J. Phys. Chem. 1983, 87, 2960-2971
2960
Mechanism of Oxygen Evolution on Perovskites John O'M. Bockrls' and Takaakl Otagawat Department of Chemistry, Texas A&M Univefslfy, College Station, Texas 77843 (Recelved December 28, 1982)
An electrode kinetic study of the electrolytic evolution of oxygen has been made on perovskites ABOB(A is a lanthanide, and B is a first-rowtransition metal). The Tafel slopes of oxygen evolution were 2RT/3F, R T I F , and 2RTIF for nickelates,cobaltates, and ferrites or manganites, respectively. The reaction order with respect to OH- was close to unity. A formal approach to determine the stoichiometric number was found to be unsatisfactory. It was concluded that a common mechanism for oxygen evolution occurs on these perovskites, i.e., the electrochemical adsorption of OH-, followed by the rate-determiningelectrochemical desorption of OH, forming H202as an intermediate, which then undergoes catalytic decomposition to 02.This mechanism is an analogue of the electrochemical desorption mechanism for hydrogen evolution and can also account for the systematic change in the catalytic activities of these perovskites. Proton transfers may play an important role in the first and second step of this mechanism.
Introduction The contributions of Henry Eyring to fundamental electrode kinetics were deep and wide. They included the second1 molecular formulation of electrode kinetic proc e s s e ~one , ~ ~of~the few methods for the determination of the potential of zero charge,44 and several critical mechanistic studies, including particularly that on the reduction of C02.7 These studies contributed much to increasing the relation between electrode kinetics and general heterogeneous kinetics, and one of the stronger characteristics of these contributions-stress on the primacy of rate-determining step evaluation-is emphasized in the present work. The oxygen evolution reaction serves as an excellent example of such reaction kinetics studies which involve rate-determining step evaluation. The reaction on metals has been well studied."17 One t Present address: Argonne National Laboratory, EES Division, Building 362, Argonne, IL 60439. (1) The quantum mechanical treatment by Gurney was published in 1931. R. W. Gurney Proc. R. SOC.London, Ser. A, 134, 137 (1931). (2) S. Glasstone, K. J. Laidler, and H. Eyring, "The Theory of Rate Processes", McGraw-Hill, New York, 1941, pp 13, 27, 146-50, 190. (3) H. Eyring, J. Walter, and G. E. Kimball, 'Quantum Chemistry", Wiley, New York, 1944, pp 282-98. (4) T. N. Anderson, R. S. Perkins, and H. Eyring, J.Am. Chem. Soc.,
86. - - ,4496 ~~~- (1964). ,~ ( 5 ) R. S. Perkins, R. C. Livingston, T. N. Anderson, and H. Eyring, J. Phys. Chem., 69, 3329 (1965). (6) D. D. Bode, Jr., T. N. Andersen, and H. Evrina. - - J. Phys. Chem., 71, 792 (1967). (7) T. Paik, T. S. Andersen, and H. Eyring, Electrochim. Acta, 14, 1217 (1969). (8) F. P. Bowden, Proc. R. SOC.London, Ser. A, 126, 107 (1929). (9) T. P. Hoar, Proc. R. SOC. London, Ser. A, 142, 628 (1933). (10) M. Breiter in "Advances in Electrochemistry and Electrochemical Engineering", Vol. 1, P. Delahay, Ed., Interscience, New York, 1961, p 123. (11) J. P. Hoare in 'Advances in Electrochemistry and Electrochemical Engineering", Vol. 6, P. Delahay, Ed., Interscience, New York, 1967, p 201. (12) J. P. Hoare, "The Electrochemistry of Oxygen", Interscience, New York, 1968. (13) J. P. Hoare, 'Encyclopedia of Electrochemistry of the Elements", Vol. 2, A. J. Bard, Ed., Marcel Dekker, New York, 1974, p 191. (14) A. Damjanovic in "Modern Aspects of Electrochemistry", Vol. 5 , J. O'M. Bockris and B. E. Conway, Ed., Plenum, New York, 1969, p 369. (15) A. Damjanovic and A. T. Ward in 'Electrochemistry: The Past -
~
I
Thirty and the Next Thirty Years", H. Bloom and F. Gutmann, Ed., Plenum, New York, 1977, p 89. (16) A. J. Appleby in "Modern Aspects of Electrochemistry", Vol. 9, J. O'M. Bockris and B. E. Conway, Ed., Plenum, New York, 1973, p 369. (17) J. Horiuti in "Physical Chemistry, An Advanced Treatise", Vol. 9B, H. Eyring, Ed., Academia Press, New York, 1970, p 543.
