hr. J. Hwh+yw Energ,v, Vol. 21. No. 3, pp. I71 178, 1996 Copyright @ International Association for Hydrogen Energy

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PREP,4RATION OF THIN Co,O, FILMS ON Ni AND THEIR ELECTROCATALYTIC SURFACE PROPERTIES TOWARDS OXYGEN

EVOLUTION

S. P. SINGH, S. SAMUEL, S. K. TIWARI and R. N. SINGH*

Electrochemical Laboratory, Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India

(Receiuedfor publicurion 3 Muy 1995)

Abstract-Thin films of Co,O, of varying thicknesses have been prepared on an Ni substrate by spray pyrolysis and sequential solution coatings and investigated for their physicochemical and electrochemical properties towards 0, evolution. The study indicated that the oxide roughness factor tended to level off with increasing thickness. Further, with an increase in oxide loading, the apparent electrochemical activity enhanced considerably at low range of loadings and then became relatively constant over a range of loadings. The higher loadings were found to be harmful. The layered oxide films (2.4-4.2 mg cm-‘) prepared in sequential solution coatings exhibited greatest electrocalalytic activity in 1 M KOH. However, the oxide electrode (loading: 3- 5.7 mg cm 2, prepared by rotating spray pyrolysis produced the lowest oxygen overpotential~&360 f 3 mV against a current density of 1 A cm-* in 30 w/o KOH at 70°C. The oxygen evolution reaction at these film electrodes exhibited two Tafel slopes: 51-68 mV decade- ’ at low overpotentials and 12Gl40 mV decade- ’ at high overpotentials. The reaction orders with respect to OH- concentration were found to be - 1 for the layered and -2 for the sprayed films. Based on the results, a suitable mechanism is also suggested for oxygen evolution.

INTRODUCTION

In previous reports [l-4], the use of spray pyrolysis has been demonstrated for obtaining adherent, relatively more compact, and crack-free uniform thin films of Co,O, and NiCo,O, on glass/Cd0 and Ti substrates. The conductivity of these films was many times greater than those prepared b:y other methods [l]. Although the film electrodes were qluite stable and showed interesting electrode kinetic features, they have not been studied for optimum oxide loading, which, in fact, is desired for water electrolysis applications. No systematic study of the Co,O, loading has appeared in the literature.

We have prepared Co,O, films of varying thicknesses on Ni by spray pyrolysis and also by the sequential solution coating method and investigated the influence of oxide loading as well as that of the deposition technique on the electrocatalytic properties of the oxide/KOH solution interface with regard to oxygen evolution. De- tails of the results of this investigation are presented in this paper.

*To whom correspondence should be addressed.

EXPERIMENTAL

The methods of spray pyrolysis [S] and sequential solution coating [6,7] were used to obtain Co,O, in thin film form on Ni plates (1.5 cm x 1 cm). Before use, these plates were polished mechanically, washed with distilled water, and cleaned ultrasonically in acetone.

The sprayed Co,O, films were obtained in two ways: in one case, the spray nozzle was kept stationary; and in the other, it was rotated mechanically with simultaneous left-right and forward&backward movements (5 rpm). In either case, the nozzle, which was of Pyrex glass, was kept at a height of 20 cm from the substrate and an aqueous solution of cobalt nitrate (0.2M) was sprayed vertically at a flow rate of - 1 ml min- ’ on to the metal substrate maintained al. 300 + 1O’C using compressed air at 2 bar. In the case of the stationary spray, spraying was carried out in steps :jo as to get a homogeneous and uniform film. The sprayed films were annealed at 360°C for 1 h, and then the furnace was switched off and the sample was taken out when it attained nearly 5O’C. The oxide films prepared at a temperature ~360“ were compara- tively less active.

