Direct ammonia solid oxide fuel cells based on spinel ACo2O4 (A=Zn, Fe, Ni) composite cathodes at intermediate temperature

Direct ammonia solid oxide fuel cell (DA-SOFC) is a promising device to realize high-efficiency NH3-to-power. In this study, we fabricate a series of cobalt-based spinel ACo2O4 (ACO, A = Zn, Fe, Ni) oxide as DA-SOFC cathodes using the citric acid complexing method. We prepare three cobalt-based spinel oxides with different A-site elements, i.e., Zn, Fe, and Ni, to establish the relationships between the valence and active site of ACO spinel oxide. The structural analysis based on XRD, XPS, TEC, and UV-Vis diffuse reflectance confirms that NCO exhibits smaller band gap, higher electronic conductivity, and better compatibility with electrolyte. The electrochemical results indicate that the cathodic surface of NCO containing rich Co3+ and Oads leads to better oxygen reduction reaction (ORR) activity. The maximum power density of the DA-SOFC using NCO cathode reaches 1.06 W·cm−2 at 800°C, superior to those of using ZCO (0.83 W·cm−2) or FCO (0.92 W·cm−2) cathode. Kinetic analysis based on the distribution of relaxation time (DRT) and electrochemical impedance spectroscopy (EIS) indicates OTPB+e−↔OTPB− is rate-limiting.