Moisture-driven CO₂ pump for direct air capture

Direct Air Capture, the chemical separation of carbon dioxide (CO₂) from air, is considered a necessary approach to generate negative carbon emissions and in turn limit global warming, though energetic and monetary costs must continue to decline to meet the needed gigaton scale. The promise of energy-efficient and continuous carbon dioxide membrane separation has only recently been investigated with the development of facilitated chemical transport and novel electrochemically driven methods. This work demonstrates the potential for continuous pumping of CO₂ from air using commercial anion exchange membranes driven solely by a water vapor gradient. A reactive transport model that couples ionic transport with moisture sensitive bicarbonate chemistry in charged polymers, also termed moisture swing sorption, was developed alongside experimentally measured equilibrium and transport properties of CO₂ and water vapor in Fumasep FAA-3 to determine the mass transport limits of such a system. The results show that the commercial membrane is kinetically limited by the moisture sensitive chemistry, enabling a CO₂ current of 1.1 μmol/m² ∙ s. In diffusion limited materials, a CO₂ pumping flux can reach 11 μmol/m² ∙ s in and peak performance is expected with a dry air relative humidity around 50% pumping into a saturated water vapor permeate. In all configurations, water loss becomes the major cost in such a separation and must be managed through optimal polymer design or water recovery mechanisms. The work demonstrates the applicability of a new driving force, the evaporation of water, to meet the energetic demand of CO₂ separation from air.