A Pair-Electrosynthesis for Formate at Ultra-Low Voltage Via Coupling of CO₂ Reduction and Formaldehyde Oxidation

Formate can be synthesized electrochemically by CO₂ reduction reaction (CO₂RR) or formaldehyde oxidation reaction (FOR). The CO₂RR approach suffers from kinetic-sluggish oxygen evolution reaction at the anode. To this end, an electrochemical system combining cathodic CO₂RR with anodic FOR was developed, which enables the formate electrosynthesis at ultra-low voltage. Cathodic CO₂RR employing the BiOCl electrode in H-cell exhibited formate Faradaic efficiency (FE) higher than 90% within a wide potential range from - 0.48 to - 1.32 V_RHE. In flow cell, the current density of 100 mA cm^-2 was achieved at - 0.67 V_RHE. The anodic FOR using the Cu₂O electrode displayed a low onset potential of - 0.13 V_RHE and nearly 100% formate and H₂ selectivity from 0.05 to 0.35 V_RHE. The CO₂RR and FOR were constructed in a flow cell through membrane electrode assembly for the electrosynthesis of formate, where the CO₂RR//FOR delivered an enhanced current density of 100 mA cm^-2 at 0.86 V. This work provides a promising pair-electrosynthesis of value-added chemicals with high FE and low energy consumption.