HYDROGEN OVERPOTENTIbL
July, 1956
AT
ELECTRO-DEPOSITED NICKELC A T H O D E S
837
HYDROGEN OVERPOTENTIAL A T ELECTRODEPOSITED NICKEL CATHODES IN HYDROCHLORIC ACID SOLUTIONS BY I. A. AMMAR AND S. A. AWAD Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt Received August 16, 1866
Hydrogen overpotential, 7, a t electrodeposited Ni cathodes has been measured, under pure conditions, in aqueous HC1 solutions (0.01-0.5 N ) . Measurements have been carried out in the current density range 10-6-10-2 a. om.? Two Tafel line slopes of bl = 0.053-0.063 v. (at the low current densit range) and b2 = 0.105-0.119 v. (at the high current density range) are obtained for the linear logarithmic section of the ‘8afel line. At lower current densities, dissolution of Ni interferes with the ooverpotential results. The effect of temperature on 7 is studied for 0.01, 0.1 and 0.5 N aq. HCl solutions between 25-55 The heat of activation, AHo*, at the reversible potential is calculated using two methods: (i) the method employing ZI log i&(l/T) where io is the exchange current, and (ii) the method using (&/ZIT)i. The results obtained by the first method are found to be more reproducible than those obtained by the second method. Attempts to fit the results of electrodeposited Ni within the framework of the mechanisms, previously suggested for hydrogen evolution on Nil have not been successful. For this reason a dual electrochemical catalytic mechanism is suggested to account for the lower slope b,. The higher slope, bz, is accounted for.by a simple mechanism.
.
r
Introduction Recent advances in the technique of hydrogen overpotential measurements, have made it possible to determine, with certainty, parameters such as slope, exchange current, stoichiometric and electron numbers2 which are important criteria for establishing the mechanism of the cathodic hydrogen evolution reaction. Existing evidence1 points out to the fact that the mechanism of the above reaction is dependent, among many other factors, on the nature of the electrode-solution interface and consequently on the nature of the electrode surface. Few studies3have been carried out on the hydrogen overpotential of electrodeposited electrodes, whereas wire electrodes have attracted much attention. The aim of the present investigation is t o report on the behavior and characteristics of the Tafel lines on electrodeposited Ni cathodes. previouswork on ~ iin, the wire form, has been carried Out by Frumkin and co-workers4and by Bockris and Potter.6 The results of Frumkin and ao-workers correspond to a mechanism in which the reaction is controlled Over partof the electrode by a discharge step and Over the remaining area by the recombination of atomic hydrogen. Bockris and Potter’s results indicate a rate-determining discharge reaction followed by atomic hydrogen recombination. At high current densities a decrease in the rate of the recombination reaction, relative to that of the discharge step, is suggested by the authors. Experimental Procedure
B C
G
T I
T
*
I
Fig. l.--F~lectrolytic cell.
dermic syringes F, which enabled the electrodes to be lowered into the solution in B, without allowing any leakage of atmospheric oxygen into the cell. One of the two electrodes (both joined to the pistons of the syringes) was used for preelectrolysis, while the other was used as a test electrode. A platinized platinum hydrogen reference electrode in the same solution was used as a reference. Electrical contact between the reference electrode and the polarized cathode was made through a Luggin capillary G , with an internal diameter of 1 mm. Purified hydrogen was introduced into the cell through H and was divided between the compartments A, B and C. Hydrogen was allowed to escape to t8he atmosphere through three bubblers I, filled with conductance water. An exit J a t the bottom of the cathode compartment.was used to empty the ce!l when re uired. The electrolytic solution was introduced into the ce8 through K . I n doing 80, the anode compartment was separated from the rest of the cell by closing the adjacent taps, and the compartment was completely filled with the solution. This was Electrolytic Cell.-The Cell was Similar to that Of Bockris followed by bubbling purified hydrogen into the solution in and Potter,S and a diagram of i t is shown in Fig. 1. I t A for three hours, to minimize the amount of dissolved oxyconsisted of three compartments: the anode compartment gen,6 before the solution was divided between A and B. A, the cathode compartment B and the hydrogen electrode The compartments B and C were previously washed and compartment C. A sintered glass disc D was inserted be- completely filled with conductance water and then emptied tween A and B to minimize the diffusion of gaseous anodic with a stream of purified h drogen The cell Was made of products toward the cathode. The anode E was in the arsenic-free glass, technical$ know; as “ ~ ~ ~ i l~. 1”taps 1 form of a platinum disc, with an area of 1 Cm.’. On top of and ground glass joints were of the water-sealed type. After the cathode compartment B, were fixed two barrels of hypo- each run, the cell was cleaned with a mixture of h a l a r HNOa and Analar HzSOd followed by washing with equilib’ (1) J. O’M. Bockris. Chem. Revs.,43, 525 (1948). rium and conductance water. (2) J. O’M. Bockris and E. C. Potter, J. Electrochem. SOC.. 99, 169 hfific-tion of Hydrogen.-Cylinder hydrogen was puri(1952). fied from 0 2 by reduced copper furnaces n t 450’ from C 0 2 (3) J. O’M. Bookris and R. Parsons, Trans. Faradag Soc., 44, 860 by soda lime, from CO by “Hopca1ite” (MnOz: Cuo)? at (1948): w.&nett and e. Hiskey, J . A m . Chem. SOC., 74, 3754 (1952). room temperature, and was then dried. The purification (4) P. Lukovzew, S. Lewina and A. Frumkin, Acta Physicochim. U.R.S.S., 11, 21 (1939). (5) J. O’M. Bockris and E. C. Potter, J. Chem. P h y s . , 20, 614
1952).
