Metal-Nitrogen-Carbon Electrocatalysts for CO₂ Reduction towards Syngas Generation

Shifting syngas (an H₂ /CO mixture) production away from fossil-fuel-dependent processes (e.g., steam methane reforming and coal gasification) is mandatory, as syngas is of interest as both a fuel and as a value-added chemical precursor. With appropriate electrocatalysts, such as silver-based and metal-nitrogen-carbon (M-N-C) materials, the electrochemical CO₂ reduction reaction (CO₂ RR) allows for the production of CO alongside H₂ (from the hydrogen evolution reaction), and thus leads to syngas generation. In this Minireview, the application of M-N-C electrocatalysts for syngas generation is discussed. The mechanisms leading to different faradaic selectivities for CO are reviewed as a function of the nature of the metal, by using both computational and experimental approaches. The role played by the metal-free moieties in the M-N-C electrocatalysts is underlined. Since M-N-C electrocatalysts only recently entered the CO₂ RR field (as opposed to Cu-, Ag-, or Au-based nanostructures), they have been mainly characterized in static liquid environments, in which the reaction rate is significantly hampered by CO₂ -dissolution/diffusion limitations. Therefore, the design of CO₂ RR electrolyzers for M-N-C electrocatalysts is addressed, and designs such as zero-gap electrolyzers with anionic membranes and humidified CO₂ gas feed at the cathode are highlighted.