Electrocatalytic behavior of polyaniline in the dark and under illumination

EleclrochimicaActa, Vol. 36, NO. 2, pp. 361-367, 1991 Printed in Great Britain.

0013-4686/91$3.00+ 0.00 Q 1991.Pergamon Press plc.

ELECTROCATALYTIC BEHAVIOR OF POLYANILINE THE DARK AND UNDER ILLUMINATION

IN

JOHANN DESILVESTBOand OTTO HAAS Paul Scherrer Institute East, CH-5232 Villigen-PSI, Switzerland (Received 19 March 1990; in revisedform

14 May 1990)

Abstract-A reexamination of polyaniline (PANI) under illumination with visible light reveals the occurrence of two kinds of photo-induced currents: (i) the slowly-rising and decaying currents show the same dependence on electrode potential and concentration of the redox species, eg 0,. Fe(CN):-‘4-, or S,O:- , as the dark currents; (ii) faster-rising, transient photocurrents are observed only in the potential range where polyaniline is electrically conducting. In contrast to previous investigations, we attribute the slowly-rising photo-induced currents to photothermally increased convection at the PAN1 electrode and the electrocatalytic behavior of PANI, as opposed to photoelectrochemical energy conversion with PANI. The transient photocurrents are discussed in terms of a qualitative molecular orbital model. In the second part of the paper, the electrocatalytic characteristics of PAN1 are investigated with rotating disk electrodes and related to the behavior under illumination. It is shown that PAN1 is, in comparison to Pt. a superior electrocatalyst for peroxodisulfate reduction or for the reduction and oxidation of Fe(CN):-“- in acidic media. Key words: polyaniline, electrocatalysis, electrochemical peroxodisulfate reduction, photothermal, electrochemical effects at polyaniline.

EXPERIMENTAL

INTRODUCTION In a recent communication, Shen and Tian[l] described a significant effect of redox species such as Fe(CN):-‘4-, I; /I-, or S,Gon the photocurrents of polyaniline (PANI). From their experimental part and their results,* we assume that the authors used stationary electrodes. However, we know from work of Noufi et uf.[2] and our experience that Fe(CN):-‘*can be reduced and oxidized quasi-reversibly on PANI in acidic solution and in the dark and that PAN1 is a good electrocatalyst for SzOi- reduction. These facts, together with the likelihood that visible light absorption by PANI increases temperature and therefore convection at stationary electrodes, lead to our suspicion towards Shen and Tian’s interpretation of the observed effects as photoelectrochemical conversion of light to electricity. As will be discussed below, previous work by various investigators[3] has not been conclusive on the origin of photo-induced currents with PANI. Therefore, we reinvestigated the electrochemistry of PANI under illumination with visible light in the presence and absence of redox species in solution. Experiments were performed under various mass transport conditions in order to get a better understanding of photocurrents induced currents.

and

photothetmally-

*Unfortunately no explicit information on eg electrode size, orientation, mass transport conditions, or light intensity on the electrode is given in Ref. [l]. EA 36,2--K

photo-

Aniline (Fluka, ~99.5%) was distilled under vacuum and kept under Ar in darkness at 5°C. All other chemicals were analytical grade and used as supplied. Water was purified by means of a “Milli-Q” system (Millipore Corporation). If not otherwise stated, all solutions were thoroughly purged with Ar. Platinum electrodes were polished with increasingly finer grades of alumina (down to 0.05 pm), followed by brief ultrasonication in H,O, and washing in 30% HN03. Finally, the electrodes were cycled for 1 h in 1 M H,S04 between -0.2 and 1.25 V rs see at a scan rate of 100mV s-’ and then transferred to the test solution. PANI films were deposited electrochemically from solutions of 0.2 M aniline in 1 M HCI or 1 M HzS04 by cycling the electrode potential between -0. I5 and 0.78 V DSsee at a scan rate of 50 mV s-i [4]. PAN1 films were grown to 50-60 mC cmv2, based on coulometry between -0.2 and 0.5 V (first oxidation wave). These values correspond to a total film charge of cu IOO-120mC cme2 (oxidation to 1.1 V). Standard three-electrode cells were used. Electrode potentials were controlled by a Wenking POS 73 potentiostat. All potentials were measured and are quoted us the saturated calomel electrode (see). Disk electrodes were rotated by a Tacussel ED1 rotator and controlled by a Tacussel Controvit unit. Rotation speeds were calibrated with a IKA-TRON model DZM I frequency meter. Experiments with light involved a 60 W tungstenhalogen lamp. Infrared irradiation was removed by a 361

