The first stages of the anodic oxidation of polycrystalline copper electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Cu20 was found to be preceded by electrosorption of oxygen species, that occurs in two successive stages, each represented by a current peak, corresponding to a different submonolayer state with a different adsorption energy. This surface oxide was formed via random electrodeposition. The width of the first current peak indicates the presence of lateral attractive interactions in the chemisorbed layer. The surface layer did not show any ageing effect.

The first stages of the oxidation of polycrystalline silver electrodes in NaOH solutions were studied by potential sweep voltammetry and ellipsometry. Formation of bulk Ag20 was found to be preceded by dissolution of silver species and deposition of a surface oxide. The equilibrium oxide coverage depended on the electrode potential and occurred within a few seconds Surface oxide formation probably took place via a process of random electrodeposition. No ageing effect was observed in the chemisorbed layer.(1) INTRO DUCTION The use of silver oxide electrodes in storage batteries has stimulated many investigations of the anodic oxidation of silver in alkaline electrolytes. Nevertheless, much remains to be discovered about the oxidation mechanisms and the structure of the silver oxides formed. (See refs. 1--12 for reviews.)Cyclic voltammograms ( Fig. 1) generally show four peaks in the anodic direction (A1 to A4) and two in the cathodic direction (C3 and C4). A3 and A4 are related to the formation of Ag20 and AgO and C4 and C3 are assigned to reduction of AgO and Ag20, respectively [11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. The small peak A2 has been variously attributed to the formation of AgOH [13,15], to oxidation of adsorbed hydrogen [ 18], to preferential oxidation of an activated lattice [19,25], and to the dissolution of silver as Ag(OH)~ with diffusion of the product into the solution [22,23].The minor peak A1 has attracted relatively little attention. Stonehart and Portante [15,16] took it to be an artefact arising from the method of metal fabrication, because this peak was not observed upon oxidation of silver, electrodeposited from a plating bath. B~ezina et al. [28], rather surprisingly, stated that at potentials which are 200 to 700 mV more negative than that of the A1 peak, surface oxides are formed. According to them the A1 peak is related to formation of a larger amount of Ag20. Giles et al. [29] concluded from impedance studies that the initial step in the oxidation, at potentials negative to the reversible Ag/Ag20 potential (140 mV vs. Argenthal, in 1 M NaOH) is the formation of a soluble species Ag(OH)~ which diffuses away from the electrode. They stated that this was followed by phase formation on the electrode which

The anodic oxidation of silver electrodes in NaOH solution and the reduction of the silver oxides formed were studied by potential step chronoamperometry. Oxidation of Ag to Ag20 is a diffusion-controlled reaction, the diffusion control being established in the solid phase. Oxidation of Ag20 to AgO proceeds via a nucleation and growth-controlled process. The amount of AgO decreased with increasing step height. The current--time curves for this reaction have been analysed with the Kolmogoroff--Avrami equation. Reduction of AgO to Ag20 occurs initially on the outside of the electrode, and the rate of the reaction is limited by diffusion of ions across the thickening layer of Ag20. Reduction of Ag20 to Ag proceeds via a nucleation and growth reaction.

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