Lanthanum Phosphate-Bonded Composite Nickel-Rhodium ...

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ABSTRACT. The electrocatalytical properties of the lanthanum phosphate-bonded composite nickel-rhodium electrodes for the hydrogen and oxygen evolution ...
Lanthanum Phosphate-BondedComposite Nickel-Rhodium Electrodes for Alkaline Water Electrolysis H. Dumont and P. Los~ DEpartement de chimie, Universitd de Sherbrooke, Sherbrooke, QuEbec, Canada J I K 2R1

L. Brossard Institut de recherche d'Hydro-Qudbec (IREQ), Varennes, QuEbec, Canada J3X 1S1

A. Lasia and H. M~nard Departement de chimie, Universitd de Sherbrooke, Sherbrooke, QuEbec, Canada J I K 2R1 ABSTRACT The electrocatalytical properties of the lanthanum phosphate-bonded composite nickel-rhodium electrodes for the hydrogen and o x y g e n evolution reactions were investigated in 1M alkaline solution at 25~ The electrodes were prepared from chemical deposition of r h o d iu m on nickel particles and subsequent polymerization of Ni/Rh powder with l an t h an um phosphate. The influence of the r h o d i u m content and other experimental conditions during the electrode preparation were considered in relation with the electrocatalytical activity. The ac impedance and steady-state polarization m et h ods were used to investigate t h e m e c h a n i s m and kinetics of the hydrogen evolution reaction.

Alkaline water electrolysis calls for low hydrogen and oxygen overpotentials. High surface area nickel-based electrodes are k n o w n to be a m o n g the most active electrocatalysts in hot concentrated alkaline solutions. Such electrodes can be obtained by sintering, v a c u u m or plasma spray deposition, electrodeposition, electrocodeposition, or high-temperature deposition ~s. Recently, a n ew process of cementation of metallic powders to produce high surface area electrodes has been reported 9n. The metallic particles have been binded by an inorganic polymer: a l u m i n u m or lanthanum phosphate. The electrodes have been shaped into useful pellets by pressing t h e m u n d e r vacuum. The spiky filamentary Ni powders were used for the fabrication of lanthanum phosphate-bonded nickel (LPBN) electrodes having a roughness factor of 3900 H. The L P B N electrodes with 2 weight percent (w/o) LaPO4 show no disintegration in alkaline solution after several hours of i m m e r s i o n at open-circuit potential or u n d er an applied cathodic current of -150 m A cm -2. The hydrogen overpotential at 150 m A cm -2 was 200 mV. The electrocatalytic activity of such an electrode ma y be theoretically i m p r o v e d by the deposition of a metallic el em en t onto the nickel particles, if the deposited metal has a low overvoltage for the hydrogen evolution reaction (HER). In addition the electrode material should be chemically stable in contact with alkaline solutions. The present investigation deals with L P B N electrodes (2 w/o LaPO4) with chemically deposited r h o d i u m on the spiky filamentary nickel particles with respect to their performance during electroreduction of water in 1M alkaline solutions. R h o d i u m is k n o w n to be a very active metal for the H E R in alkaline solution~2-~s; in addition, it is chemically stable in such solutions ~2. The electrode material was also characterized for the oxygen discharge.

Theoretical Considerations The m e c h a n i s m of the H E R on nickel and nickelrhodi u m electrodes was studied by the ac i m p e d a n ce and Tafel curve methods. The ac i m p e d a n c e m e t h o d is considered as the m o s t suitable t e c h n iq u e for studying electrode kinetics on solid electrodes. There is a significant difference b et ween the geometric and real surface area of rough electrodes. Roughness of the electrode causes nonuniform distribution of current density and affects freq u e n c y dispersion of i m p e d a n c e results. The experimental data of ac m e a s u r e m e n t s of rough electrodes could be fita On leave from Department of Pharmacy, Medical University, 50-139 Wroclaw, Poland.

ted using the constant phase element (CPE) in parallel with the faradaic i m p ed an ce or fractal models 14,16-26.In this work, the CPE m o d el gives the best approximation of experimental ac curves using the co m p l ex nonlinear least squares fitting program (CNLS) written by MacDonald et al. ~. The constant phase el em en t is defined by the equation 1 ZcpE = [1] T(jr w h er e 4~ is the phase shift, o,-angular frequency, and T a constant. The value of 4~is related to the surface roughness and is equal to 1 and T = C~ for ideally flat electrodes. The values of A, T, and 4~ parameters are determined from ac m e a s u r e m e n t s 27. The value of A is equal to 1/Rct where R~ is the charge-transfer resistance. The fitting procedure of the d ep en d en ces of log i and log A on the overpotential is used to determine the m e c h a n i s m of the HER. The details of the calculation procedure were shown in Ref. s-26.

