Surface Technology, 4 ( 1 9 7 6 ) 277 - 290 277 © Elsevier Sequoia S.A., Lausanne - - P r in ted in the Ne the r l ands

THE HYDROGEN ELECTRODE REACTION MECHANISM ON PALLADIUM AND ITS RELEVANCE TO HYDROGEN SORPTION*

M. ENYO and T. M A O K A

The Research Institute for Catalysis, Hokkaido University, Sapporo (Japan)

(Received December 1, 1975)

Summary

The mechanism of the hydrogen electrode reaction on palladium in 2N H2SO4 has been investigated mainly through polarization behaviour and analysis of overpotential decay transients. The Volmer-Heyrovsky mechanism is concluded with the first step about 20 times more rapid than the second step. Hydrogen diffusion in solution away from the electrode was kinetically important only on highly activated electrodes. Relevance of the mechanism to hydrogen sorption into palladium during cathodic electrolysis is discussed.

1. Introduct ion

Some metals placed in aqueous solution, or in a gas phase with an appreciable degree of humidity, may absorb hydrogen. Occasionally, the hydrogen dissolved causes changes in the mechanical properties of the metal, typically the phenomenon called hydrogen embritt lement. It is widely accepted that this is connected with the hydrogen electrode reaction (HER) taking place on the surface, usually coupled with metal corrosion. There may be two possible sources of hydrogen, hydrogen molecules which are products of the HER, and hydrogen adatoms or the like which are reaction intermediates; it is believed that the latter are more condUcive to hydrogen absorption. In this case, the intermediate must be supplied from H2 or H÷B (B = H20 or OH- ) and, accordingly, one has to look at the phenomenon of hydrogen absorption from a kinetic rather than an equilibrium point of view.

The relation between hydrogen absorption and the kinetics of the HER has been discussed [1] . Briefly, the (maximum) activity of hydrogen absorbed in a metal is to be related to the activity of the hydrogen intermediate of the HER, presumably the hydrogen adatom H(a). In the steady state of the

*This paper was p r e sen t ed at the Chemical Society , Fa raday Division, discussion on

Hydrogen in Metals, he ld at B i r m i n g h a m Univers i ty (Gt. Bri ta in) , J a n u a r y 5 - 7, 1976.

278

- ~ (mY)

6 ~ On l o f f (0)

0 , , t (rain) 0 I 2 3

- ~ ( m V )

80

60

40

20

0

(b)

--!i .... -i}:ii-iiii .....

o {o 20 t (sec]

30

Fig. 1. Typica l ga lvanos ta t ic ca thod ic t rans ien ts : (a) a t low cu r ren t densi ty , i = 3 .29 × 10 - 5 A/cm2; (b) at high cu r r en t dens i ty , i = 3.1 x 10 - 3 A / c m 2. 2N H2SO4, 25 °C, e l ec t rodepos i t ed Pd e lec t rode (on 0.3 m m ~ Ag wire).

HER (but the quasi-equilibrium state of the absorption process), one may express

PH(metal) = PH(a) (1)

where Pn(a) is in turn related to the detailed kinetics of the HER. Palladium has been extensively used in studies of hydrogen sorption in

electrochemical systems. Obviously, its high hydrogen solubility and high electrocatalytic activity for the HER renders the metal one of the best for examining the problem in question.

Frumkin and Aladjalova [2] reported extensive studies on the HER on Pd, particularly the time course of the overpotential after switching on or off the polarization current on one face of a Pd foil and "di f fus ion" of overpotential to the non-polarized back side of the membrane. They concluded from their observations that n~ in the decay transient (Fig. 1) corresponds to the overpotential component for the discharge reaction (the Volmer process),

H+B + e -~ H(a) + B (2)

and n~ to the Nernstian concentration polarization due to accumulation of H2 in the solution in the vicinity of the electrode with simultaneous accumula- tion of hydrogen absorbed in the Pd. In other words, n~ is understood as being due to an insufficient rate of diffusion of H2 in solution away from the electrode. They considered that H2 is formed by the recombination process (the Tafel process)

2H(a) -~ H 2 (3)

but with a negligible component of overpotential for this step [3]. The conclusion of slow H2 diffusion was supported later by a number of investigators [4 - 6].

