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Effect of solution pH on hydrous oxide growth and reduction on polycrystalline platinum

Effect of solution pH on hydrous oxide growth and reduction on polycrystalline platinum

301 J. Electroannl. Chem., 305 (1991) 301-312 Elsevier Sequoia S.A., Lausanne Effect of solution pH on hydrous oxide growth and reduction on polycry...

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301

J. Electroannl. Chem., 305 (1991) 301-312 Elsevier Sequoia S.A., Lausanne

Effect of solution pH on hydrous oxide growth and reduction on polycrystalline platinum L.D. Burke and M.M. Murphy Department (Received

of Chemistry, 3 September

University

College, Cork (Ireland)

1990; in revised form 17 October

1990)

Abstract The study of hydrous oxides produced on platinum by the potential cycling technique is complicated by the presence, in many instances, of more than one component in such deposits. Where possible, conditions favouring the growth of single component films were employed in the present work; it was confirmed that a portion (in some cases up to 70%) of these hydrous oxide deposits was resistant to reduction in base at low potentials, e.g. 0.13 V (RHE) for 10 min; this was attributed to the anionic character of such materials. The presence of a thick hydrous oxide film on the anode surface had a slight inhibiting effect on the Kolbe reaction on platinum in aqueous acetate solution.

INTRODUCTION

The electrochemistry of hydrous oxides [l] is of considerable interest at the present time as these species may be of relevance in metal dissolution [2], electrocatalysis [3], battery [4] and electrochromic [5] processes. The possibility of enhancing the catalytic activity of platinum by the intermediate formation of a thick hydrous oxide layer which is subsequently reduced to yield a finely divided metal overlayer, i.e. by changing the surface morphology, was first demonstrated by Burke and Roche [6]. In subsequent, more extensive, investigations of this process Arvia and coworkers [7-lo] have confirmed this effect and demonstrated the formation of preferentially oriented crystallite surfaces in overlayers of considerable roughness. In a recent account of the behaviour of platinum in base [lo] these authors have indicated that there is an anomaly between their results and earlier reports from this laboratory [6,11] concerning the stability of platinum hydrous oxide deposits in base; they also queried an earlier proposal [ll] that the species present in hydrous oxide deposits on platinum are of anionic character. The object of this report is to clarify our view of this area in the light of continuing research on this topic. 0022-0728/91/$03.50

0 1991 - Elsevier Sequoia

S.A.

302

In a recent publication from this laboratory [12] it was pointed out that hydrous oxide films grown on platinum in acid by the potential cycling technique are frequently composed of two components which will be designated here as HO1 and HO2 (i.e. Hydrous Oxide of type 1 and 2, respectively). In acid solution HO1 reduces at ca. 0.37 V (RHE) and HO2 at ca. 0.20 V (RHE); since these reduction processes occur in an irreversible manner (even if the initial step, i.e. the formation of the metal adatom, takes place [ll] quasi-reversibly) the peak potentials usually exhibit a significant sweep-rate dependence [6]. Using the same experimental procedures as outlined earlier [12] we have reinvestigated briefly the formation of these two species in both acid and base. We have also examined the reduction behaviour of these deposits in acid and base, and at a range of intermediate pH values. Before each oxide growth experiment the surface of the wire electrode was usually etched for 5 sec. in warm aqua regia and then washed with triply distilled water; the only exception was in the growth of HO2 where (as described later) hydrous oxide growth and reduction was carried out repeatedly, without removing the electrode from the cell, in order to attain a steady-state condition [12]. The cell solution in all cases was at 25°C. The following is a synopsis of the results. RESULTS

