Pergamon hr. J. Hdrogen Energ~~. Vol. 20, No,. 3. pp. X3- 210, 1995

International Associatmn for Hydrogen Energy Elsewer Science Ltd

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PHYSICOCHEMICAL AND ELECTROCHEMICAL CHARACTERIZATION OF ACTIVE FILMS OF LaNiO, FOR USE AS ANODE IN ALKALINE WATER

ELECTROLYSIS

S. P. SINGH,* R. N. SINGH,*t G. POILLEARATt and P. CHARTIER;

*Electrochemical Laboratory, Department of Chemistry Faculty of Science, Banaras Hindu University, Varanasi 2Zi 005, lndia ZLaboratoire d’Electrochemie et de Chimie-Physique du Corps Solide, U.R.A. au C.N.R.S. no. 405. UniversitC L(& Pasteur.

67000 Strasbourg, France

(Received for publication 3 1 March 1994)

Abstract-Thin films of LaNiO, were prepared on Ni by the sequential coating method, and the effects of film preparation variables, such as concentration of the mixed metal nitrates solution, temperature and oxide loading, were examined for the electrocatalytic activity of the film regarding oxygen evolution in concentrated KOH solution. The oxide film with a loading of -6 mg cm-’ was found to be the best electocatalyst among the electrodes prepared in situ. The activity of this film electrode was better than those prepared by othermethods , following sequential coatings of solution or mixed oxide coatings. Electrode kinetic parameters, such as Tafel slope, reaction order and activation energy, were found to be -65 mV decade-‘, m 1.2 and 18.4 kcal, respectively. Cyclic voltammetric studies. prior to the onset of oxygen evolution, exhibited two characteristic peaks corresponding to a diffusion-controlled surface redox process: Ni’+ + Ni3+ Based on these results, a probable mechanism for oxygen evolution 011 the oxide film electrode is suggested.

INTRODUCTION

Concerted efforts have been made during recent years [I 11 J to search for low cost, active and thermodynami- cally stable oxygen anodes, so as to replace the traditional nickel anode of commercial alkaline water electrolysis cells to decrease the energy consumption of electrolytic hydrogen production. Considering the activity, long-term stability and cost, mixed transition metal oxides, particu- larly spine1 type (Co,O,, NiCo,O,) or perovskite type, were described as very promising electrocatalysts for oxygen evolution in most papers [ll, 12-141. Noticeable variations in their electrocatalytic properties have been observed [8, 9, 14-161 by simply changing the method of preparation or by introducing partial substitution of constituent metals of the oxide by some other metal ions [.12. 131. Bockris and Otagawa reported [S, 6) that LaNiO,, synthesized by the hydroxide co-precipitation technique, was much more active than that obtained by high temperature solid state reactions. Recently, Balej [3] also prepared LaNiO, using the same co-precipita- tion technique and found high electrocatalytic activity similar to that reported by Bockris and Otagawa [6], though the initial activity decreased considerably with time. Similar problems were also noted [3] with Teflon-

t To whom correspondence should be addressed.

bonded LaNiO, electrodes. In these studies. the oxide electrodes used were in massive pellet form.

Recently, we have prepared [ 1.51 thin films of LaNiO, on Pt by spray pyrolysis and also by sequential coatings of solution of mixed metal nitrates. Preliminary investi- gations indicated that these film electrodes were more active than conventional pellet electrodes obtained by solid state reactions. It is noteworthy that the sprayed film showed a Tafel slope of 40 mV decade- ’ and second order kinetics in OH- concentration, while the layered film shows 6.5 mV decade -’ and first order kinetics. However, due to the high cost of the substrate, only limited studies were made.

We have now obtained similar films on ii low cost nickel substrate and studied the effects of oxide prepara- tion variables, such as concentration of solution used in coatings, preparation temperature and oxide loading, on the physicochemical and electrocatafytic properties of the catalytic film. These studies were undertaken with a view to obtain better electrocatalysts. Similar films on Ni were also produced by Balej [3], but they were not investigated for oxygen evolution in detail. Furthermore, this film showed a considerably higher oxygen overpotential com- pared to the film obtained by us.

In this work, suitable preparation conditions for an active LaNiO, film on Ni were established, and physicochemical and electrochemical properties of the film electrode in relation to oxygen evolution in KOH solution were investigated in detail.

