Freezing Hydrogen in Nanoconfinement

Abstract

Liquid hydrogen confined in pores of nanofoams crystallizes at lower temperatures than in the unconfined, bulk state. Here, we summarize results of our recent systematic relaxation calorimetry studies of the liquid–solid phase transition of hydrogen and deuterium in various materials with open-cell pores. These include spinodal-decomposition-derived silica glasses and nanoporous gold, conventional silica aerogels, and carbon foams with ligaments made from nanotubes and graphene sheets, all of which were studied previously. We present new hydrogen thermoporometry data for polymeric norbornene-based aerogels. Results show that hydrogen freezing temperatures inside all the porous materials studied are depressed. The average depression of the freezing point scales linearly with the ratio of the internal surface area to the pore volume. The average freezing point depression is limited to ≲1.6 K for foams with monolith densities ≲50 mg·cm. Details of the freezing behavior, however, depend nontrivially on the choice of the porous material and on the hydrogen-filling fraction, reflecting phenomena that are beyond the Gibbs-Thomson formalism and pointing to the complexity of pore architectures in the low-density materials of interest to thermonuclear fusion energy applications.