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Oxygen overvoltage on bright gold—I
Electrochimica Acta, 1966, Vol. 11. pp. 311 to 320.
OXYGEN
Pergamon Press Ltd.
OVERVOLTAGE
Printed in Northern Ireland
ON BRIGHT GOLD-I*
J. P. HOARE Research Laboratories, General Motors Corporation Warren, Mich. U.S.A. Abstract-Oxygen overvoltage measurements have been made on Au/Au-O and Au/Au,08 electrodes in oxygen-saturated 2 N sulphuric acid solution. The rate-determining step in the reduction of oxygen on an Au/Au-O electrode is O,(ads) + e + O,-(ads), with an i,, of I.3 x lo-l1 A/cm*. An Au/Au-O electrode is a poly-electrode system, and the opencircuit corrosion current is 3.6 x 1O-8 A/cm%. Oxygen evolution could not be studied on an Au/Au0 electode because the anodization process converts the electrode to an Au/Au,O, electrode. The Au/Au,Oll electrode is a single electrode system with an E0 of + 1.360 V, and the electrode process is the formation and reduction of Au,Os, AttaOs + 6H+ -I- 6e z= 2Au + 3H80. At hiah anodic current densities two parallel and independent processes occur-the evolution of the oxygei and the formation of Aua08. *Gold is a poor catalyst for oxygen reactions in acid solutions because the adsorbed oxide films are poor electronic conductors, and the electrode surface is a poor peroxide-decomposing catalyst. RCsm&--On a mesure la surtension d’oxygene sur des electrodes Au/Au-O et Au/Au,O, plongees dans une solution 2N d’acide sulfurique saturee en oxygene. L’etape regulatrice dans la reduction de l’oxygene SIX une electrode Au/Au-O est O,(ads) + e + O,-(ads), avec un i. &gal a 1.3 x lo-" A/cm*. L’electrode Au/Au-O est une polyelectrode et son courant de corrosion a circuit ouvert est Bgal & 3.6 x 1O-8 A/cm*. IX degagement d’oxygene n’a pas pu &tre Ctudie sur l’electrode Au/Au-O parce que le processus d’anodisation transforme cette electrode en une electrode Au/Au,O,. Celle-ci est une electrode simple avec un E. de 1360 mV et le processus d’electrode est la formation de AttaOs suivie de sa reduction, Au,O, + 6H+ + 6e f
2Au + 3H,O.
Aux hautes densites de courant anodique deux processus paralleles et independants ont lieu: le degagement d’oxygene et la formation de Au,O,. L’or est un catalysateur mediocre pour les reactions de l’oxygene dans les solutions acides parce que les films d’oxyde adsorb6 sont de mauvais conducteurs electroniques et que la surface de l’electrode ne catalyse pas effectivement la decomposition de l’eau oxygenee. tisammenfassung-Es wurden Messungen der Sauerstoffiiberspannung an Au/Au-O und Au/Au108Elektroden in sauerstoffgeslttigter 2 N Schwefels%ure durchgeftihrt. Der geschwindigkeitsbestimmende Schritt bei der Reduktion von Sauerstoff an der Au/Au-O-Elektrode ist : O,(ads) + e + O,-(ads) ist ein Polyelektrodensystem, deren mit einem i. von 1,3 x lo-" A/cma. Die Au/Au-O-Elektrode unbelasteter Korrosionsstrom 3,6 x 1O-8 A/cm* betragt. Die Sauerstoffabscheidung konnte an der Au/Au-O-Elektrode nicht untersucht werden, da der anodische Vorgang die Elektrode in eine Au/Au,08-Elektrode tiberftibrt. Die Au/Au,O,-Elektrode ist ein Einelektrodensystem mit einem E. von -f- 1,360 V; Elektrodenvorgang ist die Bildung und Reduktion von AupOs nach Au,O, + 6H+ + 6e + 2Au + 3H,O. Bei hohen anodischen Stromdichten spielen sich zwei parallele und unabh;ingige Reaktionen ab: die Abscheidung von Sauerstoff und die Bildung von AuBOJ. Gold ist ein schlechter Katalysator fiir Sauerstoffreaktionen in sauren Losungen, da die adsorbierten Oxydfilme schlechte Elektronenleiter sind und die Elektrodenoberhache ein schlechter Katalysator fiir die Peroxydzersetzung darstellt. * Manuscript
received 1 March 1965. 311
J. P. HOARE
312
I
0
7. I
I
(a)
1500 -
-
I I
I (b)
I
1400 -
1300 -
-05
APPARENT
Fm. 1. (a) Polarization
(b) Polarization
-1000
CURRENT
0
DENSITY,
pA/cm’
data at low current density on an Au/Au-O electrode. data at low current density on an Au/Au,O, electrode.
-6 LOG APPARENT%JRRENT
DEN&
(Ahme
1
Disc -3
FIG. 2. Oxygen overvoltage for the reduction of oxygen on an Au/Au-O electrode. The dashed curve is a plot of the fkst cycle of polarization from open-circuit conditions. The average of four additional cycles of polarization is shown by the solid curve which was reproducible within f10 mV.
