Ordering and phase control in epitaxial double-perovskite catalysts for the oxygen evolution reaction

The complex oxide compound praseodymium barium cobalt oxide (PBCO) is an efficient catalyst for the oxygen evolution reaction (OER) during electrochemical water splitting, with an activity that is mainly ascribed to PBCO's inherent atomic structure and band alignment. Here, we report on epitaxial PBCO thin films showing electrocatalytic properties, with current densities of up to 10 mA/cm² at 1.8 V vs RHE. Dense PBCO thin films are synthesized in a disordered perovskite phase as well as in a coherently oxygen vacancy ordered (double) perovskite phase, in which oxygen vacancies are incorporated in every second CoO_2-δ atomic plane along the out-of-plane direction. The transition from disordered to ordered growth occurs with temperature control during the growth process and can be directly monitored in situ by means of reflection high-energy electron diffraction. The epitaxial fabrication process allows the control of the structure and phase of the oxide catalysts, providing model systems for exploring structure-property relations and atomistic processes of catalysis during the OER. For all structural compositions, we demonstrate remarkably similar catalytic properties, indicating a negligible effect of the structural bulk phase on OER catalysis. Rational design routes for perovskite catalysts derived merely from bulk properties should therefore be met with suspicion.