The plot of z~ against the logarithm of the polarization current i is shown in Fig. 2. It gives a slope of approximately 30 mV/decade, in good harmony with the above conclusion, but it then tends to show a limiting overpotential at high current density.

3OO

250

200

150

I00

5O

0

(mY) /[~

. . . . . Pt

/ I /

I ogi

-5 - 4 - 3 - 2 - I 0 i (h/cm z)

Fig. 2. P o l a r i z a t i o n c u r v e s o n Pt a n d e l e c t r o d e p o s i t e d Pd e l e c t r o d e s (Pt , a f t e r a n o d i c a c t i v a t i o n ) .

2 7 9

Vetter [7] takes the appearance of a limiting V~ as an indication that the process responsible is not hydrogen diffusion but the Heyrovsky process of H2 formation:

H(a) + H÷B + e-~ H2 + B (4)

He demonstrated that the behaviour can quantitatively be accounted for on the basis of the equation derived for the Volmer-Heyrovsky mechanism• On the other hand, others who maintain the diffusion theory suggest that such a "sa tura t ion" overpotential is due to saturation of hydrogen in Pd: there would be a practical limit on the hydrogen concentration, C~(pd), in Pd and hence a limiting value of ~ , which would be expressed as

- ~ = (1/f)ln(c~(pd),eq) (5)

where f - F/RT and CH(pd),eq is the concentration in a system at equilibrium with 1 arm hydrogen.

It is intended in the present work to improve our understanding of hydrogen sorption in Pd. The work mainly concerns the elucidation of the HER mechanism on Pd through polarization studies and analysis of over- potential decay transients. The relevance of the mechanism to hydrogen absorption into Pd will then be discussed briefly.

2. Experimental

Pd foil and Pd electrodeposits on Ag wire were used as the test electrodes. The foil used was thin (~10 um) so that the time constant of the hydrogen

280

71 (mY)

8 0

60

40

20

t (sec)

o ; ,'o ,; 2'° Fig. 3. Dependence of overpotent ia l decay upon polarizat ion t ime; e lec t rodeposi ted Pd, i = 0.636 m A / c m 2.

absorption process would not be too long. The electrodeposition (~0.8 C/cm 2 at 20 pA/cm 2) was carried out from a dilute (~1 × 10 4 mol) PdSO4 solution in 1N H2SO4. The polarization behaviour was studied in 2N H2SO4 and the temperature was kept at 25 °C by means of a water thermostat, except for the temperature dependence studies.

The cell and electrodes were cleaned with hot chromic acid mixture overnight or, in the cases with Ag substrate wires, with hot 2N NaOH. The hydrogen was purified by a Pd/Ag membrane hydrogen purifier and introduced into the cell via a liquid N 2 trap followed by a trap containing the same solution as used in the experiment. The overpotential transients were followed by an ordinary oscilloscope, a chart recorder, or a transient recorder (Biomation 8100) with a preamplifier.

3. Results and discussion

3.1. Overpotent ial c o m p o n e n t 7~ 3.1.1~ 71 versus 71 As already reported in the literature [2], .~1 (in the build-up curve) and

7~ (in the decay curve) are nearly equal to each other at low current densities (Fig. l(a)), but nl is definitely larger than n~ at high current densities (Fig. l(b)). It is suggested in the literature [8, 9] that this indicates an increase in the catalytic activity of the Pd electrode for the Volmer process, probably due to lattice expansion of the Pd caused by dissolution of hydrogen in the Pd. This interpretation is plausible, as seen in Fig. 3, where 7] decreases with duration of the polarization. However, this cannot be a complete interpretation because the variation n] with time appears to be discontinuous at t = 0. It is likely that the difference between 71 and 7~ is partly caused by the fact that 71 must have sufficient magnitude to meet the applied current, whereas this is not required of 7~.