J. DESILVESTRO and 0. HAAS

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Fig. 1. Current-time behavior of PtjPANI electrodes in the dark and under polychromatic irradiation as a function of electrode potential and electrolyte composition. ‘Ihe electrode was in a vertical orientation and the solutions were not stirred unless indicated by asterisks (agitation by a magnetic stirring bar for a few seconds). Numbers indicate potentials in volts and arrows mark when the CW light was switched on (1) or off (7) by a mechanical shutter. (a) 1 M H,SO,, saturated with Ar. (b) I M H,SO,, saturated with air. 0.4’: the potential was stepped from 0.400 to 0.394V and back to 0.400 V. (c) 5 mM K,Fe(CN),/SmM K,Fe(CN), in 1 M H,SO,, saturated with Ar. (d) 0.1 M Na,S,O, in 1 M H,SO,, saturated with Ar. A fresh PtjPANI electrode was used after the experiments involving Fe(CN)i-‘4m.

1Ocm water filter. The light beam was slightly focused and measured 1.3 cm in diameter at the site of the electrode. A light intensity of 390mW cme2 was determined by an OPHIR power meter. Two irradiation geometries were chosen: (i) electrode vertical: a 0.7 cm x 0.7 cm Pt flag, covered on one side with PANI (the other side was insulated) was illuminated in a 2 cm x 2cm optical cuvette; (ii) electrode horizontal: a 0.2 cm diameter rotating disk electrode (rde), embedded in a 1.1 cm diameter Kel-F rod, was irradiated in a 7 cm diameter cell with a flat Pyrex base from the bottom.

RESULTS

AND DISCUSSION

In the first part of this investigation, we tried to reproduce Shen and Tian’s experiments under con-

*Net photocurrents are defined as the difference between the currents under illumination and in the dark.

ditions which we assume to be similar to Ref. [I]. In addition we will present our results on an absolute current scale in order to relate dark and photocurrents. Experiments were performed at vertical and also at horizontal electrodes. Dark and photocurrents at vertical electrodes

Figure 1 shows currents recorded with PANI electrodes in the dark and under illumination in various H,SO,-based, unstirred solutions. In Ar-saturated 1 M H,SO, (Fig. la) and for potentials co.1 V, only very small net photocurrents* of I 1 PA cmm2 are observed. In the potential range of 0.145 V, where the first oxidation wave occurs, transient photoanodic (at 0.1 V) and photocathodic currents (0.2-0.5 V) are recorded. At 0.6 V, anodic dark currents do not decrease to < 10 PA cme2, even after waiting periods of > 30 min. This current is probably due to PAN1 oxidation to polyquinone imine which partially undergoes an irreversible hydrolytic reaction to form soluble benzoquinone[S]. Therefore, potentials more positive than 0.6 V were avoided in most