(a) Films grown in acid

HO1 may be produced in 1.0 mol dm-’ H,SO, by repetitive triangular potential cycling [6], 0.58 to 1.90 V at 100 V s-’ for 1 to 3 min. Reduction of the resulting deposit in the same medium occurred in a single sweep (1.60 to 0 V, 10 mV sP’) at ca. 0.4 V, Fig. l(a). With a similar film, again grown in acid, reduction in base (1.0 mol drnm3 NaOH), dashed line in Fig. l(a), yielded a peak at the same potential, usually of slightly lower magnitude. The reduction in base was not quite complete at the end of this experiment, i.e. at 0 V, as a small peak at ca. 0.4 in was observed, dotted line in Fig. l(a), when a further reduction sweep was recorded after transferring the electrode back into acid solution. HO2 was produced in 1.0 mol dmm3 H,SO, on cycling the potential between 0.58 and 2.2 V at 100 V s-’ for 3 min; on the first few attempts the resulting deposit yielded two peaks on the subsequent reduction sweep in acid; apparently the product was a mixture of HO1 and H02. To obtain HO2 alone, i.e. a single reduction peak at ca. 0.22 V, full line in Fig. l(b), it was necessary to repeat the cycling and reduction procedure several times (as pointed out previously [12]) without removing the electrode from the cell. After ca. five consecutive hydrous oxide growth and reduction experiments only one reduction peak, at ca. 0.2 V, was observed. However, when using a shorter oxide growth time of 1 min the reduction sweep, even in the steady-state case, indicated the presence of a mixture of HO1 and H02. When a HO2 acid grown film was transferred to base a relatively small hydrous oxide reduction peak was observed at ca. 0.25 V, dashed line in Fig. l(b). The reaction was obviously incomplete; the remainder of the yellow film reduced readily

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E V(RHE) Fig. 1. Reduction sweeps (10 mm s-‘) recorded for acid-grown (0.58 to 1.90 V, films at 25T. (a) HO1 deposit reduced in 1.0 mol dm-’ H,SO., () and in (- - - - - -); data for a further sweep in acid (. . . . .) at the end of the reduction shown here. (b) HO2 deposit reduced in acid () and in base (- - - - - -); data acid (. . -) at the end of the reduction sweep in base is also shown here.

100 V s-’ for 3 mm) 1.0 mol drn-’ NaOH sweep in base is also for a further sweep in

during a subsequent reduction sweep, dotted line in Fig. l(b), transferring the electrode back into acid solution.

recorded

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(b) Films grown in base Attempts to grow single component hydrous oxide deposits in base (1.0 mol dm-3 NaOH) were not always successful. Using the lower value for the upper limit, which for acid solution yielded HOl, the hydrous oxide deposit, produced on cycling (0.45 to 1.95 V at 40 V s-l for 2-3 mm; the choice of conditions here was influenced by earlier work [6]) the potential of a freshly etched electrode in 1.0 mol dmP3 NaOH yielded a single reduction peak, full line in Fig. 2(a), at 0.38 V following transfer to acid. The same electrode, after a few hydrous oxide growth and reduction experiments in acid (to attain a constant level of HO1 formation), on being subjected to the same hydrous oxide growth experiment in base yielded a deposit which on reduction in acid showed an overlapping doublet, dashed line in Fig. 2(a), in the region of 0.4 V. When such a film was again grown in base and then reduced in base a relatively broad peak, with a maximum at ca. 0.32 V, full line in Fig. 2(b), was observed. Reaction in this case was also incomplete as when the electrode, after the reduction sweep in base, was subjected to a further sweep in

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E V(RHE) Fig. 2 Reduction sweeps (10 mV s-l) recorded for base-grown films at 25’C. (a) (), film grown on a freshly etched electrode (0.45 to 1.95 V, 40 V s-l for 2 min) and reduced in acid; (- - - - - -), repeat of this experiment using an electrode subjected to prior hydrous oxide growth and reduction as described in the text. (b) ( -), film grown on a freshly etched surface (0.45 to 1.95 V, 40 V s-’ for 2 min) and reduced in base; (- - - - - -), data for a further sweep in acid carried out at the end of the reduction sweep in base. (c) ( -), film grown on a freshly etched surface (0.45 to 2.25 V, 40 V s-l for 3 min) and reduced in acid; (- - - - - -), repeat of this experiment, the reduction being carried out in base; (. . . . ), data for a further sweep in acid carried out at the end of the reduction sweep in base.

