faradaic and nonfaradaic mechanisms of electrochemical processes

Electrochimica Acta 46 (2001) 4083–4094

On the faradaic and non-faradaic mechanisms ofelectrochemical processes in conducting polymers and some

other reversible systems with solid-phase reagents

V.Z. Barsukov a,*, V.G. Khomenko a, S.V. Chivikov a, I.V. Barsukov b,T.I. Motronyuk c

a Kie� State Uni�ersity of Technologies & Design, 2 Nemiro�ich-Danchenko str., Kie� 02011, Ukraineb Superior Graphite Co, 4201 W. 36th Street, Chicago, IL 60632, USA

c National Technical Uni�ersity of Ukraine, 37 Pobedy prosp., Kie� 03056, Ukraine

Received 15 January 2001; received in revised form 15 May 2001

Abstract

The electrochemical peculiarities of classical redox systems with solid-state reagents (non-soluble quinones,intercalation compounds of graphite) as well as polyaniline-type conducting polymers have been considered. Theconducting polymers show a significant non-faradaic component of the electrochemical mechanism. The essentialdifferences of faradaic and non-faradaic systems in equilibrium behavior, trends of galvanostatic charge–dischargecurves and cyclic voltammograms have been shown, and criteria for the identification of these mechanisms areproposed. Our investigations of the current-producing mechanism for the polyaniline electrode have shown that atleast within a narrower range of potentials �En from 0.30–0.40 to 0.80–0.90 V versus SHE (depending on pH value)the ‘capacitor’ model of ion electrosorption/desorption in well conducting emeraldine salt phase is more preferable.Nevertheless, such a model should take into account the transport of both anions and protons (cations in a generalcase). Besides the possibilities of redox processes at the limits and beyond this range of potentials �En should be takeninto account. At the same time, these processes can lead to the fast formation of thin passive layers of new poorlyconducting phases (leucoemeraldine salt, leucoemeraldine base, etc.) near the current collector. The formation of suchphases, even in a small amount, rapidly inhibits and discontinues the electrochemical process. © 2001 Elsevier ScienceLtd. All rights reserved.

Keywords: Conducting polymers; Polyaniline; Quinones; Graphite; Mechanisms

www.elsevier.com/locate/electacta

1. Introduction

Reversible electrochemical systems with solid-phasereagents (for instance, non-soluble quinoid compounds,intercalation compounds of graphite or some carbonswith cations and anions, conducting polymers, etc.) aremore and more widely used in modern technology as

active materials for primary and rechargeable batteries,double-layer super-capacitors, electrochemical sensors,electro-chromic devices and so on.

That is why the investigation of the nature andmechanisms of electrochemical processes in such sys-tems has a significant fundamental and applied value.

One of the central problems in the investigation ofthe mechanisms in such systems is the identification offaradaic and non-faradaic criteria of the current-pro-ducing process, which is of great theoretical and ap-

* Corresponding author. Tel.: +380-44-2901603.E-mail address: [email protected] (V.Z. Barsukov).

0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0013 -4686 (01 )00715 -0

V.Z. Barsuko� et al. / Electrochimica Acta 46 (2001) 4083–40944084

plied importance for the optimal development of appro-priate devices. For example, in double-layer super-ca-pacitors the compounds with mainly non-faradaic(‘capacitor’) current-producing mechanisms are nor-mally used as active materials to enable fast charging–discharging and creation of capacitors with very highcurrent densities and specific power. In the effectiverechargeable batteries compounds with mainly faradaic(‘redox’) current-producing mechanism are used toachieve a relatively stable voltage during the charge andespecially discharge (so called ‘horizontal’ galvanostaticcharge–discharge curves) and a high specific energy.

The formal equations which describe the overall pro-cesses in such systems in terms of ‘redox’ and ‘capaci-tor’ mechanisms are quite similar:

S1+ne− +nH+�S3 (1)

S1+ne− −nA−�S2 (2)

where Si (i=1, 2, 3) is a solid phase; A− is an anion(AZ− in the general case); H+ is a proton (cation in thegeneral case).

However, real mechanisms are different. To elucidatethe difference needs answering the following questions:1. Are electrochemical charging–discharging processes

connected with mass (H+; A−) transfer across thephase boundary (the ‘redox’ model) or only withelectron and ion charge redistribution in the doublelayer (the ‘capacitor’ model)?

2. Does the proton form a strong chemical bond withthe solid phase S1 giving a new solid phase S3 ordoes only the formation of a hydrogen bond, elec-trostatic or other weak interaction take place?

3. Does the anion A− form a strong chemical bondwith the solid phase S1 or does only electrostaticinteraction take place?

Let us answer these questions as applied to the solid-state reagents under consideration.

The electrochemical process in quinoid compounds isaccompanied by transfer of protons across the phaseboundary and causes modification of the whole systemof chemical bonds in the solid phase:

(1a)

The initial quinone phase (S1) transforms into thecompletely new hydroquinone phase (S3). The equi-librium (stationary) electrode potential E° correspondsto the quinone/hydroquinone equilibrium. This equi-librium is described Eq. (1a), which is similar to Eq. (1).

No doubt has been found in literature that thesolid-phase electrochemical mechanism of reactions in

such slightly-soluble quinone derivatives as chloranil,duroquinone, anthraquinone-9,10, etc., has a faradaicnature (see [1–3] and references therein).

The electrochemical processes in graphite intercala-tion compounds (GIC) can involve not only cations H+,Li+, etc. (so called ‘donor-type’ of GIC) but also strongacids anions AZ− =SO4

2−, BF4−, etc. (so called ‘accep-

tor-type’ of GIC). Moreover, these anions are solvatedby the molecules of acid and/or H2O; thus, they arerather large particles.

