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

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-HfO₂, space group: P6₃mc (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-HfO₂). For a stark comparison, the hydrogen spillover is strongly forbidden when using stable monoclinic phase HfO₂ (M-HfO₂, space group: P2₁/c (14), coordination number: seven) as the substrate. When applied in an acidic hydrogen evolution reaction (HER), Ru/Hex-HfO₂ 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-HfO₂, indicating that H is more likely to spillover on Hex-HfO₂. Furthermore, the strong interaction between Ru and Hex-HfO₂ 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.