acid, dashed line in Fig. 2(b), a small, relatively broad, hydrous oxide reduction peak was again observed at ca. 0.4 V. When the hydrous oxide film was produced in base using a higher value for the anodic limit (0.45 to 2.25 V at 40 V s-l for 3 min) a hydrous oxide peak was observed at ca. 0.28 V, full line in Fig. 2(c), on reducing the film in acid. Although the film was totally reduced in the latter case, the charge involved in hydrous oxide reduction was relatively small, i.e. the hydrous oxide formation reaction was not very efficient in this case. When a similar film was reduced in base, dashed line in Fig. 2(c), an even smaller peak was observed at ca. 0.43 V; a curious feature in this case was the absence of any indication of a monolayer oxide peak which was present at ca. 0.62 V in the previous experiment when the reduction was carried out in acid. Hydrous oxide reduction in base was incomplete as the electrode, on transfer to acid, gave a response for hydrous oxide reduction, dotted line in Fig. 2(c), at ca. 0.33

305

V; the relative magnitude of these peaks suggested oxide layer was reduced in base,

that only ca. 50% of the hydrous

(c) Film reduction at constant potential Visintin and coworkers [lo] grew their oxide films in base and then reduced them later at a constant potential in the range 0.3 to -0.1 V (the time for reduction was not stated). Some data from our work in this area, using two reduction potentials quoted by the above authors, are summarized in Figs. 3 and 4. The films in the present case were produced by triangular cycling of the potential of a platinum electrode in either acid or base; HO1 and HO2 films were examined separately; the films were held at a constant reduction potential in base for given lengths of time prior to transfer to acid for analysis of the residual hydrous oxide. The values for the latter are expressed as percentages; the amount of oxide present initially was estimated in an experiment run just prior to the base stability test in which the coulometric charge for total oxide reduction was estimated in acid (without any reduction in base). The hydrous oxide growth in these experiments was reproducible; in the case of both HO1 and HO2 the oxide growth and reduction process in acid was repeated initially until steady-state oxide growth behaviour was observed. As shown in Fig. 3 most of the oxide films were reduced on holding the potential in base at 0.13 V for 1 to 3 mm; the major exception was with the acid-grown HO2 deposit where ca. 70% of the film remained on the surface even after 10 min. of

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Fig. 3. Variation of hydrous oxide coverage with polarization time at 0.13 V (RHE) in 1.0 mol dm-’ NaOH: (a) HO2 (0.58 to 2.2 V, 100 V s-’ for 1 min, 1.0 mol dm- 3 HaSO,); (b) HO2 (0.45 to 2.25 V, 40 V s-’ for 2 mm, 1.0 mol drne3 NaOH); (c) HO1 (0.45 to 1.95 V, 40 V s-t for 2 mm, 1.0 mol dm-3 the oxide growth NaOH); (d) HO1 (0.58 to 1.9 V, 100 V s- ’ for 1 mitt, 1.0 mot dme3 H,SO,): conditions are given in brackets, T = 25T.