(6) With this simple method, no oxygen depolariaation was detected in the results. (7) G. Schwab and G . Drikos, 2. physik. Chem., 186A, 405 (1940).
838
I. A. AMMARAND S. A. AWAD
stage distillate was used for the solution preparation. The distillation apparatus was cleaned with a mixture of Analar HNOI and Analar H2S04, followed by washing with equilibrium and conductance water and was finally steamed. All coqductance water used had a specific conductivity of 9 X 10-7 ohm-’ cm.-’. Pre-electrolysis.-For the electrolytic purification0 of ’the solution a pre-electrolysis electrode of the same material as the test electrode was used. The adequate conditions, current density and time, of pre-electrolysis were determined by trial and error. Sufficient pre-electrolysis was attained when further increase of the extent of pre-electrolysis did not cause more than 110 mv. change in the overpotential a t any current density between 10-6and a.cm.-2 on electrodeposited Ni. Pre-electrolysis was performed a t 10-2 a.cm.-Z for about 20 hours. With these conditions, reproducible results (within f 1 0 mv.) were obtained in all concentrations studied. Measurements.-After each run, the cell was cleaned as
0.30 0.25 ~
0
6 Y 0.15
0:,25” 0 16 o,,,0,J5°
. ~
.
.
~
~
.
0.10 .
0.05
. ‘
0.0
between the anode and the cathode compartments, the preelectrolysis electrode was lowered into the solution in the 0.35 . cathode compartment, with a p.d. corresponding to a current density of 10-2 a.cm.+ imposed on it. At the end of the pre-electrolytic period, the pre-electrolysis electrode was drawn out of the solution with the current still flowing. 0.30 Part of the pre-electrolyzed solution from B was pushed with a stream of hydrogen into the compartment C. Hydrogen 0.25 . was left to bubble over the platinized platinum electrode r; for about 20 minutes. The test Ni electrode, with a p.d. 5 Y0.20 0.30 . 0.15 0.25 . 0.10 .
A
-
0.20 .
Vol. 60
0.05
0
HYDROGEN OVERPOTEKTIAL A T ELECTRO-DEPOSITED NICKELCATH0I)ES
July, 1956
a35-
The values are given in Table I1 together with their 95% confidence limits. AH: has been termed by Bockris'O as "the virtual heat of activation."
0.01 0.10 0.50 a Values of 0.1 kcal.
(ANo*)l
(AHo*)2
limits 95%
ajo.
a35- I 0.30- as a25. 0300.25- 0.30
95% limits
fl.4 10.4 f l . O f1.4 9.6 f1.3 11.6 f1.l f0.4 ( A H $ ) l and (AH,*)z are given to the neareRt 10.0 11.0 11.6
0.20h
v
0.25-
0.20-
(alt,wgi = (AH?
+ c u l t ~ ) / a ~ ~(2)
t
range
(oc.)
AH@*)^
95% limits
(AHO*)~
95 % limits
0.01
25-35 14.2 f1.6 13.2 f0.7 13.6 f5.2 13.0 f6.2 35-45 f5.3 3t3.5 13.5 16.7 45-55 0.10 25-35 14.7 f8.8 13.2 f6.5 13.1 f5.1 11.5 fl.1 35-45 15.3 f6.2 f2.6 13.2 45-55 0.30 25-35 14.8 f6.9 12.5 f3.9 13.7 f0.7 14.1 13.3 35-45 45-55 12.7 f6.7 12.9 f4.8 a Values of ( A H ? ) , and (AH$)z are given to the nearest 0.1 kcal. (10) J. O'M. Bockris, Ann. Rev. Phys. Chenz., 5, 477 (1954).
a25
azo015- azo 0.10- ais-
c 015-
0.10-
AH,*is also calculated from2
Concn N ','
0'
n
W r
TABLE 11' Concn., N
839
0.15
0.05- 0100.05- 010 0.05-
The effect of substrate on 7 is studied by measuring Tafel lines on Ni electrodeposited on a Ni wire. The result is shown in Fig. 6 for 0.10 N HC1 a t 25". It is clear from Figs. 2, 3, 4 and 6 that the substrate has no effect on the nature of the results obtained for electrodeposited Ni. The electron number, X, defined as the number of electrons necessary to complete one act of the rate-determining step, is calculated using the approximate formula2 exp.