0022-3854/83/2087-2960$0 1.50/0
of the principal difficulties is the universal presence of thin oxide films on virtually all the surfaces studied.lsZo The subject has been extensively reviewed, particularly by HoareI2 and Appleby.16 On platinum surfaces, there seems to have been a consensus that water (OH-) discharge is rate d e t e ~ n i n i n g . ' ~ J ~ J ~ However, applying the dual activation barrier model of MacDonald and Conway,21Damjanovic and JovanovicZ2 have shown that a mechanism in which a chemical step, following a charge transfer step, is rate determining could also give rise to a transfer coefficient of 1/2. Srinivasan et aLa have examined the kinetic parameters for oxygen evolution on Pt, Ru, Ir, and their equiatomic alloys. The rate-determining step is deduced as a chemical reaction. Among the oxides, RuOz has been investigated most i n t e n s i ~ e l y . ~ The ~ - ~ Tafel ~ slope and reaction order with respect to OH- are usually 40-60 mV decade-' and unity, respectively. O'Grady et al.29have proposed a unique mechanism to explain these results, in which the ratedetermining step is the second discharge with a simultaneous oxidation of the reaction site. Damjanovic et al.31,32 (18) A. Damjanovic, A. Dey, and J. O'M. Bockris, Electrochim. Acta, 11, 791 (1966). (19) A. J. Appleby, J. ElectroanaL Chem., 24, 97 (1970). (20) S. Gottesfeld and S. Srinivasan, J. Electroanal. Chem., 86, 89 (1978). (21) J. J. MacDonald and B. E. Conway, Proc. R. Soc. London, 269, 419 (1962). (22) A. Damjanovic and B. Javanovic, J. Electrochem. Soc., 123, 374 (1976). (23) M. H. Miles, E. A. Klaus, B. P. Gunn, J. R. Locker, W. E. Serafin, and S. Srinivasan, Electrochim. Acta, 23, 521 (1978). (24) S. Trasatti and G. Lodi in "Electrodes of Conductive Metallic Oxides", Part B, S. Trasatti, Ed., Elsevier, New York, 1981, p 521. (25) G. Lodi, E. Sivieri, A. DeBattisti, and S. Trasatti, J. Appl. Electrochem., 8, 135 (1978). (26) C. Iwakura, K. Hirao, and H. Tamura, Electrochim. Acta, 22,335 (1977). (27) M. H. Miles, Y. H. Huang, and S. Srinivasan, J. Electrochem. Soc., 125, 1931 (1978). (28) L. D. Burke, 0. J. Murphy, J. F. O'Neill, and S. Venkatesan, J. Chem. SOC.,Faraday Trans. 1,73, 1659 (1977). (29) W. O'Grady, C. Iwakura, J. Huang, and E. Yeager in
'Electrocatalysis", M. W. Brieter, Ed., The Electrochemical Society Softbound Proceedings Series, Princeton, NJ, 1974, p 286. (30) S. Trasatti and G. Lodi in "Electrodes of Conductive Metallic Oxides", Part A, S. Trasatti, Ed., Elsevier, New York, 1980, p 301. (31) J. F. Wolf, S. L. Soled, and A. Damjanovic in "Extended Abstracts", Vol. 79-1, The Electrochemical Society, Princeton, NJ, 1979, p 893. (32) A. Damjanovic, J. F. Wolf, and S. L. Soled, to be submitted for publication.
0 1983 American Chemical Society
Oxygen Evolution on Perovskltes
The Journal of Physical Chemistry, Vol. 87, No. 75, 1983 2961
have suggested the participation of lattice oxygens in the Preparation of Electrodes. Electrodes were made by reaction path on Ru02. pressing the powders into 13-mm-diameter pellets (BeckOxygen evolution on some spinels ( T ~ e u n g ~and ~ - ~ ~ ) man Model K-13 die) at a pressure of 300 kg cm-2. The pero~skites~ has ~ - been ~ ~ studied in alkaline solutions. pellets were sintered at 1000 "C for 48 h. No binder was Tafel slopes tend to be low, 40-60 mV decade-l. The used, to avoid the complications of the phase boundary. rate-determining step is generally concluded to be chemAn ohmic contact was made to one side of an electrode ical, e.g., OH,d, OHOad; H2O. pellet by means of a conducting silver epoxy resin (EThe present work is part of a study of the electrocataSOLDER adhesives, ACME Chemicals and Insulation Co., lysis of oxygen evolution on 18 perovskites. In this paper, catalogue no. 3021) with a copper lead embedded in the the mechanistic aspects are described. resin. The pellet was then fixed into a Teflon cap with an epoxy resin. The cap was screwed into a Teflon holder, Experimental Section which fitted into the electrochemical cell. The surface areas of electrodes were determined by Preparation of Perouskite-Type Oxides. The transidouble-layer charging curves, using the cyclic voltammetry tion-metal oxides with the substituted perovskitic structechnique. The detailed description and discussion of the ture, A,,A',BO,, where A is a lanthanide (mainly La), A' method applied to perovskites will be given in a subsequent is an alkaline earth (mainly Sr), and B is a first-row p~blication.