Engineering high-capacity hydrogen storage in pristine Ca₁₂O₁₂ nanocages via cooperative adsorption

A comprehensive in silico investigation of pristine Ca₁₂O₁₂ nanocages as potential hydrogen storage materials is presented. Our study focuses on assessing the structural, electronic, and adsorption properties of Ca₁₂O₁₂ relative to its Be₁₂O₁₂ and Mg₁₂O₁₂ analogs. The larger internal cavity, resulting from increased interatomic distances in Ca₁₂O₁₂, not only supports stable endohedral adsorption—a capability not observed in Be₁₂O₁₂ or Mg₁₂O₁₂—but also facilitates a greater number of exohedral H₂ adsorption events compared to the more compact nanocages. Notably, our results reveal a clear preference for end-on coordination in Ca12O12, in contrast to the side-on adsorption observed for the smaller cages. Although the overall adsorption process remains exothermic across a broad range of hydrogen loadings, the most favorable cooperative effects occur at moderate coverages, with the lowest normalized adsorption enthalpy observed at 13H₂ molecules and favorable interactions maintained up to 32. Additionally, mixed adsorption configurations incorporating one endohedral H2 alongside multiple exohedral H₂ molecules yield a maximum hydrogen uptake of 9.24 wt%, exceeding the U.S. DOE target of 5.5 wt%. These findings highlight the potential of pristine Ca₁₂O₁₂ nanocages as effective hydrogen storage media and offer strategic guidance for the development of high-performance storage systems.