Oxygen overvoltage on bright gold-1
313
INTRODUCTION
FROM rest-potential studies,l it was concluded that an oxygen-free surface may adsorb a partial monolayer of adsorbed oxygen atoms from an oxygen saturated acid solution at a potential of about +0*900 V. Deborin and Ershle? and Schmid and O’Brien3 made similar observations. Double-layer-capacitance measurements made on gold electrodes4 support these conclusions. Such a system, not previously anodized, is referred to as an Au/Au-O electrode. However, if the adsorbed oxygen layer is formed on an Au surface by anodic polarization above + 1.360 V, a hydrated film of reddish brown Au,O, is produced .5 This type of system is referred to as an Au/Au,O, electrode. It is believed1 that the Au/Au-O electrode is a polyelectrode6 and its rest potential is a mixed potential,’ whereas the Au/Au,O, electrode is a single electrode and its rest potential is an equilibrium potential. Oxygen-overvoltage studies may yield information about the electrochemical kinetics of the processes occurring at these electrodes and may provide evidence to support the conclusions outlined above. A discussion of the results of such an investigation is presented in this report. EXPERIMENTAL
TECHNIQUE
Overvoltage measurements were carried out on small bead electrodes (0~02-003 cm2 in area) melted at the end of gold wires (99*9+ % pure) in the manner described elsewhere.* The cell, solutions, and electrode preparations are the same as those described before.l** Only Teflon was used in mounting the electrodes, since the use of polyethylene was avoided in the presence of peroxides. Partial pressure of oxygen, po,, data were obtained by diluting the 0, stream with N, and monitoring the gas composition on a Beckman Oxygen Analyzer. All solutions were stirred with gas flowing at a rate of at least 275 ml/min. Potentials were recorded with respect to the normal hydrogen electrode (nhe), and the temperature was 25” f 1” C. The Au/Au-O electrode was obtained by bubbling H, over a clean Au electrode until the H, potential was reached (0.0 V USPt/HJ, after which 0, was bubbled until a relatively steady potential was obtained. An Au/Au,O, electrode was obtained by anodizing a gold electrode strongly (about 10 mA/cm2 for 2 or 3 hr). RESULTS
Polarization data obtained at very low current densities for an Au/Au-O electrode are given in Fig. la and for an Au/Au,O, electrode in Fig. lb. The cathodic and anodic overvoltage curves are presented in Figs. 2 and 3 for an Au/Au-O electrode and in Figs. 4 and 5 for an Au/Au,O, electrode. Both the anodic and the cathodic polarization curves, taken on the same Au,O, electrode, are plotted in Fig. 6. Figure 7 contains the partial pressure of oxygen (PoZ) data on an Au/Au-O electrode. It is observed that the cathodic overvoltage on an Au/Au-O electrode depends on log po, with a coefficient of 60 mV per logarithmic unit at high values of po,, although at the lower values of p,,,, the relationship is no longer logarithmic. Therefore, the overvoltage in Figs. 2 and 3 was calculated from a value of l-299 V, the reversible oxygen potentials However, both the rest potential and the overvoltage of an Au/Au,O, electrode are independent of the partial pressure of oxygen. Because the extrapolated slopes of the anodic and cathodic polarization curves of an Au/Au,O, electrode cross at I.355 V (see Fig. 6) the overvoltage data of Figs. 4 and 5 were
314
J. P. HOARD
61
-
,’
LOG
APPARENT
CURRENT
DENSITY
(A/cm2
)
FIN. 3. Oxygen overvoltage for the evolution of oxygen on an Au/Au-O electrode. The dashed curve is a plot of nonsteady state points obtained when an Au/Au-O electrode was anodized from open circuit. The solid line is a plot of the steady-state data. This curve shifts to more noble values with each cycle of polarization until it becomes identical with the curve shown in Fig. 5.
calculated from l-360 V, the reversible Au/Au,O, electrode potential,lJO instead of l-229 V. The top curve in Fig. 2 is a plot of the cathodic overvoltage data obtained on an Au/Au-O electrode from the first cycle of polarization, and the bottom curve is the average of four additional cycles with a deviation from the mean within f 10 mV. When an Au/Au-O electrode is first anodized, the potential drifts slowly with time. A plot of these non-steady state points is shown by the dotted curve in Fig. 3. If the system is allowed to come to a steady value (the vertical dotted line in Fig. 3), the overvoltage curve described by the solid line in Fig. 3 is obtained. For each cycle of polarization the overvoltage of each point increases by 3 or 4 mV so that a family of parallel curves is obtained. Finally, the curve for an Au/Au,O, electrode (Fig. 5) is approached. The data presented in Figs. 4 and 5 are the average for at least 4cyclesof increasing and decreasing polarization with a deviation from the mean within f 10 mV.
315
Oxygen overvoltage on bright gold-1
-3oo-
-6 LOG
-5 APPARENT
CURRENT
DENSIT:
(A/cm’
)
-3
Fro. 4. Gold oxide overvoltage for the reduction of Au,O, on an Au/Au,O, electrode. Overvoltage determined from 1.360 V. Points are the average of four cycles of polarization and are reproducible within f 10 mV.
7‘00 -
6ioo-
z --c ioOf
4 oo-
3 00
%
L
-6 LOG APPARENT
-5 CURRENT
-4 DENSITY
-3 (Ahe)
FIO. 5. Gold oxide overvoltage for the formation of AulOl on an Au,O, electrode. Overvoltage determined from I.360 V. Points are the average of four cycles of polarization and are reproducible within &lo mV.
