Kinetic theory of pseudo-capacitance and electrode reactions at appreciable surface coverage

Electrochemical kinetic studies have been carried out at the nickel oxide electrode showing that the reversible potential for the N i u N i l n system is independent of the state of oxidation of the bulk oxide in the electrode over a wide range of degrees of oxidation The properties of the electrode are shown to be determined by the state of a surface phase, which is completely charged when the bulk oxide material in the electrode has been charged to 10% of its total charge capacity Experiments on sparingly charged electrodes have proved that charging of the bulk oxide does not commence significantly until the electrode is charged to about 1 5 % Consecutive electrochemical reactions possibly involved in the charging process are discussed INTRODUCTION The real reversible potential of the nickel oxide (N^-Ni 1 1 1 ) electrode has been the subject of controversy for some years and previously reported values are misleading since they have not been based on well-defined equilibrium conditions or a well-defined state of the system Determination of the reversible potential is complicated by the self-discharge processes which occur at the electrode in the oxidized form and also by the fact that the electrode may exist in a range of chemical states corresponding to the extent of conversion of Ni to Ni (and possibly Ni under certain conditions) in the hydrated oxide In an earlier paper (1), the true reversible potential of partially charged nickel oxide electrodes at a single controlled state of oxidation "NiOi 25" was examined as a function of potassium hydroxide and water activities in aqueous alkali solutions, and distinguished from the mixed potential assumed in previous work (2, also 12, 13) to be the reversible potential A polarization decay method was employed (1, 3, 4) in which the reversible potential was approached both from the anodic and the cathodic directions In the present phase of our work, the same method has now been applied to the study of the potential of partiall) charged nickel oxide electrodes as a function of the degree of charge, at constant electrolyte composition This method was found to be applicable only over a limited range of degree of oxidation of the electrode ("N1O1 iu"-"NiOi 30") owing to problems of extrapolation to the reversible potential to be discussed below. Stationary potentials, taken at very long times on open circuit, were therefore also measured as a function of the degree of charge over a much wider range of degree of oxidation ("INiOi 02B"-"NIOI 50") The role of the surface and bulk phases in determining the measured potential was deduced It will be shown that three well-defined regions in the charging process can be distinguished experimentally. In the first, the surface phase alone is being charged, in the intermediate region, the Faradaic current is used to charge both phases, until the surface phase is fully charged, after which oxidation of the bulk is the main Faradaic process (Most of the charging process occurs at a potential anodic to the reversible oxygen electrode, so that oxygen evolution must occur to a certain extent as a parallel process in all three regions described above It does not, however, become appreciable until most of the bulk material has been charged ) Canadian Journal of Chemistry Volume 40 (1902) 1933 1934 CANADIAN JOURNAL OF CHEMISTRY VOL 4