Surface Restructuring and Stability of Perovskite Oxide Electrocatalysts Studied by Surface X-Ray Diffraction and Grazing Incidence X-Ray Absorption Spectroscopy

In recent years, the trend towards clean and renewable energy sources has led to an increased interest in water-based electrocatalysis (i.e., producing green hydrogen from water as fuels and chemicals) for energy conversion and storage, but a key barrier for efficient water splitting is the high overpotential of the sluggish oxygen evolution reaction (OER).^1-3 To overcome this, earth-abundant perovskite oxides of chemical formula AMO₃ with compositional substitutions have shown drastically improved OER activities and are particularly attractive due to their high activity, low cost, high tunability of composition, and controllable electronic structures.^2,3 For many metal oxides it was discovered that the surface can reconstruct under the oxidative conditions imposed by OER, forming (hydr)oxides prior to the onset of the reaction, and resulting in a different surface termination than that expected from the bulk. This restructuring is varied among materials and plays a critical role in determining the stability and activity of an electrocatalyst material during and after electrochemical cycling. Thus, understanding the drivers of transformation at electrocatalyst interfaces towards the development of materials design is a key research direction in many fields.¹ In this work we examine the impact of electrochemical cycling on surface reconstruction of Lanthanum Nickel Iron Oxide (LaNi_1-xFe_xO₃: x=0-0.375) and Lanthanum Strontium Nickel Iron Oxide (La_0.5Sr_0.5Ni_1-xFe_xO₃: x=0-0.625) epitaxial thin films. Surface X-ray diffraction (SXRD) is employed to investigate the relationship between complex oxide bulk composition and terminal surface OER activity and stability. X-ray reflectivity (XRR) is used to probe the electron density of surface layers and crystal truncation rod (CTR) is used to study atomic reconstruction at the surface. In select compositions, in-situ XRR and CTR illuminate the reconstruction and amorphization process during cycling under OER conditions. Furthermore, grazing incidence X-ray absorption spectroscopy (GIXAS) is performed to capture the evolution of local coordination environments with increasing compositional substitutions and soft XAS is used to explore local electronic structures. Our findings uncover the role of underlying bulk descriptors in modulating OER performance through cycling-induced restructuring and unearth the fundamental driving forces behind surface transformations in perovskite oxide materials which will provide invaluable understanding to aid in the development of electrocatalytic surfaces under OER conditions for effective materials design towards high-performance electrolyzers and batteries for renewable energy storage and conversion.<br/><br/><b>References</b><br/>1. Baeumer, C., et al. (2021). <i>Nature Materials</i>, <i>20</i>(5), 674–682.<br/>2. Liu, D., et al. (2021). <i>Small </i>(Vol. 17, Issue 43).<br/>3. Song, H. J., et al. (2021). <i>Advanced Energy Materials</i> (Vol. 11, Issue 27).