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

Abstract

High-purity CO₂ rather than dilute CO₂ (15 vol %, CO₂/N₂/O₂ = 15:80:5, v/v/v) similar to the flue gas is currently used as the feedstock for the electroreduction of CO₂, 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 (<b>Bi-HHTP</b>, HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) can not only efficiently capture CO₂ from the dilute CO₂ under high humidity but also catalyze the electroreduction of the adsorbed CO₂ 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 CO₂ atmosphere was close to that under a high-purity CO₂ atmosphere. This is the first catalyst that can maintain exceptional eCO₂RR performance in the presence of both O₂ and N₂. Moreover, by using dilute CO₂ as the feedstock, a 1 cm^-2 working electrode coating with <b>Bi-HHTP</b> 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 CO₂ capture ability of the micropores on <b>Bi-HHTP</b> and the lower Gibbs free energy of formation of the key intermediate *OCHO on the open Bi sites.