An enhanced oxygen evolution reaction on 2D CoOOH via strain engineering: an insightful view from spin state transition

Abstract

Cobalt oxyhydroxide (CoOOH) has attracted great attention in electrochemical water splitting. However, the mechanism behind its catalytic performance and how to improve its activity are still under debate. In the work, we propose that strain engineering is an effective and simple way to achieve the purpose. Based on density functional theory (DFT), we investigate the effects of strain engineering on the electronic structure and catalytic performance of two-dimensional (2D) CoOOH and the underlying mechanism of the oxygen evolution reaction (OER). We find that strain engineering is effective to tailor the electronic configuration of Co³⁺ ions in CoOOH, which can be transferred from low spin (LS: t⁶_2ge⁰_g) to high spin (HS: t⁴_2ge²_g) at a tension of 9%. Importantly, we show that LS CoOOH is a poor OER catalyst, because it is ineffective for O₂ release with a large energy (1.35 eV). However, HS CoOOH is much more active in the OER because of smaller O₂ release energy (0.03 eV) and more effective O–O bond coupling (1.21 eV) in the intramolecular oxygen coupling mechanism. The overpotential for LS CoOOH is 0.66 V according to the hydroxide ion attack mechanism, while the direct intramolecular coupling is hard to occur. HS CoOOH shows low overpotentials, 0.32 and 0.5 V, for the intramolecular coupling and hydroxide ion attack, respectively, which are comparable to those of the best OER catalysts (0.25 to 0.4 V). Our work demonstrates that the spin state transition of Co³⁺ ions tuned by strain engineering is an effective way to improve the OER activity of 2D CoOOH.