The current-producing process for the acceptor-typeof GIC can be described by Eq. (2a), which is similar toEq. (2):

Cx+yHA−e− +A−�[CxAyHA] (2a)

The existence of so called ‘graphite salts’ [CxAyHA]was proved by X-ray phase, electrochemical and chemi-cal analysis [4–8].

Intercalation of graphite occurs over a wide CxArange: from C�A and up to the maximum attainabledegree of intercalation C2A through several sequentialstages: C96A, C72A, C48A, C24A. Certain peaks orwaves in cyclic voltammograms (CV) correspond toeach of these compounds [4,5].

To describe the mechanism of the current-producingprocess in conducting polymers (CP) is much moredifficult. The ratio of the faradaic and non-faradaiccomponents of the polymer charge–discharge processcontinues to be a point at issue.

Numerous spectral and spectroelectrochemical stud-ies published do not usually take into account equi-librium and galvanostatic behavior of CP and fullyignore the capacitor process. For example, in veryinteresting papers [9,10] in situ ESP and UV–Vis spec-troelectrochemical studies of polyaniline (PAN) filmswere carried out. The theoretical base for both papersserved the model based on interaction of polaronic andbipolaronic states and on the description of soliton–soliton interactions derived by Brazovskii and Kirova[11].

In Ref. [10] there were pointed out three phenomenagiving a shift of the driving force during the chargingprocess: (a) the distribution of different conjugationlengths in the electrochemically prepared conductivepolymers due to some structural units different fromthe linear model; (b) the interaction between polaronicand bipolaronic segments in the chain and (c) theacid/base equilibrium connected with a change of thepH value inside the polymer layer during charging.Nevertheless, the authors concluded that the processes(a) and (b) cannot describe the large peak separation inthe cyclic voltammograms and the hysteresis in thespectroscopic signals. The third effect (c) should bedepressed at low pH values, but the current plateaudoes not disappear with of 0.1 M sulphuric acid added.

V.Z. Barsuko� et al. / Electrochimica Acta 46 (2001) 4083–4094 4085

It seems to be not very convincing to explain anelectrochemical mechanism and peculiarities of CP elec-trochemical behavior using only ESP/UV–Visspectroelectrochemical studies and the polaronic/bipolaronic approach, which we try to show below.

Let us suppose, for example, that reactions (1) and(2) in CP proceed not by ‘redox’, but only by the‘capacitor’ mechanism. In this case, only electron andion charge redistributions will take place in the doublelayer, solid and liquid phases of electrochemical systemunder applied external potential E.

If the current collector receives a negative charge, itcan lead to the formation of anion-radicals in thesolid-phase facing of the dl capacitor according tofollowing equation:

S1+ne−�S−1 (3)

At the same time, the cations will be attracted to thisnegative facing S1

− and the anions will be withdrawnfrom this one in a liquid part of such a dl capacitorduring its charging–discharging.

Of course, an efficient ESP/UV–Vis spectral signalcan be recorded, which corresponds to the formation ofthe anion-radical S1

− and change in the electronic stateof the solid phase S1. Nevertheless, such a signal doesnot prove the redox mechanism of the electrochemicalprocess, because such charging–discharging processesare not accompanied with mass transfer (H+; A−)across the phase boundary.

Polaronic/bipolaronic models (see, for instance [11])can often be useful to account for the nature of conduc-tivity in some solid-phase systems. In our opinion,however, it is not necessary to use such quasi-particlesfor the description of systems with poly-�-conjugatedbonds like some forms of PAN (emeraldine salt), whichhave high electronic conductivity. The conductivity ofsuch forms can be explained in most cases by analogyto that for graphite due to the possibility of �-bonddelocalization within the limits of the whole macro-molecule. It is interesting to note that the electronicinteraction between intercalated ions and carbon layersof graphite leads to substantially increasing specificconductivity in the direction which is parallel to theorientation of the carbon layers [12]. The appearance ofnoticeable conductivity in CPs just after doping by ionshas, probably, a similar nature.

Feldberg [13] was among the first scientists who drewattention to the necessity of taking into account thecapacity of the double electric layer (dl). In [13] it wassuggested that peaks in CV should correspond to redoxreactions and the flat region corresponds to the dlrecharge. However, this theoretical model does notexplain the separation and large asymmetry of CVpeaks, does not fit the experimental data even at smalldeviations from the specific conditions formulated inhis model.

Based on investigations by impedance and CV meth-ods, Tanguy and Hoclet proposed a model of chargeredistribution in a polymer [14]. During the electro-chemical cycling of a polymer film, which is accompa-nied by the intercalation and withdrawal of anions, thepolymer becomes divided into aggregates 6 nm in di-ameter. The capacity of the dl that is formed in thiscase reaches almost 200 F/cm2. In terms of this ap-proach the capacitive behavior of CPs can be onlydictated by their porosity and conductivity. Volfkovichet al. [15] established the porous structure of PANusing the standard porosimetry method (SPM). A highfilm porosity (several tens of per cent) and large specificsurface areas of 80–400 m2/cm3 were demonstrated. Itwas confirmed that high dl capacity values are observedin CPs.

