June, 1959
OVERPOTENTIAL ON ACTIVATED PLATINUM IN SODIUM HYDROXIDE SOLUTIONS
983
OVERPOTENTIAL ON ACTIVATED PLATINUM CATHODES IN SODIUM HYDROXIDE SOLUTIONS BY I. A. AMMARAND SOHEIR DARWISH Department of Chemistry, Faculty of Science, University of Cairo, Cairo, Egypt Received Mau 19, 1968
Hydrogen overpotential has been measured on platinum in 0.1 N sodium hydroxide a t 25”. It has been found that anodic activation up to 10-2 coulomb/cm.2: (ij decreases both the overpotential values and the Tafel slope; (ii) increases coulomb/ both the exchange current and the electron number. With the use of quantities of electricity higher than cme2,the direction of change of the overpotential parameters is revised.
Measurements of hydrogen overpotential on Pt in acid solutions have established the necessity of anodic activation as a means of removing surface impurities. The agreement between the results obtained for activated electrodes1 and those for non-activated electrodes cleaned by ultrasonoration3 suggests that the anomalous behavior of Tafel lines for P t 4 in pure solutions may be attributed to the presence of impurities on the elecbrodes surface. In alkaline solutions, however, the situation is still unclear. Two Tafel line slopes (varying between 0.116 and 0.138 v. for the lower value, and between 0.220 and 0.252 v. for the higher one) have been observed in the pH range 8.2-10.8.6 Above pH 10.9, one slope (0.126-0.128 v.) has been observed. In strongly alkaline solutions (above pH 12.1) it has not been possible to obtain consistent results even with the use of highly purified sodium amalgam for the solution preparation, and with extensive pre-electrolysis lasting for weeks.5 In the present investigation, hydrogen overpotential is measured on activated bright Pt in 0.1 N NaOH a t 25”, and the conditions of activation, leading to reproducible results, are discussed. Experimental The experimental technique was essentially similar to that of Bockris, et a1.186 The cell was similar to that used by Bockris a?d Potter.6 It was constructed of arsenicfree glass and incorporated ungreased water-sealed taps and ground glass joints. A platinized platinum electrode was used as a reference. To decrease anodic polarizatjon, the anode was in the form of a large Pt sheet 6 cm.* in area. This electrode was far removed from the cathode compartment. The anode and cathode compartments were separated by a sintered glass disc and a tap which was kept closed during pre-electrolysis and measurements. The pre-electrolysis electrode was in the form of a P t wire (area 0.5 cm.z) sealed to glass. A platinized Pt sheet (area 5-6 cm.*) was dipped in the solution during pre-electrolysis t o adsorb impurities. The solution was vigorously agitated by pure H?. Ha was purified by passing it into three furnaces containing Cu heated t o 450°, then over soda lime and silica gel. Pt cathodes were cut from a spectroscopically pure Pt wire of diameter 1 mm. They were directly sealed to glass and were then cleaned with chromic acid followed by conductance water. In some experiments the bulb technique was employed.? NaOH solutions were prepared from pure crystallized NaOH (under Hz) by dilution
with conductance water ( K = 5 x 10-7 ohm-’ cm.-l) and were pre-electrolyzed in a separate cell before being transferred by HZpressure to the overpotential cell, and preelectrolysis was again carried out a t amp./cm.* for 25-30 hours. A further increase in the time of pre-electrolysis had no effect on overpotential. A successful experiment was characterized by the constancy of q, a t a constant current densit , with time and by the agreement between the “up” a n f “ d o w n ” Tafel lines.8 The electrolyte concentration was checked by titration after each experiment. The direct method of measurements was employed up t o about 10-3 amp./cm.z making use of a luggin capillary. Stirring had no effect on 11 at low current densities, but it decreased it at high current densities ( e . g . , above amp./cm.2).. Most of the measurements were, therefore, carried out in unstirred solutions, and only for measurements above lo+ ampJcm.2 was the solution stirred. At low current densities, the current was checked by measuring the p.d. across a standard 0.1 megohm resistance. The apparent surface area was used to calculate the current density. Anodic activation was carried out a t various quantities of electricity. After activation, the electrode was cathodically polarized, a t the highest current density used for overpotential measurements, with Hz vigorously bubbling in the solution. The electrode potential was then measured as a function of time, and the steady-state value was attained in 20-30 minutes. Following this the Tafel line was measured.