2 3

x \ x

I O0 ~ ~"XXx~x x x

0 i I "N, i -4 -3 -2 -I log t

Fig . 4. T e s t o f e q n . ( 6 ) f o r t h e V o l m e r p r o c e s s ; e l e c t r o d e p o s i t e d Pd , i = 0 . 1 1 8 A / c m 2.

2 8 1

3 .1 .2 . D e c a y t r a n s i e n t

It is generally argued among various investigators that 7~ ,corrected for the ohmic pseudo-overpotential, is the overpotential component corresponding to the Volmer process. This view is well supported in this work through an analysis of the initial part (~10 msec) of the overpotential decay. Assuming that this decay represents discharge of the electrode double layer capacitance C through the Volmer process, viz. d 7 / d t = I i t/C, and that the rate of discharge during decay is the same with the steady state current at the corresponding 7 value, viz. 7 = a - - b log I i J, we readily obtain (7 is negative for the cathodic polarization }

77 = a + b log (10 ('0-a)/b + ( 2 . 3 0 3 / b C ) t } (6)

where a = b log io, v, b is the Tafel slope, io, v is the exchange current density of the Volmer process, and 7o is the steady state overpotential before the current interruption.

The plot of eqn. (6) is shown in Fig. 4. At relatively large values of t. the - - ~ vs. log t curve is linear with a slope b = 78 mV, which compares well with the slope of the Tafel line at a moderate overpotential region (cf . Fig. 2). Also, the plot of the whole logarithmic term with those values of b and i0,v evaluated from the Tafel line gives an excellent line, yielding a slope , and intercept values which are close to those used in calculating the term. The interpretation of ~?~ can therefore be taken as satisfactory. This also indicates, in the high overpotential region in which the analysis was conducted, that the HER is practically controlled by the Volmer process. Analysis presented below indicates that the Volmer pr.ocess is relatively rapid; io, V is roughly 20 times the i o of the overall HER. That the Tafel line of 120 mV slope at high current densities may appear in spite of such a value of io,v has been discussed elsewhere [1, 10].

282

3. 2. Overpotential component 7?2

3.2.1. Diffusion of hydrogen across the Brunner-Nernst layer 3.2.1.1. Comparison with active Pt electrode. It is repeatedly proposed

in the literature that the overpotential component 72 is due to slow hydrogen removal f rom the electrode by a diffusion process. This is probably certain for electrodes of high electrocatalytic activity, e.g. palladized Pd [5] , but not necessarily so for Pd of ordinary activity. This is particularly impor tant in this work where we hoped to investigate the HER mechanism and hence we preferred to employ electrodes with ordinary degrees of activity.

The theory can be tested first by comparison of 7~ with the analogous quanti ty obtained on highly activated Pt (or Pd) electrodes. Although the time constant of the overpotential decay on Pt is much shorter than on Pd, because there is no large hydrogen reservoir as with Pd, one can still distinguish 72 in the time region below 1 sec. With a Pt wire electrode of nearly the same dimension as the Pd-deposited Ag wire, it was observed that ~ after anodic activation was much smaller than on Pd (Fig. 2).

The decay transient of 7~ on Pt was analysed. The decay with t ime of concentrat ion Co of hydrogen in the vicinity of the electrode is [ 11]

c 0 = c ~ + - ~ f f - , = 0 ( 2 n + 1 ) 2 exp 452 t

where c o is the initial value of c 0, c~ is the bulk concentrat ion, D is the

diffusion coefficient and 5 is the thickness of the diffusion layer. Neglecting terms higher than the second and employing a Nernst equation for a concentra- tion cell to express 72 in accordance with the present model, we obtain

8c o D~ 2 ln{exp(--2fT~) -- 1} = In - - t (7)

~2C~ 4~ 2

Application of eqn. (7) to the decay transient generally yielded satisfac- tory linearity. Also, the value of 5 ~ 0.005 cm (Fig. 5) obtained f rom the slope can be accepted as reasonable [12] .

Hence, it is concluded that 72 on the active Pt electrode represents (the maximum limit of) the rate of hydrogen diffusion across the Brunner- Nernst layer under the present experimental conditions. It then follows that 7~_ on Pd, which is much larger, cannot be due to the hydrogen diffusion process.