To prepare a layered film, two drops of 0.2M cobalt nitrate solution in 75 v/o isopropanol were placed on one side of the plate with the aid of a 1 ml pipette and

171

172 S. P. SINGH rt al.

Fig. 1. SEM micrographs of (a) Ni/Co,O, (A-type), I = 5.5 mg cm-‘; (b) Ni/Co,O, (B-type), I = 3.2 mg cm-‘; (c) Ni/Co,O, (B-type), I = 4.6 mg cm -‘; (d) Ni/Co,O, (C-type), I = 3.3 mg cm-‘; and (e) Ni/Co,O, (C-type), I = 8.1 mg cm-‘.

it was then uniformly sprayed over the whole surface. For the sake of simplicity in representation, the oxide The solution-coated plate was dried by a hot air blower films prepared by the methods of rotating and stationary (- 50°C) for 10 min and then placed in a furnace at 300°C spray pyrolysis and sequential solution coating, are for 10 min. The procedure was repeated several times to denoted by A, B and C, respectively. The ohmic contact get the desired oxide loading. Finally, the samples were to the oxide film was taken from the metal surface by a annealed as before for 1 h. thin copper wire as mentioned elsewhere [3,7]. For the

PREPARATION OF THIN Co,O, FILMS 173

studies, only 0.5 cm’ area of the film was used and the remaining area including the free metal surface, was covered with Araldite.

All electrochemical measurements were made in a conventional three-electrode Pyrex glass cell as described elsewhere [3]. The potential of the working electrode was measured against an Hg/HgO/lM KOH electrode. In the case of the studies carried out in 30 w/o KOH, the reference electrode was the Hg/HgO-30 w/o KOH. In each experiment, the reference electrode was separated from the test solution by using a Luggin capillary containing an aqueous agar-agar (KCl) salt bridge. The tip of the Luggin capillary was placed close ( - 2 mm) to the working electrode.

Current-potential measurements were made using an electrochemical impedance system (Model 273A, EG&G PARC). In cyclic voltammetry (CV), the test electrode was first cycled between 0 and 0.65 V for 2-3 min at a potential scan of 50 mV s ’ in 1 M KOH and CV curves were then recorded. The IR free anodic polarisation curves, in current-interrupt mode, were recorded at a slow potential scan of 0.2 mV s I. Before recording each curve, the electrode was invariably anodised at 0.65 V (Hg/HgO) for 5 min and it was then brought back to the open circuit conditions and subsequently, the E-log I curve was recorded.

To determine the approximate order of the ohmic resistance of the catalytic film, the resistance of the test electrode plus the electrolyte (R) was determined by impedance measurements. As mentioned earlier [3,7] the electrochemically active surface area of the electrode was determined by recording the double-layer charging curves, on a bipotentiostat (Model 2000, Houston Instru- ments). The ionic strength (p) of the medium, wherever necessary, was maintained by adding KNO, as the supporting electrolyte. The morphology of the films as deposited on the conductive supports was studied using a scanning electron microscope (JEOL).

RESULTS AND DISCUSSION

Morphology

Figure 1 shows the effect of the deposition technique on the morphology of the Co,O, film. Observation of this figure indicates that the layered films (C-type) are much more porous than the sprayed ones (A- and B-type). Further, films of types A and C appear to be more compact. The B-type film exhibits a cracked dried-mud look [Fig. l(b)]. The size of the cracks is observed to be enlarged with increasing the oxide loading [Fig. l(c)]. The deposits in both A-type [Fig. l(a)] and C-type [Fig. l(d)] films are more or less flat in nature, however, unlike in the former film [Fig. l(a)], deposits in the C-type film are porous. It is observed that deposits in the latter film become granulated [Fig. l(e)] at high loadings (8.1 mg cm-‘).

Cyclic voltummetry

The effects of oxide loading (1) were investigated on

Potential /V

Fig. 2. The eE^ect of oxide loading on the cyclic voltammetric behaviour of sprayed Ni/Co,O, films prepared by (a) rotating and (b) stationary spray nozzles; scan rate = 20 mV s-‘,

[KOH] = IM (25 -C).

