the

positive and negative aged ACC electrodes is 3.6 and 2.2 mmol g-1

, respectively, as

compared to 0.9 mmol g-1

for the as-received ACC. The cumulated released CO is 3.9,

1.1 and 0.9 mmol g-1

, for the positive, negative and untreated ACC electrodes,

respectively.

Figure 39 TPD on pristine ACC (full line) and on positive (dashed line) and negative

(dotted line) ACC electrodes after 120 hours of floating at 1.7 V in 1 mol L-1

Li2SO4: (a)

CO2 evolution; (b) CO evolution.

A multiple Gaussian function was used for the deconvolution of the CO2 and CO

patterns and to determine the types of oxygenated complexes formed on the surface of

the aged positive electrode [92, 93]. Figure 40a presents the CO2 desorption peaks at

270 °C (peak 1), 500 °C (peak 2) and 620 °C (peak 3), which are attributed to

carboxylic and two kinds of peroxide groups, respectively [193]. The quite stable

oxygenated complexes desorbed as CO at 710 °C (peak 1) and 920 °C (peak 2) (Figure

40b) are assigned as carbonyl/quinone groups and pyrone-type structures, respectively

[193, 194]. The deconvolution of the TPD patterns for the positive electrode (Figure 40a

and b) includes sharp peaks noticeable at around 700 °C which, together with a



discontinuity in the TG curve (Figure 39), are probably associated to the catalytic

desorption of oxygenated functionalities, due to the presence of metallic impurities

resulting from the corrosion of the positive stainless steel collector. The new

oxygenated groups appearing on the surface of the negative electrode can be recognized

as peroxide groups, desorbing as CO2 at 550-600°C [193].

Figure 40 Deconvolution of TPD patterns for the positive ACC electrode after 120

hours ageing at 1.7 V: (a) CO2 pattern; (b) CO pattern (—, TPD experimental data; ---,

individual peaks; , sum of the individual peaks).

Hence, the resistance increase which is observed during floating might be at

least partly related to the formation of surface groups on the porous carbon electrodes.

Similarly, these groups also contribute to the decay of capacitance during floating

(Figure 34).

To better demonstrate the destructive effect of accelerated ageing in 1 mol L−1

Li2SO4, nitrogen adsorption/desorption isotherms at -196 °C have been recorded on a

fresh S30 electrode (pellet with 85 wt. % of DLC Super 30), and on positive and

negative electrodes aged by 120 floating hours at 1.7 V (Figure 41). The micro Vmicro

and mesopore volumes Vmeso were obtained directly from the calculated cumulative pore

size distribution (PSD) determined using the 2D non-local density functional theory

(2D-NLDFT) [107]. The porous texture data were referred to the total mass of one

electrode. Table 1 shows, as expected, that SBET of the positive aged electrode decreases

after ageing.



Electrode

BET

surface

area

Vmicro

< 2 [nm]

Vmeso

2-50 [nm]

Average

micropore

size

[m2 g

-1] [cm

3 g

-1] [cm

3 g

-1] [nm]

fresh electrode 1348 0.528 0.125 0.92

aged positive 895 0.355 0.074 0.89

aged negative 1345 0.525 0.128 0.92

Table 1 Porosity data obtained by nitrogen adsorption at -192°C on a fresh electrode

and on positive and negative electrodes aged by floating a S30/ S30 capacitor during

120 hours at 1.7 V in 1 mol L-1

Li2SO4.

The 2D-NLDFT pore size distribution presented in Figure 41b does not reveal

any significant change in the porous texture of the negative electrode, whereas all pores

of the positive electrode are affected by floating at 1.7 V (as shown in Table 1, the

average micropore size L0 remains unchanged). The volume of micropores (Vmicro) and

mesopores (Vmeso) is around 1.5 times lower for the positive electrode compared to the

fresh one. The reduction of SSA and pore volume for the aged positive electrode

supports the assumption that capacitance decay and resistance increase of S30/ S30 cells

is due to a partial blockage of pores by oxygenated functional groups and/or

decomposition and corrosion products.

Figure 41 (a) Nitrogen adsorption/desorption isotherms at -196 °C and (b) 2D-NLDFT

pore size distribution of a fresh S30 electrode and of aged positive and negative

electrodes after 120 h of floating at 1.7 V in 1 mol L-1

Li2SO4.



2.1.3. Effect of temperature on ageing

Since the application of 1.7 V potentiostatic floating was found to be deleterious

for the positive electrode, and consequently for the cell, and accelerated ageing at 1.5 V

indicates no important resistance increase after 120 hours at 24 °C (Figure 34b), we

have investigated the effect of raising the temperature to 35 °C and 40 °C, while

floating at 1.5 V. After ageing a S30/S30 capacitor in 1 mol L-1

Li2SO4 at 1.5 V and 40

°C and opening the cell, a russet colour attributed to corrosion has been perceived

essentially on the positive stainless steel current collector and on the separator (Figure

42).

Figure 42 Corroded components of a S30/ S30 capacitor in 1 mol L-1

Li2SO4 after 120

hours of floating at 1.5 V and 40°C.

Figure 42 shows the effect of floating at 1.5 V and different temperatures on

relative capacitance and resistance evolution of S30/ S30 cells in 1 mol L-1

Li2SO4; for

comparison, the data of Figure 34 obtained at 24°C are also reported. During the first 50

hours of floating at 1.5 V and 40 °C, the capacitance increases while resistance slightly

decreases. Such behaviour is attributed to a better mobility of ions at higher

temperature, which enhances the electrolyte penetration in the porosity of S30.

However, as the floating time proceeds at 40 °C, the capacitance starts to decline and



the resistance increases more rapidly than at lower temperature, at which the parasitic

reactions in the system are reduced. Nonetheless, it is important to notice, that even at

1.5 V, the perturbation phenomena are triggered when increasing the temperature by

around 15 °C. These so-called perturbations are essentially related to electrolyte

decomposition, which leads to carbon oxidation and decreases its conductivity, and the

formation of corrosion products at the interface between the current collector and the

carbon electrode. Therefore, gas evolution has been monitored during cycling and

accelerated ageing at RT, 35 °C and 40 °C, and is presented in part 2.2.

Figure 43 Effect of temperature increase on the accelerated ageing of S30/ S30

capacitors in 1 mol L

-1 Li2SO4 at 1.5 V: (a) relative capacitance; (b) relative resistance.

2.2. Gas evolution during floating

At positive electrode potential higher than the value for electrolyte oxidation,

gases such as di-oxygen, CO and CO2 may evolve, and activated carbon be oxidized

[131]. Likewise, below the reduction potential of water at the negative electrode,

hydrogen is produced. The generation of gases at the carbon electrodes results in a rise

of cell internal pressure and may contribute to reducing its lifetime. When a capacitor

device is extremely overcharged, excessive gassing at the electrodes may cause leakage,

cracks and permanent damages of cell constituents or even explosion. The installation

of safety vents, which open if the overpressure limit is exceeded, generally solves the

security issues [195]; however, it does not solve the loss of electrolyte accompanying its



decomposition. For this reason, to investigate the SOH of an EC, it is necessary to

simultaneously monitor pressure inside the cell during long time performance at

overcharged state.

A special cell was designed for in-situ monitoring pressure evolution during

GCPL or floating tests. It is made from a stainless steel 316L case with an outlet to

which a pressure sensor can be connected (Figure 44a). The cell was assembled by

sandwiching a 16 mm diameter separator (AGM, Bernard Dumas, thickness = 0.52 mm)

between S30 pellet electrodes (16 mm in diameter), and then introducing the sandwich

in a PTFE guide sleeve which is placed in the lower case of the cell. Then, the separator

and the carbon electrodes were soaked with the electrolyte and pressed by a stainless

steel plate and a spring, before screwing the upper cover together with the lower one. In

order to improve the accuracy of pressure measurements, the system was completely

filled with electrolyte (around 3 mL) through the upper outlet, such a way that the dead

volume is minimized. A digital pressure sensor KELLER 35X Ei (pressure range 0–3

bars; total error band of 0.05 %) (Figure 44b) was then connected to the upper outlet. A

climatic chamber (Suszarka SML 25/250 ZALMED, Poland) was used to stabilize the

cell temperature at ± 1 ºC. The pressure values were recorded using the READ30

software.

Figure 44 (a) Main components of the pressure test cell; (b) Test cell inside the climatic

chamber connected to the KELLER 35X Ei pressure sensor and to the potentiostat.



The internal overpressure was first measured during few GCPL cycles with a

current density of 1 A g-1

at room temperature (Figure 45). The plot clearly indicates a

higher rate of electrolyte decomposition and higher internal pressure evolution at 1.8 V

than at 1.6 V and 1.5 V, which can finally lead to electrolyte depletion and/or lower

cohesion of the electrode material.

Figure 45 Internal overpressure during galvanostatic (1 A g

-1) cycling of a S30/ S30 cell

in 1 mol L-1

Li2SO4 at 24°C up to 1.5 V, 1.6 V and 1.8 V.

The pressure variations at 24 °C were measured under ageing consisting of 4 2-

hour potentiostatic periods at 1.5 V, interspaced with five galvanostatic cycles at 1 A g-

1. Thereafter, the EC was maintained at open circuit voltage (OCV, self-discharge) for

24 hours (Figure 46a). A floating sequence at 1.5 V, preceded and followed by

galvanostatic cycling up to 1.5 V, is shown versus time in Figure 46b. During the 2-

hour voltage hold period at 1.5 V, pressure increases linearly by ~207 mbar. During

charging at constant current, the pressure is first stable and it starts to increase as

voltage is higher than ~1 V (Figure 46c); a steep pressure growth is observed above

around 1.25 V. This latter value is in agreement with the data obtained by galvanostatic

cycling on a two-electrode assembly with reference electrode (Figure 30), where the

positive S30 electrode reaches the thermodynamic oxygen evolution potential. During

galvanostatic discharge, the pressure declines very slightly, indicating that even



galvanostatic cycling at 1.5 V can reduce the lifetime on account of electrolyte depletion

and/or loss of electrode cohesion. Moreover, when considering the self-discharge at

OCV from 1.5 V (Figure 46a), the pressure slightly decreases. Such profile can be

assigned to very slow gas recombination into water and/or partial dissolution of gases in

the electrolyte [196, 197].

Figure 46 a) Internal overpressure evolution at 24 °C during full ageing protocol at 1.5

V on a S30/ S30 cell in 1 mol L-1

Li2SO4; (b) magnification of pressure evolution during

one 2-hour sequence preceded and followed by galvanostatic cycling at 1.5 V (1 A g-1

);

(c) magnification of pressure evolution during galvanostatic cycling (1 A g-1

) up to 1.5

V.



To observe the effect of temperature on gas evolution, potentiostatic floating has

been performed at 1.5 V and different temperatures on S30/ S30 cells in 1 mol L-1

Li2SO4, with simultaneous measurement of pressure evolution (Figure 47); for

comparison, the data of Figure 46b obtained at 24 °C are reported. For the three ECs

analyzed at 24 °C, 35 °C and 40 °C, the pressure increases linearly with floating time.

Considering the SOH of the S30/ S30 cells in 1 mol L-1

Li2SO4, it can be easily seen

that, even during the first floating period at 1.5 V, the gas evolution increases when

increasing the temperature by around 15 °C.

Figure 47 Internal overpressure evolution at 24 °C, 35 °C and 40 °C during a 2-hour

floating period at 1.5 V on a S30/S30 cell in 1 mol L-1

Li2SO4.

These pressure evolution data disclose that the factors which lead to long time

performance deterioration of S30/S30 cells in aqueous Li2SO4 at different temperatures

are related to electrolyte decomposition. This proved phenomenon entails carbon

oxidation and its conductivity decrease, while corrosion products are formed at the

interface between the current collector and the carbon electrode.

III.3. Conclusion

The performed experiments revealed that, to know perfectly the state of health

(SOH) of an electrochemical capacitor with stainless steel collectors in aqueous lithium

sulfate electrolyte, it is necessary to monitor simultaneously the pressure inside the cell,

capacitance and resistance at various lifetimes of the system and at various

temperatures. The monitoring of these parameters under potentiostatic floating allowed



to assess that ECs in 1 mol L-1

lithium sulfate electrolyte can operate with a very long

cycle life at voltage as high as 1.5 V at room temperature; such voltage is around 0.3–

0.4 V lower than the values mentioned in the literature when using gold current

collectors. Notwithstanding, the value of 1.5 V for the system in 1 mol L-1

Li2SO4 is

remarkably high when compared to 0.7–0.8 V for standard aqueous electrolytes (KOH

or H2SO4).

At voltage higher than 1.5 V, the decrease of capacitance during floating is

related to: (i) reduced accessible surface area due to oxidation of the carbon surface or

pore blockage by electrolyte decomposition or corrosion products; (ii) electrolyte

decomposition leading to ionic starvation in the electrode. The resistance increase under

accelerated ageing is generally due to: (i) electrolyte decomposition which leads to

deposition of corrosion products in the separator and on the positive electrode surface

(ionic contribution) and may be also caused by increased contact resistance between the

electrodes and current collectors (electronic contribution); (ii) gas products evolution

leading to weakening of the adhesion between the active mass and the current collector,

and also to the electronic contribution to contact resistance at the electrode/current

collectors interface.

Taking into account the results of this chapter, to improve the long time

performance of carbon-based electrochemical capacitors in neutral salt aqueous

electrolyte, strategies should be particularly intended to: (i) reduce the corrosion of

stainless steel collectors and decrease its destructive effect on ECs operation; (ii) and

avoid the decomposition of aqueous electrolyte through a shift of operating electrodes

potentials towards lower values. The results of these strategies will be presented in the

next chapter.



CHAPTER IV

STRATEGIES FOR IMPROVING

THE LONG TIME PERFORMANCE

OF HIGH VOLTAGE CAPACITORS

IN AQUEOUS ELECTROLYTES



The investigations performed in chapter III allowed disclosing the most

important factors contributing to ageing during high voltage operation of ECs based on

activated carbon electrodes and aqueous lithium sulfate electrolyte, which are: (i)

carbon oxidation reducing the accessible surface area, and/or electrolyte decomposition

leading to ionic starvation in the electrode [198], both causing a loss of capacitance; (ii)

formation of resistive decomposition products on the current collectors or gas products

evolution leading to weakening of the active mass adhesion on the current collector and

to increasing of resistance. Therefore, this chapter focuses on approaches intending to

cope with these possible perturbation phenomena which appear during high voltage

operation of ECs based on aqueous electrolytes. To eliminate or reduce the formation of

corrosion products at the interface between the AC electrode and the current collector,

three tactics have been particularly introduced: i) replacement of stainless steel current

collectors by nickel; ii) coating of the metallic foils with a conductive carbon layer; iii)

addition of sodium molybdate to the electrolytic solution to inhibit the corrosion of

steel. Finally, cells with asymmetric configuration of electrodes and coupled kinds of

current collectors have been used to avoid the decomposition of aqueous electrolyte

through down-shifting the operating electrodes potentials. The validity of the proposed

strategies was verified by electrochemical techniques, such as cyclic voltammetry and

impedance spectroscopy, as well as accelerated ageing by floating and monitoring of

internal pressure evolution.

