metal oxide materials and collector efficiency in...

Defence R&D Canada – Atlantic

DEFENCE DÉFENSE&

Metal Oxide Materials and Collector

Efficiency in Electrochemical Supercapacitors

Second Annual Report

Gwenaël Chamouland, Tarik Bordjiba and Daniel BélangerUniversité du Québec à Montréal

Université du Québec à MontréalDépartement de ChimieCase postale 8888, Succ. Centre-villeMontréal (Québec) H3C 3P8

Project Manager: Daniel Bélanger, 514-987-3000 ext. 3909

Contract Number: W7707-063348/001/HAL

Contract Scientific Authority: Colin G. Cameron, 902-427-1367

The scientific or technical validity of this Contract Report is entirely the responsibility of the contractor and thecontents do not necessarily have the approval or endorsement of Defence R&D Canada.

Contract Report

DRDC Atlantic CR 2008-215

April 2009

Copy No. _____

Defence Research andDevelopment Canada

Recherche et développementpour la défense Canada

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Metal Oxide Materials and Collector Efficiencyin Electrochemical SupercapacitorsSecond Annual Report

Gwenael ChamoulaudUQAM

Tarik BordjibaUQAM

Daniel BelangerUQAM

Prepared by:

Universite du Quebec a MontrealDepartement de ChimieCase postale 8888, Succ. Centre-villeMontreal (Quebec) H3C 3P8

Project Manager: Daniel Belanger 987-3000 ext. 3909Contract Number: W7707-063348Contract Scientific Authority: Colin G. Cameron 902-427-1367

The scientific or technical validity of this Contract Report is entirely the responsibility of the contractorand the contents do not necessarily have the approval or endorsement of Defence R&D Canada.

Defence R&D Canada – AtlanticContract ReportDRDC Atlantic CR 2008-215April 2009

Approved by

Leon ChengHead/Dockyard Lab (A)

Approved for release by

Calvin HyattChair/Document Review Panel

c© Her Majesty the Queen in Right of Canada as represented by the Minister of NationalDefence, 2009

c© Sa Majeste la Reine (en droit du Canada), telle que representee par le ministre de laDefense nationale, 2009

Original signed by Jeffrey Szabo for

Original signed by Ron Kuwahara for

Abstract

This report deals with the development of electrochemical supercapacitor based on MnO2and binary manganese and ruthenium oxides with the use of various current collectors. Thebinary oxides were prepared and characterized by physicochemical and electrochemicaltechniques. The effect of the heat-treatment on the capacitance of the binary oxides wasinvestigated as well as the effect of the supporting electrolyte, the current collector andthe composition of the composite electrode. Finally, carbon nanotubes coated on a carbonpaper was used as support for the spontaneous formation of MnO2.

Resume

Ce rapport traite du developpement de supercapacites electrochimiques basees sur desoxydes de manganese et des oxydes binaires RuO2/MnO2 et utilisant divers collecteurs decourant. Les oxydes binaires ont ete prepares et caracterises par diverses methodes physico-chimiques et electrochimiques. L’effet du traitement thermique sur la capacite des oxydesbinaires a ete etudie ainsi que l’effet de la nature de l’electrolyte support, du collecteur decourant et de la composition de l’electrode composite. Finalement, un collecteur de courantobtenu par deposition de nanotubes de carbone sur un papier carbone a ete utilise commesubstrat pour la deposition spontanee de MnO2.

DRDC Atlantic CR 2008-215 i


ii DRDC Atlantic CR 2008-215

Executive summary

Metal Oxide Materials and Collector Efficiency inElectrochemical Supercapacitors: Second AnnualReport

Gwenael Chamoulaud, Tarik Bordjiba, Daniel Belanger; DRDC AtlanticCR 2008-215; Defence R&D Canada – Atlantic; April 2009.

Background: The objectives of this project are to develop supercapacitor electrode ma-terials based on high surface area metal oxide materials and to optimize the interface be-tween electrode materials and the current collector in order to improve equivalent seriesresistance. Single oxide (MnO2) and binary oxides (RuO2/MnO2) were prepared by co-precipitation. RuO2 is characterized by excellent capacitive and charge storage propertiesand its use in conjunction with a low cost oxide such as MnO2 could lead to the develop-ment of materials with a better performance-to-cost ratio. The spontaneous deposition ofMnO2 on a current collector was also investigated. Titanium and stainless steel substrateswere used as current collectors as well as carbon paper coated with carbon nanotubes.

Principal Results: Binary RuO2/MnO2 oxides were prepared by co-precipitation fromcorresponding metal salts. A Ru:Mn atomic ratio of 1:3 was used. The specific capacitanceof composite electrodes made with the binary oxides was compared to that obtained bysimple mechanical mixing of RuO2 and MnO2 powders. It was found that similar specificcapacitance values were obtained and no synergic effect was observed. It was also foundthat RuO2/MnO2 composite electrode are only stable in neutral electrolyte and that eitheracidic or alkaline media have detrimental effect on the stability of the oxides. MnO2 alonewas deposited directly on a carbon nanotubes coated carbon paper (CP/CNT) electrodeby the spontaneous reduction of KMnO4. This method was developed to avoid the useof a binder for the preparation of the electrode and the CP/CNT was used as the currentcollector directly.

Significance: The finding that CP/CNT can be used as current collector for MnO2 is use-ful. The preparation of the electrode relies on a simple method that could be easily scaledup.

Future Work: Future work will focus on the spontaneous deposition of MnO2 and binaryoxides on CP/CNT electrode and the characterization of the resulting electrode.

DRDC Atlantic CR 2008-215 iii

Sommaire

Metal Oxide Materials and Collector Efficiency inElectrochemical Supercapacitors: Second AnnualReport

Gwenael Chamoulaud, Tarik Bordjiba, Daniel Belanger ; DRDC AtlanticCR 2008-215 ; R & D pour la defense Canada – Atlantique ; avril 2009.

Contexte : L’objectif de ce projet est le developpement de nouvelles supercapacites elec-trochimiques basees sur de nouveaux materiaux tout en ameliorant la qualite de l’inter-face entre le materiau d’electrode et le collecteur de courant. Des oxydes simples (MnO2)et mixtes (RuO2/MnO2) ont ete preparees par coprecipitation. Le RuO2 est deja connupour posseder de tres bonnes proprietes capacitives et l’utilisation simultanee de cet oxydeet d’un oxyde peu couteux tel le MnO2 pourrait conduire a des materiaux presentant unmeilleur rapport performance/cout. La deposition spontanee du MnO2 directement sur uncollecteur de courant a egalement ete etudiee dans ce travail. Finalement, des grillages detitane et d’acier inoxydables ont ete utilises comme collecteur de courant ainsi qu’un papierde carbone recouvert de nanotubes de carbone.

Resultats principaux : Les oxydes binaires de type RuO2/MnO2 ont ete obtenus par co-precipitation en utilisant des sels metalliques comme precurseurs. Un rapport atomiqueRu/Mn de 1/3 a ete utilise. La capacite specifique des electrodes composites contenantces oxydes binaires a ete comparee a celles obtenues pour des electrodes fabriquees parmelange de poudres des deux oxydes. Malheureusement, aucun effet de synergie a eteobserve et les valeurs de capacites specifiques etaient tres semblables. L’etude portantsur les electrolytes a demontre que les meilleures performances et stabilite des oxydessont observees en milieu neutre. La deposition spontanee du MnO2 directement sur uneelectrode de papier de carbone recouverte de nanotubes de carbone (PC/NTC) a egalementete realisee a partir d’une solution aqueuse de KMnO4. Cette methode presente l’avantagede permettre la preparation d’une electrode en une seule etape et sans l’ajout d’un liant.Ainsi l’electrode de PC/NTC agit comme collecteur de courant.

Portee : L’utilisation de PC/NTC presente un avantage indeniable par rapport a une elec-trode composite en vertu de sa simplicite de fabrication, permettant une extrapolation fa-cile.

Recherches futures : Les travaux futurs porteront sur la deposition spontanee du MnO2et des oxydes mixtes sur le PC/NTC. Les electrodes resultantes seront caracterisees pardiverses methodes notamment par electrochimie pour determiner la capacite specifique.

iv DRDC Atlantic CR 2008-215

Table of contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Resume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Executive summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Sommaire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

List of figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

List of tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

1 Q1: 04/2007 – 06/2007 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Electrode Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.1 RuO2 preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2.2 MnO2 preparation . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2.3 RuO2/MnO2 preparation . . . . . . . . . . . . . . . . . . . . . . 2

1.2.4 Composite electrode preparation . . . . . . . . . . . . . . . . . 2

1.3 Electrochemical Characterization . . . . . . . . . . . . . . . . . . . . . . 3

1.3.1 Electrochemical characterization in H2SO4 . . . . . . . . . . . . 3

1.3.2 Electrochemical characterization in K2SO4 . . . . . . . . . . . . 3

1.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Q2: 07/2007 – 09/2007 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7


2.2.1 RuO2 preparation . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 MnO2 preparation . . . . . . . . . . . . . . . . . . . . . . . . . 8

DRDC Atlantic CR 2008-215 v

2.2.3 RuO2/MnO2 preparation . . . . . . . . . . . . . . . . . . . . . . 8

2.2.4 Composite electrode preparation . . . . . . . . . . . . . . . . . 8

2.3 Electrochemical Characterization . . . . . . . . . . . . . . . . . . . . . . 8

2.3.1 Influence of the composite electrode composition on thepotential window . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.3.2 The effect of composite electrode composition on specificcapacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3.3 The effect of composition on electrode thickness . . . . . . . . . 14

2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3 Q3: 10/2007 – 12/2007 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16


3.2.1 Preparation of oxides . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2.2 Composite electrode preparation . . . . . . . . . . . . . . . . . . 17

3.3 Electrochemical characterization . . . . . . . . . . . . . . . . . . . . . . 17

3.3.1 Effect of the electrode composition . . . . . . . . . . . . . . . . 18

3.3.2 The effects of heat treatment . . . . . . . . . . . . . . . . . . . . 23

3.3.3 The effects of the electrolyte . . . . . . . . . . . . . . . . . . . . 28

3.3.4 The effects of the current collector . . . . . . . . . . . . . . . . . 28

3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4 Q4: 01/2008 – 04/2008 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . 33

vi DRDC Atlantic CR 2008-215

4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Symbols and Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Distribution list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

DRDC Atlantic CR 2008-215 vii

List of figures

Figure 1: Cyclic voltammograms of acetylene black, RuO2, MnO2, mixedRuO2+MnO2 and co-precipitated RuO2/MnO2 composite electrodes.Cyclic voltammograms are recorded at 10 mV/s in 0.65 M K2SO4(aq). . . 4

Figure 2: Cyclic voltammograms of acetylene black, RuO2/MnO2 synthesized byco-precipitation by fast mixing in NaOH (method 1), dropwise mixingin NaOH (method 2), and dropwise mixing in HCl (method 3). Cyclicvoltammograms are recorded at 10 mV/s in 0.65 M K2SO4(aq). . . . . . 6

Figure 3: Example of the variation in specific capacitance, calculated from cyclicvoltammetry recorded at different scan rates in 0.65 M K2SO4 aqueoussolution, for RuO2+MnO2 composite electrodes containing 70% and80% active material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 4: Cyclic voltammograms of (a) RuO2, (b) MnO2, (c) RuO2+MnO2(oxide mixture), and (d) RuO2/MnO2 (binary oxides synthesized byco-precipitation), for different electrode compositions (60, 70, 80 and90% w/w of active material), recorded at 5 mV/s in 0.65 M aqueousK2SO4 solution. Currents are reported normalized to the total mass ofthe composite electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 5: Thickness and standard deviation of RuO2/MnO2 composite films. . . . 14

Figure 6: Cyclic voltammograms of (a) RuO2, (b) MnO2, (c) RuO2+MnO2(oxide mixture), and (d) RuO2/MnO2 (binary oxides synthesized byco-precipitation), for different electrode compositions (60, 70, 80 and90% active material), recorded at 5 mV/s 0.65 M K2SO4 aqueoussolution. Currents are normalized to the total mass of the compositeelectrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 7: Cyclic voltammograms of amorphous (300C) and annealed (600C) of(a) RuO2, (b) MnO2, (c) RuO2+MnO2 (oxide mixture), and (d)RuO2/MnO2 (co-precipitated binary oxides), recorded at 5mV/s in0.65 M K2SO4(aq). Currents are normalized to the total mass of thecomposite electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 8: Cyclic voltammetry of RuO2/MnO2 composite electrodes, containing80% active material, recorded at 5 mV/s in (a) 0.65 M K2SO4, (b)0.5 M Na2SO4, (c) 0.5 M H2SO4, and (d) 0.5 M H2SO4 + 0.5 MNa2SO4 aqueous solutions. Currents are normalized to the total mass ofthe composite electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . 29

viii DRDC Atlantic CR 2008-215

Figure 9: Cyclic voltammograms of RuO2+MnO2 (oxide mixture) compositeselectrodes, containing 80% active material, with titanium or stainlesssteel current collectors, recorded at 5 mV/s in 0.65 M K2SO4 aqueoussolution. Currents are normalized to the total mass of the compositeelectrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 10: SEM images of (a) microfibrous carbon paper (CP), and (b) Highmagnification image of one microfibers. (c) EDX analysis ofmicrofibrous carbon paper. . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 11: (a—c) SEM images of the nanocomposite carbon nanotubes carbonpaper, and (d) EDX analysis. . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 12: Cyclic voltammetry of CP and CP-CNT electrodes in 0.65 MK2SO4(aq) at ν = 20 mV/s. . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 13: (a–b) SEM images of the composite carbon paper manganese oxide(CP-MnO2), and (c) EDX analysis. . . . . . . . . . . . . . . . . . . . . 36