Electrocatalytic behavior of polyaniline experiments. Under illumination at 0.6V, a net anodic photoresponse of 3-4 PA cm-* is induced. The only conceivable impurities in our solution could be traces of O2 which were not fully removed, even after purging with Ar for at least 30 min, and traces of HzOz possibly formed from O2 reduction. However, experiments with rotating disk electrodes showed that H202 is electrochemically reduced and not oxidized on PAN1 at 0.6 V. Therefore, the anodic photoresponse at this potential appears to be due to enhanced irreversible PANI oxidation. In comparison with Ar-saturated H,SO,, airsaturated solutions show higher cathodic dark currents at potentials < 0.2 V (Fig. 1b). Under illumination, the currents increase slowly for several tens of seconds, eg by 10 PA cm-* at -0.2 V. When the light is switched off the currents decay again to their initial value. Photo-induced, as well as dark cathodic currents, become smaller at more positive potentials and vanish at 0.2 V. For the potential range of 0.2-0.6 V, practically the same current-time behavior is observed as in Ar-saturated H,SO,. Peak currents of 50 PA cme2 are obtained at 0.4 V (measured with an oscilloscope). Very similar current-time curves are obtained when the electrode potential is switched negatively by 0.006 V and back to 0.400 V after a period of relaxation (cJ Fig. lb). In both cases the currents decay exponentially with time. The peak charges correspond to 120 PC cm-*, ie to ca 0.1% of the total film charge. Qualitatively similar results as shown in Fig. lb were presented by Shen and Tian in Fig. la of Ref. [I]. Also Kaneko and Nakamura[3a] described slowly-rising and decaying, photo-induced cathodic currents when PAN1 was prepared at pH 6 and then illuminated in an aqueous solution of pH 6 at potentials < 0 V. The latter authors attributed these effects to a p-type behavior of PANI. Unfortunately it was not stated in the papers of Shen and Tian[l] and Kaneko and Nakamura[3a] whether the solutions were deaerated or not. On the other hand, Wan et a1.[3b] pointed out that PAN1 prepared at pH 6 behaves electrochemically quite differently from PAN1 obtained from acidic solutions. Therefore, Kaneko’s results cannot be compared directly with findings at lower pH values. Wan et a1.[3b] illuminated PAN1 in a Zn/PANI galvanic cell at closed circuit. In the presence of oxygen, they observed that PANI, in its completely, but also in its partially reduced state, yields slowlyrising and decaying, photo-induced cathodic currents. The latter authors ascribed this effect to photoelectrochemical oxygen reduction and argued against photothermally enhanced currents: they transferred a PANI/Pt electrode from an oxygenated electrolyte solution at 24°C to one at 0°C and back to 24°C and noticed that the “thermal” equilibration of the PAN1 electrode with the 24°C electrolyte occurred within ca 1 min, as opposed to ca 3 min for the decay of the photocurrents. However, we would like to point out that the kinetics of the current decay after such a transfer is different from the current decay due to thermal equilibration in a quiescent solution. Turbulences are induced by immersion of the electrode which increase mass transport of oxygen towards the electrode and also increase the heat

363

transfer rate. Therefore, we do not agree with the conclusions of Wan et al. and are not convinced that the photocurrent decay within 3 min rules out a photothermal effect. Genies and Lapkowski[k] reported photocurrent-time curves for PANI in NH4 F/HF ,which are similar to our results. Also the latter authors observed relatively slow photocathodic effects at potentials where PANI is fully reduced. Fast transient photocathodic as well as photoanodic currents were observed in the potential range where the oxidation to the emeraldine state occurs and, finally, slowly-rising photoanodic currents at more positive potentials where PANI is completely oxidized to the polyquinone imine state. In addition, it was noted[3c] that the photo-induced currents depend on film thickness and also on the direction of the incident light (ie illumination from the solution side or from the back through the transparent substrate). We observed that in the potential range of -0.2-0.1 V, dark, as well as photo-induced currents, increase by a factor of almost five when O,-saturated H2S04 is used instead of an air-saturated solution. However, the behavior at more positive potentials is not affected by the presence of 0,. On the other hand, electrochemical O2 reduction on PANI has been investigated without illumination by Mengoli et a1.[6]. These authors presented experimental evidence that the reaction is first order with respect to O2 concentration and a two-electron process, resulting in H202 formation. From the present data it seems that the slowlyrising photocurrents are proportional to the dark currents. It is tempting to assume that illumination enhances the transport-limited dark currents due to photothermally enhanced convection at the electrode. This view is well supported by our work and even by Shen and Tian’s experiments with further redox species such as Fe(CN)i-‘4- (Fig. 1b in Ref. [I]; Fig. lc, this work), I;/I(Fig. lc in Ref. [I]), or S,Oi- /2HSO; (Fig. 2 in Ref. [l]; Fig. Id, this work). In every case, the slowly-rising photocurrents show the same potential dependence as the dark currents and they increase with increasing concentration of the redox species. Although we derive different conclusions from our measurements, we want to point out that our experimental results are very similar to those of Shen and Tian. In our opinion, the slowly-rising photocurrents reported by Kaneko and Nakamura[3a] and Wan et a1.[3b] are also due to photothermal oxygen reduction. In addition, the photo-induced steady-state cathodic currents, observed by Genies and Lapkowski[k] with PANI in NH4 F/HF solution containing trichloroacetaldehyde, may also be explained by increased convection due to photo-induced heating of the PANI film. In some experiments (in the dark), mass transport was deliberately increased by stirring with a magnetic stirring bar for a few seconds (c$ stars in Fig. lc/d). After switching off the stirrer, current relaxation occurs with tmvz kinetics, as expected for a diffusioncontrolled process. In comparison, the decay of the photo-induced currents takes place in the same time domain. However, it does not follow a simple t - i’2 law and occurs somewhat slower. The heat absorbed by PAN1 during illumination and transferred to Pt