IV.1. Corrosion reduction of positive current collector

In analogy to experiments presented in chapter III, a commercially available

carbon powder DLC Super 30 (Norit, S30) with a specific surface area of 1843 m2 g

-1

has been chosen as electrode active material for manufacturing pellet electrodes. The

electrodes were composed of 85 wt% S30, 10 wt% polyvinylidene fluoride as binder

(PVdF, Kynar HSV900, Arkema) and 5 wt% carbon black (C65, Timcal). To proceed in

the optimization of the system, coated electrodes (see paragraph 1.2.) were realized by

spreading the electrode material layer on the current collectors with an automatic

applicator using a Doctor blade. In this case, the electrode composition was 83.5 wt%

activated carbon YP80F (Kuraray Chemicals Co, YP80F), 8.5 wt% carbon black (C65,

Timcal) and 8 wt% polyvinylidene difluoride (PVdF, Kynar HSV 900, Arkema).



1.1. Alternative nickel current collectors

One of the strategies to improve the long-term stability of cheap electrochemical

capacitors in aqueous electrolyte involves the replacement of stainless steel by less

corrodible current collectors. In this study, nickel was used as alternative material to

stainless steel, due to its availability and immunity to corrosion under negative

polarization, as shown by the Pourbaix diagram in Figure 48 [199]. Besides, due to the

pH shift to higher values (pH = 10) observed previously during operation of AC/AC

capacitors in 1 mol L-1

Li2SO4, the use of nickel, even under positive polarization,

should not be a problem, since corrosion occurs only for pH lower than 8. For these

reasons, at first, paragraph 1.1 will present the performance of ECs with (-)

nickel/nickel (+) assembly, and then, paragraph 2.2. will show the examination of cells

with (-) nickel/stainless steel (+) combination of collectors.

Figure 48 Pourbaix diagram of nickel; the dashed lines show the equilibrium potentials

for (a) H2/H2O and (b) O2/H2O [199].

To establish the performance differences due to the use of the two types of

collectors, ECs were realized in Swagelok-type PTFE vessel, using 1 mol L-1

Li2SO4 as



electrolyte, and investigated by the electrochemical techniques. Cyclic voltammograms

of S30/S30 capacitors in 1 mol L-1

Li2SO4 with stainless steel and nickel collectors

(Figure 49) demonstrate nearly square shaped curves with good charge propagation at

10 mV s-1

scan rate.

Figure 49 Cyclic voltammograms (up to 1.7 V at scan rate of 10 mV s-1

) of S30/S30

cells in 1 mol L-1

Li2SO4 with stainless steel and nickel collectors.

Since the dynamic behavior of both ECs is similarly good during

charging/discharging, the S30/S30 cells in 1 mol L-1

Li2SO4 have been subjected to

potentiostatic floating at 1.6 V over a total time of 120 hours. Contrary to the fresh cells

(dotted lines), the CVs recorded after 120 hours of potentiostatic floating at 1.6 V reveal

a more resistive character of the systems with both types of collectors (full lines)

(Figure 50). After floating of the cell with nickel collectors, the capacitive current is not

diminished at voltages higher than 1 V, as it can be observed in the case of stainless

steel. It suggests that the porosity saturation observed in chapter III due to the reduction

of positive electrode pore volume (as a consequence of carbon oxidation or/and

formation of corrosion products) is not demonstrated for the cell with nickel collectors.




) of S30/S30 capacitors in 1 mol L-1

Li2SO4 with stainless steel and nickel collectors: (a) fresh cells (dotted lines); (b) cells

aged by floating at 1.6 V during 120 hours (full lines).

However, when opening the cell with electrodes coated on nickel after 120 hours

of potentiostatic floating at 1.6 V, black and pale green deposits were noticed on the

separator and at the edges of the positive current collector, and pale green on the

negative one. These residues are probably associated with the observed pH variations

during ageing: the pH increased to 7-8 and 10 on the surface of the positive and

negative electrode, respectively. To discern the oxidation states of nickel in the

discovered deposits, it is important to measure the electrode potential values during

charging from 0 V to voltage of 1.6 V, which are 0.15 V-1.04 V and -0.15 V-0.56 V vs

NHE, for the positive and negative electrodes, respectively (Figure 51). Taking into

account the Pourbaix diagram of nickel (Figure 48) [199], these residues correspond to

Ni(III) (black) and Ni(II) (pale green) compounds on the positive electrode and Ni(II)

(pale green) compound on the negative one. The oxidation of Ni metal to Ni(OH)2,



appearing as a pale green residue on the edges of the positive electrode, is evidenced by

an increase of the anodic current at potential of 1.04 V vs NHE [200]. Afterwards, part

of the green Ni(II) can be oxidized to Ni(III) forming black Ni2O3 or NiOOH deposits

[201, 202].


) of individual electrodes of S30/S30 ECs in 1 mol L-1

Li2SO4

with nickel collectors, recorded up to voltage of 0.8 V, 1.0V, 1.2 V, 1.5 V, and 1.6 V;

the vertical dashed lines corresponds to the thermodynamic limits for water

decomposition.

Due to the good conductive properties of NiOOH, its presence will not much

impede the electrochemical performance of the EC. Figure 52 shows that, during

floating of the EC with nickel collectors at 1.6 V, the resistance remains stable till the

end of the test, suggesting that the performance of the cell is not affected by the

appearance of the residues. However, the exact nature of the deposits and their real

effect on the cells constituents during long time operation has not been yet investigated.

Notwithstanding, while the maximum voltage for long term operation of S30/S30 ECs

with stainless steel collectors under floating is 1.5 V (chapter III), the S30/S30 system

with nickel collectors can operate up to 1.6 V without deterioration of the

electrochemical performance after 120 hours of accelerated ageing (Figure 52).



Figure 52 Capacitance and resistance evolution during floating at 1.6 V of a S30/S30

capacitor with nickel collectors in 1 mol L-1

Li2SO4.

1.2. Improvement of the current collector/electrode interface

1.2.1. Carbon electrodes glued to stainless steel current collectors

As it was noticed in Figure 43, the increase of resistance and decrease of

capacitance may be triggered above 35 °C, although floating at 1.5 V. After increasing

the temperature to 40°C during floating at 1.5 V, corrosion products are clearly

observed on the positive stainless steel current collector (Figure 42). In order to

eliminate the deposition of these products at the interface between the carbon electrodes

and the current collectors, pellet carbon electrodes were stick to the stainless steel

collectors with a conductive glue (Carbon Conductive Adhesive 502,

Electron Microscopy Sciences, CG) consisting of carbon particles in a fluoroelastomer

dissolved in methyl-ethyl-ketone (MEK) [203]. The capacitance and resistance of the

obtained AC/AC capacitor in 1 mol L−1

Li2SO4 were measured during floating at 1.5 V

and 24 °C or 35 °C (Figure 53). At both temperatures, the profiles of capacitance and

resistance evolution remain identical to the case when pellets are in direct contact with

the current collectors. However, at the end of floating, the resistance values are lower in

presence of conductive glue as compared to the cell without CG at both temperatures.



Figure 53 (a) Relative capacitance and (b) resistance of ECs with stick S30 electrodes

in 1 mol L

-1 Li2SO4 during floating at 1.5 V and 24°C or 35°C.

The resistance values calculated from the 5th

galvanostatic discharge at 1 A g-1

before and after 60 floating sequences at 1.5 V and 24 °C or 35 °C, with and without the

presence of conductive adhesive, are given in Table 2. The presented data reveal that

CG improves the contact between the electrodes and the current collectors, resulting in

almost twice lower initial values of resistance at both temperatures. Moreover, the

contact between the active mass and the current collectors is not weakened during

floating by forming an insulating layer of any corrosion product at this interface.

However, some decomposition products created during prolonged floating at 35 °C

probably still block the pores of S30, which entails a capacitance decrease by 8% as

observed in Figure 53.

Resistance, Ω

with CG without CG

24 °C 35 °C 24 °C 35 °C

before floating 0.8 1.0 1.6 1.6

after floating 0.9 2.0 2.8 4.5

Table 2 ECs in 1 mol L

-1 Li2SO4 with S30 electrodes placed on the stainless steel

collectors with and without conducting glue (CG): resistance values determined before

and after floating at 1.5 V and 24 °C or 35 °C.



1.2.2. Nickel foil substrate

Since some improvement was given by the use of conducting glue, and to

proceed further in the optimization of the system, we decided to follow the industrial

way of electrodes realization, where the active material layer is spread on the current

collectors with an applicator using a Doctor Blade. Moreover, since stable resistance

and capacitance was observed during ageing the EC with nickel collectors at 1.6 V

(Figure 52), we decided to use nickel foil (200/201 grade, Schlenk, thickness = 20 μm)

as substrate to prepare the coated electrodes. For this study, the commercially available

carbon YP80F (Kuraray Co.) with a high specific surface area of 2270 m2 g

-1 and

L0=1.05 nm has been chosen as electrode active material (see experimental annex

A.1.1).

Unfortunately, accelerated ageing at 1.5 V and 24 °C revealed an insufficient

contact between the nickel foil substrate and the electrode material, which peeled off

from the foil during ageing. In the literature concerning EDLCs in aqueous electrolyte,

it has been demonstrated that the performance, especially the contact resistance, is

dramatically improved by etching the current collector in order to better anchor the

coating layer [204].

Figure 54 Scanning electron microscopy (SEM) images of (a, c) nickel foil 200/201,

and (b, d) soft-annealed nickel foil current collectors.



In case of plain nickel (200/201) foil, the scanning electron microscopy (SEM)

images show a relatively even surface with rows parallel to the extrusion direction

(Figure 54a and c) where the coating layer cannot be anchored. For this reason, we have

used soft annealed nickel (Schlenk, thickness = 25 μm) as substrate for the carbon

coating. The SEM images on this material (Figure 54b and d) clearly show a rough

surface, with homogeneously distributed sub-micrometric grains. The observed grains

with a size of around 500-800 nm, formed from the recrystallized nickel structure

during annealing, are expected to ensure well-anchored coatings when using this foil.

The differences in electrochemical properties of the two kinds of capacitors

made with plain and soft annealed nickel collectors are demonstrated in Figure 55 by

the Nyquist plots, obtained from impedance spectroscopy at open circuit voltage

(OCV). Since the separator, electrolyte, and carbon electrode material are the same in

both cells, the ESR values are equal in the two systems. The slight decrease of

equivalent distributed resistance (EDR) by 0.25 Ω in the case of heat-treated nickel is

attributed to lower contact resistance between the coating electrode material and the

current collector.

Figure 55 Nyquist plots at OCV of YP80F/YP80F cells in 1 mol L-1

Li2SO4 made with

(o) nickel 200/201 and () soft-annealed nickel foil.



The most prominent difference between the two capacitors is visible by the

decreased slope in the low frequency branch for the EC with soft annealed nickel

collectors (Figure 55). As mentioned in chapter III.2, the presence of a CPE indicates a

non-uniform thickness of EDL, inhomogeneity or adsorption processes [186]. The more

developed surface area of the soft annealed nickel collector can lead to higher reactivity

during electrochemical ageing in aqueous medium and to the formation of corrosion

products at the interface between the active material and the collector, which is not

protected by a CCI pre-coating layer.

Figure 56 presents the evolution of capacitance and resistance during floating at

1.5 V and 1.6 V and room temperature on ECs with AC electrodes coated on the soft

annealed nickel. A significant increase of resistance by around 100% is observed after 6

cumulated floating hours, both at 1.5 V and at 1.6 V. The decrease of capacitance by

20% after 43 potentiostatic sequences at 1.6 V is attributed to the accumulation of

products in the pores of the positive YP80F electrode. Due to the well-developed

surface area of annealed nickel, the reactivity during electrochemical ageing in aqueous

medium is certainly much higher than in case of plain nickel, leading to the formation of

resistive corrosion products which can deposit in the bulk of the electrode material,

causing capacitance decay.

Figure 56 (a) Capacitance and (b) resistance evolution during floating at 24°C and 1.5

V and 1.6 V on ECs in 1 mol L

-1 Li2SO4 with YP80F electrodes coated on soft-annealed

nickel collectors.



1.2.3. Carbon conductive sub-layer

In order to both improve the adhesion of the carbon coating to the substrate and

protect the electrode/nickel foil interface, a conductive carbon ink (CCI, Electrodag PF-

407A, specially designed for the production of low voltage circuitry) with finely divided

carbon particles dispersed in a thermoplastic resin has been applied as a so-called ‘pre-

coating’ layer. Apart from good electrical conductivity, CCI provides resistance to

abrasion, scratching, flexing, and improves the contact of the electrode material to the

substrate, as proved visually and mechanically by scratching and cross-cut testing. The

SEM image presented in Figure 57a reveals a rough surface of the CCI layer on nickel

substrate, favourable to improve adhesion of the subsequently coated electrode material.

The magnified image of Figure 57b shows carbon agglomerates connected to each other

by polymer fibres which ensure good mechanical features of the layer and provide good

conductivity of the carbon ink.

Figure 57 Scanning electron microscopy (SEM) images of a conductive carbon ink

(CCI, Electrodag PF-407A) pre-coating of 15 μm thickness: (a) general view of the CCI

surface; (b) polymer fibres connecting the carbon-based agglomerates.

A cell with electrodes made of YP80F coating on nickel foil 200/201 previously

covered by a 15 μm thick CCI layer has been investigated by EIS at open circuit

voltage. The low frequency line almost parallel to the imaginary part of the Nyquist plot

(Figure 58) at low frequency reveals a good penetration of ions in the pores of the

electrodes and a uniform thickness of the double-layer. The reduced CPE discloses that

the conductive CCI sub-coating prevents from the adsorption of resistive deposits on the



surface of nickel. As compared to the cell with the same nickel 200/201 substrate

without CCI, the increase of low frequency branch slope results from the decrease of

equivalent distributed resistance (EDR) (Figure 58 and Table 3).

Figure 58 Nyquist plot at OCV of a YP80F/YP80F cell in 1 mol L-1

Li2SO4 made with

nickel 200/201 collectors with () and without (o) CCI pre-coating.

