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 (j CO) 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 j CO 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 j CO 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 j CO of −67 mA cm–2 at −0.61 V vs RHE at 1 M KOH. Adding KI to the base electrolyte significantly magnified the j CO 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 j CO also achieved >200 mA cm–2 at a cell voltage of around 2.7 V.

Electrochemical CO2 or CO reduction to chemicals and fuels using renewable energy is a promising way to reduce anthropogenic carbon emissions. The gas diffusion electrode (GDE) design enables low-carbon manufacturing of target products at a current density (e.g., 500 mA/cm2) relevant to industrial requirements. However, the long-term stability of the GDE is restricted by poor water management and flooding, resulting in a significant hydrogen evolution reaction (HER) within almost an hour. The optimization of water management in the GDE demands a thorough understanding of the role of the gas diffusion layer (GDL) and the catalyst layer (CL) distinctively. Herein, the hydrophobicity of the GDL and CL is independently adjusted to investigate their influence on gas transport efficiency and water management. The gas transport efficiency is more enhanced with the increase in hydrophobicity of the GDL than the CL. Direct visualization of water distribution by optical microscope and micro-computed tomography demonstrates that the water flow pattern transfers from the stable displacement to capillary fingering as GDL hydrophobicity increases. Unfortunately, only increasing the hydrophobicity is not sufficient to prevent flooding. A revolutionary change in the design of the GDE structure is essential to maintain the long-term stability of CO2/CO reduction.

The identification of the rate-determining step (RDS) in the electrochemical CO/CO 2 reduction to multi-carbon (C 2+ ) products has been complicated by the deficiency of rigorous reaction kinetic data. This work describes an experimental analysis of the key reaction steps by exploring the effect of CO partial pressure on the activity of C 2+ products. With the aid of a flow electrolyzer integrated with a gas diffusion electrode, the distinct reaction orders of CO and reaction mechanisms in forming different C 2+ products were determined. Specifically, *CO dimerization is identified as the RDS for ethylene and ethanol production, as evidenced by the gradual transition of measured CO reaction order from second to zero as CO partial pressure increases from 0.05 to 1 atm. The formation of npropanol is suggested to proceed via the *CO trimerization mechanism. The acetate generation mechanism might involve a critical step of *CO hydrogenation before C−C coupling. Kinetic studies reveal that product-specific active sites are responsible for activity and selectivity toward specific C 2+ products over oxide-derived copper.