Oxygen Interstitial Atoms as Diagnostics and Potential Participants in the Electrochemical Oxygen Evolution Reaction

Much remains unknown about the electrochemical oxygen evolution reaction (OER) catalyzed by metal oxides. Some proposed mechanisms posit participation of lattice oxygen from the oxide, mediated in part by diffusive exchange of oxygen atoms with the bulk oxide. Little consideration has been given to the lower thermodynamic formation energies for O interstitials than O vacancies at the surface of some metal oxides. Furthermore, lowered chemical coordination at clean metal oxide surfaces facilitates the creation of interstitial O atoms (O_i) from adsorbed O atoms with energy barriers near or even below 1 eV.¹ The atomic configurations for interstitial injection resemble those for site hopping in the bulk, with barriers only slightly higher. The modest hopping barriers of O_i in oxides, coupled with those for injection, make clean surfaces effective pathways for populating the nearby bulk with O_i near room temperature. Surfaces of several different oxides generate the requisite adsorbed O when submerged in liquid water. Since adsorbed O is a vital intermediate species in the OER, the injection phenomenon may be used to probe the surface concentration of this species. Furthermore, it is possible that O_i participates in the reaction mechanism itself. Using oxygen in rutile TiO₂ single crystals as a model system, we describe isotopic self-diffusion measurements near room temperature to show that (1) use of a conventional 3-electrode electrochemical cell accelerates the injection rate of O_i, and (2) dissolved O₂, if present, contributes a significant fraction of the injected O_i. We greatly refine interpretation of the experimental results via multiscale modeling that couples atomic-scale simulations of O_i injection and diffusion by density functional theory with a mesoscale microkinetic representation of O_i diffusion, lattice exchange and trapping within the solid. In addition, we present experimental evidence that even good-quality single crystal surfaces present a variety of sites exhibiting a broad spread of activities for full water deprotonation and/or injection.<br/> <br/>References<br/><br/>1. Heonjae Jeong, Elif Ertekin and Edmund G. Seebauer, “Surface-Based Post-synthesis Manipulation of Point Defects in Metal Oxides Using Liquid Water,” <i>ACS Appl. Mater. Interfaces</i>, <b>14</b> (2022) 34059-34068.