Ba0.95La0.05Fe0.8Zn0.2O3-δ cobalt-free perovskite as a triple-conducting cathode for proton-conducting solid oxide fuel cells

Abstract

A triple-conducting Ba0.95La0.05Fe0.8Zn0.2O3−δ (BLFZ) cobalt-free perovskite was evaluated as cathode for proton-conducting solid oxide fuel cells based on BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) electrolyte. BLFZ had a cubic perovskite structure and good chemical compatibility with BZCYYb electrolyte at temperatures below 1100 °C. The average thermal expansion coefficient of BLFZ is 20.4 × 10−6 K−1 at 30 °C-1000 °C. The polarization resistance (Rp) of BLFZ decreased from 2.072 Ω cm2 in air to 1.334 Ω cm2 in wet air (3% H2O) at 600 °C due to the introduction of water. The single-cell power density with BLFZ–30 wt%BZCYYb composite cathode reached 329 mW cm2 at 750 °C, and the corresponding Rp is 0.083 Ω cm2. Based on the impedance and distribution of relaxation time analyses, proton diffusion and incorporation to the electrolyte lattice (Had+(Ni)→H+(surface,electrolyte) and H+(surface,electrolyte)→H+(bulk,electrolyte)) were the rate-limiting steps above 650 °C, whereas the oxygen incorporated into the lattice (O2−(BLFZ)→O2−(bulk,electrolyte)) and proton bulk diffusion to the triple-phase boundary (H+(bulk,electrolyte)→H+(TPB)) were the rate-limiting steps below 650 °C. The single cell with BLFZ–30 wt% BZCYYb composite cathode showed good stability at 700 °C during a 100-h test.