Since the path of ions to the active surface area is the same in both cells with and

without CCI pre-coating, they display almost identical ESR values (Table 3).

Notwithstanding, the current is distributed more evenly when the electrode coating is

anchored to the substrate with help of the conductive ink. The charge transfer resistance

value Rf, (responsible for the radius of the high-frequency semi-circle), is a bit lower for

the cell with pre-coating as compared to the cell with plain nickel collectors.

In conclusion, anchoring of coating is improved in fresh cells either by CCI pre-

coating of plain nickel or by use of annealed nickel. However, one should not forget

that floating tests have revealed a high surface reactivity of annealed nickel, and

demonstrated its incompatibility with lithium sulfate electrolyte. Therefore,

potentiostatic floating is necessary to validate the improvement observed on the fresh

cells using CCI pre-coated nickel collectors.



Nickel 200/201 Pre-coated

nickel 200/201

ESR 0.77 0.76

EDR 1.78 1.27

Rf 0.41 0.38

Table 3 ESR, EDR and Rf values obtained from the Nyquist plots of YP80F/YP80F

capacitors in 1 mol L-1

Li2SO4 made with nickel 200/201, with and without CCI pre-

coating.

Figure 59 shows the evolution of relative capacitance and resistance during

potentiostatic floating at 1.5 V and 1.6 V on YP-80F/YP-80F cells in 1 mol L-1

Li2SO4

with electrodes coated on nickel pre-coated by CCI. Definitely, the comparison with

Figure 56 reveals a dramatic improvement in stability of electrochemical performance;

after 120 cumulated hours of floating at 1.5 V or 1.6 V, the values of resistance and

capacitance are almost identical to the initial values. As for the previously examined

ECs without CCI, the more pronounced initial capacitance increase for the cell

examined at 1.6 V is the most probably attributed to a better penetration of ions in the

porosity of electrodes (Figure 59a); nonetheless, further floating series do not influence

ageing of the cell.

Figure 59 (a) Capacitance and (b) resistance evolution of YP80F/YP80F cell in 1 mol

L-1

Li2SO4 made with pre-coated nickel foil with CCI during floating at 24°C and 1.5 V

and 1.6 V.



Stable C/C0 profile suggests that neither any decomposition products block the

carbon active surface, nor the good contact between the electrodes and the current

collectors is disturbed. This statement is supported by the resistance evolution presented

in Figure 59b, which values are maintained below 1 Ω during the whole 60 series both

at 1.5 V and at 1.6 V. Besides, after opening the cell, the AC electrodes were still well

attached to the nickel foils pre-coated with the conductive ink.

Hence, the floating tests reveal the necessity of conductive pre-coatings in the

manufacturing of carbon electrodes, in order to ensure satisfactory long-term

performance of high voltage electrochemical capacitors in neutral aqueous electrolytes.

To sum up, when nickel collectors are used with self-standing S30 electrodes (part 1.1),

as well as with YP-80F electrodes coated on CCI/nickel substrate (part 1.2), ECs in 1

mol L-1

Li2SO4 can operate up to 1.6 V without deterioration of electrochemical

performance.

1.3. Addition of corrosion inhibitor

According to the investigations performed by Q. Abbas in our research group,

sodium molybdate (Na2MoO4) as additive to lithium sulfate electrolyte reduces the

corrosion of current collectors in AC/AC capacitors, whereas the capacitance is

enhanced through faradaic contributions [205]. Therefore, we have prolonged this

research on lifetime improvement, by investigating the performance of S30/S30 cells

with 0.1 mol L-1

Na2MoO4 + 1 mol L-1

Li2SO4 electrolyte at 24 °C and 40 °C.

Cyclic voltammograms of the capacitors with S30 electrodes in the form of

pellets in 1 mol L-1

Li2SO4 (pH = 6.5, conductivity = 64 mS cm-1

) and 1 mol L-1

Li2SO4

+ 0.1 mol L-1

Na2MoO4 (pH = 6.7, conductivity = 72 mS cm-1

) performed at 10 mV s-1

scan rate up to 1.5 V are presented in Figure 60. Due to redox processes involving the

molybdate ions, the capacitance is higher in Li2SO4 + Na2MoO4 than in Li2SO4. This

enhancement of capacitive current, as well as more rectangular shape of the CV curve

for EC with Li2SO4 + Na2MoO4, as compared to the one with Li2SO4, could be also

partly attributed to the higher conductivity of the electrolyte with the molybdate

additive.




) of S30/S30 capacitors in 1 mol L-1

Li2SO4 and 1 mol L-1

Li2SO4 + 0.1 mol L-1

Na2MoO4 electrolytes up to 1.5 V.

To determine the impact of the molybdate additive on ageing, ECs in 1 mol L-1


Na2MoO4 were submitted to floating at 1.5 V at 24 °C or 40 °C

with simultaneous monitoring of cell capacitance and resistance (Figure 61). As for the

previously presented capacitors in Li2SO4 (Figure 43), capacitance of ECs with Li2SO4

+ Na2MoO4 increases during the first 20 floating hours. However, when the floating is

prolonged, the capacitance remains almost constant, while it decreased for the capacitor

in 1 mol L-1

Li2SO4 at 40°C.

Figure 61 Effect of floating at 1.5 V and 24°C or 40°C on the evolution of (a) specific

capacitance and (b) relative resistance of a S30/S30 capacitor in 1 mol L

-1 Li2SO4 + 0.1

mol L

-1 Na2MoO4.



The beneficial effect of sodium molybdate on the long-term electrochemical

performance of a capacitor in the salt aqueous electrolyte is more quantitatively

presented in Table 4. The ECs in 1 mol L-1


Na2MoO4 reveal stable

capacitance, which is higher than in Li2SO4 by 19 F g-1

and 37 F g-1

at 24 °C and 40 °C,

respectively.

The impact of the electrolyte mixture is also revealed by the resistance evolution

presented in Figure 61, with almost constant value at 24 °C and only a slight increase by

40% at 40 °C. The resistance increase for the cell with the additive is much lower than

for the EC in 1 mol L-1

Li2SO4, by 43% and 60% at 24 °C and 40 °C, respectively (see

Figure 43b). As presented in literature, the corrosion provoked by aggressive anions,

such as chlorides or sulfates, can be inhibited by molybdate addition. The additive

strengthens the hydrated iron oxide layer on the stainless steel surface in neutral

aqueous solutions [206]. The interaction between MoO42-

and Fe2+

results in the

formation of FeMoO4, which in the presence of dissolved di-oxygen is further

transformed into insoluble complex preventing from corrosion and related resistance

increase [207].

Li2SO4 + Na2MoO4 Li2SO4

24 °C 40 °C 24 °C 40 °C

Capacitance, F g-1

before floating 98 101 82 74

after floating 105 106 86 69

Relative resistance, -

before floating 1 1 1 1

after floating 1.14 1.36 1.63 2.18

Table 4 Capacitance values and relative resistance for ECs in 1 mol L

-1 Li2SO4 and 1

mol L-1


Na2MoO4 determined before and after floating at 1.5 V

and 24°C or 40°C.



In Figure 62 showing the aspect of the stainless steel current collectors after

floating at 1.5 V and 40°C in 1 mol L-1


Na2MoO4, a grey deposit is

apparent essentially on the positive current collector and separator. According to the

Pourbaix diagram of molybdenum [199], the reaction (39) occurs at neutral pH of the

electrolyte:

MoO2 + 2H2O → MoO42-

+ 4H+ + 2e

- (39)

Therefore, the deposit on the positive electrode is probably assigned to a protective

passive film, composed mainly of MoO42−

, MoO2 and some traces of MoO3. The

transformation of molybdate ions into HMoO4- and further into MoO3 can originate

from water oxidation at the positive electrode, which contributes to a locally decreased

pH and shift of equilibrium potentials [199].

Figure 62 Collectors and separator of a S30/S30 cell after 120 hours of floating at 1.5 V

and 40°C in 1 mol L-1


Na2MoO4.



Beside the faradaic contribution and inhibition of corrosion, the presence of

sodium molybdate in neutral aqueous electrolyte is supposed to reduce also another

important factor contributing to ageing of the system at high voltage, namely electrolyte

oxidation. Indeed, the investigations on two-electrode S30/S30 cell equipped with a

reference electrode presented by Abbas et al. [205] showed that the potential reached by

the positive electrode (E+) in presence of Li2SO4 + Na2MoO4 electrolyte is shifted by

around -0.162 V as compared to Li2SO4. For this reason, whilst in the cell with lithium

sulfate electrolyte, the potential of the positive electrode reaches the thermodynamic

oxygen evolution limit of 0.84 V vs NHE at voltage of only 1.35 V, the system with

sodium molybdate additive can operate at room temperature up to 1.6 V with positive

electrode potential 0.042 V below the thermodynamic oxygen evolution potential. In

conclusion from the referred study [205], the shift of potentials toward negative values

should practically prevent from electrolyte decomposition and positive stainless steel

current collector corrosion during ageing at 1.5 V.

To verify the above statement, the evolving rate of gases during one 2-hour

floating period at 1.5 V at room temperature (24 °C) and at 40 °C was measured with a

pressure sensor connected to the electrochemical cells with Li2SO4 and Li2SO4 +

Na2MoO4 (see Figure 44 for the system construction), and the results are shown in

Figure 63. Considering the experiments performed at 24 °C, the addition of molybdate

to the electrolyte dramatically reduces the internal pressure increase by ~25 mbar. At

higher temperature of 40°C, the impact of the additive is less significant, and the

pressure increases in both cells (with and without molybdate) by 130 mbar and 140

mbar, respectively. This diminished effect of the corrosion inhibitor to reduce gases

evolution can be attributed to the inactivity of MoO42-

to form molybdenum complexes

at 40 °C. The curves presented in Figure 63 exhibit a linear pressure growth,

demonstrating the destructive effect of ageing at higher temperatures, which occurs just

from the beginning of floating. Consequently, the increase of resistance displayed

during prolonged potentiostatic voltage hold at 40°C in Figure 61b seems to be

essentially related to the evolution of gases which might worsen the electrical contacts

between electrodes and current collectors.



Figure 63 Internal overpressure evolution in S30/S30 cells with and without sodium

molybdate additive during two hours potentiostatic floating at 1.5 V and 24 ⁰C or 40 ⁰C.

As observed in chapter III.3, the increase of resistance during floating, as well as

pressure evolution, due to electrolyte decomposition, suggest a possible oxidation of the

positive carbon electrode. Figure 64 presents the mass loss, and CO2 and CO profiles

obtained by TPD for the fresh ACC and the positive and negative ACC electrodes of an

EC in 1 mol L-1


Na2MoO4 electrolyte aged by 120 floating hours at

1.7 V and RT. The data also reveal surface oxidation of carbon electrodes as compared

to the case of the cell without molybdate addition (Figure 39). The mass loss at 950 °C

is 44.8 % for the positive and 18.5 % for the negative electrode, as compared to 43.6%

and 14.5%, respectively, for the EC in 1 mol L-1

Li2SO4. However, the surface

functionality of the positive and negative aged electrodes is different. It is rich in new

oxygenated groups, releasing 4.8 mmol g-1

and 1.2 mmol g-1

of CO2, respectively. The

cumulated released CO is 6.8 mmol g-1

and 4.9 mmol g-1

, for the positive and negative

electrode, respectively, which is actually few times more than for the ACC electrodes

operating in Li2SO4. During floating in presence of molybdate ions, the carbon surface

is oxidized with formation of new CO-evolving groups identified as carbonyl/quinone

groups and pyrone-type structures at 820 °C and 940 °C, respectively [193, 194].



Figure 64 TPD on pristine ACC (black line) and on positive (red line) and negative

(blue line) aged carbon electrodes after 120 hours of floating at 1.7 V in 1 mol L-1


Na2MoO4: (a) CO2 evolution; (b) CO evolution.

Hence, the addition of 0.1 mol L-1

Na2MoO4 to 1 mol L-1

Li2SO4 inhibits the

corrosion of the positive stainless steel collectors and enhances capacitance through

faradaic contributions. Moreover, the internal pressure increase is reduced, even at 40

°C. However, to reduce the influence of electrolyte decomposition on the lifetime and

performance of the AC/AC electrochemical capacitor, further improvement of the cell is

needed.

IV.2. Shifting of electrodes operating potentials

2.1. Asymmetric configuration

According to formula (40) [129]:

𝒎+𝑪+∆𝑬+ = 𝒎−𝑪−∆𝑬− (40)

expressing equality of the electric charge passed through each electrode (where m+ and

m- are the active carbon mass, C+ and C- - the specific capacitance, ΔE+ and ΔE- - the

potential range of the positive and negative electrodes, respectively), the electrodes

potential range (and consequently the electrodes potential extrema) can be shifted by

adjusting the mass ratio of electrodes or/and by using different materials of different

capacitance. Therefore, to improve long-term performance of ECs, by avoiding



decomposition of electrolyte, electrochemical capacitors with asymmetric configuration

of electrodes with different carbons in 1 mol L-1

Li2SO4 were built. Such effect of

shifting electrodes operating potential has been previously observed with (-) AC /MnO2

(+) capacitors in 0.5 mol L−1

Na2SO4 using a mass ratio m+/m- = 2.5 and voltage of 2 V

[208]. Although the short time experiments in two-electrode cell with reference

electrode showed the possibility to reach 2.2 V with the latter system, galvanostatic

cycling revealed that the maximum voltage for this system needs to be reduced by 0.2 V

for a good cycle life. Therefore, in the present study, after the basic electrochemical

investigations on the realized asymmetric carbon/carbon cells, both the SOH of ECs and

the actual values of potentials reached by the positive and negative electrode have been

simultaneous monitored during accelerated ageing by floating at 1.5 V.

Taking into account equation (40), our objective has been to realize a capacitor

with electrodes of same mass of two carbons with different capacitance. The cyclic

voltammograms of two symmetric cells built with the S30 and Burley800 (SBET = 1651

m2 g

-1; L0 = 0.86 nm) [78] (further named as B800) carbons in 1 mol L

-1 Li2SO4

demonstrate higher capacitance for B800 (Figure 65); from galvanostatic

charge/discharge measurements, the capacitance values are 82 F g-1

and 125 F g-1

for

S30 and B800, respectively. Therefore, an asymmetric capacitor has been built with

B800 as positive electrode and S30 as negative one, in order to get ΔE->ΔE+ and

consequently to shift the potential extrema of electrodes towards lower values.

Figure 65 Cyclic voltammograms at 10 mV s-1

for symmetric cells based on the S30

and B800 carbons in 1 mol L-1

Li2SO4.