316
J. P. HOARE DISCUSSION
Polarization at low current density From the slope of the linear part of the polarization curve at low current densities, the rate of the electrochemical reaction, K = di/dy, may be obtained.ll If the rest potential is a mixed potential, the slope, di/dq, represents the rate of the over-all local cell reaction.8 Rest-potential studies1 indicate that the O,/H,O reaction proceeds with difficulty on a gold surface because the rest potential is far removed from 1.229 V and is independent of PO,. The data in Fig. la support this conclusion, since the rate of the electrode reaction on an Au/Au-O electrode is even smaller (I.4 x IO+ mho) than that on a Pt/Pt-0 electrode (l-25 x 1O-5mho).s A sketch of the local cell diagram for the gold-oxygen-acid system deduced from rest-potential studies1n12may look something like that presented in Fig. 8. E,’ is the equilibrium potential of the 02/H20 reaction, 0, + 4H+ + 4e + 2H,O, E,,’ = 1.229 V (1) and E,,” that of the Au/Au-O reaction, Au-O + 2H+ + 2e %GAu + H,O,
E,” = ?.
(2)
E,,” is not known because the Au-O layer is only a very small fraction of a complete monolayer. Consequently, the activity of the Au-O layer, which changes with time1 because the amount of oxygen adsorbed on the surface changes with the length of time that the Au is in contact with oxygen-saturated acid solution, is less than unity and is not known. Most likely E,” is very close to 1 V. The local cell current, I,, at the steady-state mixed potential, Em, is defined as that point where 1, = I’ = I”, where I’ and I” are the partial currents for the two half-reactions. Since the partial currents are equal, an external current at E = Em is not observed. If the potential is shifted from Em to El or E,, however, the observed external current is the algebraic sum of the partial currents. When the system is cathodized galvanostatically, the potential shifts from E, to El until the constant cathodic current Zr, equals ]Zr’]- Ii”. In the region E,,” < El < E,,,, the polyelectrode overvoltage, qs may be defined as 7, = E - E,,,, and it may be shown that the rate equation takes the linear form,* i=rj,$$[-
z’u’ - ~“(1 - a”)],
where A is the electrode surface area, a’ and a” the transfer coefficients of the 02/H20 and the Au/Au-O half-reactions respectively, and z’ and z” are the numbers of electrons transferred in the rate-determining step of these two half-reactions. Since the region E,” < El < Em is narrow, according to Fig. 8, one would expect the polarization curve to be linear over a small current density range, as observed in Fig. la (from 0 to -0.2 pA/cm2). If the system is anodized galvanostatically, the potential is shifted from E,,, to E, so that the external anodic current, Z,, equals 1,” - I1,‘l. However, the electrode process increases the amount of oxygen adsorbed on the gold surface. Because the adsorbed oxygen layers on gold are poor electronic conductors1*4*13in contrast to those on Pt, the potential is shifted to more noble values in order to maintain the demanded
Oxygenovervoltageon bright gold-1
317
current. As a result, steady potential values are not observed until the system has been converted to an Au/Au,O, electrode, as shown in Fig. 3. Therefore, it is not possible to study the evolution of oxygen on an Au/Au-O electrode because the electrode processes convert the system to an Au/Au,O, electrode which is an entirely different electrode system from the Au/Au-O electrode. The linear polarization region in Fig. la on the anodic side is, consequently, very short, if not entirely absent. On open circuit, the corrosion current which causes the observed change in the activity of the Au-O layer1 may be estimated from (3). Assuming that tc’ = CC” = 4 and that z’ = z” = 1, since it is generally believed that only one electron is transferred at a time, then i, = &,,/A= -(di/dy,)
y,
where dildq, is the slope of the linear curve. From Fig. la, -di/dy, = 1.4 x low6 mho and i, = 3.6 x 10-s A/cm2. This is a very small current indeed, and could account for the observed slow change in the rest potential with time.l Reduction of 0, on the Au/Au-O electrode
As the Au/Au-O electrode is cathodically polarized to potential values less noble than E,“, the kinetics are determined by the O,/H,O reaction, and the system is no longer a polyelectrode but a single electrode system. A good reproducible curve with a Tafel slope of 0.116 is obtained after the first cycle of polarization, as shown in Fig. 2. This value of the slope agrees with that14 which is associated with a mechanism in which an electron-transfer step is rate-determining. In this case, the rate-determining step may be the first electron-transfer step, O,(ads) + e + O,(ads).
(5)
The partial pressure of oxygen data in Fig. 7 support this conclusion, since a slope of 60 mV in the q-log po, plot indicates that a one-electron reaction involving O2 is the rate-determining step. The deviation from the logarithmic curve observed at the low values of PO2 most likely indicates that a mass transfer step is becoming rate-determining. After the transfer of the second electron, peroxide is formed, which builds up to a steady-state concentration in solution. The magnitude of this steady-state concentration depends on the ability of the electrode surface to decompose the peroxide catalytically. At the end of the run described in Fig. 2, a test for peroxide using TiS04 reagent15 was made. By comparing the density of the colour of this test with that of spot tests of standard solutions, it was estimated that the steady-state concentration of peroxide was about lo4 M which is about one or two orders of magnitude higher than that found in the solution surrounding a Pt/O, cathode. It is concluded that Au is a much poorer catalyst for the decomposition of peroxide than Pt. This is one of the reasons why the overvoltage for the reduction of 0, on Au (Fig. 2) is so high and the i, (I.3 :< lo-l1 A/cm2) is so small. During the first cycle of polarization (dashed curve), this steady-state concentration of peroxide was set up, and subsequent cycles gave reproducible (solid curve) polarization points, as shown in Fig. 2.
