July, 1957
HYDROGEN EVOLUTION REACTION ON PT, AG AND W SURFACES IN ACIDS
879
THE MECHANISM OF THE HYDROGEN EVOLUTION REACTION ON PLATINUM SILVER AND TUNGSTEN SURFACES I N ACID SOLUTIONS BY J. O’M. BOCKRIS,I. A. AMMAR AND A. K. M. S. HUQ Department of Chemistry, University of Pennsylvania, Philadelphia, and Department of Inorganic and Physical Cheinistr~, Imperial College of Science and Technology, London, S.W. 7 Received June 19, 1966
The parameters of the H2 evolution reaction on Pt have been determined as a function of current density (c.d.) pH, salt addition, mode of electrode preparation, aging and degree of preelectrolysis; and as a function of c.d., and degree of preelectrolysis for W and Ag. On Pt, Tafel slopes ( b ) and exchange currents (io) varied with mode of preparation, that in air giving a slope of 2.303 R T / F compared with the more usually observed slope of 2.303 RT/2F; anodic activation gives high and reproducible io’s. Aging decreases i o and extends the c.d. range in which b = 2.303 R T / F . Successive increase of preelectrolysis causes i o to increase to a limiting value. Anodic activation occurs only at a c.d. > 10-2 amp. cm.-2 and is independent of the amount of current passed and subsequent reduction. The stoichiometric number ( v ) is unity. On W and Ag Tafel lines had two distinct slopes; u = 1 in 0.1-0.4 N solution. There is no intrinsic screening effect due t o a Luggin capillary. Previous work on Pt in pure solutions indicating b > 2.303 R T / F is reinterpreted in terms of partial diffusion control. Anodic activation on Pt is due to both mechanical cleaning and oxidation. The limiting c.d. for activation is that at which H Zdepolarization is negligible. Tafel slopes of R T / F are consistent with migration of H atoms over the surface as rate determining. Calculation of the rate of change of the potential of the layer of ions in contact with the electrode with that of the electrode indicates that near the electrocapillary maximum Tafel slopes may be changed to lower values.
Experimental Establishment of the validity of methods for the determination of the mechanism of the hydrogen The cell, of As-free Hysil glass, contained Pt electrodes evolution reaction (h.e.r.) and their application is of in the anode compartment with which to measure the of the solution. It contained a breaker for the importance to electrode process chemistry because strength bulbs in which electrodes were sealed, which could be restudy of the h.e.r serves as a model for the study of tracted from the cell without exposing the latter to the air. other electrode reactions, and because it is a rate- The cell was attached via a glass frame to paxolin board, and stood on wax. It was kept in chromic-sulfuric acid controlling process in certain types of corrosion. some hours (after glass blowing, for some days) and The mechanism of the h.e.r. has now been es- for washed with equilibrium and conductance water after each tablished for several systems; it changes with elec- run (no tap water was used). Elimination of grease from trode material and with conditions at a given elec- the walls, indicated by the easy streaming of water without trode-solution interface. However, the technique the formation of drops, is essential. A dust-free laboratory and absence of cigarette smoke are desirable. of avoiding side reactions due to trace impurities a t atmosphere Electrodes were arranged in a “six electrode cap”‘; each low c.d.’s, in spite of its essential role in a recently of five electrodes (and a pre-electrolysis electrode) could be evaluated technological electrode process, is still consecutively lowered into the solution through ground often neglected2 so that electrode phenomena ex- glass joints. Whilst preelectrolytic purification was proceeding the five test cathodes, protected from the solution amined are not always those of a single reaction. by surrounding glass bulbs (handled after washing by PtIt is, therefore, necessary to show directly not only tipped forceps), were pushed to the bottom of the cathode that pre-electrolytic purification of solutions allows compartment so that adsorbed grease films on the glass bulbs measurements on a single electrode reaction, but cpntact the solution a t the commencement of pre-electrolyAfter pre-electrolysis, the bulb on one electrode was also that the kinetic parameters of the reaction 81s. broken and measurements made after positioning the elecchange rationally with increased pre-electrolysis, trode near the Luggin capillary. After\ observation of the Mechanisms hitherto evaluated have been mainly current-potential relation, the first electrode was raised