Figure 14: SEM images of the carbon paper/carbon nanotubes/manganese oxidenanocomposite CP-CNT-MnO2 . . . . . . . . . . . . . . . . . . . . . . 38

Figure 15: Cyclic voltammogramms of CP-CNT, CP-MnO2, and CP-CNT-MnO2in 0.65 M K2SO4(aq) at ν = 20 mV/s. . . . . . . . . . . . . . . . . . . . 39

Figure 16: Effects of scan rate on the cyclic voltammetry of (a) CP-MnO2 and (b)CP-CNT-MnO2 in 0.65 M K2SO4(aq) solution. . . . . . . . . . . . . . . 39

Figure 17: Voltammetric charge q∗ as a function of sweep rate . . . . . . . . . . . . 40

Figure 18: Electrochemical impedance spectra of CP-MnO2 and CP-CNT-MnO2electrodes in 0.65 M K2SO4(aq) solution. DC voltage: 0 V vsHg/HgSO4, frequency range 100 kHz to 10 mHz. (a) Nyquist plots and(b) Zoom of the high-frequency region. . . . . . . . . . . . . . . . . . . 41

DRDC Atlantic CR 2008-215 ix

List of tables

Table 1: Capacitance and potential windows for RuO2, MnO2, mixedRuO2+MnO2 and co-precipitated RuO2/MnO2 composite electrodes. . . 4

Table 2: Capacitance and potential windows values for RuO2/MnO2 binaryoxide composite electrodes synthesized using different co-precipitationmethods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Table 3: Composition of RuO2, MnO2, and RuO2/MnO2 composite electrodes. . . 8

Table 4: Cathodic (hydrogen evolution) and anodic (oxygen evolution) potentiallimits and the potential window (∆E) for different compositions ofRuO2, MnO2, RuO2+MnO2, and co-precipitated RuO2/MnO2 electrodes. 11

Table 5: Electrode specific capacitance (Csp,E , F/g) and active material intrinsicspecific capacitance (Csp,M, F/g) for different compositions of RuO2,MnO2, RuO2+MnO2, and co-precipitated RuO2/MnO2 electrodes. . . . . 13

Table 6: Composition of RuO2, MnO2, RuO2+MnO2, and co-precipitatedRuO2/MnO2 composite electrodes. . . . . . . . . . . . . . . . . . . . . 17





Table 11: Specific capacitance (F/g) as a function of scan rate . . . . . . . . . . . . 39

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1 Q1: 04/2007 – 06/20071.1 IntroductionIn our previous annual report [1], we showed that the co-precipitation of RuOy ·n(H2O)(hereafter simplified to RuO2) and MnO2 can be achieved by mixing KMnO4 and RuCl3in aqueous NaOH or HCl, followed by the addition of equivalent amount of HCl or NaOH.

The reaction of KMnO4 with RuCl3 aqueous in NaOH leads to the formation of a solubleintermediate species. This formation is optimal for a Mn:Ru ratio equal of 3:2. It was alsoshown that the properties of the MnO2/RuO2 oxides depend on the preparation method:

• Binary oxides formed by mixing in NaOH media are amorphous. Thermogravimetricanalyses have also shown that those oxides are hydrated (around 20% by mass).

• Binary oxides prepared in NaOH media are more homogenous (Mn/Ru = 3) than bi-nary oxides prepared in HCl. These MnO2/RuO2 oxides also form smaller particles(60–150 nm) when prepared in NaOH.

• Finally, TGA and DTA data for the binary oxides prepared by dropwise mixing differfrom that of single MnO2 and RuO2 oxides. Therefore, the binary oxide is probablynot a simple mixture of those two oxides.

This present report deals with the electrochemical characterisation of composite electrodesprepared from RuO2/MnO2 binary oxide powders. These electrodes were characterized byelectrochemistry (capacitance) and were compared to those made with RuO2, MnO2 and aphysical mixture of RuO2 and MnO2 (with atomic ratio Mn/Ru = 3) composite electrodes.

1.2 Electrode PreparationDifferent composite electrodes were prepared from RuO2/MnO2 binary oxide powders syn-thesized using three different co-precipitation methods as previously described [1]. The bi-nary oxide composite electrodes were compared to RuO2 and MnO2 composite electrodes.Because synthesized RuO2/MnO2 binary oxide powders have a Mn/Ru atomic ratio equalto 3, an additional composite electrode was prepared by mixing RuO2 powder and MnO2powder with a similar Mn/Ru atomic ratio. The preparation of the oxide powders follows.

1.2.1 RuO2 preparation

RuO2 was synthesized by adding 0.51 g of RuCl3 to 200 mL of 1 M NaOH. The solutionwas then stirred for 12 hours. A dark precipitate slowly appeared, in accordance withthe literature [2, 3]. The precipitate was filtrated (Whatman #54 filter paper) and washedwith 200 mL of water, 100 mL of methanol, and finally 100 mL of acetone. The resultingpowder was dried and stored under vacuum at 40C.

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1.2.2 MnO2 preparation

MnO2 powder was synthesized using a simple co-precipitation method [4, 5]. KMnO4(99% purity) was dissolved in deionized water. While the solution was stirred, MnSO4 wasadded. The KMnO4/MnSO4 molar ratio was 2:3. A dark brown precipitate was immedi-ately obtained according to:

2KMnO4 +3MnSO4 +2H2O−−→ 5MnO2 +2K+ +4H+ +3SO2−4 (1)

The mixture was filtered (Whatman #54 filter paper). The precipitate was then dispersedin deionized water and stirred for 5 minutes using an ultrasonic bath. The operation wasrepeated twice in methanol and once more in acetone. The resulting powder was then driedat 40C for 12 hours under vacuum.

1.2.3 RuO2/MnO2 preparation

The RuO2/MnO2 binary oxide was synthesized using three different co-precipitation meth-ods as described previously [1].

1. (coarse mixing) 0.49 g of KMnO4 was added to 200 mL of 1 M NaOH. The solutionwas stirred for 10 minutes. Next, 0.51 g of RuCl3 was added and the mixture wasstirred for 10 minutes. Finally, 200 mL of 1 M HCl was added to the alkaline solutionand the mixture was stirred for 12 hours.

2. (dropwise mixing) 25 ml of 10 mM aqueous KMnO4 solution was added dropwiseto 100 mL of 1 M NaOH. The solution was stirred for 10 minutes. Next, 25 ml of10 mM RuCl3 aqueous solution was added dropwise and the mixture was stirred for10 minutes. Finally, 100 mL of 1 M HCl was added to the alkaline solution and themixture was stirred for 12 hours.

3. (dropwise mixing) 25 ml of 10 mM aqueous RuCl3 solution was added dropwise to100 mL of 1 M HCl. The solution was stirred for 10 minutes. Next, 25 ml of 10 mMaqueous KMnO4 solution was added dropwise and the mixture was stirred for 10minutes. Finally 100 mL of 1 M NaOH was added to the alkaline solution and themixture was stirred for 12 hours.

All solutions led to the formation of a precipitate. Each precipitate was filtered andwashed with 200 mL of water, 100 mL of methanol, and 100 mL of acetone. Thepowders were then dried and stored under vacuum at 40C.

1.2.4 Composite electrode preparation

Composite electrodes were prepared by mixing 90 wt% of the active material (one of RuO2,MnO2, or RuO2/MnO2 binary oxide) powder with 5 wt% of acetylene black (Alfa Aesar,

2 DRDC Atlantic CR 2008-215

> 99.9%) and 5 wt% of poly(tetrafluoroethylene) dried powder (PTFE). The three con-stituents were mixed together until a homogeneous black powder was obtained. A fewdrops of ethanol were then added. This resulted in a rubber-like paste that was rolled intoa film (100–200 µm thick) on a flat glass surface. A piece of this film (2 mg) was cut andpressed at 9 t/cm2 onto a stainless steel grid which was used as current collector.

1.3 Electrochemical CharacterizationElectrochemical characterization was performed on a Solartron 1470 potentiostat operatedunder Corrware II software (Scribner Associates). An Ag/AgCl (KCl saturated) assemblyand a platinum gauze were used as the reference and counter electrodes, respectively. Theelectrochemical characterization was done in H2SO4 and K2SO4 aqueous media.

The specific capacitance was calculated by integrating either the oxidative or the reductivepart of the cyclic voltammogram over the potential window of the CV. All the specificcapacitances are reported per gram of active material. It must be noted that due to themethod used for the preparation of the electrodes, an uncertainty of ±10% is estimated forthe reported capacitance values.

1.3.1 Electrochemical characterization in H2SO4

The results of electrochemical characterization made in 0.5 M H2SO4 aqueous media werenot reported here due to the decomposition of the RuO2/MnO2 composite electrodes in thismedium. This decomposition is only observed upon potential cycling of the electrode. Thisdecomposition is not observed for the RuO2, MnO2 and RuO2+MnO2 mixture electrodes.This tends to show that the properties and electrochemical stability of the co- precipitatedRuO2/MnO2 binary oxides differ from those of the single oxides.

1.3.2 Electrochemical characterization in K2SO4

The cyclic voltammetry (CV) of bare acetylene black and the RuO2, MnO2, RuO2+MnO2,or RuO2/MnO2 composite electrodes in 0.65 M K2SO4 at a scan rate of 10 mV/s are com-pared in Figure 1. The capacitance values and potential window of electroactivity estimatedfrom the cyclic voltammetry are reported in Table 1.

Figure 1 shows that all the oxide composite electrodes display a quasi-rectangular shape,which is characteristic of capacitive behaviour. The capacitance of the RuO2, RuO2+MnO2and RuO2/MnO2 composite electrodes is clearly larger than that of the MnO2 electrode(Table 1).

Table 1 shows that the capacitance of RuO2 and MnO2 electrodes are 179 and 91 F/g re-spectively. These values are lower than the capacitance values reported in literature for


-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-4

-3

-2

-1

0

1

2

3

4

i / A.g-1

E / V vs. Ag/AgCl

- acetylene black

- - RuOy.n(H2O)

…. MnO2

- -mixed RuOy.n(H2O)/MnO2

- - co-precipitated RuOy.n(H2O)/MnO2

Figure 1: Cyclic voltammograms of acetylene black, RuO2, MnO2, mixed RuO2+MnO2and co-precipitated RuO2/MnO2 composite electrodes. Cyclic voltammograms arerecorded at 10 mV/s in 0.65 M K2SO4(aq).

Table 1: Capacitance and potential windows for RuO2, MnO2, mixed RuO2+MnO2 andco-precipitated RuO2/MnO2 composite electrodes.

RuO2 MnO2 RuO2+MnO2 RuO2/MnO2Capacitace (F/g) 179 91 184 188

∆E (V) 1.10 0.95 1.15 1.25


Table 2: Capacitance and potential windows values for RuO2/MnO2 binary oxide compos-ite electrodes synthesized using different co-precipitation methods.

Method 1 Method 2 Method 3(fast mix/NaOH) (dropwise mix/NaOH) (dropwise mix/HCl)

Capacitance (F/g) 142 188 134∆E (V) 1.25 1.25 1.25

composite electrodes which are around 350 and 200 F/g, respectively [5, 6]. These dif-ferences can be explained by the fact that our experimental conditions are not optimized.The electrode composition, ratio of active material, acetylene black, and PTFE (90:5:5), isslightly different from those commonly used in literature. For RuO2 and MnO2 compositeelectrodes, a composition of 70:25:5 is generally used [5, 7]. The low quantity of con-ducting material (here, acetylene black) may not be sufficient to obtain good conductivityand electrochemically access all the active material. Similarly, the scan rate (10 mV/s) forthe cyclic voltammetry may be too high. For MnO2 composite electrodes, a scan rate of5 mV/s is more generally used [5]. It should be noted that these preliminary experimentsare only for the purpose of comparing the single and binary oxide composite electrodes.The optimisation of the composition of the composite electrodes and the scan rate used forcyclic voltammetry parameters are currently being investigated.

The capacitance of RuO2+MnO2 and RuO2/MnO2 composite electrodes are 184 and 188 F/g,respectively (Table 1). The simultaneous presence of ruthenium oxides and MnO2 seemsto enhance the capacitance of the material. Effectively, with one quarter of the rutheniumoxide of the RuO2 electrode, capacitance values for the RuO2+MnO2 and RuO2/MnO2composite electrodes were slightly higher.

The RuO2+MnO2 and RuO2/MnO2 composite electrodes are characterized by a wider po-tential window relative to the RuO2 and MnO2 electrodes. The presence of rutheniumshifts the hydrogen evolution to lower potentials, around −0.3 V vs. Ag/AgCl, whereasthe presence of manganese shifts the oxygen evolution to higher potentials, around 0.9 Vvs. Ag/AgCl (Figure 1). This produces a wider potential window for both RuO2/MnO2 andRuO2+MnO2 composite electrodes (from 1.15 to 1.25 V) relative to the RuO2 and MnO2electrodes which are 1.10 and 0.95 V, respectively (Table 1).

The RuO2/MnO2 composite electrode shows a capacitance similar to that exhibited by theRuO2+MnO2 composite electrodes. The co-precipitation method did not seem to have asignificant effect on the capacitance of the material. However, the RuO2/MnO2 electrodesdid give a potential window wider by around 100 mV (Figure 1 and Table 1).

Electrochemical data for composite electrodes prepared with RuO2/MnO2 binary oxidessynthesized by using various co-precipitation methods are reported in Figure 2 and Table 2.