the cyclic voltammetric behaviour of Co,O, supported on Ni between 0 and 0.6 V in 1M KOH (Fig. 2). It was observed that with the exception of electrodes of type B, all the electrodes prepared in situ (A- and C-type) exhibited a single anodic and corresponding cathodic peak [Fig. 2(a)]. Further, the position of the peaks did not change noticeably with the oxide loading. The values of the peak potentials (&and E,,) at 20 mV s-l were found to be 533 + 23 and 474 k 19, and 556 & 26 and 492 i 20 mV for the C- and A-type films, respectively. Similar values of peak potentials were also observed for Co,O, films on glass with an interlayer of Cd0 (E,, - 520 and E,, - 510 mV) [l] and on Ti (E,, - 550 and E,, - 5 IO mV) [3] supports. Thus, the observed redox peaks ton these Co,O, films may be ascribed [ l,S] to the formation of the Co(IV)/Co(III) redox couple.

Cyclic voltammograms for films of type B [Fig. 2(b)] indicated two anodic (E,,(I) = 467 +_ 14 and E,,(H) = 540 f 15 mV) and two corresponding cathodic peaks (E&II0 = 504 & 10 and E,,(IV) = 346 k 13 mV). As mentioned above, the II and III peaks correspond to the surface redox couple Co(W) -+ Co(III). The compari- son of redox peaks I and IV with those already reported for pure Ni (E,, - 490 and E,, - 380 mV) [9] for the

174 S. P. SINGH et al.

3.2

1.6

-1.6

E/V

Fig. 3. Typical cyclic voltammograms for the Ni/Co,O, (B-type) elctrode at varying scan rates in 1M KOH (25°C).

Ni(OH), film on Pt (E,, - 470 and E,, - 370 mV) [l] in 1M KOH shows that the I and IV peaks are produced due to the oxidation of the nickel substrate. The B-type oxide films, perhaps, do not protect the nickel surface well and the electrolyte comes into contact with the surface.

Roughness factor

As shown in Fig. 3, the double-layer charging curves were recorded on each oxide electrode at varying scan rates in a small potential range of 50 mV, near the open-circuit potential, where the charge transfer reactions

Fig. 4. Plot of icap vs scan rate for the Ni/Co,O, electrodes; 1 (mg cm-*); O-55; O-6.8; 04.4.

, I I 3.0 6.0 9.0

Oxide loading / mg.cni2

12.0

Fig. 5. The effect of oxide loadings on the roughness factor

were practically absent. Values of the double layer capacitance (C,,) were estimated as mentioned earlier [3] from the slope of charging current vs scan rate plot (Fig. 4). The roughness factor was calculated, assuming a C,, of 60 PF cm-’ for a smooth oxide surface and values, so obtained, are displayed as a function of the catalytic film thickness (mg cmm2) in Fig. 5. The electrodes used in this experiment were galvanostatically pre-anodised at 25 mA cme2 for 30 min.

The results showed that increase in oxide loading enhanced the roughness factor until a constant value was obtained. This complex behaviour may be attributed [6,8] to the fact that at low loading, almost the entire crystallites of the oxide come into contact with the electrolyte, but when loading exceeds a certain critical value, some crystallites get excluded from contact and the number of such crystallites increases as the layer grows.

Observation of Fig. 5 further shows that at the same oxide loading, the sprayed B-type film has a higher roughness than the other films. Thus, with suitable oxide loadings, one can obtain an oxide film of roughness as high as -480, which is - 100 and - 50 times greater than those obtained for the similar films on glass/Cd0 [2] and Ti [3] supports, respectively.

Electroctalytic activity

The electrocatalytic activity of oxide electrodes was tested towards oxygen evolution by determining the IR-free anodic Tafel polarisation curves in 1M KOH at 25°C. Values of R were found to fall between 0.4 and 2.0 Q. The polarisation curve for each electrocatalyst in- dicated two Tafel regions: one at low, and the other at high overpotentials. Values of the first Tafel slope (b,) ranged between 53 and 68 mV decade-’ and of the other (b,) ranged between 100 and 140 mV decade-‘. Details

PREPARATION OF THIN Co,O, FILMS 175

Table 1. Eaectrocatalytic activity of thin Co,O, film electrodes towards oxygen evolution in 1 M KOH at 25°C

Oxide loading (mg cm-*)

