Ni-Substituted Sr₂FeMoO_6-δ as an Electrode Material for Symmetrical and Reversible Solid-Oxide Cells

This work develops a novel perovskite Sr₂FeNi_0.35Mo_0.65O_6-δ (SFN_0.35M) simultaneously using as a fuel electrode and oxygen electrode in a reversible solid oxide cell (RSOC). SFN_0.35M shows outstanding electrocatalytic activity for hydrogen oxidation, hydrogen evolution, oxygen reduction, and oxygen evolution. In situ exsolution and dissolution of Fe-Ni alloy nanoparticles in SFN_0.35M is revealed. In a reducing atmosphere, SFN_0.35M shows in situ exsolution of Fe-Ni alloy nanoparticles, and then the Fe-Ni alloy is reoxidized into SFN_0.35M while converting into an oxidizing atmosphere. The polarization resistances of SFN_0.35M electrode are 0.043 Ω cm² in 20% O₂-N₂ and 0.064 Ω cm² in H₂ at 850 °C. Moreover, symmetric fuel cells using the SFN_0.35M electrode achieves a maximum power density of 0.501 W cm^-2 at 850 °C in H₂ fuel, while the symmetric electrolysis cell has an electrolysis current density of 0.794 A cm^-2 at 1.29 V in 90% H₂O-10% H₂ at 850 °C. It is the first time we demonstrate that the cell voltage of symmetrical cell at 0.5 A cm^-2 in the fuel cell mode and -0.5 A cm^-2 in the electrolysis cell mode can be fully recovered in 10 electrode alternating cycles and therefore demonstrate the possibility that SFN_0.35M can be used in a fully symmetric RSOC stack with electrode alternating functions.