318
J. P. HOARE
Polarization of the Au/Au,O,
electrode
As pointed out above, the anodic polarization of an Au/Au-O electrode does not give steady-state polarization points until the surface is covered with AusO,. At this point, however, the electrode system is no longer an Au/Au-O but an Au/Au,Os electrode. The extrapolated slopes of the anodic and cathodic polarization curves on an Au/Au,O, electrode (Fig. 6) cross at the reversible potentiallO of an Au/Au,O, couple, Au,O, + 6H+ + 6e + 2Au + 3H,O, E, = 1.360 V. (6)
2.0-
-10
-9
-9 LOG
-6
-7 APPARENT
CURRENT
DENSITY
-5 (A/cm’
-4
-3
)
FIG. 6. Cathodic and anodic polarization curves taken on the same Au/Au,O, electrode. The extrapolated Tafel lines cross at 1.355 V, very near the reversible potential of the Au/Au,O, reaction.
It is concluded that the Au/Au,O, electrode is a true metal/metal-oxide electrode with an E, of 1.360 V and is not a polyelectrode. Since the Au,O, layers are poor conductors1*4*13and poorly adherent to the Au surface,1*6 the electrochemical formation and reduction of Au,O, should be a relatively slow process. This is confirmed experimentally. From Fig. lb it is seen that the rate of reaction (3.9 x 1W mho) is the same order of magnitude as the reduction of oxygen on an Au/Au-O electrode. Although the Au/Au,O, electrode exhibits a linear polarization curve over a greater current density range, it is still short. However, the i, (4.4 x lo-lo A/ems) for the reduction of Au,O, is an order of magnitude greater than that for the reduction of oxygen on Au/Au-O. Since the Tafel slopes for the anodic formation and cathodic reduction of Au,O, are not the same, the rate-determining step is different in each case, even though the over-all electrode reaction (6) is the same for both. Additional data beyond that presented here is needed before identitkation of the rate-determining steps can be made in this very complicated process. It is to be remembered that the
Oxygen overvoltage on bright gold-1
319
1 lb)
(a)
1 -
J””
(A-5 LOG
APP
-4
0
CD (A/cm21
-0.5
LOG
-1.0
PO2 (Atm)
FIG. 7. (a) Variation of overvoltage with changes in P,,* on an Au/Au-O electrode. Pan = 1 atm, 0; 0.6, A; @3, V ; 0.08, 0. (b) Plot of n as a function of log PO2(data obtained from Fig. 7a).
I
ly 1 LOCAL
I
II
I
1; &II CELL
II; CURRENT
c
1
FIG. 8. Sketch of the mixed potential diagram for the Au-O-acid system as a plot of potential against the absolute value of the local cell current. The polarization curve for the O,/H,O halfreaction is given by the upper curve and that for the Au/Au-O half-reaction by the lower. The rest potential is the point at which E = E,,, and I = I,.
320
J. P. HOAR@
overvoltage in this case is independent of po,, so that oxygen reactions are unimportant to the rate-determining step. As the current density is increased, the electrode mechanism changes, as indicated by the change in slope from O-115 to O-047. An obvious choice for the new mechanism would be the evolution of oxygen on an Au,O, surface. Barnart@ has shown that the film of Au,O, is loosely held and flakes off easily. In this way, new gold sites are made available for the continued production of Au,O, at a given constant current density. When the current density becomes large enough, 0, may be evolved along with Au,O, formation, giving the resultant change in slope. A Tafel slope of CO45 has been observed by MacDonald and Conway16 up to a current density of nearly 0.1 A/cm2. Barnarttl’ has obtained similar results. Since two parallel reactions occur in the region of the O-045 slope, it appears that a simple mechanism in terms of a definite rate-determining step cannot be interpreted from this 0.045 value. It is interesting that a slope of O-045 may be obtained over three orders of magnitude of the current density. Such behaviour could be explained in terms of two types of sites. On one, the evolution of oxygen predominates, whereas on the other, the formation of the Au,O, predominates. The ratio of the number of these two types of sites remains nearly constant, and the two parallel reactions occur independently of one another. However, it is impossible to determine an i,, for this region as done in the literature16*17 (1O-22A/cm2) because a single reference state cannot be chosen. author is grateful to Dr Raymond Thacker of General Motors Research Laboratories for his helpful suggestionsand comments.
Acknowledgement-The
REFERENCES 1. J. P. HOARE,J. electrochem. Sot. 110, 245 (1963). 2. G. DEBORINand B. ERSHLER, Actu Physicochim. CJRSS 13,347 (1940). 3. G. M. SCHMIDand R. N. O’BRIEN,J. electrochem. Sot. 111,832 (1964). 4. J. P. HOARE,Electrochim. Actu 9, 1289 (1964). 5. S. BARNARTT,J. electrochem. Sot. 106, 722 (1959). 6. E. LANGEand H. G&R, 2. Elektrochem. 63, 74 (1959). 7. C. WAGNERand W. TRAUD,Z. Elektrochem. 44,391 (1938). 8. J. P. HOARE,J. electrochem. Sot., 112, 602 (1965). 9. W. H. LATIMER,Oxidation Potentials, Prentice-Hall, New York (1952). 10. A. HICKLING,Trans. Furaduy Sot. 42, 518 (1946). 11. P. DOUN, B. ERSHLER and A. FRUMKIN,Acta Phys.-chim. URSS 13,782 (1940). 12. J. P. HOARE,J. electrochem. Sot. 111, 988 (1964). 13. H. A. LAITINENand M. S. CHAO, J. electrochem. Sot. 108, 726 (1961). 14. J. O’M. BOCKRIS, J. them. Phys. 24,817 (1956). 15. F. D. SNELLand C. T. SNELL,Calorimetric Meihods of Analysis, Vol. II, 3rd ed. (1949). 16. J. J. MACDONALDand B..E. CONWAY, Proc. R. Sot. 269A, 419 (1962). 17. S. BARNARIT,J. electrochem. Sot. 106, 991 (1959).
