Characterization of Doped Yttrium Chromites as Electrodes for Solid Oxide Fuel Cell by Impedance Method

Abstract

Redox-stable ceramic electrodes, Co and Ni doped yttrium chromites, for solid oxide fuel cell (SOFC) are characterized by electrochemical impedance spectroscopy (EIS) in various PO2 and PH2 atmosphere. The polarization resistances for Y0.8Ca0.2Cr0.8Co0.2O3-δ (YCCC) and Y0.8Ca0.2Cr0.9Ni0.1O3-δ (YCCN) on yttrium stabilized zirconia (YSZ) electrolyte are 0.96 and 8.4 Ωcm2 in wet H2, 1.2 and 36.7 Ωcm2 in air at 850°C, respectively. For the anode application, the rate-limiting steps (RDS) are identified as charge transfer and surface diffusion for both YCCC and YCCN. The primary active zone in YCCN is three-phase boundary (3PB) but extends to a small portion of the electrode bulk in the YCCC anode. For the cathode application, O2 dissociative adsorption or diffusion is one of the RDS for both electrodes, and the active zone is limited to the 3PB area. The influence of electrolyte to electrode performance is investigated by replacing YSZ electrolyte with scandium stabilized zirconium (SSZ). Smaller polarization resistances are observed on each electrode in both wet H2 and air atmospheres. Replacement of electrolyte can alter not only the rate of charge transfer process but also in some cases other surface processes not related to the electrolyte directly. It is proposed that the impact of the electrolyte on each electrode process is passed down as in a chain and the charge transfer step functions as the first ring in the chain. The best performance is obtained with the YCCC/SSZ combination, 0.49 and 1 Ωcm2 in wet H2 and air at 850°C, respectively, making YCCC a promising electrode.