Characterization of electrocatalysis in the oxygen evolution reaction at

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Langmuir 1990, 6, 268-276

268

low pH

hgh pH

Figure 10. Schematic representation for reversible interpolymer complexation at the air-water interface. Conclusion The present study demonstrates the following: (i) Welldefined amphiphilic block copolymers composed of a PAA segment ( n = 7.2) or a POE segment ( n = 9.7) and a DP,-controlled PSt segment ( m = 7-119) can be prepared by using a catalytic system of halogen-containing oligomer with Mn,(CO),,. (ii) Well-behaved monolayers are formed from both types of amphiphilic copoly-

mers a t the air-water interface, and II-A profiles are profoundly influenced by the chain length of PSt. (iii) Monolayers of 2 are considerably affected by varying the pH in the subphase, due to a conformational change of the PAA segment. (iv) Interpolymer complexes between such monolayers and PVP or PAA in the subphase are formed a t the air-water interface a t an appropriate pH and for 3-PAA-complexed monolayer; particularly, the formationdeformation of the complex is reversibly controlled by pH change (Figure 10). These findings have made it possible for the first time to control a pH-induced interpolymer complexation a t the air-water interface. We believe that the LB films composed of such complexed monolayers might be useful for an application as an organic thin film due to the predictable enhancement of thermal and mechanical stability. Registry No, TBOE, 122948-27-0; PVP, 9003-39-8; (TBOE)(St) (block copolymer), 122948-28-1; (oxirane)(St) (block copolymer), 107311-90-0; Mn,(CO),,, 10170-69-1; tribromoacetyl chloride, 34718-47-3; tribromoacetic acid, 75-96-7; sodium w-methoxypoly(oxyethylene), 31494-81-2.

Characterization of Electrocatalysis in the Oxygen Evolution Reaction at Platinum by Evaluation of Behavior of Surface Intermediate States at the Oxide Film B. E. Conway* and T.-C. Liu Chemistry Department, University of Ottawa, 32 George Glinski St., Ottawa K I N 6N5, Ontario, Canada Received April 1, 1989. I n Final Form: July 30, 1989 The anodic 0, evolution reaction (OER) at noble-metal anodes provides an important case of electrocatalysis where the electrode surface on which the reaction proceeds depends on potential, through the state of the surface of a thin oxide film developed upon it. An essential but little examined aspect of electrocatalysis is the behavior of the kinetically involved intermediates of the reaction a t the electrode's surface in relation to the potential dependence of the electrode reaction rate, characterized by the Tafel slope. Digitally recorded potential relaxation transients following current interruption are used, as in our previous studies on cathodic H, evolution, to evaluate the pseudocapacitance (CJ of intermediates generated in the steady state of the OER at Pt and hence the potential dependence of coverage by the kinetically significant intermediate states in the reaction. In the case of the OER proceeding as it always does on an oxidized surface of the metal, the intermediate states are not only OH and 0 species but probably also two or more oxidation states of Pt ions in the oxide film acting as a mediator couple in the 0, evolution kinetics. The involvement of such a couple may influence electron transfer through the oxide film. The derived C, is found to be logarithmically related to potential over two distinguishable ranges, corresponding to the observed changes of the Tafel slope of the OER. The slopes of these log C, vs potential plots are, however, unexpectedly small, a result that may originate from strong lateral interaction effects or partial charge transfer in the deposition of the intermediates. Introduction Adsorbed Intermediates in Electrocatalysis. While e l e c t r ~ c a t a l y s i shas ~ ~ commonly been treated in terms of the kinetics of the rate-determining step of the reaction concerned, and hence of its exchange-current density,'" io, in relation to electrode material, until re~ently,~.' (1) Bockris, J . OM. Modern Aspects of Electrochemistry; Bockris, J. O'M. Ed.; Butterworths: London, 1984; Vol. 1, Chapter 4.

0743-~463/90/2406-0268~02.50 f0

with the exception of the important paper of Gerischer and Mehl,' virtually no experimental information has (2) Srinivasan, S.; Bockris, J. O'M. In Fuel Cells; Their Electrochemistry; McGraw Hill: New York, 1969; Chapter 6. (3) Conway, B. E.; Bockris, J . OM. J. Chem. Phys. 1957,26, 532. (4) Kita, H. J . Res. Inst. Catal., Hokkaido Uniu. 1965, 13, 151. (5) Ruetschi, P.;Delahay, P. J. Chem. Phys. 1955,23, 195. (6) Conway, B. E.; Bai, L. J. Chem. Soc., Faraday Trans. 1 1985, 82, 1841. (7) Conway, B. E.; Liu, T . C. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 461.

0 1990 American Chemical Society

Langmuir, Vol. 6, No. 1, 1990 269

Electrocatalysis in Oxygen Evolution at Pt

existed on an essential aspect of electrocatalysis-the behavior of the kinetically involved adsorbed intermediates and/or surface states in the steady states of the reaction a t various potentials. Since the potential (V) dependence of the surface density of such intermediate states is a major factor determining the Tafel slope, b, of the process, which is as importantg as io in characterizing the electrocatalysis, we have developed, in recent papers, the technique20 and interpretative basis'' for evaluating the adsorption behavior of intermediates in electrocatalytic reactions proceeding a t appreciable net rates. In the present paper, this approach is applied to the study of the anodic 0, evolution reaction (OER) at Pt in alkaline solution, which, like the OER at all metals, proceeds at an oxidized surface of the metal. While empirical reactions involving io and b are useful in characterizing kinetics of electrode processes, the io values do not represent, in any directly accessible way, one of the fundamental aspects of heterogeneous catalysis that is involved in many electrochemical reactions, viz., the adsorption behavior of the transient intermediates that are involved in the main pathway of the reaction under nonequilibrium conditions of appreciable net current flow. For practical purposes, and in regard to mechanistic aspects of electrode processes, we have emphasized' that the Tafel slope factor b, or the corresponding transfer coefficient a, is as important as, if not more important than, the io value for characterization of "electrocatalysis" a t practically significant high current densities, as it is more particularly determined by the reaction mechanism and conditions of coverage, 0, by intermediates through the equation b-' = d(ln %)/dV+ PF/RT

