Efficient Capture and Electroreduction of Dilute CO₂ into Highly Pure and Concentrated Formic Acid Aqueous Solution

Abstract

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.