Modulating Metal–Oxygen Bond Energy by Valence State Engineering in 2D High Entropy Oxides for Enhanced Water Electrolysis

Abstract

Valence state engineering has emerged as a powerful strategy to optimize catalytic performance by modulating the electronic structure of metal active sites. However, the valence state regulation in high-entropy compounds (HECs) remains elusive due to their complex multi-element components and electronic interactions. Here, the valence states of different metals in two-dimensional (2D) high entropy oxide (HEO) (FeNiMoRuV)O_2−x are precisely modulated through controlled pyrolysis of corresponding 2D high entropy hydroxide (HEHO) (FeNiMoRuV)(OH)₂ under varying temperatures. Temperature-controlled pyrolysis selectively reduces the oxidation state of Ru, while simultaneously increasing the valence state of other constituent metals (Fe, Ni, Mo, and V), suggesting a competitive redox equilibrium. Notably, these low-valence Ru sites with oxygen vacancy in 2D HEO significantly reduce Ru–O bond energy and promote the generation of O–*O intermediates, thereby enabling oxygen evolution with a lattice oxygen mediated-oxygen vacancy site mechanism. 2D HEO with low-valence Ru exhibits superior electrolytic water performance (HER/OER) compared to HEHO and other HEO with high-valence Ru, achieving a current density of 1000 mA cm⁻² at 1.923 V, which exceeds the commercial Pt/C||RuO₂ system. Therefore, this study reveals the valence state regulatory mechanism of HECs and provides a solid hammer for the catalytic mechanism of valence state engineering.