Harnessing Collective Magnetic Forces for Enhanced Modulation of Oxygen Diffusion in CO₂/O₂ Separation toward Direct Air Capture

Membrane-based direct air capture (m-DAC) has recently been introduced as a simple, scalable, and environmentally friendly method to capture CO₂ from the atmosphere. The captured CO₂ is considered to be a carbon source for chemical reduction to other value-added chemicals. However, the chemical reduction of CO₂ is disrupted by any O₂ in the captured gas. Therefore, membranes with high CO₂/O₂ selectivity are essential for the m-DAC process. In this work, we design magnetic mixed matrix membranes (MMMs) with magnetic nanoparticle (MNP) fillers in polymer matrices, which exhibit room-temperature trapping of gaseous O₂ within the membrane to achieve high CO₂/O₂ selectivities. We found that the CO₂/O₂ selectivity increased with both the MNP content and the externally applied magnetic field strength, signifying the magnetic interaction of paramagnetic O₂ with MNP, while the permeation of CO₂ remained unaffected. The experimental results were supported by our mathematical model. Overall, the magnetic PolyActive-MMMs containing 40 wt % MNPs achieved the highest CO₂/O₂ selectivity of 35 under a magnetic field of 800 mT, corresponding to a selectivity enhancement of 60% over pure PolyActive membranes. Our findings demonstrate the potential of using magnetic fields to control gas transport for applications that require the separation of O₂ from other gases.