Arresting the transient microstructural evolution of carbon molecular sieve membranes via ultrafast thermal engineering

Abstract Carbon molecular sieve (CMS) membranes combine molecular‐level discrimination with robust carbon frameworks, yet rational micropore design is hindered by pore collapse and structural densification during conventional slow carbonization. Here we report a Joule‐heating ultrafast carbonization process (UCP) that arrests the polymer‐to‐carbon evolution with second‐level temporal control. Reducing the thermal treatment from hours to seconds suppresses high‐temperature strand mobility, preserving non‐equilibrium open‐pore microstructures that are otherwise lost. Time‐resolved tuning along the UCP window further exposes a sequential transformation pathway, offering a high‐resolution experimental handle on pore formation and refinement. As a result, UCP‐derived CMS membranes reach a CO 2 permeability of 10,863 Barrer (2.4‐fold higher than tube‐furnace analogues) and surpass the 2019 upper bounds for CO 2 /N 2 and CO 2 /CH 4 separations. These findings establish UCP as a versatile, high‐throughput platform for precision engineering of CMS membranes for advanced gas separations.