Tailoring Ba0.5Sr0.5FeO3-δ cobalt-free perovskites with zinc as triple-conducting cathodes for protonic ceramic fuel cells

Abstract

Developing triple-conducting cathode is an available way to enhancing the electrochemical performance of protonic ceramic fuel cells (PCFCs). Here we report a systematical investigation on the performance of cobalt-free perovskites Ba0.5Sr0.5Fe1-xZnxO3-δ (x = 0, 0.1, and 0.2; BSF, BSFZ0.1, and BSFZ0.2) as triple-conducting cathodes for PCFCs. Zn doping improves the hydration capacity, oxygen vacancy concentration, surface exchange coefficient, and bulk diffusion coefficient of BSFZx (x = 0.1 and 0.2) as compared to the bare BSF. Theory calculations show that Zn doping benefits the oxygen vacancy formation, which is conducive to the promotion of cathodic oxygen reduction reaction (ORR). Among materials investigated, the BSFZ0.2 perovskite possesses an abundance of oxygen vacancies, strong hydration capacity and exceptional triple conductivity (H+/O2−/e−), and thus showing the lowest polarization resistance (Rp) of 0.328 Ω cm2 at 700 °C under wet air. At 700 °C, the cell reaches a maximum power density of 497 mW cm−2 in H2 using BSFZ0.2-30 wt% BaZr0.4Ce0.4Y0.15Zn0.05O3-δ (BZCYZ) composite cathode based on 25-μm-thick BZCYZ electrolyte, and there is no obvious deterioration in performance after operation for 100 h. The rate-limiting steps of ORR on BSFZ0.2-BZCYZ composite cathode are determined to be the surface oxygen incorporated into lattice reaction processes. These findings demonstrate that the proposed BSFZ0.2 holds great potential as a cobalt-free triple-conducting cathode for PCFCs.