Roughness factor

Tafel slope i (mA cm 2, (mV decade-‘) - n o = 400 mV 2 b, b, APP Tr

?I o2 = 450 mV

APP Tr

Ni/Co,O,,: A-type 0.79 * 0.03 1.40 * 0.20 1.93 * 0.03 2.74 + 0.13 2.98 f 0.03 4.35 + 0.24 5.63 i 0.11

Ni/Co,O,: B-type 0.7 1.5 2.5 3.6 5.2 6.8 8.1

10.3

Ni/Co,O,: C-type 1.2 2.4 3.6 4.2 6.1 9.0

11.6

55 * 2 60 138 9.0 106 k 8 55 118 13.4 114&4 54 107 24.7 116 + 18 55 114 33.8 190* 1 53 116 44.6 260 i 10 50 124 46.8 302 f 2 54 119 59.8

12s 68 140 10.8 0.086 27.2 0.218 196 64 140 18.9 0.096 64.4 0.329 208 60 130 20.1 0.097 77.4 0.312 296 62 135 42.5 0.144 123.6 0.418 314 62 135 41.9 0.133 120.8 0.385 450 62 133 47.0 0.104 176.0 0.391 459 67 135 19.4 0.042 69.6 0.152 483 64 131 17.5 0.036 62.0 0.128

64 217 231 244 248 260 310

53 183 21.7 53 125 95.0 55 130 97.5 54 128 95.5 60 129 46.2 64 130 14.6 56 141 26.4

0.164 0.126 0.217 0.210 0.235 0.180 0.198

0.339 0.438 0.422 0.391 0.186 0.056 0.085

25.1 0.456 43.9 0.414 77.3 0.678

161.6 0.817 167.8 0.883 179.0 0.688 146.2 0.484

44.0 0.687 288.2 1.328 289.6 1.254 301.2 1.234 134.0 0.540 46.8 0.180 90.8 0.393

of the results are shown in Table 1. It is worth mentioning that the results were quite reproducible as can be seen in the case of A-type films in Table 1.

Table 1 shows an improvement in the electrocatalytic activity with increasing oxide loading, particularly, at low levels. The activity of films seems to be practically constant in a range of intermediate oxide loadings. The higher loadings were found to be harmful from an electrocatalysis point of view. The decrease in apparent current density at higher loadings can be attributed to the change in morphology of the film, leading to blocking of the effective surface area of the catalyst. Similar results were also obtained by Hall in a study of oxygen evolution on Ni(OH), impregnated anodes in an alkaline medium [lo]. Considering both the apparent as well as true current density at a given overpotential, the layered Co,O, film electrode (C-type) with oxide loadings be- tween 2.4 and 4.2 mg cm-‘, were the best among all electrodes prepared in situ. Based on values of the current density at a given overpotential, the electrocatalytic activity of active Co,O, film electrodes can be put in the following order: C-type film (2.444.2 mg cm-‘) > A-type film (1.93-2.98 mg cm-‘) > B-type film (6.8 mg cm-*).

To determine the reaction order (p) with respect to OH- concentration, the active oxide anode of each preparation, e.g. Ni/Co,O, (A-type), Ni/Co,O, (B-type)

and Ni/Co,O, (C-type), possessing oxide loadings of 3.0, 6.8 and 4.2 mg cmm2, respectively, were chosen and investigated for oxygen evolution at different KOH concentrations (O.lLlM), keeping the ionic strength of the medium constant (p = 1.5). The order was computed by measuring the slope of linear log i vs log [OH-] curves constructed at a constant applied potential across the oxide/KOH interface (E = 0.68 V) as shown in Fig. 6 and found to be - 1.2 for layered and - 1.8 for sprayed films.

Values of the Tafel slope and the reaction order obtained for C-type films on Ni are in fair agreement with those found for the sprayed Co,O, films on glass/Cd0 supports [2] but differ from those obtained for similar catalytic films on Ti [3]. It is noteworthy that the activity of films supported on Ni was many times greater than those obtained on other substrates [2,3]. Taking into account the roughness factors, Ni/Co,O, (C-type) electrodes were found to be - 53 times more active than the Ti/Co,O, electrode [3] and -6 times more active than the glass/CdO/Co,O, electrode [2] at the 0.45 V overpotential in 1M KOH at 25°C.