For the same potential window, Table 2 indicates that the capacitance of methods 1 and


-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

-3

-2

-1

0

1

2

3

i / A

.g-1

E / V vs. Ag/AgCl

____ acetylene black

- - - - fast mixing in NaOH (sample 1)

⋅ ⋅ ⋅ ⋅ dropwise mixing in NaOH (sample2)

⋅ - ⋅ - dropwise mixing in HCl (sample 3)

Figure 2: Cyclic voltammograms of acetylene black, RuO2/MnO2 synthesized by co-precipitation by fast mixing in NaOH (method 1), dropwise mixing in NaOH (method 2),and dropwise mixing in HCl (method 3). Cyclic voltammograms are recorded at 10 mV/sin 0.65 M K2SO4(aq).

3 composite electrodes (142 and 134 F/g, respectively) are lower than the capacitance ofmethod 2 electrode (188 F/g). These capacitances are intermediate between those obtainedfor RuO2 and MnO2 electrodes but lower than that of the RuO2+MnO2 electrode (Table 1).

The RuO2/MnO2 binary oxide prepared by dropwise mixing in NaOH followed by theaddition of HCl (method 2) shows the highest capacitance. This result could be relatedto previous XRD and EDX results (see the first Annual Report [1]) where it was shownthat method 2 produced the most homogeneous binary oxide and the one with the smallestparticles by SEM (size between 60 and 150 nm).

1.4 ConclusionsThis report has shown that the RuO2/MnO2 binary oxides synthesised by co-precipitationmethods display specific capacitances between 134 and 188 F/g. The RuO2/MnO2 binaryoxide synthesised by dropwise mixing in NaOH followed by the addition of HCl exhibitsthe highest capacitance.

The composite electrodes are not yet optimized and a number of parameters remain to betweaked:


• The composition of the composite electrodes (e.g., the ratios of active material,acetylene black and PTFE) will be studied to optimize the capacitive properties.

• The thermal dehydration of the RuO2/MnO2 binary oxides will be performed. Thesynthesized materials are hydrated at around 20%. Water removal by heat treatment(around 300C) could have a positive effect on the capacitance (CV studies) and onthe material structure (XRD studies).

• The particle size seems to have an effect on the capacitance (e.g., differences betweenmethods 1 and 2). The specific surface area of the synthesized RuO2/MnO2 binaryoxides will be measured using the Brunauer, Emett, and Teller method (BET).

2 Q2: 07/2007 – 09/20072.1 IntroductionIn the previous section (Section 1), we showed that the RuO2/MnO2 binary oxides syn-thesized by co-precipitation methods displayed specific capacitances between 134 and188 F/g. The RuO2/MnO2 binary oxide synthesized by drop-wise mixing in NaOH fol-lowed by the addition of HCl shows the highest capacitance. The corresponding synthe-sized composite electrodes were not optimized.

The present report deals mainly with the study of the effect of the composite electrodecompositions (e.g., ratio of active material, acetylene black, graphite and PTFE) on thecapacitive properties (potential window, capacitance values) and film thickness.

2.2 Electrode PreparationComposite electrodes were prepared from RuO2/MnO2 binary oxide powders synthesizedusing a co-precipitation method by drop-wise mixing in NaOH followed by the additionof HCl. This produces the highest capacitance values as previously described (Section 1).These binary oxides composite electrodes were compared with RuO2 and MnO2 compositeelectrodes. Because synthesized RuO2/MnO2 binary oxide powders have a Mn/Ru atomicratio equal to 3, an additional composite electrode was prepared by mixing RuO2 powderand MnO2 powder with a similar Mn/Ru atomic ratio. The oxide powders were preparedas described below.

2.2.1 RuO2 preparation

Refer to §1.2.1 on page 1


Table 3: Composition of RuO2, MnO2, and RuO2/MnO2 composite electrodes.

Composite Active Material∗ Acetylene Black Graphite PTFEElectrode % w/w % w/w % w/w % w/wAM-60 60 17.5 17.5 5AM-70 70 12.5 12.5 5AM-80 80 7.5 7.5 5AM-90 90 5.0 0.0 5

2.2.2 MnO2 preparation

Refer to §1.2.2 on page 1

2.2.3 RuO2/MnO2 preparation

Method 2 from Section 1.2.3 (page 2) was used, including the filtering and rinsing stepslisted at the end of the Section.


The procedure given in Section 1.2.4 (page 2) was used, producing the formulations listedin Table 3.

2.3 Electrochemical CharacterizationElectrochemical characterization was carried out as described in Section 1.3, but usingaqueous 0.65 M K2SO4 as the electrolyte.

Figure 3 illustrates the variation in the calculated specific capacitance for cyclic voltam-mograms recorded at different scan rate. The specific capacitance depends on the scan rateand is therefore limited by diffusion phenomenon. Consequently, the specific capacitancevalues reported here were determined from cyclic voltammograms recorded at a slow scanrate (1 mV/s).

2.3.1 Influence of the composite electrode composition on thepotential window

Chialvo et al. have proposed that interactions between manganese and ruthenium in amixed oxide (RuxMn1-xO2) can be used to explain the larger potential window observedfor Ti/RuxMn1-xO2 electrodes, compared to RuO2 and MnO2 single oxide electrodes [2].


1 10 100

20

40

60

80

100

120

140

160

180

70% RuO2+MnO

2

80% RuO2+MnO

2

Capacitance / F

.g-1

Scan rate / mV.s-1

Figure 3: Example of the variation in specific capacitance, calculated from cyclic voltam-metry recorded at different scan rates in 0.65 M K2SO4 aqueous solution, for RuO2+MnO2composite electrodes containing 70% and 80% active material.

In this work, the effect of the composition of composite electrodes on anodic and cathodicpotential limits of various single and mixed oxides was studied.

Representative cyclic voltammograms of RuO2, MnO2, RuO2+MnO2, and RuO2/MnO2electrodes are shown in Figure 4. These cyclic voltammograms show that the anodic andcathodic potential limits depend on the composition of the electrode and the nature ofthe active material. For RuO2 and MnO2 electrodes, the anodic and cathodic potentiallimits are only slightly affected when the composition of the oxides changed from 60%to 90%. The potential window varies between 750 and 850 mV for RuO2, and 850 and900 mV for MnO2 electrodes. For RuO2+MnO2 and RuO2/MnO2 binary oxide electrodes,the variation of the potential limit is more significant (over 250 mV). For RuO2+MnO2electrodes, the largest potential window (950 mV) is obtained for electrodes containing90% active material. The lowest potential window (700 mV) is obtained for electrodescontaining 70% RuO2+MnO2. Electrodes containing 60% and 80% RuO2+MnO2 have anintermediate electrochemical window of 800 mV. For RuO2/MnO2 electrodes, the highestpotential window (1000 mV) is obtained for electrodes containing 60% active material. Thelowest potential window (700 mV) is obtained for electrodes containing 70% RuO2/MnO2.

The potential limits occur just before hydrogen and oxygen evolution reactions start. Itappears that the electrode composition seems to have an effect on the potential limits, butthere is no apparent trend. The limits are summarized in Table 4.


-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

RuO2-60

RuO2-70

RuO2-80

RuO2-90

i / A

.g-1

E / V vs. Ag/AgCl

-0.2 0.0 0.2 0.4 0.6 0.8

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

MnO2-60

MnO2-70

MnO2-80

MnO2-90i

/ A

.g-1

E / V vs. Ag/AgCl

(a) (b)

c)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

RuO2+MnO

2-60

RuO2+MnO

2-70

RuO2+MnO

2-80

RuO2+MnO

2-90

i / A

.g-1

E / V vs. Ag/AgCl

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

RuO2/MnO

2-60

RuO2/MnO

2-70

RuO2/MnO

2-80

RuO2/MnO

2-90

i / A

.g-1

E / V vs. Ag/AgCl

(c) (d)

Figure 4: Cyclic voltammograms of (a) RuO2, (b) MnO2, (c) RuO2+MnO2 (oxide mix-ture), and (d) RuO2/MnO2 (binary oxides synthesized by co-precipitation), for differentelectrode compositions (60, 70, 80 and 90% w/w of active material), recorded at 5 mV/s in0.65 M aqueous K2SO4 solution. Currents are reported normalized to the total mass of thecomposite electrodes.


Tabl

e4:

Cat

hodi

c(h

ydro

gen

evol

utio

n)an

dan

odic

(oxy

gen

evol

utio

n)po

tent

iall

imits

and

the

pote

ntia

lwin

dow

(∆E

)fo

rdi

f-fe

rent

com

posi

tions

ofR

uO2,

MnO

2,R

uO2+

MnO

2,an

dco

-pre

cipi

tate

dR

uO2/

MnO

2el

ectr

odes

.

Pote

ntia

llim

itsfr

omcy

clic

volta

mm

etry

(Vvs

.Ag/

AgC

l)E

lect

rode

RuO

2M

nO2

RuO

2+M

nO2

RuO

2/M

nO2

Com

posi

tion

Eca

thE

ano

∆E

Eca

thE

ano

∆E

Eca

thE

ano

∆E

Eca

thE

ano

∆E

AM

-60

-0.1

5+0

.60

0.75

-0.0

5+0

.80

0.85

-0.1

0+0

.75

0.85

-0.2

0+0

.80

1.00

AM

-70

-0.2

0+0

.60

0.80

-0.0

5+0

.80

0.85

-0.0

5+0

.65

0.70

-0.0

5+0

.65

0.70

AM

-80

-0.2

0+0

.55

0.75

-0.1

0+0

.75

0.85

-0.1

0+0

.75

0.85

-0.2

0+0

.65

0.85

AM

-90

-0.2

0+0

.65

0.85

-0.1

0+0

.80

0.90

-0.2

0+0

.75

0.95

-0.2

5+0

.70

0.95


2.3.2 The effect of composite electrode composition on specificcapacitance

The effect of electrode formulation on specific capacitance was studied by cycling voltam-metry. The values of the specific capacitance for the whole composite electrode and onlyfor the active material are reported in Table 5.

Electrodes containing 90% active material show the highest specific capacitances, greaterthan 140 F/g, and as high as 157 F/g for RuO2/MnO2. Not surprisingly, as the fractionof active material decreases, so does the specific capacitance. While formulations such asAM-60 have a greater capacitance due to the higher carbon content, clearly the pseudoca-pacitance contributed by the oxide is the far more dominant factor.

The specific capacitances for MnO2 composite electrodes are similar to those reported inliterature in K2SO4(aq) solutions [4]. The specific capacitances of the RuO2 compositeelectrodes reported here are lower than those reported for RuO2 electrodes in a H2SO4electrolyte (around 700 F/g [3]) due to the use of K2SO4.

Generally, the specific capacitances for electrodes prepared with the RuO2+MnO2 mixture(with a 1:3 ratio) lie somewhere between specific capacitances of simple RuO2 and MnO2electrodes.

The specific capacitances of RuO2/MnO2 electrodes, synthesized by co-precipitation, seemslightly higher than those obtained by mixing RuO2 and MnO2 (for the same Ru/Mnatomic ratio). Moreover, specific capacitances for the RuO2/MnO2 electrodes are alsoslightly higher than the theoretical values derived from the specific capacitances of RuO2and MnO2. However it should be noted that the capacitances are almost similar in somecases and a wider study needs to be undertaken to ascertain the statistical significance ofthis difference. Possible reasons for the enhancement are: (i) Better homogeneity in theco-precipitated binary oxide powder compared to mechanical mixing, and (ii) some sort ofinteraction between the oxides.

It is also useful to know the effect of the electrode composition on the specific capacitanceof the active material itself. For this purpose, a carbon/acetylene black composite electrodewas tested in 0.65 M K2SO4(aq) solution. Its specific capacitance was 3.9 F/g, in goodagreement with the literature [8]. With this information, it was possible to estimate theintrinsic specific capacitance values for RuO2, MnO2, RuO2+MnO2, RuO2/MnO2, whichare also presented in Table 5.

As was the case with Csp,E , the material specific capacitances Csp,M of RuO2+MnO2 andRuO2/MnO2 binary oxide electrodes lie somewhere between the specific capacitance ofRuO2 and MnO2. Specific capacitances obtained for RuO2/MnO2 electrodes, synthesizedby co-precipitation, were again higher than those obtained by mechanical mixing of RuO2and MnO2 (for the same Ru/Mn atomic ratio). Furthermore, specific capacitances obtained


Tabl

e5:

Ele

ctro

desp

ecifi

cca

paci

tanc

e(C

sp,E

,F/g

)an

dac

tive

mat

eria

lint

rins

icsp

ecifi

cca

paci

tanc

e(C

sp,M

,F/g

)fo

rdi

ffer

ent

com

posi

tions

ofR

uO2,

MnO

2,R

uO2+

MnO

2,an

dco

-pre

cipi

tate

dR

uO2/

MnO

2el

ectr

odes

.C

ompo

site

RuO

2M

nO2

RuO

2+M

nO2

RuO

2/M

nO2

RuO

2+M

nO2

(the

o)∗

Ele

ctro

deC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,M

AM

-60

9215

079

129

106

174

120

198

8213

4A

M-7

013

018

411

716

612

818

212

117

212

017

0A

M-8

012

916

014

417

912

415

413

216

314

017

4A

M-9

016

518

214

115

614

716

215

717

414

716

2∗

The

oret

ical

valu

eca

lcul

ated

from

RuO

2an

dM

nO2

spec

ific

capa

cita

nce

fora

bina

ryox

ide

com

posi

teel

ectr

ode

with

aR

u/M

nat

omic

ratio

equa

lto

1/3.


60 65 70 75 80 85 90

50

100

150

200

250

RuO

2/M

nO

2 film

Thic

kness

/ µm

Composition in active material / %

Figure 5: Thickness and standard deviation of RuO2/MnO2 composite films.

for the RuO2/MnO2 electrodes are slightly higher than predicted from theoretical values.The highest specific capacitance for RuO2/MnO2 material was obtained for the electrodecontaining 60% of RuO2/MnO2, Csp,M = 198 F/g. There is no clear trend for Csp,M, andthis is potentially significant. One would expect an increase in the fraction of carbon andacetylene black to lead to an increase of the electrode conductivity. A high conductivityshould provide better access to electrochemically active material. Since there seems to beno relationship, one can conclude that the conductivity of the oxide is not a limiting factorunder these experimental conditions where the scan rate is slow.

