Operando HERFD-XAS of Bimetallic Perovskite Thin Film Interfaces

Area of Science:

• Materials Science
• Electrochemistry
• Spectroscopy

Background:

• Understanding the dynamic changes in material properties during electrochemical reactions is crucial for developing efficient catalysts.
• Thin films of bimetallic perovskite oxides are promising candidates for electrocatalytic applications, particularly the oxygen evolution reaction (OER).

Purpose of the Study:

• To investigate the valence state dynamics of cobalt (Co) and manganese (Mn) in La2CoMnO6 thin films during the electrocatalytic oxygen evolution reaction.
• To correlate these valence changes with applied potential and reaction conditions using advanced spectroscopic techniques.

Main Methods:

• Operando high-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XAS) was employed to probe the electronic structure of the thin film.
• Spectra were recorded at Co and Mn K-edges in situ during OER in a 0.1 M KOH solution under applied potential.
• Measurements were performed on a 10 unit cell (approx. 4 nm) La2CoMnO6 thin film grown on SiNx membranes, with XAS data collected from the backside.

Main Results:

• HERFD XAS spectra indicated an increase in the oxidation state of Co from Co2+ (ex situ) to predominantly Co3+ under open-circuit voltage (OCV) conditions.
• Voltage-dependent studies showed Co oxidation towards a higher state, while Mn remained primarily as Mn4+.
• An average valence change of 0.20 was observed for Co and Mn combined, with the Co valence increasing by 0.40 while Mn valence remained constant.
• The observed spectral changes were reversible upon removal of the applied potential.

Conclusions:

• The study successfully identified element-specific valence dynamics of Co and Mn in La2CoMnO6 thin films during the intrinsic oxygen evolution reaction.
• The findings highlight the distinct roles of Co and Mn in the catalytic process, with Co undergoing significant oxidation state changes.
• Operando HERFD XAS is demonstrated as a powerful tool for elucidating the reaction mechanisms of advanced electrocatalysts at the nanoscale.