(1)

for the case of a charge-transfer-controlled desorption step, with a symmetry factor p. Considering electrocatalysis in terms of exchange current densities, Conway and Bockris3 showed that the demonstrable relations of io to metallic properties such as the electron work function must arise indirectly through a primary dependence of io on the energy of adsorption of the chemisorbed intermediate in the reaction, e.g., H in the H, evolution reaction (HER) (cf. refs 3 and 8), t h e chemisorption energy usually being dependent3 on or the electronegativity difference3 between H and the metal (the Eley-Pauling r e l a t i ~ n ' ~ ) . Such relations between In io and the standard Gibbs energy of chemisorption, AGHO, e.g., of H in the HER, originate for various metals through the dependence of equilibrium coverage, OH, of the intermediate on AGHo and lead to "volcano relations" between In io and AGHo values, as shown by Parsons.16 With regard to the 0, evolution reaction (OER) at various electrodes, Ruetschi and Delahay" found that the overvoltage at a given current density was linearly related to the metal-to-0 bond strength but, as expected for the OER, not to the work function 9 of the metal, as was also concluded5 for the H, evolution reaction (HER), con(8) Gerischer, H.; Mehl, W. Z . Elektrochem. 1955, 59, 1049.

(9) Conway, B. E.; Bai, L.; Sattar, M. A. Int. J. Hydrogen Energy 1987, 12, 607. (10) Conway, B. E.; Bai, L.; Tessier, D. F. J. Electroanal. Chem. 1984, 161, 39. (11) Harrington, D. A.; Conway, B. E. J. Electroanal. Chem. 1987, nn,

.

L41, 1.

(12) (13) (14) (15) (16)

Ruetachi, P.; Delahay, P. J. Chem. Phys. 1956, 23, 556. Busing, W. R.; Kauzmann, W. J. Chem. Phys. 1952, 20, 1129. Butler, J. A. V. Proc. R . Soc., London 1936, A157,423. Eley, D. D. Discuss. Faraday SOC.1950,8, 34. Parsons, R. Trans. Faraday SOC.1958,54, 1053.

trary to the apparent experimental indication^.'-^ For the OER, relations to metal-oxygen bond energies can be expected since all anodic gas evolution reactions (02, Cl,, N, from N3-, and products of the Kolbe reaction) proceed on an oxide-film covered metal surface, the catalytic and electronic properties of which are entirely different from those of the underlying metal. While many theoretical considerations on electrode reaction mechanisms have been published which take into account the role of adsorbed intermediate~,~.'~*l~,'~ it is remarkable that until recently (cf. ref 6, 7, 18, and 19) almost no experimental information was available, with the exception of that in ref 8 and 20, about the states of adsorption and the coverage behavior of intermediates in some of the multistep reactions of main significance in electrochemistry, e.g., the HER, the OER, anodic C1, evolution, etc., under conditions of appreciable steadystate current flow. In the literature on regular heterogeneous catalysis, on the other hand, this important aspect of the reaction behavior has received much attention, e.g., in ref 21. Some Previous Work on the OER at Pt. As with the HER a t Pt, there is considerable interest in the OER from the points of view of the role of surface intermediate states in the reaction m e ~ h a n i s m ~ . 'and ~ - ~also ~ the influence of oxide film thickness.30 For satisfactory experimental studies, the latter factor must be known or otherwise controlled, as has been recognized in various recent works on anodic 0, and C1, evolution k i n e t i ~ s . ~ ' ,It~ ~ , ~ ~ is well-known that at Pt the oxide film itself is relatively stable down to much less positive potentials than those required for 0, evolution and, in fact, is not reduced until potentials near, or in, the UPD H potential range for Pt are approached, as is seen from typical cyclic voltammograms illustrated in Figure 1. There is hence a necessity for distinguishing between the oxide film a t Pt anodes and the OER intermediate states at its surface. This is also indicated by the recent results of Willsau et al.,38 who found by means of "0-labeling experiments that gaseous O,, electrolytically generated a t Pt from water, does not contain significant quantities of 0 atoms from the oxide film itself. However, this conclusion is a t variance with that of ref 39. (17) Bockris, J. OM. J. Chem. Phys. 1956,24,817. See also: Bockris, J. OM.; Huq, A. K. M. s.h o c . R. SOC.London 1956, A237, 277. (18) Willems, H.; Kobussen, A. G. C.; Vinke, I. C.; de Wit, J. H. W.; Broers, G. H. J. J. Electroanal. Chem. 1985,194, 317. See also: Kobussen, A. G. C. Ibid. 1980,115, 131; 1981, 126, 199. (19) Conway, B. E.; Bai, L. J. Electroanal. Chem. 1986,198, 149. (20) Breiter, M. W.; Knorr, C. A.; Volkl, W. 2.Elektrochem. 1955, 59, 681. (21) Discuss. Faraday SOC.1950,8; 1966,41, various papers therein. See also: Stoltze, P. Phys. Scr. 1987, 36, 824; J. Vac. Sci. Technol. A 1987,5(4),581. (22) Armstrong, R.; Henderson, M. J . Electroanal. Chem. 1972,39, 81. (23) Gileadi, E.; Conway, B. E. J. Chem. Phys. 1963,39,3420. (24) Conway, B. E.; Gileadi, E. Trans. Faraday SOC.1962,58, 2493. Faraday Trans. 1 (25) Conway, B. E.; Liu, T. C. J. Chem. SOC., 1987,83,1063. (26) Conway, B. E.; Liu, T. C. submitted to Proc. R. SOC.London. (27) Conway, B. E.; Bourgault, P. L. Can. J. Chem. 1960, 38, 1557; 1962,40,1690. (28) Trasatti, S. J.Electroanal. Chem. 1980, 111, 125. (29) Tseung, A. C. C.; Jasem, S. Electrochim. Acta 1977,22, 31; J. Electrochem. SOC.1977, 22, 31. (30) Schultze, J. W.; Haga, M. 2.Phys. Chem., N.F.1977, 104, 73. (31) Vetter, K. J.; Schultze, J. W. J. Electroanal. Chem. 1973, 34, 131, 141. (32) Birss, V.; DamjanoviC, A. J. Electrochem. SOC.1983, 130, 1688; 1987,134, 113; 1986,133, 1621. (33) Roscoe, S.; Conway, B. E. J. Electroanal. Chem. 1987, 224, 163. (34) Tamura, M. Electrochim. Acta 1978, 23,9; 1979,24,993. See also: Denki Kagaku 1980,48, 173.