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Fig. 4. Variation of hydrous oxide coverage with polarization time at 0.3 V (RHE) in 1.0 mol dmw3 NaOH: (a) HO2 (0.45 to 2.25 V, 40 V s-l for 2 min. 1.0 mol drn-j NaOH); HO1 (0.45 to 1.95 V, 40 V s-’ for 2 min, 1.0 mol dm-3 NaOH): T= 25°C.

polarization. Earlier work in this laboratory [13] has established that hydrous oxide film reduction on platinum in base is not very effective when the electrode is held at potentials in the regions of -0.1 to 0.1 V; the process occurred much more efficiently on polarizing within the range 0.15 to 0.35 V. Data for base-grown HO1 and HO2 films held at 0.3 V in base are shown in Fig. 4. Film reduction in this case, especially for HO1 material, was quite rapid at short times. However, there was invariably a residue of HO2 material, and quite often a much lower, though still significant, quantity of HO1 material on the electrode surface after 10 min of polarization (Figs. 3 and 4). The only exception was the acid-grown HO1 material (bottom trace in Fig. 3) where little oxide remained after 7 min of polarization. However, it may be stated that in all cases examined here there was a component in these films (the percentage varied with the type of oxide and the medium in which it was formed) that exhibited a considerable resistance to reduction in solutions of high pH. Therefore, the earlier assumption that hydroxide incorporation, or the generation of anionic species of increased stability, occurs in these systems still appears to be valid. It was pointed out earlier [14] that such incorporation may be limited largely to the outer regions of such films. There is as yet no conclusive evidence as to the nature of the anionic interaction leading to the increased stability of these films in solutions of high pH. The excess hydroxide ions may simply be adsorbed in a non-specific (or non-localized) manner in the dispersed hydrated solid which is assumed to have an unusually high surface area, especially in the outer regions of the layer where the latter is in contact with the aqueous phase. The alternative view is that the interaction is more specific; e.g.

307

loss of protons from water molecules coordinated to Pt’” cations in the film (the latter being an aggregate of PtO, .4 H,O or Pt(OH), .2 H,O species) may be assumed to yield hexahydroxyplatinate species, viz., Pt(OH),.

2 Hz0 + 2 OH-(aq)

+ Pt(OH)z-

+ 2 H,O(aq)

(1)

In this case the outer region of the film is considered as a negatively charged anionic framework structure (of a type well known [15] in other areas of chemistry) composed of an aggregate of a well characterised [l] oxidation product of platinum, i.e. platinic acid or its salts. In both cases the counterions, balancing the negative charge on the oxide, are assumed to be present in water-flooded pores of the low-density solid. (d) Effect of solution pH on the oxide reduction processes The following is a summary of data obtained for reduction of hydrous oxide films grown in 1.0 mol dmp3 H,S04 or 1.0 mol dmp3 NaOH solution and reduced in a single sweep (1.6 to 0 V, 10 mV s-‘) following transfer of the platinum electrode to acetate (pH range 2.7 to 5.5) or phosphate (pH range 5.5 to 10.7) buffer solutions. Some examples are given in Fig. 5(a) of the type of responses observed for acid-grown HO1 films on reduction in 1.0 mol dmp3 H,SO,, 0.5 mol drnp3 CH,COOH (plus added Na,SO, to ensure high ionic strength; without the sulphate salt the reduction peak was much broader) and acetic acid/sodium acetate buffer solutions. Only one oxide reduction peak was observed in all cases; the peak

(b)

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electrodes (grown by cycling, Fig. 5. Reduction sweeps (1.6 to 0 V, 10 min s-r) recorded for HOl-coated 0.58 to 1.9 V, 100 V s-r for 1 mm, 1.0 mol dmm3 H2S04) in (a) 1.0 mol dmm3 H,SO,, pH=1.6, ); 0.5 mol dm-3 CH,COOH (+ 1.0 mol dme3 Na2S0,), pH = 3.0, (- - - - - -); 0.5 mol dmd3 (CH,COOH +0.5 mol dmm3 CH,COONa, pH = 4.5 (. . . .): (b) phosphate buffer, pH = 6.8, (- - - - - -); T= 25T. phosphate buffer, pH = 9.8 ( -):

