Unprecedented size-sieving ability in polybenzimidazole doped with polyprotic acids for membrane H₂/CO₂ separation

Polymers with efficient and tight chain-packing and thus strong size-sieving ability are of great interest for H₂/CO₂ separation. Herein, we demonstrate a new approach to manipulating polymer structure by acid doping, leading to superior H₂/CO₂ separation performance. We have doped polybenzimidazole (PBI) with polyprotic acids, specifically H₃PO₄ and H₂SO₄. These acids cross-link PBI chains and drastically decrease free volume, improving the material's H₂/CO₂ selectivity to far surpass the Robeson's 2008 upper bound for membrane performance. For example, PBI doped with H₃PO₄ at a molar ratio of 1:1 exhibits an unprecedented H₂/CO₂ selectivity of 140 at 150 °C, which exceeds that of previously known polymeric materials and is superior or comparable to that of state-of-the-art 2D materials with sharp size separation, such as graphene oxide, MoS₂, and metal-organic frameworks. This facile approach to enhancing polymer chain-packing efficiency opens up a new avenue for designing strong size-sieving polymers for membrane gas separations.