Experimental Preparation.--Chemical deposition of rhodium on nickel particles.--The spiky filamentary nickel particles (Inco 255) were put in a beaker containing 50 ml of Barnstead Nanopure water. The solution was deaerated by bubbling nitrogen. The correct a m o u n t of r h o d i u m was added to the solution in the form of RhCls - 3H20 (Aldrich Co.). A few drops of concentrated HC1 was added to the solution to solubilize r h o d i u m complexes. The red-ox reactions to obtain Rh deposited onto Ni particles are ~2 RhCI~- (aq) + 3e --> Rh + 6CI- (aq)

[2]

Ni --~ Ni 2+ + 2e

[3]

and the overall reaction is 3Ni + 2RhCI~- --* 2Rh + 3Ni 2+ + 12C1-

[4]

The deposition reaction en d ed when the initially dark brown solution b e c a m e green due to the presence of dissolved nickel. UV/vis analysis of the solution shows the complete disappearance of dissolved r h o d i u m in solution, and the x-ray fluorescence (XRF) of the electrode showed that all the Rh present initially in the solution was deposited on nickel. In a different set of experiments, Ni/Rh particles were heated u n d e r an inert atmosphere to promote the formation of a Ni-Rh alloy 28 through the diffusion of both elements.

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Preparation of the binder.--Lanthanum phosphate was p r o d u c e d by co m b i n in g acid l a n th a n u m phosphate and l a n t h a n u m h y d r o x i d e u. Two overall reactions are involved in the formation of acid phosphate l a n th a n u m La(H2PO4)3 and its s u b s e q u e n t transformation into LaPO4 1/2 La203 + 3HaPO4 --~ La(H2PO4)3 + 3/2 H20

[5]

La(H2PO4)3 + 2La(OH)3 ~ 3LaPO4 + 6HzO

[6]

T he La(H2PO4)3 was synthesized by adding 48.88 g La203 to 103.7 g H=PO4 (85%) in a rectangular Teflon cell measuring 10 • 5 x 5 cm, since a reactive intermediate reacts with glass rather than Teflon. The reactive m i x t u r e was stirred constantly during the addition of La203 until a very thick, silicon-like paste was obtained. Since the reaction is very fast, strongly e x o t h e r m i c and autocatalytic, even w h e n the c o m p o n e n t s are m i x e d at room temperature, the La203 m u s t be completely incorporated within a few minutes. As soon as the reaction ended, the Teflon reactor was heated in an oven at 150~ for 24 h. The reaction product, in the form of a viscous paste, was transferred to a flask and vacu u m dried to r e m o v e traces of water resulting from the reaction. T h e addition of 0.883 g La(OH)3 per 1 g of La(H=PO4)3 m a d e it easier to crush and m i x the two components in a ball mill. The m i x t u r e was stored for several days in a dessicator before final polymerization. It should be noted that polymerization is very slow at room temperature.