308

maximum potential ( EP) was significantly higher with the solution of lowest pH, ca. 0.4 V as compared with ca. 0.15 V. Some typical responses for acid-grown HO1 films on reduction in phosphate buffer solutions are shown in Fig. 5(b). Usually two oxide reduction peaks were observed in the presence of phosphate, one at ca. 0.3 V and the other just below 0.2 V; such behaviour may be due to phosphate anion involvement in the reduction process occurring at the lower potential. In any given buffer solution system, acetate or phosphate, there was no significant variation of Ep with pH. Evidently the bulk of the acid-grown HO1 material was in the neutral PtO, . n H,O, rather than the anionic, form. Some typical reduction profiles for base-grown HO1 films in various buffer solutions are shown in Fig. 6. In the acetate system, pH = 4.5, a symmetrical reduction peak at ca. 0.14 V was observed. At a slightly higher pH, 5.5, the reduction peak commenced at the same potential but significant tailing of the peak was noted after the maximum; reduction was incomplete at 0 V. Incomplete oxide reduction was also observed with phosphate buffer solutions of higher pH even though the reaction in this case commenced at a higher potential (ca. 0.4 V); evidence for more than one reduction peak was again observed in some instances. It is assumed here that HO1 films grown in base retain a significant level of anionic character due to OH- ion incorporation. At the low pH value (4.5) there was evidently a sufficient proton concentration in the solution to neutralize this excess and reduction occured smoothly to yield the sharp peak. This occurred less readily in solutions of higher pH where the retention of anionic character lowered the reduction potential of part of the film [ll], hence the tailing effect.

EN(RHE)

), (- - - - - -) and (. . . .), reduction of base-grown HO1 films (0.45 to 1.95 V, 40 V s-l Fig. 6. (for 2 min, 1.0 mol drnm3 NaOH) in acetate buffer (pH = 4.5), acetate buffer (pH = 5.50). and phosphate buffer pH = 10.4, respectively: (. -. - .), reduction of an acid-grown film (0.58 to 2.2 V s-’ for 3 min, 1.0 mol dm- 3 H,SO,) in acetate buffer, pH = 4.5: T = 25°C; same reduction sweep conditions as in Fig. 5.

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The response for an acid-grown HO2 film is also shown in Fig. 6. Reduction of such films in 1.0 mol dmp3 H,SO, occurred at ca. 0.14 V (see Fig. l(b)). The same process in acetic acid/sodium acetate buffer solution, pH = 4.5, gave rise to a peak (Fig. 6) at a slightly lower potential, Ep = 0.1 V, but reaction was incomplete; an oxide film, which was subsequently reduced in acid, was visible on the electrode surface at the end of the reduction sweep (even though the reduction current had dropped to virtually zero at this point). (e) Hydrous oxide films and the Kolbe reaction The Kolbe reaction, which has broad synthetic applications, has been extensively investigated [16]; however, certain aspects of the process, such as the role (if any) of the surface oxide on the platinum anode and its influence on adsorption of intermediates, are still a matter of debate [16]. In the present case, since acetate solutions were employed as buffer systems, it was decided to survey briefly the influence of hydrous oxide deposits, grown initially by potential cycling of a Pt electrode in acid, on the rate of the Kolbe process. Films of different thickness were produced by varying the cycling time; then, after transferring the electrode to an aqueous acetic acid/sodium acetate solution, current/ potential profiles were recorded at a slow sweep rate. Typical examples of the data obtained are shown in Fig. 7: the rate of reaction at the upper limit of the scan, Fig. 7(a), was slightly less in the presence of the hydrous oxide deposit. The presence of the latter on the surface during the reaction was confirmed by the appearance of a large peak at ca. 0.2 V, Fig. 7(b), on a reduction sweep recorded after the Kolbe reaction following transfer of the electrode back into 1.0 mol dme3 H,SO,. In a further series of experiments, involving oxide growth times of up to 5 min, the slightly inhibiting effect of the hydrous oxide deposit on the rate of the Kolbe reaction at 2.2, 2.4 and 2.6 V (RHE), was confirmed. In this series of experiments the hydrous oxide was reduced after the Kolbe reaction in the acetate solution: however, this was unsatis-