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-0 3 1

04

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Potentioi / V, RHE

Figure 1. Cyclic voltammetry plots for formation and reduction of surface oxide at Pt for various conditions of preformation of the oxide film in 1 mol dm-3 aqueous NaOH at 298 K: (1) anodic limit, V , = 1.60 V, holding time th = 60 min; (2) V , = 1.90 V, t , = 100 min; (3) V , = 2.10 V, t h = 100 min; (4) V , = 2.34 V, t , = 100 min. Sweep rate is 25 mV s-'.

That electrochemical characterization of anodically formed oxide film at Pt is chemically meaningful was shown by Anson and Lingane,30 who demonstrated that results from chemical analysis of such films were in agreement with data derived from chronopotentiometry. It was also noted, in this early work, that the presence of oxide films at Pt inhibited the oxidation of I- to IO3and oxalate to CO,. The importance of the oxide film at Pt anodes on which the OER proceeds was recognized by Schultze et al.30 and treated in terms of electron tunnelling,30B5in particular by DamjanoviE et al.32in relation to the origin of the "3/2" reaction order in [OH-] in acid solution associated with Tafel slope values of 60 and 120 mV per decade of current d e n ~ i t y . ~ ' A "dual-barrier" model was considered3' which provided an explanation of this behavior. However, the essential information for examination of electrocatalysis in the OER in terms of the adsorption behavior of the intermediate states in the OER reaction pathway is still lacking and will be provided in the results of the work described in the present paper for the case of Pt in alkaline solution. For the OER, we proposed in an earlier paper27that the intermediate states are not only discharged OH or 0'' but also higher oxidation states of the metal ions in the surface of the oxide film, e.g., a t NieOsOH, for which it was shownz7 how such states can act as mediators in the OER; this mechanism was later applied to the C1 evolution reaction34 and in other work on the OER.29,38 In fact, changes of Tafel slope for the OER are associated with the potentials for change of oxidation state of oxide film^.'^*^^ A general scheme of formal reaction pathways in the OER was given by B o ~ k r i s , with ' ~ diagnostic criteria for distinction of mechanisms. (35) Schmickler, W.; Schultze, J. W. 2. Phys. Chem., N.F. 1978, 110, 277. (36) Tilak, B. V.; Conway, B. E. Electrochim. Acta 1956, 21,745. (37) Butler, J. A. V.; Armstrong, J. F. Trans. Faraday Soc. 1933, 2.9. - - , 12fiI. ~ ~ (38) Willsau, J.; Wolter, 0.; Heitbaum, J. J . Electroanal. Chem.

1985, 195, 299. (39) Rozenthal, K. I.; Veselovskii, V. I. Dokl. Akad. Nauk. USSR 1956, 111, 647.

Basis of the Potential Relaxation Method. In recent we have proposed approaches based on potential relaxation analysis for providing information on the kinetically involved adsorbed intermediates in electrocatalytic reactions. Details have been given previously,6,10and a full theoretical discussion of the significance of potential relaxation measurements was presented recently in the paper of Harrington and Conway." Applications to processes with various reaction mechanisms were made by Tilak and C ~ n w a yCon,~~ way and Bai,6p19and Kobussen et a1.18 In the potential relaxation method, it is the open circuit potential relaxation transients themselves, recorded following interruption of polarizing current, that provide the necessary timedependent information (as also may be obtained from ac impedances'22) complementary to that from the steadystate polarization characteristic, which together enable the adsorption behavior of the surface intermediate states, that are kinetically involved in the principal reaction pathway, to be quantitatively determined. The information on the kinetically involved intermediate states in the reaction is derived in the form of the potential-dependent adsorption p s e u d o ~ a p a c i t a n c e of ~ ~the * ~ ~ouerpotentialdeposited (OPD) species in the steady states prior to interruptions of polarizing currents. We emphasize that the nature and states of such OPD species will generally be different from those of UPD species generated at Pt at lower potentials, K1.23 V EH2. The basic equation representing the potential decay rate, dV/dt, -C(dV/dt) = io exp(cuVF/RT) E iss(V) (2) where V is the measured electrode potential referred to a stable reference electrode, t time, io the exchange current density of the process, cy its transfer coefficient, and is, the initial steady-state current density. C is the net interfacial capacitance which is characterized mainly by the adsorption pseudocapacitance, for the process when appreciable potential-dependent surface coverages, 0, of the intermediate states of the reaction arise over the potential range of interest in the kinetic study of the reaction. Formally, C = C, Cdl, where c d l is the interfacial double-layer capacitance. As was discussed previously," three types of adsorption pseudocapacitance of the OPD species are to be distinguished: (i) a steady-state (ss) pseudocapacitance defined, as in the work of Gileadi and C ~ n w a y , 'by ~ C,,23724

+

c,,,= s(d0ss/dVss)

(3)

(ii) a transient pseudocapacitance''

