Nickel–Nitrogen–Carbon Molecular Catalysts for High Rate CO₂ Electro-reduction to CO: On the Role of Carbon Substrate and Reaction Chemistry

Metal–nitrogen–carbon (M–N–C) molecular catalysts with NiN4 active structure have been extensively studied as selective and active catalysts toward electrochemical reduction of CO2 to CO. The key challenge for a practical M–N–C catalyst is to increase the density of atomic metal active sites that achieves the partial current density of CO (jCO) relevant to the industrial level at lower overpotentials. Here, we revealed the effect of physical and chemical properties of carbon substrates and synthetic processes on the tuning of the density of atomic metal active sites as well as the role of reaction chemistry in enhancing the jCO and reducing the overpotential. The achievable loading of NiN4 active site in the Ni–N–C is determined by the combined content of pyridinic and pyrrolic N functionalities and Ni–N coordination efficiency derived from the pyrolytic step rather than the uptake capability of Ni2+ in the adsorption step in the case of carbon black with high specific surface area (>1000 m2/g). The N dopant content can be improved by modifying oxygen functional groups on the surface of carbon black, optimizing the pyrolytic temperature, and iterating the doping step. Through a combination of all optimum factors, the resultant Ni–N–C catalyst has a maximum loading of ∼4.4 wt % for atomic Ni. This Ni–N–C catalyst exhibited Faradaic efficiency (FE) of CO of 97% and jCO of −152 mA cm–2 at −0.93 V vs RHE in a flow cell using 0.5 M KHCO3 electrolyte while showing 93% FE of CO and jCO of −67 mA cm–2 at −0.61 V vs RHE at 1 M KOH. Adding KI to the base electrolyte significantly magnified the jCO to larger than −200 mA cm–2 at a potential of −0.51 V vs RHE while maintaining the almost unity FE of CO. The Ni–N–C is compatible with the membrane-electrode-assembly-based electrolyzer in which the jCO also achieved >200 mA cm–2 at a cell voltage of around 2.7 V.