446 SHORT COMMUNICATIONS

The dependence of the separation factor for the hyd rogen evolution reac- tion upon potential: Mechanism evidence for transition metal substrates

In the electrolytic hydrogen evolution reaction, recent work by BOCKRIS AND MATTHEWS 1 showed that the separation factor of hydrogen and tritium on mercury electrodes varies with the potential. This variation seems to depend upon a change in shape of the potential energy barrier of the rate-determining proton discharge step with potential, resulting in a change in the degree of tunnelling for proton and tritium.

Here, results of an examination of the potential-dependence of the separation factor when a variety of electrode catalysts is used are discussed. Generalizations are ( T a b l e I ) :

(i) I f t h e m e t a l s u n d e r c o n s i d e r a t i o n a r e i n t h e g r o u p o f " s o f t " m e t a l s (e .g . , H g ,

S n , Cd) , f o r w h i c h a r a t e - d e t e r m i n i n g p r o t o n t r a n s f e r s t e p h a s b e e n e s t a b l i s h e d 2,3,

t h e n t h e r e i s a v a r i a t i o n o f t h e s e p a r a t i o n f a c t o r w i t h p o t e n t i a l s i m i l a r t o t h a t o b s e r v e d

b y BOCKRIS AND MATTHEWS ( e x c e p t f o r t h e s p e c i a l c a s e o f C d p r e p a r e d b y e t c h i n g

i n H N 0 3 ) .

(2) I f t h e m e t a l s c o n c e r n e d a r e t h e t r a n s i t i o n m e t a l s (e .g . , N i , A g ) , t h e r e i s a

n e g l i g i b l e v a r i a t i o n o f t h e s e p a r a t i o n f a c t o r w i t h p o t e n t i a l e x c e p t w h e n t h e r e i s a

T A B L E 1

SEPARATION FACTORS AND TAFEL PARAMETERS FOR A NUMBER OF METALS AT ROOM TEMPERATURE

Metal Solution Separation Overpotential Magnitude -- b -- logio factor range of separation (m V) current variation (mV) factor ( A c m -~) with potential?

Tin 1.o N HC1 Tin~ i .o N N a O H

C a d m i u m b I.O N HCI C a d m i u m o I.O N HC1 Mercu ry i .o N HC1 Mercu ry i .o N H C I + zo N

(CsH~)4HC1 Mercury i .o N HCIO4 Mercu ry o. 5 N H2SO4 Silver a i .o N HC1

Silver I.O N N a O H Nickel i .o N HC1

Nickel I.O N NAOH I ron I.O N HC1

T u n g s t e n a i .o N HC1 No

T u n g s t e n a i .o N N a O H No

P l a t i n u m i .o N HC1 No

Yes 655 to -- IOOO 8 .2 -3 .6 z2o lO.2 No - -95o to --ILOO 8.0 116 8.0

220 5.4 Yes 99o to - -112o 6 .2-4. 5 115 9.5 No - - 9 2 0 to -- 112o 6.8 12o lO.2 Yes - -92o to - -115o 8 .8-6 .2 115. 12.o Yes - - I I o o to - -12oo 6 .6-5 . 3 i i o 12. 4

Yes - -94 ° to - -115o 8 .4-5 .8 112 11.9 Yes 95o to - -115o 8 .6-5. 7 113 12.3 No 660 to - - i o o o 6.3 68 7.2

- -22o to - -360 18 I15 5.4 No - - 4 8 0 to - -950 21 121 6.6 No - -560 to - -800 i8 113 5.8

-- 395 to -- 715 No - -25 ° to - -385 4.6 IOO 5,5 No - -45 ° to - -455 3.9

500 to - -580 5.0 12o 5.5 - -580 to --Too 15.9

380 to - -575 3.7 66 7.6 116 5.7

- -4o5 to - -775 6.7 7 ° 7.0 122 6.6

- -45 to 19o 6.7 28 3.o

At h igh overpoten t ia l s , SnH4 is be ing formed. b Surface p repa red b y h e a t i n g in H~ a t 200 °. e Surface p r epa red b y chemica l etct i ing in cone. HNO3. d Separa t ion fac tor changes occur a t t he po ten t i a l where t h e Tafel s lope is c o n s t a n t . . . .

J. Electroanal. Che~n~,, I 9 (z968) 446-447

SHORT COMMUNICATIONS 447

change in mechanisms of reaction, a change in the rate-determining step, or both. Possible causes of the variation of the separation factor (s) (cf. ref. 3) with po-

tential such as: (i) collection inefficiency of hydrogen and tritium; (it) the existence of various exchange reactions; (iii) specific adsorption effects; and (iv) limiting diffusion of tritium, were eliminated. Thus, the existence of a dependence of s on potential seems to occur with systems having only a simple discharge step as the rate-determining process. Under such conditions, the classical factor of the separation factor is almost potential-independent, alld the variation of the separation factor with potential can be explained I by a variation of the quantum mechanical contribution (a highly mass sensitive factor) of the separation factor with potential.

For those metals in which the separation factor is independent of potential over a certain potential range but then changes to another value at a certain Potential and remains constant, it seems likely that a change of mechanism, rate-determining step, or both, cause the change of separation factor. For these metals, one must con- clude that the quantum mechanical ttmnelling contribution to the separation factor is constant with potential. Such a lack of the effect of proton tunnelling may aris, e from the different form of the potential energy surface associated with the rate-determ- ining step:

H + + H a d s + e --> H~

(probable for transition metalsY), compared with that for:

H + + e -+ Haas

(probable for metals such as Hg, Sn, Cd, PbY).

Acknowledgement Thanks are due to Dr. S. SRINIVASAN for discussion, and to the National

Aeronautics and Space Administration for financial support, under contract (NsG- 325) 4-o3114-3-5133.

Electrochemistry Laboratory, J. O'M. BOCKRIS University of Pennsylvania. E. GILEADI Philadelphia, Pa. 191o 4 (U.S.A.) R. HAYNES

I J. O'M. BOCKRIS AN'D D. B. ~&TTI-IEWS, Electrochim. Acta, i i (1966) 143. 2 B. E. CONWAY AND J: O'M. B0CKRIS~ J. Chem. Phys., 26 (i957) 532. 3 J. O'M. B0CKRIS, S. SRINIVAS•I'r AND D. B. MATTHEWS, Discussions Faraday Soc., 39 (1965) 239.

Received April I l t h , 1968

J. Electroanal. Ct~em., 19 (1968) 446-447