EN(RHE)

Fig. 7. (a) Variation of current with potential for the Kolbe reaction on Pt in 0.5 mol drnm3 HAc+0.5 ml smooth Pt; (- - - - - -), hydrous oxide-coated Pt dme3 NaAc at 25T; scan rate = 1 mV s-l: ( -), (oxide growth conditions, 0.58 to 1.9 V, 100 V s-l for 1.5 min, 1.0 mol dme3 H2S04). (b) Oxide reduction sweep (10 mV s-‘) for the latter, recorded after the Kolbe reaction, in 1.0 mol dmm3 H,SO,.

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factory as the peak maximum was below 0.1 V and the process was incomplete at the end of the scan (0 v). CONCLUSIONS

(1) The study of hydrous oxide films produced on platinum by the potential cycling technique is complicated by the fact that more than one component may be present in such deposits. This means that some earlier conclusions, such as those on the electrical properties of hydrous oxide films [17], may need to be reinvestigated as the observed low electrical conductivity may be characteristic of only one component. Procedures are outlined here for producing hydrous oxide films in which either HO1 or HO2 predominates. (2) Hydrous oxide formation and reduction occurred with fewer complications in acid as compared with base. Single sweep (10 mV SC’) reduction of acid-grown films in base was invariably incomplete at the lower limit of 0 V; this was particularly true with HO2 deposits; the effect is attributed to the presence of anionic character in the outer regions of the oxide due to OH- ion incorporation which increases the stability of the oxidized form of the couple. The greater stability of HO2 compared with HO1 in base was confirmed in experiments involving film reduction at constant potential. The unusual resistance to reduction of hydrous oxide films on platinum in base is also evident from the work of Visintin and coworkers [lo]; in the cyclic voltammograms shown in Fig. 6 of their publication cathodic currents are evident on the anodic scan even after 100 cycles between 0 to 0.6 V. (3) Hydrous oxide growth in base was complex in so far as it was more difficult to produce HO1 in the single component form while the extent of HO2 formation in this case was quite limited. The latter effect may be due to incomplete separation of the monolayer oxide and hydrous oxide components (the absence of a clear monolayer oxide reduction peak when such films were reduced in base was pointed out earlier). The complication here may again be due to significant OH- ion incorporation when such films are produced in solutions of high pH. The presence of a significant anionic character also explains the difficulty associated with basegrown hydrous oxide, even HOl, reduction in base under constant potential conditions. (4) Hydrous oxide growth on platinum by the potential cycling technique is virtually impossible to achieve [6] at intermediate pH values (ca. 4.0 to 9.0). The reduction of the vast bulk of acid-grown HO1 deposits in the various buffer solutions occurred without significant change in peak potential; the bulk (or interior) of such deposits is evidently non-anionic. There was an appreciable shift in reduction peak potential on changing from 1.0 mol dmp3 H,SO, (pH = 1.6) to the acetic acid/sodium acetate buffer system (E, dropped from ca. 0.4 V to 0.15 V). An unusual shift in the onset potential for monolayer film formation (which may also involve (181 hydrous oxide species) was reported earlier (see Fig. 2 in ref. 18) for the same region. Minor variations in peak potential values on altering the composition