on metals manifesting either particularly high or above the solution, and the bulb surrounding a second etc. particularly low overpotentials. This choice avoids broken, Pt cathodes were wires of diameter 0.014 in. and area 0.1 those metals which have a high heat of adsorption They were welded to W wires of comparable diamefor hydrogen from the gas phase (e.g., W; it may ter so that a tight seal with Hysil glass could be obtained.6 be noted that Pt has a comparatively low heat of (a) Cathodes were treated with chromic acid, washed with conductance water, heated to 600” in HLfor 10 minutes and adsorption of H), and where, therefore, the fre- sealed to Hysi16 glass bulbs. (b) Cathodes were sealed to quently observed mechanism of rate-controlling glass in air and subjected to acid dips simulating conditions proton discharge on to a bare surface is less prob- used by Yeager.’ Ag cathodes were spectroscopically pure able. Theoretical progress is also demanded for wires, 2 cm. long and 0.1 cm. diameter, spot welded to thin wires. Both the Ag and Pt were sealed into glass and the interpretation of Tafel slopes not approximately Pt the seals tested under vacuum. The Ag was heated in Hz equal to RT/2F, 2RT/3F and 2RT/F in pure solu- in a quartz furnace for one hour a t 700’; and introduced tions and for sharp changes in slope of Tafel lines, into glass bulbs. “Spectroscopically pure” W wire was which resemble those arising from limiting cur- heated electrically in a vacuum of 10+ mm. to 3000” for one to remove surface carbides. The W electrode was rents. The h.e.r. has been examined on Pt sur- minute then removed in pure Hz into a quartz tube, heated to 1000’ faces, which are particularly sensitive to impurities; for a few minutes, and sealed into As-free glass. on W, where the heat of adsorption for H is particH2 was purified by passage through a strain containing ularly high; and on Ag where two distinct Tafel (4) J. O’M. BockriB and B. E. Conway, J . Sci. Inst., 26, 283 (1948); slopes occur in pure solutions depending on current E. C. Potter, i b i d . , 29, 160 (1952). density. a ( 5 ) Relatively thick Pt wires do not make a tight seal with Hysilin (1) M. A. Steinberg, S. S. Carlton, M. E. Sibcrt and E. Wainer, Eleclrochem. Soc., 102, 332 (1955). (2) M. B,reiter and R. Cl!amroth, Z.Eleklrochem., 68, 493 (1954). (3) J. O’M. Bookris and B. E. Conway, Trans. Faraday Sac., 48, 724 (1952).
Hz b u t do in 02. (6) If Pyrex is used, traces of AsrOs may volatilize and contaminate the electrode surface. (7) E. Yeager, T. 8. Oey and F. Hovorka, THISJOURNAL,17, 268 (1953).
J. O’M. BOCKRIS, I. A. AMMAR AND A. K. M. S. HUQ
880
I
powdered glass a t -80” to condense Hi304 vapor. HtSOd (Asp03 content, 0.5 parts per million) was twice distilled under reduced pressure and part of the middle portion was used to obtain the anolyte. &SO4 was twice crystallized from conductance water and solutions were pre-electrolyzed before addition to the main solution. Potential Measurements.-Current and potential measurements were conventional. Build up and decay curves were traced on a c.r.0. screen and ZR drops thus determined by means of a mechanical commutator. Currents less than 10-6 amp. cm.-e were measured from the p.d. across a standard megohm resistance. Pre-electrolysis Conditions.-An electrode of the material of the test cathode was used a t a c.d. of that of the highest used in measurements. The time was that after which preelectrolysis did not further alter results (Fig. 1).
0
P
-5
3-
0
”
Vol. 61
2-
.a
I -
Results Platinum : Electrode Preparation. Electrodes TIME OF PREELECTROLYSIS ( H O U R S ) , in Hz (Table I).-Pre-electrolysis was for 24-63 Fig. 1.-Values of exchange current for electrodes of the t o lo-’ amp. cm.-2. The io values same initial surface preparation (unactivated) as a function hours a t of degree of pre-electrolysis of the solution. (2-5.10-4 amp. cm.-2) were comparatively high, and the Tafel slope was 0.028 v. between c.d.’s and 10-3 amp. Tafel lines attained 0.240 higher sloDes above c.d.’s of amD. cm.-2 but ofupoor reproducibility. Electrodes Prepared in Air (Table 11).-Preelectrolysis was as for electrodes prepared in Hz. The io values (2-3 X amp. cm.-2) were low; o.120~ the slope of the Tafel line was 0.028 0.002 v. between and amp. cm.-2, with a repro6 0.080 ducible higher slope of 0.053 over the c.d. range > loF4t o amp. cm.-2. Increased pre-electro0.040t /d / lysis did riot alter this result. Figure 2 shows typTafel lines. (Preparation “in air” is clearly - ical equivalent to preparation “in the presence of conLOG C.D. (ornp. cm;‘). taminants”). Electrode Anodically Activated (Table 111).Fig. 2.