2.3.3 The effect of composition on electrode thickness

The thickness of the active material film can affect the determination of the specific capac-itance of an electrode, since the thickness has a direct impact on the diffusion phenomenoninside the electrode. Figure 5 presents the measured thickness of several RuO2/MnO2 films(before being pressed between the stainless steel grids) for different formulations. Thick-nesses where measured with a Mitutoyo Digimatic Micrometer (accuracy≈± 1µm) on tendifferent samples.

Figure 5 indicates that RuO2/MnO2 films from the same paste batch do not yield a consis-tent thickness. The variations could be linked to difference in the homogeneity of the pastesor to inconsistencies in the preparation method itself. For example, the quantity of ethanolis not carefully controlled between samples. Similarly, the thickness of the assembled elec-trodes (including the steel current collectors) varies as well. Clearly, it is difficult to control


the true thickness of the electrode material, and this could have significant consequencesfor the reproducibility of specific capacitance measurements.

The thickness of the composite electrodes (pressed film + current collector) was also mea-sured. The thickness of a single stainless grid (not pressed) is 262±16 µm. Two stainlessgrids, pressed together under 9 t/cm2, have a thickness of 229±27 µm. When the activematerial film is pressed between two stainless grids (under 9 t/cm2 pressure), the result-ing composite electrode thickness is around 287±44 µm. For such electrodes, the activematerial film thickness can be estimated at 58±17 µm, but a non-trivial quantity of activematerial must also be present within the stainless steel mesh. In these conditions it is dif-ficult to know the true thickness of the active material film. This thickness is estimatedbetween 40 and 330 µm depending how the active material paste was flattened and spreadduring the pressing step. This uncertainty could account for the significant variations in thespecific capacitances (Table 5), since thicker electrodes may not permit electrochemicalaccess to the entire volume of active material.

2.4 ConclusionsThis report showed that combining RuO2 and MnO2 as the active capacitor material (with aRu:Mn atomic ratio of 1:3) produces a useable electrochemical potential window of about1000 mV. On the other hand, single RuO2 and MnO2 oxide materials are characterized bypotential windows of about 850 and 900 mV, respectively. The RuO2/MnO2 compositeelectrode (prepared by the co-precipitation method) offers the largest potential window.

The highest electrode specific capacitance (Csp,E = 157 F/g) for a RuO2/MnO2 compositeelectrode results from an electrode containing 90% RuO2/MnO2. The highest materialspecific capacitance (Csp,M = 198 F/g) for RuO2/MnO2 was obtained from a compositeelectrode formulation containing 60% RuO2/MnO2. These results ought to be taken withsome caution due to the absence of (i) any correlation between specific capacitance andelectrode formulation, and (ii) the inconsistent electrode thickness.

When the preparation of the composite electrodes can be made more reproducible, theeffects of other parameters on the capacitance of the synthesized electrodes will be studied:

• The nature of the current collector will be studied. The use of titanium could allowthe extension of the anodic potential limit to higher values, and thereby increasethe potential window. Moreover, using the titanium nanotube structure presented inan earlier report [1] as a current collector may diminish the contact and/or chargetransfer resistance present at the electrode–collector interface.

• The dehydration of the RuO2/MnO2 binary oxides will be performed. The synthe-sized materials are approximately 20% hydrated. Water removal by heat treatment at


around 200–300C could have a positive effect on the capacitance (CV studies) andon the material structure (XRD studies).

• The particle size seems to have also an effect on the capacitance. The specific sur-face area of the synthesized RuO2/MnO2 binary oxides will be measured using theBrunauer, Emett and Teller method (BET).

3 Q3: 10/2007 – 12/20073.1 IntroductionIn Section 2, the effect of composition on the specific capacitance of composite electrodeswas investigated. The absence of any trend was attributed to significant variations in thethicknesses of the prepared composite electrodes (±44 µm) and to non-homogeneity ofthe active material paste (dispersion of the different constituents of the electrode and thequantity of ethanol used for the dispersion of the PTFE). The present report deals withthe study of new composite electrodes synthesized by using a different preparation proto-col, giving a better homogenization of the active material and an improved control of theelectrode thickness. The effect of the electrode composition (e.g., ratio of active material,acetylene black, graphite, and PTFE), the effect of the heat treatment temperature on theoxide powders (RuO2, MnO2 and coprecipitated RuO2/MnO2), the effect of the supportingelectrolyte, and the effect of the current collector were also studied.

3.2 Electrode PreparationComposite electrodes were prepared from RuO2/MnO2 binary oxide powders synthesizedusing a co-precipitation method by drop-wise mixing in NaOH followed by the additionof HCl which produces the highest capacitance values as previously described (Section 1).These binary oxides composite electrodes were compared with RuO2 and MnO2 compositeelectrodes. Because synthesized RuO2/MnO2 binary oxide powders have a 3:1 Mn:Ru, anadditional composite electrode was prepared by mixing RuO2 powder and MnO2 powderwith a similar Mn:Ru atomic ratio.

3.2.1 Preparation of oxides

The preparation methods used for RuO2, MnO2, and RuO2/MnO2 oxide powders were thesame as described earlier in Section 2.2.


Table 6: Composition of RuO2, MnO2, RuO2+MnO2, and co-precipitated RuO2/MnO2composite electrodes.

Composite Active Material∗ Acetylene Black Graphite PTFEElectrode % w/w % w/w % w/w % w/wAM-60 60 17.5 17.5 5AM-70 70 12.5 12.5 5AM-80 80 7.5 7.5 5AM-90 90 2.5 2.5 5

∗Active materials are RuO2, MnO2,or RuO2/MnO2 binary oxides (mixture or co-preciptated)


Due to the thickness and homogeneity reproducibility problems noted in Section 2, anew protocol was developed. The composites electrodes were prepared by mixing dif-ferent quantities of active material (RuO2, MnO2, or RuO2/MnO2 binary oxide) powderswith acetylene black (Alfa Aesar, > 99.9%), graphite (Alfa Aesar, > 99.8%) and 5 wt%poly(tetrafluoroethylene) dried powder (PTFE). Table 6 shows the composition and namingconvention of the electrode composition.

The composite electrode materials (except PTFE) were mixed in an excess of ethanol (ap-proximately 5 ml) for 5 minutes using an ultrasonic bath. PTFE (5 wt%) was then added,and the mixture was sonicated for another 10 minutes. The excess of ethanol was thenslowly evaporated under weak vacuum at 25C.

The resulting rubbery paste was then rolled into a film on a flat glass surface. The thicknessof this film was measured at 110±10 µm using a Mitutoyo Digimatic Micrometer. Next, apiece of this film (about 2 mg) was cut and pressed under 9 t/cm2, on a stainless steel grid,which was used as current collector. The thickness of all electrodes (film and grid) wasalso measured using the micrometer and was 268±26 µm.

In our previous study (Section 2), the thickness variations of the films and the compositeelectrodes were ±30 and ±44 µm, respectively. In this study, those variations were bettercontrolled: ±10 µm for the films and ±26 µm for the composite electrodes, alleviatingsomewhat the non-reproducibile electrode thickness.

3.3 Electrochemical characterizationThe electrochemical characterization of the composite electrode was performed on a So-lartron 1470 potentiostat operated under Corrware II software (Scribner Associates). AnAg/AgCl (KCl saturated) assembly and a platinum gauze were used as the reference and


counter electrode, respectively. The electrochemical characterization was performed in a0.65 M K2SO4 aqueous solution. The specific capacitance was calculated by integratingeither the oxidative or the reductive part of the cyclic voltammogram over the potentialwindow of the CV. All the specific capacitances are reported per gram of active material.It must be noted that due to the method used for the preparation of the electrodes, an un-certainty of ±10% is estimated for the reported specific capacitance values. These valueswere all calculated from cyclic voltammograms recorded at slow scan rate (1 mV/s).

3.3.1 Effect of the electrode composition

Chialvo et al. have proposed that interactions between manganese and ruthenium in amixed oxide (RuxMn1-xO2) can be used to explain the larger potential window observedfor Ti/RuxMn1-xO2 electrodes, compared to RuO2 and MnO2 single oxide electrodes [9].

Our previous study of the effects of electrode composition on specific capacitances andpotential window has shown that RuO2/MnO2 composite electrodes (prepared by the co-precipitation method) offer larger potential windows (approximately 900 mV) than singleoxides electrodes. For binary oxide electrodes, the highest specific capacitances were ob-tained for electrodes containing 90% active material (approximately 157 F/g) and highestintrinsic specific capacitances were obtained for electrodes containing 60% active mate-rial (approximately 198 F/g). However, these results were questionable due to the lack ofany correlation between specific capacitance and electrode composition, which in turn waslikely connected to inconsistent thickness and inhomogeneity within the electrode. Usingthe new synthesis protocol described in Section 3.2.2, the composition of the electrodeswas expected to be more homogeneous and the thickness of those electrodes more con-trolled. Figure 6 illustrates the effects of electrode composition on the cyclic voltammetryof RuO2, MnO2, RuO2+MnO2, and RuO2/MnO2 composite electrodes.

As previously observed, the shape of the CV varies with the electrode composition. Cyclicvoltammograms of electrodes containing a large quantity of active material (90%) are lessrectangular than CV of electrodes containing a smaller quantity of active material (60%).Those shape differences are linked to the conductivity of the electrode and related to thequantity of carbon present in the electrodes. The cyclic voltammogram cathodic and anodicpotential limits, and the potential window are reported in Table 7.

The potential windows of the RuO2 composite electrodes are slightly higher than for theother electrodes. This indicates that the potential windows for binary oxide electrodes isdetermined primarily by MnO2 and the presence of RuO2 has little effect.

There is no clear correlation between potential limits and electrode composition. This isin contrast with electrodes prepared by using the old protocol, where a small increase inthe potential window accompanied an increase in the fraction of active material in the elec-trode. It is possible that this is due to the present formulations being more homogenous.


a)

-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8

-1,2

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

RuO2/graphite/acethylene black/PTFE composite electrodes

in 0.65 M K2SO

4 aqueous solution

i /

A.g

-1

E/V vs Ag/AgCl

60%

70%

80%

90%

b)

-0,2 0,0 0,2 0,4 0,6 0,8 1,0

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

MnO2/graphite/acethylene black/PTFE composite electrodes

in 0.65 M K2SO

4 aqueous solution

i /

A.g

-1

E/V vs Ag/AgCl

60%

70%

80%

90%

c)

-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0

-1,2

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

RuO2+MnO

2/graphite/acethylene black/PTFE composite electrodes

in 0.65 M K2SO

4 aqueous solution

i /

A.g

-1

E/V vs Ag/AgCl

60%

70%

80%

90%

d)

-0,2 0,0 0,2 0,4 0,6 0,8 1,0

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

RuO2/MnO


in 0.65 M K2SO

4 aqueous solution

i /

A.g

-1

E/V vs Ag/AgCl

60%

70%

80%

90%

Figure 6: Cyclic voltammograms of (a) RuO2, (b) MnO2, (c) RuO2+MnO2 (oxide mix-ture), and (d) RuO2/MnO2 (binary oxides synthesized by co-precipitation), for differentelectrode compositions (60, 70, 80 and 90% active material), recorded at 5 mV/s 0.65 MK2SO4 aqueous solution. Currents are normalized to the total mass of the composite elec-trodes.


Tabl

e7:

Cat

hodi

c(h

ydro

gen

evol

utio

n)an

dan

odic

(oxy

gen

evol

utio

n)po

tent

iall

imits

and

the

pote

ntia

lwin

dow

(∆E

)fo

rdi

f-fe

rent

com

posi

tions

ofR

uO2,

MnO

2,R

uO2+

MnO

2,an

dco

-pre

cipi

tate

dR

uO2/

MnO

2el

ectr

odes

.

Pote

ntia

llim

itsfr

omcy

clic

volta

mm

etry

(Vvs

.Ag/

AgC

l)E

lect

rode

RuO

2M

nO2

RuO

2+M

nO2

RuO

2/M

nO2

Com

posi

tion

Eca

thE

ano

∆E

Eca

thE

ano

∆E

Eca

thE

ano

∆E

Eca

thE

ano

∆E

AM

-60

-0.3

50+0

.700

1.05

0-0

.125

+0.8

500.

975

-0.1

00+0

.850

0.95

0-0

.100

+0.9

001.

000

AM

-70

-0.3

75+0

.850

1.22

5-0

.150

+0.9

501.

100

-0.1

25+0

.850

0.97

5-0

.125

+0.8

250.

950

AM

-80

-0.4

25+0

.600

1.02

5-0

.125

+0.8

000.

925

-0.1

25+0

.800

0.92

5-0

.125

+0.8

000.

925

AM

-90

-0.4

00+0

.650

1.05

0-0

.100

+0.9

001.

000

-0.2

00+0

.775

0.97

5-0

.075

+0.9

501.

025


Additionally, this absence of trend could be explained by the error in identifying the poten-tial limits where gas evolution (i.e., H2 or O2) begins. This is certainly the case when thecyclic voltammograms are not perfectly rectangular (for example, see the CVs of electrodescontaining 90% active material in Figure 6b, c, and d). The precision of the measurements(5 mV) reported in Table 7 are consequently probably over-optimistic. If there had beenindications of trends, repeat experiments with statistical analysis would have been useful.

The specific capacitance values of composite electrodes determined from the CV are re-ported in Table 8. For RuO2 composite electrodes, the specific capacitances are lowerthan those reported for RuO2 composite electrodes in a H2SO4 electrolyte, approximately700 /F/g [6]. This large difference is due to the use of K2SO4 instead of H2SO4 as elec-trolyte. RuO2 composite electrodes containing 90% active material show the highest spe-cific capacitance, 182 F/g. Not surprisingly, this specific capacitance decreases when theamount of active material increases.

