Rotating Ring–Disk Electrode Study of Oxygen Evolution at a Perovskite Surface: Correlating Activity to Manganese Concentration

Transition-metal oxides with the perovskite structure are promising catalysts to promote the kinetics of the oxygen evolution reaction (OER). To improve the activity and stability of these catalysts, a deeper understanding about the active site, the underlying reaction mechanism, and possible side reactions is necessary. We chose smooth epitaxial (100)-oriented La0.6Sr0.4MnO3 (LSMO) films grown on Nb:SrTiO3 (STNO) as a model electrode to investigate OER activity and stability using the rotating ring−disk electrode (RRDE) method. Careful electrochemical characterization of various films in the thickness range between 10 and 200 nm yields an OER activity of the epitaxial LSMO surface of 100 μA/cm2ox at 1.65 V vs RHE, which is among the highest reported for LSMO and close to (110)-oriented IrO2. Detailed post-mortem analysis using XPS, XRD, and AFM revealed the high structural and morphological stability of LSMO after OER. The observed correlation between activity and Mn vacancies on the surface suggested Mn as the active site for the OER in (100)-oriented LSMO, in contrast to similar perovskite manganites, such as Pr1–xCaxMnO3. The observed Tafel slope of about 60 mV/dec matches the theoretical prediction for a chemical rate-limiting step that follows an electrochemical pre-equilibrium, probably O–O bond formation. Our study established LSMO as an atomically flat oxide with high intrinsic activity and high stability.