(g)
= 0.05
(3)
where q s is the overpotential a t which the Tafel line departs from linearity due to the appreciable rate of
I. A. AMMAR AKD S. A. AWAD 0.35
.
0.30 . 0.25 . ,A
d
0.20 -
v
c
0.15 . 0.10
-
0.05
.
0.0
Fig. 6.-Tafel
-5
-4 -3 log c.d. line for Ni electrodeposited on a Ni substrate.
ioiiization of adsorbed atomic hydrogen. Calculation of X using the formula2 could not be done, bex = -(RT/ioF)(bi/b,J)?)+o (4) Cause q becomes constant a t the lowest range of current densities examined. The mean values of X (calculated according to equation 3) are given in Table IV, together with their 95% confidence limits. TABLE IV" 0.01
mechanism is distinguished by: (i) a Tafel line slope of 0.12 v. a t 30°, (ii) a value of X equal to unity, and (iii) by the fact that q is independent of pH in the concentration range where Stern's theory holds, Le., in dilute solutions." Although most of the values of X lie near to unity (Table IV), yet the occurrence of two slopes rules out the existence of a simple slow discliarge mechanism for electrodeposited Ni. It must be noted that the values of X given in Table IV are only approximate since equation 3 is an approximate one. As an attempt to explain the results on electrodeposited Ni, the following scheme may be suggested. If a t the low current density range the slow discharge step is fast, and the two desorptive steps (catalytic and electrochemical) have comparable speeds, a dual electrochemical-catalytic mechanism is rate determining when the rate of the discharge step is much greater than the rates of the desorptive steps. A similar mechanism has been discussed before. The condition for this mechanism may be expressed by
.r
7 -6
Concn., N
Vol. 60
t
("e.)
x 1.2 1.3 1.5 1.8 1.3 1 .o 1.1 1.2 1.4
25 35 45 55 0.05 25 0.10 25 35 45 55 . 0.50 25 0.7 35 0.8 45 0.9 55 1.0 X i s given to the nearest first decimal figure.
95%
limits
10.2 f0.2 f0.2 f0.3 10.2
fO.l *0.2 10.2 *0.2 *O.l *o. 1 f O .1 *o. 1
Discussion The mechanism of hydrogen evolution a t Ni cathodes in acid solutions has been given by Bockris and Potter6 as a rate-determining slow discharge step followed by a catalytic desorption. This
VI
= vz >> v3
v 4
(5)
where VI is the rate of discharge, VZis the rate of ionization of adsorbed atomic hydrogen, Va is the rate of the catalytic desorption and V4 is the rate of the electrochemical desorption. The energy barrier for the discharge step, I (Fig. 7a) corresponds to that of a reversible process, while the barriers I1 and I11 for the two desorptive steps are far removed from reversibility and of nearly equal height. For this reason, the reverse rates of the two desorptive steps are neglected in (5). The condition given by ( 5 ) can be derived easily when the expression for the steady state is taken as: (VI V2) = (V3 - V,) (VC- V6),where Vs and V6 are the reverse rates for the catalytic and the electrochemical desorption steps, respectively. The cathodic current V , is, therefore, proportional to V Band to "4, I n fact V, may be given by
+
Vo = 2 v * = 2VC
(6)
The rates V 8and V4 (taking a = 0.5; cf. ref. 2) are given by v3 = k3X2 where A+ is the p.d. between the Helmholtz double layer (initial state) and the metal, X is the fraction of the surface covered with adsorbed atomic hydrogen and ( a H + ) d . l . is the activity of hydroxonium ions in the double layer. From (6), (7) and (€9, one gets
x
(- Fjj@) A@
(h/k3)(aH+)d.l.
(9)
for the fraction of the surface covered with adsorbed atomic hydrogen as a function of potential, under the condition given by (5). From (6), (7) and (9) and similarly from (6), (8) and (9), the cathodic current, i, is given by i (8) (b) Fig. 7.-(a) Energy barriers for the dual electrochemicalcatalytic mechanism; (b) energy barriers for the electrochemical mechanism.
=
const.
(aH+)*d.l.
exp.