The specific capacitance for MnO2 composite electrodes are similar to those reported inthe literature for K2SO4 aqueous solutions [10]. The highest specific capacitances wereobtained for a composition of 80% active material Csp,E = 141 F/g, and decreasing forthe 90% active material formulation. Presumably in this case, not all the active materialin the electrode can be accessed electrochemically, due to shortage of conducting material(carbon and acetylene black). For compositions of 60% and 70%, the specific capacitancedecreases with the quantity of active material as expected.

For binary oxide composites electrodes (RuO2+MnO2 and RuO2/MnO2), the capacitancevalues are of the same order as theoretical ones, calculated from the capacitance of theindividual oxides (RuO2 and MnO2) their relative proportions in the composite electrode(1:3). The trend in specific capacitance for the RuO2+MnO2 and RuO2/MnO2 electrodesis similar to that observed for the MnO2 electrodes (Table 8), peaking at around 80% activematerial loading. These specific capacitance maxima, already known in the literature [10]for MnO2 composite electrodes having around 75% loading, were not observed previouslyusing our old synthesis protocol (see Section 2). This indicates that the new protocol yieldsmore accurate and reproducible results.

Comparing the RuO2+MnO2 and RuO2/MnO2 composite electrodes shows that the capac-itances of binary oxides electrodes prepared by co-precipitation method are slightly higherthan those prepared by simple mixing. However, the difference is probably not meaningful.

It is useful to determine the material specific capacitance Csp,M, which is also presented inTable 8. Csp of carbon black was previously determined (Section 2.3.2) to be 3.9 F/g, ingood agreement with the literature, and this value was used to calculate the Csp,M values inTable 8.

No obvious trend can be observed for the specific capacitance values of Table 8. Increasingthe content of carbon and acetylene black should lead to an increase of the electrode con-


Tabl

e8:

Ele

ctro

desp

ecifi

cca

paci

tanc

e(C

sp,E

,F/g

)an

dac

tive

mat

eria

lint

rins

icsp

ecifi

cca

paci

tanc

e(C

sp,M

,F/g

)fo

rdi

ffer

ent

com

posi

tions

ofR

uO2,

MnO

2,R

uO2+

MnO

2,an

dco

-pre

cipi

tate

dR

uO2/

MnO

2el

ectr

odes

.C

ompo

site

RuO

2M

nO2

RuO

2+M

nO2

RuO

2/M

nO2

RuO

2+M

nO2

(the

o)∗

Ele

ctro

deC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,M

AM

-60

116

190

9615

798

160

101

166

101

165

AM

-70

136

193

8912

510

214

413

719

510

114

2A

M-8

018

022

414

117

513

817

114

217

715

118

7A

M-9

018

220

211

612

913

114

514

015

513

214

7∗

The

oret

ical

valu

eca

lcul

ated

from

RuO

2an

dM

nO2

spec

ific

capa

cita

nce

fora

bina

ryox

ide

com

posi

teel

ectr

ode

with

aR

u/M

nat

omic

ratio

equa

lto

1/3.


ductivity. A high conductivity may lead to better electrochemical access to active material.Therefore, an increase of the specific capacitance of the oxide is expected when the amountof oxide in the composite electrode decreased. This was not the case here, suggesting that,in these conditions, the electrode composition is not the major factor for the electrodeconductivity. Particle size may be significant factor in that large particles may not allowelectrochemical access to their inner core, and this could account for the inconsistencies.

As expected, the highest capacitances were obtained for RuO2 composite electrodes, withCsp,M > 190 F/g, the highest value coming from the electrodes having 80% RuO2 (224 F/g).The capacitances values for RuO2 electrodes are higher than those obtained from the oldmethod, suggesting that the current protocol produces more homogeneous composite elec-trodes with more active material accessible, leading to higher capacitance values. Similarly,the capacitance values for MnO2 composite electrodes are higher than previously.

The intrinsic specific capacitances for the binary oxide formulations (RuO2+MnO2 andRuO2/MnO2) lie somewhere between those of the RuO2 and MnO2 composite electrodes.For both binary oxide electrodes, the highest capacitance values are obtained at 80% load-ing, with no significant difference between the two. The Csp,M of the RuO2+MnO2 andRuO2/MnO2 electrodes are close to the theoretical value, showing that there is no syner-gistic improvement in intrinsic specific capacitance obtained by mixing RuO2 and MnO2.Nor is there any notable increase in the CV potential window (Table 7).

Since it appears that 80% loading offers the best response, this formulation will be used infuture experiments.

3.3.2 The effects of heat treatment

Previous XRD analyses of RuO2, MnO2 and RuO2/MnO2 synthesized powders (first an-nual report [1]) have shown that these oxides are amorphous. The thermogravimetric anal-yses have also shown that a significant decrease in mass occurs between 25 and 300Cdue to water loss. The mass loss was approximately 12% for RuO2 and 17% for MnO2,and somewhere in-between for the binary oxides. Differential thermal analysis (DTA) hasalso shown an exothermic peak at 380C for MnO2, attributable to the reorganisation ofthe oxide structure. The binary oxides prepared by co-precipitation do not show this peak,suggesting the co-precipitated binary oxides are distinct materials and may not be a simplemixture of RuO2 and MnO2.

To confirm these differences, RuO2, MnO2, RuO2/MnO2 and RuO2+MnO2 powders (syn-thesized as previously described in Section 2.2) were heat treated at 300 and 600C for12 hours under a nitrogen atmosphere using a high-temperature programmable furnace(MTI GSL 1300X), before the preparing the composite electrodes. Subsequently, RuO2,MnO2, RuO2/MnO2 and RuO2+MnO2 composite electrodes were prepared from amor-phous (300C) and annealed (600C) powders using the protocols described in Section 2.2.


a)

-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8

-1,0

-0,5

0,0

0,5

1,0

RuO2/graphite/acethylene black/PTFE composite electrodes

in 0.65 M K2SO

4 aqueous solution

I /

A.g

-1

E / V vs. Ag/AgCl

amorphous

300oC

600oC

b)

-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2

-1,0

-0,5

0,0

0,5

1,0

MnO2/graphite/acethylene black/PTFE composite electrodes

in 0.65 M K2SO

4 aqueous solution

I /

A.g

-1

E / V vs. Ag/AgCl

amorphous

300oC

600oC

c)

-0,2 0,0 0,2 0,4 0,6 0,8 1,0

-1,0

-0,5

0,0

0,5

1,0

RuO2+MnO


in 0.65 M K2SO

4 aqueous solution

I / A

.g-1

E / V vs. Ag/AgCl

amorphous

300oC

600oC

d)

-0,2 0,0 0,2 0,4 0,6 0,8

-1,0

-0,5

0,0

0,5

1,0

RuO2/MnO


in 0.65 M K2SO

4 aqueous solution

I / A

.g-1

E / V vs. Ag/AgCl

amorphous

300oC

600oC

Figure 7: Cyclic voltammograms of amorphous (300C) and annealed (600C) of (a)RuO2, (b) MnO2, (c) RuO2+MnO2 (oxide mixture), and (d) RuO2/MnO2 (co-precipitatedbinary oxides), recorded at 5mV/s in 0.65 M K2SO4(aq). Currents are normalized to thetotal mass of the composite electrodes.

The effects of the oxide heat-treatment temperatures on the cyclic voltammetry, on theanodic and cathodic potential limits, and on electrode specific capacitance were examined.Representative cyclic voltammograms for are shown in Figure 7.

For RuO2 composite electrodes (Figure 7a), heat-treatment of the active RuO2 material at300 and 600C led to a significant decrease in the current of the cyclic voltammograms.Generally, the high specific capacitance of amorphous RuO2 is attributed to proton inter-calation into the bulk material [11]. In contrast, crystalline RuO2, where only the surfaceis involved, gives low specific capacitances [6], as seen here. On the other hand, no signif-icant change in the CV occures when MnO2 is annealed at 300C. However, when MnO2is annealed at 600C, a significant decrease of CV current was observed. Furthermore, tworedox waves appear around 0.25 and 0.75 V. The nature of these two waves is not clear,but it could be related to two new manganese oxides, Mn2O3 and Mn3O4, which havepreviously been observed during the heat treatment of amorphous MnO2 [12, 13].

For annealed RuO2+MnO2 composite electrodes prepared by mechanical mixing (Fig-ure 7c), the voltammetric features of both single RuO2 and MnO2 oxides are observed.


For RuO2+MnO2 annealed at 300C, a slight decrease of the current is observed, as wasthe case for RuO2 (Figure 7a). The decrease is lower than that of the single RuO2 oxidedue to the presence of MnO2 which still has good capacitive properties. The compositeelectrode made with RuO2+MnO2 annealed at 600C shows the two redox waves at 0.25and 0.75 V as was seen for MnO2 alone (Figure 7b).

Cyclic voltammograms for annealed RuO2/MnO2 composite electrodes, prepared by co-precipitation (Figure 7d) differ significantly from those prepared by mechanical mixingof the oxides. For RuO2/MnO2 annealed at 300C, there is a small decrease in current.This decrease is less than the one observed for the RuO2+MnO2 composite electrodes. ForRuO2/MnO2 annealed at 600C, a substantial decrease of the capacitive area is observed,and the two redox waves have disappeared. This confirms the thermogravimetric analysisdata which suggested that the co-precipitated RuO2/MnO2 binary oxide is not a simplemixture of the two constituent oxides.

Variations in the anodic and cathodic potential limits of the cyclic voltammograms and thepotential windows are detailed in Table 9. A decrease in the potential window for RuO2and RuO2/MnO2 composite electrodes was observed following heat-treatment. Conversely,when annealed at 600C, the MnO2 and RuO2+MnO2 composite electrodes show an in-creased potential window, by over 300 mV. This increase seems related to the presence ofthe two redox systems. Possibly Mn2O3 and/or Mn3O4, formed from MnO2 during theheat treatment, produce wider potential windows.

The specific capacitance of the composite electrodes prepared with RuO2, MnO2, and bi-nary oxides are reported in Table 10. RuO2 experiences a drop in capacitance when an-nealed at 300C, and recovers somewhat when annealed at 600C, and the reasons forthis are not understood. MnO2 and composite electrodes exhibit monotonic decreases incapacitance at increasing anneling temperature.

Mixed oxide RuO2+MnO2 electrodes exhibit capacitance consistent with predicted values.Co-preciptated RuO2/MnO2 electrodes show a near-total loss of capacitance after treatmentat 600C, yet the drop in capacitance of the material treated at 300C is not as significant asthat seen for RuO2+MnO2. With the absence of peaks in the cyclic voltammogram, this isfurther evidence that the co-precipitated RuO2/MnO2 is not the same material as a simplemechanical mixture of RuO2+MnO2.

The results are consistent with the earlier postulates: (i) Annealing RuO2 leads to crystal-lization, which in turn inhibits proton intercalation, and (ii) co-precipitated RuO2/MnO2is not the same material the mechanical mixture of RuO2+MnO2. Finally, the annealingexperiments indicate that amorphous oxide has superior performance.


Tabl

e9:

Cat

hodi

c(h

ydro

gen

evol

utio

n)an

dan

odic

(oxy

gen

evol

utio

n)po

tent

iall

imits

and

the

pote

ntia

lwin

dow

(∆E

)fo

rdi

f-fe

rent

com

posi

tions

ofR

uO2,

MnO

2,R

uO2+

MnO

2,an

dco

-pre

cipi

tate

dR

uO2/

MnO

2el

ectr

odes

.

Pote

ntia

llim

itsfr

omcy

clic

volta

mm

etry

(Vvs

.Ag/

AgC

l)A

nnea

ling

RuO

2M

nO2

RuO

2+M

nO2

RuO

2/M

nO2

Con

ditio

nsE

cath

Ean

o∆

EE

cath

Ean

o∆

EE

cath

Ean

o∆

EE

cath

Ean

o∆

Eno

ne-0

.425

0.60

01.

025

-0.1

250.

800

0.92

5-0

.125

0.80

00.

925

-0.1

250.

800

0.92

530

0C

-0.2

500.

600

0.85

00.

000

0.90

00.

900

-0.0

500.

800

0.85

0-0

.050

0.80

00.

850

600

C0.

165

0.75

00.

915

-0.1

401.

150

1.29

0-0

.170

1.00

01.

170

-0.1

000.

650

0.75

0


Tabl

e10

:Ele

ctro

desp

ecifi

cca

paci

tanc

e(C

sp,E

,F/g

)an

dac

tive

mat

eria

lint

rins

icsp

ecifi

cca

paci

tanc

e(C

sp,M

,F/g

)fo

rdi

ffer

ent

com

posi

tions

ofR

uO2,

MnO

2,R

uO2+

MnO

2,an

dco

-pre

cipi

tate

dR

uO2/

MnO

2el

ectr

odes

.C

ompo

site

RuO

2M

nO2

RuO

2+M

nO2

RuO

2/M

nO2

RuO

2+M

nO2

(the

o)∗

Ele

ctro

deC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,MC

sp,E

Csp

,M

AM

-am

orph

ous

180

224

141

175

138

172

142

177

151

187

AM

-T30

031

3811

316

178

9711

514

392

130

AM

-T60

079

9840

4945

558

850

61∗

The

oret

ical

valu

eca

lcul

ated

from

RuO

2an

dM

nO2

spec

ific

capa

cita

nce

fora

bina

ryox

ide

com

posi

teel

ectr

ode

with

aR

u/M

nat

omic

ratio

equa

lto

1/3.