Results An example of the results for non-activated electrodes (sealed in air) is shown in Fig. 1 from which it is clear that the Tafel slope b is 0.26 v. It is also clear that the departure from linearity occurs at an appreciable overpotential of about 0.2 v. The “up” and “down” Tafel lines are identical, and 7 is not affected by stirring. Although the measurements a t low current densities indicate a linearity between 7 and the current density up to a value of about 50 mv., yet the value of the electron number X 9 calculated for this electrode is about 0.2. X is defined as the number of electrons necessary to complete one act of the rate-determining step and it is related to the stoichiometric number v by9
x
= 2/Y
Neither the above value of nor the value of the Tafel slope correspond to any theory of overpotential. The mean parameters of six separate results obtained under similar conditions are given in Table I, together with the corresponding mean deviation from the mean. (1) J. O’M. Bockris, I. A. Ammar and A. K. H u q , THISJOURNAL, Anodic activation of Pt electrodes (sealed in air) 61, 879 11957). 10-2, 10-1 and 5 X lo-‘ was carried out a t (2) E. Wicke and W. Weblus, Z . Elektrochem.. 56, 159 (1952). (3) E. Yeager, T . Oey and F. Hovorka, THIS JOURNAL, 67, 268 coulomb/cm.2 of the electrode surface. The current (1953). was varied and the time of activation was kept, con(4) Associated with Tafel line slopes of 0.19-0.3 v.; c f . J. O’M. stant a t 2 minutes. The mean results are given in Bockris, Chem. Revs., ra, 525 (1948). ( 5 ) S. Schuldiner, J . Electrochem. Soc., 101, 426 (1954). (6) J. O’M. Bockris and E. C. Potter, J . Chem. Phya.,
PO, 614
(1952). (7) J. O’M. Bockris and B. Conway, J . Sci. Instr., 25, 283 (1948).
(8) Tafel lines measured from low to high current densities and vice vema. (9) J. O’M. Bookris and E. C. Potter, J . Elsctrochem. Soc., 99, 169
(1952).
I. A. AMMARAND SOHEIRDARWISH
984
Vol. 03
TABLEI PARAMETERS OF HYDROGEN EVOLUTION ON Pt ELECTRODES SEALED IN AIR Condition
Kon-activated Activated a t 10-8 coulomb/cm.2 Activated a t 10-2 Activated nt 10-1 Activated at 5 x
No. of
b (rnv.)
io (amp./cni.2)
4
250 f 10 161 f 5
( 2 . 7 f 0.4)10-6 (8.0 f O . 6 ) l O - 6
x 0 16 f 0.02 . 2 4 f 07
11 11 G
1 1 4 f 11 l l 6 f 14 127fT
( 6 . 8 f 0.7)10-5 (2.2 f 1.5)10-5 (1.5 i 1.1)10-&
, 9 2 1 .18 . 5 6 f .27 , 3 8 5 .OG
expts.
6
10-4 ainp./cni.2
(rnv.) at 3.10 X 10-4
10-3
385 16 173 f 2
508 f 12 254 1 4
333 f 8
32 1 5 9 7 f 14 131 A 2 2
76 1 9 1 5 3 f 12 2 0 0 1 10
133 f 13 209& 15 271 f 3
...
10-1
TABLEI1 PARAMETERS OF HYDROGEN EVOLUTION O N Pt ELECTRODES SEALED IN H, n (mv.) a t Condition
Non-activated Activated a t aoulomb/cm.
No. of expts.
4 4
b
io
(mv.)