-Tafel lines on Pt showing various slopes according to methods of preparation (I, electrode preparation in Pre-electrolysis was for GO hours at lo-‘ amp. Hp; 11, elect,rode preparation in air; 111, activated electrode cm.+. Reproducibility greatly improved; zo inprepared in H2). creased to 10-3 amp. and slopes were about 0.03 up to lo-’ amp. cm.-2. Aging and Activation.-With increased aging” (electrode in solution without net current flow), io values decreased from about lod4 to lod6 amp. cm.-2 over about 4 hours, the slopes of 0.053 occupied a larger c.d. range, and the change of parameters caused by subsequent pre-electrolysis was greater than for fresh electrodes. Reproducibility fell. For electrodes previously anodically “activated,” the aging effect was slower. The onset I I I of aging was hindered by increased pre-electrolysis -4 -3 -2 of the solution. LOG C.D. “ A C T I V A T I O N ” ( a m p . cm:‘). After anodic activation, electrodes were reduced Fig. 3.-Effect of c.d. of activation on io value obtained cathodically a t a c.d. of amp. cm.-2 for 100 for Pt (constant conditions of pre-electrolysis). seconds (in a solution ’ pre-electrolyzed past the solid reagentss (avoids carry-over of vapor, e.g., of pyro- limit of Fig. 1). The Tafel lines were identical to gallol), and four traps containing activated charcoal a t liquid air temperatures. Distilled water was refluxed and those before reduction. When the amount of thrice distilled in CO1-free Np, using a Thiessen and Her- electricity passed during anodic activation was. manng still (water of minimum specific conductance 8.IO-* consbant (10-3 coulomb) and the c.d. of activamhos. cm.-l a t 25” was obtained). After removal from the tion was varied from 10-1 to 2 X amp. final vessel of this still, it was distilled in purified H, into the cell. Water of specific conductance greater than 3 X IO-’ the io values increased with increase of c.d. of acmhos cm.-’ was rejected. The conductance water still was tivation and attained the constant limiting value cleaned with chromic-sulfuric acid followed by equilibrium of 1 X amp. cm.-2, for an activation c.d. of and conductance water about every six runs. about lo-? amp. cm.-Z. Further increase of c.d. HCl gas was prepared from KCI and HZS04(both Analar). KC1 was preheated to 500’ for several hours to eliminate or- of activation or time of passage of current did not ganic impurities. HCI was passed through traps containing affect io (Fig. 3). Pre-electrolysis.-Electrodes prepared in Hzwere (8) I. A. Azzam. J. O’M. Bockris, B. E. Conway and H. Rosenberg, studied after varying periods of pre-electrolysis of Trana. Faraday Soe., 46, 918 (1950). the solution. The i;s €or electrodes having the (9) P. A. Thieasen and K. Hermann, Z.Elektrochem., 43, 66 (1937). d20 ;
t
. I
0
I 40
I 60
I
80
881
HYDROGEN EVOLUTION REACTION ON PT, AG AND W SURFACES IN ACIDS
July, 1957
TABLEI" PARAMETERS OF THE HYDROGEN EVOLUTION REACTION ON PLATINUM (ELECTRODE PREPARED I N HI)'' Straight section of Tafel line -log is
Electrolyte HCl, d
Pre-elec trolysis Time Cad., (hr.) amp. cm.-g
No. of Tafel linea
io
(amp. cm.-a)
b
0.028 rt 0.002 0.1 24 3 x 10-2 12 2 f 2 x 10-4 3.5-2.4 1.0 24 1 x 10-2 8 1.8 f 0.9 x 10-4 ,030 =!= .022 3.5-2.2 1.5 14-63 1 x 10-1 5 4 . 5 f 1 . 2 x 10-4 ,029 k ,001 All results are quoted for the laboratory temperature, which was 23 =!= 2". The uncertainty thus introduced is less than the experimental reproducibility. b These results are uoted with reference to current densities calculated per apparent sq. cm. On the smooth Pt electrodes used (as also on and Hg)the likely ratio of true to apparent surface area is 2-3. 3.9-3
3
same initial (unactivated) condition but in soh- cm.-2 in stirred solutions. Stirring effects a t low tions which have been subjected to different de- c.d.'s were negligible. grees of pre-electxolysis are shown in Fig. 1. The exchange current increases with pre-electrolysis until a limiting value is reached. With activated electrodes, pre-electrolysis had less effect on the io value, but increased reproducibility and eliminated appreciable change of overpotential with time at a given c.d. Parameters on Platinum (Tables I-III).-No O.Oe0 significant difference between HC1 and HzS04 z o'060 solutions and over the concentration range 0.1-2.5 exists. The stoichiometric number, Y, for anodi~ 0 4 0 + cally activated Pt cathodes in 0.1-2.5 N HC1 s o h tions is 1.0. Figure 4 shows a typical Tafel line o,020 B with a plot of 4 against i a t low overpotentials. The overpotential is linear with c.d. up to only 5 -4 -3 -2 -I mv.; the plot passes through the origin. LOG '2.0.( a m p . c m r ' ) .