3.3.3 The effects of the electrolyte

The capacitance of Ru and Mn oxides in aqueous media has been widely studied. RuO2 hasbeen examined for the most part in acid such as H2SO4 [2,6,7] due to rapid proton insertionin the matrix, although some studies have been done in KOH [14]. The capacitance ofMnO2 has been examined in several different media, including K2SO4 [10,15], Na2SO4 [5,12, 16], H2SO4 [10], KCl [17], NaCl [13], and LiClO4 [18]. Binary systems of RuO2 andMnO2 have been studied in HCl + NaCl and H2SO4 electrolytes [9]. In the present study,80% RuO2/MnO2 electrodes were studied in the following media: 1.0 M HCl + NaCl,1.0 M NaOH, 0.5 M H2SO4, 0.65 M K2SO4, 0.5 M Na2SO4, and 0.5 M H2SO4 +Na2SO4.

Experiments in 1.0 M HCl + NaCl and in 1.0 M NaOH suffered from stability problems.In the former, the electrode material dissolved at open circuit. In the latter, it dissolved atanodic potentials.

The cyclic voltammetry the RuO2/MnO2 electrodes is presented in Figure 8. For electrodesin Na2SO4, only the capacitive contributions of RuO2 and MnO2 with Na+ are expected;the redox reaction of RuO2 with protons is not expected. The H2SO4 +Na2SO4 electrolytewas investigated to observe the capacitive contribution of RuO2 with both H+ and Na+.

In 0.65 M K2SO4, the RuO2/MnO2 composite electrodes are very stable (1400 cycles)between −0.10 and 0.75 V (Figure 8a). The composite electrodes are also stable in 0.5 MNa2SO4 over the−0.25 to 0.85 V window, as shown in Figure 8b. In 0.5 M H2SO4, a redoxcouple is observed at 0.55 V (Figure 8c). This redox system is attributed to the followingreaction [7]:

RuOx(OH)y +δH+ +δe− −−−− RuOx−δ(OH)y+δ

(2)

The decreasing current in Figure 8c is attributable to the slow dissolution of MnO2 only,since the RuO2 peak persists.

The mixed electrolyte H2SO4 +Na2SO4 system exhibits some voltammetric characteristicssimilar to those in the individual electrolytes. Most notable is the rapid decrease in current,indicating rapid dissolution of the electrode material. Consequently, H2SO4 +Na2SO4 is apoor electrolyte.

In summary, RuO2/MnO2 composites are very unstable in base, and somewhat unstable inacid. The electrodes are stable in media containing only Na+ or K+ ions, but the cyclicvoltammograms lack the strong capacitance that arises from the intercalation of protons inRuO2.

3.3.4 The effects of the current collector

Our previous annual report [1] discussed the use of titanium as the current collector. It wasconcluded that titanium could be nanostructured using different chemical and electrochem-


a)

-0,2 0,0 0,2 0,4 0,6 0,8

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

1,0

RuO2/MnO

2 / graphite / acethylene black / PTFE composite electrodes

in 0.65 M K2SO

4 aqueous solution

i / A

.g-1

E / V vs. Ag/AgCl

b)

-0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0

-2,5

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

RuO2/MnO


in 0.5 M Na2SO

4 aqueous solution

i / m

A.g

-1

E / V vs. Ag/AgCl

c)

0,0 0,2 0,4 0,6 0,8 1,0

-1,0

-0,8

-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

0,8

RuO2/MnO


in 0.5 M H2SO

4 aqueous solution

I / A

.g-1

E / V vs. Ag/AgCl

d)

-0,2 0,0 0,2 0,4 0,6 0,8 1,0

-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0

RuO2/MnO


in 0.5 M H2SO

4+Na

2SO

4 aqueous solution

i / A

.g-1

E / V vs. Ag/AgCl

Figure 8: Cyclic voltammetry of RuO2/MnO2 composite electrodes, containing 80% ac-tive material, recorded at 5 mV/s in (a) 0.65 M K2SO4, (b) 0.5 M Na2SO4, (c) 0.5 MH2SO4, and (d) 0.5 M H2SO4 + 0.5 M Na2SO4 aqueous solutions. Currents are normal-ized to the total mass of the composite electrodes.


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0,2

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0,6

0,8

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1,2

RuO2+MnO

2/graphite/acethylene black/PTFE composites electrodes

in 0.65 M K2SO

4 aqueous solution

i / m

A.g

-1

E / V vs. Ag/AgCl

Titanium collector

Stainless steel collector

Figure 9: Cyclic voltammograms of RuO2+MnO2 (oxide mixture) composites electrodes,containing 80% active material, with titanium or stainless steel current collectors, recordedat 5 mV/s in 0.65 M K2SO4 aqueous solution. Currents are normalized to the total mass ofthe composite electrodes.

ical processes. After a suitable electroreduction of the titanium oxides formed during thenanostructuring, the resulting material could be used as current collector.

All the previous data for RuO2/MnO2 composite electrodes were obtained by using a stain-less steel grid as the current collector. Figure 9 compares the cyclic voltammograms ofamorphous RuO2+MnO2 composite electrodes (containing 80% active material) in 0.65 MK2SO4 with either stainless steel or titanium current collectors.

Current densities obtained with stainless steel and titanium current collectors are the same.However, the potential window with a titanium current collector is wider, 1.05 V vs. 0.90 V.Furthermore, the cyclic voltammograms have similar shapes, indicating that there is no sig-nificant resistance in the titanium collector due to titanium oxides formed during cycling.These preliminary results are encouraging, and more experiments (such as impedance spec-troscopy) need to be carried out for a more detailed characterization of the current collectinterface.

3.4 ConclusionsThis section showed first that an improved protocol for the preparation of the compositeelectrodes (where thickness is controlled) yields more reproducible results. The study ofthe effect of the composition of those electrodes showed that the highest capacitance whereobtained in those containing 80% active material. However, this study demonstrated thatthe synthesis of RuO2/MnO2 active material using the co-precipitation method leads to ca-pacitances similar to those from a simple mechanical mixing of RuO2 and MnO2 powders,and that they are also similar to theoretical values. The co-precipitation method used for


RuO2 and MnO2 does not give a synergistic effect on specific capacitance and potentialwindows.

One possible limiting parameter could be the use of acetylene black and graphite as conduc-tive materials of the composite electrodes, since the potential windows seem to be limitedby the hydrogen and oxygen evolutions reaction on those two carbons. If this is the case,the effect of the combination of RuO2 and MnO2 on the potential windows may not beobservable and this could explain why the results differ from the literature [9]. A potentialfuture investigation could be the replacement of the acetylene black and graphite by someother conductive material.

The study of the effect of the heat treatment showed that the highest capacitances are ob-tained for the composites electrode made with amorphous materials. The heat-treatment ofamorphous RuO2, converting it to its crystalline form, is detrimental to all composite elec-trodes containing ruthenium. This study also confirmed previous thermogravimetric anal-yses which show that a RuO2/MnO2 powder prepared by co-precipitation is not a simplemixture of RuO2 and MnO2. XRD characterization will have to be performed on annealedRuO2, MnO2, and binary oxides to determine the structure of the materials.

The study of several aqueous electrolytes showed that the RuO2/MnO2 composite elec-trodes are only stable in K2SO4 and Na2SO4 electrolytes. The presence of protons or hy-droxide ions has a detrimental effect on the stability of MnO2 and RuO2. The optimizationof these solutions (e.g., concentration, pH) remains to be done.

Finally the use of titanium grid as the current collector with RuO2/MnO2 composite elec-trodes shows encouraging results, with a shape similar to those obtained with a stainlesssteel grid, but with a wider potential window. Additional experiments will be performed(electrochemical impedance spectroscopy, for example) to understand these findings better.

4 Q4: 01/2008 – 04/20084.1 IntroductionElectrochemical capacitors are grouped into three classes according to their active elec-trode material: (i) carbons which use double layer capacitance arising from the separationof charge at the interface between the electrode and an electrolyte [19–21], (ii) electroni-cally conducting polymers [19,22–24], and (iii) metal oxides [4,5,13,15,16,18,19,25–64].The last two rely on pseudo-faradic reactions. Among all the oxides studied, ruthenium ox-ide presents the highest specific capacitance. Capacitance values around 760 F/g have beenreported [62, 63], but high cost and environmental concerns limit its commercial applica-tion [65]. With its low cost, natural abundance, and environmental benignancy, amorphoushydrous manganese oxide (hydrous MnO2) is an attractive alternative to ruthenium oxidein capacitor applications. If one Mn atom in hydrous MnO2 is assumed to store one elec-


tron, the specific capacitance of hydrous MnO2 ought to be around 1370 F/g [16]. Butin practice practically, these oxides exhibit specific capacitance of only around one-fifththis value. The causes of this diminished capacitance stem from the material’s intrinsicallypoor electronic conductivity and dense morphology [5, 66]. Therefore, the challenge withthe hydrous MnO2 lies in maximizing its electrochemical utilization. Because of their ex-cellent electric conductivity and high surface area, carbon nanotubes are now often usedwith hydrous MnO2 to form nanocomposites. Recently, several MnO2/CNT nanocompos-ites [8,48,53,65,67–70] have been synthesised with the aim of improving the electrochem-ical utilization of MnO2 and overall conductivity of the electrode.

Conventionally, these nanocomposites are prepared in three steps: (i) the dispersion of car-bon nanotubes at the manganese oxide surface, (ii) the formation of composite film withan appropriate binder, and (iii) the assembly of the composite film on a current collec-tor. It is well known that the dispersion is rather difficult and the adhesion of nanotubesto the MnO2 matrix material still presents considerable challenges. Indeed, the effectiveutilization of CNTs in composite applications depends strongly on the ability to dispersethe CNTs individually and uniformly throughout the host matrix without destroying theirintegrity or reducing their aspect ratio. Therefore, the formation of a composite film isnot always straightforward and usually requires several trial-and-errors iterations to obtaina good composite film. Finally, experimental conditions must be found to obtain a lowcontact resistance between the metal oxide/CNT composite and the current collector.

This work aimed to overcome these problems by proposing a new strategy for the prepa-ration of the MnO2/CNT nanocomposite. For the first time, the direct redox depositionof manganese oxide on multi scaled CNT/microfiber carbon is reported. The resultingmaterial can be used as binder-less electrode for electrochemical capacitors.

4.2 Experimental4.2.1 Materials

Purified (> 90%) carbon nanotubes (Multiwall, O.D.×I.D.×length: 10–15 nm × 2–6 nm× 0.1–10 µm) that had been prepared by chemical vapor deposition (CVD) method werepurchased from Aldrich, and used without further purification. All other chemicals werepurchased from Aldrich and used as received. Deionized water (18 MΩ·cm, obtained froma Branstead Nanopure II unit) was used to prepare the solutions. Carbon paper (CP) waspurchased from Spectracarb (SC 2050 A), and used without further treatment.

CNTs (100 mg) were dispersed in 100 mL of ethanol by ultra-sonication (FS 30, FischerScientific) for 30 min, without any surfactant. A sheet of carbon paper (CP) was coatedwith carbon nanotubes by using dip-coating for 30 min. Afterwards, the sheets were driedin vacuo at room temperature. The resulting material is denoted as CP-CNT.

The solution for redox deposition (0.25 M KMnO4 + 0.5 M H2SO4) was prepared accord-


ing to literature methods [51]. The as-prepared CP-CNT and CP sheets were dipped in abeaker containing the freshly prepared KMnO4 + H2SO4 solution for 30 minuntes. After-wards, the samples were dried in vacuum at room temperature for 24 hours. Thereafter,the samples were cleaned with distilled water and dried in vacuo at room temperature. Theresulting materials are denoted as CP-MnO2 and CP-CNT-MnO2.

4.2.2 Characterization

The morphologies and chemical composition of the samples were examined by scanningelectron microscopy (SEM) and energy dispersive X-ray analysis (EDX) using a HitachiS-4300SE/N (VP-SEM) apparatus. Electrochemical measurements were carried out usinga three-electrode cell with the reference electrode and counter electrode being Hg/HgSO4(saturated K2SO4) and platinum foil, respectively. The mass loading (m) per unit of areaof electrode was 0.1, 0.45, and 0.15 mg/cm2 for CP-CNT, CP-MnO2, and CP-CNT-MnO2,respectively. The mass loading is the mass of CNT for CP-CNT and the mass of MnO2 forCP-MnO2 and CP-CNT-MnO2. In our experiment, the area of the samples were 2 cm2, halfof which was immersed in the electrolyte. All the experiments were conducted at 25C.

The electrolyte was 0.65 M K2SO4 for all experiments. It was degassed with N2 priorto electrochemical measurements and a blanket of the gas was maintained throughout thecourse of the experiments. A Solartron 1470 battery tester operated under Corrware IIsoftware (Scribner Associates) was used for the cyclic voltammetry measurements. Theelectrochemical impedance measurements were conducted in the constant potential modeby sweeping the frequencies from 100 kHz to 0.01 Hz range at an amplitude of 10 mVusing a Solartron 1255B Frequency Response Analyzer. The values of specific capaci-tance Csp (F/g) were estimated by integrating the cyclic voltammogram curve to obtain thevoltammetric charge (Q), and subsequently dividing this charge by the mass of the activematerial (m) and the width of the potential window ∆E:

Csp =Q

m∆E(3)

4.3 ResultsFigures 10a and 10b show SEM images of the bare microfibrous carbon paper which con-sists of a highly porous three-dimensional network of carbon microfibers, each having adiameter of about 5 µm. EDX analysis, Figure 10c, shows the presence of C and O atatomic concentrations 97% and 3%, respectively. Figure 11a shows an image of the mi-crofibrous carbon paper with coated carbon nanotubes. CNTs are deposited on the microfi-brous carbon paper, either as networks which form bridges between microfibers of carbonpaper (Figure 11b) or simply dispersed on the surface of the carbon paper (Figure 11c).The EDX analysis of the CNT network, shown in Figure 11d, also indicates the presenceof C and O at atomic concentration of 97% and 3%, respectively. It should be noted that no


a c

b

Figure 10: SEM images of (a) microfibrous carbon paper (CP), and (b) High magnificationimage of one microfibers. (c) EDX analysis of microfibrous carbon paper.

metal was detected, within the detection limit of EDX, confirming the low metal content ofthe CNTs.

