HYDROGEN OVERVOLTAGE O F PLATINUM APiD COPPER
1401
INFLUENCE OF CERTAIX SURFACE-ACTIVE COMPOUNDS OK THE HYDROGEN OVERVOLTAGE OF PLATINUM AKD COPPER APPARATUS FOR OVERVOLTAGE STUDIES RAPIDRECORDING R. D. MISCH'
AND
R B. BERXSTEIN
Department of Ckemisli y, Illznozs Institute of Technology, Ckzcago 16, Illznuis Received August 16, 1960 INTRODUCTION
Although considerable attention has been devoted to the problem of hydrogen overvoltage in mixed solvents, there appear to be few data showing the influence of small amounts of surface-active compounds upon the overvoltagecurrent density relationship. The usefulness of solvent studies in elucidating overvoltage mechanisms has been stressed by Bockris (3) in a review. The importance of the solvent lies in its influence on solvation of the hydrogen ion, adsorption on the cathode, and the interfacial tensions. The energy of solvation is a major consideration in the slow-discharge theories of overvoltage (1, 7). The concept of adsorption of hydrogen atoms or molecules is also important. Interfacial tension, however, is no longer 'considered a primary factor, but may be responsible for secondary effects (2). Comparison of overvoltages under appropriate conditions have shown both positive and negative potential changes which have been attributed to the presence of the nonaqueous component at the electrode-solution interface. Hickling and Salt (IO) measured the hydrogen overvoltage for platinized platinum, tungsten, tin, lead, and mercury in solutions of ethylene glycol. They reported increases in overpotential up to 0.3 v. at current densities in the range of 1 ma./sq. cm. Bockris (2) studied the effects of a large number of miscible organic compounds upon the overvoltages of lead, copper, and nickel. For lead and copper, a decrease in overvoltage with increasing concentration of nonaqueous solvent was found, while the opposite effect was observed for nickel. The overvoltage dependence of current density was shown to satisfy the Tafel equation for all solvents. Evidently the complexity of the hydrogen-discharge process in media of mixed solvents makes it difficult to deduce any generalized interpretation. The purpose of the present investigation was to obtain further information bearing on the hydrogen-discharge process under conditions where various foreign species are present at low concentration in the electrolyte. The reported sensitivity of the overvoltage to the surface condition of the cathode (4), as well as the observed time-dependence of the overvoltage in the case of most solid metals ( 5 , 9, 12, 14), suggests that small amounts of surface-active compounds may be influencing the conditions of hydrogen discharge. In this study,
' Present
address: Argonne National Laboratory, Chicago 80, Illinois.
1402
B. D. MIBCB AND R. B. BERNSTEIN
the composition of the bulk of the electrolyte was essentially unaltered, since small quantities of surface-active compounds were added to the solution. EXPERIMENTAL
A rapid method was developed for obtainiig the dependence of the overvoltage upon the current. This may have general interest in connection with other studies involving solid metallic electrodes. A continuously varying potential difference was applied between the electrode under investigation and a saturated calomel half-cell which did not polarize with the currents studied. The calomel electrode served as both the auxiliary and the reference half-cell. A Sargent Model XX polarograph was found suitable for supplying the variable voltage, simultaneously amplifying and recording the electrolysis current. The cell was arranged to provide low internal resistance and minimum contamination of the cathode compartment. It was used to measure the overvoltage over a range of current densities from lo-‘ t o 1 ma./sq. om. within a period of approximately 15 min. This made it possible to study the influence of small amounts of surface-active compounds where prolonged electrolysis was undesirable. Reagent grade chemicals were used throughout. Platinum foil (Baker), 0.005 cm.thick, and electrolytic copper sheet, 0.02 cm. thick (stated punty 9939 per cent, were used as electrodes. Electrolytic hydrogen was passed through a purification train consisting of a “De-Oxo” unit (Baker), alkaline permanganate solution, and platinized asbestos at 425°C.) followed by distilled water at the ’temperature of the electrolysis cell. The electrolyte used throughout thia investigation was I N hydrochloric acid. The cell, shown in figure 1, consisted of a large vacuum-type stopcock whose large opening provided a low-resistance path. The lower portion of the cell served as a reservoir for a saturated calomel cell; the tap was opened only when an experiment was actually being performed. The saturated potwium chloride solution was changed at suitable intervals. A low-resistance diffusion barrier was provided with three layers of finely cut Pyrex-glass wool held in place by three porcelain Witt plates. The gas bubbler was arranged to keep the top plate in position. The thickness of the glass wool barrier was 4.0 cm.; the lower tip of the electrode was 8 cm. from the potassium chloride junction. The hydrogen was directed upwards, escaping through the glass bearing for the stirrer. The end of the stirrer was kept at about 0.5 cm. from the electrode d a c e . The speed of rotation was found not to be critical at the current densities used. The electrode area was 1.00 & 0.05 cm.’in all c m . The rubber stoppers were paraffined completely. The polarograph Was slightly modified by using a storage battery instead of dry cella to supply the electrolysk current. The potential of the large calomel electrode agreed within 1 mv. with a Beckman standard calomel half-cell. The resistance of the electrolysis cell was approximately 20 ohms. The estimated uncertainty in current as determined from the calibrated records was &2 per cent. The entire cell assembly was immersed in a bath maintained at 25.OoC. f