The electrode’s kinetic parameters, particularly the Tafel slope and the reaction order, indicate a mechanism which involves a slow chemical step after the primary discharge of OH- on the oxidised electrode surface.

176

-1.00 - 0.75 -0.50 - 0.25 0.0

log con- Similarly, the rate for chemical step (2) can be given as:

Fig. 6. Plot of log i vs log Co,- at a constant potential (0.68 V) k, = ki exp[(l - j)rB,/RT], (10) across the oxide-KOH interface (25°C).

where p is the symmetry factor. Now, considering step (1) under quasi-equilibrium

Details of the mechanistic steps are given in the following scheme where M = Co’“,

conditions and neglecting the term ln(0/1 - 0,) at inter- and is an active site on the mediate values of coverage (0.2 < 0r < 0.8) one obtains

oxide surface. This mechanism is similar to that proposed t by Krasilshchikov for oxygen evolution on oxides [ 111 rO, = RT In KyCoHm + FE. (11)

k, M+OH- Z+ MOH+e- (1)

Substituting the value of rfl, into equation (10) and then

k-1 k, into equation (5) the final expression for the rate

rds becomes

MOH + OH- + MO- + H,O (2) k, v = k~O,,C,,~ exp[(l - j?)(RT In KyC,,- + EF)/RT.

k,

MO- + MO+e- (3) (12)

k-3 Now, considering that QoH is relatively invariant under 2M0 z$ 2M+O, (4) intermediate coverage conditions and that fl N 0.5, equa-

tion (14) gives a Tafel slope and reaction order of In the above scheme, if the total surface coverage by approximately 120 mV decade-’ and 1.5, respectively.

adsorbed intermidiate (0, = 0ou + B. + 0,) follows As the primary discharge step (1) is followed by a slow Langmuirian behaviour, the overall rate for oxygen chemical step, there seems to be the possibility for evolution (u) can written as establishment of said adsorption conditions, particularly

v = k,O,,&,,- u k,KIC&m exp(FE/RT). at high overpotentials. The experimental Tafel slope and

(5) the reaction order obtained at high overpotentials, in fact, support this mechanism.

Therefore, the net electrolysis current can be given as: The Tafel slope of 60 mV decade- ’ and approximately first-order kinetics in the OH- concentration, can be

i = nFv = nFk, K,C& exp(FE/RT), (6) explained by considering steps (2) and (3) as a single step,

where K,( = k,/k- r) is the adsorption-desorption equi- MOH + OH- --* MO + H,O + e-, libruim constant for step 1.

The rate expression (6) gives a second-order reaction and assuming it as the rate-determining step. This mech- in OH- concentration and a Tafel slope of -60 mV anism now becomes similar to Bockris’s electrochemical decade- ‘. This explains the kinetic parameters observed path [ 1 l] and gives, under intermediate coverage condi- on the sprayed film. tions, a Tafel slope of 60 mV decade- ’ and a reaction

However, when or is appreciable, and influences the order of 1.5. The details of this treatment can be found heats of adsorption and hence the free energies of in Refs [2,12]. activation of species to be adsorbed, the following rela- Studies of oxygen evolution from Co,O, films have tionship between the apparent equilibrium constant (K) been carried out by several workers and were reviewed and QT is found to hold well [12] in Refs [13,14]. Depending upon the method, the experi-

S. P. SINGH et al.

K, = KY exp( - rQ,/RT), (7)

where KY is the standard equilibrium constant for the adsorptiondesorption process and r is a coefficient determining the variation of the heat of adsorption with h.

