Improving Ruddlesden-Popper electrocatalysts through interstitial fluorination-driven rearrangements of local coordination environment

Anion-doping is an attractive strategy to regulate the local coordination environment of oxygen electrodes in solid oxide fuel cells (SOFCs), which is generally regarded as a substitution for O-sites. In this work, the interstitial fluorine anion-doping in Sr₃Fe₂O_7-δ, a Ruddlesden-Popper oxide with a two-layered structure, has been observed for the first time. The X-ray absorption fine structure analysis and Rietveld refined X-ray diffraction results demonstrate that the fluorine anion dopants occupy the rock-salt layer forming the stronger metal-fluorine bonds to activate oxygen ions. Density functional theory calculations further verify fluorine anions' ability to insert into tetrahedral intervals consisting of four Sr²⁺ in the rock-salt layer. The elevated proportion of O₂²⁻/O⁻ and Fe⁴⁺ are obtained, which are commonly deemed in favor of oxygen reduction reaction. The fluorine anion-doped oxygen electrode exhibits more than a 20% drop in area specific resistance measured from 600 to 750 °C. In addition, the symmetric cell with Sr₃Fe₂O_7-δF_0.1 single-phase cathode shows excellent durability within a 1000 h thermal cycling and long-term tests in air atmosphere. A high peak power density of 888 mWcm⁻² is achieved at 700 °C, showing a 35% improvement.