203

204 S. P. SINGH et al.

EXPERIMENTAL

Thin films of LaNiO, on nickel plates (1 x 1.5 cm) were prepared at different temperatures by the method of sequential coatings of mixed metal nitrate solutions. Nickel plates were cut from a nickel foil (Aldrich, 99% purity, 0.025 mm thick), polished mechanically with emery papers, treated with cont. HNO, for 20 s, washed with distilled water, cleaned ultrasonically in acetone and then used for film preparation.

The oxide films used were prepared by dissolving equimolar mixtures of L4NW3. 6H,O and Ni(N0,),.6H,O in ethanol. The concentration of each of the metal nitrates in the mixture ranged from 0.025 to 0.25 M. Three drops of the mixture were placed on one side of the nickel plate with the aid of a 1 ml pipette and then uniformly spread over the whole surface. The solution-coated Ni was dried by a hot air blower for 20 min and then placed in an electronically controlled furnace at 500°C for 10 min. This procedure was repeated several times to get the desired oxide loading. Finally, the oxide film was heated at a higher temperature for 2 h so as to obtain the LaNiO, film. The furnace was then switched off and the sample was removed only when it reached room temperature.

As mentioned in Ref. [ 111, an electrical connection to the oxide film was taken from the nickel surface which was not used for the film preparation. For electrochemical studies, only 0.5 cm2 area of the film was used.

Electrochemical studies were carried out in a three- electrode conventional Pyrex glass cell as described elsewhere [ll]. The potential of the working electrode was measured against an Hg/HgO-1 M KOH electrode. All potential values for the test electrodes are given with respect to the reversible potential for this reference electrode, unless mentioned otherwise. The reference electrode was brought into contact with the cell solution through a Luggin capillary (KCL agar-agar salt bridge).

Cyclic voltammetry studies were carried out using a bi-potentiostat (Model RDE4, Pine Instruments Com- pany, U.S.A.) connected to an X-Y recorder (Model 200, Houston instruments). Before recording voltammograms, the electrode was first cycled for 2-3 min at a potential scan rate of 40 mV s- ‘.

The electrochemically active surface area of electrodes was determined from double layer charging curves using a cyclic voltammetric technique [S, 11, 131.

The oxygen evolution studies were carried out at the oxide electrode by recording the iR-free anodic Tafel curves at a potential scan rate of 0.2 mV s- ’ with an electrochemical impedance system (Model 273A, EG & G PARC). Before recording each curve, the electrode was polarized at a potential of 0.65 V for 10 min and then brought back to the open circuit conditions, and after 2 min or so the curve was recorded. To know the approxi- mate order of the ohmic resistance of the catalytic film, the resistance of the electrode, including the contribution of the electrolyte (R), was determined by impedance measurements.

RESULTS AND DISCUSSION

Cyclic voltammetry

Figure 1 shows the cyclic voltammograms between 0 and 0.7 V for the LaNiO, films of different oxide loadings ranging from 3.59 to 13.2 mg cmm2 at a potential scan of 60 mV s-l in 1 M KOH (25°C). Each curve in this figure exhibits a single anodic peak and a corresponding cathodic peak before the oxygen evolution peak. The nature of voltammograms was almost the same regardless of the oxide loadings. Matsumoto et al. [ 171 also observed a pair of oxidation and reduction waves at potentials adjacent to oxygen evolution with massive LaNiO, electrodes. The cyclic voltammetric behaviour of the electrode was also examined at varying scan rates. Typical cyclic voltammograms as a function of scan rate for an oxide loading of 6.17 mg cmm2 are shown in Fig. 2. Different voltammetric curves were analysed and the results are summarized in Table 1. The values of anodic and cathodic current peaks shown in the table have been corrected for the contribution of background current using the procedure demonstrated in Ref. [18].

Table 1 shows that the increase in LaNiO, loading on Ni has practically no influence on the redox potential (E”) for oxidation-reduction of surface sites, as well as on the position of anodic and cathodic peaks. However, with the exception of the electrode of 13.2 mg cm-* loading, all the electrodes show an increase in anodic and cathodic peak current values with increasing oxide load- ing. The anodic peak potential (E,,) becomes slightly

Scan Rate = 0.06 Vk

0.0 0.2 0.4 0.6 o-3 Potential/V

Fig. 1. Cyclic voltammograms of the LaNiO, films of different loadings at a potential scan of 60 mV s-l in 1 M KOH (25’C).

liSE OF ACTIVE FILMS OF LaNiO, IN ALKALINE WATER ELECTROLYSIS 205

Table I, Effects of preparation temperature and oxide loading on cyclic voltammograms of the Ni/LaNiO, electrode at a potential scan of 60 mV s-l in 1 M KOH (25°C)

Preparation Oxide temperature loading

(“C) (mg cm-‘)

E,, DE E A-% 'W : :,, , pi’ ‘Pi ED.5 + E,, EOEl-- (VI w (VI (mA cm ‘) (mA cm ‘!

