The oxygen evolution reaction on rhodium and iridium electrodes in 85% orthophosphoric acid

Electroanalytical Chemistry and lntelfacial Electrochemistry, 60 (1975) 101-t08 (~ Elsevier Sequoia S.A.,Lausanne

101

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The oxygen evolution reaction on rhodium and iridium electrodes in 85% orthophosphoric acid

A. J. APPLEBY* and C. J. VAN DRUNEN

Institute of Gas Technology, IIT Center, 3424, S. State Street, Chicago, Ill. 60616 (U.S.A.) (Received 2nd December 1974)

Introduction The oxygen evolution reaction on anodized noble metals in acid solution has been most widely studied on platinum electrodes I 6. Iridium and rhodium have received much less attention 7 9. Data on the temperature dependence on current density have only been reported for platinum a'4'6. This note compares briefly previous data for all three materials, and new work is presented on the tempei'ature dependence of the process on Ir and Rh electrodes. It is shown that evidence exists for a compensation effect between experimental Arrhenius preexponential terms and activation energies for the oxygen evolution process, as has been demonstrated elsewhere for other electrocatalytic processes l°aa.

Experimental Apparatus, experimental technique, and method of preparing reproducible anodized surfaces before measurements were identical with those described previously for platinum electrodes 6. Rh and Ir electrodes and gold suspension wires, together with their polishing and cleaning procedure, have been described 12. As in earlier work, 85% orthophosphoric acid chemically purified by treatment with H 2 0 2 w a s used as electrolyte 6. After reproducible preanodization 4"6'9, descending galvanostatic Tafel plots for the oxygen evolution reaction were obtained on the oxidized Rh and Ir surfaces, allowing sufficient time (usually 15 min) between points to ensure true steady-state conditions. Typical Tafel plots obtained are given in Figs. 1 and 2, over a temperature range from 25.0 to 136.0°C. Experimental Tafel slopes, and apparent exchange current densities obtained by extrapolation to the calculated reversible potentials under the prevailing temperature and pressure conditions 6 are given in Table 1. Unlike pt 1 6, Tafel slopes on Rh and Ir electrodes increase to higher values at high overpotentials: only the lower slopes are recorded in Table 1. A similar change in slope has been noted by other workers in dilute acid electrolytes v9. In the present work, the transition to the higher slope on Rh electrodes occurred at a c.d. of ca. 4 × 1 0 -5 A cm 2 (1850 mV HRE) at 25.0°C, and at * Present address: Laboratoires de Marcoussis, 91-Marcoussis, France.

20]

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1.9

C

112

1.7

~

B

1.5] 1160° 1.3 -7

136.~""

-'6

log10( i/a ¢r52) -'5

-~

23

Fig. 1. Tafel plots for oxygen evolution on iridium: 85% H3PO 4. (A) Lower Tafel region; (B) upper Tafel region; (C) (at 25.0°C): region showing high polarization.

2oj

B

1.8-

g

A 25.0'

1.6-

1.4"

1.35 -6

-5

-4

-3

Fig. 2. Tafel plots for oxygen evolution on rhodium: 85% HaPO 4. (A) Lower Tafel region; (B) upper Tafel region.

increasingly higher current densities as the temperature was increased to 136.0°C. The slope for the upper Tafel region was about 145 mV/decade (2.4 RT/F) at 25°C. As far as could be established the value of the slope in the lower Tafel region was not significantly temperature-dependent, indicating a temperature-dependent transfer coefficient. Such effects have been frequently noted 6'12 14. Values of 1/c~ (i.e., the R T/F multiplier) for Rh in the lower Tafel region are given in Table 1. For Ir electrodes, the upper slope was about 140 mV/decade at 25.0°C (i.e., 2.3 RT/F). The lower slope (Table 1) had a value that was rather less than RT/F at all the temperatures studied: in contrast to Rh, the value of 1/c~ increased slightly