Preparations of the electrodes.--A h o m o g e n e o u s mixture of La(H2PO4)3 and La(OH)3 was added to a desired a m o u n t of Ni/Rh powder, and m i x e d thoroughly manually. Th e electrodes were pressed u n d e r v a c u u m in a m o u l d 1.3 c m in diameter and Ni foil was introduced into the pellet to ensure a proper electrical contact. After 12 h of preheating at 120~ polymerization was achieved by heating the pellets for 3-4 h u n d e r a flow of argon at different temperatures (200 to 800~ The electrodes were coated on one side and on the edge with Epofix resin (Struers) to obtain a 1.33 cm 2 geometric surface area. The r h o d i u m content in the electrode materials represented 0.8, 1.5, 3.5, 8, or 12 w/o c o m p a r e d to 2 w/o for the binder with the remaining consisting of nickel. Electrochemical characterizat~on.--The experiments were p er f o rm ed in a dual-compartment glass cell separated by a D u p o n t Nation e m e m b r a n e (Electrosynthesis Co.). The t e m p e r a t u r e of the cell c o m p a r t m e n t containing the w o rk i n g electrode was kept constant by circulating of t h e r m o s t a t e d water. A Luggin capillary probe was placed less than 1 m m from the vertical working electrode. A nickel grid with a projected geometric surface area of 40 cm 2 was used as the counterelectrode and the reference electrode was Hg/HgO/1M KOH. The measured value of the reversible potential of the H E R in 1M KOH was -925 m V at 25~ with respect to the reference electrode. Barnstead N a n o p u r e water with 17.5 Ml2-cm resistivity was used to prepare KOH (Fischer-certified ACS reagent grade) solutions w h i c h were deaerated by bubbling nitrogen. Th e ac m e a s u r e m e n t s were carried out using 1M NaOH (Aldrich, 99.99%) solutions. The steady-state and ac m e a s u r e m e n t s were m a d e using the P A R 273 potentiostatgalvanostat and a P A R 5208 lock-in analyzer controlled by a C o m m o d o r e PC2.

current densities from 250 m A cm -2 to 1 9 10 5 m A cm -2. The Tafel parameters then became constant during further cycling, and reproducible ac measurements were obtained.

Determination of OER kinetic parameters.--The measurements for Ni electrodes were performed at 25~ after 30 min preanodization at 70~ This procedure gave a stable and reproducible oxide surface ~1. The electrodes were further stabilized by cycling from - 1 to 0.4 V vs. Hg/HgO/1M KOH at 10 m V s -1 until the v o l t a m m o g r a m s became reproducible. It is k n o w n that a build-up of an oxide layer is possible u n d er such conditions 29. The Tafel parameters of the OER were obtained on Ni and Ni/Rh electrodes in 1M KOH at 25~

Results and Discussion Characterization of materials.--The mechanical strength of the L P B N electrodes and their chemical stability is closely related to the experimental conditions under which the polymerization was carried out. Special attention was placed on the influence of the polymerization temperature on the electrode material characteristics. The curve obtained by differential thermal analysis (DTA) on nickel particles covered by r h o d i u m (12 w/o) is shown in Fig. 1. F r o m this curve it has been d ed u ced that the rate of formation of Ni/Rh alloys becomes significant close to 510~ Fu r t h er nickel particles covered by r h o d i u m (12 w/o) were heated at 400 or 600~ during 3 h under an argon atmosphere prior to being investigated by x-rays diffraction (XRD). At 400~ the formation of a Ni/Rh alloy was not observed after this treatment. At 600~ a Ni/Rh alloy is formed from the interdiffusion of both metals. The above observations are in ag r eem en t with Ref.28; the interdiffusion of both metals is facilitated by the fact that both metals are centered cubic. When the p o w d e r was heated from 300 to 800~ SEM analysis (Fig. 2) reveals that the needles present on Ni fractal particles tend to disappear and the specific surface area of the powder decreases according to B E T measurements. However, the B E T surface area of Ni/Rh powder tends to increase with higher Rh content (Table I). This behavior is ascribed to the presence of small aggregates of Rh or Ni/Rh alloy on Ni particles. As the temperature is increased from, e.g., 200 to 600~ the needles present on the Ni particles and the aggregates of Rh or Ni/Rh possibly present on those particles m ay tend to disappear, resulting in a decrease of BET surface area. The roughness factors (R~) for electrodes containing from 0 to 12 w/o of Rh are also summarized in Table I. The Rf is defined as a specific real (BET) area of the electrode divided by its projected geometric area measured in m 2 g-'.

