Scalable synthesis of coordinatively unsaturated metal-nitrogen sites for large-scale CO electrolysis.

Practical electrochemical CO-to-CO conversion requires a non-precious catalyst to react at high selectivity and high rate. Atomically dispersed, coordinatively unsaturated metal-nitrogen sites have shown great performance in CO electroreduction; however, their controllable and large-scale fabrication still remains a challenge. Herein, we report a general method to fabricate coordinatively unsaturated metal-nitrogen sites doped within carbon nanotubes, among which cobalt single-atom catalysts can mediate efficient CO-to-CO formation in a membrane flow configuration, achieving a current density of 200 mA cm with CO selectivity of 95.4% and high full-cell energy efficiency of 54.1%, outperforming most of CO-to-CO conversion electrolyzers. By expanding the cell area to 100 cm, this catalyst sustains a high-current electrolysis at 10 A with 86.8% CO selectivity and the single-pass conversion can reach 40.4% at a high CO flow rate of 150 sccm. This fabrication method can be scaled up with negligible decay in CO-to-CO activity. In situ spectroscopy and theoretical results reveal the crucial role of coordinatively unsaturated metal-nitrogen sites, which facilitate CO adsorption and key *COOH intermediate formation.