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TABLE 1 OXYGEN EVOLUTION ON RHODIUM AND IRIDIUM ELECTRODES IN 85°,;; H3PO4: TAFEL PARAMETERS AND EXTRAPOLATED EXCHANGE CURRENTS AS A FUNCTION OF TEMPERATURE

Electrode

Temperature/

Tqfel slope/mV(decade) 1 1/~

io/A e m - 2

:C

Iridium

25.0 52.0 75.0 95.0 116.0 136.0

52.5 57.5 65 68 73 81

0.88 0.89 0.94 0.94 0.95 1.0

5.8 x 10 12 4.2 x I0 11 2.1 x 10 1o 9.3 x 10 1o 2.3 × 1 0 - 9 7.5 x 10 9

Rhodium

25.0 52.0 75.0 95.0 116.0 136.0

94 94 98 97 97 96

1.59 1.46 1.42 1.33 1.28 1.19

3.6x 10 11 1.7 x 10-l° 8.5 x 10- ao 2.6 x 10- 9 6.9x 10-9 1.9 x 10-s

with temperature. In addition, at very high overpotentials, a further relationship involving a sudden increase in overpotential occurred. This will be considered in m o r e detail below.

Discussion D a t a for oxygen evolution on preanodized Rh and Ir electrodes, taken in other acid electrolytes under c o m p a r a b l e conditions to those used here, are comparatively scarce. O n Rh electrodes, a double T a M slope was noted in 1 M H2SO4 by H o a r e 7, and in 1 M H C 1 0 4 by D a m j a n o v i c et al. 9, under ambient temperature conditions. The lower slope was reported to be 108 mV/decade in 1 M H2SO, , (ref. 7), but only 60 mV/decade ( R T / F ) in 1 M HC1Oa (ref. 9). The region of transition was 1.6 V H R E in both cases, but the current density in this region differed: 6 x 10 -3 A cm -2 in 1 M H2SO4, 10 4 A cm 2 in 1 M H C 1 0 4. The upper Tafel slopes were 180 mV/decade (3 R T/F) and 125 mV/decade (2 R T/F), respectively. In the present work, in 8 5 ~ H3PO4, the slopes lie between the above values, but the transition potential is ca. 200 mV higher, lr electrodes have been studied by the same authors, and in the same electrolytes 8'9. In 1 M H C I O 4 the lower slope was 40 mV/decade (2 R T / 3 F), with a transition to the upper region at 1.5 V H R E (3 x 10 -4 A cm z), where the slope was 120 mV/decade (i.e., 2 R T / F ) 9. In 1 M H 2 S O 4 (at least up to 1.6 V H R E ) , only one slope was observed, with a value of 80 mV/decade s. A dual slope plot is apparent in the present work, the slopes being close to those noted by D a m j a n o v i c et al. 9, t h o u g h at overpotentials of a b o u t 150 mV higher. The difference between the results of H o a r e 78 and D a m j a n o v i c et al. 9 for oxygen evolution on Rh and Ir electrodes p r o b a b l y depends on the different experimental conditions used: H o a r e polarized anodically in an ascending m a n n e r to 2.0 V HRE, the electrodes being then allowed to equilibrate at various applied c.d.

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or potential values to obtain the descending Tafel plot. Damjanovic et al. initially polarized for extended times at the highest anodic potential studied to build a reproducible anodic film on the electrode surface. The same procedure was used in this work. The higher activity and different TaM slopes on Hoare's electrodes are thus explained by the presence of thinner, less reproducible films, whose thickness may have been potential-dependent. The rapid jump in overpotential at the highest c.d. values studied on Ir electrodes in the present work appears to represent a transition to a further potential region where another process predominates, e.g., as noted by Frumkin 15 in other oxyacids, where the oxyanion, rather than water, is involved in the reaction. However, it is difficult to see why similar effects are not noted on Pt and Rh, if this is indeed the explanation. The effect is best explained in terms of the observation of Damjanovic et al. 9 that the oxide film on Ir increases very rapidly in thickness at potentials higher than about 1.3 V HRE: this may give rise to a "barrier layer ''16 effect at high overpotentials. This is supported also by the observation made in the present work that potentials under galvanostatic conditions continue to rise slowly with time on Ir electrodes, indicating oxide film thickening. By contrast, those on Rh (and Pt 6) remain stable. Reaction mechanisms For well anodized Pt, oxygen electrode Tafel slopes are close to 2 R T / F in both anodic and cathodic directions in acid solution 2 6. The most likely path is the so-called oxide pathS'6:

4 ( S + H 2 0 ~ S O H + H + + e )(rate-determining) 4 SOH ~ 4 S + 2 H 2 0 + O 2

(I) (II)

where S is a surface adsorption site. An alternative mechanism, the "electrochemical oxide path"4:6 may also participate 6' 17: 2 ( S + H 2 0 ~ S O H + H + + e -) 2(SOH ~ S O + H + + e -) 2 SO ~- 2 S + O 2

(III) (IV) (V)

This process has been proposed to account for the apparent change of path in going from acid to alkaline solution 4'17, and its Partial occurrence in the cathodic process in acid solution is supported by reaction order measurements 6. Under Langmuirian adsorption conditions at low coverage, Reaction I (oxide path) as rate-determining step will have Tafel slopes 4 of 2 RT/F (/~=½). If however the same step applies, but with IV and V in quasi-equilibrium (electrochemical oxide path), the TaM slopes will be 2 R T / F (anodic) and 2 RT/3 F (cathodic), under low coverage conditions. If IV is rate-determining in the same mechanism, the slopes will be inverted 4'9. In this basis, it has been suggested that the two reactions may have similar equilibrium rates 17, with one controlling anodically, the other cathodically, according to their Tafel slopes. Another interesting speculation concerning the reaction mechanism is possible. If OH coverage is limitingly high, but still under Langmuirian conditions, then rate-determining Reaction III (electrochemical oxide path) will have a Tafel slope of 2 R T / F (/?=½) under cathodic conditions, as is

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experimentally noted 3 6 It is therefore possible that Reaction III is the sole ratedetermining step, with the OH intermediate coverage changing from a high value in the cathodic region to a low value anodically. The transition region will then be close to the (normally inaccessible) reversible potential. It will be shown below that this explanation is supported by the available electrocatalytic evidence. Ruetschi and Delahay 18 examined the effect of metal hydroxide bond-strength (obtained from chemical data) on the rate of oxygen evolution on a range of metals. Their analysis showed that rate increased with increasing bond-strength. If we consider an electrochemical process involving a discharge reaction to give an adsorbed product as rate-determining step, followed by rapid, quasireversible desorption to give the final product (or, in the opposite sense, quasireversible adsorption followed by discharge), then the corresponding rate equations for the reaction under different conditions of adsorbate coverage (at constant potential) are given by: i = nFk exp - f i A G / R T

(low coverage)

(1)

and i = nFk' exp + (1 - fi) AG/R T exp n'F V / R T (high coverage)

(2)