VI
Fig. 4.-Typical Tafel lines for hydrogen evolution on plat. h u m in 1.5N HC1 (no I R correction).
TABLEI1 PARAMETERS OF THE HYDROQEN EVOLUTION REACTION ON PLATINUM (ELECTRODES PREPARED IN AIR) Straight Presection Electro- electrolysis of Tafel 1 te C.d line ZCi, Time a m i : -logic N (hr.) om.-'
5-4 4-2 4.6-3.8 4.0-2.0
No. of Tafel lines
io X 106
(3 f 2)
1.0
24
lo-*
5
1.5
60 lo-"
4
I
0.160
(amp. cm.-P)
I
0.140
b
0.028i0.001 .053 f .005 (2 f 2) . 0 3 0 f .001 ' ; .054 i .004
o.'20
2
< 2
Satisfactory Tafel lines can be obtained (Fig. 5 ) :at higher c.d.'s by I R drop corrections, measured ',by means of a mechanical commutator applicable :because of the large io values (cf., the theory of the +error introduced in commutator measurements10). A linear Tafel line for Pt in 0.2 N Hi304 exists up ko a c.d. of 5 X lo-' amp. cm.-2.11 Agreement between the results of Hammett12 :and those of Bockris and Aazam" for "non-activated" electrodes is excellent (Fig. 6). Yeager' ,.and Schuldiner13 on unactivated e]ectrodes ob;tained lower io values than those of H ~or :Bockris and Azzam. Yeager's values during -ultrasonoration of the cathode7 are near to those re;ported here for activated electrodes. Schuldiner l 3 vi a
E
250-
200 -
2.
5 0.5 1.0 1.5 2.0 LOG C.D. ( a m p . c n r 2 ) .
J
g
Eg W
150-
c
100-
50
-6 LOG
C.D.
-5 ( a m p . cm;'),
-4
-3
Fig. 8.-Applicability of Tafel equation for W electrode in 0.1 N HC1 a t limitingly high conditions of pre-electrolysis.
0.120
I
4
LOG C.D. ( a m p . em:'
).
Fig. 9.-Inflection in Tafel lines for H2 evolution on Pt in unstirred solution compared with theoretical plots for activation-diffusion control: I, results of Schuldiner,*3 11, theoretical Tafel line for io = lo-' amp. cm.-2 and i~ = amp. ern.+; 111, theoretical Tafel line for io = 10-4 amp. and i~ = 10-lamp. cm.-2. Theoretical values from ref. 35.
0.06-0.07 are observed over cad. ranges up two powers of 10 and are hence not due to concentration overpotential. For rate determining electrochemical desorption,z3 b = 2.303RT/(l+@)F, but tends to 0.06 only if p tends to zero. If the combination mechanism is rate determining and the (23) R. Parsons, Trana. Faraday Soc., 47,1332 (1951),
884
J. O’M. BOCKRIS, I. A. AMMAR AND A. K. M. S. HUQ
+
surface is approaching saturation the H H combination may become a first-order reaction (Breiter and C1amrothZ4). However, degree of coverage (and hence order), depends on potential in a known wayz5and changes rapidly with change of potential, so that a first-order combination would not be expected over a long potential range (Fig. 2).z6 However, consider 2H30. (2MH) (2MH)
+ 2e; +2MH + 2Hz0 --f
before migration
(2MH)
after migration
+ Hz + 2M
after migration
(1) (2) (3)
in which the rate-controlling step (reaction 2) is the migration of H atoms from the point on the electrode a t which they are discharged, either (a) to meet another H atom which is also mobile on the electrode surface; or (b) to diffuse to an immobile H atom. (Tafel coefficients resulting from both modifications are the same.) For the latter case, if A+ is the galvani p.d. between metal and the ions i n the Helmholtz double layer, and CH+ the concentration of hydroxonium ions in the Helmholtz double layer (and regarding as negligible under the given conditions the rate of dissociative adsorption of H2) 29 211
= kl(1
- k’CH)CH+B-BA@/RT
= k-,cne(l-B)AdF/RT
21-1
V2 v-2
= kZCH = k-ZCH’
va =
~~CH’CH
(4) (5) (6) (7)
(8)
where CHI is the surface concentration of immobile H and CH the total concentration. The concentration of the migrating atsms is CH - CHI but presumably C H I