Ref. [16] proposed a ‘pure capacitor’ model of cur-rent-producing process in PAN. This model explainswell the trend of galvanostatic charging–dischargingcurves and CVs of PAN in the main working potentialrange and allows one to estimate theoretically a maxi-mum possible dl capacity Cmax�400–800 F/g. It wasalso shown that in the case of a slightly narrower of thepotential sweep range �En�En from �0.30–0.40 V to0.80–0.90 V versus SHE (depending on pH value) allpeaks on CV disappear. The experimental CV in such arange has shown a shape close to a parallelogram withtwo smoothed over angles, which corresponds ideally tothe theoretical one. It seems to convincing proof thatthere is an absence of redox processes at least over the�En range of potentials. Moreover, a very simplifiedmodel proposed in Ref. [16] makes it possible to ac-count for the two sharp peaks in the CV over acomplete possible potential range, which are connectedwith a sharp change (by several orders of magnitude[17]) in PAN resistance at the limits of the completepotential range �Emax. However, there exist somedifficulties in describing more diffuse small CV peaks inCV and trends of galvanostatic charging–dischargingcurves in the whole �Emax range as well as in somespecial conditions of testing. It stimulates further inves-tigation into the nature of the electrochemical process,first of all, outside the �En range.

It can be supposed that the defined theoretical modeladequately characterizes the electrochemical behaviorof the system over the complete possible potential range�Emax if it is capable to explain the following minimumnecessary complex of experimentally observable proper-ties of the system with a solid-state reagent:1. electrochemical equilibrium (the value and nature of

equilibrium (stationary) potential E°; character ofE° variation with time and the degree of systemoxidation/reduction);

2. trend of cyclic voltammograms;3. trend of galvanostatic charge–discharge curves.


Properties (1)– (3) were usually reported as studied withdifferent theoretical and applied goals. This is why itappears to be practically impossible to find in referencesall the properties (1)– (3) simultaneously for the sameobject in the similar conditions.

That is why the paper goals are as follows:1. to consider this main complex of properties for

various typical systems, which can be described byEqs. (1) and (2);

2. to determine what mechanism corresponds to theappropriate complex of the properties (1)– (3).

2. Experimental

Electrochemical measurements were performed byusing three-electrode electrochemical cells. The refer-ence electrode was an Ag/AgCl electrode; a Pt wire wasthe counter electrode.

The cyclic voltammetric measurements were carriedout on a PI-50-1 potentiostat. The chrono-amperomet-ric experiments were carried out using an automaticbattery test system with computer-aided recording ofresults.

Twice-distilled water was used to prepare all solu-tions. All standard chemicals were analytical reagentgrade and were used without further purification.

2.1. Technique for the preparation and in�estigation ofanthraquinone and graphite electrodes

For investigations of anthraquinone-9,10 or graphiteactive mass, we have developed a special ‘plug’ type ofworking electrodes. An active mass was pressed into apolyethylene plug of 11 mm inside diameter. Beforepressing, a spiralled thin platinum wire was insertedinto an opening, pierced in a plug wall.

For the preparation of an acti�e anthraquinone-9,10mass we used a mixture of 50 wt% anthraquinone-9,10

and 50 wt% acetylene black. A ‘plug’ type an-thraquinone electrode contained MAQ=200 mg of ac-tive mass.

The acti�e graphite mass consisted of natural graphiteflakes (88 wt%) from Kropfmuhl Normalflocke (Mu-nich) and acetylene black (12%). Acetylene black wasused for increasing of porosity in the electrode due toits more dispersed structure. Before pressing into thepolyethylene plug 1.5–2.0 wt% PTFE emulsion wasadded to the active mass. A ‘plug’ type graphite elec-trode contained MC=280 mg of active mass.

We used H2SO4 or HBF4 as a working electrolyte forthe ‘plug’ type of anthraquinone and graphiteelectrodes.

2.2. Technique for the preparation and in�estigation ofPAN electrodes

Aniline was purified by simple distillation and thefraction (colorless) boiling at 180 °C was used for theformation of the polymer. PAN was precipitated usingmultiple cyclic voltammetry (�=100 mV/s) in the range0.0–1.0 V versus SHE. Polymer films were preparedusing 0.1 M of aniline in 1 M CF3COOH acid solution.The thickness of the films was estimated consideringthat films with a passed charge of 0.1–1.0 mC/cm2 forthe 0–300 mV voltage range in 1 M HCl solution havea thickness of about 200–1000 A� .

The working electrode was Pt, the geometric areabeing 0.2 cm2. The useful volume of the cells was 2 ml.

Cyclic voltammetric and chrono-amperometric exper-iments were carried out at 20 °C in an acidified (pH�0.5) 1 M KCl solution. The acidification of thesolution was performed using CF3COOH.

3. Results and discussion

3.1. Anthraquinone-type quinoid compounds

Anthraquinone-type quinoid compounds belong tothe group of quinones, which are practically non-solu-ble in aqueous solutions (solubility less than 10−8 M/l).

The stationary potential of electrode in 1.5 M H2SO4

is E°�0.05 V versus Ag/AgCl. Moreover, an electrodeacquires this potential in any case when the current isswitched off.

Fig. 1 shows the typical CVs for the anthraquinoneelectrode in 1.5 M H2SO4 during the first 300 cycles ofpotentiodynamic charging–discharging. CV has twosharp peaks. E° can be easily determined from the CV.The ‘background’ current is negligible.

Fig. 2 shows typical galvanostatic charge–dischargecurves for the anthraquinone electrode taken at thecurrent density j=5 mA/cm2. The charge and dischargepotentials demonstrate a very high stability. The charge

Fig. 1. Typical experimental CVs for the anthraquinone-9,10electrode in 1.5 M H2SO4 during the first 300 cycles ofpotentiodinamic charging–discharging. Sweep rate �=10 mV/s. MAQ=200 mg. E vs Ag/AgCl electrode.


Fig. 2. Typical experimental galvanostatic charge–dischargecurves for the anthraquinone-9,10 electrode in 1.5 M H2SO4

taken at the current density j=5 mA/cm2. MAQ=200 mg. Evs Ag/AgCl electrode.

is switched off, the electrode potential moves to itsstationary value E° in this solution.