364

J. DE~ILVE~TRO and 0. HAAS

and the electrode holder is probably released relatively slowly to the solution, leading to a longer time required to reach steady-state convection conditions. In quiescent solutions, limiting mass transport rates, k,, amount to ca 3.3 x 10-4cm s-’ in the case of Fe(CN)ireduction (Fig. lc) and to 5.3 x 10m4cm s-’ for S,O:- reduction (Fig. Id). k, values are calculated from j = k,Fc (j = current density, F = Faraday constant, c = bulk concentration of redox species). The mass transport rate increases under illumination by a factor of 2.4-2.5. On rotating disk electrodes, these rates would correspond to rotation speeds of 2-4 rpm in the dark and to 12-24 rpm under illumination.* In addition to these “slow” photoeffects, much faster-rising, although transient photocurrents are observed in the potential range of 0.1-0.6 V. The direction of these photocurrents depends on the electrode potential and the redox species present (cJ Fig. la-c, 0.1-0.5 V). A more detailed discussion of these phenomena will be presented below. Photoeffects at horizontal electrodes

In comparison with vertical electrodes, dark currents at stationary disk electrodes (horizontal orientation) are slightly higher and the net, slowly-rising photocurrents significantly smaller. In addition, slow current fluctuations due to ill-defined mass transport conditions are more pronounced. In the case of 0.1 M Na,S,O, for example, cathodic currents of ca 8 and 10.2mA cm-’ are measured at -0.2V in the dark and under illumination, respectively. These currents would correspond to rotation speeds of 5 and 8 rpm (see above). This observation shows that the lightinduced convection is, as expected, weaker at the horizontal PANI electrode compared to the vertical configuration. On the other hand, the fast-rising, transient photocurrents in the potential range of 0.110.6 V are practically identical to the currents represented in Fig. la/b. Interpretation of the photoeffects at PANI

From literature reports[l, 31 and our own experiments we have seen two kinds of photoeffects: (i) currents increasing and decreasing relatively slowly (within tens of seconds) and reaching steadystate values which are proportional to the dark currents; (ii) much faster-rising transient photocathodic or photoanodic currents which decay within a few seconds. The seemingly peculiar dependence of the slowlyrising photocurrents on potential, nature and concentration of redox species, and even electrode orientation, as well as their practically diffusioncontrolled decay, can be explained by the fact that the PANI film is heated up due to light absorption, resulting in increased convection at the electrode surface. These photoeffects reflect primarily the properties of the electrolyte solution rather than solidstate characteristics of the polymer. In addition to *These values were calculated from the Levich equation, the diffusion coefficients of Fe(CN)i(7.6 x 10m6cm2 s-I in I M KCl[7]) and S,O$- (9.2 x 10m6cm* s-’ in 0.5 M HClO,[8]), and the kinematic viscosity of 1 M HISO (0.01143 cm s-‘[9]).

