Electrochemical impedance spectroscopy was used to measure the diffusion coefficient of protons, in hydroxide films at room temperature, as a function of the state-of-charge of a battery. A model for the complex faradaic impedance of the active material was developed and was used to extract the proton diffusion coefficient from the electrochemical impedance spectroscopic data. It was noted that impedance data for a range of frequencies could be used to extract a constant diffusion coefficient without having any knowledge of the initial mobile proton concentration or the form of the charge-transfer kinetic expression. It was found that the proton diffusion coefficient was a sensitive function of the state-of-charge, and decreased by some 3 orders of magnitude (3.4 x 10-8 to 6.4 x 10-11cm2/s) as the electrode changed from the completely charged to the completely discharged state. The condition of the hydroxide films suggested that the measured diffusion coefficients corresponded to the  phase of the active material. The diffusion coefficient was measured for film thicknesses of 1.5, 1.2 and 1.0, and was found to be independent of the film thickness, as predicted by theory. The 3 orders of magnitude decrease in the diffusion coefficient was explained by assuming that the protons moved mainly through the oxidized phase, NiOOH, which was interdispersed with the reduced phase, Ni(OH)2, in the active material.

S.Motupally, C.C.Streinz, J.W.Weidner: Journal of the Electrochemical Society, 1995, 142[5], 1401-8