Plasma engineered carbon molecular sieve membranes for precise and stable hydrogen permeation

Carbon molecular sieve (CMS) membranes represent a high-performance alternative to polymers, surpassing the trade-off limits between permeability and selectivity typically seen in conventional polymeric membranes. Their efficacy stems from a precise molecular sieving mechanism, enabling exceptional gas separation performance. However, a challenge persists in fine-tuning their microporous structure to achieve effective kinetic separation of gas molecule pairs with small differences in kinetic diameter. We demonstrate that simple plasma treatment can fundamentally alter the CMS structures, drive ultraselective molecular hydrogen (H₂) separation over carbon dioxide (CO₂) and methane (CH₄), and stabilize the structures over a year of physical aging. Two types of plasma, nitrogen- and argon-derived, were used, both of which induced surface oxidation by creating defects or incorporating nitrogen atoms into the carbon layer and strands, leading to distinct development of ultramicroporosity compared to pristine CMS structures. These structural modifications resulted in marked improvements in separation performance. The plasma-treated membranes achieved over 40-fold increases in H₂/CO₂ selectivity and over 137-fold increases in H₂/CH₄ selectivity compared to the pristine membrane, while maintaining high H₂ permeabilities of over 1000 Barrers, surpassing the current upper bound set by CMS membranes. When combined with high-temperature pyrolysis, the plasma-treated membrane exhibited a record-high combination of H₂ permeability (exceeding 100 Barrers) and H₂/CO₂ selectivity (exceeding 260). Plasma-treated CMS membranes demonstrated exceptional stability over 14 months of aging, with less than 10% variation in observed performance. This study establishes plasma treatment as a powerful strategy, delivering CMS membranes with both enhanced separation performance and remarkable long-term stability.