Separation and concentration of CO2 from air using a humidity-driven molten-carbonate membrane

Abstract Separation processes are substantially more difficult when the species to be separated is highly dilute. To perform any dilute separation, thermodynamic and kinetic limitations must be overcome. Here we report a molten-carbonate membrane that can ‘pump’ CO 2 from a 400 ppm input stream (representative of air) to an output stream with a higher concentration of CO 2 , by exploiting ambient energy in the form of a humidity difference. The substantial H 2 O concentration difference across the membrane drives CO 2 permeation ‘uphill’ against its own concentration difference, analogous to active transport in biological membranes. The introduction of this H 2 O concentration difference also results in a kinetic enhancement that boosts the CO 2 flux by an order of magnitude even as the CO 2 input stream concentration is decreased by three orders of magnitude from 50% to 400 ppm. Computational modelling shows that this enhancement is due to the H 2 O-mediated formation of carriers within the molten salt that facilitate rapid CO 2 transport.