550 6.15 0.530 0.380 0.150 0.455 600 3.w 0.500 0.385 0.115 0.440 600 4.51 0.515 0.410 0.105 0.460 600 6.17 0.510 0.400 0.110 0.455 600 9.00 0.540 0.370 0.170 0.455 600 1 !3.‘0 0.480 0.380 0.100 0.430 650 6.31 0.520 0.385 0.135 0.453 700 5.Yi 0.525 0.420 0.105 0.470

I 19 I!8 1 (,h I .29 1 !9 1 .3? i)c)J 0.89

more positive, while the cathodic peak potential is less positive with increasing scan rate. It was noted that with ten-fold increase in the scan rate (i.e. from 10 to 100 mV s- ‘), the peak separation potential (A$) increased nearly 55-100 mV. Furthermore, a plot of peak current (i,) as a function of scan rate indicated a non-linear relation between i, and scan rate. On the other hand, the plot q vs (scan rate)-‘j2 (Fig. 3), where q is the charge used for oxidizing or reducing surface sites of the oxide obtained by the integration of the voltammetric curve between 0 and 0.65 V, was found to be linear. The i, vs (scan rate)“2 curves were also linear (Fig. 4). Figures 3 and 4 clearly indicate that the surface redox processes are diffusion controlled. Cyclic voltammograms were also obtained on the catalyst film prepared at different temperatures, i.e.

0.10 _

0.06

0.06

-40’ 0.0 0.2 0.4 0.6 0.8

Potential/V

Fig. 2. Cyclic voltammogram of the LaNiO, film (6. I7 mg cm -‘) at different potential scans in I M KOH (25°C).

o.,L- i 0.1 0.2 o-3

(Scan mtep, m l%“2

k-i&. 3. Plot ot’ charge (y) VI; (scan rate1

550, 650 and 700°C and the electrochemical spectrum so obtained followed similar experimental findings, except that i,,/i,, values were reduced to 0.80 + 0.08 in the case of the film prepared at 700°C’.

Similar voltammograms and E,,, E,, and E” values were also reported for Ni [ 191. M/Ni(OH), (M L= Ni or Pt) [20] and NiCo,O, [ 11, 213 in 1 M KOH. Thus, the observed anodic and cathodic current peaks on the LaNiO, surface before the oxygen evolution current peak are mainly due to the oxidation-reduction of Ni(lI) sites following a diffusion-controlled process. Similar sharp peaks were not, to our knowledge, reported with massive pellets of LaNiO,.

The roughness fbrtor

Cyclic voltammograms of the oxide electrodes were recorded in a 35&400 mV potential range at varying scan rates and in 1 M KOH (25°C). Tha charging current density (i,,,) was measured at the middle of the scan range

206 S. P. SINGH er al.

Table 2. Effect of the preparation conditions on the electrocatalytic properties of the Ni/LaNiO, electrode towards oxygen evolution in 1 M KOH (25°C)

Concentration of each metal nitrate in solution (M)

Preparation temperature

(“Cl

Oxide loading Roughness Tafel slope (mg cm-*) factor (mV decade-‘)

Eat100 mA cm-*

(mv)

0.025 700 5.85 220 70 3.4 0.050 700 6.00 366 68 5.3 0.10 700 5.93 393 60 3.5

0.15 700 6.10 406 80 4.5 0.20 700 6.15 416 82 3.4 0.25 700 6.18 423 65 4.4 0.10 650 6.20 590 62 2.5 0.10 550 6.15 733 55 0.6

875 860 820

(low 845 850 900 810 790 (940) 883

(low 820

(957) 760

(895) 764

(900) 776

(926)

0.10

0.10

600

600

3.59

4.51

363 66 0.7

493 65 0.7

0.4

0.3

0.10

0.10

600 6.17 666 64

600 9.00 867 64

0.10 600 13.20 980 67 0.3

Values of E shown in parentheses are measured at 300 mA cm-*.

