Unexpectedly Strong Size-Sieving Ability in Carbonized Polybenzimidazole for Membrane H₂/CO₂ Separation

Polymers with high permeability and strong size-sieving ability are needed for H₂/CO₂ separation at temperatures ranging from 100 to 300 °C to enable an energy-efficient precombustion CO₂ capture process. However, such polymers usually suffer from a permeability/selectivity tradeoff, that is, polymers with high permeability tend to exhibit a weak size-sieving ability and thus low selectivity. Herein, we demonstrate that carbonization of a suitable polymer precursor (i.e., polybenzimidazole or PBI) generates microcavities (leading to high H₂ permeability) and ultramicroporous channels (leading to strong size-sieving ability and thus high H₂/CO₂ selectivity). Specifically, carbonization of PBI at 900 °C (CMS@900) doubles H₂ permeability and increases H₂/CO₂ selectivity from 14 to 80 at 150 °C. When tested with simulated syngas-containing equimolar H₂ and CO₂ in the presence of water vapor for 120 h, CMS@900 exhibits stable H₂ permeability of ≈36 barrer and H₂/CO₂ selectivity of ≈53 at 150 °C, above Robeson's 2008 upper bound and demonstrating robustness against physical aging and CO₂ plasticization.