Electrochemical results.--HER kinetic parameters.--The Tafel curves were registered in order to determine electrode activity of (Tables II, III, and IV). It should be noted that the Tafel parameters obtained after galvanostatic electrolysis (Table III) differ from those presented in Table IV w h i ch were calculated after the multiple cycle (multicycle) polarization treatment previously described.

sl0 ~ c

Determination of HER kinetic parameters.--Cathodic polarization curves were obtained by decreasing the applied current galvanostatically from 250 to 0.01 m A cm -2 after 30 rain u n d er a cathodic current density of 250 m A c m -2. The electrode potentials were corrected for the ohmic drop which was determined by the currentinterruption and the ac i m p e d a n c e techniques; both methods gave similar results n,24. The influence of both the Rh contents and the LaPO4 polymerization temperature on the m e c h a n i s m and kinetics of the H E R and OER were investigated. In another set of experiments, the ac measurements were carried out after approximately 30 cycles, each cycle consisting of 30 m i n of galvanostatic electrolysis at 125 m A c m -z and 10 rain of Tafel curve m e a s u r e m e n t in cathodic

-~ -o 2S

-7

13~ 3

266 7

a00

TEMP~

533 3 (Heating}

666 7

8O0

Fig. 1. Differential thermal analysis of the Ni/Rh alloy formation.

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Fig. 2. SEM pictures of Ni particles in LPBN electrodes: (a, top left) heated at 200~ at 600~ and (d, bottom right) heated at 800~ T h e i n f l u e n c e o f t h e m u l t i c y c l e t r e a t m e n t is s i g n i f i c a n t f o r n i c k e l - r h o d i u m e l e c t r o d e s p r e p a r e d a t 600~ I n a d d i t i o n , the Tafel parameters for Ni/Rh powder electrodes depend

Table I. Specific surfaces and roughness factor of materials

obtained by BET.

Powder type

Polymerization a temperature (~

Specific b surface area (m2gr -x)

Ni Ni Ni Ni Ni Ni/Rh 0.8% Ni/Rh 1.5% Ni/Rh 3.5% Ni/Rh 8.0% Ni/Rh 12% Ni/Rh 3.5% Ni/Rh 3.5% Ni/Rh 3.5%

RTd 200 300 400 600 RT RT RT RT RT 300 400 600

0.6 -0.6 0.55 0.37 1.6 1.3 1.7 2.4 -1.9 1.0 0.5

R o u g h n e s sc factor

(b, top right) heated at 400~

8,200 30,000

a For - 3 h. b Powder. c L P B N electrode. d RT: p o w d e r at r o o m t e m p e r a t u r e , no polymerization.

Table III. Effect of polymerization temperature and Rh contents on HER parameters of Ni powder electrodes. Temp. ~

0.0 0.8 1.5 3.5 8.0 12.0

300

Polymerization temperature (~

Tafel slope (mV/decade)

200 300 400 600 800

133 132 130 115 126

16.6 12.0 3.1 0.3 0.5

157 177 247 334 345

600

io

I]250

b

1o

"q250

b

io

11250

132 106 122 77

12 17 20 27

177 125 132 74

130 100 120 115 112 90

3.1 7.0 7.8 12 20 5.6

257 145 184 150 130 146

114 145 124 138

0.3 1.8 7.8 8.3

334 305 190 205

81

29

76

PureRh powder

b Tafel slope (mV/decade). I E x c h a n g e c u r r e n t d e n s i t y (mA/cm2). ~0 Overpotential at 250 m A / c m 2 (mV).

Table IV. Tafel parameters of the HER after multicycle polarization experiment.

Electrode Overpotential at 250 m A / c m 2 (mV)

400

b

Table II. Effect of polymerization temperature on HER parameters of Ni powder electrodes. Exchange current density ( m A / c m 2)

(c, bottom left) heated

strongly on the polymerization temperature while the T a f e l s l o p e o f p u r e N i (-~130 m V / d e c a d e ) w a s n o t a f f e c t e d significantly by the polymerization temperature. It should b e a d d e d t h a t t h e 0250 f o r p u r e n i c k e l L P B N d e c r e a s e d sig-

%Rh

6,500

2145

Ni (300~ Ni (600~ Ni-Rh (300~ Ni-Rh (600~ a As m e a s u r e d .