~~ transition metal, were chosen in order to study the elecElectrochemical Cell and Solutions. A two-comparttrochemical kinetics of oxygen evolution systematically. ment glass cell was used. In a double-walled main comThe addition of A' was necessary to give sufficient conpartment, a working electrode was placed below a cylinductivity. drical platinum gauze counterelectrode (diameter = 3.5 cm, The perovskites were synthesized by high-temperature height = 5 cm). A saturated calomel electrode (Calomel solid-state r e a c t i ~ n s .Binary ~ ~ oxides or carbonates with Internal with Ceramic Junction, Beckman no. 93-003-94high purity (-99.999%, Aldrich or Alfa) were used as 02) was used as reference. All stopcocks were made of starting materials. The appropriate amount of each Teflon to avoid the use of grease. The temperature of the reagent required to give the desired metal ion stoichiommain component was controlled by a constant-temperature etry was weighed and mixed mechanically by means of a circulator (HAAKE Model FK). Vortex-Genie stirrer (Fisher Model K550-G) for 2 h. All experiments, except for the study of temperature Lanthanum oxide was dried at 500 OC before weighing. dependence, were made at 25 OC. All electrode potentials The uniformly mixed powder was then placed in a Ptwere converted to the normal hydrogen electrode scale. covered Alundum combustion boat (Fisher) and fired in The electrolytes used were 0.1,0.32,1, and 3.2 M NaOH, an appropriate atmospherea4, at 1100-1400 "C for a total prepared by mixing double distilled water with 10 N of 30-70 h, with frequent regrinding and refiring of the NaOH solution (Medical Chemical Corp., catalogue no. products. The pure lanthanum nickelate crystals were 486A). No excess salts were added in the electrolytes. synthesized by a coprecipitation technique.54 Before measurements, the solution was preelectrolyzed X-ray diffraction patterns were taken for each powder at -1 mA cm-2 (a Pt plate and an auxiliary Pt gauze of to confirm the compound concerned. 100 cm2were used) for at least 20 h, with stirring by slow (33)A. C. C. Tseung and S. Jasem, Electrochim. Acta, 22,31 (1977). purified N2 bubbling. (34)S.Jasem and A. C. C. Tseung in 'Electrode Materials and ProAll measurements, except for the study of pH depencesses for Energy Conversion and Storage", J. D.E. McIntvre. F. G. Will. dence, were carried out in 1 M NaOH solution. A slow and S. SrinivG-an, Ed., The ElectrocGemical Society Softbound Pro: ceedings Series, Princeton, NJ, 1977,p 414. stream of purified oxygen was directed toward the working (35)S. M. Jasem and A. C. C. Tseung, J. Electrochem. Soc., 126,1353 electrode throughout the measurements. The oxygen (1979). partial pressure (0.07-1 atm) for the determination of (36)P. Rasiyah, A. C. C. Tseung, and D. B. Hibbert, J. Electrochem. reaction order was adjusted by mixing appropriate Soc., 129,1724 (1982). (37)Y. Mataumoto and E. Sato, Electrochim. Acta, 24, 421 (1979). amounts of O2 and N2 by means of flow meters (Curtin (38)Y. Mataumoto, J. Kurimoto, and E. Sato, J.Electroanul. Chem., Matheson Scientific, catalogue no. 106-716). 102,77 (1979). Electrochemical Measurements. Preliminary charac(39)Y. Mataumoto, H. Manabe, and E. Sato, J.Electrochem. SOC., 127,811 (1980). terizations of perovskite electrodes were carried out by (40)Y. Matsumoto and E. Sato, Electrochim. Acta, 25, 585 (1980). linear sweep voltammetries. The potential was swept by (41)A. G.C. Kobussen, J.Electroanal. Chem., 126, 199 (1981). connecting the output of a PAR Model 175 universal (42)A. G.C. Kobussen and G. H. J. Broers, J. Electroanal. Chem., 126,221 (1981). programmer to the external potential signal input of a PAR (43)These relatively rare O2evolution studies should be contrasted Model 173 potentiostat. An X-Y recorder (Hewlettwith the comprehensive studies of 02 reduction on perovskites; e.g., B. Packard Model 7010B) was used to plot the current/poC. Wang, J. Molla, W. Aldred, and E. Yeager in "Extended Abstracts", Vol. 81-2,The Electrochemical Society, Princeton, NJ, 1981,p 280. tential curves. (44)A. S. Galasso, 'Structure, Properties and Preparation of PerSteady-state polarization characteristics for oxygen evovskite-Type Compounds",Pergamon, Oxford, 1969. olution and/or reduction was determined with a PAR (45)G.V. Subba Rao, B. M. Wanklyn, and C. N. R. Rao, J. Phys. Chem. Solids, 32,345 (1971). Model 173 potentiostat with a PAR Model 376 current/ (46)C. N. R. Rao, J. Indian Chem. Soc., 51,979 (1974). voltage converter module. The output potential and (47)H.Obayashi, T. Kudo, and T. Gejo, Jpn. J. A p p l . Phys., 13, 1 current was measured with two HP3466A digital multim(1974). (48)D.S.Rajoria, V. G. Bhide, G. R. Rao, and C. N. R. Rao, J. Chem. eters. The output current was also monitored on an X-t SOC.,Faraday Trans. 2,70,512 (1974). recorder (Fisher Recordall series 5000) to ensure obser(49)T. Kataura, K. Kitayama, T. Sugihara, and N. Kimizuka, Bull. vation of the steady state. The time required to reach a Chem. Soc. Japn., 48,1809 (1975). steady state was usually about 2 min, while 5-10 min was (50)B. C. Tofield, J. Solid State Chem., 12,270 (1975). (51)R.J. H. Voorhoeve, J. P. Remeika, and L. E. Trimble, Ann. N.Y. necessary in the lower overpotential region. Before each Acad. Sco., 272,3 (1976). measurement, the working electrode was held at the open (52)C. P. Khattak and D. E. Cox, Mater. Res. Bull., 12,463 (1977). circuit condition under a stream of purified O2for 1h. The (53)T. Nakamura, M. Misono, and Y. Yoneda, Bull. Chem. SOC.Jpn.,
+
-
+
55,394 (1982). (54)T. Otagawa and J. O'M. Bockris, J . Electrochem. Soc., 129,2391 (1982).