HYDROQEN OVERVOLTAGE OF PLATINUM AND COPPER
1403
0.1", unless stated otherwise. The potential data represent measured values referring to the saturated calomel cell plus the junction potential. The equilibrium hydrogen potential, shown as a dotted line in some of the figures, includes a small correction for a junction with strong acid (8). The calomel potential waa assumed constant, since the area of the mercury exceeded 25 sq. cm. (6) and the maximum current was 1 ma. In correcting the calomel potential to 25°C. a temperature coefficient of -7.6 X IO-' v./"C. was used (8). The platinum electrodes were cleaned by rinsing successively in concentrated acids. Immersion in each of the following was preceded and followed by a rinse in hydrochloric acid : sulfuric acid, distilled water, nitric acid, distilled water. A final rinse in distilled water preceded insertion in the cell. Some etching reTo
POLAROGRAPH
5 cm.
FIG.1 . Cell for overvoltage measurements
sulted from this procedure, as evidenced by the appearance of a grain pattern. The copper was cleaned in sulfuric acid, rinsed with distilled water and then with dilute nitric acid, and fmally rinsed in distilled water. The entire cell was cleaned initially with chromic acid cleaning solution; between successive experiments, however, the upper cell chamber only was cleaned, using a detergent solution. After rinsing, the cell was partly filled with electrolyte and the Pyrex-glass wool was inserted. Excess solution was drawn off and fresh electrolyte added. The electrode was transferred to the cell immediately after final rinsing. Hydrogen was introduced within 10 min., and bubbling was continued for a t least 30 min. before opening the cell to the calomel chamber. During this period the potential of the electrode with respect to calomel was measured to observe the changes with time indicative of the approach to equilibrium. The stopcock waa then opened and a steadily increasing negative potential was applied to the electrode. At suitable intervals the voltage sweep was
1404
R. D. MISCH AND R . B . BERNSTEIN
stopped and the potential difference measured with a Rubicon portable potentiometer to 1 mv. RESULTS
Consistent results were obtained when the overvoltage curve was determined within about 20 min. after opening the stopcock. When the electrolysis was prolonged beyond 30 min., the overvoltage curves appeared to change appreciably. This was attributed t o the electrodeposition of mercury. In the lower current range, from 0 to IO-* ma./sq. cm., a small current of the type generally observed in polarography was noted. This residual current was found to be approximately linear with the applied potential. Currents were measured above this reference line. Reproducible curves were only obtained if the residual current at zero applied potential was less than about 1 microamp. u1
0 z
*I
? c -a 0
c Y
L
0
QlW 0 In am NEGATIVE POTENTIAL v% SAT'D CALOMEL CELL
I
5
0
YI
m
500
CURRENT. CHART D I V I S I W S
FIG. 2 FIG. 3 FIG.2. Tracing of experimental curve for platinum in 1 N hydrochloric acid FIG.3. Data of figure 2 plotted with log i as abscissa
In the higher range of currents, the residual current could be neglected. Here, however, more time was required before a steady current was reached at a given applied voltage. The total drift in the 2- to 5-min. period thus required at each of the points was less than 5 per cent'of the total current, however. Sufficient time was allowed to attain a steady current at each point where the voltage was measured. At currents exceeding 0.2 ma., a small I R voltage correction was made. Figure 2 shows a portion of the recorded experimental curve for smooth platinum in 1 N hydrochloric acid at 28.4"C. This result is typical of several such curves in the lower range of current density. The bridge potential was measured at the indicated points. The values of the applied potential have been corrected for temperature to refer to the calomel cell at 25°C. In figure 3 the same data are shown in a semi-logarithmic plot, with several additional points. Despite the precautions exercised throughout, only fair reproducibility was obtained in this study. Figure 4 shows a comparison of two curves determined under presumably identical conditions. Although the semi-logarithmic plots of
HYDROGEN OVERVOLTAQE O F PLATINUM AND COPPER
1405
the overvoltage curves were found to be linear in every case, considerable variation in slope and intercept was noted, perhaps owing t o slight differences in surface condition. Table 1 summarizes the data obtained for platinum in 1 N hydrochloric acid at different temperatures. The columns list the temperature, io (ma./sq. cm.), defined as the current density at the intersection of the curve with the dotted line representing the equilibrium potential, and b, the slope of the semi-logarithmic line, A E / A log (i) (volts). Except for the result at l.l"C., all the curves lie in a rather narrow band. A temperature coefficient could not be obtained from these data.
