The Role of the German Federal Constitutional Court Law and Politics (“The Role of the Supreme Court and Consitutional Courts in Japan, Korea and Germany” DAAD Joint Symposium of Ritsumeikan University (RU), Ludwig-Maximilians-Universität München (LMU), Seoul National University (SNU) in Kyoto/Japan)

departmental bulletin pape

Proton Diffusion in Nickel Hydroxide Films: Measurement of the Diffusion Coefficient as a Function of State of Charge

Electrochemical impedance spectroscopy (EIS) was used to measure the solid-state diffusion coefficient of protons in nickel hydroxide films at room temperature as a function of state of charge (SOC). A model for the complex faradaic impedance of the nickel hydroxide active material is presented and used to extract the diffusion coefficient of protons from the EIS data. Impedance data over a range of frequencies can be used to extract a constant diffusion coefficient without the knowledge of the initial mobile proton concentration or the form of the charge-transfer kinetic expression. The proton diffusion coefficient is a strong function of SOC and decreases approximately three orders of magnitude from 3.4 × 10–8 to 6.4 × 10–11 cm2 s–1 as the electrode discharges from the completely charged to the completely discharged state. The measurements were performed on well-conditioned nickel hydroxide films and therefore it is likely that the diffusion coefficients measured correspond to the -phase of the active material. The diffusion coefficient of protons was measured for three different film thicknesses, 1.5, 1.2, and 1.0 µm. The diffusion coefficient is independent of the thickness of the film as predicted by theory. The three orders of magnitude decrease in the diffusion coefficient of protons can be explained on the assumption that the protons move predominantly through the oxidized phase [NiOOH] which is interdispersed along with the reduced phase [Ni(OH)2] in the active material

Proton Diffusion in Nickel Hydroxide: Prediction of Active Material Utilization

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