Therefore, the rate for the adsorption (equation 8) and the desorption (equation 9) steps can be written as:

and

k, = ky exp( -pr&/RT), (8)

k-, = ky, exp[(l - &Q,/RT]. (9)

PREPARATION OF THIN Co,O, FILMS 171

Ni /Co30,, ; 30 w/o KOH

Type 25’C 7o”c

A 0

0.725

2 0.675

0.625

0.525

-6.0 -3.0 -2.0 -1.0 0.0

log i / A.cm-’

Fig. 7. Tafel plots for oxygen evolution on the active Ni,‘Co,O, electrode in 30 w/o KOH at 25 and 70 C.

mental conditions and nature of support employed in the preparation of the films, varying values of Tafel slope (40-80 mV decade-‘) and reaction order (l-2) were obtained. Iwakura rt ul. [15] have studied the oxygen evolution reaction for the Co,O, film prepared on a variety of metals such as Ti, Fe, Co, Ni, Nb, Ta and Pt by sequential coatings of an aqueous solution of Co(NO,),. They observed Tafel slopes of -60 mV decade-’ for the catal,ytic films on substrates such as Ti, Co, Ni, Nb and Ta, while an average slope of -45 mV decade ’ was observed for Co,O, films on Fe. Of these, Fe/Co,O, has the lowest oxygen overvoltage, e.g. 0.350 V at 10 mA cme2 in 1M KOH at 3O”C, however, these tests were limited to a current density of 10 mA cmm2 only. The Co,O, loading was 1 x 1O-5 mol of Co cmm2 (- 1 mg of Co,O, cm 2).

Rasiyah and Tseung [ 163 observed a Tafel slope of 60 mV decade-’ and nearly second-order kinetics in OH- concentration for Teflon-bonded Co,O,. prepared by the freeze-drying method. On the other hand, Burke and McCarthy [ 171 reported a Tafel slope of 40 mV decade- 1 for RuO, and Co,O, mixtures (RuO, content > 20 m/o) coated on Ti. However, the thin catalytic layers on gold [18] showed a Tafel slope and reaction order of 50 mV decade-’ and 1.3 at low overpotentials and 100 mV decade- ’ and 0.7 at high overpotentials.

The performance of active film electrodes was also tested up to a current density of 1 A cme2 in 30 w/o KOH at 70°C. The electrodes also showed quite satis- factory performance under practical conditions (Fig. 7). It is noteworthy that A-type film on Ni (loading: 3-5.7 mg cm-‘), which was observed to be less active compared with the C-type films on Ni in 1M KOH, showed the highest activity in 30 w/o at 70°C. It only produced an oxygen overpotential of 360 f 3 mV, while others (types B and C), regardless of their methods of preparation, gave a considerably higher oxygen overpotential of 402 f 5 mV, at 1 A cme2 (7O’C).

The stability of the active Co,O, film on Ni obtained by sequential solution coating (C-type) has also been

i : 0.1 A cn? (Without iR correction)

“I------

Fig. 8. Variation of electrode potential with time at a constant current densitj (0.1 A cm ‘) in 30 w,‘o KOH (25 ‘C), I = 3.6 mg

cm 2.

examined by anodising at a current density of 100 mA cm m2 in 30 w/o KOH at 25 C for a period of up to 45 h (Fig. 8). The results have shown that the electrode was reasonably stable during the period of investigation.

CONCLUSION

This study has shown that thickness of the oxide layer has a strong influence on electrocatalytic activity as well as on performance of the electrode, particularly at higher current densities. The enhanced electrochemically active area and also the electrocatalytic activity of Co,O, film on Ni might be due to some dispersion of metal ions into the oxide layer during film preparation. In the case of the Fe/Co,O, electrode prepared by a thermal decom- position method at 35O’C, Iwakura et al. [ 151 had already observed a fairly homogeneous distribution of Fe species into the Co,O, layer by a secondary ion mass spec- trometry.

Acknowledgemcnts~ Theauthorsare grateful to the Indo-French Centre for the Promotion of Advanced Research (Centre Franco- lndien pour la Promotion de la Recherche Avancee), New Delhi, India, for providing the 273A IX&G PARC electrochemical impedance sytem. One of the authors (SPS) is also grateful to U.G.C., New Delhi, for awarding a research fellowship to carry out the work.

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