Operando HERFD-XAS of Bimetallic Perovskite Thin Film Interfaces

We investigated the valence dynamics of thin films during the electrocatalytic oxygen evolution reaction using operando high-energy-resolution fluorescence-detected (HERFD) X-ray absorption spectroscopy (XAS) in a 0.1 M KOH solution under an applied potential. We show that it is possible to measure the 10 unit cell (∼4 nm) bimetallic perovskite oxide La₂CoMnO₆ thin film grown on Pt-covered SiN_x membranes. Operando Co and Mn K-edge HERFD XAS spectra on the active surface species were recorded from the backside of the SiN_x membrane. The HERFD XAS Co K-edge spectra reveal a (partial) increase in the oxidation state from Co²⁺ in ex situ conditions to mainly Co³⁺ in the solid-liquid open-circuit voltage (OCV). Voltage-dependent Co K-edge HERFD XAS white line intensities show that Co is oxidized toward the higher oxidation state, while Mn remains Mn⁴⁺. Interestingly, the Co and Mn main edge shifts with the average valence change of 0.20; the valence of Mn is constant, implying that the valence of Co increases by 0.40. The OCV spectra were reversible when switching off the potentials. These observations clearly identify element-specific valence dynamics in the intrinsic oxygen evolution reaction.