THE ELECTROCHEMICAL BEHAVIOR OF THE NICKEL – NICKEL OXIDE ELECTRODE: PART I. KINETICS OF SELF-DISCHARGE

Abstract

The nickel – nickel oxide electrode forms the positive plate in the charged nickel–cadmium battery. After "charging" the electrode to a chemical state represented by the non-structural formula NiOx, where x can vary from about 1.4 to 1.8 depending on the current density and temperature, loss of oxygen and a fall of potential on open circuit occurs. In the present work this "self-discharge" effect has been examined by study of (i) the rate of decay of e.m.f. on open circuit, (ii) rate of oxygen evolution on open circuit, (iii) the electrochemical capacity of the electrode, and (iv) the build-up or charging curves for the electrode. The decay behavior has been studied in aqueous KOH solutions from 0.0015 to 15 M. Tafel slopes are obtained from the plots of e.m.f. vs. log (time of decay), and abrupt changes occur at certain electrode potentials which indicate changes of rate-determining mechanism in the self-discharge process. The slopes observed are interpreted in terms of a new scheme of consecutive reactions for anodic oxygen evolution by deducing, by means of the Christiansen method, the relevant Tafel slopes. It is shown that the scheme proposed uniquely accounts for the experimental behavior and that the change of mechanism observed in the self-discharge can only be explained if two consecutive and not alternative processes are involved. The dependence of the rates of self-discharge upon OH− ion and water activity is deduced and the significance of these results is discussed.