In order to prove the effectiveness of the S30(-)/B800(+) construction, Figure 66

compares the electrodes potential ranges vs cell voltage for three different couplings of

the two carbons: S30 (-) /S30 (+), B800 (-)/B800 (+) and S30(-)/B800(+).

Figure 66 Electrodes potential range vs voltage during galvanostatic cycling at 1 A g-1

on: (a) S30 (-) / S30 (+); (b) B800 (-) / B800 (+) and (c) S30 (-) / B800 (+) cells in 1

mol L-1

Li2SO4. The measurements were realized in two-electrode cells with reference

electrode.



For the two symmetric systems in 1 mol L-1

Li2SO4, lower value of positive

electrode potential is reached at voltage of 1.6 V with B800 (-) /B800 (+) (Figure 66b)

as compared to S30 (-) /S30 (+) (Figure 66a) , 0.78 V and 0.96 V, respectively. Due to

the higher capacitance of the B800 carbon as compared to S30, the shift of positive

electrode potential is more pronounced for the S30 (-) /B800 (+) asymmetric

configuration (Figure 66c). Taking into account the limit for water reduction and

noticeable H2 evolution in 1 mol L-1

Li2SO4 (-0.8 V vs NHE estimated by three-

electrode cell measurements as presented in Figure 20), at voltage of 1.2 V, the potential

of the negative electrode is lower than the potential for di-hydrogen evolution (lower

horizontal line at -0.8 V vs NHE in Figure 66c). However, on the CVs of the negative

electrode of the S30 (-) /B800 (+) system, no oscillations due to bubbling were observed

below the potential of -0.8 V vs NHE. Notwithstanding, the shift of potentials higher

than eventually expected using this S30 (-)/B800 (+) construction may require slight

adjustment of electrodes masses. A statement about that will be presented once floating

experiments have been realized with this system.

In order to accurately state on the effects of electrodes potential shift, floating at

1.5 V has been realized on the three cell configurations at 24 ⁰C, with simultaneous

monitoring of the SOH of the systems (Figure 67). The symmetric EC based on the

B800 carbon exhibits the best performance, when considering the evolution of both

capacitance and resistance. Due to the well-developed microporosity of the tobacco

carbon, the B800 (-) / B800 (+) capacitor exhibits the highest initial capacitance value,

which is maintained during 120 floating hours of the test. Overall, the B800 (-) / B800

(+) capacitor largely outperforms the S30 (-)/S30 (+) one [78]. The asymmetric cell,

obtained by coupling the positive electrode from microporous B800 carbon with the

negative one from S30, exhibits an intermediate capacitance value of 102 F g-1

. When

compared to the symmetric cells, the asymmetric system displays the worst long-term

performance at 1.5 V, both for capacitance and resistance evolution. It suggests that

either down-potential shifting resulting from the asymmetric construction is too high

(although, as noticed, detrimental effects of gas bubbling are not observed at the

negative electrode) or that important changes occur in the electrodes potential range

during floating.



Figure 67 Effect of the floating voltage at 24 ⁰C on (a) capacitance and (b) resistance of

the cells with three different configurations of carbon electrodes in 1 mol L

-1 Li2SO4.

In order to better understand what happened during accelerated ageing of the

asymmetric EC, the evolution of electrodes potentials was monitored during the

repeated floating sequences at 1.5 V. As it can be seen for a two-hour potentiostatic

period (Figure 68a), the potentials of positive and negative electrodes increase

remarkably at the beginning of the period and then the shift is less pronounced as the

system tends to an equilibrium state. Due to the applied polarisation, the ions attracted

to the active surface of the carbon electrodes reach the more highly confined porosity

and are further pushed from the diffusion layer to the compact one, ordering the

structure of the EDL. During the further floating sequences, the electrodes potentials

shift by around +0.3 to +0.4 V after 120 hours of floating (Figure 68b), which can

finally lead to subsequent possible effects: i) positive electrode oxidation; ii)

accumulation of corrosion products. The potential of the positive electrode exceeds the

thermodynamic limit for water oxidation after around 40-50 hours of floating at 1.5 V,

which is in agreement with Figure 67b, where the resistance of the asymmetric cell

begins to suddenly increase from this time.



Figure 68 Electrodes potential profile of a S30 (-) / B800 (+) capacitor in 1 mol L-1

Li2SO4 electrolyte during (a) one 2-hour floating period at imposed voltage of 1.5 V

during the third sequence of accelerated ageing; (b) 60 2-hour sequences at 1.5 V.

Figure 69 presents the capacitance evolution of the asymmetric S30 (-) / B800

(+) cell and of the individual positive B800 and negative S30 electrodes during floating

at 1.5 V. It can be easily noticed, that the capacitance decay of the whole system is

essentially related to the positive electrode degradation, while the negative electrode is

not much influenced by the ageing.

Figure 69 Capacitance evolution of the S30 (-) / B800 (+) cell and individual electrodes

during floating at 1.5 V in 1 mol L-1

Li2SO4.



The TPD data show that, as compared to S30, the B800 carbon has a higher

amount of surface oxygenated functional groups (Table 5). Hence, the higher reactivity

observed at the positive B800 electrode during floating must be attributed to the higher

number of active sites of this carbon. It is likely that additional annealing treatment

could reduce the oxygen content [78] and probably would improve the cyclability of the

positive electrode.

Carbon material

CO2 CO H2O O

µmol g-1

µmol g-1

µmol g-1

wt%

S30 317 249 44 1.5

B800 440 598 416 3.0

Table 5 TPD analysis data on carbons S30 and B800.

The previous interpretation suggesting decomposition reactions at the positive

B800 electrode are confirmed when comparing the shape of voltammograms recorded

after floating the S30 (-) / B800 (+) cell (Figure 70a) and the S30 (-) /S30 (+) one

(Figure 38c) at 1.5 V. In both cases, the CVs deviation from the rectangular shape

indicates worse charge propagation after floating. However, the narrowing of

voltammogram at high voltage for the S30 (-) / B800 (+) cell is more pronounced than

with S30 (-)/S30 (+), which indicates higher porosity saturation for the former cell,

related with decomposition reactions at the positive electrode. The CVs of the

individual electrodes for the S30 (-) / B800 (+) cell, before (Figure 70b) and after 120

hours of floating at 1.5 V (Figure 70c), clearly demonstrate the non-EDL behaviour of

the positive electrode after floating. Additionally, due to the shift of potentials towards

higher values, the potential of the positive electrode exceeds the limit for water

oxidation (Figure 70c), which can lead to electrode oxidation and/or accumulation of

corrosion products in the porosity of carbon, finally causing a reduction of positive

electrode active surface.




) recorded before and after 60 sequences

of 2-hour floating at 1.5 V, using a S30 (-) / B800 (+) cell with reference electrode in 1

mol L-1

Li2SO4 : (a) full cell; (b) individual electrodes before floating and (c) individual

electrodes after floating .



The presented data have revealed that, when compared to symmetric

configurations of cells, the investigated asymmetric system displays worse long time

performance at 1.5 V. Even though, coupling the positive electrode from microporous

B800 carbon with the negative one from S30 results in satisfactory shift of electrodes

operating potentials towards lower values, the whole cell exhibits an intermediate

capacitance which is decreased by 20% after 110 hours of floating at 1.5 V.

Notwithstanding, it seems that the reactivity of the B800 carbon could be at the origin of

the performance decay. Therefore, some further efforts should be dedicated to better

stabilize this material by annealing. The too strong potential shift given by this

construction cannot be rejected as additional cause of the poor cycle life. Better control

of the potential shift by finely tuning the electrodes mass ratio should be also further

investigated.

2.2. Current collectors coupling

Considering the optimization of electrochemical capacitors in salt aqueous

electrolyte by adapting the components, besides asymmetric configuration of the cells

utilizing carbon electrodes with different mass and/or nature, or various kinds of

electrolytes, coupling of different current collectors can be applied to shift the

maximum potential of the positive electrode towards lower values. As presented in

section IV.1, nickel was found as an alternative and promising material to stainless steel

to improve the long-term stability of electrochemical capacitors in aqueous electrolyte.

To verify the difference between the implemented collectors configuration in 1 mol L-1

Li2SO4, ECs were realized in PTFE Swagelok-type assembly with YP-80F coated

electrodes either on stainless steel or nickel 200/201 foil (previously pre-coated by

CCI), using the corresponding cylindrical current collectors, either from stainless steel

or nickel.

The investigations previously performed in symmetric cells (part 1.1.) served as

a basis to propose coupling of stainless steel and nickel collectors. Table 6 presents

capacitance values [F g-1

] determined from galvanostatic discharge at -1 A g−1

from 1.6

V to 0 V according to equation (43) in experimental annex. The data reveal the same

discharge capacitance for the two cells with stainless steel (−) /stainless steel (+) and

nickel (−) /nickel (+) collectors. However, when considering the individual electrodes,

the positive electrode displays a high capacitance in the stainless steel (−) /stainless steel



(+) cell, while it is the negative one in the nickel (−) /nickel (+) cell. Therefore, we

decided to investigate if the performance could be enhanced by a nickel (−) /stainless

steel (+) assembly, and to verify if the corrosion of the stainless steel positive collector

corrosion could be reduced when nickel is applied as a negative collector.

steel (-) /steel (+) nickel (-) /nickel (+)

positive electrode 125 72

negative electrode 63 100

whole cell 87 85

Table 6 Capacitance [F g-1

] of electrochemical capacitors and individual electrodes

coated on stainless steel or nickel foils in 1 mol L-1

Li2SO4 determined by galvanostatic

cycling (1 A g-1

) up to 1.6 V of stainless steel (−) /stainless steel (+) and nickel (−)

/nickel (+)configuration of current collectors.

Figure 71 presents cyclic voltammograms of the three cells with different

configuration of collectors. Although capacitance of the whole cell (85 F g-1

) is not

enhanced by coupling the two kinds of collectors, the voltammogram of the EC with

nickel (−) /steel (+) configuration exhibits a slightly improved shape with a diminished

current leap at the charged state, when compared to the steel (−) /steel (+) one.


) of electrochemical capacitors with

carbon electrodes coated on stainless steel or nickel foils in 1 mol L-1

Li2SO4.



The size of the high-frequency semi-circle in the Nyquist plot of the three cells

(Figure 72a) reveals that the cell with the combined collectors presents the lowest

charge transfer resistance value Rf (0.94 Ω), when compared to 1.1 Ω and 1.24 Ω for

nickel (−) /nickel (+) and stainless steel (−) /stainless steel (+) configurations,

respectively. For high power, Rf and time constant are crucial parameters indicating the

electrical losses taking place in all resistive components of the cell during charging and

discharging. The time constants of 0.80 s for the steel (-) / steel (+) and 0.57 s for nickel

(-) / steel (+) configurations reveal a harmful effect of steel on the dynamics of charge

exchange in the ECs, when compared to the nickel (-) / nickel (+) cell for which τ is

0.46 s. This impact is also demonstrated in the Bode plots (Figure 72b) where in the

frequency range 0.1 Hz – 1 Hz the phase angle increases more rapidly for the steel (-

) / steel (+) cell. The two configurations with nickel collectors reveal similar

performance up to around 1 Hz, above which the impact of the stainless steel collector

in the coupled cell is observed by the lower phase angle. Nevertheless, it is important to

note that the three ECs exhibit almost ideal capacitive behaviour at low frequency

represented by the value of phase angle very close to -90°.

Figure 72 (a) Nyquist and (b) Bode plots of the three YP-80F/YP-80F cells in 1 mol L-1

Li2SO4 with different configurations of collectors.

To analyse the SOH of the three cells with different combination of the current

collectors during long time performance, floating at high voltage of 1.6 V has been

applied (Figure 73). The most stable capacitance and resistance values during floating

are revealed by the cell with the coupled nickel (-) / steel (+) collectors.



Figure 73 (a) Capacitance and (b) resistance evolution during floating at 1.6 V of YP-

80F/YP-80F capacitors in 1 mol L-1

Li2SO4 with electrodes coated on stainless steel and

nickel collectors.

Once opening the YP-80F_nickel (-) / YP-80F_steel (+) cell after 120 hours of

floating at 1.6 V, no corrosion of the positive current collector was observed, although

some green deposits appeared on the negative one. The mixed-conductive nickel

compounds, formed on the surface of the negative current collector during floating, did

not affect the cell performance. The decrease of capacitive current at voltage higher than

1 V is much less pronounced as in the case of the two other cells using stainless steel

collectors (Figure 74).


) recorded after 120 hours of floating at 1.6 V on YP-

80F/YP-80F capacitors with different current collectors in 1 mol L-1

Li2SO4.



From CVs of individual electrodes recorded on two-electrode cells with

reference electrode, the reduced corrosion of the positive stainless steel collector in the

nickel (−) / stainless steel (+) system can be attributed to the −105 mV shift of the

electrode potentials at a voltage of 1.6 V (Figure 75a), as compared to the steel (−) /

stainless steel (+) combination (Figure 75b).


) of individual electrodes (coated on stainless steel or nickel

foils) of YP-80F/YP-80F cells in 1 mol L-1

Li2SO4 at 1.6 V with different collectors

combinations: (a) nickel (-) / nickel (+); (b) steel (-) / steel (+); (c) nickel (-) / steel (+).



The CV of the positive electrode in the nickel (−) / stainless steel (+) system

(Figure 75c) discloses a smaller anodic current leap, as compared to the symmetric

collector combinations (Figure 75a and b). Such characteristics traduce diminished

electrolyte decomposition, electrode oxidation and/or formation of corrosion products

on the positive current collector. Moreover, the accumulation of Ni(II) and Ni(III)

derivatives may contribute to polarize the surface of electrodes, and thus, cause a shift

of the EOCP from 0.285 V (Figure 75b) to 0.224 V vs NHE (Figure 75c) [209].

The performance of the capacitors in 1 mol L−1

Li2SO4 electrolyte is improved

by using both nickel (-) / nickel (+) and nickel (-) / steel (+) collectors configuration

when high voltages are applied. However, it would be worth to characterise the nature

of the nickel deposits formed after long-term operation, their actual effect on the cell’s

performance and to investigate strategies for reducing their creation.

IV.3. Conclusion

Strategies to improve the long time performance of AC/AC electrochemical

capacitors in neutral salt aqueous electrolyte were presented in this chapter. The

undertaken tactics intended to reduce the corrosion of stainless steel collectors and to

avoid the decomposition of aqueous electrolyte by shifting the operating electrodes

potentials to lower values.

The reduction of ECs lifetime due to collectors corrosion can be prevented by:

(i) using non-corrodible nickel collectors; (ii) avoiding deposition of the corrosion

products on the electrode-collector plane by coating the electrode material on metallic

foils; (iii) adding a corrosion inhibitor to lithium sulfate electrolyte.