OXYGEN
Pergamon Press Ltd.
OVERVOLTAGE
Printed in Northern Ireland
ON BRIGHT GOLD-I*
J. P. HOARE Research Laboratories, General Motors Corporation Warren, Mich. U.S.A. Abstract-Oxygen overvoltage measurements have been made on Au/Au-O and Au/Au,08 electrodes in oxygen-saturated 2 N sulphuric acid solution. The rate-determining step in the reduction of oxygen on an Au/Au-O electrode is O,(ads) + e + O,-(ads), with an i,, of I.3 x lo-l1 A/cm*. An Au/Au-O electrode is a poly-electrode system, and the opencircuit corrosion current is 3.6 x 1O-8 A/cm%. Oxygen evolution could not be studied on an Au/Au0 electode because the anodization process converts the electrode to an Au/Au,O, electrode. The Au/Au,Oll electrode is a single electrode system with an E0 of + 1.360 V, and the electrode process is the formation and reduction of Au,Os, AttaOs + 6H+ -I- 6e z= 2Au + 3H80. At hiah anodic current densities two parallel and independent processes occur-the evolution of the oxygei and the formation of Aua08. *Gold is a poor catalyst for oxygen reactions in acid solutions because the adsorbed oxide films are poor electronic conductors, and the electrode surface is a poor peroxide-decomposing catalyst. RCsm&--On a mesure la surtension d’oxygene sur des electrodes Au/Au-O et Au/Au,O, plongees dans une solution 2N d’acide sulfurique saturee en oxygene. L’etape regulatrice dans la reduction de l’oxygene SIX une electrode Au/Au-O est O,(ads) + e + O,-(ads), avec un i. &gal a 1.3 x lo-" A/cm*. L’electrode Au/Au-O est une polyelectrode et son courant de corrosion a circuit ouvert est Bgal & 3.6 x 1O-8 A/cm*. IX degagement d’oxygene n’a pas pu &tre Ctudie sur l’electrode Au/Au-O parce que le processus d’anodisation transforme cette electrode en une electrode Au/Au,O,. Celle-ci est une electrode simple avec un E. de 1360 mV et le processus d’electrode est la formation de AttaOs suivie de sa reduction, Au,O, + 6H+ + 6e f
2Au + 3H,O.
Aux hautes densites de courant anodique deux processus paralleles et independants ont lieu: le degagement d’oxygene et la formation de Au,O,. L’or est un catalysateur mediocre pour les reactions de l’oxygene dans les solutions acides parce que les films d’oxyde adsorb6 sont de mauvais conducteurs electroniques et que la surface de l’electrode ne catalyse pas effectivement la decomposition de l’eau oxygenee. tisammenfassung-Es wurden Messungen der Sauerstoffiiberspannung an Au/Au-O und Au/Au108Elektroden in sauerstoffgeslttigter 2 N Schwefels%ure durchgeftihrt. Der geschwindigkeitsbestimmende Schritt bei der Reduktion von Sauerstoff an der Au/Au-O-Elektrode ist : O,(ads) + e + O,-(ads) ist ein Polyelektrodensystem, deren mit einem i. von 1,3 x lo-" A/cma. Die Au/Au-O-Elektrode unbelasteter Korrosionsstrom 3,6 x 1O-8 A/cm* betragt. Die Sauerstoffabscheidung konnte an der Au/Au-O-Elektrode nicht untersucht werden, da der anodische Vorgang die Elektrode in eine Au/Au,08-Elektrode tiberftibrt. Die Au/Au,O,-Elektrode ist ein Einelektrodensystem mit einem E. von -f- 1,360 V; Elektrodenvorgang ist die Bildung und Reduktion von AupOs nach Au,O, + 6H+ + 6e + 2Au + 3H,O. Bei hohen anodischen Stromdichten spielen sich zwei parallele und unabh;ingige Reaktionen ab: die Abscheidung von Sauerstoff und die Bildung von AuBOJ. Gold ist ein schlechter Katalysator fiir Sauerstoffreaktionen in sauren Losungen, da die adsorbierten Oxydfilme schlechte Elektronenleiter sind und die Elektrodenoberhache ein schlechter Katalysator fiir die Peroxydzersetzung darstellt. * Manuscript
received 1 March 1965. 311
J. P. HOARE
312
I
0
7. I
I
(a)
1500 -
-
I I
I (b)
I
1400 -
1300 -
-05
APPARENT
Fm. 1. (a) Polarization
(b) Polarization
-1000
CURRENT
0
DENSITY,
pA/cm’
data at low current density on an Au/Au-O electrode. data at low current density on an Au/Au,O, electrode.
-6 LOG APPARENT%JRRENT
DEN&
(Ahme
1
Disc -3
FIG. 2. Oxygen overvoltage for the reduction of oxygen on an Au/Au-O electrode. The dashed curve is a plot of the fkst cycle of polarization from open-circuit conditions. The average of four additional cycles of polarization is shown by the solid curve which was reproducible within f10 mV.