C , , = q(de/dt) / (d V/dt) (4) and (iii) an operational pseudocapacitance," C,,?, as measured here through use of eq 2 with an experimentally recorded Tafel polarization characteristic is,( V) (in eq 2) and the experimental potential decay transient V(t ) (cf. eq 2 ) : C , , = -i,,(V)/(dV(t)/dt) (5) In eq 2-4, q1 signifies the formal charge required to complete a monolayer of the kinetically significant intermediate species involved in the reaction pathway. (Note that this charge q1 is not necessarily identical with the charges for monolayer surface oxidation of Pt by OH or 0 since here the species of kinetic significance are new states on or at the already oxidized Pt surface that are participating in the anodic formation of O,.) Kinetic stimulation calculations by Harrington and Conway" showed

Langmuir, Vol. 6, No. 1, 1990 271

Electrocatalysis in Oxygen Evolution a t P t

that the theoretically significant C , , and the experimentally measurable C , , quantities are quite similar, both in magnitude and in their dependence on potential, for mechanisms that involve a rate-controlling electrochemical desorption step. Hence, for such conditions, the C results provide information on the surface density orntermediate states in the OER, as a function of potential. Applications were made in previous work to the HER at Ni, Ni-Mo cathodes? and Ptl’ and to the OER at oxides of Ni25and Co7 and provided for the first time quantitative information about the potential dependence of coverage by the participating OPD intermediates adsorbed at the electrode during passage of appreciable net currents. I n this way, information concerning electrosorption of intermediates, essential to the understanding of electrocatalysis under conditions of practical densities of current flow (up to 100-200 mA cm-2), was ~ b t a i n e d . ~ ” . ~Here ’ . ~ ~ we apply the potential relaxation method to the study of the surface intermediate states in the OER from alkaline solution at Pt and compare the results with those recently obtainedz6 for the OER at Pt in acid (H2S0,) solutions. Experimental Section General Procedure. The general experimental procedure involvingdigital recording of potential relaxation transients over a time range of ca. 6 decadaes from ps to s and digital plotting of Tafel relations using a computer-controlled potentiostat was as described previously.6J0 Preformation of Pt Surface Oxide Film. As in other studies of anodic processes at Pt electrode^,^*^' it is important that the experiments on the OER and potential relaxation be conducted on oxidized Pt surfaces at well-defined, controlled potentials and/or where a definite thickness or state of the oxide film has been previously established by polarization for a controlled period of time to potentials above the highest to which the electrode will be taken in the kinetic measurements. Such a procedure was adopted in the present work, as in other work on the OER at Pt30932 and in our work on the C1, evolution reaction at Pt.33 The stability of such “preoxidized” surfaces is demonstrated by well-known cyclic voltammetry behavior, which shows that oxide films on Pt formed at high potentials (up to 1.8-2.2 V EH, in the present work) do not become reduced until potentials near those for the H UPD region are attained (cf. Figure 1). Therefore, the preformed oxide film itself, on which the OER is caused to take place, does not suffer reduction during the potential relaxation transients, but there is an adjustment of surface density of intermediates on the film during the open circuit potential relaxation transient as the potential declines spontaneously toward the 0, reversible potential. Solutions. Aqueous solutions of NaOH (1mol dmW3),made up from NaOH recrystallized at low temperature from twicedistilled water, were used as the electrolyte at 298 K. Pyrodistilled water,“ which is required for UPD and cathodic HER studies, is found to be unnecessary for anodic OER experiments. Solutions in the working electrode compartment were saturated with 0, bubbled through the solution while purified N, was bubbled in the counter electrode compartment. Reference Electrode. A Hg/HgO reference electrode was used in the same experimental NaOH solution but maintained in a compartment separated from the Pt working electrode by a closed, wetted stopcock. An H,/Pt electrode was not used in order to avoid diffusion of traces of H, to the Pt anode, which can give rise to depolarization effects in sensitive potential relaxation experiments. Potentials recorded in this paper are con~

(40) Conway, B. E.; Sharp, W. B. A.; Angerstein-Kozlowska,H.; Criddle, E. Anal. Chem. 1973,45,1331. (41) Morley, H. B.; Wetmore, F. E. W. Can. J. Chem. 1956,34,359. (42) Conway, B. E.; Gileadi, E.; Dzieciuch, M. Electrochim. Acta 1963, 8, 143. (43) Bockris, J. OM.; Huq, A. K. M. S. Proc. R. SOC.,London 1956, A237, 277.

(a)

(b)

-8

-8

-4

-2

0

I

log i ( A cm-2)

Figure 2. Curve a: potentiostatically determined Tafel polarization line for the 0, evolution reaction at Pt in 1 mol dms aqueous NaOH at 298 K; data for descending and ascending changes of anodic potential, from 2.25 to 1.47 V RHE. Curve b plot of log (-dV/dt) from 2.00 V, RHE, determined from the potential relaxation transient taken at the same electrode. verted to the RHE scale (denoted by EHn)for the same 1 mol dm-3 NaOH solution, as also checked in separate experiments. Pt Working Electrode. High-purity Pt (99.99%) from Johnson Matthey Co. was used as the working electrode. After preliminary cleaning and sealing into a soft-glass tube, the electrode was subjected to ca. 10 anodic/cathodic potential sweep cycles at 50 mV s-l between 0.05 and 1.2 V E, until the cyclic voltammogram corresponded to that for a very clean Pt The charge for UPD H accommodation provided the real area of the electrode, in the usual way. Cyclic Voltammetry. Cyclic or linear sweep voltammetry was also performed in the usual way to check the state of, and charge for, oxide film formation (see Preformation of Pt Surface Oxide Film above) at various potentials and for controlled times of anodic polarization (see Figure 1). Results and Discussion The derivation of the primary results was as described in previous related papers.s’10,11.19.36,41 The following experimental information was obtained: (i) Potentiostatically determined Tafel relations as in Figure 2 were digitally recorded at a Pt electrode bearing a “preformed” oxide film grown at a potential of 2.3 V E, for 600 s, i.e., at a potential higher than the highest anodic pobntial at which polarization measurements were made in electrode kinetic OER runs or from which potential relaxation transients were recorded. The points shown in Figure 2 are for the descending direction of potential change below 2.3 V EH, and represent the reproducible curve for the OER in alkaline solution at the “preoxidized” Pt anode surface. The technique used here was similar to that described previously for the C1, evolution reaction3, and in the papers of DamjanoviE et aL3* The Tafel relations obtained consist of three regions: at low overpotentials, a low-slope region having b z 46 mV; at high potentials above 2.0 V, a high-slope region with b r 148 mV; and a transition region in between, covering a span of ca. 0.35 V. This behavior notably contrasts (Figure 3) with that observed in acid (0.1 mol dm-3 H,SO,) under otherwise similar conditions of oxide film formation, where the Tafel relation at potentials < 1.85 V EH, has a slope of ca. 120 mV, while above 1.9 V EH2 the slope is ca. 57 mV. This change, corresponding formally to kinetics determined by two alternative parallel reactions, has been interpreted as due to onset of resonance t ~ n n e l i n g ~ ’or, ~mediation ~ of the OER by higher

