One approach to reduce CO₂ emissions is the decarbonization of fossil fuels. Here, the fossil fuels are converted to H₂ and CO₂ at high temperatures and under high pressures. CO₂ is then separated and sequestered and only H₂ is used further. For this method, materials that can selectively and efficiently separate the two gases are needed.

Haiqing Lin and colleagues, University at Buffalo, The State University of New York, USA, have developed an acid-doped polymer membrane with an unprecedented H₂/CO₂ selectivity. The team immersed films of poly[2,20-(m-phenylene)-5,5-bibenzimidazole] (PBI) in solutions of H₃PO₄ or H₂SO₄. The acid molecules act as hydrogen-bonded bridges between imidazole rings in the polymer. This causes crosslinking between the polymer chains and improves the chain-packing efficiency.

The researchers used the synthesized crosslinked polymer membranes for H₂/CO₂ separation. They found that the H₃PO₄-doped membrane has an H₂/CO₂ selectivity of 140 at 150 °C, an unprecedented value for polymeric materials and one that is comparable to ordered 2D materials such as graphene oxide or metal-organic frameworks.

• Unprecedented size-sieving ability in polybenzimidazole doped with polyprotic acids for membrane H₂/CO₂ separation,
  Lingxiang Zhu, Mark T. Swihart, Haiqing Lin,
  Energy Environ. Sci. 2018.
  https://doi.org/10.1039/c7ee02865b