Figure 12 compares the cyclic voltammetry of CP and CP-CNT electrodes in K2SO4 at20 mV/s. The CV of the CP-CNT electrode shows a rectangular shape over a 1 V range,and the current is 13 times greater than that of the bare CP electrode of an equivalentgeometric area. The CP-CNT electrode shows a low specific capacitance of about 10 F/g,which is comparable to CNT in the literature [20]. The specific capacitance of the CP-CNT electrode is unchanged when the scan rate is increased because the carbon nanotubesexhibit almost entirely electrochemical double layer capacitance, have low resistivity, anda highly accessible surface area.

The CP and CP-CNT electrodes were used as a substrate (and reducing agent) for thespontaneous formation of MnO2 from MnO–

4 ions. The SEM image (Figure 13) of a CP-MnO2 electrode shows an aggregation of MnO2 particles on the carbon microfibres. Thedensity of these aggregates is low and a higher magnification image (Figure 13b) revealsthat they have a diameter of around 1 µm. Since it is well known that the pseudocapacitivereaction of MnO2 is a surface process [16], small aggregates or thin films of MnO2 arevery desirable for maximum performance. The EDX analysis of the CP-MnO2 electrode(Figure 13c) shows that the atomic concentration of Mn is very low, only around 0.3%.


a

c

b

d

Figure 11: (a—c) SEM images of the nanocomposite carbon nanotubes carbon paper, and(d) EDX analysis.

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-0.00002

0.00000

0.00002

0.00004

0.00006

I(A

/cm

2)

E(V)

CP

CP+CNT

Figure 12: Cyclic voltammetry of CP and CP-CNT electrodes in 0.65 M K2SO4(aq) atν = 20 mV/s.


77

c

a b

Figure 13: (a–b) SEM images of the composite carbon paper manganese oxide (CP-MnO2), and (c) EDX analysis.


Presumably, microfibrous carbon paper is not a good reducing agent for MnO–4 anions.

The morphology of deposited manganese oxide on a carbon nanotubes/microfiber carbonsubstrate was also analyzed by SEM. The micrographs and EDX analysis of this electrodesystem are shown in Figure 14. At low magnification (Figure 14a), manganese oxidesare seen on the substrate. More importantly, the manganese oxide is concentrated on theCNTs. At higher magnification, Figure 14b, it is evident that the CNTs are well-coatedwith MnO2. Figure 14c shows a carbon microfibre coated with carbon nanotubes, actingas deposition sites for MnO2 aggregates. Higher magnification images (Figure 14d) con-firm that the CNTs are coated with manganese oxide. In Figure 14e, one can clearly see asingle CNT partially coated with manganese oxide, and the thickness of MnO2 layer canbe estimated at 10 nm. The deposition of manganese oxide on carbon nanotubes in thinlayers allows its maximum electrochemical utilization, which is very important for elec-trochemical capacitor applications. The EDX analysis (Figure 14f) reveals the abundanceof Mn to be 7.6%, much higher than the MN content of CP-MnO2. The atomic ratio ofO:Mn in CP-CNT-MnO2 is 2. Therefore, it is reasonable to conclude that the coating isamorphous manganese oxide. Chen and coworkers [51] have reported that immersing agraphite disc electrode in an acidic solution of KMnO4 results in the deposition of MnO2on the electrode surface. Based on these observations, it appears that the carbon nanotubesare the preferred sites for the reduction of MnO–

4 .

It is worth considering the mechanism of forming MnO2 from MnO–4 in the presence of

CNT and a carbon microfibre electrode. The spontaneous reduction of metal ions to metal-lic form on CNTs has been reported in aqueous solutions of noble metal ions [71]. Inaddition, the spontaneous formation of MnO2 from MnO–

4 ions on CNTs [61,65] has beenexplained by the difference in the reduction potential between the CNT and the MnO–

4ions [70]. The work function of CNTs has been determined to be 5 eV [8, 71]. Therefore,the Fermi level of CNTs is approximately 0.5 V above the potential of the standard hy-drogen electrode (SHE), which is well above the reduction potential of MnO–

4 ions, whichis +1.692 V vs. SHE. These relative potential levels provide a thermodynamic explanationfor the spontaneous electron transfer from the CNT to the MnO–

4 [8].

Figure 15 shows the CVs of CP-CNT, CP-MnO2, and CP-CNT-MnO2 in 0.65 M K2SO4(aq)at ν = 20 mV/s. The CV of the CP-CNT electrode is almost rectangular in shape due tothe double layer behaviour of CNT [20, 21], however the CP-MnO2 and CP-CNT-MnO2electrodes show broad redox waves. The CP-CNT electrode is characterized by a verylow current density. The current is somewhat larger for the CP-MnO2, electrode but thenanocomposite CP-CNT-MnO2 electrode shows the highest current density of the three.Hu et al. have reported similar observations for CNT-MnO2 electrodes prepared by a dif-ferent approach [72]. The CV of CP-MnO2 and CP-CNT-MnO2 electrodes exhibit anodicand cathodic waves centered at about 0.55 and 0.25 V vs. Hg/HgSO4, respectively. Thesewaves can be tentatively attributed to cation deintercalation upon oxidation and cation in-sertion upon reduction. Yuan et al. have observed these two redox peaks for MnO2 elec-


a

c

b

d

e

f

Figure 14: SEM images of the carbon paper/carbon nanotubes/manganese oxide nanocom-posite CP-CNT-MnO2


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0

2

4

6

I(A

/g)

E (V )

C P -C N T

C P -M n O2

C P -C N T -M n O2

Figure 15: Cyclic voltammogramms of CP-CNT, CP-MnO2, and CP-CNT-MnO2 in0.65 M K2SO4(aq) at ν = 20 mV/s.

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30

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CP-CNT-MnO2

I(A

/g)

E(V)

1 mV/s

5 mV/s

10 mV/s

20 mV/s

50 mV/s

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200 mV/s

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/g)

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20 mV/s

50 mV/s

100 mV/s

200 mV/s

Figure 16: Effects of scan rate on the cyclic voltammetry of (a) CP-MnO2 and (b) CP-CNT-MnO2 in 0.65 M K2SO4(aq) solution.

trode in LiOH electrolyte, and they attribute them to the insertion and extraction of Li+ inMnO2 with charge transfer at the electrode/electrolyte interface [73]. Brousse et al. havealso observed redox peaks for MnO2 electrodes in K2SO4 electrolyte, and these were alsoattributed to the deintercalation and intercalation of the cations [10].

Figures 16a and b show the effect of the scan rate on the CV response of CP-MnO2 andCP-CNT-MnO2 electrodes. An increase of the current with an an increase of the scan rateis observed for the two electrodes with no significant change in the shape of the cyclicvoltammograms at high scan rate. The specific capacitances of CP-MnO2 and CP-CNT-MnO2 electrodes are reported Table 11.

Table 11: Specific capacitance (F/g) as a function of scan rate

Scan Rate (mV/s)Electrode 1 5 10 20 50 100 200

CP-CNT-MnO2 322 219 180 150 118 97 79CP-MnO2 125 70 54 44 33 27 23


0.0 0.1 0.2 0.3 0.4 0.5

0

40

80

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240

a

q* (C

/g)

SR-1/2

(mV/s)-1/2

CP-MnO2

CP-CNT-MnO2

0 2 4 6 8 10

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b

1/q

* (g/C

)

SR1/2

(mV/s)1/2

CP-MnO2

CP-CNT-MnO2

Figure 17: Voltammetric charge q∗ as a function of sweep rate

From CV curve recorded at 1 mV/s, the peak specific capacitance of CP-CNT-MnO2 com-posite is 322 F/g, significantly higher than that of CP-MnO2, which is 125 F/g, and thisis true at higher scan rates as well. Csp drops off notably at higher scan rates, and this ispresumably due to high resistance (electric and/or ionic). The CP-CNT-MnO2 electrodesappears to be somewhat less vulnerable to this effect; it suffered a 75% loss in capacitanceat 200 mV/s, compared to 82% in the CP-MnO2 material. Similar results were found forrepeat experiments. The superior performance of the CP-CNT-MnO2 materials could beexplained by: (i) Better electrode conductivity resulting from the added CNTs, (ii) SmallerMnO2 aggregates on the CNTs, allowing for better electrochemical use of the materials,and (iii) a more porous structure that facilitates the flow of ions within the matrix.

The voltammetric charge data extracted from CV measurements performed at various scansrates can be analyzed by using the approach developed by Trasatti and coworkers [74, 75].The voltammetric charge q∗ has been found to decrease with increasing potential scanrate ν. The total voltammetric charge q∗T is defined as [74]:

q∗T = q∗o +q∗i (4)

where, q∗o and q∗i are charge related to outer (more accessible) and inner (less accessible)surface, respectively. If only reversible redox transitions take place, the effect of scan rateon q∗ is normally attributed to diffusion of charge-compensating ions into pores, cracksand grain boundaries (i.e., the inner surface) of the material. The extrapolation of q∗ ν = 0provides q∗T which is the total charge proportional to the whole active surface [75]. Ex-trapolation of q∗ to ν = inf yields q∗o, the charge proportional to the outer active surface.The extrapolation procedure, based on a phenomenological approach as discussed else-where [5, 74, 75], consists of plotting 1/q∗ vs. ν1/2 to obtain q∗T , and q∗ vs. ν−1/2 to obtainq∗o. This is shown in Figures 17a and 17b, respectively.

The plots are satisfactorily linear over a large range of scan rate. From the plots of Fig-ure 17, q∗T and q∗o may be determined. The contribution of the charge related to outer (more


0 200 400 600 800 1000 1200 1400

0

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-800

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-1400

a

Z''(

Oh

m)

Z'(Ohm)

CP-MnO2

CP-CNT-MnO2

0 1 2 3 4 5 6 7 8

0

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-3

-4

-5

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b

Z''(

Oh

m)

Z'(Ohm)

CP-MnO2

CP-CNT-MnO2

Figure 18: Electrochemical impedance spectra of CP-MnO2 and CP-CNT-MnO2 elec-trodes in 0.65 M K2SO4(aq) solution. DC voltage: 0 V vs Hg/HgSO4, frequency range100 kHz to 10 mHz. (a) Nyquist plots and (b) Zoom of the high-frequency region.

accessible) surface with respect to the total voltammetric charge were found to be to 13.4%and 21.8% for the CP-MnO2 and CP-CNT-MnO2 electrodes, respectively. In this electrodesystem the contribution of inner surface (less accessible surface) in the charge storage ismore important than that one of outer surface (more accessible surface). The increasingcontribution of outer surface from 13.4% for the CP-MnO2 electrode to 21.8% for CP-CNT-MnO2 electrode leads to the improvement of the specific capacitance from 125 F/g to322 F/g. Presumably, this electrode system can reach higher specific capacitance if moreouter surface is involved in the charge storage process.

Electrochemical impedance spectroscopy (EIS) measurements for CP-MnO2 and CP-CNT-MnO2 nanocomposite electrodes were performed at 0 V vs. Hg/HgSO4 and the resultingNyquist plots are presented in Figure 18a, with an enlargement of the high frequency regionin Figure 18b. Key features of the Nyquist plots are: (i) a semicircle in the high-to-mediumfrequencies region only for the electrode not containing CNTs, (ii) a short 45 region in themedium-low frequency range corresponding to the semi-infinite Warburg impedance stem-ming from ion penetration in the porous structure of the electrode [76–78], and (iii) at verylow frequencies, a vertical line which is due to the accumulation of ions at the bottom of thepores of the electrode [19]. The nearly vertical line demonstrates good capacitive behaviourwithout diffusion limitation. The semicircle reflects the sum of the electrolyte resistanceand the contact resistance between the active material and the current collector, i.e., theCP-CNT-MnO2 electrode has better electrical contact between the current collector andthe active material. The extrapolation of the low frequency vertical line of the impedanceto the real axis yields a value that is related to the conductivity of the electrode [19] and allthe other resistances in the system.

Recently, the direct redox deposition of MnO2 on carbon substrates was developed basedon the redox reaction between aqueous permanganate and either planar graphite [51] or


dispersed powders of high surface area carbons, including acetylene black [79], templatedmesoporous carbon powders [80], carbon aerogels [81,82], and carbon nanotubes [8]. Theaddition of CNT to the MnO2 matrix by a dispersion method has been shown to lead to animprovement of conductivity, but also to a decrease in the specific capacitance [48,68]. Thesimple nanocomposite synthesis method presented in this work not only to improves theelectrical conductivity but also increases the specific capacitance. This method can be usedto synthesise other metal oxide/carbon nanotubes nanocomposites, and the resulting ma-terials can be employed in electrochemical capacitors and insertion electrodes for lithiumion batteries.

4.4 ConclusionsTo summarize, new low-cost nanocomposite materials were created through a simple in situtechnique where manganese oxide was deposited on a carbon nanotube/carbon microfibresubstrate. There materials were examined for electrochemical supercapacitor applications.The substrate is an efficient reducing agent of MnO–

4 to MnO2, and superior to microfi-brous carbon alone. The specific capacitance of nanocomposite CP-CNT-MnO2 is as highas 322 F/g, more than double that of the simple CP-MnO2 material at 125 F/g. Electro-chemical impedance spectroscopy shows that the presence of carbon nanotubes improvesthe electrical contact between the active material and the current collector.