3. 2.1.2. Heat of activation. The temperature dependence studies indicated that the heat of activation corresponding to 7~, as obtained from the polarization resistance, is roughly 7 kcal on ordinary Pd electrodes, but decreases to as low as 3 kcal on a highly active Pd electrode obtained by a strong electrodeposit ion current (Fig. 6). The latter value is reasonable for the diffusion process but the former value is hardly so.

283

log ( I / R )

( x l O - 3 c m )

×

x x

-J3 I - 2

X X X

X

I I log i - I 0 (Alcnn z )

- / 0

- 1 5

I I (llT]xl03

3 3.,5 4.0

- 2 0

Fig. 5. Dependence of thickness of Brunner-Nernst layer upon polarization current; Pt wire electrode (0.3 mm ¢), 2N H2SO4, 25 °C.

Fig. 6. Arrhenius plot of reaction admittance corresponding to ~2; electrodeposited Pd, R in ~ cm 2.

3.2.1.3. Limiting value o f ~ . This experimentally observed phenomenon indicates a saturation value of Co/C~ at high polarization currents. This has often been explained in connection with limiting concentrat ion of hydrogen dissolved in Pd. However, this view is in doubt since what directly determines ~ in the diffusion theory is Co in the solution, and not the hydrogen concentrat ion in Pd, and also Co would increase with increase of polarization current irrespective of at tainment of the hydrogen concentrat ion limit in Pd.

The above arguments altogether suggest that ~ , at least its major part, should be attr ibuted to an electrocatalytic process taking place on the electrode surface.

3. 2. 2. Reaction control 3.2.2.1. The Volmer-Tafel mechanism. If the formation of H2 from

2H(a) is not sufficiently rapid [13, 14], it may cause appreciable values of overpotential to remain at the beginning of the overpotential decay, viz. 77~. Then, ~ will decay with time according to the relation

- - --2F dnH(Pd) /O'T I ( a l l ( a ) ) 2 A - - d t - - aH(a),eq -- 1 I (8)

where nH(Pd) is the number of moles of H(Pd), A is the electrode surface.area, all(a) is the activity of H(a) and /O,W is the exchange current density of the Tafel process.

2 8 4

The i so the rm of hyd rogen absorp t ion in Pd is r ep o r t ed by F rumkin and his co l labora tors to be logari thmic [15] :

CH(Pd) = Cn(ed),l + K log P (cm3H2/cm 3 Pd, P in a tm) (9)

with Cn(Pd), 1 = 956 and K = 68 (by in te rpo la t ion of thei r da ta to 25 °C). Stackelberg and Bischoff [9] also r epo r t ed the same func t iona l fo rm with C H ( P d ) , 1 = 850 and K = 63. We will employ this empirical re la t ion in the fol lowing analysis.

F/H(Pd ) in half thickness l o f Pd foil is

Al 273 r/H(Pd) -- 2.24 × 104 T CH(Pd ) ---- ]~lCH(Pd) (moles)

or, with eqn. (9),

l /H(Pd) = F/H(Pd), 1 + (Kkl /2 .303) lnP (10)

When the absorp t ion and adsorp t ion equil ibria are bo th established,

PH: = 2 P H ( P d ) = 2 P H ( a )

o r

lnP = 21n(aH(Pd)/aH(Pd),eq ) = 2 ln(aH(a)/aH(a),eq) (11)

where the subscr ipt " e q " signifies equi l ibr ium at 1 arm. Accordingly , f rom the last two equat ions ,

rimed) = nH(ed)n + 2 (Kkl /2 .303) ln(aH(a)/an(a),eq) (12)

We shall assume tha t such equil ibria are main ta ined during the V~_ decay. We also assume tha t the Volmer process is pract ical ly in equi l ibr ium during the decay; on ly a very small a m o u n t o f ioniza t ion of H(a) would be needed to adjust the e lec t rode po ten t ia l to fo l low the slow var ia t ion of all(a). We then have

F

aH(a)/aH(a),eq = exp(--f~ 2 ) ( 13 )

F rom eqns. (12) and {13),

r/H(Pd ) = k l C H ( P d ) , 1 - - 2(Kk1/2.303)fi?2 (14)

