Galvanostatic charge and discharge experiments reveal that the active material in nickel electrodes cannot be fully accessed at high currents or for thick films. It has been proposed that the utilization of the active material is controlled by the diffusion rate of protons through the film. This hypothesis is supported by the good agreement between mathematical simulations of material utilization and experimental data over a range of charge and discharge currents and film thicknesses. Furthermore, the fraction of material utilized is larger on charge than on discharge. The asymmetry on charge and discharge is due to a diffusion coefficient that is a function of the state-of-charge of the active material. The mathematical model is used to perform a parametric study of material utilization as a function of charge and discharge currents, and material loading (i.e., film thickness, concentration of nickel sites) in order to improve battery design and operation.