of each voltammogram and icap was plotted vs scan rate. It was noted that the anodic and cathodic current densities were nearly equal in magnitude at each scan rate and icap vs scan rate curves were linear. Typical cyclic voltammograms and icap vs scan rate curves for the

6.0

N ‘6 2.0

2 ‘, 0.0 B .-

-2.0

-4.0

-6.0

0:35 0.40

Potent id IV

Fig. 5. Cyclic voltammograms of the Ni/LaNiO, electrode (6.17 mg cm-‘) in the potential range of 0.35-0.40 V at varying scan

rates in 1 M KOH (25°C).

Ni/LaNiO, electrodes with a loading of 6.17 mg cm-* are displayed in Figs 5 and 6, respectively. Values of the double layer capacitance were determined by measuring the slopes of ifap vs scan rate plots. The roughness factor was calculated by assuming a double layer capacitance of 60 PF cm-* for a smooth surface [22].

Values of the roughness factor for the catalyst films prepared under varying conditions are given in Table 2, in which one can see that, with the increase in concen-

( Scan Rate ;” / m j”z:‘2

Fig. 4. The variation of anodic and cathodic peak currents as a function of the square root of the scan rate.

USE OF ACTIVE FILMS OF LaNiO, IN ALKALINE WATER ELECTROLYSIS ?!I7

oort 0 20 LO 60 80 100

Scan Rote / m v s-’

Fig. 6. Plot of capacity current vs scan rate for LaNiO, films (If different oxide loadings in I M KOH (25°C).

tration of the precursors, the roughness factor increases. The latter approximately doubles with a two-fold increase in concentration, the roughness factor also increasing when the oxide loading increases. Data shown in Table 2 and Fig. 7 demonstrate that as the oxide loading is increased from 3.59 to 13.2 mg cm-‘, the roughness factor increases almost linearly at the beginning and then approaches a constant value. This complex behaviour may be due to the fact [23] that at low loadings almost the entire crystallites of the oxide come into contact with

Sintering Temperature /“C

600 700 I I

Oxide Loading / mg cm-*

Fig. 7. The effect of sintering temperature and oxide loadings on the roughness factor.

the solution, but when loading exceeds a certain critical value some crystallites are excluded from contact with the solution and the number of such crystallites increases as the layer grows. This explains the observed asymptotic behaviour of the roughness factor at higher loadings.

The increase in roughness at high concentrations of precursors may be due to the higher volume of gas (NO,, 0,) which evolves during the decomposition of more concentrated nitrate coatings [24]. The increase in the oxide preparation temperatures from 550 to 700°C de- creased the electrochemically active area of the oxide, as shown in Fig. 7. This behaviour is well known in catalysis [25] and is attributed to crystallite growth and sintering when the temperature rises.

Tqfel plots

The iR-corrected E vs log i plots for LaNiO, films on Ni prepared under varying conditions were recorded at a potential scan rate of 0.2 mV s- ’ in 1 M KOH at 25°C. Each Tafel line exhibited two linear regions with slopes of 55-80 mV (i < 100 mA cm-*) and 100 130 mV decade-’ (i > 100 mA cm-‘), respectively. Values of the initial Tafel slope and of the oxygen overpotential at a current density of 100 mA cm - * for different film electrodes are shown in Table 2.

The resistance of the film electrodes (R), including the contribution of the electrolyte (1 M KOH), was also determined and values ranged between 5.3 and 0.3 ft (Table 2).

Based on values of the overpotential at 100 mA cm - 2, the LaNiO, film obtained at 700°C using a solution containing 0.1 M of both metal nitrates [20 ml 0.2 M La(N0,),.6H,O + 20 ml 0.2 M Ni (N0,),.6H,O] was electrocatalytically more active than those prepared from other solution concentrations. Furthermore, Table 2 shows that the oxide films synthesized at 600°C are more active than those made at other temperatures. The resistance of this film electrode was also quite low. The higher values of R for oxide films prepared at 650 and 700°C may be due to the formation of NiO at the Ni/LaNiO, interface. The study of oxygen evolution on oxide films of different loadings indicated that the eiec- trodes with a loading smaller than 6.17 mg cnM2 (i.e. 3.59 and 4.51 mg cm-*) were electrocatalytically less active since they showed considerably higher oxygen overpotentials in 1 M KOH as compared to those of higher loadings. Values of the oxygen overpotential for the film electrodes containing the oxide loadings 6.17,9.0 and 13.2 mg cm-* were not much different at 100 mA cm-* in 1 M KOH, and these were 760, 764 and 776 mV, respectively. However, at a current density of 300 mA cm- *, the electrode of 13.2 mg cm - * loading gave -40 mV higher oxygen overpotential. Thus, the elec- trodes prepared at 600°C with the oxide loadings of 6.17 and 9.0 mg cm-’ were considered as the best oxygen anodes among the electrodes used in situ.