(amp./cln.*)
x
168 5~ 10 111 1 5
( 2 . 1 f 0.2)10-6 ( 7 . 3 f 1.2)10-6
0 48 f 0 06 0.91 f O 13
10-4 amp./cm.2
3.16 X 10-4
10 - 8
280 f 6
364 f 9 183 f 6
445 f 7 239 f 5
128f7
than the corresponding values for the lion-activated electrode, and a further decrease in q and b is observed for the electrode activated at coulomb/ cm.2. Anodic activation also decreases the overpotential at which the Tafel line departs from linearity (Fig. 1). Above coulomb/cm.2, both q and b increase (Table I). The relation between the exchange current io and the quantity of electricity passed during activation, Q, is opposite to that between q and Q. The maximum value of X is coulomb/ observed for electrodes activated at cm.2. Since in alkaline solutions the rate-determining step is the discharge from water molecules6,Iowhich is characterized by a slope of 0.118 v. at 25’ and by a value of X = 1, the results for eleccoulomb/cm.2 represent the trodes activated a t best approach of the experimental results to the theoretical requirements. No iR drop is observed in t,hese results (cf. curve 1, Fig. I), and stirring effects are negligible up to amp./cm.2 For this reason iR correction was not necessary for the present investigation. The mean results for electrodes sealed in Hz are given in Table I1 which indicates that both q and b decrease, while io and X increase, for activated electrodes as compared with the results for non-activated electrodes. The effect of anodic activation on q may be attributed to changes in the nature of the surface such that, at low values of Q, the heat of activation for the discharge reaction is decreased with the consequent increase of the exchange current and the decrease of q.ll This may be visualized if anodic activation cleans the electrode surface from impurities and exposes sites of adsorption for the cathodic -7 - 6 - 5 -4 -3 hydrogen evolution reaction. At high values of Q, fog c d however, oxidation may take place, and the presFig. 1.-Hydrogen overpotential on Pt in 0.1 N XaOH a t ence of oxide films increases q.12 Although, before 259: I, activated at 10-2 coulomb/cm.2; 11, activated a t the measurements of q, the activated Pt electrode 111, non-activated. was made the cathode for 20-30 minutes a t the Ta,ble I, and two examples of the resu1t)sfor elec(10) R. Parsons a n d J. O’M. Bockris, Trans. Farads?, Sac.. 47, 914 trodes activated a t and l o + coulomb/cm.2 are shown in Fig. 1 for comparison. It is clear from (1951). (11) E. Conway a n d J. O’M. Bockris, Nature, 178, 488 (1956); this figure that the values of q and b for the elec- 3 . Chem. P h w . , 26, 532 (1957). (12) A. Frurnkin, Disc. Paraday Soc., 1, 57 (1947). coulomb/cm.2 are smaller trode activated at,
June, 1959
EXPERIMENTAL CHECKOF THEORIES OF VISCOSITIES OF SOLUTIONS
highest current density used for overpotential measurements (cf. Experimental), yet this procedure
985
might have been incapable of reducing the surface oxide film probably produced at high values of Q.
THE EXPERIMENTAL CHECK OF THEORIES OF THE VISCOSITIES OF SOLUTIONS1 BY WLADIMIR PHILIPPOFF AND FREDERICK H. GASKINS T h e Franklin Institute Laboratories for Research and Development, Philadelphia, Penna. Received J u l y 7 , 1958
The solutions of the synthetic polypeptide, poly--pbenxyl-L-glutamate (PBLG), reported by Yang* are practically monodisperse either as tightly wound a-helixes (similar to rigid ellipsoids) in m-cresol as a solvent or random coils in dichloroacetic acid. This gives the experimental possibility of testing the existing theories of the viscosities of suspensions without the disturbing influence of polydispersity. The theory for rigid ellipsoids requires a departure from the initial viscosity VU a t low rates of shear D with the square of either the shear stress 7 or of D,rather than linearly with T or D . This was confirmed for the nz-cresol solutions of PBLG. The dichloroacetic acid solutions, however, gave a linear dependence on r . This shows that even monodisperse solutions of random coils give a linear deviation from 70, This may he the result of an “internal polydispersity” for each single coil in the sense introduced by Rouse3 for the dynamic behavior of polymer solutions. The normal linear departure from i ofor polymer solutions is therefore very probably caused by polydispersity in a general sense.
