Oxygen Vacancies Can Drive Surface Transformation of High-Entropy Perovskite Oxide for the Oxygen Evolution Reaction as Probed with Scanning Probe Microscopy

Epitaxial transition-metal oxide perovskite catalysts form a highly active catalyst class for the oxygen evolution reaction (OER). Understanding the origin of chemical dissolution and surface transformations during the OER is important to rationally design effective catalyst. These changes arise from complex interactions involving dynamic restructuring and electronic/structural adaptations. Although initial instability is common, surfaces can reach equilibrium through chemical transformations. High entropy perovskite oxides (HEPOs), which incorporate multiple 3d metal cations in near-equimolar ratios, have emerged as promising catalysts due to their enhanced OER activity compared to single-cation variants, attributed to their high configurational entropy and compositional flexibility. To advance HEPO catalyst applications, understanding the mechanisms governing their surface (in)stability is important. In this work, we examine surface degradation in epitaxial La(Cr,Mn,Fe,Co,Ni)O_3-<i>δ</i> thin films before and after OER using complementary scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM reveals tip-induced degradation of as-grown films under positive bias, attributed to oxygen anion removal and charge trapping-induced lattice degradation, demonstrating its utility as a probe for surface stability dynamics. Post-OER XPS analysis shows irreversible surface transformations from the initial epitaxial phase, characterized by 3d-metal leaching and formation of La and d-metal (oxy)hydroxides. Our findings indicate that oxygen vacancies and lattice strain trigger structural breakdown in these multi-cation perovskite surfaces during the OER, leading to surface restructuring and diminished catalytic performance compared to the as-grown epitaxial HEPO phase. This work identifies oxygen leaching as the likely primary driver of surface transformation during the OER. We show that STM offers an important tool to probe the transformation even before operando conditions, which can find use in similar material studies.