(55)J. OM. Bockris and T. Otagawa, J.Electrochem. Soc., submitted for publication.
2962
The Journal of Physical Chemistry, Vol. 87,
Bockris and Otagawa
No. 15, 1983
TABLE I : Kinetic Parametersa for Oxygen Evolution (Upper Row) and Reduction (Lower R o w ) AE*,e
Tafel slope, V decade-'
electrode LaNiO, LaNiO, LaCoO, 0.1
3
Lao.,Sro.'lCoO3 0. lCoo
I
3
3
Nd0.9Sr0,1C003
Gd O.Ssr
0. lCoo
3
Lao., Sr0.3FeO3
0.043 0.060 0.065, 0.060 0.070, 0.065 0.066, 0.067 0.064, 0.063 0.062, 0.064 0.065, 0.055 0.065, 0.057 0.070, 0.059 0.130 0.063
0.110 La0.5Sr0.5Fe03
LaMnO
La 0.9 Sr 0 . 1 Mn
3
La 0.6 Sr 0 .qMn0
3
La0.8K0,2Mn03
La0.2Ca0.8Mn03
La0,8Sr0.2Cr03
LaVO SrVO,
0.072 0.126 0.047 0.125 0.047 0.125 0.047 0.125 0.054 0.130 0.063 0.200 0.125 0.175
0.110 0.235 0.120
0.130 0.135 0.095 0.132 0.130 0.136 0.114 0.140
ab
1.37 0.98 0.91, 0.98 0.84, 0.91 0.89, 0.88 0.92, 0.94 0.95, 0.92 0.91, 1.07 0.91, 1.04 0.84,
1.00 0.45 0.94 0.54 0.82 0.47 1.26 0.47 1.26 0.47 1.26 0.47 1.09 0.45 0.94 0.30 0.47 0.34 0.54 0.25 0.49
reaction orderC 0.95 1.2
0.45
0.80, 0 . 7 8 1.1
0.44
0.70, 0.65 0.98, 1.2 0.84, 0.72 0.95,
0.62 0.45 0.45
0.75 0.98 0.94 0.83
0.88 0.43
0.70, 0 . 8 4 1.2 0.85, 0 . 8 5
0.52
1.1 0.42
0.68, 0.65 0.85 0.82 0.75 0.75 0.92 0.65 1.02 0.65 0.80 0.60 1.02 0.65 0.94 0.64 0.76 0.25 -0
io,d A cm-, 6.3 4.0 5.9 3.9 3.6 3.2 7.9 1.4 3.2 1.6
x 10-9 X
lo-''
10-8, 1.9 x 10-5 x 10." x l o - @5.0 , x
x X
x X
x X
lo-'' 10-9,6 . 3 x i o - 7 lo-" 10-7,4.4 x lo-''
1.0 x 4.0 x x 10-1°
3.3 4.4 1.4 4.0 6.3 4.0
x 1 0 - 9 , 2.4 x 1 0 - 6 X lo-'' x , 5.3 x 10.' x x
5.0 x
kcal mol-' 17.0 14.7 17.2 18.6 19.5 9.9 18.8 9.5
18.1 11.0 19.5 11.7 20.5 20.0 18.3 19.0 22.4
1.1 x
13.0
2.5 X 2.0 x 5.6 X 2.5 x 1.4 X 7.1 X 7.1 X 2.0 x 7.1 X 7.1 X 1.4 x 7.1 X 1.3 X 5.1 X 1.8 x 1.3 x 4.0 x
10-9 10.~
3.0 x
10-13
22.0 14.9 21.0 14.0 23.5 14.3 22.6 15.4 22.1 13.0 23.1 17.4 22.5 16.7 25.7 12.6 -10 -29 -17 -10
10-9
lo-10.'