The same conclusion about the reversible intercala-tion potential Eint�1.55 V can be made from CVs forgraphite electrode (see Fig. 3).

It follows from Fig. 3 that if the maximum oxidationpotential Emax is higher (intercalation process), a higherdeintercalation peak current and discharge capacity willbe obtained by reduction process. In any case thereversible intercalation potential Eint corresponds to thestationary potential value E°�1.55 V.

As follows from the relatively high positive potentialof intercalation, a certain rate of oxygen formation isalways coupled with the intercalation process. The pres-ence of these two processes is clearly seen from com-parison of galvanostatic charge and discharge curves(Fig. 4).

The first plateau in the charge curve corresponds tothe process of BF4

− intercalation into graphite and thesecond one to the process of oxygen formation from theelectrolyte. The plateau in the discharge curve corre-sponds to the process of BF4

− deintercalation fromGIC.

It is very simple to estimate the stationary potentialvalue E°�1.55 V from the galvanostatic charge–dis-charge curves too.

Thus, we can also observe all features (a), (b) and (c)of the faradaic mechanism during the current-produc-ing process at the graphite electrode, which occurs inaccordance with reaction (2a).

3.3. Polyaniline-type conducting polymers

3.3.1. Ion exchange in the absence of oxygenElectrochemical equilibrium has been investigated by

taking chrono-potentiometric curves at a PAN elec-trode without the presence of oxygen in the cell, aftergiving the electrode a certain charge (Fig. 5).

It is well known that when the current is switchedoff, the potential of redox electrode tends to returnback to its stationary value. In contrast to this ‘classi-cal’ variant, in the case of a PAN electrode, afterswitching off the current, the potential settled at a valuewhich was practically the same as that achieved at themoment of charging–discharging (Fig. 5a). So, PANdoes not display the potential of any own redox equi-librium in the absence of oxygen. Along with this, whenthe PAN electrode acquired even a small quantity ofelectricity Q, the value of stationary potential E°(Q)considerably changed (Fig. 5b).

The curves obtained under the above conditions (Fig.5a, b) possess one more feature: after passing a certainquantity of electricity in one direction and the samequantity in the reverse direction, we come to the initialpotential value. Thus, the experimental curves shown inFig. 5 represent a quasi-equilibrium dependence of

Fig. 3. Typical experimental CVs for the graphite electrode in4 M HBF4; �=50 mV/s; MC=280 mg. E vs Ag/AgCl elec-trode. Emax=1.6 (1), 1.7 (2), 1.8 (3), 1.9 (4), 2.0 V (5).

and discharge capacities are equal to each other. Thestationary potential can be determined from thesecharge–discharge curves too, and is E°�0.05 V.

Thus, it is clear that the current-producing process atthe anthraquinone-9,10 electrode occurs in full accor-dance with the faradaic mechanism, which can be de-scribed by equations similar to Eq. (1a).

The main electrochemical proofs of such faradaicprocess are as follows: stable and easily determinableequilibrium (stationary) potential E°; sharp peaks inCV on both sides of E°; ‘horizontal’ charge–dischargecurves on both sides of E°.

3.2. Graphite intercalation compounds

Let us consider, for example, the formation of accep-tor-type GIC with tetrafluoroborate anions.

The stationary potential of GIC electrode in 4 MHBF4 is E°�1.55 V versus Ag/AgCl. When the current

Fig. 4. Typical experimental galvanostatic charge–dischargecurves for the graphite electrode in 4 M HBF4 taken at thecurrent density j=3 mA/cm2. MC=280 mg. E vs Ag/AgClelectrode.


Fig. 5. Plots of the PAN electrode potential vs time with constant current in the system (1) and with the current switched off (2,3) (a); the stationary potential vs the degree of charging Q/Qmax (b).

electrode potential on the degree of polymer chargingQ/Qmax.

During the common electrochemical redox process,the degree of charging Q/Qmax is a measure of the ratioof concentrations (activities) of the oxidized and re-duced forms. For instance, if we assume that duringcharge–discharge the quantity of passed electricity isspent only on changing the degree of polymer oxidation(which is quite reasonable since in the potential range0–0.9 V there are no side processes at the electrode inthe absence of oxygen), the standard potential E° mustobey the classical Nernstian equation. Logarithmic de-pendence of the potential E° on the Q/(Qmax−Q) areknown to correspond to this theoretical equation,which does not agree with the experimentally observedlinear dependence E° versus Q/Qmax (Fig. 5b). Besides,as follows from Fig. 5b, when the degree of oxidationof polymer is changed by a factor of two, the electrodepotential changes by about 450 mV. If the system weredescribed by the Nernstian equation, a change of noless than seven orders of magnitude in the degree ofoxidation of polymer should correspond to this poten-tial change.

The PAN electrode potential shifts proportionallywith the quantity of electricity Q, which is typical forany capacitor. These results are in a good agreementwith the proposed capacitor model [16] and are, in ourview, difficult to explain in terms of redox processes inPAN.

Nevertheless, some redox reaction can proceed withinthe practically important potential range at a PANelectrode in an oxygen atmosphere. The effect of thecatalytic reduction of air oxygen on a PAN film elec-trode was established for the first time in Ref. [18]. To

make this effect manifest, corresponding conditionsshould be created.

3.3.2. Ion exchange under oxygen atmosphere atequilibrium and at low rates of process

If the special measures to withdraw oxygen fromsolution are not taken, a PAN electrode in a salt andacid solution (for instance in ZnCl2 at pH 5) usuallyshows a stationary potential of about 0.40 V (versusSHE).

