An efficiently tuned d-orbital occupation of IrO2 by doping with Cu for enhancing the oxygen evolution reaction activity

Abstract

The oxygen evolution reaction (OER) has been regarded as a key half reaction for energy conversion technologies and requires high energy to create OO bonds. Transition metal oxides (TMOs) seem to be a promising and appealing solution to the challenge because of the diversity of their d-orbital states. We chose IrO₂ as a model because it is universally accepted as a current state-of-the-art OER catalyst. In this study, copper-doped IrO₂, particularly Cu_0.3Ir_0.7O_δ, is shown to significantly improve the OER activity in acidic, neutral and basic solutions compared to un-doped IrO₂. The substituted amount of Cu in IrO₂ has a limit described by the Cu_0.3Ir_0.7O_δ composition. We determined that the performance of Cu_0.3Ir_0.7O_δ is due primarily to an increase in the Jahn–Teller effect in the CuO₆ octahedra, and partially to oxygen defects in the lattice induced by the IrO₆ octahedral geometric structure distortions, which enhance the lift degeneracy of the t_2g and e_g orbitals, making the d_z² orbital partially occupied. This phenomenon efficiently reduces the difference between ΔG2 and ΔG3 in the free energy from the density functional theoretical (DFT) calculations and can yield a lower theoretical overpotential comparable to that of IrO₂. The proposed method of doping with foreign elements to tune the electron occupation between the t_2g and e_g orbital states of Ir creates an opportunity for designing effective OER catalysts using the TMO groups.