10-12 10-7
lo-" lo-''
3.6 x 10-5 5.6 x
f Vopen, V vs. NHE 0.489 0.343 0.361 0.193 0.432 0.177 0.320 0.174 0.340 0.373 0.360 0.302 0.311 0.332 0.302 0.344 0.170 0.066
-0.520
a Data were taken 0,-saturated 1 M NaOH at 25 'C, except for the determinations of reaction orders and activation energies. Parameters in lower and higher overpotential ranges are indicated in the first and second columns, respectively, when available. Transfer coefficient: 01 = 2.3RT/bF, where b is a Tafel slope. Reaction order with respect to OH-for oxygen evolution (upper row), and with respect to PO for oxygen reduction (lower row). Exchange current densities are given in terms of geometric area. e Arrhenius activatidn energy at the reversible potential. Open circuit potential. g LaNiO, prepared by a coprecipitation technique. In reality, X-ray analysis showed that this substance consisted of LaNiO,, La,NiO,, and NiO, as a mixed multiphase.
electrode potential was then changed in steps of 20 mV in the direction of decrease in overpotential. The sum of the solution resistance and electrode resistances was determined from the impedance measurements, and iR-free data were obtained by subtracting the ohmic drop from the measured potential value. The potential step technique56was used to determine the coverage of OH- species as a function of potential and/or pH. The electrode was held at a certain potential for 100 s, under a stream of O2 and/or N,; then a cathodic potential step function (-0.2 V vs. NHE) was applied, and the coverage was measured by integrating the i / t curve over a relatively long period (-60 s), by means of a fast electronic integrator (ECO Instruments, Model 721 integrator) connected to a PAR Model 376 current/voltage converter. Results The steady-state current/potential data were plotted to obtain Tafel lines for oxygen evolution and reduction on (56) E. Gileadi, E. Kirowa-Eisner, and J. Penciner, "Interfacial Electrochemistry, and Experimental Approach", Addison-Wesley, Reading, MA, 1975, p 438.
the various perovskite-type oxides (Figure la-h). Current densities are expressed with respect to geometric surface areas. The electrode kinetic parameters of oxygen evolution and/or reduction are summarized in Table I. The Tafel slopes for oxygen evolution were 2RT/3F, RTIF, and 2RT/F for the nickelates, cobaltates, and ferrites and/or manganites, respectively. The 2RT/3F slope was observed only in the pure nickelate, while a multiphase sample of the same substance gave an R T I F slope (Figure la). A change in Tafel slopes from R T I F to 2RTIF in higher overpotentials was observed on cobaltates (Figures lb-d). Unusual Tafel slopes, 175235 mV decade-l (Figure lh), with little evidence for oxygen evolution, were observed on vanadites and/or chromites. These electrodes also exhibited abnormal and nonreproducible cyclic voltammograms and large deviations (-300-400 mV) of the intersection of Tafel lines from the reversible potential (Figure lh). The reaction order, with respect to OH- ion in the oxygen evolution, was determined from the slope (a log i / a log uOH-)V,T as near unity (Figure 2a). La1-xSrxCo03has been studied by Matsumoto et al.39
Oxygen Evolution on Perovskites
I'
No. 15, 1983 2983
The Journal of Physical Chemistty, Vol. 87,
'
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'
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.
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Bockris and Otagawa
The Journal of Physical Chemistfy, Vol. 87, No. 15, 1983
2064
a)
LaNi03
1 1
0 70V 0 66V
-5
0 12v
Reduction
0 70V
-4
-
-
N
0 66V
0 62V
5
Evolution
9 -3 ._
0 66V
0
0 -
0 70V
-2 -
1
-1 -
,
i
'
"
'
I
'
0
LaMn03
30
31
32
I/T (K-')
33
34
x io3
Flgure 3. Temperature dependence of current density on LaNiO, for oxygen evolution and reduction, at constant potentials, in 0,-saturated 1 M NaOH.
-7
'
'
'
'
'
'
'
'
'
'
-05 log Poz (atm)
-1
'
.
' 00
Flgure 2. log i vs. pH plots at constant potentials on LaNiO, and
La,,9Sr,,,Co03(a). Log i vs. log Po? plots in 1 M NaOH on LaMnO, at various potentials (b).