The discharge curve at a low current density ( j�0.5mA/cm2) exhibits a plateau of non-estimated length inthe same range of potential (Fig. 6).

Taking into consideration basically possible ranges ofoxygen reduction equilibrium potentials, it is possibleto assume that electrochemical process normally occurswith hydrogen peroxide formation (E°�0.3–0.5 V for

Fig. 6. Galvanostatic discharge curve for a thin PAN filmunder conditions of good oxygen supply, j=0.25 mA/cm2.


Fig. 7. Typical experimental galvanostatic discharge (1) andcharge (2, 3) curves for a PAN electrode at j�1 mA/cm2.Curve 3 recorded on a PAN electrode with the initial treatingat the potential E=0 V. The curves obtained in a 1 Maqueous solution of KCl and CF4COOH (pH 0.5).

Experiments show that in the film PAN electrode thecurrent density (cd) of oxygen reduction is usually lessthan 0.5–1.0 mA/cm2. That is why an optimal carbonsubstrate with a high conductivity, stability to oxida-tion, low density and sufficient specific surface is neces-sary for developing a gas-diffusion porous electrodewith a practically interesting cd (a few dozen of mA/cm2 depending on potential). PAN composites withdifferent types of graphite and carbon blacks wereprepared and selected. The best results in such com-posites were demonstrated by thermally exfoliatedgraphite from Superior Graphite Co., Chicago, IL.

By the way, the above results can find practicalapplication for development of PAN/exfoliatedgraphite catalysts in quite efficient air–metal powersources [20]. Specific energy attained in mockups ofair–Zn and air–Mg batteries is about 150–200 W·h/kg.The discharge curve of such batteries is practicallyhorizontal since it is determined by the oxygen reduc-tion potential.

3.3.3. Gal�anostatic charge–discharge cur�es in anyatmosphere at the real currents

At discharge (charge) current density values of j�1mA/cm2, the electrode potential decreases (increases)with time practically linearly both in the absence and inthe presence of oxygen in the main working range ofpotentials 0�E�0.9 V (Fig. 7, curves 1, 2).

This effect can be easily explained by the fact that theshare of oxygen reduction current is usually much lessthan that of the main current for the electrode charg-ing–discharging process.

According to the ‘capacitor’ concept [16], the maincurrent-producing process is bound up with thecharge–discharge capacity of the inner double electriclayer, which is abnormally high in the system underconsideration due to the peculiarities of its structure.

For instance, under the positive electrode potentialanions from a solution are incorporated in the structureof polymer. In the same time, cations inside polymericfilm go out from polymer in a solution.

There is only electrostatic interaction between anionsand solid state. Thus, it is not necessary to apply greatenergy (or voltage) for the effect of such a electrosorp-tion–desorption process, which occurs with a very lowpolarization. By the way, that is why the conductingpolymers can find an effective application for extractionof ions out of sewage [21].

A deviation from the linear shape of charge–dis-charge curves is observed at the limits and outside ofthis range (Fig. 7).

We can observe an appearance of sharp electrodepassivity outside a working range of potentials 0�E�0.9 V both with discharging (1) and with charging (2).Moreover, we performed the initial treating of the PANelectrode at the ‘edge’ potential E=0 V. For such a

usual oxygen concentrations in the solution). Such atheoretical assumption was confirmed experimentallyusing the reaction of H2O2 with KI solution [19]. Thisqualitative reaction proved the formation of H2O2 inthe solution. H2O2 concentration increases with timeand on application of current. Addition of KI indicatorto the electrolyte confirmed that the polymer electrodeis the source of H2O2 formation.

In the case of a good oxygen supply to the PAN filmand relatively slow processes, CVs taken at sweep po-tential rate �=1 mV/s exhibit an external pair of peakswith the average potential in between about 0.40 V.Nevertheless, this pair of peaks can not be recorded atmore rapid sweep potential rates (for instance, at �=10mV/s). This effect can be easily explained by the factthat the share of oxygen reduction current is usuallymuch lesser than that of the current for the meanprocess of charging–discharging of electrode. However,decrease of sweep potential rate makes this processmore pronounced.

The phenomenon of catalytic reduction of air oxygenat PAN film observed, first of all has a great signifi-cance for understanding the mechanism of current-pro-ducing processes in conducting polymers. The pair ofpeaks (with average potentials of about 0.40 V) thatappear at slower sweep potential rates must not beinterpreted from the viewpoint of main redox processof charging–discharging the electrode.

It is possible to explain the phenomenon of PANcatalytic activity in terms of the electronic theory ofcatalysis by Pisarzhevsky and the ‘capacitor’ concept[16]. Let us suppose that the system of poly-�-conju-gated bonds for a conductive form of PAN persistsunchangeable during the electrode charge–discharge.Such a system is characterized by the possibility of easydelocalization of electrons. An oxygen molecule can be,for instance, easily ionized under the sorption on such aconductive surface. This creates conditions for PANcatalytic activity towards oxygen reduction.


case an additional delay of a potential (plateau) isrecorded during the process of charging (curve 3).

So, a further development of a model [16] for wholepotential range �Emax is necessary for explanation ofthese ‘edge effects’.

3.3.4. Cyclic �oltammograms of a PAN electrode in awhole potential range

Cyclic voltammetry is widely used to study conduc-tive polymers. The shape of CV is, first of all, a test forthe quality of polymer films and characterized well thecharge–discharge process.

A typical CV for our PAN film in a 1 M aqueoussolution of KCl and CF4COOH (pH 0.5) is shown inFig. 8. The curve obtained by us on a PAN electrode issimilar to CVs that are reported by many investigators.This corroborates the fact that our PAN is similar inchemical nature. A feature peculiar to the CV is thesteep anodic peak at the onset of the process. A flatplateau follows after that peak.

