Revealing the critical pore size for hydrogen storage via simultaneous enclathration and physisorption in activated carbon†

Hydrogen hydrate is regarded as an ideal hydrogen storage medium, but it faces unsolved challenges related to its extreme formation and stabilization conditions. Recent experiments have demonstrated that utilizing porous host materials can ease hydrate formation conditions and enhance stability. However, there is an urgent need to examine the nanoscopic interactions between hydrogen hydrates and solid porous materials to understand and optimize the mechanisms of hydrogen storage in the combined system. Here, the effect of pore size on hydrogen storage in hydrates confined in porous activated carbon is explored through molecular dynamics simulations, with initial validation by ab initio calculations. The results revealed a critical pore size of ∼2 nm, below which hydrogen hydrate decomposes. This critical size is primarily influenced by temperature and oxygen surface groups, while being largely unaffected by other properties of the activated carbon host. Furthermore, pockets of physisorbed H₂ gas were found to occupy the smallest pores (around 1 nm). The results revealed a previously unreported dual-storage mechanism for H₂ gas in hierarchical porous structures, where H₂ can simultaneously be stored in micropores through physisorption and larger meso- and macro-pores through enclathration. Given further research and experimental verification, the hybrid mechanism could enable hydrogen storage capacities in hydrate-filled porous media to match or exceed that of pure hydrogen hydrate, while simultaneously offering milder formation and stability conditions.