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

Abstract

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−Cx structure. A model of the reduction mechanism of CO2 on the active site was proposed.