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log I ( A cm-2)

Figure 3. Comparison of Tafel relations for anodic 0, evolution on preoxidized Pt electrodes in (b) 1 mol dm-3 aqueous NaOH and (a) 0.5 mol dm-3 aqueous H,SO, at 298 K (the latter from ref 26). oxidation of Pt in the oxide film surface,26 as was proposed by Conway and Bourgault" for the OER a t oxidized Ni. While the forms of the two Tafel relations in Figure 3 are qualitatively different, it may be significant that the transition in slope a t higher overpotentials occurs a t a potential V,,, which is almost the same (relative to the H, electrode in each solution) in acid and alkaline solution, viz. ca. 1.85 V EH2. (ii) Potential relaxation transients, as plotted out from the digital data acquisition system over the time range ps-s, are shown in log time in Figure 4. The same data, subjected to a computer-programmed differentiation procedure to obtain log (-dV/dt) as f ( W ,are plotted in Figure 5. Another log (-dV/dt) vs V plot, from a transient taken from 2.23 V (near the upper limit of the Tafel line a in Figure 2), is shown as curve b in Figure 2 to provide comparison with the Tafel relation, curve a, over a comparable potential range. The relation between the shapes or the slopes of the V vs log (-dV/dt) profiles (curves a and b of Figure 2) can be understood through inspection of eq 2, which indicates that In (-dV/dt) = In (io/C)+ aVF/RT (6) i.e., In (-dV/dt) as f ( V ) has the same slope, aF/RT, as the Tafel relation only if C is independent of otential. Generally, when C,,, is appreciable and thence2l?24 potential-dependent, In i,/C in eq 6 must also be potentialdependent, so the plots of In (-dV/dt) vs V will generally not be identical in form with the Tafel relation In i vs V , as in fact is the case for Figure 2 and the results in Figure 5. However, generally from eq 2 it is seen that In i(V) - In (-dV/dt) = In C(V), and it is this relation that enables C to be evaluated as f( V) (see below). The potential dependence of In io/C in eq 5 arises because the coverage factors 6' or 1 - 6' for the electroactive intermediate(s) are expected to be potential-dependent, limitingly exponential in VF/RTm," tests of such behavior are shown later. It is found that the logarithmic decay plots of Figure 4 do not become superimposable when properly converted to plots (cf. ref 6, 10, 13, and 41) in log ( t + r ) , where r is the integration constant for eq 2, determined (44) Allen, G. C.; Tucker, P. M.; Capon, A,; Parsons, R. J . Electroanal. Chem. 1974,512,335. (45) Dickinson, T.: Povev, A.: Sherwood, P. M. A. J . Chem. Soc., Faraday Trans. I 1975,71, 298.

I

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1

tog [t (SI]

Figure 4. Potential relaxation transients in log time, t , for the OER polarization at Pt in 1mol dm-3 aqueous NaOH from five initial polarization potentials: (1)1.98 V; (2) 1.91 V; (3) 1.85 V; (4) 1.80 V; (5) 1.75 V. Dashed line shows the corresponding plot for curve 1 vs log ( t + 7).

1

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log [ [ -

0 dV/dt

1, V

I 2

5-'1

Figure 5. log (-dV/dt) plots vs electrode potential from the respective potential relaxation transients of Figure 4. appropriately for each initial current density. This is also confirmed by the log (-dV/dt) plots of Figure 5, which avoid evaluation of r but are not coincident throughout the range of potentials covered. This means that, at higher initial polarization potentials in the OER (e.g., curves 1 and 2 in Figure 5), states of the surface are generated that are different from those involved in potential decay from lower initial potentials (e.g., curves 4 and 5 in Figure 5) over common ranges of potential (e.g., VI to V, in Figure 4) covered after the potential has begun to relax following the current interruption. In acid solution, on the other hand, the potential decay plots in log ( t + 7) taken from various initial potentials 11.85 V EH, and the corresponding log (-dV/dt) vs V plots are coincident, as is also found for cathodic polarization in the H, evolution reaction a t Ni.6 Capacitance Behavior. We have shown in a theoretical paper published elsewhere" that the pseudocapacitance (C,) behavior associated (cf. ref 23) with electroactive adsorbed intermediates in faradaic reactions is to be represented in terms of three not exactly equivalent "CVnquantities, defined by eq 2-4. The operational pseudocapacitance, C , , (eq 4), that is experimentallyaccessible is calculated from the experimental data points for log i(V) and dV/dt. It must be mentioned that derivation of the C,,, behavior relies in part, and necessarily, on the i( V) characteristic itself (right-hand side of eq 2); i.e., evaluation of C,,,( V) from V ( t )transients requires

Electrocatalysis in Oxygen Evolution at Pt

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RHE

Figure 7. Test of linearity of a log plot for the two regions of the C,, vs potential relation for the OER at Pt at an initial current density of 1.61 mA cm-', with the relation superimposed on a Tafel plot similar to curve a in Figure 2. Table I. Values of the Derivative -d V/d log Cu.o

curve no. in Figure 8

initial potential, V 1.97 1.85 1.80 1.74

18

V/V,

Figure 6. Plots of the operational pseudocapacitance, C,,, for the OER at Pt as a function of electrode potential for the more anodic range of potentials derived from the potential relaxation transients of Figure 4,curves 1, 3, 4, and 5.