Tafel slope

Exchange current density

Overpotential at 250 m A c m -2

(mV/decade)

( m A c m -2)

(mV)

124 118 92 111

4.14 0.28 50.0 0.11

216 283 a 65 330

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-0.5

0.2

o

O0

-1.0

oE - 0 . 2

r I

E - t .5

~P - 0 . 4

0

~-0.6

~--2.0

0

)

[7]

A s s u m i n g t h a t t h e c a p a c i t y v a l u e o f t h e s m o o t h m e t a l surface e q u a l s 20 ~ F / c m 2,10,23,a0,31, t h e r o u g h n e s s f a c t o r R w a s e s t i m a t e d ( T a b l e IV). B y d i v i d i n g t h e r o u g h n e s s f a c t o r obt a i n e d f r o m B E T ( T a b l e I) b y t h e e l e c t r o c h e m i c a l v a l u e r o u g h n e s s f a c t o r for t h e m o s t e l e c t r o c a t a l y t i c e l e c t r o d e , w e d e d u c e d t h a t a p p r o x i m a t e l y 44% o f t h e B E T r e a l surface a r e a is a c t i v e e l e c t r o c h e m i c a l l y . T h i s v a l u e is i n a g o o d a g r e e m e n t w i t h t h e r e s u l t s o f P o t v i n et a l ) ~ for p h o s phate-bonded nickel powder electrodes. It should be n o t e d t h a t t h e a c r e s u l t s a r e i n a g r e e m e n t w i t h t h e Tafel m e a s u r e m e n t s . T h e r a t e c o n s t a n t v a l u e s for N i - R h elect r o d e s o b t a i n e d a t a l o w e r t e m p e r a t u r e (300~ a r e a f e w orders of magnitude higher than those for the same electrode p r e p a r e d a t 600~ T h e d e p o s i t i o n o f r h o d i u m o n t o p u r e

Table V. Rate constant values obtained from ac measurements. Composition (w/o)

Temp. (~

kl 108 (mol cm -2 s -I)

k-1 108 (mol cm -2 s -I)

k2 107 (mol cm -2 s -I)

cq

az

Ni Ni Ni-Rh(3.5) Ni-Rh(3.5)

300 600 300

2.86 _+ 0.61 0.21 +- 0.01 84.5 -+ 8.5

600

0.056 -+ 0.004

2.92 -+ 2.63 a 24.5 +- 1.7 "

1.04 +- 0.73 a 11.1 -+ 0.6 a

0.46 +- 0.01 0.51 +- 0.01 -b 0.53 -+ 0.01

b b b b

a Very large value, precise determination is impossible. b a = 0.5 was assumed.

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?

-0.50 0.06

-075

-1,00

I

jj

c~J I

E

E 004

0

< -1 2 5

O LL

& - I 50 0

~0.02

-1 7 5 -200 -008

-006

-004

-002

~/v

000

nickel electrode improves electrocatalytic activity and changes the calculated surface coverage, 0, by adsorbed hydrogen (Fig. 8). The influence of polymerization t e m p e r a tu r e on the electrode activity is noted w h e n the rate constants divided by the roughness factor (Table VI) are considered. The highest value was obtained for the most active (Ni-Rh 3.5 w/o, 300~ electrode. F r o m these results it is obvious that the increase in nickel powder electrode activity after the addition of nickel p o w d e r covered by r h o d iu m is related to the increase of both the intrinsic activity and the real surface area. L o w electrocatalytic activity of the electrodes polymerized at high t e m p e r a t u r e is understandable w h e n we consider the above-mentioned changes in the morphology of nickel particles. Moreover, in the case of the Ni/Rh system, the formation of a solid solution occurs, which causes the decrease in electrocatalytic activity o f these electrodes. This conclusion is obvious w h e n we compare the value k]Rf (Table VI) for Ni/Rh electrodes prepared at 300 and 600~ OER parameters.--Table VII shows that OER kinetic parameters were slightly affected by polymerization temperatures up to 400~ for pure Ni powder electrodes and one Tafel region is observed at each temperature. These electrodes exhibit the same electrocatalytic activity at 250 m A c m -2 as in our earlier investigationsn for L P B N electrodes bonded with 2 w/o of LaPO4. T w o Tafel regions are observed at a polymerization temperature of 600~ while only one Tafel region is observed again at a polymer-

Table Vl. The rate constant values divided by the surface roughness for Ni and Ni/Rh electrodes.

Ni Ni Ni-Rh(3.5) Ni-Rh(3.5)

300 600 300 600

)5

-02

-01

O0

v/v

Fig. 6. Tafel plot obtained in 1M NaOH at 25~ for nickel-rhodium (3.5 w/o) 300 C electrode (11), calculated (--).

