Oxygen reduction kinetics of high performance BaCo0.4Fe0.4M0.1Y0.1O3-δ (M = Mg, Zr) positrode for protonic ceramic fuel cells
Hirofumi Sumi, Konosuke Watanabe, Aman Sharma, M. Fujioka, Hiroyuki Shimada, Yasunobu Mizutani, Md. Saiful Alam, Isao Kagomiya
Abstract
Protonic ceramic fuel cells (PCFCs) should exhibit high performance at intermediate temperatures in the range of 400–600 °C. To reduce the operating temperature, more active air electrodes (positrodes) are needed. In the present work, BaCo0.4Fe0.4Mg0.1Y0.1O3-δ (BCFMY) is investigated as a positrode material for application in PCFCs as well as solid oxide fuel cells (SOFCs). For SOFCs, the polarization resistance ascribed to the oxygen reduction reaction is proportional to pO2−1/4 (pO2: oxygen partial pressure), suggesting that the rate-determining process is the charge transfer on the mixed ionic-electronic conductors. For PCFCs, this polarization resistance is proportional to pO2−1/2, suggesting that the rate-determining process is the oxygen dissociation. The total polarization resistance for the PCFCs using the BCFMY positrode is 0.066 Ωcm2 at 600 °C, lower than that using the BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) positrode. The higher oxygen nonstoichiometry of BCFMY promotes the oxygen dissociation process on the PCFC positrode surface. Perovskite BaCo0.4Fe0.4Mg0.1Y0.1O3-δ is a promising material as a positrode (air electrode) of protonic ceramic fuel cells (PCFCs). Here, the oxygen reduction kinetics in PCFCs is clarified through the analysis of the distribution of relaxation times using electrochemical impedance spectra.