Next-gen membranes for carbon capture

CO₂ produced from burning fossil fuels is still mostly released into the atmosphere, adding to the burden of global warming. One way to cut CO₂ levels is through carbon capture, a chemical technique that removes CO₂ from emissions ("postcombustion"), preventing it from entering the atmosphere. The captured CO₂ can then be recycled or stored in gas or liquid form, a process known as sequestration.

Carbon capture can be done using high-performance membranes, which are polymer filters that can specifically pick out CO₂ from a mix of gases, such as those emitted from a factory's flue. These membranes are environmentally friendly, they don't generate waste, they can intensify chemical processes, and can be used in a decentralized fashion. They are now considered as one of the most energy-efficient routes for reducing CO₂ emissions.

Scientists led by Kumar Varoon Agrawal at EPFL Valais Wallis have now developed a new class of high-performance membranes that exceeds post-combustion capture targets by a significant margin. The membranes are based on single-layer graphene with a selective layer thinner than 20 nm, and have highly tunable chemistry, meaning that they can pave the way for next-generation high-performance membranes for several critical separations.

Current membranes are required to exceed 1000 gas permeation units (GPUs), and have a CO₂/N2 separation factor above 20—this is a measure of their carbon-capturing specificity. The membranes that the EPFL scientists developed show six-fold higher CO₂ permeance at 6,180 GPUs with a separation factor of 22.5. The GPUs shot up to 11,790 when the scientists combined optimized graphene porosity, pore size, and functional groups (the chemical groups that actually react with CO₂), while other membranes they made showed separation factors up to 57.2.

"Functionalizing CO₂-selective polymeric chains on nanoporous graphene allows us to fabricate nanometer-thick yet CO₂-selective membranes," says Agrawal. "This two-dimensional nature of the membrane drastically increases the CO₂ permeance, making membranes even more attractive for carbon capture. The concept is highly generic, and a number of high-performance gas separations are possible in this way."