I. Introduction At present two theories treating the viscosity of suspensions are applicable to solutions of high polymers: the theory of rigid ellipsoids (rods) and the one of coiled molecules. Both theories have been extensively treated ; also, the dependence of the intrinsic viscosity on the molecular weight of the polymer has been tested experimentally. However, the theoretical relationships are explicitly valid for infinite dilution and usually for a rate of shear approaching zero. Until recently all polymer solutions were more or less polydisperse. This polydispersity influences the relationships derived for monodisperse solutions, but one had no way of eliminating it. The solutions of the synthetic polypeptide (PBLG) reported by Yang (ref. 2) are practically monodisperse, either as rods or random coils dependent on the solvent. They give the possibility of testing experimentally the existing theories of the viscosities of suspensions without the disturbing influence of polydispersity. The dependence of viscosity on the rate of shear (lion-Newtonian viscosity) has been theoretically treated for both models. The results of the theory of rigid ellipsoids require the viscosity to be an even-powered function of the rate of shear; in other words, the decrease of viscosity from its limiting value a t low rates of shear must be proportional to the square of the rate of shear D. Until now, in practically all of the investigated cases, n linea?, decrease of viscosity with D was found. This discrepancy could possibly have been caused by polydispersity, but it was impossible to check the validity of the theory since there were no monodisperse high polymers. The conditions for coiled molecules with regard to the non-Newtonian viscosity have not yet been calculated to make definite predictions in this respect. The PBLG gave us the experimental possibility of testing the departure of the
The material used was a synthetic polypeptide, poly-ybenzyl-L-glutamate (PBLG), which has been described by Yang in reference 2. We investigated several concentrations of PBLG: 0.5, 0.8 and 1.1%-by-weight in m-cresol (MC) and 0.5 and 0.82% by weight In dichloroacetic acid (DCA). All of these were measured a t 25”; in additio!, the 0.5% m-cresol solution was measured a t 15 and 35 . Yang has repoorted these data with the exception of the data a t 15 and 35 . The viscometer used is a high pressure capillary viscometer4for which the principle of operation has been summarily described.6 We could use two calibrated capillaries interchangeably at the rates of shear D required for this investigation, which ranged from 4 to about 350,000 see.-’ (a range of about 100,000 to I ) corresponding to shear stresses T of 6 to 65,000 dynes/cm.* (a range of 10,000 to 1 ) . The precision of the mensurenients was in the mean i=l%as determined by the deviation from a smoothed curve.
(1) Presented a t the Meeting of the American Chemical Society. September, 1957,New York City. (2) J. T. Yang, J . A m . C h e m . Soc., 80, 1783 (1958). (3) P. E. Rouse, Jr.. J . Chem. P h y s . , a l , 1272 (1953).
Trans. Soe. Rheology. 11, in print (1958). (5) W. Philippoff, F. H. Gaskins nnd J. G . Brodnyan, J . A p p l .
viscosity from the “initial” viscosity (Newtonian viscosity) a t low rates of shear for both rigid rods and coils of the same material in monodisperse solutions. The experimental check described below showed that the requirements of the theory of rigid rods are indeed fulfilled for monodisperse systems. Investigation of the monodisperse coils showed that their behavior is qualitatively different from the one of rigid rods, since the solutions of random coils give a linear deviation from qo. 11. Experimental
111. Results The flow curves (log D us. log T ) have been given as Figs. 2 and 3 in Yang’s pager; t,herefore, they will not be repeated here. In Fig, 1, we have plotted the specific viscosity llsp
=
[g
- 13
where 7 0 is the soln. viscosity va is the solvent viscosity
linearly as a function of log T (in dynes/cmS2). This plot allows a large range of shearing stresses to be plotted in a limited space, but tends to (4) J. G. Brodnyan, F. H. Gaskins. W. Philippoff a n d E. G . Lendrat
P h y s . , 28, 1118 (1957).
