Tailoring sub-3.3 Å ultramicropores in advanced carbon molecular sieve membranes for blue hydrogen production

Carbon molecular sieve (CMS) membranes prepared by carbonization of polymers containing strongly size-sieving ultramicropores are attractive for high-temperature gas separations. However, polymers need to be carbonized at extremely high temperatures (900 degrees to 1200 degrees C) to achieve sub-3.3 angstrom ultramicroporous channels for H-2/CO2 separation, which makes them brittle and impractical for industrial applications. Here, we demonstrate that polymers can be first doped with thermolabile cross-linkers before low-temperature carbonization to retain the polymer processability and achieve superior H-2/CO2 separation properties. Specifically, polybenzimidazole (PBI) is crosslinked with pyrophosphoric acid (PPA) via H bonding and proton transfer before carbonization at <= 600 degrees C. The synergistic PPA doping and subsequent carbonization of PBI increase H-2 permeability from 27 to 140 Barrer and H-2/CO2 selectivity from 15 to 58 at 150 degrees C, superior to state-of-the-art polymeric materials and surpassing Robeson's upper bound. This study provides a facile and effective way to tailor subnanopore size and porosity in CMS membranes with desirable molecular sieving ability.