Shen and Tian’s work, older investigations[3] must probably be reinterpreted. The slow photoeffects reported by Kaneko and Nakamura[3a] and by Wan et a1.[3b] are, in our opinion, due to photothermallyinduced reduction of 02. The transient photo-induced currents, on the other hand, are limited to potentials where PAN1 is electronically conducting(lO]. They seem to be determined by the solid state properties of PANI. Peak currents of < 50 PA cm-’ are observed, corresponding to a very low light-to-electric current conversion efficiency of <3 x 10m4.This figure is obtained if we assume, for a rough estimation, an average energy of 2.3 eV for our polychromatic light source (390 mW cm-‘). The transient photocurrents lead to the reduction of up to 0.1% of the PAN1 film. In the following qualitative discussion we regard PAN1 as a solid-state material with a band structure (cf [I I]). Conductivities, u, of up to 7.7 S cm-’ have been determined for the emeraldine state by in situ measurements[lOa]. From this value and a charge carrier mobility, p, of ca 0.2 cm* V’ SC’[ 121,a carrier density, n, of 2 x 1020cm-3 is estimated from equation (1) (e = electron charge): 0 = nep.

(1)

This carrier density lies between values for typical metals (1022-1023cm--‘) and typical, doped semiconductors (10’6-10’9 cm--‘). The width of the space charge layer, d,, in PAN1 can be estimated from equation (2)[13] if we assume an extrinsic semiconductor in depletion mode: d,, = [2~~~V,/ne]“*,

(2)

where so is the permittivity of free space. The absolute potential drop, I’,, through the space charge layer is expected to be considerably lower than the band gaps of emeraldine of 1.5 and 2.8 eV which have been determined by optical spectroscopy and related to band structure calculations[l I]. If we assume a V, value of < 1 V and a dielectric constant, E, of roughly 20 for PAN1 we obtain barrier widths in the nanometer range. No effective light absorption and charge separation can be achieved within such a thin space charge layer. According to the Gartner model[ 141,the total photocurrent is the sum of the field-induced current and the diffusion of minority carriers from the bulk towards the space charge layer. For aL < 1 and ud, < I, the photocurrent is given by equation (3) where I, represents the photon flux, c( the optical absorption coefficient, and L the diffusion length of the minority carriers: i,, = ef,cc(L + d,).

(3)

The low conversion efficiencies of light to electricity may be explained by L and d,, values being much smaller than in conventional semiconductor systems. Electron micrographs of PAN1 obtained from H2S04 and HCl solutions showed that the films consist of a three-dimensional network of fibers of ca 0.2 pm diameter[4, 151.Therefore the electrochemically active area of PAN1 is considerably higher than the projected area. Light penetrating into the pores of the polymer network might explain the observation that the photocurrents increase with increasing film thickness[3c]. A more quantitative description of the

Electrocatalytic behavior of polyaniline

properties of PANI, which is beyond the scope of this work, would require a detailed investigation of photocurrents as a function of wavelength and film thickness. In a different approach we may regard the polymer as chains of electronically rather independent chromophores. After excitation of protonated emeraldine/anion units [EM+A-] by visible light, the excited state can relax by internal conversion, resulting in heat production, or by undergoing an electron transfer reaction (Fig. 2). As shown in the qualitative molecular orbital (MO) scheme in Fig. 2, an electron can be transferred, in the absence of any redox species in solution (cJ Fig. la), from the electrode to [EMtAm]*, leading to a photo&ho& current. The resulting leucoemeraldine (LEM) is oxidized electrochemically under potentiostatic conditions. Therefore the photocurrent decays until a steady-state is reached which is given by the photo-induced generation rate of LEM and the rate of LEM oxidation in the dark. On the other hand, we observed small transient photoanodic currents at 0.1 V in the absence of any electron donor (Fig. la/b). Genies and Lapkowski reported an even more complex potential dependence of the direction of photo-induced currents at PAN1 in NH4 F/HF solution[3c]. These results probably rely on the photochemistry of PAN1 involving various excited and ground states in different oxidation states. More experiments are needed in order to understand the complex photochemistry of PANI. We have also observed transient photoanodic currents in the presence of an electron donor, eg with Fe(CN& in the potential range of 0.3 V < E < 0.6 V (cf Fig. Ic). It appears that Fe(CN);1- quenches the excited state according to reaction (4a):