As presented in chapter III, the maximum voltage for long term operation of

S30/S30 electrochemical capacitors with stainless steel collectors under floating is 1.5

V, while the S30/S30 system with nickel collectors can operate up to 1.6 V without

deterioration of electrochemical performance after 120 hours of accelerated ageing.

Due to the well-developed surface area of annealed nickel, the reactivity during

electrochemical ageing in aqueous medium is certainly much higher than in case of

plain nickel, leading to corrosion of the collectors and deterioration of electrochemical

performance



A good adhesion of carbon coating to the substrate and protection of the

electrode/substrate interface from accumulation of decomposition products is a key

factor in manufacturing of carbon electrodes by coating. Therefore, the application of

conductive carbon ink (CCI) pre-coating between the metallic substrate and the

electrode material appears to be necessary, in order to ensure satisfactory long-term

performance of electrochemical capacitors in neutral aqueous electrolytes at high

floating voltage.

The addition of 0.1 mol L-1

Na2MoO4 to 1 mol L-1

Li2SO4 inhibits the corrosion

of the positive stainless steel collectors and enhances capacitance through faradaic

contributions. However, in order to avoid oxidation of carbon electrodes after long time

performance at high voltage, further improvement of the AC/AC electrochemical

capacitor in neutral aqueous electrolytes with corrosion inhibitor is needed.

Coupling a highly microporous B800 carbon as positive electrode with industrial

S30 as negative one results in satisfactory shift of electrodes operating potentials

towards lower values. However, when compared to symmetric configurations of cells,

the investigated asymmetric system displays worse long time operation at 1.5 V. Since

the reactivity of the B800 carbon seems to be at the origin of the performance decay,

annealing of this material should be performed.

By using nickel (-) / nickel (+) and nickel (-) / steel (+) collectors configuration,

the performance of the capacitors in 1 mol L−1

Li2SO4 electrolyte is improved.

Although, the appearance of the residues does not affect the performance of the cell

during 120 hours of floating at 1.6 V, it would be worth to characterise the nature of the

nickel deposits formed after long-term operation.



CHAPTER V

TOWARDS A NEW CONCEPT

OF HIGH VOLTAGE AC/AC CAPACITOR

IN AQUEOUS ELECTROLYTES



As presented in the literature review, aqueous electrolytes offer several

important advantages for electrochemical capacitors (ECs) application compared to

solutions in organic solvents. Nonetheless, pure water has a thermodynamic stability

window of only 1.23 V, and it was observed that the practical cell potential in

conventional aqueous electrolytes used in batteries (H2SO4, KOH) is even limited to

lower values. Notwithstanding, for the electrochemical capacitor application, the over-

potential of di-hydrogen evolution at the negative electrode may in certain conditions

enhance the stability potential window when using an aqueous electrolyte. As it was

lately observed, this over-potential on an AC electrode is higher in neutral aqueous

electrolytes than in aqueous KOH or H2SO4; a voltage of 1.6 V during 10,000

charge/discharge cycles was claimed with a symmetric AC/AC capacitor in aqueous

Na2SO4 with gold current collectors [131]. However, as it was presented in the previous

chapters of this dissertation, the operating voltage of ECs with stainless steel current

collectors in neutral aqueous electrolytes is essentially dictated by the positive electrode,

due to oxidation of the electrode material and corrosion of current collectors, when the

thermodynamic limit of water oxidation is exceeded. Therefore, in order to increase the

overall voltage of the EC, asymmetric systems should be more extensively investigated

to optimize the operating potential range of both electrodes.

For extending the operating voltage of carbon-based ECs, this chapter presents

a new concept of AC/AC cell using KOH and Na2SO4 as catholyte and anolyte,

respectively. Besides, developing this new cell will help to validate our interpretations

for the over-potential observed at the negative electrode of AC/AC capacitors in salt

aqueous electrolytes.

III.1. The new concept of high voltage cell in aqueous

electrolytes

The concept cell which will be presented in this chapter is based on the fact

that the potentials of water oxidation and reduction are dependent on the electrolyte pH.

According to the Nernst law, one can imagine to extend the potential difference between

water oxidation and reduction (i.e. the operating voltage of an EC cell) by using a

catholyte with higher pH than the anolyte, both electrolytes being separated by a cation

exchange membrane (CEM). In the study, a homogeneous electro-dialysis membrane

used in standard demineralisation applications (FKS-PET-130, FuMA-Tech GmbH) has



been chosen as CEM, due to its high chemical stability (pH 5-14) and relatively good

selectivity (>96%). The nature and concentration of the electrolytes were practically

imposed by the characteristics of the selected CEM. They were selected for keeping

chemical inertness vs the CEM and relatively constant pH in both anodic and cathodic

compartments during the whole electrochemical experiment. After systematic

investigations based on the latter criteria, our final choice was 0.5 mol L-1

KOH as

catholyte and 1.0 mol L-1

Na2SO4 as anolyte.

The electrolyte decomposition potentials E vs NHE of half-cells in 0.5 mol L-1

KOH (pH= 13.2) and in 1.0 mol L-1

Na2SO4 (pH= 6.6) at equilibrium are given by

Nernst equations (41) and (42), respectively [210]:

𝑬− = −𝟎. 𝟎𝟓𝟗𝟏𝒑𝑯 = −𝟎. 𝟕𝟖𝟎 𝑽 (41)

𝑬+ = 𝟏. 𝟐𝟑 + (−𝟎. 𝟎𝟓𝟗𝟏𝒑𝑯) = 𝟎. 𝟖𝟒𝟎 𝑽 (42)

When the negative electrode potential is below E-, di-hydrogen evolves from the

catholyte. In turn, if the potential of the positive electrode is higher than E+, water from

the anolyte is oxidized producing nascent oxygen which may: (i) provoke corrosion of

the positive stainless steel current collector; (ii) oxidize the AC carbon electrode; (iii) or

evolve as di-oxygen. It follows, that the full cell should be theoretically able to operate

safely up to 0.840 – (-0.780) = 1.620 V, which is much more than the thermodynamic

limit of water decomposition (1.23 V).

Accordingly to the theory of the extended stability window due to the existing

pH difference between the cathodic and anodic compartment, one might ask to replace

the neutral Na2SO4 by, e.g., acidic solution. However, if sulfuric acid would be used as

anolyte, during charging, protons would migrate through the membrane towards the

negatively polarized electrode and cause neutralization of the initially basic catholyte.

Figure 76 shows the principle of cell operation, when it is charged with a

power generator. Due to the electrical potential difference between the electrodes, the

SO42-

ions migrate towards the positive electrode, where they are stored in the pores of

carbon. The sodium ions from the anolyte migrate through the CEM towards the

catholyte and are adsorbed together with potassium ions on the active surface of the



negative electrode. During, discharge of the cell, the ions may move in opposite

directions. Since the CEM contains in its matrix negatively charged groups, it controls

the transport of ionic species and prevents the OH- ions to flow from the catholyte to the

anolyte.

Figure 76 Charging principle of the concept cell with potassium hydroxide and sodium

sulfate as catholyte and anolyte, respectively, activated carbon electrodes, and a cation

exchange membrane as separator.

The electrochemical investigations on the new concept cell were realized in

PTFE case, using stainless steel (316L grade) collectors and two reference electrodes,

namely (Pt) Hg/Hg2SO4 in 1 mol L-1

H2SO4 and Hg/HgO in 6 mol L-1

KOH, for the 1.0

mol L-1

Na2SO4 anolyte compartment and 0.5 mol L-1

KOH catholyte compartment,

respectively (Figure 77). By sweeping/monitoring the voltage between the two activated

carbon electrodes, the system can be investigated as a two-electrode cell. Moreover, in

this two-electrode assembly, the data of either positive or negative carbon electrode can

be monitored vs the corresponding reference electrode, using the other carbon electrode

as counter one. If not referred otherwise, a commercially available activated carbon

powder (YP 80F, Kuraray Chemicals Co, further named as YP80F), with SBET=2270 m2

g-1

and L0=1.05 nm, has been chosen as electrode active material for the study on the

concept cell (see experimental annex A.1.1).



Figure 77 Schematic representation of the new concept AC/AC electrochemical

capacitor using two aqueous electrolytic solutions of different pH separated by a cation

exchange membrane (CEM), stainless steel collectors and reference electrodes.

The CEM membrane (diameter of 2.7 cm) was then placed in a cave secured

by silicone ring gaskets from both sides and retained by joining the two PTFE bodies of

the device together by screws, in the way to prevent from any leak and movement of the

membrane. The area of CEM exposed to electrolyte after assembling was 1.77 cm2. For

optimal performance with minimal wrinkling and lowest electrical resistance, the cell

was filled with demineralized water for 24 h at room temperature.

Figure 78 compares the cyclic voltammograms (CVs) of the new concept (-)

YP80F-KOH / YP80F-Na2SO4 (+) capacitor and of the (-) YP80F-Na2SO4 / YP80F-

KOH (+) cell with reversed configuration of electrolytes. The (-) YP80F-KOH / YP80F-

Na2SO4 (+) cell displays a near-ideal rectangular CV typical for an EDL capacitor up to

1.6 V (Figure 78a) contrary to the other cell where the sign of redox contributions is

easily visible even for low values of voltage (Figure 78b). The charging CVs of the

concept (-) YP80F-KOH / YP80F-Na2SO4 (+) cell are not featured by any current leap,

related to catholyte reduction and/or electrochemical oxidation of the positive carbon



electrode, and thereof proves the possible extension of operating voltage by an adapted

selection of the electrolytes.

Figure 78 CVs (0.4 mV s−1

) of YP80F/YP80F electrochemical capacitors in: (a) (-) 0.5

mol L-1

KOH / 1.0 mol L-1

Na2SO4 (+); (b) (-) 1.0 mol L-1

Na2SO4 / 0.5 mol L-1

KOH

(+) electrolytes for voltage windows from 0 V up to 0.8, 1.0, 1.2, 1.4, 1.5 and 1.6 V.

Galvanostatic cycling at 40 mA g-1

with simultaneous monitoring of E- and E+

(vs the respective reference electrodes introduced in each compartment) was performed

to verify the potential range of electrodes (Figure 79). According to formulae (41) and

(42), at a voltage of 1.6 V, the potential reached by the positive electrode (E+) should

not theoretically exceed 0.84 V vs NHE. However, even if the electrodes have an equal

mass, their capacitance values are uncontrolled. According to formula (40), the potential

range of the positive electrode may be higher than expected [129], leading the

maximum potential of this electrode to be higher than the value calculated from

equation (42) and represented by the upper horizontal dashed line on Figure 79.

From this figure, it is clearly seen that the maximum possible voltage of the

YP80F/YP80F capacitor with equal electrodes masses should be 1.5 V. Since the

electrodes potentials are shifted towards higher values than indicated by the

thermodynamic assumptions (equations (41) and (42)), the potential of the negative

electrode is higher than -0.78 V vs. NHE at a voltage of 1.6 V. It means that, in the

present configuration of the system, the maximum possible voltage range is not fully



exploited; further, we will show that a change of relative masses of electrodes, as well

as ACs, can help in shifting the electrodes potentials.

Figure 79 Electrodes potential extrema vs voltage measured during galvanostatic

cycling at 40 mA g-1

on a (-) YP80F-KOH / YP80F-Na2SO4 (+) cell with equal

electrode masses. EOCP - open circuit potential.

The cyclic voltammograms of individual activated carbon electrodes in of the

(-) YP80F-KOH/YP80F-Na2SO4 (+) cell were recorded in the potential ranges

determined from galvanostatic cycling (see Figure 79) for voltages up to 1.4 V, 1.5 V

and 1.6 V and are presented in Figure 80. These CVs confirm that, in practice, the

potential of the negative electrode does not reach the di-hydrogen evolution potential in

KOH (-0.78 vs. NHE), where oscillations on the CVs would be visible. The CVs of the

positive electrode are not featured by a significant current leap, even at the highest

voltage (1.6 V). It correlates with the previous observation made in two-electrode

assembly (Figure 78), that the system is able to operate up to 1.6 V, with almost ideal

rectangular shape of CVs and without electrolyte decomposition to gaseous products in

the form of O2 and H2. The minor current increase during positive polarization in



Na2SO4 with increasing voltage can be also attributed to the redox reactions between the

oxygenated surface groups and the electrolyte [131].

Figure 80 CVs of the individual electrodes in a (-) YP80F-0.5 mol L-1

KOH / YP80F-

1.0 mol L-1

Na2SO4 (+) cell for maximum voltages up to 1.4, 1.5, and 1.6 V (scan rate

for the cell 0.4 mV s−1

).

Taking into account Figure 79, the maximum voltage of the cell with YP80F

electrodes of equal mass should not be higher than 1.5 V, to avoid water oxidation at the

positive electrode. Therefore, galvanostatic (current density of 100 mA g-1

) cycling has

been performed up to 1.5 V, to verify the previous conclusion about high voltage

operation of the new concept cell. Figure 81 presents the evolution of capacitance and

resistance vs number of galvanostatic cycles, after initially conditioning the EC cell by

10 CV cycles at 0.4 mV s−1

and 10 galvanostatic charge/discharges at 40 mA g-1

. The

increase of capacitance during the first 200 cycles is attributed to better wetting of the

electrode material by the electrolyte, allowing narrow pores to be accessed by ions [211,

212]. Afterwards, capacitance very slowly decreases to reach 98% of the initial value

(C0) after 1,000 cycles. Likewise, the value of resistance at the end of cycling is



increased by only 20 % as compared to the ESR in the first cycle (R0). Overall, within

this number of cycles, the SOH of the cell is still very good, with capacitance and

resistance variations below the generally accepted end-of-life criteria [187].

Figure 81 Effect of galvanostatic (100 mA g-1

) cycling up to 1.5 V on capacitance and

resistance of the (-) YP80F-KOH /YP80F-Na2SO4 (+) cell.

After cycling of the new concept cell at 1.5 V, no traces of corrosion on the

positive stainless steel collector were noticed. During the long time performance, the

anolyte pH increased up to 8.9 after 550 galvanostatic cycles, and then did not change

anymore until the end of cycling; taking into account the initial pH value of 6.5, this pH

increase traduces almost negligible OH- migration to the anolyte during cycling.

Notwithstanding, the process of steel corrosion remains inhibited in the anolyte pH

range close to neutrality (6.5-8.9).

