The significant effect of the phase composition on the oxygen reduction reaction activity of a layered oxide cathode

Layered oxides of Sr₄Fe₄Co₂O₁₃ (SFC2) which contains alternating perovskite oxide octahedral and polyhedral oxide double layers are attractive for their mixed ionic and electronic conducting and oxygen reduction reaction properties. In this work, we used the EDTA-citrate synthesis technique to prepare SFC2 and vary the calcination temperature between 900 and 1100 °C to obtain SFC2, containing different phase content of perovskite (denoted as SFC-P) and (Fe,Co) layered oxide phases (SFC-L). Rietveld refinements show that the SFC-P phase content increased from ∼39 wt% to ∼50 wt% and ∼61 wt% as the calcination temperature increased from 900 °C (SFC2-900) to 1000 °C (SFC2-1000) and 1050 °C (SFC2-1050). At 1100 °C (SFC2-1100), SFC-P became the dominant phase. The oxygen transport properties (e.g. oxygen chemical diffusion coefficient and oxygen permeability), electrical conductivity and oxygen reduction reaction activity is enhanced in the order of SFC2-1000, SFC2-1100 and SFC2-1050. The trend established here therefore negates the hypothesis that the perovskite phase content correlates with the oxygen transport property enhancement. The results suggest instead that there is an optimum composition value (e.g. 61 wt% of SFC-L for SFC2-1050 in this work) on which synergistic effects take place between the SFC-P and SFC-L phase.