Equa t ion (8) is now conver t ed to

4F 2 Kkl d~72 - - - /OuT {exp(--2fv~) - - 1}

A 2.303 R T dt

Solving

I - - e x p ( 2 f ~ , o ) in = k ' t (15) t

1 -- exp(2f~2)

where ~2,o is the initial value of V~ and

k ' =2 .303 io,wA/4FKkl (sec -1 ) (16)

285

2 0

I 0

V-T

o I 7 6 x IO-S(A/cm z ) x I . 7 6 x I O - 4

z~ 0 . 7 1 x 1 0 -3 [] I 7 6 x 1 0 -2 ® 7 0 5 x l O -2

0 X z~

Ox ~ o ~o

5 IO 15

ox z~

OR A

[ t (sec)

2 0

- ~ ] (mV)

180 ~ _ 5

120 2

, o Q 0 L

0 rO

I 0 6 4 (mA/cm z)

2 3 0 9

3, 3 0 7 7

I L I 2 0 50 4 0

t ( sec )

Fig. 7. Test of eqn. (15) for the Vo lmer -Ta fe l mechanism:

1 -- exp(2f~?'2,0) YV-T -= In

1 -- exp(2fn~,o)

Fig. 8. Super-polar iza t ion behaviour in ca thodic galvanostat ic t ransients ; e l ec t rodepos i t ed Pd.

Equation (15) was tested (Fig. 7). Linearity of the log term vs. t relation was fair but constancy of the slope with variation of polarization current was not very satisfactory as compared with the case of Fig. 9 presented below.

The diffusion model can also be analysed analogously. Starting with

2 F dnH(Pd ) C 0 - - C~ - - - - = D - -

A dt 5

employing eqn.(10)and - - 2 f ~ = In Co/C~, and assuming Henry's law for dissolution of H2 in aqueous solution, we obtain

1 -- exp(2f~,o) _ t In

1 -- exp (2 /~ ) r

where r - klS/Dc~. This equation has the same form as eqn. (15). Thus, although the diffusion model should not be applicable in this case, the diffusion equation closely simulates the V~ decay.

3.2.2.2. The Volmer-Heyrovsky mechanism. Vetter [7] derived the following relation for the Volmer-Heyrovsky process:

, l + M e x p ( - - f ~ ) M = 1 iio,v +io,H) exp(--f~?2"°) = exp(--f~ ) + M 2- , - / O , H ~),V

where ~,v and/O,H are the exchange current densities of the processes. Equation (17) assumes a limiting value of ~ at large overpotentials:

(17)

+- [r~,~m = In M (18)

286

which explains the "saturat ion" of r~,0. The value of io,v/ io, H was found from the observation of---02,Um ~ 42 mV by Clamroth and Knorr [4] to be either 10.1 or 0.1. We may exclude io,v/ io, H = 0.1 because, if this is the case, it can be shown that the activity of the hydrogen adatom, and hence the amount of hydrogen absorption in Pd, should decrease with increase of cathodic overpotential [ 1 ] ; this contradicts the experimental observations. These authors also demonstrated a good fit of eqn. (17) with the experimental 72,0 vs. log i relation employing io,v/io,H = 10.1.

The Volmer-Heyrovsky mechanism was supported in this work for the following reasons. (a) The 72,0 vs. log i relation, including its limiting behaviour, was roughly explained by eqn. (17), with io,v/io, H ~ 20. (b) "Super polarization" was observed during cathodic galvanostatic transients at high current densities (Fig. 8); such a behaviour is expected in this mechanism [7, 16] . (c) Interpretation of the ~ decay curve on the basis of this mechanism was found to be satisfactory (see below).