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semiconductor

EM + Fe(CN)t-A -

LEM + Fe(CN)i-

(4a)

EM.

(4b)

dark -e-

LEM-

+e-

The photogenerated LEM is then oxidized electrochemically (4b), giving rise to a photoanodic e!?ect.

-80 -100

0.5

0.0

Fig. 3. Rotating disk voltammograms for Pt (..., + + +) and Pt/PANI I-. ---) in 1 M H,SO.-based solutions. (. . ., -1-): 0.1-M I(,Fe(dN),/O.l M *K4fie(CN&; starting potential was -0.2 V. (+ + + , -): 0.1 M Na,S,O,; starting potential was 0.8 V. Electrode rotation speed was 1000 rpm and scan rates were 5 mV s-l, with the exception of 2mV s-l for curve (+ + +). Photocurrents occur only in a narrow range of potentials because fermcyanide is significantly oxidized only at potentials ~0.3 V. For E > 0.55 V on the other hand, the diffusion-limited oxidation current is reached (cjYalso Fig. 3), resulting in zero Fe(CN)tconcentration at the surface. Scheme (4) is completely analogous to reactions occurring. in a photogalvanic cell with the chromophores being attached to the electrode. It has been shown that photogalvanic cells yield generally low light-to-electricity conversion efficiencies of < 1% due to thermal back reactions of the products[ 161. Electrocatalytic investigations with rotating disk electrodes So far we have described the behavior of PAN1 under illumination and shown that light absorption, thermal conversion and the electrocatalytic behavior of PAN1 are responsible for the slowly-rising photoinduced currents. In this second part of our investigation, we will concentrate on the electrochemistry of Fe(CN)i-/4- and &O:- on PAN1 electrodes in the

t-

ihv _,

4-k EM PANI/Pt

Pt

PANI

1.0

E/V vs. SCE

PANI/Pt

electrolyte

Fig. 2. Left: scheme for the generation of photocathodic currents from illuminated emeraldine (EM+A-) in the absence of redox species in solution. With increasing photo-induced concentration of leucoemeraldine (LEM), electrons are transferred back to the Pt electrode until a new steady-state is reached. Right: qualitative molecular orbital model for electron transfer processes involving the two highest occupied molecular orbitals.

