Unlocking Hydrogen Spillover: Dynamic Behavior and Advanced Applications

Hydrogen spillover, the simultaneous diffusion of protons and electrons, has recently emerged as a key phenomenon in the functionalization of hydrogen in cutting-edge research fields. Its occurrence has been found to significantly impact hydrogen-related fields of science, such as catalysis, reduction, and hydrogen storage. Since the discovery of hydrogen spillover more than half a century ago, although many scientists have reported its unique properties and have attempted to utilize them, no practical advanced applications have been established yet. The biggest issue in realizing such applications is unraveling how spilled atomic hydrogen behaves. Although observation techniques have greatly improved in recent years, a comprehensive understanding of the behavior of spilled hydrogen, such as which pathways it follows and at what temperature it occurs, has not yet been achieved. This is because its behavior can vary depending on the characteristics of the platform materials. Uncovering the dynamics of the hydrogen spillover phenomenon is expected to pave the way toward the creation of new and versatile hydrogen-handling technologies.

In this Account, we report the comprehensive dynamic behavior of spilled hydrogen on various platform materials and potential advanced applications. For reducible metal oxides, which are the ideal platform for hydrogen spillover, the diffusion pathway for spilled hydrogen is found to depend on the platform material. For TiO₂ and CeO₂, the preferential diffusion pathway is along the surface, whereas for WO₃ it is through the bulk region. For graphene oxide, the ether groups generated by calcination in air enable energetically feasible hydrogen spillover on its basal plane. In the case of MgO, a moderate amount of Al doping provides abundant hydrogen spillover pathways within the bulk of the material.

Hydrogen spillover induces a strong reduction field on the surface of platform materials, leading to simultaneous reduction of metal ions with different redox potentials. This facilitates the fabrication of nonequilibrium alloy nanoparticles composed of two types of elements with a positive mixing enthalpy, such as Ru–Ni and Rh–Cu. This strategy can be applied to multiple kinds of metal ions and enables the facile synthesis of high-entropy alloy nanoparticles, which exhibit unique catalytic properties. This review establishes guidelines for utilizing the material-dependent behavior of hydrogen spillover and describes advanced applications.