Gas Diffusion Mechanism in Two-Dimensional Lamellar Membranes

Two-dimensional (2D) materials, e.g., graphene oxide (GO), covalent organic frameworks (COF), based lamellar membranes have shown outstanding gas separation properties, surpassing upper bound for polymeric membranes especially in hydrogen separation. However, it is challenging to directly and accurately measure gas diffusion coefficients in such ultra-thin laminated membranes. As a result, the gas diffusion mechanism therein remains unclear due to lack of diffusivity and discussion of thickness dependent gas separation properties reported in the literature. In this work, the authors developed a time lag method to determine the individual diffusivities of mixed gases in 2D lamellar membranes by on-line mass spectrometry. Pure gas and equimolar (H₂/CO₂) mixture tests were conducted on GO membranes. The dependence of diffusivity on membrane thickness, temperature, pressure and GO flake size was studied. The temperature and thickness dependence of diffusivities showed good agreement with thickness dependent diffusion activation energy calculated based on molecular sieving model. A two-pathway model (inter-sheet and inner-sheet) was used to explain the obtained results. This study provides an insightful roadmap for the design of other new 2D lamellar membranes.