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dark. Rde experiments were performed with Pt/PANI and with clean Pt electrodes. It is known that Fe(CN)i-‘4-[17] and SzOi- [18] are decomposed slowly in acidic media. Therefore, the measurements were performed immediately after preparation of the solutions. The dotted curve in Fig. 3 shows the first cathodic-anodic voltammogram for freshly cleaned Pt in 1 M H,S04 containing Fe(CN):-“- . With subsequent scans, the electrochemical kinetics become slower and slower due to irreversible adsorption of Fe and cyanide-containing species[l9] on the Pt surface. Interestingly, the kinetics of Fe(CN)z- oxidation and Fe(CN)i- reduction (dashed curve in Fig. 3) are significantly faster on PANI compared to Pt electrodes when acidic electrolytes are employed. The voltammograms are not affected at all by repeated cycling. Thus the electrocatalytic behavior of PAN1 surfaces seems to be less sensitive to poisoning by ferro- and ferricyanides. Plateau currents are identical for Pt and PAN1 electrodes. From 5 mM ferri-/ferrocyanide solutions in 1 M H, SO,, diffusion coefficients of 7.6 x lo-” and 6.2 x 10m6cm2 s-’ are determined for Fe(CN)i- and Fe(CN)z- , respectively (cJ earlier footnote for kinematic viscosity of I M H,SO,). These figures are very similar to literature values reported for 1 M KCl[7]. The electrocatalytic effect of PAN1 on peroxodisulfate reduction is even more pronounced. S20i- is reduced on PAN1 (full curve in Fig. 3) at potentials 0.6V more positive as compared to Pt (crosses in Fig. 3). From the transport-limited currents, which are reached on PAN1 electrodes for E < 0.2 V, a S,O:- diffusion coefficient of 7.6 x 10-6cm2 s-’ is calculated for a solution of 0.1 M Na,S,O, in 1 M H, SO4 by assuming a two-electron process. A similar diffusion coefficient has been determined from rde measurements in 1 M KOH[20], whereas a slightly higher value of 9.2 x 10e6 cm2 s-l has been reported, based on polarography in 0.5 M HC104[8]. The slight hysteresis in the voltammogram for peroxodisulfate reduction on PAN1 is partly due to the underlying PAN1 voltammogram but also due to slow decomposition of S,Oi- solutions, as indicated by the gradual decrease of the plateau current. The electrochemical response of PAN1 films, as monitored in S,Oi--free media, remains unaffected when peroxodisulfate reduction is performed at potentials < 0.5 V. Even after extended electrolysis times corresponding to turnover numbers of >4000, no film deterioration is noticeable. At more positive potentials, however, the electrochemical activity of PAN1 decreases slowly due to S,O{--induced, irreversible PAN1 oxidation. From literature data it is known that S20i- reduction on Pt occurs at very high overpotentials (cf: E”(S,Oi-/2HSO;) = 1.82 V L’Ssce[21]) and is partially blocked by anodic platinum oxide films [8,22]. The hysteresis in the voltammogram on Pt (crosses in Fig. 3) can be explained by the partial reduction of surface platinum oxides during the cathodic scan. For the case of S20i- reduction on gold in alkaline media, we showed in a previous investigation that S,Oi- shifts the potential for surface oxide reduction to more negative potentials

[20]. A similar mechanism may be operative for the S,O:- /Pt system so that surface oxides are reduced only at rather negative potentials around OV. In analogy to the Fe(CN):-j4- system, PAN1 is also less sensitive than Pt to surface impurities with respect to S20i- reduction. Electrocatalysts for S20i- reduction may become very interesting, eg for reserve batteries (cj Ref. [18]), if the overpotential for S20i- reduction can be lowered significantly. So far, promising preliminary results have been reported with upd monolayers of Pb, TI, and Bi on Pt[8] or with metal oxides such as nickel oxyhydroxides[20]. Conductive polymers represent a new class of electrocatalysts which have not been exploited enough so far. CONCLUSIONS We were able to reproduce the photoeffects at PAN1 electrodes described by Shen and Tian[l]. However, we do not agree with the interpretation in earlier investigations[ 1,3] that the photo-induced currents are due to photoelectrochemical conversion of light to electricity. We showed that most of the photoeffects are due to photothermally enhanced convection at the PAN1 electrode. The transient photocurrents, on the other hand, could be rationalized by the photochemical behavior of PANI. Due to the fast decay of these currents and their low lightto-electricity conversion efficiencies ( < 3 x 10m4), PAN1 seems to be of no practical interest for photovoltaic devices or light detectors. The rather high, slowly-rising photocurrents rely upon the good electrocatalytic activity of PANI. We showed that a relatively cheap material such as PAN1 may give a significantly superior behavior, eg for peroxodisulfate reduction, when compared to platinum. In addition to numerous other applications, PAN1 appears to be a promising candidate for electrochemical detectors and for electrodes in reserve batteries, such as Al/PAN1 or Mg/PANI. Such applications are presently being investigated in our laboratory. Acknowledgements-This

work was supported by the Swiss (grant no. 2.676-0.87). We Prof. M. Grltzel for interesting

National Science Foundation thank Dr H. Kiess and discussions.

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