The Active Center of Co–N–C Electrocatalysts for the Selective Reduction of CO2 to CO Using a Nafion‑H Electrolyte in the Gas Phase

To contribute a solution for the global warming problem, the selective electrochemical reduction of CO2 to CO was studied in the gas phase using a [CO2(g), Co–N–C cathode | Nafion-H | Pt/C anode, H2/water] system without using carbonate solutions. The Co–N–C electrocatalysts were synthesized by partial pyrolysis of precursors in inert gas, which were prepared from various N-bidentate ligands, Co­(NO3)2, and Ketjenblack (KB). The most active electrocatalyst was Co–(4,4′-dimethyl-2,2′-bipyridine)/KB pyrolyzed at 673 K, denoted Co–4,4′-dmbpy/KB­(673K). A high performance of CO formation (331 μmol h–1 cm–2, 217 TOF h–1) at 0.020 A cm–2 with 78% current efficiency was obtained at −0.75 V (SHE) and 273 K under strong acidic conditions of Nafion-H. Characterization studies using extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy–energy-dispersive X-ray (TEM-EDX), X-ray diffraction (XRD), and temperature-programmed desorption with mass spectrometry (TPD-MS) indicated the active site as Co coordinated with four N atoms bonding the surface of KB, abbreviated Co–N4–C x structure. A model of the reduction mechanism of CO2 on the active site was proposed.