The nickel -nickel oxide electrode forms the positive plate in the charged nickel-cadmium battery. After "charging" the electrode t o a chemical state represented by the non-structural formula NiO,, 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 bee11 examined by study of (i) the rate of decay of e.m.f. on open circuit, (ii) rate of oxygen e v o l u t i o~~ on open circuit, (iii) the electrochemical capacity of the electrode, and (ill) the build-up or charging curves for the electrode. The decay behavior has been studied in aqueous ICOH 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 a t 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 e v o l u t i o~~ by deducing, by nleans of the Christiansen method, the relevant Tafel slopes. I t is shown that the scheme proposed uniquely accounts for the esperimental behavior and that the change of mechanism observed in the self-discharge can only be explained if two consecutive and not alternative processes are irlvolved. The dependence of the rates of self-discharge upon OH-ion and water activity is deduced and the significance of these results is discussed.
INTRODUCTIONInterest in the behavior of the nickel -nickel oxide electrode arises from its use as the positive plate in the nickel-cadmium and nickel-iron batteries; a t the same time, study of the system affords further insight into the fundamental kinetics of anodic processes, which in the case of the nickel system are complex owing to the existence of several possible oxidation states of the metal in corresponding hydrated oxides (1, 2). One of the features of interest is the loss of charge of the nickel oxide electrode when it is left standing in pure alkaline solution on open circuit. T h e loss of charge is accompanied by oxygen evolution and decay of e.m.f. of the electrode (e.g., measured with respect to the Hg/HgO electrode in the same s o l u t i o~~) .Hitherto, studies of the kinetics of decay have not been made rigorously and only sorneu~hat arbitrary rate measurements have been obtained (i) by determining the total time required for the e.m.f. of the electrode to decay over about 0.G v (3), and (ii) by determining the total volume of oxygen liberated from an electrode a t the end of a period of 13 hours (4). Since it is clear that over the potential range studied (3) more than one process is involved in the decay, because the e.m.f.-time curve passes through an inflection, and since in the oxygen evolution process (4) the rate of oxygen evolution depends on the electrode potential, it must be concluded that the kinetic significance of "rates" measured by these procedures is unclear. Furthermore,...