CO2-selective polymeric chains anchored on graphene effectively pull CO2 from a flue gas mixture. Credit: KV Agrawal (EPFL)

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, , and (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 drastically increases the CO2 permeance, making membranes even more attractive for . The concept is highly generic, and a number of high-performance gas separations are possible in this way."

More information: High-permeance polymer-functionalized single-layer graphene membranes that surpass the postcombustion carbon capture target. Energy & Environmental Science 26 July 2019. DOI: 10.1039/c9ee01238a

Journal information: Energy & Environmental Science