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||RuO 2 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.