Composition Temperature of (w/o) polymerization (~

000

Roughness (kl/R)• 101~ factor (mol cm -2 s -1) 1000 1650 3250 1000

0.286 0.013 2.600 0.006

Fig. 7. Dependence of the double layer capacities on the overpotenfial: (O) nickel, 300 C; (O) nickel, 600~ (~) nickel-rhodium (3.5 w/o) 600~ (11} nickel-rhodium {3.5 w/o} 300~

07 06 05

..........

N i I : R h .......... ........ I I ~

05 Ni

02 01 O0 -04

i

-0.5

i

i

i

-0.2

-0.1

~/v

b

200 300 400 600 800

93 109 96 86 --

Low

~oz io

5.3 • 1 0 - 3 14.0 • 10 -s 6.8 • 10 -s 1.0 • 10 -s --

"Considering the region of high overvoltage (~ >420 mV). b Tafel slope (mV/decade). I ~Exchange current density (mA/em2)

0.0

Fig. 8. Calculated dependence of the surface coverage on the overpotential for nickel, 300~ (--) and nickel-rhodium (3.5 w/o), 3oooc ( ...... ).

ization t em p er at u r e of 800~ A dual Tafel slope (40 m V dc -1 in the region of low ~o2 compared to 170 mV dc -1 in the region of high 71o2was also reported in Ref. s2 for a smooth, polished Ni wire electrode. This slope variation probably means a change in the O ER mechanism, although the exact nature of this difference is not clearly understood. The O ER kinetic parameters for Ni/Rh electrodes are shown in Table VIII. The best performance is also obtained for the electrodes containing 3.5 w/o of Rh and polymerized at 300~ At 300, 400, and 600~ only one Tafel region is observed regardless of the values of ~o2 (Table VIII). The Tafel parameters for the OER are reminiscent of those reported for Ni p o w d e r electrodes in the region of low overpotential. A slightly beneficial effect of Rh content is observed from 0.8 w/o Rh to 3.5 w/o Rh despite the

Table VII. Effect of polymerization temperature on OER parameters of Ni powder electrodes. Polymerization temperature (~

/

o04

High b

~o2 io

--

170 159

--2 2 5 • 10 -s 15.0 • 10 -s

Overpotential at 250 mA/emz (mY) 437 463 438 518 a 670

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Table VIII. Effectof temperature and Rh contentson OER parameters of Ni powder electrodes. Temp. ~ Rh 0.8 1.5 3.5

300

400

600

b

I ~ • 10

~250

b

Io • 10-3

~2s0

b

I o • 10 -8

~2so

78 77 90

1.2 1.3 2.0

416 406 369

97 94 --

2.2 4.0 --

490 452 --

86 105 90

1.5 8.9 3.6

450 470 435

96

0.2

580

Pure Rh powder b Tafel slope (mV/decade). I~ Exchange current density (mA/cm2) ~50 Overpotential at 250 mA/cm2(mV).

fact that a pure Rh powder electrode has a lower electrocatalytic activity at 250 mA/cm 2 than Ni/Rh electrodes.

Conclusions A n e w type of l a n t h a n u m phosphate-bonded nickelr h o d i u m electrodes was studied for the HER and OER. Detailed studies of the electrode structure were carried out using XRD, SEM, BET, DTA, and XRF. It was shown that electrode electrocatalytic activity depends significantly on the temperature of polymerization and on the rhodium content. The most active electrode containing 3.5 w/o Rh was obtained at a polymerization temperature of 300~ The values of ~2~0 - 70 mV and io - 50 m A cm -2 are the same as for pure Rh powder and among the most active s. On the basis of ac measurement, it was shown that the HER proceeds via Volmer-Heyrovsky reaction. The increase in electrode activity was related to increases in both the intrinsic electrocatalytic activity and real surface area of l a n t h a n u m - p h o s p h a t e b o n d e d Ni/Rh electrodes. The presence of r h o d i u m in L P B N electrodes slightly improved the activity of this type of electrode for the OER.

Acknowledgments The Natural Science and Engineering Research Council of Canada, IREQ, and the Quebec government are acknowledged for their financial support. We t h a n k Mr. L. Timberg of Inco Metals C o m p a n y for furnishing the characterized nickel powders. Manuscript received Dec. 24, 1991.

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