During backward potential sweep, a cathodic peakwith a potential shift relative to the anodic peak ap-pears at the end of the plateau. The current magnitudeof the cathodic peak is lower than that of the anodicpeak (the areas of the anodic and cathodic portions ofthe CV being approximately equal). At more negativepotentials, the currents are practically zero, and thereare no maxims because the film has a low electronicconductivity in this potential range [22].

This behavior is not typical of ordinary redox films.The CV has a relatively large plateau with approxi-mately constant current magnitude and exhibits a po-

tential shift between the cathodic and anodic peaks aswell as an asymmetry of the peaks.

To elucidate the nature of the processes that occur inthe polymer film during discharge and charge, it isnecessary to answer the following questions:1. What processes take place in CV?2. How do they affect the electroactivity of polymer?3. What factor predominates?Examination of the CV in Fig. 8 allows three pairs ofpeaks to be distinguished.

When discussing the electrochemical behavior ofPAN, it should be noted that the second pair of peaks(E�0.7–0.8 V versus SHE) is not specific for PANproper. Its appearance is marked by the beginning ofloss of PAN electroactivity, which is observed onoveroxidation. In an aqueous medium, degradationleads to the formation of soluble products, which maybe a quinone/hydroquinone couple [23]. Ref. [24] sug-gests, on the basis of theoretical considerations, thatelectroactive oligomers with a length of the order of tenaniline members may exist in this potential range. Theappearance of the peak can be avoided by removingthese oligomers.

The study of the third pair of peaks (E 0.9–1.1 Vversus SHE) is difficult because an irreversible oxida-tion of PAN is observed on cycling, which also givesrise to a loss of polymer electroactivity.

In view of that said above, we shall consider theelectrochemical behavior of a polymer film in the work-ing potential range 0–0.9 V versus SHE, where there isa so-called ‘window’ of PAN stability. In this potentialrange, PAN can be cycled many times, for there are noside processes, which lead to degradation.

3.3.5. Theoretical models and additional experimentsThe fact that emeraldine salt is the only electrically

conductive form of PAN is considered now to beexactly established. Therefore, when analyzing variousprocess schemes, one should pay special attention to thefact that during any redox process a practically non-conducting phase must be formed. The electric fieldstrength in the solid phase, which is the driving force ofthe process in similar systems, is maximal near thecurrent collector and decreases rapidly with distance[25,26]. Therefore, if the faradaic process is initiated atdefinite potentials, it will occur at the highest rate nearthe current collector. As soon as the adjacent polymermonolayer (or several monolayers) changes to practi-cally non-conducting state, the circuit opens, and fur-ther occurrence of the electrochemical process isimpossible.

Therefore, the predominant process in the electro-chemical behavior of PAN must be, in our view, dlcharge–discharge, and the share of the faradaic processbe very small since it gives a sharp decrease of polymerelectroactivity.

Fig. 8. Typical CV recorded on a PAN electrode with a scanrate �=100 mV/s in a 1 M aqueous solution of KCl andCF4COOH (pH 0.5).


Fig. 9. The scheme of positive/negative ions transfer duringthe charging of polymer which takes into account the forma-tion of the poorly conducting layer near the current collector.

The nature of the electrochemical behavior of PANcan be approximately described by the scheme reportedin our previous work [16], which is, nevertheless to besupplemented with the following considerations:1. taking into account the possibility of formation of a

poorly conducting layer near the electrode surface;2. taking into account the fact that dl charge–dis-

charge can result both from proton and anion trans-port in the polymer film [27,28].

A basic feature of this dl formation model is that eachof the polymer chains in principle can be regarded as amicroelectrode. Thus, dl is formed at a molecular level.

Taking into account the scheme in Fig. 9, the equiva-lent circuit of a thin PAN electrode can be presented byfollowing an electric equivalent (Fig. 10). The poorlyconducting very thin layer in the substrate acts as avariable resistor or even as a switch in such anequivalent.

Let us analyze the main peculiarities of the electro-chemical behavior of a PAN electrode, which followfrom the above models.

It is easy to understand that for galvanostatic condi-tions the function E(t) must be linear for the mainworking potential range. A deviation from linear shapeobserved at the potentials of about 0 and 0.9 V, is dueto an holmic drop since when these potentials arereached, the poorly conducting layer fully blocks thecurrent collector surface. It should be noted that whenthe current is switched off, the potential reached by theelectrode remains constant.

Taking into account the capacitive nature of currentvariation in CV, we calculated the current traversingthe capacitor C as a function of potentials. This currentIc is called ‘capacitive’ current. In Fig. 11 we plotted thecurrent obtained by cyclic voltammetry and the previ-ously calculated current Ic from equation [29]:

Ic=Cd�/dt [1−exp(− t/RC)] (4)

The striking point is that the capacity curve beingsymmetric in the oxidation and reduction processes, the‘capacitive’ current is symmetric, too. On the contrary,the ‘non-capacitive’ Inc current exhibits a large hys-teresis. This can be interpreted in terms of the forma-tion of an insulating monolayer near the electrodesurface which leads to the disconnection of the bulk ofpolymer from the electrode. Therefore, in the case oflinear sweep in the anodic direction, a �E potentialshift is needed to switch the insulating layer to theelectrically conducting state.