-
who also obtained a Tafel slope of 65 mV decade-'. In this work, a Tafel slope of 120-130 mV decade-' in the higher overpotential region was also observed. However, there is some discrepancy concerning the reaction order with respect to OH- on cobaltates. Matsumoto et al.39reported 1.8 for Lal-,Sr,CoO, ( x = 0.2, 0.4), and Kobussen and Masters5' reported -1 for L%,5B%,5C003,while all cobaltates showed -1 in this work. The exchange current densities obtained in this work for the 18 perovskites measured ranged over the values 10-'-10-9 A cm-2 (with respect to the geometric area), which are consistent with the values obtained on cobaltates studied by both Matsumoto et al.39and Kobussen and master^.^' The Tafel slopes of 125 mV decade-' and the reaction order of -0.7, obtained on Lal_,SrxMnO3in this work, are in fair agreement with the values of 130-140 mV decade-' and -1 reported by Matsumoto and sat^.^' The Tafel slopes for oxygen reduction were R T I F for most of the electrodes. However, the observed Tafel regions were narrow (1-1.5 decade), and the extrapolated exchange current densities appeared to be lower than deduced from oxygen reduction rather than from oxygen
-
-
(57)A. G. C. Kobussen and C. M. A. M. Mesters, J. Electroanal. Chem., 115, 131 (1980).
evolution. Thus, the nature of the surfaces on cathodically and anodically polarized surfaces were evidently different. The measurements became erratic and fluctuating below geometrically based current densities of about lo4 A cm-2 (ca. lo4 on a real area scale). This is probably atrributable to nonequilibrium phenomena, such as oxygen ion diffusion58r59and/or competing electrochemical reactions, occurring simultaneously with oxygen reduction. The reaction order with respect to oxygen partial pressure, Po,, in oxygen reduction was determined from the slope, (a log i/d log Po,)v,r (Figure 2b). The activation energies, AE*,were calculated by using the equation, AE* = -2.3R[d log i/a(l/T)lv, at constant potentials (Figure 3). The activation energies at the reversible potential were determined by extrapolating the values of 7 (overpotential) = 0. The effect of pH on cyclic voltammograms was investigated on the Nd,,gSr0.1C003electrode (Figure 4a). The curves became broader with the increase of pH. The coverage (obtained by integrating voltammograms between -0.4 and 0.7 V) vs. pH is plotted in Figure 4b. A linear relation was obtained. The values of coverage was normalized by using the roughness factor of 1.2 X 1.4 X l O I 5 sites cm-2 was used for the full surface coverage.60 Figure 5 gives the plots of coverage vs. pH at different potentials on the N&.gSro,lCo03electrode. A linear relation between the coverage and pH was obtained. The presence of O2gave a slight increase in coverage, indicating that the area occupied by O2groups is a small fraction of the coverage. Therefore, the main species covering the surface is OH-. (58)P.R.van Buren, G. H. J. Broen, C. Boesveld, and A. J. Bouman, J. Electroanal. Chem., 87, 381 (1978). (59)T. Kudo, H. Obayashi, and M. Ymhida, J. Electrochen. SOC.,124, 321 (1977). (60) M. Crespin and W. K. Hall, J. Catal., 69,359 (1981).
Oxygen Evolution on Perovskites
30
-
1-
20
300
-
1
-
./
/
N2
E
..-. 10-
E
E
/
/
/
O2Saturated
I
a
/
Ndo&3ro ,Coo3
2 ;200
v
m
2
>
0
......
-30
0 1 M NaOH
- . O32M --1.0 - 3.2
{
100 -
I
Potential ( V vs. NHE)
ol
I
I
,
,
,
0
13
8
10 0
1
14
PH
Figure 5. Coverage vs. pH relations in 02and/or N,-saturated NaOH solutions, as a function of the initial potential. 1.5
1.o
e A
-16
tL, -08
"
13
'
'
"
14
"
IE
0
0.5
01
N
0
J 0
PH Flgure 4. Cyclic voltammograms on Nd0,,Sro,,CoO3 in different pH solutions (a). Coverage vs. pH plot (b).
-04
00
-00
04
08
Potential ( V v s NHE)
Discussion
Figure 6. Capacitance vs. potential plot and Mott-Schottky plot on Ndo9Sr,,COO, in O,-saturated 1 M NaOH at 25-100 Hz. Values are expressed in terms of geometric surface area.
Stoichiometric Numbers. An attempt was made to obtain the stoichiometric number from the method which uses the reciprocals of the Tafel slopes.6l Although values were obtained mostly in the region of 2, they have been rejected in the mechanism analysis because of the evidence from the extrapolation of the Tafel lines that the mechanism was different in reduction as compared with that of oxidation (for example, Mott-Schottky plots showed the perovskites to be metallized in the anodic region due to surface state concentration, see the following section). Metallization of the Anodic Perovskite Surfaces. The Mott-Schottky plot and the capacitance vs. potential curve on Nd0,9Sr0.1C003 are shown in Figure 6. A lock-in analyzer technique was used in the impedance measurements.62 The sign of the slope of the Mott-Schottky plot in Figure 6 indicates a p-type semiconductor behavior at the surface between -0.2 and +0.3 V (NHE). Thus, the
cathodic reduction of oxygen takes place in a potential region of depletion. The slope of the C-V curve as the potential becomes increasingly anodic to 0.2 V (NHE) may be due to increasing specific adsorption on OH- ions on the electrode surface. A plot of eOH-as a function of potential in 1 M NaOH is shown in Figure 7. The characteristic exponential increase with potential is seen.@ The capacitance maximum at about 0.6 V (NHE) is not reached by the 8-V data (i.e., no expected inflection can be seen), but it seems reasonable to infer that the capacitance maximum arises from lateral repulsion among adsorbed anions.64 It is noteworthy that, as the flat-band potential occurs at 0.3 V (NHE), the maximum is displaced to a value of +0.3 V with respect to the zero charge region, therefore, the fact is not well in accord with the prediction of the water rotation
(61)R. Parsons, Trans. Faraday Soc., 47, 1332 (1951). (62)These measurements will be reported in a separate publication: J. O'M. Bockris and T. Otagawa, J . Solid State Chem., submitted for publication.