Oxygen overvoltage on bright gold-1
313
INTRODUCTION
FROM rest-potential studies,l it was concluded that an oxygen-free surface may adsorb a partial monolayer of adsorbed oxygen atoms from an oxygen saturated acid solution at a potential of about +0*900 V. Deborin and Ershle? and Schmid and O’Brien3 made similar observations. Double-layer-capacitance measurements made on gold electrodes4 support these conclusions. Such a system, not previously anodized, is referred to as an Au/Au-O electrode. However, if the adsorbed oxygen layer is formed on an Au surface by anodic polarization above + 1.360 V, a hydrated film of reddish brown Au,O, is produced .5 This type of system is referred to as an Au/Au,O, electrode. It is believed1 that the Au/Au-O electrode is a polyelectrode6 and its rest potential is a mixed potential,’ whereas the Au/Au,O, electrode is a single electrode and its rest potential is an equilibrium potential. Oxygen-overvoltage studies may yield information about the electrochemical kinetics of the processes occurring at these electrodes and may provide evidence to support the conclusions outlined above. A discussion of the results of such an investigation is presented in this report. EXPERIMENTAL
TECHNIQUE
Overvoltage measurements were carried out on small bead electrodes (0~02-003 cm2 in area) melted at the end of gold wires (99*9+ % pure) in the manner described elsewhere.* The cell, solutions, and electrode preparations are the same as those described before.l** Only Teflon was used in mounting the electrodes, since the use of polyethylene was avoided in the presence of peroxides. Partial pressure of oxygen, po,, data were obtained by diluting the 0, stream with N, and monitoring the gas composition on a Beckman Oxygen Analyzer. All solutions were stirred with gas flowing at a rate of at least 275 ml/min. Potentials were recorded with respect to the normal hydrogen electrode (nhe), and the temperature was 25” f 1” C. The Au/Au-O electrode was obtained by bubbling H, over a clean Au electrode until the H, potential was reached (0.0 V USPt/HJ, after which 0, was bubbled until a relatively steady potential was obtained. An Au/Au,O, electrode was obtained by anodizing a gold electrode strongly (about 10 mA/cm2 for 2 or 3 hr). RESULTS
Polarization data obtained at very low current densities for an Au/Au-O electrode are given in Fig. la and for an Au/Au,O, electrode in Fig. lb. The cathodic and anodic overvoltage curves are presented in Figs. 2 and 3 for an Au/Au-O electrode and in Figs. 4 and 5 for an Au/Au,O, electrode. Both the anodic and the cathodic polarization curves, taken on the same Au,O, electrode, are plotted in Fig. 6. Figure 7 contains the partial pressure of oxygen (PoZ) data on an Au/Au-O electrode. It is observed that the cathodic overvoltage on an Au/Au-O electrode depends on log po, with a coefficient of 60 mV per logarithmic unit at high values of po,, although at the lower values of p,,,, the relationship is no longer logarithmic. Therefore, the overvoltage in Figs. 2 and 3 was calculated from a value of l-299 V, the reversible oxygen potentials However, both the rest potential and the overvoltage of an Au/Au,O, electrode are independent of the partial pressure of oxygen. Because the extrapolated slopes of the anodic and cathodic polarization curves of an Au/Au,O, electrode cross at I.355 V (see Fig. 6) the overvoltage data of Figs. 4 and 5 were
314
J. P. HOARD
61
-
,’
LOG
APPARENT
CURRENT
DENSITY
(A/cm2
)
FIN. 3. Oxygen overvoltage for the evolution of oxygen on an Au/Au-O electrode. The dashed curve is a plot of nonsteady state points obtained when an Au/Au-O electrode was anodized from open circuit. The solid line is a plot of the steady-state data. This curve shifts to more noble values with each cycle of polarization until it becomes identical with the curve shown in Fig. 5.
calculated from l-360 V, the reversible Au/Au,O, electrode potential,lJO instead of l-229 V. The top curve in Fig. 2 is a plot of the cathodic overvoltage data obtained on an Au/Au-O electrode from the first cycle of polarization, and the bottom curve is the average of four additional cycles with a deviation from the mean within f 10 mV. When an Au/Au-O electrode is first anodized, the potential drifts slowly with time. A plot of these non-steady state points is shown by the dotted curve in Fig. 3. If the system is allowed to come to a steady value (the vertical dotted line in Fig. 3), the overvoltage curve described by the solid line in Fig. 3 is obtained. For each cycle of polarization the overvoltage of each point increases by 3 or 4 mV so that a family of parallel curves is obtained. Finally, the curve for an Au/Au,O, electrode (Fig. 5) is approached. The data presented in Figs. 4 and 5 are the average for at least 4cyclesof increasing and decreasing polarization with a deviation from the mean within f 10 mV.
315
Oxygen overvoltage on bright gold-1
-3oo-
-6 LOG
-5 APPARENT
CURRENT
DENSIT:
(A/cm’
)
-3
Fro. 4. Gold oxide overvoltage for the reduction of Au,O, on an Au/Au,O, electrode. Overvoltage determined from 1.360 V. Points are the average of four cycles of polarization and are reproducible within f 10 mV.
7‘00 -
6ioo-
z --c ioOf
4 oo-
3 00
%
L
-6 LOG APPARENT
-5 CURRENT
-4 DENSITY
-3 (Ahe)
FIO. 5. Gold oxide overvoltage for the formation of AulOl on an Au,O, electrode. Overvoltage determined from I.360 V. Points are the average of four cycles of polarization and are reproducible within &lo mV.
316
J. P. HOARE DISCUSSION
Polarization at low current density From the slope of the linear part of the polarization curve at low current densities, the rate of the electrochemical reaction, K = di/dy, may be obtained.ll If the rest potential is a mixed potential, the slope, di/dq, represents the rate of the over-all local cell reaction.8 Rest-potential studies1 indicate that the O,/H,O reaction proceeds with difficulty on a gold surface because the rest potential is far removed from 1.229 V and is independent of PO,. The data in Fig. la support this conclusion, since the rate of the electrode reaction on an Au/Au-O electrode is even smaller (I.4 x IO+ mho) than that on a Pt/Pt-0 electrode (l-25 x 1O-5mho).s A sketch of the local cell diagram for the gold-oxygen-acid system deduced from rest-potential studies1n12may look something like that presented in Fig. 8. E,’ is the equilibrium potential of the 02/H20 reaction, 0, + 4H+ + 4e + 2H,O, E,,’ = 1.229 V (1) and E,,” that of the Au/Au-O reaction, Au-O + 2H+ + 2e %GAu + H,O,
E,” = ?.