where k, k' are substrate-independent constants, AG is the standard free energy of adsorption of the intermediate (at V=0), and /? is a symmetry factor (Bronsted slope), n' is the number of electrons per molecule of intermediate involved in the electrochemical adsorption isotherm (positive for reductive processes, as step IV, equal to zero for chemical isotherms, as step II). The derivation of these expressions will not be given here: it is analogous to that used by Parsons 19 for the hydrogen electrode reaction. Since, for high coverage, n F V A G must be negative, stronger adsorption will cause a decrease in rate (the converse of the effect at low coverage), and a change in Tafel slope. In consequence of Ruetschi and Delahay's observation, the intermediate involved in the oxygen evolution process is therefore present under low coverage conditions at high anodic potentials. This implies that OH is the controlling intermediate: its coverage will be low for all paths in which OH and O are related by a proton discharge process (e.g., step IV). It also implies that O sites and metal oxide sites are essentially equivalent (see ref. 20). On Ir electrodes, Damjanovic et al. propose that the 2 R T / F Tafel slope at high anodic potentials is caused by the transition of Reaction IV, rate-determining in the electrochemical oxide path, from low to high intermediate coverage. In view of the foregoing, this suggestion seems improbable: their alternative suggestion of a change of rate-determining step to Reaction III 9 is more likely. In both acid and alkaline solution, the most active surface for oxygen evolution among the three metals is the anodized Ir electrode, Pt being the least activeg: in the cathodic direction the converse is true. This is most striking in the case of Rh and Ir electrodes in alkaline solution, for which the same Tafel slopes are observed, and for which the ~ m e mechanism and rate-determining step have been postulated 9. Following the argument used for eqns. (1) and (2) above, this implies again that intermediate coverage has changed between the cathodic and anodic regions. According to this argument, Ir electrodes have Reaction III rate-determining in acid electrolyte throughout the range of potentials explored, with low OH coverage up to 1.5-1.6 V HRE. This is consistent with pH dependence data 21. Whether the

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same process applies on Rh as on Ir electrodes in acid solution is more doubtful. It seems probable however that the same inversion of coverage of the OH intermediate between anodic and cathodic regions will take place on both metals, in the same way as in alkaline solution. If this occurs, then the 2 R T / F Tafel slope observed at extreme anodic potentials (low coverage) and at cathodic potentials corresponds to the same rate-determining step as on Ir and Pt electrodes: i.e. a step corresponding to III in a similar path involving two separate charge transfer steps overall. Again, this is supported by the inversion of activity between Rh and Ir in the anodic and cathodic regions. The R T / F Tafel slope observed at lower anodic potentials on Rh may be due to a rate-determining chemical step after the primary charge transfer (as Damjanovic et al. suggest9), in a path of Krasilsh'chikov type 22, with a step similar to III followed by: SOH ~ S O - + H + (rate-determining) SO - ~ SO + e-

(VI) (VII)

Step VII is then followed by step V. An alternative explanation is that step III remains rate-determining, but under Temkin coverage conditions 6' 23 in the transition region between high and low coverage, in which case the overall mechanism is the same as that proposed on Pt and Ir. In concentrated H3PO4, results on all the metals, particularly Rh, are complicated by the temperature variation of c~, hence the temperature dependence of the entropy of activation 1'~. In all cases, the Tafel slopes observed are higher than in dilute acids 3'4"9, in the case of Rh and lr by as much as 30-50% . These higher slopes probably result from barrier-layer effects involving phosphate or polyphosphate adsorption, which is potential and temperature dependent. However, there is no reason to suppose that the reaction mechanisms are different in 85% H3PO4 from those in the dilute acids. Activation ener(lies Arrhenius plots for the extrapolated i o values in Table 1 are shown in Fig. 3. Included for comparison are the figures previously obtained for Pt 6 under similar conditions. Activation energies (in kcal tool 1) are Pt, 13.1; Rh, 13.9; Ir, 16.0, with preexponential terms expressed as logl0(A'/A cm 2) of -0.52, -0.33, and +0.36 respectively. At the reversible potential, the heat of activation on Ir is greater than on Pt, as would be expected from eqn. (2) (high coverage conditions, with AG or AH for adsorption of the intermediate more negative on Ir) for step III. In view of the different Tafel slopes on the two metals ( ~ 2 R T/F on Pt, < R T/F on Ir) the activation energies at higher potentials (varying by c~Fr/) will change differently, Ir finally having a lower activation energy than Pt for oxygen evolution under low coverage conditions (eqn. (1)). For Ir, low coverage will start at about 1.6 V HRE, where the higher Tafel slope commences. At a constant potential of 1.6 V HRE, only a 1.8 decade change in rate takes place on Ir in the temperature range studied. On Pt 6, the corresponding change is about 2.8 decades, indicating an activation energy under these conditions about 5 kcal mol-1 higher on Pt than on Ir. This is strong evidence for the inversion of OH coverage proposed here. The corresponding value for the rate change on Rh is 3.7 decades, but, as discussed above, this is