To summarize, the presented concept of carbon-based electrochemical capacitor

using two aqueous electrolytic solutions (KOH (-) / Na2SO4 (+)) separated by a CEM

has been validated by the electrochemical investigations. Due to the pH difference

between 0.5 mol L-1

potassium hydroxide as catholyte and 1.0 mol L-1

sodium sulfate as

anolyte, the theoretical potential difference between water oxidation and reduction is

increased to 1.62 V. However, when the system with two identical carbon electrodes is

charged up to a voltage of 1.6 V, there is still a waste range of negative potential which



is not utilized and, at the same time, the positive electrode potential exceeds the

thermodynamic limit of anolyte oxidation. The YP80F/YP80F capacitor with equal

electrodes masses operates with a good stability only up to 1.5 V.

III.2. Extension of voltage range by electrodes asymmetry

Although a maximum voltage slightly higher than 1.6 V is theoretically

predicted for the (-) YP80F-KOH/YP80F-Na2SO4 (+) capacitor, the previous

experiments have demonstrated that, in practice, this value cannot be reached due to the

high potential of the positive electrode leading to carbon oxidation. Therefore, we now

suggest balancing the electrodes in order to reduce the potential range ∆E+ of the

positive electrode, and consequently to lower its maximum potential below the

oxidation limit of the anolyte; according to equation (40), this can be realized by

increasing either m+/m- or C+/C-. Several examples of electrodes potential window

adjustment by this strategy are available in the literature, e.g., for the asymmetric

carbon/MnO2 capacitors [208], or for symmetric ECs in neutral [213] and organic

electrolytes [214] using the same activated carbon in both electrodes.

2.1 Adjustment of electrodes potential window by increasing

m+/m-

In our attempts to reduce ΔE+, we have applied different values of mass ratio

m+/m-. Although a small increase of m+/m- should theoretically be sufficient, it turned

out that, in practice, m+/m- should be increased up to 2.25 in order to sufficiently shift

the potential of the positive electrode. With this mass ratio, at a voltage of 1.6 V, the

potential of the positive YP80F electrode is E+ = 0.852 vs NHE (Figure 82), i.e., close

to the value of 0.840 V calculated from equation (42). While comparing with Figure 79,

it is clear that ΔE+/ΔE- is significantly reduced in Figure 82, but at the same time EOCP is

shifted to higher values. Hence, it can be concluded that the difficulty to reduce the

maximum potential of the positive electrode is related to this shift of EOCP. It can be

anticipated that the important change of positive electrode thickness accompanying its

mass increase can lead to reduced charge propagation in this electrode and correlatively

perturbations in EOCP.



Figure 82 Electrodes potential extrema vs voltage measured during galvanostatic (40

mA g-1

) cycling of (-) YP80F-KOH / YP80F-Na2SO4 (+) cell with unequal electrode

masses (m+/m-=2.25). EOCP - open circuit potential.

The consequence of applying a thicker positive electrode can be seen in the

comparison of GCPL curves (40 mA g-1

) for cells with equal and unequal electrodes

masses (Figure 83), where the voltage drop at 1.5 V is 28 mV and 32 mV, respectively.

Additionally, for the asymmetric cell, the discharge time is reduced as compared to the

symmetric one, traducing a diminishing of gravimetric capacitance as consequence of

increasing the total mass of carbon electrodes.

Figure 83 Galvanostatic (40 mA g-1

) charge-discharge profiles of (-) YP80F-

KOH/YP80F-Na2SO4 (+) cell and of the individual electrodes for cells with: (a) equal

electrode masses (m+/m-=1), (b) unequal electrode masses (m+/m-=2.25).



Likewise, when the cell with unequal electrode masses is charged, polarization

distribution across the positive electrode differs from the negative one [215]. The

hindrance in charge propagation during charging of the cell with thicker positive YP80F

electrode is visible by a more rounded CV as compared to the cell with symmetric

electrodes (Figure 84).

Figure 84 CVs (0.4 mV s−1

) of YP80F/YP80F electrochemical capacitors in (-) 0.5 mol

L-1

KOH / 1.0 mol L-1

Na2SO4 (+) with equal (m+/m-=1) and unequal electrode masses

(m+/m-=2.25).

The results with the unequal electrodes masses reveal that it is possible to shift

the operating potential range of the carbon electrodes; however, the impediments

resulting from different thicknesses of carbon electrodes suggest performing further

experiments with positive and negative electrodes made of different carbons.

2.2. Voltage extension by use of different carbon electrodes

Accordingly to equation (40), the ΔE+/ΔE- ratio can be also reduced by

increasing the ratio C+/C- between the specific capacitances of the positive and negative

electrodes. With this objective in mind, we have selected the YP 80F carbon (Kuraray

Chemicals Co, with SBET=2270 m2 g

-1 and L0=1.05 nm) for the positive electrode, with

YP50F (Kuraray Chemicals Co, with SBET=1522 m2 g

-1 and L0=0.86 nm) for the

negative one. The lower SSA of YP50F, essentially due to a lower content of mesopores

(see experimental annex A.1.1), should result in lower capacitance and different kinetics

in the pores of the negative electrode.



Successfully, the potential of the positive YP80F electrode in the (-) YP50F-

KOH / YP80F-Na2SO4 (+) cell is diminished to E+ = 0.844 vs NHE at a voltage of 1.6

V (Figure 85a), value which is close to of the theoretical one of 0.840 V calculated from

equation (42). Moreover, contrary to the (-) YP80F/YP80F (+) cell with unequal

electrode masses (m+/m-=2.25) (Figure 82), EOCP is not shifted to higher values when

the two different carbon electrodes are used (Figure 85a). Overall, as seen in Figure 10a,

the potential ranges of both electrodes for the (-) YP50F-KOH / YP80F-Na2SO4 (+) cell

perfectly fit within the thermodynamic stability limits represented by the dashed lines.

The near-rectangular shape of CV (0.4 mV s−1

) up to 1.5 V for the (-) YP50F-

KOH / YP80F-Na2SO4 (+) cell with different carbon electrodes in Figure 85b proves

good charge propagation. Contrarily, since the potential of the positive electrode in the

(-) YP80F-KOH / YP80F-Na2SO4 (+) capacitor exceeds the thermodynamic limit for

water oxidation in Na2SO4 (Figure 79), the CV of the cell with the same YP80F

electrodes is featured by a current leap related to carbon electrochemical oxidation.

Since, such phenomenon is not revealed by the (-) YP50F-KOH / YP80F-Na2SO4 (+)

cell, which exhibits nearly constant capacitive current during the CV scan, the possible

extension of operating voltage by an adapted selection of the electrode materials seems

to be proved.

Figure 85 (a) Electrodes potential extrema vs. voltage measured during galvanostatic

(40 mA g-1

) cycling of a (-) YP50F-KOH / YP80F-Na2SO4 (+) cell; (b) CVs (0.4 mV

s−1

) of (-) YP50F-KOH / YP80F-Na2SO4 (+) and (-) YP80F-KOH / YP80F-Na2SO4 (+)

cells.



As discussed above, the margin before reaching the di-hydrogen evolution

potential at the negative electrode can be utilized either by balancing the mass of the

electrodes or by using different optimized carbons for the positive and negative

electrodes. An adjustment of positive and negative electrodes mass ratio (m+/m-=2.25)

enables to extend the operating cell potential to 1.6 V; however, the higher resistance of

this system due to different thickness of the carbon electrodes suggests optimization of

the cell through different AC electrodes. An application of carbon with different pore

size distribution as electrode active material for positive and negative electrode enables

to extend the operating voltage, and at the same time, to keep good electrochemical

properties of the EC. Since the (-) YP50F/YP80F (+) configuration revealed promising

results in the voltage extension, further experiments, i.e., cycling or accelerated ageing

are obviously planned.

III.3. Conclusion

According to thermodynamic considerations, due to the pH difference between

the basic catholyte and the neutral anolyte, an (-) AC-KOH / AC-Na2SO4 (+) capacitor

can theoretically operate up to 1.62 V. The effect of Galvani potential difference for AC

electrodes in KOH and Na2SO4, as catholyte and anolyte, respectively, has been

validated for a system with equal electrodes masses, which demonstrates good cycle life

up to 1.5 V. The experiments on the (-) YP50F-KOH / YP80F-Na2SO4 (+) cell

confirmed that the operating voltage is essentially limited by the positive carbon

electrode.

The exploration of the effects of electrode materials porosity on voltage

expansion is found to be crucial for enhancing the voltage range in the new concept

KOH (-) / Na2SO4 (+) capacitor. Obviously, there is still plenty of room for future

experiments and for a subsequent design of the cation exchange membrane and current

collectors which appear as ways for developing a new electrochemical capacitor

generation. Notwithstanding, the performed experiments initiated a new direction for

further studies based on this new concept cell, taking into account the advantages of

both alkaline and neutral aqueous media (possibility to use stainless steel or nickel

collectors) to develop high voltage and cheap AC/AC electrochemical capacitors.



GENERAL CONCLUSION



Improving the energy density, while keeping high power density and long

cycle life, is the main objective in AC/AC electrochemical capacitor development. For

being industrially implemented, such devices should be also cheap and easily

manufactured, environmentally friendly and fulfill all the security requirements. To

design and develop ECs in neutral aqueous electrolyte with stainless steel current

collectors, three directions were particularly explored in this research work: i) the main

factors contributing to ageing of AC-based electrochemical capacitors in lithium sulfate

aqueous electrolyte with stainless steel collectors were disclosed by accelerated ageing

based on potentiostatic floating; ii) strategies were proposed and verified to improve the

long-term performance of the ECs at high voltage while implementing cheap

constituents; iii) a new concept cell, based on two aqueous electrolytic solutions of

different pH has been suggested and validated in order to extend the operating voltage

window. The results obtained by the various physico-chemical and electrochemical

investigations allow the following conclusions to be formulated.

Potentiostatic floating including two-hour floating periods is an accurate method

for accelerated ageing of electrochemical capacitors based on carbon electrodes in

aqueous electrolyte. Owing to the longer periods at high voltage, this test is more

effective for determining ECs operation stability limits than galvanostatic cycling.

The failures which mainly appear during operation of the ECs are an increase of

equivalent series resistance, capacitance loss and electrolyte decomposition. The post-

floating investigations reveal carbon oxidation and accumulation of corrosion products

on the positive electrode as subsequent factors causing ageing. The formed oxygenated

surface groups block the pores, limiting the access of ions to the electrode active

surface, and causing a drop of capacitance, whereas the accumulation of corrosion

products at the electrode/collector interface causes a resistance increase. Moreover, the

gases generated at the electrodes shorten the cell life due to electrolyte depletion and/or

loss of electrode cohesion.

The reduction of ECs lifetime due to collectors’ corrosion has been, at least in

part, prevented by: (i) using non-corrodible nickel collectors; (ii) coating the electrode

material on the metallic collector in order to avoid the accumulation of corrosion

products at the electrode-collector interface; (iii) adding a corrosion inhibitor to lithium

sulfate electrolyte. The deposition of oxidized nickel compounds on the collectors and



separator during ageing at 1.6 V is probably associated with the observed pH variations.

Nevertheless, the appearance of these residues does not affect the performance of the

cell which exhibits stable resistance values up to 120 hours at 1.6 V floating voltage.

The application of a conductive carbon ink (CCI) pre-coating on the metallic substrate

improves the adhesion of the carbon coating and prevents from the accumulation of

decomposition products at the electrode/substrate interface. The corrosion of stainless

steel collectors was diminished by adding sodium molybdate inhibitor, which

additionally enhances the cell capacitance.

The other strategy to improve the long-term operation of capacitors in aqueous

media is a downshift of electrodes operating potential to avoid oxidation phenomena at

the positive electrode (i) either by asymmetry of electrode materials; (ii) or by

combining two kinds of cheap collectors. As expected, the maximum potential of the

positive electrode could be reduced by coupling highly microporous Burley tobacco

carbon (B800) as positive electrode with the industrial DLCS30 one as negative

electrode. However, due to important surface functionality and correlated reactivity of

B800, the potential of the positive electrode shifted to higher values during floating,

with subsequent deterioration of electrochemical performance. Finally, the corrosion of

the positive stainless steel collector disappeared by combining (-) nickel and (+)

stainless steel collectors. The potential shift towards lower values results in negligible

electrolyte decomposition, electrode oxidation and/or formation of corrosion products

on the positive current collector, allowing the cell to reach up to 1.6 V.

Due to the pH difference between potassium hydroxide and sodium sulfate

separated by a proton exchange membrane, the new concept AC/AC capacitor is able to

operate up to 1.5 V with a good stability while using stainless steel collectors. By

asymmetry of electrodes, the operating voltage could be extended to 1.6 V.

As research perspectives on AC-based electrochemical capacitors in neutral

salt aqueous electrolytes, in-situ analysis of evolved gases during accelerated ageing, by

coupling gas chromatography (GC) with mass spectrometry (GC/MS), would be useful

to elucidate the real decomposition mechanisms and to suggest adapted strategies (for

example components enhancing recombination processes) to reduce internal pressure

increase and oxidation of electrodes. Analysis by, e.g., X-ray photoelectron

spectroscopy (XPS) of the nickel deposits formed after accelerated ageing on the



collectors and separator could provide information about their exact nature and allow

strategies to be proposed for reducing their creation during long-term performance. On

the basis of the aqueous lithium sulfate solution, another important objective would be

to work out an electrolyte formulation for covering a wide temperature range, i.e. from -

40°C to +60°C. The ultimate work on cells implementing aqueous lithium sulfate would

be to build pouch cells including all the optimized components previously identified and

to investigate their electrochemical properties in various environments.

The performance of the new concept cell with two kinds of electrolytes could be

improved by modifying the geometry and also by implementing more stable and highly

conductive CEM with a good selectivity, in order to enhance cycle life and reduce the

cell resistance. Applying gel electrolytes could be also a way to reduce some of the

difficulties inherent to the use of a cationic exchange membrane.



EXPERIMENTAL ANNEX



A.1. Cell construction

1.1. Materials and chemicals

Electrode materials:

(i) The commercial activated carbons:

DLC Super 30 (Norit) was used for manufacturing pellet electrodes (named in

the main text as S30)

YP-80F (Kuraray Chemicals Co) was used for manufacturing coated electrodes

(named in the main text as YP80F)

YP-80F and YP-50F (Kuraray Chemicals Co) were used as self-standing

electrodes for the new concept cell (named in the main text as YP80F and

YP50F, respectively)

(ii) The tobacco carbon:

Burley carbon (named as B800) was prepared from the leaves’ stems wastes of

tobacco industry carbonized in a tubular furnace under nitrogen flow rate of 100

mL min-1

and heated at 10 °C min-1

up to 800 °C for one hour. The detailed

process of sample preparation is given in the reference [78].

For the post-floating analysis of electrodes by thermoprogrammed desorption

(TPD), self-standing electrodes from activated carbon cloth (ACC 507-20,

Kynol) were selected to avoid the interference of the electrode binder.