To investigate the reaction order with respect to OH - concentration, two film electrodes of nearly the same loadings (-6 mg cm-*, one prepared at 600°C and the

208 S. P. SINGH er al.

A 0.5M 1) 0.760 - , 1.0~ ~1

0 2.0 M *>

0.540. I I I -3.750 -2.750 -1.750 -0.750

log i /A cm2

Fig. 8. Tafel plots for oxygen evolution on LaNiO, film (6.17 mg cm-‘) at varying KOH concentrations (25°C).

other at 700°C) were chosen and tested for 0, evolution at different KOH concentration at 25°C; the results in the case of the oxide prepared at 600°C are shown in Fig. 8. Using data of this figure, log i vs log aOH- plots (Fig. 9) were obtained at different constant applied potentials across the Ni/LaNiO,/KOH solution interface. From the slope of these linear plots, the reaction order was found to be approximately 1.2 in each case.

Similar values of the Tafel slope and the reaction order were also observed [15] on LaNiO, film obtained on Pt at 700°C. Balej observed [3] 0.072 V as the Tafel slope for a similar film on Ni in 10 M KOH at 700°C. However, the film obtained on Pt by spray pyrolysis yielded a low Tafel slope of 40 mV decade- ’ over a current density of about 100 mA cme2 and second order reaction kinetics in OH- concentration [15]. Bockris and Otagawa also found [6] a Tafel slope of 45 mV decade-’ but a first

-l.O-

Ni /LaNi@

-4.0 - 0 0.62 mV 6ooQc

A 044mV }

-5.0 I I I I 1

-1.2 -0.6 -0.4 0.0 0.4 0.6

b OOH-

Fig. 9. Plot of log i vs log uOHm at different constant applied potentials across the LaNiO,-KOH solution interface (25°C).

0.760 I I I

30 W/O KOH

0 E?P

0.700 t

. *Lx A 50% A 6O’C Cl 7o’c

> ’ 0,620- l&J

0.460

-3.250 -2.250 -1.250 -0.250

log i /A cm‘*

Fig. IO. Tafel plots for oxygen evolution on LaNiO, film (6.17 mg cm-‘) at different temperatures in 30 w/o KOH.

order reaction in OH- ion concentration on LaNiO,, prepared by a co-precipitation technique. On the other hand, the same oxide prepared by a high temperature solid state reaction had a Tafel slope of 65 mV decade- ’ and first order reaction kinetics [S].

The study of 0, evolution on the active LaNiO, film has also been carried out at higher tempreatures (40, 50, 60 and 70°C) in 30 w/o KOH (Fig. 10). The Arrhenius plots (log i vs l/T) at different constant applied potentials across the oxide film-KOH solution interface were also constructed (Fig. 11) and the activation energy (18.40 kcal) was determined from the slope of each straight line plot.

The results of cyclic voltammetry and of electrode kinetic studies suggest the following mechanism for 0, evolution at the Ni/LaNiO, electrode in KOH solutions:

0 0.52v -

A 049v

0 0.47v

-3.0 -

1 I 1 ! 2.9 3.1 3.3

; x 103/‘K-l

Fig. 1 I. Arrhenius plots at different constant applied potentials across the LaNiO,-KOH solution interface.

LISE OF ACTIVE FILMS OF LaNiO, IN ALKALINE WAIF-R ELECTROLYSIS ?iW

- 0.75

0000 0 0 0 0 0 0 0 0 0 0 00

c

I”‘-:

00 E ; m -2 z

i z 100 mA cm

a (Without ii?- correction)

0.65 0

I I 15 30 L5

Time /hours

Fig. 12. Variation of electrode potential with time at a constant current density (100 mA cm-*) in 30 w/o KOH (25°C).

