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,...

The significance of observations of rates of e.m.f.-decay on opencircuit at the nickel oxide electrode is examined in terms of the potential dependefice of the surface capacitance associated with the processes occurring during decay. Results are compared with those reported by Lukovtsev and Temerin and it is concluded that a capacity based on the properties of the surface layer (e.g., adsorbed oxygen radicals or a highly oxidized nickel oxide surface phase) must be used rather than one based on the oxidation-reduction behaviour of the bulk phase oxide. Direct micro-volumetric determinations of volumes of oxygen evolved from the electrode during opencircuit e.m.f.-decay allow quantitative comparisons to be made between true Tafel slopes and open-circuit e.m.f.-decay slopes. These slopes are shown to be identical only when the volume of oxygen evolved from the surface is a linear function of e.m.f. When the e.m.f. is a logarithmic function of the oxygen evolved, Tafel and decay slopes are shown to differ in the theoretically expected direction. The linear and logarithmic behaviour with respect to e.m.f. and volumes of oxygen evolved can be shown to follow as limiting conditions of the adsorption behaviour of oxygen at the electrode surface, depending upon the extent of surface coverage. The kinetic significance of the surface capacitance is discussed in relation to the e.m.f. decay behaviour. 593* This is based on a figure of 50 mA per actual plaque used (7-5 x 2 5 x 0.1 cm in size).* Very recent work by Conway and Gileadi indicates that when the true reversible potentials (as distinct from mixed potentials 2) are deduced, the Nernst redox slope for the bulk material is, in fact insignificantly merent from zero within the experimental error of f 5 mV.

Further evidence that the rate-controlling process in self-discharge of the iiickel oxide electrode is the anodic partial reaction of oxygen evolution is reported and is based on: True Tafel slopes are deduced and interpreted in terms of possible mechanisms of oxygen evolution, taking account of the dependence of activation energy upon surface coverage by adsorbecl intermediates.

The electrode potential of the nickel oxide electrocle has been determined as a function of water and solute activity in aqueous solutions of potassium hydroxide. The electrode, which was charged to a mean state of oxidation corresponding to 5 0 y 0 Ni I1 and 50yo Ni 111, was examined after long periods of time and by cathodic and anodic e.m.f. decay meas~irements after polarization in order to establish the quasi-reversible potentials by two methods. The results are discussed in terms of the stoichiometry of the potential-determining reaction, and distinction between mixed and true reversible potentials is made. INTRODUCTIONWe have previously examined in part I (1) the kinetic behavior of the "overcharged" nickel oxide electrode with regard to rates of self-discharge and oxygen evolution. In the present paper we present data on the quasi-equilibriunl potentials of the nickel oxide electrode in aqueous potassium hydroxide solutions. The behavior of the electrodes is referred to as "quasi-equilibrium" since the electrodes are continuously losing charge on open-circuit by the corrosion type of process described previously (1).This can affect the electrode in two ways: firstly to produce a continuous but slow decrease of the state of oxidation of the oxide and secondly to give rise to a n electrode potential which is more cathodic than the true equilibrium potential corresponding to the formal state of oxidation of the oxide. In fact, the potentials measured on opencircuit are analogous to mixed potentials observed in corrosion, since there is a slow anodic evolution of oxygen and a corresponding cathodic self-reduction of the oxide. This factor does not seem to have been realized in previous work (2, 3, 4). However, by suitable choice of conditions and inethods of study it is possible to determine the potential of the electrode under conditions very near to equilibrium. Thus, if the electrodes are left in dilute potassium hydroxide (0.001 M) for several days, the potential ceases to change by more than 0.1 111v per hour and the potential of the electrode may then be examined as a function of potassiunz hydroxide activity without further significant selfdischarge during the measurements. Also in the solutions of higher allcali concentration the electrode is lllore stable* since the open-circuit potential becomes less anodic to the reversible ox).gen potential and the rate of self-discharge (already allowed to become small in the 0.001 M solution) becomes negligible.Electrode potentials and stoichiometric reactions for the potential-determi~li~~g reaction a t the nickel oxide electrode have been discussed in earlier work by 1;orster (2) and Zedner (3) and more recently by Kornfeil (4). ' 4 potential of 0.40 v has been attributed to the nickel oxide electrode by Latimer ( 5 ) , but no eviclence that the potential-determining species (e.g. NiOz) are those written by Latimer (5) has been given; in fact there is no evidence that NiOz exists as such in the electrocl~emicall~ active material in the nickel

No abstract