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

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 metal-organic framework-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 h, representing state-of-the-art performance. Using air as feedstock, it achieves 98.2% FE_HCOOH 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 5000 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.