i
, mo/cm'
I,
FIG. 4
mo/cm'
FIG.5
FIG.4. Reproducibility of overvoltage curves for platinum in 1 N hydrochloric acid FIG.5. Overvoltage curves for platinum in 1 N hydrochloric acid, showing effect of thiourea concentration.
-
TABLE 1 Overvoltage data for platinum i n 1 N hydrochloric acid at various temveratures IEYPXPAmE
10%x i o
'C.
ma.lcm.'
ooltr
1.1 15.3 16.0 28.3 28.4 46.3 49.2
4.80 1 .oo 0.48 0.67 1.45 1.50 0.95
0 050 0.033 0 039 0 032 0 039 0 037 0 035
SLOPE b
The influence of the surface-active compounds thiourea and benzyl sulfide was studied, since these compounds had shown significant effects upon the process of hydrogen deposition upon a palladium cathode (11). Figure 5 shows the data for thiourea in concentrations of lo-', lo-', and lO-'M in the electrolyte. The lower curve is an arbitrarily chosen "standard" curve obtained from a consideration of all the results for platinum. Figure 6 shows the effect of benzyl sulfide present at its saturation concentration of 4 X M . The lower line is the "standard" for platinum. The data for benzyl sulfide happen to coincide with the line for thiourea at lo-* M . The
1406
R. D. MISCH AND R. B. BERNSTEIN
slope of this curve is approximately 30 per cent greater than for platinum without additive. Table 2 summarizes the overvoltage data for platinum with the additives at '25°C. The overvoltage studies with copper were carried out in a higher range of current densities, from lo-' to 1rna./sq. cm. The electrode was initially polarized negatively to prevent dissolution of the copper. A fairly constant residual current of less than 50 microamp. was observed. The overvoltage data for two experiments are shown in figure 7. The slope of this "standard" curve for copper is 0.083 v. A special copper electrode was prepared by plating copper of 0.1 mrn.
FIQ.6 FIQ.6. Overvoltage curves for platinum in
FIQ.7 1 N hydrochloric acid, showing effect of
benzyl sulfide. FIG.7. Reproducibility of overvoltage curves for copper in 1 N hydrochloric acid
TABLE 2 Overvoltage data for platinum i n I N hydrochloric acid at 86°C. w i t h additives
1
ADDITIVE
MNCLMPATION
molcs/lifn
Thiourea. . . . , . , , .
.....,..,,..
10-7
Benzyl sulfide .... . . . . . . . . . . . . . .
lo-' 10-2 10-6
,
4
x
1
101 X i o
m(l./cm.:
0.22 0.14 0.16 0.15
1
SLOPE
b
VOlfI
0.034 0.040
0.048 0.049
1407
HYDROGEN OVERVOLTAGE OF PLATINUM AND COPPER
The effect of a layer of sulfide was studied for comparison with the preceding data. To produce such a film, electrodes of platinum and copper were treated with ammonium polysulfide solution to coat the surfaces as thoroughly as possible. The effect of the adsorbed sulfide ion on platinum is shown in figure 10 and for the copper sulfide in figure 11. In the latter case the top curve is taken from figure 7, where the points are given.
-a5 1
I 5.16'
5x10'
5.w-'
i, malcm'
FIG. 8 FIG. 9 FIG.8. Overvoltage curve for copper-plated electrode compared with figure 7 FIQ.9. Overvoltage curves for copper in 1 N hydrochloric acid, showing effect of thiourea and benzyl sulfide. 0 , thiourea; 0 , benzyl sulfide.
-0.11
I 5,lU'.
5x10"
i
,
5.10''
mulcm?