SOC.London, Ser. A , 274, 55 (1963). (64) J. O'M. Bockris and M. A. Habib, 2.Phys. Chem. (Frankfurt a m Main), 98,43 (1975).
(63)J. O'M. Bockris, M. A. V. Devanathan, and K. Muller, Proc. R .
The Journal of Physical Chemistry, Vol. 87,
2988
300 I
1
I
1
I
!
a
1
b2 -
0.6
-
0.4
'00
1
02
03 04 05 06 Potential (V vs. NHE)
Figure 7. Coverage vs. potential relation on Nd,,BSr, ,COO, in 1 M NaOH, constructed from Figure 5.
A C-V relation of similar shape has been reported by Tench and Yeagere' for the Li-NiO system. The maximum capacitance which lies at a potential near the voltammetry peak was interpreted by these workers in terms of the Ni2+ Ni3+ oxidation. However, the cyclic voltammogram for N&.9Sro,1C003 (Figure 4a) shows no peaks corresponding to a similar oxidation. Therefore, the observed capacitance peak here is presumably related to an adsorption-capacitance associated with OH-species. A similar conclusion has been deduced by Kobussen and B r ~ e r in s ~the ~ impedance study on Lq,5Bh.5C003. The evidence for copious OH- adsorption at least on Ndo,gSro,,Co03 is manifest (Figure 7 ) . Thus, insofar as surface states are induced on the perovskite surface by the OH- ions, there will be a tendency to diminish the Shottky barrier and semiconductor character of the material and increase metallization. Correspondingly, the variation of the electrode potential would occur increasingly in the Helmholtz layer and decreasingly within the oxide (Fermi level pinning).6s Such a transition was deduced first by Greeneg who showed that the ratio of changes of the potential drop in the Holmholtz layer (AVH)to those in the semiconductor (AV,,) could be calculated by the equation:
-
-dAVH - --ka,, dAV,,
tHtokT
H
q
b
H 0
o 1
H, ,ti?
H
0
0
'
0 A on l l a n t r a n d e l 0 B om [ t r a n s i t i c i m e i a M'I o oxrae I O ? io:
- 0.2 0'
Bockris and Otagawa
No. 15, 1983
(1)
With 6 = 3 A,EH = 6, and N , as 1.0 X 1015cm-2 (the value at 0.45 V from Figure 7 ) ,dAVH/dAV,, N 350. Thus, even if (asindicated by the high coverage) the OH-ions undergo charge transfer adsorption, a very high degree of metallization is suggested. A Model for the Perovskite Surface in Solution. The effect of the variation of A in ABO, on kinetic parameters was examined on cobaltates and manganites. The results (Table I) reveal that the kinetic parameters for oxygen evolution on perovskites are independent of these sub(65) N. F. Mott and R. J. Watts-Tobin, Electrochim. Acta, 4, 79
( 1961).
(66) S.Levine, G.M. Bell, and A. L. Smith, J . Phys. Chem., 73,3534 (1969). (67) D.M. Tench and E. Yeager, J.Electrochem. Soc., 120,164 (1973). (68) A. J. Bard, A. B. Bocarsly, F. F. Fan, E. G . Walton, and M. S. Wrighton, J . Am. Chem. Soc., 102, 3671 (1980). (69) M. Green in "Modern Aspects of Electrochemistry", Vol. 2, J. O'M. Bockris, Ed., Butterworth, London, 1959, p 343.
Figure 8. Schematic model of the surface of perovskttes: high coverage of OH- (a) and low coverage of OH- (b).
