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 CO 2 to CO was studied in the gas phase using a [CO 2 (g), Co-N-C cathode | Nafion-H | Pt/C anode, H 2 /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(NO 3 ) 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-N 4 -C x structure. A model of the reduction mechanism of CO 2 on the active site was proposed.