Porous ices with ultralow-density assembled from water polyhedral cages and ice nanotubes

Porous ice represents an emerging class of crystalline ice phases characterized by extensive nano-cavities and/or nano-channels within their hydrogen-bonded water frameworks. The prediction of ultralow-density porous ices is of particular interest, as their high surface-to-volume ratio could significantly expand their practical applications. In this work, we assembled two series of ultralow-density porous ices from polyhedral water cages and ice nanotubes, designated as SOD_n and LTA_n, respectively. Their density decreases progressively with the increasing length of the constituent ice nanotubes. The SOD_n structures were predicted to be stable under negative pressure by first-principles calculations using the vdW-DF2 functional. By evaluating the mechanical stability of these porous ices, we reconstructed a new water phase diagram under negative pressure. Furthermore, the guest-assisted growth of the SOD framework and the formation of SOD_n (n = 1-4) were demonstrated by extensive molecular dynamics simulations. Notably, SOD_5, with a remarkably low density of 0.2 g/cm3, exhibits promising gas storage performance at 100 bar and 77 K. Its predicted capacities are 36.29 wt. % (105.88 g/l) for H2 and 0.65 g/g (489 cm3 [STP]/cm3) for CH4. These results demonstrate the potential of ultralow-density porous ice as a novel medium for gas storage.