Graphene-Coated Halloysite Nanoclay Membrane for the Enhanced Separation of Hydrogen from a Hydrogen-Helium Mixture

This study highlights the separation of hydrogen from H₂-He mixture gas by a graphene-coated halloysite nanoclay membrane. The graphene-coated clay membrane along with its pure clay counterpart is successfully developed and studied for gas separation using hydrogen (H₂)-helium (He) single and mixture gases. Hydrothermal and nonhydrothermal methods were applied for the synthesis of a ″coated″ membrane on a porous alumina substrate from the graphene and halloysite clay. To date, nanoporous zeolites are the potential materials for gas separation based on a molecular sieving mechanism. A similar separation mechanism for hydrogen and helium from mixture gases may not work efficaciously due to the closeness of their kinetic diameter (H₂: 2.89 Å and He: 2.6 Å). The presence of defects and torn nanopores between graphene layers along with the different surface charges of the inner and outer layer of halloysite nanotubes facilitates the ″coated″ membrane to show an appreciable H₂/He separation factor of ∼4 using H₂-He (1:1) mixture gas compared to 2.86 for the pure halloysite membrane. The available charge layer of graphene also has a significant contribution for this increased H₂/He selectivity value. The permeate flux of H₂ and He through both the graphene-coated clay membrane and pure clay membrane has also been noted. The permeate flux of pure H₂ and He was 2 × 10^-7 and 1.3 × 10^-7 mol m^-2 s^-1 Pa^-1 for the clay membrane, whereas for the ″coated″ clay membrane, the values changed to 0.1 × 10^-7 and ∼0.05 × 10^-7 mol m^-2 s^-1 Pa^-1 at 100 kPa, respectively.