Poly(hydrazide–imide) Membranes with Enhanced Interchain Interaction for Highly Selective H2/CO2 Separation

Membrane-based H2/CO2 separation plays a significant role in sustainable hydrogen production. However, due to the similar diffusion speed of H2 and CO2 in polymers, very few polymer membranes can effectively separate them, which largely limits the application of membrane technology in the hydrogen production process. In this work, a new polymer named poly(hydrazide–imide) (PHI) is synthesized by the polycondensation reaction of aromatic dianhydride with isophthalic dihydrazide and is used for H2/CO2 separation. Hydrazide, a typical polar molecule, is introduced into the polyimide backbone to induce the formation of a hydrogen bond network inside the polymer as confirmed by temperature-dependent infrared reflection spectroscopy. With further thermal annealing, intermolecular π–π interaction is simultaneously strengthened as proved by fluorescence emission spectroscopy, UV–vis absorption spectroscopy, and wide-angle X-ray diffraction (WAXD) characterization. The synergistic effect of the H-bond and π–π interaction efficiently enhances the interchain interaction and optimizes the polymer packing state. The obtained membranes demonstrate greatly improved H2 permeabilities and H2/CO2 selectivity with the comprehensive separation performance surpassing the 2008 Robeson upper bound. The membranes also exhibit excellent CO2 plasticization resistance up to 40 bar and stability in the long-time (>100 h) mixed gas separation test. The design of hydrazide-based polyimide polymer membrane provides a new route for preparing polymer materials for H2/CO2 separation.