Revealing the Intrinsic Oxygen Evolution Reaction Activity of Perovskite Oxides across Conductivity Ranges Using Thin Film Model Systems

The development of efficient electrocatalysts in water electrolysis is essential to decrease the high overpotentials, especially at the anode where the oxygen evolution reaction (OER) takes place. However, establishing catalyst design rules to find the optimal electrocatalysts is a substantial challenge. Complex oxides, which are often considered as suitable OER catalysts, can exhibit vastly different conductivity values, making it challenging to separate intrinsic catalytic activities from internal transport limitations. Here, we systematically decouple the limitations arising from electrical bulk resistivity, contact resistances to the catalyst support, and intrinsic OER catalytic properties using a systematic epitaxial thin film model catalyst approach. We investigate the influence of the resistivity of the three perovskite oxides LaNiO_3-δ (3.7 × 10^-4 Ω cm), La_0.67Sr_0.33MnO_3-δ (2.7 × 10^-3 Ω cm), and La_0.6Ca_0.4FeO_3-δ (0.57 Ω·cm) on the observed catalytic activity. We tuned the electron pathway through the catalyst bulk by comparing insulating and conductive substrates. The conducting substrate reduces the electron pathway through the catalyst bulk from the millimeter to nanometer length scale. As we show, for the large electron pathways, the observed catalytic activity scales with resistivity because of a highly inhomogeneous lateral current density distribution. At the same time, even on the conducting substrate (Nb-doped SrTiO₃), large contact resistances occur that limit the determination of intrinsic catalytic properties. By inserting interfacial dipole layers (in this case, LaAlO₃) we lifted these interface resistances, allowing us to reveal the intrinsic catalytic properties of all examined catalysts. We find that La_0.6Ca_0.4FeO_3-δ and LaNiO_3-δ exhibit a similar intrinsic overpotential of 0.36 V at 0.1 mA/cm², while their resistivities differ by 3 orders of magnitude. This finding shows that optimizing the electron pathway of the OER catalyst can lead the way to find new structure-activity relationships and to identify high-activity catalysts even if the electronic resistance is high.