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of the buffer solutions may be due to anion interaction with either the hydrous oxide or the initial product of reduction of the latter, the hydrated adatom. (5) The generation of hydrated adatoms as intermediates in the course of hydrous oxide film reduction is difficult to confirm. The need to postulate such species arose in the first instance [ll] in order to explain the high overpotential, or irreversibility, of the hydrous oxide reduction process. Prior to reduction the PttV material is assumed to be surrounded by an octahedral arrangement of six OHligands (or, alternatively, four OH- and 2H,O groups if the material is present for instance in the uncharged state as Pt(OH), .2 H,O). The metal atom produced on reduction (a reaction that probably entails simultaneous injection of both electrons and protons) is thus quite likely to be surrounded by a sphere of water molecules. The metal surface itself, i.e. the inner region of the double layer, is also commonly assumed-in the absence of specifically adsorbed species-to be covered by a monolayer of water dipoles. Water (with its two groups of lone pair electrons) is a strong Lewis base whereas the platinum atom (with vacancies in its d-orbital) is a Lewis acid; thus there may well be significant hydration energy involved in the case of discrete adatoms. The deposition of the metal atoms from the hydrous oxide layer is not unlike the discharge of hydrated metal cations in electroplating systems (a significant difference is that with the hydrous oxide the cation is less capable of migrating to favourable surface sites for discharge). According to Bockris and Reddy [19] such a reaction in the latter case is a multistep process with charge transfer being succeeded by surface diffusion to step and then kink sites and, finally, lattice incorporation. Each step in the latter sequence is accompanied by loss of water molecules, a process that is assumed here to contribute significantly to the overpotential associated with hydrous oxide reduction. In addition, one cannot ignore the possibility, mentioned by Bockris and Reddy, that there may be some charge on the adatom; this may well increase the binding energy of surrounding water dipoles. REFERENCES 1 L.D. Burke and M.E.G. Lyons in R.E. White, J.O.‘M. Bockris and B.E. Conway (Ed%), Modem Aspects of Electrochemistry, No. 18, pp. 169-248, Plenum Press, New York. 2 L.D. Burke and M.E.G. Lyons, J. Electroanal. Chem., 198 (1986) 347. 3 L.D. Burke and K.J. O’Dwyer, Electrochim. Acta, 34 (1989) 1659. 4 L.D. Burke and T.A.M. Twomey, J. Electroanal. Chem., 134 (1982) 353. 5 L.D. Burke and R.A. Scannell, J. Electroanal. Chem., 257 (1988) 101. 6 L.D. Burke and M.B.C. Roche, J. Electroanal. Chem., 164 (1984) 315. 7 A. Visintin, J.C. Canullo, W.E. Triaca and A.J. Arvia, J. Electroanal. Chem., 239 (1988) 67. 8 A. Visintin, J.C. Canullo, W.E. Triaca and A.J. Arvia, J. Electroanal. Chem., 267 (1989) 191. 9 A.E. Bolzan, A.M. Castro Luna, A. Visintin, R.C. Salvarezza and A.J. Arvia, Electrochim. Acta, 33 (1988) 1743. 10 A. Visintin, W.E. Triaca and A.J. Arvia, J. Electroanal. Chem., 284 (1990) 465. 11 L.D. Burke, M.B.C. Roche and W.A. O’Leary, J. Appl. Electrochem., 18 (1988) 781. 12 L.D. Burke, J.J. Borodzinski and K.J. O’Dwyer, Electrochim. Acta, 35 (1990) 967. 13 L.D. Burke and J.F. O’Sullivan, J. Appl. Electrochem., 21 (1991) 151.

312 14 L.D. Burke and J.F. O’Sullivan, J. Electroanal. Chem., 285 (1990) 195. 15 E. Forslind and A. Jacobson, in F. Franks (Ed.), Water, a Comprehensive Treatise, Vol. 5, Plenum Press, New York, 1975, pp. 173-248. 16 E.J. Rudd and B.E. Conway, in B.E. Conway, J.O’M. Bockris, E. Yeager, S.U.M. Khan and R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 7, Plenum Press, New York, 1983. p. 749. 17 T.H. Randle and A.T. Kuhn, Electrochim. Acta, 31 (1986) 739. 18 L.D. Burke and M.B.C. Roche, J. Electroanal. Chem., 159 (1983) 89. 19 J. O’M. Bockris and A.K.N. Reddy, Modem Electrochemistry, Vol. 2, Plenum Press, New York, 1977, pp. 1175-1182.