2D Metastable‐Phase Hafnium Oxide Triggers Hydrogen Spillover for Boosting Hydrogen Production

Abstract Hydrogen (H) manipulation plays a significantly important role in many important applications, in which the occurrence of hydrogen spillover generally shows substrate‐dependent behavior. It therefore remains an open question about how to trigger the hydrogen spillover on the substrates that are generally hydrogen spillover forbidden. Here a new metastable‐phase 2D edge‐sharing oxide: six‐hexagonal phase‐hafnium oxide (Hex‐HfO ₂ , space group: P6 ₃ mc (186)) with the coordination number of six is demonstrated, which serves as an ideal platform for activating efficient hydrogen spillover after loading Ru nanoclusters (Ru/Hex‐HfO ₂ ). For a stark comparison, the hydrogen spillover is strongly forbidden when using stable monoclinic phase HfO ₂ (M‐HfO ₂ , space group: P2 ₁ /c (14), coordination number: seven) as the substrate. When applied in an acidic hydrogen evolution reaction (HER), Ru/Hex‐HfO ₂ exhibits a low overpotential of 8 mV at 10 mA cm ⁻² and a high Ru utilization activity of 14.37 A mg _Ru ⁻¹ at 30 mV. Detailed mechanism reveals the positive H adsorption free energy on Hex‐HfO ₂ , indicating that H is more likely to spillover on Hex‐HfO ₂ . Furthermore, the strong interaction between Ru and Hex‐HfO ₂ optimizes the desorption of hydrogen intermediate, thus facilitating the surface H spillover. The discovery provides new guidance for developing metastable‐phase oxide substrates for advanced catalysis.