EFFECT OF TIME-SCALE ON REDOX-DRIVEN ION AND SOLVENT TRANSFERS AT NICKEL-HYDROXIDE FILMS IN AQUEOUS LITHIUM HYDROXIDE SOLUTIONS

We describe an EQCM study of mobile species transfers accompanying red ox switching of electroprecipitated nickel hydroxide films exposed to aqueous lithium hydroxide solutions. The films are rigidly coupled to the underlying electrodes, allowing him 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 resp onse with redox cycling; this allows us to explore the effects of time scale and electrolyte concentration for a given film. Steady state fi lm redox switching in aqueous LiOH solutions is accompanied by non-mon otonic mass changes. The extent to which ion and solvent transfers con tribute to the observed mass change is a function of electrolyte compo sition and experimental time scale; neutral species transfers are inco mplete on the time scales of most voltammetric experiments. Correlatio n 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 electrol yte concentration, hydroxide transfer also occurs. Under these conditi ons, lithium (hydroxide) ion transfer maintains electroneutrality at p otentials negative (positive) to the current peak potential. (C) 1998 Elsevier Science S.A. All rights reserved.