Temporal resolution of ion and solvent transfers at nickel hydroxide films exposed to LiOH

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor mobile species transfers on short time scales following the application of a potential step to potentiostatically deposited α-nickel hydroxide films exposed to aqueous LiOH solution. Upon film oxidation, hydroxide ions enter the film and protons are deintercalated. However, the protons are not detected directly since, upon ejection from the film, they immediately combine with hydroxide ions from the solution to yield water. The relatively rigid film lattice (which allows gravimetric interpretation of the EQCM response) imposes volume constraints. As ions are driven into/out of the film in order to satisfy electroneutrality, this volume constraint causes solvent to be driven out of/into the film by internal pressure. Consequently, solvent transfer is forced into the same time scale as ion transfer and is no longer simply diffusionally controlled. Combining the EQCM/PBD-derived fluxes of individual mobile species with their molar volumes, one can estimate the implied changes in film volume driven by redox switching. These volume changes are relatively small and correspond to a monotonic expansion on film oxidation and a transient expansion then eventual contraction upon film re-reduction. These observations are in the opposite sense to those predicted on the basis of unit cell volumes for crystalline α-Ni(OH)2 and γ-NiOOH, suggesting that the material studied is poorly represented by these idealised structures.