In Situ X-ray Absorption Spectroscopy of LaFeO 3 and LaFeO 3 /LaNiO 3 Thin Films in the Electrocatalytic Oxygen Evolution Reaction.

We study the electrocatalytic oxygen evolution reaction using in situ X-ray absorption spectroscopy (XAS) to track the dynamics of the valence state and the covalence of the metal ions of LaFeO 3 and LaFeO 3 /LaNiO 3 thin films. The active materials are 8 unit cells grown epitaxially on 100 nm conductive La 0.67 Sr 0.33 MnO 3 layers using pulsed laser deposition (PLD). The perovskite layers are supported on monolayer Ca 2 Nb 3 O 10 nanosheet-buffered 100 nm SiN x membranes. The in situ Fe and Ni K-edges XAS spectra were measured from the backside of the SiN x membrane using fluorescence yield detection under electrocatalytic reaction conditions. The XAS spectra show significant spectral changes, which indicate that (1) the metal (co)valencies increase, and (2) the number of 3d electrons remains constant with applied potential. We find that the whole 8 unit cells react to the potential changes, including the buried LaNiO 3 film.