,2l

16

-dV/d log C,,,V low V high V 0.19 0.19 0.15 0.14

0.74 0.51 0.45 0.42

the complementary and independently determined experimental data provided by the potential relaxation transients which enable dV/dt to be accurately derived as f ( v ) from the digitally recorded V(t) transients, ca. 1200 points/V, by using a differentiating subroutine previously checked for accuracy on an analytic algebraic function. From the V vs log i and the V vs log t plots in Figures 2 and 4, we thus obtain the C,,, vs V profiles of Figure 6, corresponding to curves 1,3,4, and 5 of Figure 4. These curves, especially 1and 3, taken from higher anodic potentials, are evidently composed of two distinguishable regions with an inflection around 1.85 V EH2,corresponding to the upper inflection region in the Tafel line (Figure 2, curve a). This is seen more clearly when log C,,, is plotted vs V in comparison with log i vs V, drawn on the same graph (Figure 7); the log C,,o vs V plots are best represented by two, or possibly three, linear regions having slopes dependent on the potential from which the potential relaxation was initiated, as listed in Table I. These plots give, for the first time on an experimental

I eo

RHE

Figure 8. Logarithmic plots of the operational capacitance C , , for the OER at Pt in 1 mol dm-3 aqueous NaOH derived from four of the potential relaxation transients of Figure 4,curves 1, 3, 4, and 5.

basis, a clear idea of how changes of Tafel slope for an electrode process are intimately connected with the changing conditions of adsorption of the intermediate(s) and the related potential dependence of coverage by the kinetically significant intermediate states in the OER at a Pt surface, measured here in terms of the operational pseudocapacitance, C,,,. As we have noted elsehere,^^'^' for the OER proceeding a t oxide films on transition-metal anodes, the intermediate states on and in the oxide film surface are to be identified with adsorbed, discharged OH' and 0' species (cf. ref 17), together with high oxidation states of the metal ions of the oxide film (here probably Pt2+and Pt4', see Comparison with Behavior in Acid Solution in Results and Discussion section) acting as a mediator ~ o u p l ein~ the ~ *discharge ~~ of OH-, or from H,O, depending on pH. It is evident that the relations between V and log C,,o can be represented quite well by straight lines (Figure 8), but the slopes are very different from the limiting value of ca. -59 mV expected in terms of a simple pseudocapacitance relation for a oneelectron electroactive species approaching full coverage (cf. ref 23 and 24). The fact that the slopes, dV/d log C,,, depend on the initial polarization current density or potential (Figure 8) suggests that some characteristic state of the Pt oxide surface is developed during polarization a t each oxygen evolution current density. For example, this may be due to the degree to which higher oxidation states of Pt44i45 are developed in the oxide film's surface during 0, evolution even though a definite overall extent of film growth was established in each series of experiments by "preformation" of the oxide film for controlled time and potential, before the polarization and current-interruption experiments were performed. Qualitatively, the only other factor likely to lead to a changing electrocatalytic behavior of the surface oxide, as potential relaxation transients are taken from successively lower potentials, is the continuing reconstruction of the quasi-three-dimensional surface oxide that can take place in time, without any further growth (i.e., without further increase in charge for reduction). This could cause a time-dependent change of the actual physical and/or chemical constitution of the surface in contact with the electrolyte. It was of interest to establish if C,,, exhibited any maximum at low overpotentials as was found for H in the H, evolution reaction at Ni and Ni/Mo composite electrodes.16 Potential relaxation transients, as in Fig-

274 Langmuir, Vol. 6, No. I , 1990

Conway and Liu

-59 mV, respectively, very different from the observed values in Figure 7 or Table I. Introduction of a large lateral repulsive interaction parameter g = 12RT in the adsorption isotherm (cf. ref 23 and 24) is required to account for such a high slope as -1130 mV, and, even then, the calculated relation for log C,,vs V is not as linear as that observed in Figure 7 or 8. Use of an isotherm of the form4' 8 = Kc exp[

13 -6

-4

-2

2

0

4

log t ( 5 )

Figure 9. Potential relaxation transients, as in Figure 4 but for lower ranges of potential: (1)1.86 V; (2)1.82 V; (3) 1.72 V; (4) 1.62 V, down to 1.34 V EHZ.

