Effect of pressurization on electrochemical characteristics of Ni-yttria-stabilized zirconia fuel electrodes

Abstract The effects of pressurization on Ni-Yttria-Stabilized Zirconia (Ni-YSZ) were studied in fuel-electrode-supported symmetric cells using electrochemical impedance spectroscopy (EIS) for various steam/hydrogen ratios. The data is fit well using an equivalent circuit including Warburg gas diffusion and transmission-line model elements. Increasing the total pressure from 1 to 5 atm yields a substantial reduction in the Warburg resistance, but only slightly decreases the transmission-line resistance. Existing analytical models for gas diffusion and charge transfer processes in the porous electrodes, which generally work well at 1 atm, were tested against the pressurized data. The gas diffusion model provided good fits to the data, but only assuming pore sizes that are larger than expected from microstructural data. Using a charge transfer model assuming proton spillover from Ni onto YSZ, a set of parameters was found that matched the weak dependence on pressure observed experimentally, but did not correctly predict the dependence on steam/hydrogen ratio. The results suggest the additional information provided by pressurized data may aid in the development of improved models of Ni-YSZ electrode processes.