(2)
E,,” is not known because the Au-O layer is only a very small fraction of a complete monolayer. Consequently, the activity of the Au-O layer, which changes with time1 because the amount of oxygen adsorbed on the surface changes with the length of time that the Au is in contact with oxygen-saturated acid solution, is less than unity and is not known. Most likely E,” is very close to 1 V. The local cell current, I,, at the steady-state mixed potential, Em, is defined as that point where 1, = I’ = I”, where I’ and I” are the partial currents for the two half-reactions. Since the partial currents are equal, an external current at E = Em is not observed. If the potential is shifted from Em to El or E,, however, the observed external current is the algebraic sum of the partial currents. When the system is cathodized galvanostatically, the potential shifts from E, to El until the constant cathodic current Zr, equals ]Zr’]- Ii”. In the region E,,” < El < E,,,, the polyelectrode overvoltage, qs may be defined as 7, = E - E,,,, and it may be shown that the rate equation takes the linear form,* i=rj,$$[-
z’u’ - ~“(1 - a”)],
where A is the electrode surface area, a’ and a” the transfer coefficients of the 02/H20 and the Au/Au-O half-reactions respectively, and z’ and z” are the numbers of electrons transferred in the rate-determining step of these two half-reactions. Since the region E,” < El < Em is narrow, according to Fig. 8, one would expect the polarization curve to be linear over a small current density range, as observed in Fig. la (from 0 to -0.2 pA/cm2). If the system is anodized galvanostatically, the potential is shifted from E,,, to E, so that the external anodic current, Z,, equals 1,” - I1,‘l. However, the electrode process increases the amount of oxygen adsorbed on the gold surface. Because the adsorbed oxygen layers on gold are poor electronic conductors1*4*13in contrast to those on Pt, the potential is shifted to more noble values in order to maintain the demanded
Oxygenovervoltageon bright gold-1
317
current. As a result, steady potential values are not observed until the system has been converted to an Au/Au,O, electrode, as shown in Fig. 3. Therefore, it is not possible to study the evolution of oxygen on an Au/Au-O electrode because the electrode processes convert the system to an Au/Au,O, electrode which is an entirely different electrode system from the Au/Au-O electrode. The linear polarization region in Fig. la on the anodic side is, consequently, very short, if not entirely absent. On open circuit, the corrosion current which causes the observed change in the activity of the Au-O layer1 may be estimated from (3). Assuming that tc’ = CC” = 4 and that z’ = z” = 1, since it is generally believed that only one electron is transferred at a time, then i, = &,,/A= -(di/dy,)
y,
where dildq, is the slope of the linear curve. From Fig. la, -di/dy, = 1.4 x low6 mho and i, = 3.6 x 10-s A/cm2. This is a very small current indeed, and could account for the observed slow change in the rest potential with time.l Reduction of 0, on the Au/Au-O electrode
As the Au/Au-O electrode is cathodically polarized to potential values less noble than E,“, the kinetics are determined by the O,/H,O reaction, and the system is no longer a polyelectrode but a single electrode system. A good reproducible curve with a Tafel slope of 0.116 is obtained after the first cycle of polarization, as shown in Fig. 2. This value of the slope agrees with that14 which is associated with a mechanism in which an electron-transfer step is rate-determining. In this case, the rate-determining step may be the first electron-transfer step, O,(ads) + e + O,(ads).
(5)
The partial pressure of oxygen data in Fig. 7 support this conclusion, since a slope of 60 mV in the q-log po, plot indicates that a one-electron reaction involving O2 is the rate-determining step. The deviation from the logarithmic curve observed at the low values of PO2 most likely indicates that a mass transfer step is becoming rate-determining. After the transfer of the second electron, peroxide is formed, which builds up to a steady-state concentration in solution. The magnitude of this steady-state concentration depends on the ability of the electrode surface to decompose the peroxide catalytically. At the end of the run described in Fig. 2, a test for peroxide using TiS04 reagent15 was made. By comparing the density of the colour of this test with that of spot tests of standard solutions, it was estimated that the steady-state concentration of peroxide was about lo4 M which is about one or two orders of magnitude higher than that found in the solution surrounding a Pt/O, cathode. It is concluded that Au is a much poorer catalyst for the decomposition of peroxide than Pt. This is one of the reasons why the overvoltage for the reduction of 0, on Au (Fig. 2) is so high and the i, (I.3 :< lo-l1 A/cm2) is so small. During the first cycle of polarization (dashed curve), this steady-state concentration of peroxide was set up, and subsequent cycles gave reproducible (solid curve) polarization points, as shown in Fig. 2.