$004

?a0-

YE eoou

lL

i

o

100

-

9'3

I 15

17

19

V/V, RHE

Figure 10. Plots of the operational pseudocapacitance ,C,, for the OER at Pt as in Figure 8 but for the lower ranges of potential covered in the potential relaxation transients of Figure 9. Note the development of the maxima in C,, at ca. 1.42 V E, . Data for two polarization current densities of 0.63 and 1.52 ma cm-2 in the steeply rising part of curve a in Figure 2. ure 9, were accordingly recorded down to lower potentials and gave the C,,vs V profiles of Figure 10 for two initial current densities, each exhibiting the expected (cf. ref 11and 23) maximum, which arises in this case at around 1.43 f 0.05 V EH2. The total charges under the C,,,( V) vs V profiles over the potential range 1.46 to 1.8 through to 1.99 V (Figure 6) lie between 48 and 56 p C cm-2,50 corresponding formally to about 25% site occupancy relative to the original Pt metal surface, based on the H accommodation and one electron per site (see below). This corresponds, of course, formally to every other lattice position on the oxide film in each direction on, say, a two-dimensional (100) surface, being occupied by an intermediate in the OER a t the highest potential. These figures seem reasonable bearing in mind that undischarged OH- ions are also resident in the double-layer adjacent to the surface of the Pt oxide film. One of the interesting aspects of the results presented here (Figure 7, Table I) is that the slopes of the two regions of the V vs log C,,, plots have remarkable values: a t lower potentials in curve a, Figure 7, dV/d log C,,, 2 -236 mV, while the longer region of curve a has a slope of ca. -1130 mV. Data for other polarization conditions were given in Table I. The limiting slopes of curves of log C,,, vs V, for low and high coverage, in the absence of lateral interaction effects are e ~ p e c t e d * ~to ' ' ~be + and

- $!01/2] exp(VF/RT) (7) 1- 8 corresponding to lateral repulsion of surface or image dipoles experiencing a pairwise interaction energy U(8) = g83/2gives rise to an adsorption pseudocapacitance for an intermediate produced in a quasi-equilibrium step, given by

8(l-8) RT 1 5 3 g81i2(l- 8)

q P C(8) = -

+

(8)

This does not improve the situation, as the direction of asymmetry of the corresponding curve of C(V) vs V about its maximum (cf. ref 42) is opposite to that observed in Figures 7 or 10, and a lengthy linear logarithmic region of the kind found in Figure 7 cannot be derived. It does not seem that the unusual results for the potential dependence of log C,,, are likely to be the result of a flaw in the employment of the potential relaxation method or analysis of the results therefrom, since (a) previously, we have found good agreement between results for the capacitance behavior of intermediates in the OER at Co oxide electrodes,' independently determined by means of potential relaxation and ac impedance measurements; (b) over the potential range for appreciable currents for H, evolution at Au, for which H chemisorption is very weak, the potential relaxation method gives only double-layer capacitance values (26-38 pF cm-') (corresponding, as expected for Au, to no significant adsorption pseudocapacitance for H); and (c) the experimental behavior found previously for the HER a t Ni and Pt69" by means of the potential relaxation method can be well simulated in terms of kinetic equations'' without reference to any arbitrary equivalent circuits for the interfacial processes. An alternative origin for large values of the slope dV/ d log C,,, would be if the electroactive intermediates were deposited or generated with transfer of only a small partial charge51ye, so that the limiting slopes would be RTI ye. However, the values of y required to account for the observed slopes of ca. -236 and -1130 mV would, on such a basis, have to be ca. 0.25 and 0.05 (2), respectively, and other values corresponding to the dV/d log C,,data of Table I. For the case of y = 0.05, the charge passed over the linear log region of slope -1130 mV would be ca. 9.5 p C cm-', which would correspond to a particle number density equivalent to 9.510.05, i.e., 190 p C cm-' if a full l e charge were passed per particle. Such a figure is interestingly close to a change of surface density of the intermediate involved, equivalent to monolayer coverage. For the region of higher slope a t lower potentials, the charge passed would be ca. 20 ~,LCcm-', and with y = 0.25, as above, this would correspond to coverage change by the species involved in that process equivalent to ca. 40% of a monolayer. Comparison with Capacitance Behavior Determined from ac Impedance Measurements. In order to complement the results obtained from potential relaxation transients, several ac impedance spectra were deter-

Langmuir, Vol. 6, No. I, 1990 275

Electrocatalysis in Oxygen Evolution at Pt

,

I

I

I

I .6

1.2

I

14

I 1.6

1

I

I

2.0

1.8

V/V

1

2.2

RHE

Figure 11. Comparison of plots of log C , , vs V for Pt in acid (ref 26) and alkaline (present world solutions at 298 K. 2000

A .

t i,

i

90

.

b)

.

60.

2"

frequency range 0.05-lo4 Hz are marked as dots and the calculated (Z', 2" ) values in the simulation by A symbols, in Figure 12. The agreement is good, with Cdltaken as 26 p F cm-', C, = 130 pF cm-', R, = 2550, and R, = 600 Q cm-'. Note that the value of C, for this fit is in quite good agreement with the value of C,,o a t this potential in Figure 10, as determined from the potential relaxation experiments. Comparison with Behavior in Acid Solution. As was shown in Figure 3, the Tafel relations for the OER at Pt in alkaline and acid solutions are quite different from one another, although an inflection occurs in both cases at a potential V,, around 1.85 V, but the Tafel slopes above V,, are quite different, as they also are in the lower potential range, 46 and 117 mV (see Figure 3). In the acid case, the change of slope was attributed by Schultze and Haga3' to onset of "resonance tunneling" and by Conway and LiuZ6to intervention of a redox mediation (cf. ref 27) step involving higher oxidation states of Pt ions in the surface of the Pt oxide film. These attributions are probably equivalent, as resonance tunneling can be induced by high oxidation states of ions in the oxide surface. A significant difference of reaction mechanism of the OER between acid and alkaline solutions, involving difference of activation energy, arises from the difference of source of OH or 0 species: H,O at acid pH and OH-,, in alkali. The overall energy difference for these processes corresponds to the energy of ionization of water, viz., 73 kJ mol-' in AGO; i.e., discharge of OH from H,O is expected to be kinetically more difficult than from OH-,. (Relative to the H, electrode, the overall energy for discharge of 0, is, of course, the same, independent of pH.) This seems to be consistent with the difference in the Tafel slopes a t low potentials in alkaline and acid solutions, 46 and 117 mV, respectively; the first of these values could correspond (cf. ref 17,32, and 43) to a ratedetermining step such as

-

U

120

60

180

Z ' n(cm-*)