Decay transients of rT~ are derived as follows. Based on io,v/io, H ~ 20, we may take the Volmer process to be practically in equilibrium during the r~ decay. Then,

2F dnH(Pd ) { "H(a) ) A d~-- - i0'H exp ( - - a f ~ ) -- exp ((1 -- a ) f ~ } (19)

\ aH(a),e q

The solution for a = 1/2, employing eqns. (12) and (13), is

(exp(--frT~/2) + 1} {exp(--f~,0/2) -" 1} In +

{exp(- - f~/2) -- 1} (exp(- - f~ ,0 /2) + 1}

+2 (tan-1 exp (--fv~_/2) - - t an -1 exp (--fr~'2,o/2)} = k ' t (20)

where k' is the same as in eqn. (16), except that i0 , T is now replaced by iO,H.

A typical plot of eqn. (20.) is shown in Fig. 9. The linearity was fair and, in contrast with Fig. 7, the slope was maintained constant, independent of variation of the polarization current, over a very wide range. The plot deviates from the initial straight line after about 10 sec. It is considered likely that this is due to neglect of concentration polarization in the analysis, which must not, in fact, be completely negligible. This gives rise to a somewhat higher reverse rate of the Heyrovsky process or to a decreased rate of the ~ decay. This will have some importance, especially at lower values of rT~/~,o or at later portions of the decay.

3. 3. H y d r o g e n o v e r p o t e n t i a l and h y d r o g e n a b s o r p t i o n

3. 3.1. A f f i n i t y d i s t r i b u t i o n a m o n g various p r o c e s s e s As mentioned in the Introduction, eqn. (1), the equilibrium activity of

hydrogen in Pd is determined by the steady state activity of H(a) which is in turn determined by the kinetics of the HER'. If the HER occurs through the sequence

2 0

I 0

/ 0

0 5

o I 7 6 x 1 0 -5 ( ,&/cm 2) x I 7 6 x r O -4 ,,, 0 7 t x l O "3 ® 7 0 5 x l O -2

o" A

o ~

0

I I I t (sec) I0 15 20

Fig. 9. T e s t o f eqn . ( 2 0 ) f o r t h e V o l m e r - H e y r o v s k y m e c h a n i s m

( e x p ( - - f v ~ / 2 ) + 1} (exp(- - f~2,0 /2) - - 1} Y V _ H ~ In +

( exp( - - f~ /2 ) - I} {exp(--f,72,0/2) + I}

+2 {tan - 1 e x p ( - - f n ~ / 2 ) - t an - 1 exp(--fT?~.0/2))

287

(+ e) (+ H + + e) H + > H(a) > H2,0 ~ H 2 ] V o l m e r ~ I H e y r o v s k y d i f f u s i o n I

+-----Ag~ H(Pd) >l, >E , I - - A g 2 I

all(a) will be determined by the combined processes (i.e. of Heyrovsky and diffusion). We may write

--Ag2 = (PH(a) -I-pH + + ~t e - - p H 2 , ° ) + (PH2 ~ - - P H 2 , ~ )

---- ]2H(a) + ,t/H* + /2 e - - /2H2,~

but at equilibrium

0 = PH(a),eq + PH ÷ + Pe,eq - - P H 2 , ~

Hence, defining p~ - - ~te,eq = - - F~ a l l ( a )

- - A g 2 = R T I n - - F~ (2 aH(a) , eq

Therefore, regardless of the relative magnitudes of the free energy changes of the Heyrovsky and diffusion processes, all(a) is simply related to Ag2. In other words, in the discussion of steady state absorption of hydrogen in Pd, we may treat the case as if the diffusion is included in the Heyrovsky process.

The problem of hydrogen absorption into Pd in the electrochemical system can be visualized by considering an analogy to a gas phase system.

288

We consider a hydrogen pressure P which is hypothetically in equilibrium with the hydrogen overpotential [1]. Briefly, one may say that p H 2 = 2pma ), or eqn. (11). Hence, from eqn. (21),

--Ag 2 = (RT/2) lnP-- e~? (22)

As discussed previously [ 1], we have

2raFT? Ag 2 - - - , m = Agl/Ag2 (23)

m + l

and

m + l 2f~ ( l n meq+exp(f~) )-1 = (24) meq + exp(---fv)

where

rr~q -~ ( Ag2/ Agl )e q = io,v/io,H (25)

Hence

Ag2 m~q + exp(f~) - 2 f v - - l n

RT meq + exp(--f~)

