Ordering and Phase Control in Epitaxial Double-Perovskite Catalysts for the Oxygen Evolution Reaction

Abstract

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/cm 2 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.

Epitaxial catalysts for oxygen evolution reaction: model systems and beyond

“…For one, epitaxial growth processes such as PLD allow to stabilize materials and phases away from thermodynamic equilibrium. In this way, for example, coherently ordered chains of oxygen vacancy defects have been controlled and stabilized in epitaxial double-perovskite OER catalysts, such as described in [13]. As could be shown, catalytic activity and overpotential of these compound is primarily decoupled from its polymorphous phase.…”

“…In recent literature, it was shown that it is possible to synthesize e.g. SrIrO 3 or (Pr,Ba)CoO 3−δ layers with a surface roughness below 2 nm at a layer thickness of about 100 nm [2,13], while even subnanometer surface smoothness was achieved for (La,Sr)CoO 3−δ at a similar thickness [14]. The growth process can be monitored and controlled by means of electron diffraction techniques allowing layer deposition with single monolayer precision.…”

“…Critical catalyst properties such as electrical conductivity and transport behavior can be measured as an independent property of the catalyst material in absence of chemical admixtures, and in a defined geometry allowing four-point-probe sheet resistance and Hall characterization [13,14]. Starting from a defined single crystalline catalyst, phase stability and amorphization processes during OER may be monitored based on model catalysts ( figure 1(B)).…”

“…Similar to all complex oxides, epitaxial thin film catalysts contain defects, even under ideal growth conditions. Typical defects range from oxygen and cation non-stoichiometry, but can also contain more extended defect structures, particularly stacking faults and dislocations, potentially arising from strain relaxation, non-ideal growth conditions, or as a thermodynamic requirement under applied growth conditions [13,14]. The grade of defectiveness, however, can typically be systematically controlled through a variation of growth parameters, enabling to study the effect of a defined defect structure on the catalyst properties based on a model system approach [13].…”

Engineering of the d‐Band Center of Perovskite Cobaltite for Enhanced Electrocatalytic Oxygen Evolution

“…Perovskite oxides (ABO 3 ), which incorporate rare‐earth cations (A‐site) and transition metal cations (B‐site), have been recognized as potential alternatives noble‐metal electrocatalysts owing to its high ionic and structure flexibility . In previous studies, LaCoO 3 (LCO) was found to be a durable and active OER catalyst .…”