The Electronic Structure of Transition Metal (TM) Oxides for the Oxygen Evolution Reaction: The Critical Role of TM 3d Hole State

Abstract

Electrolysis of water to produce hydrogen and oxygen is a promising pathway for the storage of renewable energy in form of chemical fuels. The efficiency of the overall process is usually limited by the sluggish kinetics of the oxygen evolution reaction (OER) due to a complex four-electron/proton transfer mechanism. Therefore, the most crucial step for water electrolysis to become a widespread industrial process is to develop efficient electrocatalysts capable of driving the OER at a low overpotential.[1,2] In this talk, I will summaries the key electronic features that induces high catalytic activity towards the oxygen evolution reaction of trivalent transition metal (TM) oxides with perovskite structure, such as LaCoO3 [3] and LaFeO3.[4] The partial substitution of La for Sr in LaTMO3 results in the oxidation of TM3+ to TM4+ and substantially enhances OER catalytic activity. A comprehensive X-ray spectroscopic study, based on in-situ XPS and XAS, reveals a strong correlation of the enhanced OER catalytic activity with the presence of cations in the 4+ oxidation state. Increasing the concentration of TM cations in the 4+ oxidation state leads to a more covalent TM–O bond due to higher TM 3d–O 2p orbital hybridization and shifts the energy position of the valence band (VB) closer to the Fermi level (E F). Such an electronic modulation optimizes the surface adsorption energetics of *OH intermediates, contributing to faster OER kinetics. Furthermore, the oxidation of TM3+ to TM4+ creates a new unoccupied state (hole state) below the conduction band, associated with the formation of TM 3d empty electronic states. This hole states reduce the energy barrier for electron transfer associated with the OER, as illustrated in the figure below, thereby facilitating charge transfer at the interface during the reaction.Figure 1: Key electronic features of transition metal (TM) oxides for the oxygen evolution reaction