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 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,...

New Approaches to the Study of Electrochemical Decarboxylation and the Kolbe Reaction: Part I. The Model Reaction With Formate

“…For the three metals studied in the present work, gold, palladium, and platinum, there is clear evidence of an arrest in the transients corresponding to the charge associated with a film of adsorbed intermediates produced in the anodic reaction. The arrests only appear after moderate and high anodic polarizatiolls and collfirm that the transition behavior in the current-potential plot is associated with a critical condition for formation of an anodic product (an anodic passive film) on the electrode surface, which can be removed either by a self-discharge process (18,29) on open-circuit or by a reverse cathodic pulse, as with adsorbed 0 or H (30). Following our initial observations in a preliminary paper (16,17), we believe this is the first direct proof of the role of an adsorbed layer of intermediates in electrochemical decarboxylation reactions.…”

Hydrous Nickel Oxide: Redox Switching and the Oxygen Evolution Reaction in Aqueous Alkaline Solution

“…The working electrode was constructed from 1 mm thick polycrystalline nickel foil (as supplied by Alfa Aesar-Johnson Matthey, purity 99.9945% (metals basis)) with a geometric surface area of 0.16 cm 2 . Prior to each experiment the surface of the working electrode was polished with 1200 grit carbimet paper, dipped in H 2 SO 4 , wiped, and polished with a slurry of 0.3 micron alumina powder until a "mirror bright" finish was achieved. A platinum wire electrode (CH Instruments, cat no.…”

“…Ni-Cd, Ni-MH and Ni-MH 2 ) since it facilitates "self discharge" and consequently leads to a decrease of charge storage capacity. 4,5 Thus in contrast to electrolyser anode research, work in the battery area has been directed toward increasing the OER overpotential at nickel hydroxide electrodes. This has been achieved by the addition of cobalt hydroxide to the nickel hydroxide, 6 however, depending on the amount of incorporated Co, this procedure can actually improve OER catalytic activity.…”

Symposium on Charge Transfer Processes: A REPORT ON A SYMPOSIUM HELD BY THE PHYSICAL CHEMISTRY DIVISION OF THE CHEMICAL INSTITUTE OF CANADA

“…outside the range of cathodic overpotentials examined. The somewhat unique polarizatioll behavior of silver therefore appears to reflect a genuine change of rate-determining mechanism with change of electrode potential (see also (12)). …”