5 I IO-'
5ilU'
i
5.04
, ma/crnc
FIG. 10 FIG. 11 FIG. 10. Overvoltage curves for sulfided platinum compared with clean platinum. 0 , sulfided platinum; 0 , clean platinum. FIG. 11. Overvoltage curve for sulfided copper compared with clean copper. -, clean copper; 0 , sulfided copper. DISCUSSION
From the data presented, the average value of the slope of the overvo1tag;e line for smooth platinum is 0.035 f 0.004 v. For copper the value of b was found t o be 0.083 f 0.008 v. at 25OC. Both of these results are lower than the customarily accepted values (13). No completely satisfactory explanation of this discrepancy has been established. It was found in all experiments with platinum and copper that the linearity of the relation between overvoltage and log current density waa unaffected by
1408
R. D. MISCH AND R. B. BERNSTEIN
the addition of surface-active compounds. An increase of the slope b and a decrease in io followed the addition of thiourea in concentrations greater than lo-' M and benzyl sulfide at 4 X lo-' M. The effects due to a surface film of sulfide on platinum were slight; however, a marked shift in the curve for copper was evident. It is of interest to note that reduction of the sulfide film on copper resulted in a potential-log current density line with a slope approximately equal to that for copper alone. These results suggest that the rate-controlling step in more complex reductions may be the same as in the simple reduction of the proton to the hydrogen molecule. Since the influence of surface films has not been found to alter the slope of the overvoltage curves by more than 50 per cent, it would appear that contamination would not explain the higher slopes often reported. With the electrolysis cell as described, an experiment could not be prolonged, since impurities were concentrated at the electrode surface. An electrolyte flowing continuously from cathode to anode to prevent contamination would seem to be required for studies involving long time effects. I t is believed that the rapid method employed in these experiments is a more suitable one for measuring overvoltage as a function of the initial surface condition of the electrode. The experiments with platinum indicate that the effect of temperature upon the overvoltage was too small to be evaluated within the limitations of the reproducibility of the method. Since b is usually defined as RTIaF, a temperature change from 1" to 49°C. would increase b by about 14 per cent. The uncertainty in slope was estimated to be about 10 per cent. Clearly the influence of the surface condition of the metal is the major factor affecting the result. SUMMARY
1. A rapid recording apparatus suitable for overvoltage studies is described.
The method utilizes a Sargent polarograph which supplies a continuously varying polarizing potential while amplifying and recording the electrolysis current. 2. The linearity of the hydrogen overvoltage versus log current density relationship has been verified for platinum and copper electrodes in 1 N hydrochloric acid. The slope b of the semi-logarithmic plot was found to be 0.035 f 0.004 v. for platinum and 0.083 f 0.008 v. for copper at 25°C. 3. Small concentrations (e.g., lo-' M ) of surface-active compouuds mere shown to cause significant changes in the overvoltage curves. The overvoltage versus log current density plot was found to be linear in all cases, however. An increase in slope 6 and a decrease in io generally resulted from trace additions of thiourea or benzyl sulfide. REFERENCES
(1) AUDUBERT,R.:Faraday Society Discussions 1,72 (1047). (2) BOCXRIS, J. O'M. : Trans. Faraday SOC. 43, 417 (1947). (3) BOCXRIS, J. O'hl.: Chem. Revs. 43, 525 (1948). J. O'M.:Faraday Society Discussions 1, 95 (1947) (4) BOCHRIS, (5) BOWDEN, F.: Proc. Roy. SOC.(London) A126, 107 (1929).
CUMMUNICA‘PION TU THE EDITOR
I 4oY
(61 U h n r , O . , A N D S ~ o o ~ s13. n ,C.: The Ekctmde Potentid Hehouiur 01
.
AND
M c l ) o ~ i . u H. , J.: J. I%ys. & Colloid Chem.
(12) S E r s ~ n u 1s.: , J . Chem. SOC.10% 1051 (1916). (13) SMITH, J.: Scienra Piogre= 96,675 (1947) (14) VOX N L n ~ r ~ S r ~ I.: n 6Z. , physik. Chem. A118, 3S5 (l!Ui)
Pm. 1 Fra. 2 Fir, I. !?kction < ! i R r w l i ~ ejmttern of ~ p i n c l ~ t yown i A Pi,;. Y . The paltc~reof the s:mDpIc giving s i hdtrr yield than that of fixur(. I
pitttCI’nSshowing t h e P of porudcr oxides of samples. It was impossible tu obtain crystallogrilphic ation regarding the catalysts from these patterns. Thc pdishat mrfxros of the catalysts were etched with a violent, ctehing reagent in the preEnt study. The flux which is used in soldering iron and steel wit,h “Easy Plo” w’o.” adopted ail etchwt in the present, study. This flux, containing Rooride, chloride, 5nd borax, esn remove the Fe& film from an iron aurfarr. The Beilby layer formed in a catalyst surfsee hy polishing WBL removed with this flux a t 600°C. Figure8 I and 2 were obtained from the two types of eatalyrt treated with the flux. Figure 1 shows 8 mixture of powder crystals of the spinel type and single cryatals giving rise to Kikuchi lines. Figure 2 shows the existence of large crystal grains of the spinel type. The sample of figure 2 gave a bettor yietd at 480°C. than that of figure I in the reaction test for ammonia
