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

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' CO₂ from a 400 ppm input stream (representative of air) to an output stream with a higher concentration of CO₂, by exploiting ambient energy in the form of a humidity difference. The substantial H₂O concentration difference across the membrane drives CO₂ permeation 'uphill' against its own concentration difference, analogous to active transport in biological membranes. The introduction of this H₂O concentration difference also results in a kinetic enhancement that boosts the CO₂ flux by an order of magnitude even as the CO₂ 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 H₂O-mediated formation of carriers within the molten salt that facilitate rapid CO₂ transport.