It follows that the capacitor C is connected withmore positive potentials, in which case a jump in cur-rent must arise. It is the factor that is responsible forthe anodic peak observed in CVs, which is often at-tributed to a pure faradaic process. Using the equation:

I=I0 exp(− t/RC)+CdV/dt [1−exp(− t/RC)] (5)

Fig. 10. A model of a thin layer of conducting polymer. RS,the resistance of poorly conducting layer controlled by poten-tial (RS�� as the switch); C, the effective DL capacitance ofthe PAN electrode; R, the effective resistance of the PANelectrode without taking into account a slightly-conductinglayer near the current collector.

Fig. 11. The fragments of experimental (dashed line) andcalculated (curves 2, 3) CVs of a PAN electrode. Dashed linerepresents the fragment of experimental CV in a 1 M aqueoussolution of KCl and CF4COOH (pH 0.5); potential limits:0.1–0.9 V; �=100 mV/s. Curves 1 and 1� represent a conven-tional capacitor current as a function of the potential calcu-lated step-by-step by the equation Ic=Cd�/dt [1−exp(− t/RC)], where d�/dt=100 mV/S. Inc, a non-capacitor cur-rent calculated as the difference between the total cathode CVcurrent and capacitor current (curve 1�). The anodic capacitorcurrent (curve 2) is calculated by the equation I=I0exp(− t/RC)+Cd�/dt [1−exp(− t/RC)] when the isolated layer dis-appears in a PAN electrode.


Fig. 12. Experimental fragment of CVs (curves 1, 2) on a PANelectrode with a scan rate �=100 mV/s in a 1 M aqueoussolution of KCl and CF4COOH (pH 0.5). Curve 2 recordedwith the initial treating of the electrode at the potential E=0V. The anodic Ic current (curve 3) is calculated by the equationI=I0exp(− t/RC)+Cd�/dt [1−exp(− t/RC)].

Fig. 14. Potential-step curves on a PAN electrode at a differ-ent range of potentials. Electrode potential stepped in thefollowing ranges (vs SHE): from 0.3 to 0.6 V and after thatfrom 0.6 to 0.3 V; from 0.0 to 0.3 V and after that from 0.3 to0.0 V.where I0=�E/R for the equivalent circuit in Fig. 10

and the experimental value of potential delay �E�0.2V, we obtain an Ic(E) curve, which is shown in Fig. 11(dotted line).

When the electrode is kept at negative potentials, athicker insulating layer is possibly formed. This is cor-roborated by galvanostatic measurements. For in-stance, the galvanostatic charge curve 3 (Fig. 7) exhibits

a constant-potential region after keeping the PAN elec-trode at the potential E=0 V. Due to the thickerpoorly conducting layer, the �E value increases in CV,which is usually attributed to the first-cycle effect. Fig.12 shows a CV which was taken after keeping theelectrode at the potential E=0 V.

Anodic curve 3 in Fig. 12 calculated by Eq. (5) showsan increase in current with increasing potential, whichis in a good agreement with experimental data (curve 2,Fig. 12). Eq. (5) fails to describe absolutely accuratelythe experiments due to the simplified cycling at poten-tials of 0.4–0.9 V, no peaks are observed in CVs (Fig.13). In this potentials range, the capacity of the poly-mer films is purely capacitive in nature as was shownearlier [16].

The effect of insulating layer formation can be clearlyobserved in chrono-amperometric measurement. Fig. 14shows current variation as a function of stepwise poten-tials variation. It is evident from the figure that whenpotentials vary forwards the formation of a poorlyconducting phase, a decrease in current magnitude inthe chrono-amperogram is observed. Calculations ofcapacity from chrono-amperograms show a decrease of37% in electrode capacity on stepwise potentials varia-tion in this direction. Thus, the thin dielectric polymerlayer causes the electrode to be blocked and the cur-rent-producing process to cease.

The model in Fig. 10 allows one to account for notonly the complex of properties (1)– (3) for PAN-typeCP, but also more specific yet important properties, e.g.

Fig. 13. Experimental fragments of CV curves on a PANelectrode at different potentials of switching: 1, E1=0; 2,E2=0.3; 3, E3=0.4 V. Electrolyte is a 1 M aqueous solutionof KCl and CF4COOH (pH 0.5); a scan rate �=100 mV/s.


‘memory’ effect (see [30–34] and references therein).Many assumptions were reported to account for thiseffect, some of them being mutually exclusive. Someassumptions were connected with the formation ofpoorly conducting layers. It is obvious that convincingdeduction can only be made in constructing a generalelectrode model and revealing factors limiting the elec-trode process on the whole. The ‘memory’ effect multi-ple manifests itself in our experiments connected withelectrode treating at the some potentials (Fig. 7, curve3; Fig. 11, curve 2; etc.). This effect can be easilyaccounted for in terms of the model in Fig. 10 withtaking into account the fact that the internal poorlyconducting layer formed at the some potentials becomesthe limiting factor of the whole process; its thicknessbeing dependent on the potential value and time oftreating.

4. Conclusion

Analysis of different systems with solid-phase reagentconfirms the fact that in such systems as slightly solublequinones, acceptor- or donor-type GIC, etc., the elec-trochemical process occurs by the classical faradaicmechanism.

In PAN-type CP the electrochemical process ismainly non-faradaic in nature.

Our investigations of current-producing mechanismsfor a PAN electrode have shown that at least within anarrower range of potentials �En from 0.30–0.40 to0.80–0.90 V versus SHE (depending on pH value) the‘capacitor’ model of ion electrosorption–desorption ina well conducting emeraldine salt phase is more prefer-able. Nevertheless, such a model should take into ac-count the transport of both anions and protons (cationsin a general case).