ScOH F”SOH+e- (1)

SOH + OH ;;;;’ S.H20, + e-~ (2)

2SH,O> d ‘S + 7H,O + 0, (3)

where S ( = Nil”) is an active site on the oxide surface. The electrode kinetic parameters such as Tafel slope

and the reaction order obtained on the layer deposited LaNiO, film on Ni can be explained by considering the involvement of Temkinian behaviour of the adsorption process. In this. the total surface coverage (0,) by the adsorbed intermediates, SOH and S. H,O,, influences the heats of adsorption and hence the free energies of activation. Such adsorption has already been considered by Thomas [26] and Parsons [27] in the case of hydrogen electrode reactions and by Conway and Gileadi [28], Conway and Salomon [29] and Bockris and Otagawa [S] in the oxygen evolution reaction.

Thus. from the above mechanism, the following ex- pression (41 for the overall current density under Temkin adsorptlon conditions for oxygen evolution can be ob- tained (details of treatment are given elsewhere [IO]):

/ = 2Fk,t~,~COlt exp[ -fRT In K, + RT In CO,,

-t F‘,lcb)(l - z)‘RT] exp[(l - /i)FA&RT], (4)

where CI and [zI are the symmetry factors, and K, is the adsorption desorption equilibrium constant for step 1. Other terms have their usual meanings.

Assuming 11, is relatively constant under intermediate coverage conditions (0.2 < 0, > 0.8). equation (4) gives values of the Tafel slope and reaction order, -60 mV decade ’ and 1.5. respectively, with z and /I each being approximately 0.5. These theoretical values are in close agreement with the experimental values found for oxygen evolution on the NiiLaNiO, electrode.

Our results reveal that the LaNiO, thin film electrode (6.17 mg cm .‘) is much more active than the massive LaNiO, electrode [S] prepared by high temperature reactions, but it is less active than the electrode prepared by a co-precipitation technique [6]. The respective oxy- gen overpotentials found on these electrodes were -0.46, - 0.5 I and -0.315 V at a current density of 100 mA cm 2 in 1 M KOH at 2S’C.

The electrocatalytic activity of the electrode prepared by us has been found to be comparatively better at higher current densities in 30 w;o KOH at 70°C. This gave an oxygen overpotential of 290 mV at 0.5 cm -’ and 320 mV at 1.0 A cm 2 (70°C). Balej found [3] approximately the same overpotentials for similar films of LaNiO, (2%) mV), La,,Sr,,.,Co,,,Ni, ,O, (288 mV) and La,, ~ Ba,,,CoO, (277 mV) on Ni at 0.5 A cm --’ in 10 M KOH at 100°C. Wendt and Plzak observed [30] ~0~ ,- 330 rn!. for La,, Srr,,i COO, and LaNi,,,C~~,,, 0 a, coatings at I .O A cm ’ m 50% KOH at 90°C‘. Fiori ant! Mari found [31] $I2 = 320 mV at 0.5 A cm ’ in 30”If KOH at 80°C for LaNi, ,Co,, ,O, coating. AI thi: Teflon-bonded NiCo,O,. oxygen overpotential amounted to 377 mV at 0.5 A cm ’ in XI06 KOH ;II 80°C [32].

The stabiliry of the film electrode for current densit! of 100 mA ~’ has also been examined for duration up to 45 h in 30 w. o KOH (Fig. 12). This study indicated that the electrode potential increased by - 50 mV during the first 5 h and became almost constant thereafter. After the test was ocer. an oxygen evolution study was also made with this electrode in the same solution and showed ncari! the same Tafel slope as obtained before: houcvixr. th: oxygen overpotential was slightly incrc:lscd

This study indicated the large inlluence of film prep- aration conditions on the oxygen evolution reaction. Based on oxygen overpotential values at 300 mA cm ’ in I M KOH (25°C~. the catalytic film prepared & 6OO”(’ has the highest activity for oxygen evolution. with an oxide loading of 6.17 mg cm ‘The clectrocatalytiu activity of this film electrode is also found to be bettci than other pcro\,skitc film elcctrodc rcporiotj in the literature.

A~knoM,/edgrn,eMt,s~ Financial support from the Indo- French Centre for the Promotion of Advanced Research (Centrc France-Indien pour la Promotion ds ia Rc-- cherche Avancee). New I)elhi. India. LS gr:ttehlliy ac knowledged.

I. 2.

3 4:

5.

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7.

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