Nano trap for CO2

Image: Matthew Lloyd/Getty

A novel high-performance membrane of nanoporous graphene could facilitate the capture of carbon dioxide from industrial waste streams. The 2D graphene structure, just 20nm thick, allows gas to transit easily through the membrane (Energy & Environmental Science, doi: 10.1039/c9ee01238a).

High-performance membranes can cut down capture costs of CO2 rich flue-gas streams, such as from steel and cement industries. Efficient carbon capture will be critical to achieve a reduction of CO2
emissions by 45% by 2030, compared with 2010, needed to restrict global temperature rise to 1.5°C from pre-industrial levels.

The new membrane comprises polymer chains grafted onto graphene. It ‘combines the benefit of atom-thick graphene membrane [leading to fast molecular transport] with CO2 selectivity of polymer chains,’ says Kumar Varoon Agrawal, materials scientist at Ecole Polytechnique Fédérale de Lausanne (EPFL). The polymer chains grab CO2 molecules selectively from a stream of flue gas consisting of a mix of CO2 and N2.

The graphene was made by chemical vapour deposition, and holes were punched into it by exposing it to oxygen plasma and ozone, giving a porosity close to 20%. Treating a stream of flue gas, the hybrid membranes permitted a six-fold higher transit of CO2 than the performance target.

Polymers can be prepared in various ways by changing the monomer building blocks and the way the monomers are connected and the polymer chains terminated. ‘Here, we used specific polymeric chains, hosting amine and ether groups, that show selectivity towards CO2 as compared to N2,’ says Agrawal.

The team is now working to scale-up graphene production, while ensuring it does not tear or crack when expanded on a lowcost membrane support. The paper reports a 1mm2 membrane, but more recently a 1cm2 membrane has been made. ‘The next phase would be to make these membranes on low-cost polymer support and use improved synthesis processes,’ Agrawal explains.

‘With this, we can make much larger membranes [tens of cm], which can be then adopted in a commercial-scale process.’ A commercial process would need several hundred, even thousands of m2 of membrane.

‘The performance is promising and comparable with the best performing polymeric membrane materials,’ comments Maria Grazia De Angelis, materials engineer at the University of Bologna, Italy. ‘[However,] as this membrane is very thin and requires several fabrication steps, the scale-up of the membrane fabrication process to the industrial level is not straightforward.’