Within the limits and outside this range of potentials�En it is also necessary to take into account the possi-bilities of redox processes. At the same time, theseprocesses can lead to the fast formation of thin passivelayers of new poorly conducting phases (leu-coemeraldine salt, leucoemeraldine base, etc.) near thecurrent collector. The formation of such phases even ina small amount inhibits and discontinues the electro-chemical process quickly and allows one to account forthe ‘memory’ effect and other peculiarities of electro-chemical behavior of CP.

Acknowledgements

The financial support of this research in the frameof the NATO c 973849 SfP project is highlyacknowledged.

References

[1] H. Alt, H. Binder, A. Kohling, et al., Electrochim. Acta17 (1972) 873.

[2] G. Matricali, A. Kergreis, B. Auclair, M. Guillon, C.R.Acad. Sci. 278 (1974) 829.

[3] O. Ksenzhek, V. Barsukov, V. Matveev, Reports ofUSSR Acad. Sci. (Doklady Akademii Nauk SSSR) 232(5)(1977) 1120 (in Russian).

[4] S. Aronson, S. Lemont, J. Weiner, Inorg. Chem. 10(1971) 1296.

[5] F. Beck, H. Junge, H. Krohn, Electrochim. Acta 26(1981) 799.

[6] J.O. Besenhard, E. Wudy, H. Mohwald, et al., Synth.Met. 7 (1983) 185.

[7] I.G. Chernysh, I.I. Karpov, G.P. Prikhod’ko, V.M. Shai,Physico-Chemical Properties of Graphite and its Com-pound, Naukova Dumka, Kiev, 1990 (in Russian).

[8] V.Z. Barsukov, T.I. Motronyuk, I.V. Barsukov, in: J.Przyluskii, S. Roth (Eds.), New Materials: ConjugatedDouble Bond Systems, vol. 191, Trans. Tech. Publica-tions, Switzerland, 1995, p. 261.

[9] G. Paasch, P.H. Nguyen, A. Fischer, Chem. Phys. 227(1998) 219.

[10] A. Neudeck, et al., Synth. Met. 107 (1999) 143.[11] S.A. Brazovskii, N.N. Kirova, JETP Lett. 33 (1982) 4.[12] M.X. Karapetyanc, S.I. Drakin, General and Inorganic

Chemistry (Obshaya i Neorganicheskaya Khimiya),Khimiya, Moskwa, 1981 (in Russian).

[13] S.W. Feldberg, J. Am. Chem. Soc. 106 (1984) 4671.[14] J. Tanguy, M. Hoclet, Synth. Met. 43 (1991) 2995.[15] Yu.M. Volfkovich, A.G. Sergeev, Electrochim. Acta 44

(1999) 1543.[16] V. Barsukov, S. Chivikov, Electrochim. Acta 41 (1996)

1773.[17] E.W. Paul, A.J. Ricco, M.S. Wrighton, J. Phys. Chem. 89

(1985) 1441.[18] S.V. Chivikov, V.Z. Barsukov, N.V. Korneev, Funda-

mental Problems on Electrocatalysis, Abstr. III All-UnionConference on Electrocatalysis, Moscow, 1992, p. 155 (inRussian).

[19] S. Chivikov, T. Motronyuk, M. Danilov, V. Barsukov,WEEPF-97 (2nd Int. Workshop on Electrochemistry ofElectroactive Polymer Films), Volume of Abstracts,Dourdan, France, 1997.

[20] V.G. Khomenko, S.V. Chivikov, V.Z. Barsukov, Prob-lems of Chemistry and Chemical Technology/VoprosyKhimii i Khimicheskoy Tekhnologii (Ukrainian Journal)1 (2000) 247 (in Russian).

[21] S. Chivikov, V. Khomenko, T. Motronyuk, V. Barsukov,49th ISE Meeting, Kitakyushu, Japan, Extended Ab-stracts, 1998.

[22] S.U. Glarum, J.H. Marshall, J. Electrochem. Soc. 134(1997) 142.

[23] T. Kobayashi, H. Koneyama, H. Tanura, Electroanal.Chem. 177 (1984) 281.

[24] S.U. Glarum, J.H. Marshall, J. Phys. Chem. 92 (1988)4210.

[25] V.Z. Barsukov, B.E. Rogoza, L.N. Sagoyan, Rus. J.Electrochem. (Electrokhimiya) 20 (1984) 1631 (inRussian).


[26] V.Z. Barsukov, B.E. Rogoza, L.N. Sagoyan, 34th ISEMeeting, Extended Abstracts, O-112, Erlangen, Germany,1983.

[27] V. Khomenko, S. Chivikov, V. Barsukov, T. Motronyuk,WEEPF-2000 (NATO ARW on Electrochemistry of Elec-troactive Polymer Films), Extended Abstracts, 14, Porajnear Czestochowa, Poland, 2000.

[28] C.M. Elliott, C.A. Salzer, WEEPF-2000 (NATO ARWon Electrochemistry of Electroactive Polymer Films), Ex-tended Abstracts, 11, Poraj near Czestochowa, Poland,2000.

[29] J. Tanguy, N. Mormilliod, M. Hocles, J. Electrochem.Soc. 134 (1987) 795.

[30] C. Odin, M. Nechtschein, Synth. Met. 43 (1991)2943.

[31] C. Odin, M. Nechtschein, R. Hapiot, Synth. Met. 47(1992) 329.

[32] C. Odin, M. Nechtschein, Synth. Met. 55 (1993) 1281.[33] M. Lapkowski, A. Zagorska, I. Kulszewiecz-Bajer, A.

Pron, Synth. Met. (1993) 1570.[34] C. Barbero, R. Kotz, M. Kalaji, L. Nyholm, L.M. Peter,

Synth. Met. (1993) 1545.

faradaic and nonfaradaic mechanisms of electrochemical processes

Documents