Tuning the transport properties of gases in porous graphene membranes with controlled pore size and thickness

Porous graphene membranes emerged as promising alternatives for gas separation applications due to their atomic thickness enabling ultra-high permeance, but suffer from low gas selectivity. Whereas decreasing the pore size below 3 nm is expected to increase the gas selectivity due to molecular sieving, it is rather challenging to generate large number of uniform small pores on the graphene surface. Here, we introduce a pore narrowing approach via gold deposition onto porous graphene surface to tune the pore size and thickness of the membrane to achieve large number of small pores. Through our systematic approach, we determined the ideal combination as pore size below 3 nm obtained at the thickness of 100 nm to attain high selectivity and high permeance. The resulting membrane showed a H₂/CO₂ separation factor of 31.3 at H₂ permeance of 2.23x10⁵ GPU (1 GPU = 3.35x10^-10 mol s^-1 m^-2 Pa^-1), which is the highest value reported to date in the 10⁵ GPU permeance range. This result is explained by comparing the predicted binding energies of gas molecules with the Au surface, -5.3 vs -21 kJ mol^-1 for H₂ and CO₂, respectively, increased surface-gas interactions and molecular sieving effect with decreasing pore size.