(- +g)
(10)
(11) 8. A. Jofa, Acto Phyaicochim., U.R.S.S., 10, 903 (1939). (12) A. Frumkin, P. Dolin a n d B. Erschler, ibid.. 18, 779 (1940);
B. Conway, Ph.D. Thesis, London,
1949;
Awad, J. Electrochem. Soc., in press (1956).
I. A. Ammar a n d 9. A.
A
July, 1956
HYDROGEN OVERPOTEKTIAL AT ELECTRO-DEPOSITED KICKELCATHODES
with the result that the Tafel line slope, a t constant (aH+)d.l., becomes
841
The fact that the lower parts of the Tafel lines become parallel to the log c.d. axis at potentials negative with respect t o the hydrogen electrode po2.303RT ( b A $ / b log i) = - -= -0.06 v. at 30’ tential is attributed to the dissolution of Ni in F HC1 s01utions.l~ The bend-up, a t high current Equation 9 indicates that X increases with in- densities, observed for a number of Tafel lines (cf. crease of cathodic polarization till a maximum Figs. 3, 4 and 5 ) may be attributed (at least in value of X very near to unity is reached, after which part5) to a resistance overpotential effect. The plot Vs remains constant while Vr increases with in- of Aq (difference between the actual Tafel line crease of cathodic potential (cf. equations 7 and 8). and the extrapolated line) against i results in a A condition will, therefore, be reached when Vc >> straight line for all cases where the bend-up is obV 3(cf. Fig. 7b, where I1 represents the barrier for served. the electrochemical desorption), and the over-all The difference between the results obtained on rate is then governed by the simple electrochemical electrodeposited Ni and those of Bockris and Potmechanism, with a Tafel line slope of 0.12 v. a t 30°, at high cathodic polarization.2 It must be ters on bulk Ni may be attributed to the difference noted that the increase of X may not affect the rate in the extent of the fractional surface coverage, X, of the discharge step, if i t is assumed that this reac- a t the start of polarization. The method of election takes place on adsorption sites different from trode preparation employed by Bockris and Potthose responsible for the desorption reactions. This ter, i.e., sealing the electrode in a glass bulb under assumption is reconcilable with the views of Hori- an atmosphere of hydrogen results in a value of X near to unity a t the start of polarization. Owing uti and P01anyi.l~ to the fact that the heat of adsorption of hydrogen The pH effect associated with the dual electrochemical-catalytic mechanism described above can on Ni is decreasedis a t high values of X , the desorpbe derived with the help of the Stern theory of the tion processes are fast. The slow discharge process double layer. I n dilute solution, under the condi- taking place on a highly covered surface is, tions when specific adsorption of ions is absent and therefore, rate determining with a slope of 0.12 v. a t when the electrode potential is far from the poten- 30”. For electrodeposited Ni, the value of X at the tial of the electrocapillary maximum, Stern’s thestart of polarization is probably much smaller than ory gives for the zeta potential unity. The desorption processes are slow by virE = const. f ( R T I F )In (UH+)B (11) tue of a decrease in the concentration of the reactwhere ( u ~ + ) Bis the activity of H30+ ions in the ants as well as a high value for the heat of adsorpbulk of solution. The activity of H30+ ions in the tion a t low values of X. The slow discharge procdouble layer is related to ( ~ H + ) Bby ess is fast for a sparsely covered surface and the (“H+)d.i.= ( ~ H + ) Bexp. ( - t F / R T ) (12) desorption processes are consequently rate determining. As given above the Tafel line slope bl = From (lo),( l l ) ,(12) and substitutingfor A$ b y 7 A$* - [(cf, ref. Z), and for A$r (the p.d. for the re- 0.053-0.063 v. cannot be explained on the basis of either a simple electrochemical or a simple catalytic versible potential) by RT/F In (“H+)B,one gets desorption but may be accounted for by a dual 7 = const. - ( R T / F ) In i (13) Equation 13 indicates that q a t constant i is inde- electrochemical-catalytic mechanism. The author’s thanks are due to Prof. A. R. pendent of pH. Similarly the electrochemical mechanism requires that q is independent of pH in Tourky for the facilities provided and for his intersolutions where Stern’s theory applies.2 Figure 5 est in the work. Thanks are also due to Prof. J. shows that this is the case in solutions below 0.5 N O’M. Bockris for helpful discussions. HC1. (14) J. Kolotyrkin and A. Frumkin, Compt. rend. Acad. Sci..
+
(13) J. Horiuti and M. Polanyi, Acta Physicochim., U.R.S.S., 305 (1935).
8,
U.R.S.S., 88, 445 (1941). (15) E. Rideal and B. Trapnell. J . chim. p h y s . , 47, 126 (1950).
