Electrochemistry and stoichiometry of thin film oxides on nickel.

Abstract

The initial stages of electrochemical oxidation of nickel surfaces have been investigated with respect to (a) anodic formation and reduction characteristics of the thin oxide films; (b) chemical characterization of the films, e.g. determination of the active oxygen to nickel content and stoichiometry; (c) relation of the chemical and electrochemical properties of the oxide films to the kinetics of anodic oxygen evolution on the oxides; (d) investigation of the origin and significance of irreversibility in the processes of formation and reduction of the oxide films (hysteresis effects). Complementary comparative work has also been carried out on silver oxide electrodes where surface oxide films in at least two easily distinguishable valence states are involved, and at low temperatures the formation of a third oxide Ag 2O3 is indicated. Since electrochemical behaviour of an electrode depends significantly on the pre-history of the electrode and the nature of the surface oxide, much attention has been given to the preparation of electrodes in comparable states. This matter was found to be of particular importance in the studies on oxygen evolution kinetics at the oxidized nickel surfaces. The kinetic observations provided evidence in support of the ion-radical type of recombination step as rate-determining in oxygen evolution kinetics. This is in agreement with the conclusions derived from studies on the behaviour of bulk oxide material studied previously in this laboratory. The potentiostatic studies on the oxygen evolution reaction at nickel, oxidised nickel surfaces and at silver revealed inhibition effects (self-passivation) analogous to those found in anodic organic oxidations at the noble metals. Here, however, the inhibiting species, surface oxides, are directly involved in the overall reaction itself. A kinetic theory of the "self-passivation" effects is presented in general terms for various supposed oxidation states of the surface region of the electrode interphase. In order to relate the nature of the surface oxides on nickel metal surfaces to the kinetics of oxygen evolution, various transient methods viz. potentiostatic step charging, potentiodynamic linear sweep and galvanostatic discharge methods have been applied. It is shown that higher nickel oxides are reduced in cathodic galvanostatic pulses only to Ni(OH)2 and hydrogen commences to evolve on this latter oxide. A potentiostatic step (anodic) charging method was therefore developed for characterisation of the formation of the surface oxide at nickel in a progressive manner over small intervals of potential. A theoretical treatment for interpretation of results obtained by the step-charging method has been developed. The kinetics of growth of nickel oxides were investigated and it has been observed that the growth kinetics obey a simple logarithmic law. A theoretical formulation of a growth law logarithmic in time has been developed. For the purpose of determining the stoichiometries of the surface oxides formed in the thin film at the metal surface, combined chemical and electrochemical methods were used. For determining micro-quantities of nickel in the thin film oxide layers, a new spectrophotometric method based on the principle of reductive stripping by aq. hydrazine has been developed. Stoichiometries of thin film layers (after allowance for self-discharge effects) are reported as a function of the formation potential. The apparent extent of oxide reduction charge in the thin films, determined electrochemically by a fast galvano-static reduction pulse, and the amount of nickel in the film determined chemically, pass through a maximum at ca. 0.9 V. This is shown to be connected with a change of the oxide layer to a more passive condition. This behaviour is closely connected with the self-passivation effect observed in the kinetics of the anodic oxygen evolution reaction at nickel oxide surfaces. Hysteresis between the charge and discharge processes has been observed both at nickel and silver (similar effects also occur at more noble metals such as platinum, palladium, etc.). This phenomenon has been studied in some detail using potentiodynamic sweep and galvanostatic discharge techniques. It is proposed that hysteresis is an intrinsic thermodynamic effect and is possibly related to the change of state of the system arising from an irreversible rearrangement of the surface oxide during or after its formation.