Figure A1 Porous texture of carbons used in the study: (a) nitrogen

adsorption/desorption isotherms recorded at 350 °C; (b) Pore size distribution

determined using the 2D-NLDFT model.



Carbon

Surface area Vmicro

Vmeso

L0

< 2 [nm]

[m2

g-1

] [cm3

g-1

] [cm3

g-1

] [nm]

DLCS30 1843 0.715 0.183 0.92

YP-80F 2270 0.827 0.237 1.05

YP-50F 1522 0.628 0.059 0.86

B800 1651 0.664 0.190 0.86

ACC 507-20 2231 0.886 0.029 0.99

Table A1 Nitrogen adsorption data of the carbons used in the study.

Carbon material

Weight loss

950 °C CO2 CO H2O O

wt% µmol g-1

µmol g-1

µmol g-1

wt%

DLCS30 2.9 317 249 44 1.5

YP-80F 2.4 245 331 246 1.7

YP-50F 5.3 731 186 43 2.7

B800 10.9 440 598 416 3.0

ACC 507-20 7.2 1347 331 133 5.1

Table A2 TPD on the carbon samples used for the experiments: weight loss at 950 °C,

amount of desorbed CO2, CO and H2O, and oxygen content calculated from the

desorbed gases.



Electrolytes:

(i) The output electrolyte in the study was 1 mol L−1

lithium sulfate (Li2SO4,

Sigma-Aldrich, >99%) (pH = 6.5, conductivity = 64 mS cm-1

)

(ii) In order to reduce the corrosion of current collectors, in some experiments in

chapter V.1, 0.1 mol L−1

sodium molybdate (Sigma Aldrich, >99.5%) has

been added to the Li2SO4 solution, (pH = 6.7, conductivity = 72 mS cm-1

)

(iii) For the new concept cell presented in chapter V, 1 mol L-1

sodium sulfate

(Na2SO4, Sigma-Aldrich, >99%) (pH = 6.6, conductivity = 68 mS cm-1

) and

0.5 mol L-1

potassium hydroxide (KOH, POCh, min. 85%) (pH = 13.2,

conductivity = 56 mS cm-1

) were used as catholyte and anolyte, respectively.

1.2. Preparation of electrodes

Pellet electrodes:

Pellet electrodes were prepared by mixing activated carbon (85 wt. %) with 5 wt.

% polyvinylidene fluoride as binder (PVdF, Kynar HSV900, Arkema) and 5 wt. %

carbon black (C65, Timcal) conductivity enhancer. The three components were mixed

with acetone (Avantor, 99.5%), then rolled to get a film, and the pellets with a

thickness of around 0.3 mm and mass 8–10 mg and 1 cm diameter were pressed under

4.870 kg cm-2

. The prepared electrodes were dried under vacuum at 110°C for 12 hours.

Coated electrodes:

Unless otherwise noted, for realizing coated electrodes, the surface of grade

1.431 stainless steel (Interbelts, thickness = 15 μm) or nickel (Schlenk, thickness = 20

μm) foils was pre-coated with a thin layer (15 μm) of carbon conductive ink (CCI)

(Electrodag™ PF-407A™, Acheson) to provide a rougher surface of the substrate.

Activated carbon YP 80F (83.5 wt. %), carbon black (SUPER C65, Timcal, 8.5 wt. %)

conductivity enhancer and polyvinylidene difluoride (PVdF, Kynar® HSV 900,

Arkema, 8 wt. %) binder dissolved in 1-methyl-2-pyrrolidone (NMP, Sigma-Aldrich)

were mixed with an homogenizer (IKA ULTRA-TURRAX® T 18 basic), and the

obtained slurry was cast with a Doctor Blade applicator (Elcometer® 3600) on the

previously prepared surface of stainless steel or nickel foil. Afterwards, the coating is

dried overnight by slow evaporation in air, followed by heating under vacuum at 120°C



for 12 h. 10 mm diameter disk electrodes (mass of active material ~3.5 mg, coating

thickness ~100 μm) were punched out from the coating.

1.3. Cell configurations

Cells with pellet electrodes

Cells with electrodes in the form of pellets were realized by sandwiching a

porous glass microfiber membrane GF/A (Whatman™, thickness = 0.26 mm) between

two pellet electrodes (DLC Super 30, Norit) and two current collectors either from

stainless steel or nickel, using PTFE Swagelok-type vessels with or without inlet for a

reference electrode (Hg/Hg2SO4 in 0.5 mol L-1

H2SO4) (Figure A2). Before being

closed, the assembled system was soaked under vacuum with 1 mol L-1

Li2SO4

electrolytic solution. The current collectors (diameter 1.2 cm) were made from a low

carbon content stainless steel 316L alloy consisting of the following major elements: Fe,

C (0.02%), Cr (16%), Ni (10%) and Mo (2%) or commercially pure nickel 200/201.

Their surface was cleaned with emery paper (P1000) before the investigations.

Figure A2 Schematic representation of the capacitors in PTFE Swagelok-type assembly

with reference electrode.



Cells with coated electrodes

Electrochemical capacitors with electrodes coated on stainless steel or nickel

were realized in PTFE Swagelok-type vessel with an additional inlet for Hg/Hg2SO4

reference electrode for measurements of CVs of individual electrodes, by sandwiching

an absorptive glass mat separator (AGM, Bernard Dumas, thickness = 0.52 mm)

between two coated electrodes (Kuraray YP-80F) and cylindrical current collectors,

either from stainless steel 316L or nickel 200/201 (in analogy to the cells with pellet

electrodes). Before being closed, the assembled system was soaked under vacuum with

1 mol L-1

Li2SO4 electrolytic solution.

All the data with reference electrode presented in the manuscript were calculated

vs normal hydrogen electrode (NHE).

A.2. Electrochemical characterizations

The electrochemical properties of the capacitors and the new concept cell were

investigated by cyclic voltammetry (CV), galvanostatic cycling with potential limitation

(GCPL) and impedance spectroscopy (EIS) at open circuit voltage (OCV) in the

frequency range 1 mHz to 100 kHz and amplitude of 5 mV, using a VMP3 multichannel

potentiostat/galvanostat (Bio-Logic Instruments, France). Data were collected using EC-

Lab V10.34 software. Capacitance was calculated from the galvanostatic discharge and

expressed per average active mass of electrodes [F g-1

] according to formula (43):

C = 2 I / [(dV/dt) m] (43)

where I is the current [A], dV/dt is the slope of the discharge curve [V s-1

], m is the

average mass of carbon active material [g] .

A.3. Physico-chemical and surface characterization

Temperature-programmed desorption (TPD) analysis

The surface oxygenated functionality of fresh and aged carbon electrodes

(Kynol, ACC 507-20) was characterized by temperature-programmed desorption (TPD),

using TG equipment (TG209 F1 Iris, NETZSCH) coupled with a mass spectrometer

(QMS 403C Aëolos, NETZSCH). To investigate the evolution of surface chemistry



after accelerated ageing, ACC electrodes were taken out from ECs after 120 h of

floating, washed carefully with distilled water to remove the electrolyte and dried under

vacuum at 120 °C. During surface functionality determination by TPD, c.a. 10 mg of

ACC was heated up to 950 °C (heating rate 20 °C min-1

) under helium flow rate of 50

mL min-1

. The surface functional groups evolving as CO2 and CO were quantified after

calcium oxalate monohydrate calibration, taking into account CO disproportionation

[216]. To determine the types of oxygenated complexes formed on the surface of the

aged positive electrode, the deconvolution of CO2 and CO patterns have been made with

a multiple Gaussian function using the Origin 9.0 software.

Porous texture characterisation

In courtesy of Mgr. Piotr Skowron help from our research group, the porous

texture of carbons was determined from nitrogen adsorption/desorption isotherms

recorded at -196 °C using an ASAP2020 (Micrometrics). Prior to the measurements, the

fresh and aged electrodes (around 60 mg) were degassed under vacuum for 36 h at

100°C. The pore size distribution (PSD) was determined using the 2D non-local density

functional theory (2D-NLDFT) [107], the micro Vmicro and mesopore volumes Vmeso

were obtained directly from the calculated cumulative PSDs. The average micropore

size (L0) was determined from the integration of the PSD area for the pores below 2 nm.

Scanning electron microscopy (SEM)

In courtesy of Dr. Eng. Tomasz Rozmanowski, scanning electron microscopy

(SEM) images of nickel 200/201, with and without carbon conductive ink (CCI), and

soft-annealed nickel foils were analysed in a high vacuum mode with the use of Hitachi

Model S-3400N Scanning Electron Microscope with secondary electron (SE) detector.

Magnifications of x50 to x5.00k were obtained with a voltage of 15.0 kV.



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SCIENTIFIC ACHIEVEMENTS



1. Chapters in scientific books

1. E. Frąckowiak, P. Ratajczak, F. Béguin; Ed.: P.N. Bartlett, R.C. Alkire, J.

Lipkowski, „Electrochemistry of Carbon Electrodes: Electrochemical Capacitors

Based on Carbon Electrodes in Aqueous Electrolytes”, Wiley-VCH, Weinheim,

2015

2. Publications

2.1. Publications in international journals from the Philadelphia list

1. Q. Abbas, P. Ratajczak, P. Babuchowska, A. Le Comte, D. Bélanger, T.

Brousse, F. Béguin, Strategies to Improve the Performance of Carbon/Carbon

Capacitors in Salt Aqueous Electrolytes, J. Electrochem. Soc. 162 (2015)

A5148-A5157

2. P. Kleszyk, P. Ratajczak, P. Skowron, J. Jagiełło, Q. Abbas, E. Frąckowiak, F.

Béguin, Carbons with narrow pore size distribution prepared by simultaneous

carbonization and self-activation of tobacco stems and their application to

supercapacitors, Carbon, 81 (2015) 148–157

3. Q. Abbas, P. Ratajczak, F. Béguin, Sodium Molybdate - An additive of choice

for enhancing the performance of AC/AC electrochemical capacitors in salt

aqueous electrolyte, Faraday Discuss., 172 (2014) 199-214

4. P. Ratajczak, K. Jurewicz, P. Skowron, Q. Abbas, F. Béguin, Effect of

accelerated ageing on the performance of high voltage carbon/carbon

electrochemical capacitors in salt aqueous electrolyte, Electrochim. Acta, 130

(2014) 344–350

5. P. Ratajczak, K. Jurewicz, F. Béguin, Factors contributing to ageing of high

voltage carbon/carbon supercapacitors in salt aqueous electrolyte, J. Appl.

Electrochem., 44 (2014) 475.

