Structural Evolution of Ultrathin SrFeO_3-δ Films during Oxygen Evolution Reaction Revealed by In Situ Electrochemical...

We report the electrochemical stress analysis of SrFeO_3-δ (SFO) films deposited on Au substrates during oxygen evolution reactions (OERs). Our in situ analysis of Au reveals conversion reactions from Au to Au(OH)₃, AuOOH, and AuO during the OER. Au reactions cause a monotonic compressive stress on surfaces assigned to the formation of Au hydroxides and oxides. Electrochemical stress analysis of SrFeO_3-δ/Au shows a dramatically different behavior during the OER, which we attribute to structural evolutions and conversion reactions, such as the conversion of SFO to iron (oxy)hydroxides. Interestingly, electrochemical stress analysis of SrFeO_3-δ/Au shows a tensile trend, which evolves with cycling history. Electrochemical stress analysis of SFO films before the onset of the OER shows in situ changes, which cause tensile stresses when cycling to 1.2 V. We attribute these stresses to the formation of Fe^2+δO_δ(OH)_2-δ (0 ≤ δ ≤ 1.5)-type materials where δ approaches 1.5 at higher potentials. At potentials higher than 1.2 V and during OER, surface stress response is rather stable, which we assign to the full conversion of SFO to iron (oxy)hydroxides. This analysis provides insight into the reaction mechanism and details of in situ structural changes of iron perovskites during the OER in alkaline environments.