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/

-7.5-

-8-

'E u -9-

Pt

< .._o

-lO-

/ /

-11

10 3 T I/ K~ -11.5

3.4

3[2

3.'0

218

216

214

Fig. 3. Arrhenius plots for extrapolated exchange currents, oxygen evolution on platinum, rhodium and iridium electrodes.

complicated by the temperature independence of the Tafel slope.It is also not clear if the rate-determining step is the same as on Pt and Ir.

Conclusions Electrocatalytic and activation energy evidence has been presented that appear to show the identity of the rate-determining step and reaction mechanism for the oxygen electrode reaction on anodized Pt and Ir electrodes in acid solution. The reaction occurs under different conditions of intermediate (OH) coverage. The experimental activation energy for the extrapolated exchange currents is higher on Ir than on Pt, but this is partially offset by the Arrhenius preexponential term, i.e., a compensation effect, as discussed elsewhere 1°11, occurs. Further investigation of the effect of temperature on these processes would be of important practical and theoretical interest, particularly in alkaline solution, in which Rh and Ir show very similar behavior 9.

Acknowledgement This paper is published with the permission of the sponsors of the TARGET fuel-cell program and Pratt & Whitney Aircraft Division of United Aircraft Corporation. REFERENCES 1 F. P. Bowden, Proc. Roy. Soc. (London), A126 (1929) 107.

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T. P. Hoar, Proc. Roy. Soc. (London), A142 (1933) 628. J. O'M. Bockris and A. K. M. S. Huq, Proc. Roy. Soc. (London), A237 (1956) 277. A. Damjanovic, A. Dey and J. O'M. Bockris, Electrochim. Acta, l l (1966) 791. J. P. Hoare, J. Electrochem. Soc., 112 (1965) 602. A. J. Appleby, J. Electroanal. Chem., 24 (1970) 97. J. P. Hoare, Electrochim. Acta, 11 (1966) 203, 311. J. P. Hoare, J. Electroanal. Chem., 18 (1968) 251. A. Damjanovic, A. Dey and J. O'M. Bockris, J. Electrochem. Soc., 113 (1966) 739. K. Gossner and U. Freyer, Z. Phys. Chem., N.F., 63 (1969) 132; K. Gossner and F. Mansfield, Ibid., 63 (1969) 143. A. J. Appleby, Cat. Rev., 4 (1970) 221. A. J. Appleby, J. Electroanal. Chem., 27 (1970) 325, 335. B. E. Conway, D. J. Mackinnon and B. V. Tilak, Trans. Faraday Soc., 66 (1970) 1203. J. N. Agar, Discuss. Faraday Soc., 1 (1947) 81. A. N. Frumkin, Electrochim. Acta, 5 (1961) 265. R. E. Meyer, J. Electrochem. Soc., 107 (1960) 847; J. J. MacDonald and B. E. Conway, Proc. Roy. Soc. (London), A269 (1962) 419. A. C. Riddiford, Electrochim. Acta, 4 (1961) 170. P. Ruetschi and P. Delahay, J. Chem. Phys., 23 (1955) 1167. R. Parsons, Trans. Faraday Soc., 54 (1958) 1053. K. I. Rozenthal and V. I. Veselovskii, Dokl. Akad. Nauk SSSR, 111 (1956) 637. A. Damjanovic and M. K. Y. Wong, J. Electrochem. Soc., 114 (1967) 592. A. I. Krasilsh'chikov, Zh. Fiz. Khim., 37 (1963) 531. B. E. Conway and P. Bourgault, Can. J. Chem., 37 (1959) 292; 40 (1962) 1960; Trans. Faraday Soc., 58 (1962) 593.