3. Conferences

3.1. Oral presentations

1. F. Béguin, Q. Abbas, P. Babuchowska, P. Ratajczak, Development of a

high energy AC/AC capacitor in aqueous electrolyte, 16th Topical Meetingof

the International Society of Electrochemistry, Brazil, Angra dos Reis 2015



2. F. Béguin, Q. Abbas, P. Jezowski, P. Ratajczak, Development of a high-voltage

capacitor prototype in environment friendly salt aqueous electrolyte, Advanced

Automotive Battery Conference AABC 2014, USA, Atlanta, 2014

3. P. Ratajczak, P. Jeżowski, F. Béguin , Performance improvement of

AC/AC capacitors in aqueous medium through modification of the current

collector/active material interface, 65th Annual Meeting of the International

Society of Electrochemistry, Switzerland, Lausanne 2014

4. P. Ratajczak, P. Jeżowski, P. Skowron, K. Jurewicz, F. Béguin, Design and

development of AC/AC supercapacitors in salt aqueous electrolyte, Winter

seminar „Latest Developments in Electrochemical Capacitors“ , Estonia, Tartu

2013

5. P.M. Kleszyk, P. Ratajczak, P. Skowron, F. Béguin, Samo-aktywacja biomasy:

nowa metodologia wytwarzania węgli aktywowanych z kontrolą rozkładu

wielkości porów, Węgiel aktywny w ochronie środowiska i przemyśle, Poland,

Białowieża 2013

6. P.M. Kleszyk, Q. Abbas, P. Ratajczak, P. Skowron, F. Béguin, Manufacturing

of nanoporous carbons by self-activation of tobacco and their application for

energy storage in supercapacitors, 5th International Conference on Carbon for

Energy Storage/Conversion, Germany, Mülheim a.d. Ruhr 2013

7. F. Béguin, Q. Abbas, P. Ratajczak, E. Frackowiak, A new generation of high

voltage and environment friendly supercapacitor using salt-based aqueous

electrolytes, The 64th Annual Meeting of the ISE, Mexico, Santiago de

Querétaro 2013

8. F. Béguin, P.M. Kleszyk, P. Ratajczak, P. Skowron, Novel nanoporous carbons

prepared by self-activation of biomass and their properties in supercapacitors,

Annual World Conference on Carbon - Carbon 2013, Brazil, Rio de Janeiro

2013

9. P. Ratajczak, K. Jurewicz, F. Béguin, Performance limits of high voltage

aqueous AC/AC supercapacitors under accelerated ageing, 3rd International

Symposium on Enhanced Electrochemical Capacitors (ISEECap2013), Italy,

Taormina 2013

10. P. Ratajczak, K. Jurewicz, F. Béguin, Monitoring the state of health (SOH) of

high voltage aqueous AC/AC supercapacitors during the life of the system,



International Conference on Advanced Capacitors (ICAC2013), Japan, Osaka

2013

11. F. Béguin, Q. Abbas, P. Kleszyk, P. Ratajczak, Towards the prototyping of

high voltage AC/AC capacitors in neutral aqueous electrolyte, International

Conference on Advanced Capacitors (ICAC2013), Japan, Osaka 2013

12. Q. Abbas, P.M. Kleszyk, P. Ratajczak, F. Béguin, Performance of

Electrochemical Capacitors with Microporous Carbon Electrodes in New Types

of Aqueous Electrolytes, 13th Topical Meeting of the International Society of

Electrochemistry - Advances in Electrochemical Materials Science and

Manufacturing, South Africa, Pretoria 2013

13. P.M. Kleszyk, Q. Abbas, P. Ratajczak, P. Skowron, F. Béguin, Novel

Nanoporous Carbons Based on Tobacco and Their Electrochemical Properties in

Supercapacitors, 13th Topical Meeting of the International Society of

Electrochemistry - Advances in Electrochemical Materials Science and

Manufacturing, South Africa, Pretoria 2013

14. P.M. Kleszyk, Q. Abbas, P. Ratajczak, P. Skowron, F. Béguin, Novel

nanoporous carbons based on tobacco and their properties in supercapacitors,

VII International Scientific and Technical Conference – Carbon Materials &

Polymer Composites, Poland, Ustroń – Jaszowiec 2012

15. P. Skowron, P. Ratajczak, M. Anouti, E. Frąckowiak and F. Béguin,

Supercapacitor application of activated carbons modified by electrografting with

pyridine-4-diazonium chloride, VII International Scientific and Technical

Conference – Carbon Materials & Polymer Composites, Poland, Ustroń –

Jaszowiec 2012

16. F. Béguin, P. Ratajczak, P. Kleszyk, P. Jeżowski, Q. Abbas, P. Skowron, K. Fic

and E. Frąckowiak, Strategies for enhancing the performance of carbon-based

supercapacitors, VII International Scientific and Technical Conference – Carbon

Materials & Polymer Composites, Poland, Ustroń – Jaszowiec 2012

17. F. Béguin, K. Fic, P. Ratajczak, K. Jurewicz, Q. Abbas, G. Lota, G. Gao, L.

Demarconnay, E. Raymundo, E. Frackowiak, Performance limits of 2 V C/C

supercapacitors in alkali sulfate aqueous media, 222nd Meeting of ECS — The

Electrochemical Society (PRiME 2012), USA, Honolulu, Hawaii 2012

18. P. Ratajczak, P. Jezowski, K. Jurewicz, G. Lota, E. Frackowiak and F. Béguin,

Influence of supercapacitors operating conditions on their performance in



aqueous electrolyte, COST Action MP1004 “Hybrid Energy Storage Devices

and Systems for Mobile and Stationary Applications”, Turkey, Kayseri 2012

3.2. Poster presentations

1. F. Béguin, Q. Abbas, A. Laheäär, P. Ratajczak, B. Górska, P. Skowron, P.

Jeżowski, P. Przygocki, P. Babuchowska, Development of high performance and

ecologically friendly supercapacitors for energy management – ECOLCAP

project, Interdisciplinary FNP conference Warsaw, Poland 2015

2. P. Ratajczak, K. Jurewicz, F. Béguin, Overcoming the electrochemical stability

limits of aqueous electrolyte capacitors, 65th Annual Meeting of the

International Society of Electrochemistry Lausanne, Switzerland 2014

3. P. Ratajczak, A. Ślesiński, K. Jurewicz, P. Skowron, E. Frąckowiak, F. Béguin,

Gas evolution and accompanying reactions - main factors contributing to

deterioration of electrochemical capacitors in salt aqueous electrolyte, 65th

Annual Meeting of the International Society of Electrochemistry Lausanne,

Switzerland 2014



International Society of Electrochemistry Lausanne, Switzerland 2014

5. L. Garcia-Cruz, P. Ratajczak, J. Iniesta, V. Montie, F. Béguin, Self-Discharge

of Carbon/Carbon Supercapacitors in Salt Aqueous Electrolyte, The World

Conference on Carbon, Jeju, Korea 2014

6. P. Ratajczak, P.M. Kleszyk, K. Jurewicz, F. Béguin, Effect of ageing

supercapacitors operating in aqueous medium on the surface chemistry of

activated carbon, 5th International Conference on Carbon for Energy

Storage/Conversion, Germany, Mülheim a.d. Ruhr 2013

7. P. Skowron, P. Ratajczak, K. Fic, M. Anouti, E. Frąckowiak, F. Béguin, Effect

of diphenols addition to protic ionic liquid electrolytes on the performance of

supercapacitors, 5th International Conference on Carbon for Energy


8. Q. Abbas, P. M. Kleszyk, P. Ratajczak, F. Béguin, Effect of pH on the

performance of activated carbons based symmetric capacitors in aqueous



electrolytes, 3rd International Symposium on Enhanced Electrochemical

Capacitors (ISEECap2013), Italy, Taormina 2013

9. K. Torchała, K. Kierzek, P. Ratajczak, F. Béguin, J. Machnikowski , Effect of

surface functionalization on the performance of activated carbon as positive and

negative electrode in asymmetric aqueous capacitor, 3rd International


Taormina 2013

10. P. M. Kleszyk, Q. Abbas, P. Skowron, P. Ratajczak F, Béguin, Self-activated

carbons based on biomass for application in supercapacitors, 3rd International


Taormina 2013

11. P. Ratajczak, P. M. Kleszyk, K. Jurewicz, F. Béguin, New insights on stability

of high voltage supercapacitors utilizing tobacco-based carbons, 3rd

International Symposium on Enhanced Electrochemical Capacitors

(ISEECap2013), Italy, Taormina 2013

12. P. Ratajczak, P. M. Kleszyk, K. Jurewicz, F. Béguin, Effect of carbons on the

performance of aqueous electrochemical capacitors under accelerated ageing,

International Conference on Advanced Capacitors (ICAC2013), Japan, Osaka

2013

4. Awards

4.1. Best poster awards



International Society of Electrochemistry, Lausanne, Switzerland 2014

2. P. Ratajczak, P.M. Kleszyk, K. Jurewicz, F. Béguin, Effect of ageing

supercapacitors operating in aqueous medium on the surface chemistry of

activated carbon, 5th International Conference on Carbon for Energy


3. P. Ratajczak, P. M. Kleszyk, K. Jurewicz, F. Béguin, New insights on stability

of high voltage supercapacitors utilizing tobacco-based carbons, 3rd

International Symposium on Enhanced Electrochemical Capacitors

(ISEECap2013), Italy, Taormina 2013



5. Participation in research projects

5.1. As a stipendee (PhD thesis):

ECOLCAP project funded in the frame of the Welcome Programme

implemented by the Foundation for Polish Science (FNP) within the Measure

1.2. ‘Strengthening the human resources potential of science’, of the Innovative

Economy Operational Programme supported by European Union.

Project leader: Prof. François Béguin

5.2. As a coordinator of the researches:

Statutory grant no 03/31/DSMK/0287

Statutory grant no 03/31/DSMK/0305

This thesis’ research was partially supported by statutory grants

5.3. Employed as a scientific assistant:

LIDER project financed by National Centre for Research and Development

(NCBiR): „Kondensator elektrochemiczny o wysokiej gęstości energii i mocy

operujący Procesy pseudopojemnościowe na granicy faz elektroda/elektrolit w

elektrochemicznych systemach magazynowania energii w roztworach

sprzężonych par redoks”.

Project leader: Dr. Eng. Krzysztof Fic

5.4. As a stipendee (Master thesis):

ECOLCAP project funded in the frame of the Welcome Programme

implemented by the Foundation for Polish Science (FNP) within the Measure

1.2. ‘Strengthening the human resources potential of science’, of the Innovative

Economy Operational Programme supported by European Union.

Project leader: Prof. François Béguin



ABSTRACT



Taking into account the numerous advantages of water-based media over

organic solutions, the ultimate aim of this doctoral dissertation is to design and develop

a carbon-based environmentally friendly and low-cost electrochemical capacitor (EC)

operating in an aqueous electrolyte and using non-noble collectors. To pursue this

objective, an accelerated ageing test has been adapted, factors contributing to failures

during operation have been determined, and finally, a number of solutions allowing the

cells performance to be optimized have been proposed. Overall, after a general

introduction, the dissertation is divided into five chapters and ends by a general

conclusion.

The first chapter presents the state-of-the-art of electrochemical capacitors

(ECs). At first, the operating principle and general properties of electrical double-layer

capacitors (EDLCs) are briefly described. Then, the common electrode materials (in

particular porous carbons) and electrolytes generally employed for ECs are introduced,

with their advantages and disadvantages. A special emphasis is placed on neutral

aqueous electrolytes exhibiting a high over-potential for di-hydrogen evolution, and

thereof allowing high operating voltages to be obtained.

Chapter II presents the experimental techniques and procedures used in the

development of the dissertation. The principles of galvanostatic cycling with potential

limitation (GCPL), cyclic voltammetry (CV) and electrochemical impedance

spectroscopy (EIS), together with the parameters determined from these methods, are

introduced. Taking into account the limited time allowed for the preparation of this

dissertation, the advantages of an accelerated ageing protocol, by so-called

potentiostatic floating, are presented to evaluate the end of life of ECs in a reduced

research time.

The adaptation of the floating protocol and the determination of maximum

operating voltage for a carbon-based capacitor in aqueous lithium sulfate electrolyte

with stainless steel collectors are presented in chapter III. To evaluate the state-of-health

(SOH) of this system, capacitance, resistance and internal pressure of the cell are

monitored during the test. The possible factors contributing to ageing of the ECs in

aqueous solution with stainless steel current collectors are identified. The alterations in

physicochemical properties of the cell constituents after long time operation, such as

modifications of surface functionality and porosity of the AC electrodes, together with

corrosion of the stainless steel current collectors, are revealed.



In chapter IV, strategies are investigated to improve the cycle-life of ECs in

aqueous lithium sulfate electrolyte, essentially for reducing the corrosion of stainless

steel collectors and decreasing its detrimental effect on cells operation. The suggested

solutions include the replacement of stainless steel by nickel collectors, the protection of

the active material/current collector interface and the addition of sodium molybdate

corrosion inhibitor to lithium sulfate electrolyte. Another tactics involves the application

of an asymmetric configuration of electrodes, i.e., different current collectors and

different mass or kind of carbon for the two electrodes, in order to shift the electrodes

operating potentials toward lower values.

Chapter V is directed to new perspectives for the research on ECs in aqueous

electrolytes. A new concept cell is proposed by implementing an anolyte and a catholyte

of different pH, both separated by a cationic exchange membrane. The application of

potassium hydroxide as catholyte and sodium sulfate as anolyte should result in a higher

voltage of the cell than the thermodynamic limit of 1.23 V for water decomposition.

Practically, the cell is able to operate up to 1.5 V with an excellent cycle life. Although

the proposed new concept cell still requires some optimizations, it opens new insights

for the R&D on ECs in aqueous electrolytes.



STRESZCZENIE



Z uwagi na liczne zalety elektrolitów wodnych nad roztworami organicznymi,

bezpośrednim celem prezentowanej pracy doktorskiej było opracowanie przyjaznych

dla środowiska oraz tanich kondensatorów elektrochemicznych, działających w

elektrolicie wodnym oraz przy użyciu kolektorów prądowych wytworzonych z metali

nieszlachetnych. Pierwszym krokiem do zrealizowania powyższego założenia było

dostosowanie do badanych układów testu przyspieszonego starzenia. Następnie

określono czynniki, które przyczyniają się do pogorszenia pracy kondensatorów w

elektrolitach wodnych. Ponadto przeprowadzono badania, weryfikujące zaproponowane

rozwiązania, mające na celu zoptymalizowanie tych układów. Po ogólnym

wprowadzeniu rozprawa podzielona jest na pięć rozdziałów i kończy się ogólnymi

wnioskami.

Pierwszy rozdział przedstawia przegląd literatury nt. kondensatorów

elektrochemicznych. Na początku krótko opisano zasadę działania oraz ogólne

właściwości kondensatorów podwójnej warstwy elektrycznej. Następnie przedstawiono

materiały elektrodowe (w szczególności porowate elektrody węglowe) oraz elektrolity

zwykle stosowane w kondensatorach elektrochemicznych, wraz z ich zaletami i

wadami. Szczególny nacisk został położony na neutralne elektrolity wodne, wykazujące

znaczący nadpotencjał wydzielania wodoru, który pozwala na uzyskanie wysokiego

napięcia pracy układu.

Rozdział II przedstawia techniki i procedury eksperymentalne użyte w

badaniach do przedłożonej pracy doktorskiej. W pierwszej kolejności zaprezentowano

techniki, które są wykorzystywane do rozpatrywania cykliczności kondensatorów

elektrochemicznych poprzez galwanostatyczne ładowanie/wyładowanie,

woltamperometrię cykliczną oraz spektroskopię impedancyjną. Ponadto, biorąc pod

uwagę ograniczoną ilość czasu, pozwalającego na przygotowanie tej rozprawy,

skupiono się również na teście przyspieszonego starzenia przez tzw. floating, dla oceny

‘końca życia’ kondensatora przy skróconym czasie badań.

W rozdziale III przedstawiono adaptację protokołu przyspieszonego starzenia

oraz określono maksymalne napięcie pracy kondensatora na bazie węgla, działającego

w wodnym roztworze siarczanu litu oraz z kolektorami ze stali nierdzewnej.

Monitorowanie takich parametrów jak: pojemność, opór oraz ciśnienie wewnętrzne,

zostały uznane za niezbędne do oceny stanu analizowanych układów. Przeprowadzone

badania pozwoliły na zidentyfikowanie przyczyn spadku żywotności kondensatora,



pracującego w roztworze wodnym z zastosowaniem kolektorów prądowych ze stali

nierdzewnej. Ponadto, po długim czasie pracy zanalizowano zmiany we właściwościach

fizyko-chemicznych poszczególnych elementów ogniwa, takich jak: redukcja

powierzchni właściwej i porowatości elektrod węglowych oraz korozja stalowych

kolektorów prądowych.

W celu poprawy długoterminowej pracy kondensatora elektrochemicznego w

wodnym roztworze siarczanu litu, w rozdziale IV przedstawiono strategie, skupiające

się głównie na zmniejszeniu korozji kolektorów prądowych ze stali nierdzewnej i

zredukowaniu szkodliwego wpływu tych depozytów na działanie ogniwa. Proponowane

rozwiązania obejmują zastąpienie stali nierdzewnej przez kolektory niklowe, ochronę

granicy faz elektroda/kolektor oraz dodanie inhibitora korozji (molibdenianu sodu) do

elektrolitycznego roztworu siarczanu litu. W celu przesunięcia potencjału elektrody

dodatniej w kierunku niższych operacyjnych wartości, asymetryczne konfiguracje

(poprzez sparowanie dwóch różnych kolektorów prądowych lub użycie różnych

elektrod węglowych dla dodatniej i ujemnej polaryzacji) zostały wykorzystane.

Rozdział V zorientowany jest na perspektywiczne badania nad kondensatorami

w elektrolitach wodnych. Nowa koncepcja ogniwa elektrochemicznego polega na

zastosowaniu anolitu i katolitu o różnym pH, oddzielonych od siebie przez membranę

kationowymienną. Użycie wodorotlenku potasu i siarczan sodu (odpowiednio, jako

katolitu i anolitu), powinno skutkować wyższym napięciem pracy układu od

termodynamicznego limitu rozkładu wody (1,23 V). W praktyce zbudowany

kondensator jest w stanie działać do napięcia 1,5 V z satysfakcjonującą cyklicznością.

Mimo że, proponowana koncepcja ogniwa wciąż wymaga pewnych optymalizacji,

otwiera ona nowe perspektywy dla badań i rozwoju nad kondensatorami w elektrolitach

wodnych.

design of high voltage ac/ac electrochemical capacitors in...

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