Unveiling Advances in Membrane Materials for CO2 Separation and Direct Air Capture (DAC): From Membrane Design to Applications.

The increase in carbon dioxide (CO2) concentration in the atmosphere has resulted in adverse and irreversible effects in terms of climate change and global warming. To limit the temperature rise to less than 2 °C by the end of this century, it is urgent to reduce the CO2 concentration in the atmosphere. Direct air capture (DAC) is considered a carbon-negative emission technology which could efficiently remove CO2 from air. Membrane gas separation is a promising technology for CO2 capture, owing to its higher energy efficiency, greater scale-up ability, and smaller carbon footprints compared with conventional sorption processes. The application of membranes in the DAC process (m-DAC) is still in its infancy, owing to the low CO2 concentration (400 ppm) in air. However, simulations and laboratory studies have demonstrated the feasibility of m-DAC. With the development of high-performance membrane materials and the design of multistage membrane processes, the implementation of m-DAC will be a promising strategy for the efficient reduction of CO2 concentration in air. This review presents current studies on the m-DAC process and recently developed membranes for CO2/N2 separation which could be potentially used in that process, as well as highlighting research gaps that currently represent obstacles to the wider use of membranes for m-DAC. In conclusion, challenges and future prospects are presented, along with a roadmap for the future development of m-DAC, to provide a deeper insight into m-DAC processes.