Introducing this relation into eqn. (22), we obtain

P = ( m e q e x p ( - - f ~ ) + l ) 2 (26)

rr~q + exp(--fv)

At sufficiently high cathodic overpotentials at which exp(--fv ) > ) rn~q > > exp(fv ) P attains a limiting value [1] :

Pure = (meq) 2 (27)

According to Vetter [7], at high overpotentials

1)f --fr~jim-~ in ~- meq + - - (28) me q

Hence, for the value --~,lim ~ 60 mV as observed above, we obtain m~,q ~ 20 or P ~ 400 atm. This means that the hydrogen concentration in Pd during strong electrolysis would be such as is in absorption equilibrium ~vith an hydrogen atmosphere of approximately 400 atm. It should be noted that this is higher than the value one anticipates directly from the ~.um value on the basis of a Nernstian equation.

3. 3.2. Compar i son w i th o t h e r meta ls The mechanism of HER concluded above on Pd is different from that

on other catalytically active metals, Pt, Rh, Ag, Au and Ni, where the Volmer- Tafel mechanism was concluded from deuterium tracer studies. (The same mechanism might also be applicable to Fe [1] . )

For the Volmer-Tafel mechanism, it can be shown that the v~ component represents the affinity value of the Tafel process and hence the hydrogen

c o n c e n t r a t i o n in the metal dur ing the electrolysis would reach such a magni tude as is in equi l ib r ium with a P value which is d i rec t ly calculable f r o m ~ , 0 by means of a Nerns t ian relat ion. This means that , at the same ~2.0 values on Pd and on the o the r metals q u o t e d above, the Pd is m o re hydrogen-absorbing.

289

4. S u m m a r y

The mechan ism of the hyd rogen e lec t rode reac t ion (HER) on Pd prepared by e l ec t rodepos i t ion on Ag wire was investigated mainly t h rough observat ions o f polar iza t ion behaviour and analysis of the galvanostat ic overpo ten t ia l transients. The initial (< 10 msec) rapid overpo ten t ia l decay, 7'1, upo n te rmina t ing the polar iza t ion cur ren t was sat isfactor i ly exp la ined on the basis of the Vo lmer process, H ÷ + e -* H(a). The rest of the overpotent ia l , ~ , decays slowly. On highly e lec t roca ta ly t ica l ly active Pd or Pt e lec t rodes , ~ is a t t r i bu ted to a slow di f fus ion process of H2 away f r o m the e lec t rode , as o f t en suggested in the l i terature , bu t this should n o t be the case on Pd e lect rodes with ord inary magni tudes of activity. Thus, the process responsible for ~ on such e lec t rodes was cons idered to be reac t ion cont ro l led .

The V o l m e r - H e y r o v s k y ( V - H ) mechan ism (H ÷ + e -* H(a), H(a) + H ÷ + e -* H2) was p roposed on the basis of the fol lowing evidence. ( l ) A sa tura t ion behaviour of ~ at high cu r ren t densit ies could be a c c o u n t e d for wi th an equa t ion der ived by Vet te r for the V - H mechanism. (2) " S u p e r po l a r i za t i on" was observed during bui ld-up of ca thodic galvanostat ic t ransients at high cur ren t densities, as e x p e c t e d with this mechanism. (3) The r equ i r emen t s of this mechan ism were well satisfied by the decay transients.

The relevance of the H E R mechan ism to hyd ro g en absorp t ion during ca thodic polar iza t ion of Pd was discussed. I t was shown tha t for the V - H mechan i sm the l imiting hyd rogen pressure, P~m, which is h y p o t h e t i c a l l y equivalent to the hydrogen overpotent ia l , is re la ted to the rat io o f the exchange cur ren t densit ies o f the Volmer and Hey rovsky processes, meq - io.v/io,H, by Pnm = rn~. On the Pd used in this work meq was evaluated to be ab o u t 20 f rom the sa tura t ion value of ~ ~- 60 mV. Thus, we ob ta ined Pnm -~ 400 atm. The figure would, however , be much smaller on e lec t roca ta ly t ica l ly active Pd, such as pal ladized e lect rodes .

References

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