Figure 12. Complex-plane impedance plots for the OER at Pt in 1 mol dm-3 aqueous NaOH at 298 K for two typical, but different anodic polarization potentials: (a) 1.54 V, RHE; (b) 1.82 V, RHE. Frequency range: 0.05-10 000 Hz. Simulated Z',2" behavior in curve a indicated by A symbols, based on Cd,l = 26 fiF cm-', C, = 130 fiF cm-', R, = 2550 S2 cm-', and R, = 800 S2 cm-' (see diagram below).

mined by means of a Solartron frequency response analyzer. Over the potential range 1.54-1.90 V, only one semicircle is developed in the complex plane plot. An example is shown in Figure 12 for 1.54 V (i = A cm-' 0, evolution current density), which can be simulated quite well by the response of the series-parallel equivalent circuit composed of a double-layer capacitance C,, and an adsorp-

+

Mox.OH + OH- Mox.O + H,O e (9) for alkaline solution having a slope of ca. 42 mV (2.3RTl (1 + p)F with 0 "= 0.5; cf. curve b of Figure 3) with the prior discharge step Mox OH- Mox.OH e having a greater rate constant, in the usual way, while, in acid, the first step, Mox H,O -+ Mo,.OH + H+ e, could account for the 117-mV slope of curve a, Figure 3. This low-slope region does not, however, pass continuously into a region of higher slope, 2.3RT/PF, expected in the simple analysis of a step such as eq 9, but passes through an extended inflection (Figure 3) over which the V vs log C,,o relation (Figures 7 and 8) has the unusually high (negative) slope referred to earlier, before the upper linear Tafel region of high slope, 148 mV, is reached (curve a, Figure 3). We have emphasized earlier in this paper, and that electrocatalysis in anodic reactions taking place from aqueous solutions must depend on the state of the oxide film (Max in eq 9) that is generated at the metal electrode rather than on the properties of the substrate metal itself. The involvement of the oxide film is modifying electrode processes a t oxidized vis h vis reduced Pt electrode surfaces was investigated in early work by Anson et al.46 For example, reduction of a previously oxidized Pt electrode led to enhanced reversibility of IO,- and V0,- reduction reactions, but this effect was attributed to a selfplatinization, Le., generation of a new active surface on

+

+

-

+ +

+ & I J s tion pseudocapacitance C,, together with faradaic reaction resistances R, (for discharge of OH) and R, (for desorption). The experimental points determined over the

(46) Anson, F. C.; King, D.M. Anal. Chem. 1962,34, 362.

276 Langmuir, Vol. 6, No. 1, 1990

the Pt following reduction of its previously oxidized surface. Correspondingly, the presence of an oxide film at Pt markedly affects the reversibility of the Fe(IJ)/ Fe(IT1) couple4' while apparent catalysis of IO; reduction accompanies reduction of the oxide film a t a previously oxidized Pt electrode leading to IO; reduction occurring some 200-600 mV more positive a t the oxidized Pt.48These effects arise, however, for quite different reasons from those involved here with the OER, where the electrocatalysis involves discharged OH' and 0' intermediates on the surface and probably Pt2+and Pt4+redox mediator states in the oxide film's surface, since the oxide film is always present during the reaction of anodic 0, evolution. An indication of the substantial difference in the adsorption behavior of the intermediate surface states that are involved in the OER on oxide films a t Pt in alkaline and acid solutions is given by the curves of log C,, vs V in Figure 11. In alkaline solution, log C , vs V plots show the higher and the lower negative d q / d log C , values indicated in Figure 11, while for acid solution2'the negative slope changes to a positive one beyond about 1.85 V. This implies, for the acid solution case, the appearance of a new electroactive species beyond 1.85 V, increasing in surface concentration with potential from some initially low value (corresponding to the slope 57 mV, Figure 3, curve a). In alkaline solution, this behavior of the OER is not exhibited, and the log C,, vs V relation remains with a high negative dV/d log b,,, value until the further linear Tafel region (slope z 148 mV, curve b, Figure 3) appears a t high potentials. The negative slopes of the V vs log C,,. relations (except for potentials below that for the maximum in Figure 10) imply increasing coverage toward a limitingly high (saturation) value of the coverage by the intermediate states involved, viz., adsorbed OH' and 0' species and probably the Pt2+/Pt4+redox mediator couple. The situation in acid solution, evidently quite different from that in alkali, suggests that the new species developed beyond 1.85 V could be a higher state of oxidation of Pt ions, e.g., Pt4+, as indicated by XPS measurement^,^^,^^ at the oxide-film surface, corresponding possibly to appearance of appreciable coverage by 0' rather than OH' species as the kinetically significant (47) Anson, F. C. Anal. Chem. 1961,33, 934. (48) Anson, F. C. J. Am. Chem. SOC.1959,81, 1554.

Conway and Liu

intermediate, with mediation of OH- discharge by Pt(IV) cations, e.g., as in the schematic sectional representation

;;+ - 5;; / 5 OH'

Bulk

pi?* OH

OH.

__c

20.

-

0,

Funhei steps

OH p[2-

OH

(49) While catalysis in electrochemical reactions was probably first specificallyrecognized by Frumkin at a conference in Leningrad in 1939, a first and perceptive definition of "electrocatalysis"seems to have been given by Busing and Kauzmann in 1952'' in terms of the ability of various electrode surfaces to promote the velocity of the rate-determining step of the reaction. In this respect, their definition preceded the common use of this term in North America in the 1960s by some years, when it was applied to the activities of fuel-cell electrodes. (50) We have considered whether this linear region in Figure 6 could be due to double-layer capacitance, but the charges involved over the potential range referred to here appear to be too large for this to be the case integrally; they would amount to ca. 150 p C V-'. (51) Of course, if formal charges < l e are passed per intermediate state generated in the reaction pathway, correspondingly greater charges must be passed in the subsequent steps requiring 4 F mol-' for overall molecular 0, formation.