A new membrane devised at MIT offers a means of continuously removing carbon dioxide from a stream of waste gases. The gas permeability of the electrochemically assisted membrane can be switched on and off as needed without the use of mechanical moving parts and with minimal energy consumption.

The membrane is designed to continuously and dynamically modulate gas transfer at a gas-liquid interface, based on reversible electrochemical zinc deposition/dissolution on conductive porous anodized aluminum oxide at the gas-liquid interface. Hexagonal openings allow gas molecules to flow in and out when in the open state, but gas passage can be controlled and blocked when a thin layer of zinc is electrically deposited to cover the pores of the membrane.

A redox-active carbon-absorbing material is sandwiched between two switchable gas gating membranes and the device is immersed in propylene carbonate electrolyte solvent to provide a medium for zinc ions to shuttle back and forth. These two gating membranes can be opened or closed electrically by switching the polarity of a voltage between them, causing ions of zinc to shuttle from one side to the other. The ions simultaneously block one side by forming a metallic film over it, while opening the other by dissolving the film.

When the sorbent layer is open to the side where the waste gases are flowing, the material readily soaks up carbon dioxide until it reaches its capacity. The voltage can then be switched to block off the feed side and open the other side, where a concentrated stream of nearly pure carbon dioxide is released.

A proof of concept membrane system was demonstrated to effectively separate and pump carbon dioxide against a concentration gradient without the need for differential pressure, making it versatile for different carbon capture applications. Compared to conventional batch processes, the membrane separation device can eliminate the need for system blowdown between absorption and regeneration, simplify reactor design, improve energy efficiency, and reduce the footprint of separation units.

The research is published in Science Advances.

The gating membrane in its open (A) and closed (B) state. Source: Yayuan Liu et al.