Embedding a self-supporting MOF-based molecular sieve membrane into an electrolyzer for boosting electroreduction of CO2 in air and flue gas to HCOOH

Abstract Efficient electrochemical reduction of CO2 (eCO2RR) in air and flue gas to high-purity products is crucial for reducing atmospheric CO2 levels. However, low CO2 concentrations (400 ppm in air, 15% in flue gas) and reducible impurities present significant challenges. To address this, we developed an electrolyzer integrating a self-supporting MOF-based mixed-matrix molecular sieve membrane (MOF-MMM), a conductive diffusion layer, and a Bi nanoparticle catalytic layer. This design enables simultaneous gas purification and eCO2RR while avoiding electroreduction of impurities (SO2, NO, O2). The MOF-MMM enriches CO2 from 15% to 82.5% in flue gas or from 0.04% to 2.05% in air. Under acidic conditions, enriched CO2 is reduced to formic acid. Using flue gas as feedstock, the eCO2RR current reaches 9000 mA with 100% Faradaic efficiency, producing 23 mL of pure HCOOH after 4 hours, representing state-of-the-art performance. Using air as feedstock, it achieves 98.2% FEHCOOH with a record partial current density of 5.3 mA cm−2 at 2.5 V, yielding 177,200 μmol h−1 g−1, which is 5,000 times higher than reported catalysts without molecular sieves. This integrated approach enables the direct transformation of dilute CO2 emissions into transportable liquid fuels, offering dual environmental benefits through carbon utilization and sustainable chemical production.