Symbols and Abbreviations

ν Scan rate

Csp Specific capacitance

Csp,E Electrode specific capacitance

Csp,M Active material specific capacitance

Q Charge

m Mass loading

q∗ Voltammetric charge

q∗T Total voltammetric charge

q∗o Outer surface charge

q∗i Inner surface charge

BET Brunauer, Emett, and Teller method of measuring surface area

CNT Carbon nanotube

CP Carbon paper

CP-CNT Carbon paper treated carbon nanotubes

CP-MnO2 Carbon paper treated with MnO2

CP-CNT-MnO2 Carbon paper treated with carbon nanotubes and MnO2

CV Cyclic voltammetry or voltamogram

DTA Differential thermal analysis

EDX Energy dispersive x-ray analysis

EIS Electrochemical impedance spectroscopy

PTFE Poly(tetrafluoroethylene)≡Teflon

SHE Standard hydrogen electrode

SEM Scanning electron microscope, microscopy, or micrograph

TGA Thermogravimetric analysis

XRD X-ray diffraction


References

[1] Chamoulaud, Gwenael and Belanger, Daniel (2007), Metal Oxide Materials andCollector Efficiency in Electrochemical Supercapacitors: First Annual Report,(CR 2007-233) Defence R&D Canada – Atlantic.

[2] Sato, Y., Yomogida, K., Nanaumi, T., Kobayakawa, K., Ohsawa, Y., and Kawai, M.(2000), Electrochem. Solid-State Lett., 3, 113.

[3] Zang, J., Jiang, D., B.Chen, Zhu, J., Jiang, L., and Fang, H. (2001), J. Electrochem.Soc., 148, A1362.

[4] Lee, H. Y. and Goodenough, J. B. (1999), J. Solid State Chem., 144, 220.

[5] Toupin, M., Brousse, T., and Belanger, D. (2002), Chem. Mater., 14, 3946.

[6] Zheng, J.P. (1999), Electrochem. Solid-State Lett., 2, 359.

[7] Dandekar, M.S., Arabale, G., and Vijayamohanan, K. (2005), J. Power Sources, 141,198.

[8] Ma, S. B., Ahn, K. Y., Lee, E. S., Oh, K. H., and Kim, K. B. (2007), Carbon, 45, 375.

[9] Fernandez, J.L., de Chialvo, M.R. Gennero, and Chialvo, A.C. (2002), J. Appl.Electrochem., 32, 513.

[10] Brousse, T., Toupin, M., Dugas, R., Athouel, L., Crosnier, O., and Belanger, D.(2006), J. Electrochem. Soc., 153, A2171.

[11] Rolison, D.R., Long, J.W., Lytle, J.C., Fischer, A.E., Rhodes, C.P., McEvoy, T.M.,Bourg, M.E., and Lubers, A.M. (2009), Chem. Soc. Rec., 38, 226.

[12] Nagarajan, N., Cheong, M., and Zhitomirsky, I. (2007), Mat. Chem. Phys., 103, 47.

[13] Reddy, R. N. and Reddy, R. G. (2004), J. Power Sources, 132, 315.

[14] Wang, Y.-G., Wang, Z.-D., and Xia, Y.-Y. (2005), Electrochim. Acta, 50, 5641.

[15] Brousse, T., Toupin, M., , and Belanger, D. (2004), J. Electrochem. Soc., 151, A614.

[16] Toupin, M., Brousse, T., and Belanger, D. (2004), Chem. Mater., 16, 3184.

[17] Lee, M.-T., Chang, J.-K., and Tsai, W.-T. (2007), J. Electrochem. Soc., 154, A875.

[18] Chin, S.F., Pang, S.C., and Anderson, M.A. (2002), J. Electrochem. Soc., 149, A379.

[19] Conway, B. E. (1999), Electrochemical Supercapacitors: Scientific Fundamentalsand Technological Applications, Kluwer Academic/Plenum Press.


[20] Pandolfo, A.G. and Hollenkamp, A. F. (2006), J. Power Sources, 157, 11.

[21] Frackowiak, E. and Beguin, F. (2001), Carbon, 39, 937.

[22] Rudge, A., Davey, J., Raistrick, I., Gottesfeld, S., and Ferraris, J.P. (1994), J. PowerSources, 47, 89.

[23] Fusalba, F., El Mehdi, N., Breau, L., , and Belanger, D. (1999), Chem. Mater., 11,2743.

[24] Naudin, E., Ho, H.A., Branchaud, S., Breau, L., and Belanger, D. (2002), J. Phys.Chem. B, 106, 10585.

[25] Fang, W. C., Chyan, O., Sun, C. L., Wu, C. T., Chen, C. P., Chen, K. H., Chen, L. C.,and Huang, J. H. (2007), Electrochem. Commun., 9, 239.

[26] Cottineau, T., Toupin, M., Delahaye, T., Brousse, T., and Belanger, D. (2006), Appl.Phys. A, 82, 599.

[27] Choi, D., Blomgren, G. E., and Kumt, P. N. (2006), Adv. Mater., 18, 1178.

[28] Kim, H. and Popov, B. N. (2002), J. Power Sources, 104, 52.

[29] Chang, K. H. and Hu, C. C. (2004), J. Electrochem. Soc., 151, A958.

[30] Soudan, P., Gaudet, J., Guay, D., Belanger, D., and Schulz, R. (2002), Chem. Mater.,14, 1210.

[31] Lee, H. Y., Manivannan, V., and Goodenough, J. B. (1999), C. R. Acad. Sci. Paris 2(serie II c), 13, 565.

[32] Wu, M. S. and Chiang, P. C. J. (2004), Electrochem. Solid-State Lett., 7, A122.

[33] Pang, S. C., Anderson, M. A., and Chapman, T. W. (2000), J. Electrochem. Soc.,147, 444.

[34] Lee, H.Y., Kim, S.W., and Lee, H.Y. (2001), Electrochem. Solid-State Lett., 4, A19.

[35] Hu, C. C. and Tsou, T. W. (2002), Electrochem. Comm., 4, 105.

[36] Jiang, J. and Kucernak, A. (2002), Electrochim. Acta, 47, 2381.

[37] Hu, C. C. and Tsou, T. W. (2002), Electrochim. Acta, 47, 3523.

[38] Jeong, Y. U. and Manthiram, A. (2002), J. Electrochem. Soc., 149, A1419.

[39] Broughton, J. N. and Brett, M. J. (2002), Electrochem. Solid-State Lett., 5, A279.


[40] Hong, M. S., Lee, S. H., and Kim, S. W. (2002), Electrochem. Solid-State Lett., 5,A227.

[41] Kim, H. and Popov, B.N. (2003), J. Electrochem. Soc., 150, D56.

[42] Hu, C. C. and Wang, C. C. (2003), J. Electrochem. Soc., 150, A1079.

[43] Chang, J.K. and Tsai, W.T. (2003), J. Electrochem. Soc., 150, A1333.

[44] Reddy, R.N. and Reddy, R.G. (2003), J. Power Sources, 124, 330.

[45] Brousse, T. and Belanger, D. (2003), Electrochem. Solid State Lett., 6, A244.

[46] Jones, D., Wortham, E., Roziere, J., Favier, F., Pascal, J.L., and Monconduit, L.(2004), J. Phys. Chem. Solids, 65, 235.

[47] Chen, Y. S., Hu, C. C., and Wu, Y. T. (2004), J. Solid State Electrochem., 8, 467.

[48] Zhou, Y. K., He, B. L., Zhang, F. B., and Li, H. L. (2004), J. Solid StateElectrochem., 8, 482.

[49] Chang, J. K., Chen, Y. L., and Tsai, W. T. (2004), J. Power Sources, 135, 344.

[50] Chang, J. K., Lin, C. T., and Tsai, W. T. (2004), Electrochem. Commun., 6, 666.

[51] Wu, M., Snook, G. A., Chen, G. Z., and Fray, D. J. (2004), Electrochem. Commun.,6, 499.

[52] Broughton, J. N. and Brett, M. J. (2004), Electrochim. Acta, 49, 4439.

[53] Pinero, E. R., Khomenko, V., Frackowiak, E., and Beguin, F. (2005), J. Electrochem.Soc., 152, A229.

[54] Wu, N.L., Wang, S. Y., Han, C. Y., Wu, D. S., , and Shiue, L. R. (2003), J. PowerSources, 113, 173.

[55] Wu, N. L. (2002), Mater. Chem. Phys., 75, 6.

[56] Wang, S. Y. and Wu, N.L. (2003), J. Appl. Electrochem., 33, 345.

[57] Prasad, K.R., Koga, K., and Miura, N. (2004), Chem. Mater., 16, 1845.

[58] Prasad, K.R. and Miura, N. (2004), Electrochem Commun., 6, 849.

[59] Lee, H.Y. and Goodenough, J. B. (1999), J. Solid State Chem., 148, 81.

[60] Liu, T. C., Pell, W. G., and Conway, B. E. (1999), Electrochim. Acta, 44, 2829.


[61] Kalu, E. E., Nwoga, T. T., Srinivasan, V., and Weidner, J. W. (2001), J. PowerSources, 92, 163.

[62] Zheng, J. P. and Jow, T. R. (1995), J. Electrochem. Soc., 142, L6.

[63] Zheng, J. P., Cygan, P. J., , and Jow, T. R. (1995), J. Electrochem. Soc., 142, 2699.

[64] Wu, M., Zhang, L., Gao, J., Zhou, Y., Zhang, S., , and Chen, A. (2005), J.Electrochem. Soc., 155, A355.

[65] Xie, X., , and Gao, L. (2007), Carbon, 45, 2365.

[66] Devaraj, S. and Munichandraiah, N. (2005), Electrochem. Solid-State Lett., 8, A373.

[67] Wang, G. X., Zhang, B. L., Yu, Z. L., and Qu, M. Z. (2005), Solid State Ionics, 176,1169.

[68] Subramanian, V., Zhu, H., and Wei, B. (2006), Electrochem. Comm., 8, 827.

[69] Huang, X., Pan, C., and Huang, X. (2007), Mater. Lett., 61, 934.

[70] Ma, S. B., Nam, K. W., Yoon, W. S., Yang, X. Q, Ahn, K. Y, Oh, K. H, and Kim,K. B (2008), J. Power Sources, 178, 483.

[71] Choi, H. C., Shim, M., Bangsaruntip, S., and Dai, H. (2002), J. Am. Chem. Soc., 124,9058.

[72] Wu, Y. T. and Hu., C. C. (2004), J. Electrochem. Soc., 151, A2060.

[73] Yuan, A. and a, Q. Zhang (2006), Electrochem. Commun., 8, 1173.

[74] Ardizzone, S., Fergonara, G., , and Trasatti, S. (1990), Electrochim. Acta, 53, 263.

[75] Pauli, C.P. De and Trasatti, S. (1995), J. Electroanal. Chem., 396, 161.

[76] de Levie, R. (1963), Electrochim. Acta, 8, 751.

[77] Keiser, H., Beccu, K. D., and Gutjahr, M. A. (1976), Electrochim. Acta, 21, 539.

[78] Macdonald, D. D., Urquidi-Macdonald, M., Bhakta, S. D., and Pound, B. G. (1991),J. Electrochem. Soc., 138, 1359.

[79] Ma, S. B., Lee, Y. H., Ahn, K. Y., Kim, C. M., Oh, K. H., , and Kim, K.-B. (2006), J.Electrochem. Soc., 153, C27.

[80] Dong, X., Shen, W., Jinlou, G., Xiong, L., Zhu, Y., Lu, H., , and Shi, J. (2006), J.Phys. Chem. B, 110, 6015.


[81] Fischer, A. E., Pettigrew, K. A., Rolison, D. R., Stroud, R. M., , and Long, J. W.(2007), NanoLett., 7, 281.

[82] Fischer, A. E., Saunders, M. P., Pettigrew, K. A., Rolison, D. R., and Long, J. W.(2008), J. Electrochem. Soc., 155, A246.


Distribution list


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DOCUMENT CONTROL DATA(Security classification of title, body of abstract and indexing annotation must be entered when document is classified)

1. ORIGINATOR (The name and address of the organization preparing thedocument. Organizations for whom the document was prepared, e.g. Centresponsoring a contractor’s report, or tasking agency, are entered in section 8.)

Universite du Quebec a MontrealDepartement de ChimieCase postale 8888, Succ. Centre-villeMontreal (Quebec) H3C 3P8

2. SECURITY CLASSIFICATION (Overallsecurity classification of the documentincluding special warning terms if applicable.)

UNCLASSIFIED

3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriateabbreviation (S, C or U) in parentheses after the title.)

Metal Oxide Materials and Collector Efficiency in Electrochemical Supercapacitors: SecondAnnual Report

4. AUTHORS (Last name, followed by initials – ranks, titles, etc. not to be used.)

Chamoulaud, G.; Bordjiba, T.; Belanger, D.

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April 2009

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Contract Report

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Defence R&D Canada – AtlanticPO Box 1012, Dartmouth NS B2Y 3Z7, Canada

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12sz07

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W7707-063348

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This report deals with the development of electrochemical supercapacitor based on MnO2 andbinary manganese and ruthenium oxides with the use of various current collectors. The binaryoxides were prepared and characterized by physicochemical and electrochemical techniques.The effect of the heat-treatment on the capacitance of the binary oxides was investigated aswell as the effect of the supporting electrolyte, the current collector and the composition of thecomposite electrode. Finally, carbon nanotubes coated on a carbon paper was used as supportfor the spontaneous formation of MnO2.

14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and couldbe helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such asequipment model designation, trade name, military project code name, geographic location may also be included. If possible keywordsshould be selected from a published thesaurus. e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified.If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.)

supercapacitor; ruthenium; manganese; mixed oxide; carbon; nanotubes

metal oxide materials and collector efficiency in...

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