Assignment of elementary reactions to impedance components for solid-oxide fuel cells and proton-conducting ceramic fuel cells with Yb and Co-doped barium zirconate as the cathode

Abstract

To clarify the elementary reactions of multiple impedance components in fuel cell electrodes, the impedance characteristics of Ni anode-supported fuel cells with BaZr_0.8Yb_0.2O_3−δ and Zr_0.84Y_0.16O_2−δ electrolytes were evaluated using BaZr_0.3Yb_0.2Co_0.5O_3−δ as the cathode. The elementary reactions were identified by separating impedance components based on isotope effects during proton/deuteron substitution. Distribution of relaxation time analysis showed that the electrode impedances of the proton-conducting ceramic fuel cells (PCFCs) and solid-oxide fuel cells (SOFCs) could be separated into five components. The isotope effect in the impedance was investigated by replacing the anode gas from H₂O–H₂ to D₂O–D₂. For PCFCs, isotope effects appeared in two high-frequency components under open-circuit voltage (OCV), indicating their relation to the anodic reaction; however, isotope effects appeared in the lowest-frequency component under bias (0.85 V). This component was attributable to water vapor formation in PCFCs. By contrast, isotope effects were observed in the three middle-frequency components under bias (0.85 V) in SOFCs, indicating their relation to the anodic reaction; however, no isotope effects appeared in the highest-frequency component among three middle-furequency components under OCV. This component was attributed to water vapor formation in SOFCs.