318
J. P. HOARE
Polarization of the Au/Au,O,
electrode
As pointed out above, the anodic polarization of an Au/Au-O electrode does not give steady-state polarization points until the surface is covered with AusO,. At this point, however, the electrode system is no longer an Au/Au-O but an Au/Au,Os electrode. The extrapolated slopes of the anodic and cathodic polarization curves on an Au/Au,O, electrode (Fig. 6) cross at the reversible potentiallO of an Au/Au,O, couple, Au,O, + 6H+ + 6e + 2Au + 3H,O, E, = 1.360 V. (6)
2.0-
-10
-9
-9 LOG
-6
-7 APPARENT
CURRENT
DENSITY
-5 (A/cm’
-4
-3
)
FIG. 6. Cathodic and anodic polarization curves taken on the same Au/Au,O, electrode. The extrapolated Tafel lines cross at 1.355 V, very near the reversible potential of the Au/Au,O, reaction.
It is concluded that the Au/Au,O, electrode is a true metal/metal-oxide electrode with an E, of 1.360 V and is not a polyelectrode. Since the Au,O, layers are poor conductors1*4*13and poorly adherent to the Au surface,1*6 the electrochemical formation and reduction of Au,O, should be a relatively slow process. This is confirmed experimentally. From Fig. lb it is seen that the rate of reaction (3.9 x 1W mho) is the same order of magnitude as the reduction of oxygen on an Au/Au-O electrode. Although the Au/Au,O, electrode exhibits a linear polarization curve over a greater current density range, it is still short. However, the i, (4.4 x lo-lo A/ems) for the reduction of Au,O, is an order of magnitude greater than that for the reduction of oxygen on Au/Au-O. Since the Tafel slopes for the anodic formation and cathodic reduction of Au,O, are not the same, the rate-determining step is different in each case, even though the over-all electrode reaction (6) is the same for both. Additional data beyond that presented here is needed before identitkation of the rate-determining steps can be made in this very complicated process. It is to be remembered that the
Oxygen overvoltage on bright gold-1
319
1 lb)
(a)
1 -
J””
(A-5 LOG
APP
-4
0
CD (A/cm21
-0.5
LOG
-1.0
PO2 (Atm)
FIG. 7. (a) Variation of overvoltage with changes in P,,* on an Au/Au-O electrode. Pan = 1 atm, 0; 0.6, A; @3, V ; 0.08, 0. (b) Plot of n as a function of log PO2(data obtained from Fig. 7a).
I
ly 1 LOCAL
I
II
I
1; &II CELL
II; CURRENT
c
1
FIG. 8. Sketch of the mixed potential diagram for the Au-O-acid system as a plot of potential against the absolute value of the local cell current. The polarization curve for the O,/H,O halfreaction is given by the upper curve and that for the Au/Au-O half-reaction by the lower. The rest potential is the point at which E = E,,, and I = I,.
320
J. P. HOAR@
overvoltage in this case is independent of po,, so that oxygen reactions are unimportant to the rate-determining step. As the current density is increased, the electrode mechanism changes, as indicated by the change in slope from O-115 to O-047. An obvious choice for the new mechanism would be the evolution of oxygen on an Au,O, surface. Barnart@ has shown that the film of Au,O, is loosely held and flakes off easily. In this way, new gold sites are made available for the continued production of Au,O, at a given constant current density. When the current density becomes large enough, 0, may be evolved along with Au,O, formation, giving the resultant change in slope. A Tafel slope of CO45 has been observed by MacDonald and Conway16 up to a current density of nearly 0.1 A/cm2. Barnarttl’ has obtained similar results. Since two parallel reactions occur in the region of the O-045 slope, it appears that a simple mechanism in terms of a definite rate-determining step cannot be interpreted from this 0.045 value. It is interesting that a slope of O-045 may be obtained over three orders of magnitude of the current density. Such behaviour could be explained in terms of two types of sites. On one, the evolution of oxygen predominates, whereas on the other, the formation of the Au,O, predominates. The ratio of the number of these two types of sites remains nearly constant, and the two parallel reactions occur independently of one another. However, it is impossible to determine an i,, for this region as done in the literature16*17 (1O-22A/cm2) because a single reference state cannot be chosen. author is grateful to Dr Raymond Thacker of General Motors Research Laboratories for his helpful suggestionsand comments.
Acknowledgement-The
REFERENCES 1. J. P. HOARE,J. electrochem. Sot. 110, 245 (1963). 2. G. DEBORINand B. ERSHLER, Actu Physicochim. CJRSS 13,347 (1940). 3. G. M. SCHMIDand R. N. O’BRIEN,J. electrochem. Sot. 111,832 (1964). 4. J. P. HOARE,Electrochim. Actu 9, 1289 (1964). 5. S. BARNARTT,J. electrochem. Sot. 106, 722 (1959). 6. E. LANGEand H. G&R, 2. Elektrochem. 63, 74 (1959). 7. C. WAGNERand W. TRAUD,Z. Elektrochem. 44,391 (1938). 8. J. P. HOARE,J. electrochem. Sot., 112, 602 (1965). 9. W. H. LATIMER,Oxidation Potentials, Prentice-Hall, New York (1952). 10. A. HICKLING,Trans. Furaduy Sot. 42, 518 (1946). 11. P. DOUN, B. ERSHLER and A. FRUMKIN,Acta Phys.-chim. URSS 13,782 (1940). 12. J. P. HOARE,J. electrochem. Sot. 111, 988 (1964). 13. H. A. LAITINENand M. S. CHAO, J. electrochem. Sot. 108, 726 (1961). 14. J. O’M. BOCKRIS, J. them. Phys. 24,817 (1956). 15. F. D. SNELLand C. T. SNELL,Calorimetric Meihods of Analysis, Vol. II, 3rd ed. (1949). 16. J. J. MACDONALDand B..E. CONWAY, Proc. R. Sot. 269A, 419 (1962). 17. S. BARNARIT,J. electrochem. Sot. 106, 991 (1959).