Efficient Capture and Electroreduction of Dilute CO 2 into Highly Pure and Concentrated Formic Acid Aqueous Solution.

High-purity CO ₂ rather than dilute CO ₂ (15 vol %, CO ₂ /N ₂ /O ₂ = 15:80:5, v/v/v) similar to the flue gas is currently used as the feedstock for the electroreduction of CO ₂ , and the liquid products are usually mixed up with the cathode electrolyte, resulting in high product separation costs. In this work, we showed that a microporous conductive Bi-based metal-organic framework ( Bi-HHTP , HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) can not only efficiently capture CO ₂ from the dilute CO ₂ under high humidity but also catalyze the electroreduction of the adsorbed CO ₂ into formic acid with a high current density of 80 mA cm ^-2 and a Faradaic efficiency of 90% at a very low cell voltage of 2.6 V. Importantly, the performance in a dilute CO ₂ atmosphere was close to that under a high-purity CO ₂ atmosphere. This is the first catalyst that can maintain exceptional eCO ₂ RR performance in the presence of both O ₂ and N ₂ . Moreover, by using dilute CO ₂ as the feedstock, a 1 cm ^-2 working electrode coating with Bi-HHTP can continuously produce a 200 mM formic acid aqueous solution with a relative purity of 100% for at least 30 h in a membrane electrode assembly (MEA) electrolyzer. The product does not contain electrolytes, and such a highly concentrated and pure formic acid aqueous solution can be directly used as an electrolyte for formic acid fuel cells. Comprehensive studies revealed that such a high performance might be ascribed to the CO ₂ capture ability of the micropores on Bi-HHTP and the lower Gibbs free energy of formation of the key intermediate *OCHO on the open Bi sites.