Redox Behavior and Transport Properties of Composites Based on (Fe,Ni)3O4 ± δ for Anodes of Solid Oxide Fuel Cells

Abstract

The Fe–Ni–O system designed for producing bimetal-containing composite anodes of solid oxide fuel cells (SOFCs) was studied. The solubility of nickel in the structure of spinel (Fe,Ni)₃O_{4 ± δ} at atmospheric oxygen pressure is ~1/3. Moderate reduction at 1023 K and p(O₂) ≈ 10^–20 atm leads to partial decomposition of spinels, forming an electron-conducting phase (Fe,Ni)_1–yO and submicron bimetallic Fe–Ni particles on the oxide surface, which have potentially high catalytic activity. The electron conductivity has a thermally activated character and increases substantially during the reduction. In the anode conditions of SOFCs, the electric conductivity reaches 30–100 S/cm, while the thermal expansion coefficients are ~12 × 10^–6 K^–1, which ensures compatibility with solid electrolytes. At the same time, significant volume changes during the redox cycling (up to ~1% on the linear scale) necessitate the introduction of additional components such as yttria-stabilized zirconia (YSZ). The polarization resistance of the model composite anode of reduced Fe₂NiO_{4 ± δ} and YSZ deposited on the YSZ solid electrolyte membrane was ~1.8 Ohm cm² at 923 K in a 4% H_2–Ar–H₂O atmosphere.