Effect of time scale on redox-driven ion and solvent transfers at nickel hydroxide films in aqueous lithium hydroxide solutions

Abstract

Abstract We describe an EQCM study of mobile species transfers accompanying redox switching of electroprecipitated nickel hydroxide films exposed to aqueous lithium hydroxide solutions. The films are rigidly coupled to the underlying electrodes, allowing film resonant frequency changes to be interpreted in purely gravimetric terms. After an initial ‘break-in’ period, the films do not show significant evolution of the EQCM response with redox cycling; this allows us to explore the effects of time scale and electrolyte concentration for a given film. Steady state film redox switching in aqueous LiOH solutions is accompanied by non-monotonic mass changes. The extent to which ion and solvent transfers contribute to the observed mass change is a function of electrolyte composition and experimental time scale; neutral species transfers are incomplete on the time scales of most voltammetric experiments. Correlation of mass and charge responses as functions of LiOH concentration and voltammetric sweep rate suggests that oxidation is always accompanied by lithium ion expulsion and some solvent transfer. At higher electrolyte concentration, hydroxide transfer also occurs. Under these conditions, lithium (hydroxide) ion transfer maintains electroneutrality at potentials negative (positive) to the current peak potential.