Engineering high-capacity hydrogen storage in pristine Ca 12 O 12 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.