Monolayer methane hydrate formation in 2D confinement with multiple plastic phases and low superionic pressure

Confined gas and ionic hydrates play vital roles in energy storage, carbon capture, and water desalination. Yet, the fundamental interactions between water films and hydrophobic molecules remain poorly understood. Here, we investigated nanoconfined monolayer methane hydrates encapsulated within graphene capillaries, exploring their phase behavior through crystal structure prediction combined with a machine-learning force field. We identified a thermodynamically stable two-dimensional tetragonal compound CH4(H2O)4 under moderate pressures. Its hydrogen bonding network markedly differs from that of 2D pure or porous ice, giving rise to multiple plastic phases in which methane and water molecules rotate. Remarkably, 2D CH4(H2O)4 transitions into a superionic state featuring proton diffusion at pressures as low as ~3 GPa, substantially lower than that required for 2D ice. The calculated phase diagram further reveals that CH4 molecule incorporation elevates the melting temperatures above 350 K while reducing the onset pressure for superionicity. These findings provide fundamental insight into hydrophobic gas hydrate under nanoscale confinement and open new avenues for applications in energy storage and hydrocarbon capture.