stitutions, indicating that the reaction site is a first-row transition-metal ion (B ion) in perovskites. Therefore, the (100)plane, containing the transition-metal ions, is likely to be an active reaction surface. A model of the surface is shown in Figure 8. At lower coverage of OH- species (Figure Bb), free sites of transition-metal ions on (100) planes would be covered with adsorbed water. Diagnostic Criteria of Proposed Paths for Oxygen Evolution. In order to elucidate the mechanism of oxygen evolution, the diagnostic criteria under Langmuir and/or Temkin conditions have been calculated and summarized in Table I1 for the five different paths most often considered in oxygen evolution studies.24 The values have been calculated for the Temkin conditions for the first time in paths 111, IV,and 142(for earlier calculations, see Conway et a1.70-73). Basic Assumptions for the Mechanism of Oxygen Euolution on Perovskites. The mechanistic interpretation is made along the following line: 1. An examination of the current density (based on real surface areas) at a constant overpotential shows a linear dependence on, e.g., the enthalpy of formation of corresponding hydroxide, for all the perovskites for which results are examined (Figure 9).55 Hence, it is likely that the rate determining step is common for the described materials (cf., the common rate determining electrochemical desorption step for hydrogen evolution on transition metals74). 2. The relation described (Figure 9) suggests that breaking of an M-OH bond is involved in the rate-determining step. 3. On the surface of most perovskites, a high coverage of OH species seems to be a general situation. The linearity observed in Figures 4b and 5 suggests that a Temkin-type adsorption isotherm may be applicable in the surface hydroxylation process. Identification of the Reaction Path. It has been shown in Table I that the Tafel slopes were 2RT/3F, R T I F , and 2 R T I F for the nickelate, cobaltates, and ferrites and manganites, respectively. The reaction order with respect to OH- was near unity for all perovskites. Table I1 shows (70) B. E. Conway and E. Gileadi, Trans. Faraday Soc., 58, 2493 (1962). (71) B. E. Conway and P. L. Bourgault, Can. J. Chem., 40,1690 (1962). (72)B. E. Conway and M. Salomon, Electrochim. Acta, 9,1599 (1964). (73) E. Gileadi and B. E. Conway in 'Modern Aspects of Electrochemistry", Vol. 3, J. O M . Bockris and B. E. Conway, Ed., Butterworths, London, 1964, p 347. (74) B. E. Conway and J. O'M. Bockris,J. Chem. Phys., 26,532 (1957).
Oxygen Evolution on Perovskites
The Journal of Physical Chemistry, Vol. 87,No. 15, 1983 2967
M-OH or M-0, are involved on both sides of the ratedetermining step. On this basis, it would, therefore, be difficult to understand the strong linear dependence of (i)n=0.3V on AHofof hydroxides (cf. Figure 9). The catalytic effects would be balanced in both directions. 3. A clue as to the appropriate rate-determining step arises, when it is noted that under Temkin conditions using the step 11-2 or IV-2, only the case rOH>> ro or rl >> r2can account for a Tafel slope of R T I F needed for the cobaltates. It would be difficult to understand these conditions for the mechanisms concerned. However, the observed dependence of (i),,=0.3von A H O f suggests that physisorbed species would be involved as the product species, i.e., in the right-hand side of a rate-determining step. Mechanism for Oxygen Evolution on Perovskites. Thus, an alternative mechanism for oxygen evolution on perovskites, would be mechanism I, steps a and b, followed mechanism I
-4L I
>
Ni
I
-51
2 I1
,m"
4 1
N
A
Cr
-240
-220
-200
-1 80
-160
step a
Mz
step b
M'-OH
AH: of Hydroxide (kcal mol-') Flgure 9. Current density (based on real surface area) for oxygen evolution on perovskites at an overpotential of 0.3 V vs. enthalpy of formation of corresponding hydroxides: M"'(OH),. Transition-metal ions are indicated by different symbols.
-
that either Bockris's electrochemical oxide path with the second step, MOH + OHMO + HzO + e-, as rate determining, or O'Grady's path with the second step, MzOH Mz+1-OH + e-, as rate determining, can account for the observed Tafel slopes. However, the following difficulties arise concerning these mechanisms: 1. The fast surface recombination of oxygen atoms in subsequent, nonrate-determining steps on perovskites seems improbable (cf. step 11-3 in Table 11). If the recombination step is to be "fast", its velocity in the forward direction must exceed that corresponding to the highest current density observed in this study. The highest real current density observed was of the order of A cm-' = 6.2 X 1013molecules cm-' s - ~ An . ~ elementary argument shows that the bimolecular recombination rate would be less than -P
2r&02v0 exp[-E*/RT]
(2)
where ro is the radius of a surface-adsorbed 0, No is the number of adsorbed 0, vo is the gas kinetic linear velocity of diffusion in one direction, and E* is the sum of the heat of activation for recombination and surface diffusion. The expression omits entropy of activation and other factors, all of which would reduce the calculated velocity compared with that given by eq 2. A value of vo can be obtained from the expression uo = ( ~ ~ T / T M and ) ' / ~is ca. lo4 cm s-l. In view of the high OH coverage, No could not be reasonably taken as cm-'. E* is likely to be greater than values available for a similar quantity for the surface diffusion velocity. The available figure~'~J~ are 14-30 kcal mol-l for 0 on W, and 34 kcal mol-l for 0 on Pt. In view of the fact that these are values in the absence of competitive OH radicals, and due to the lack of data for 0 on Ni, Co, Fe, or Mn, it seems reasonable to take the mean value (25 kcal mol-l). When eq 2 is used, the maximum velocity is
