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

Abstract

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 (CO2RR) 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 CO2RR 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 CO2RR 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.

Recent Advances in the Catalyst Design and Mass Transport Control for the Electrochemical Reduction of Carbon Dioxide to Formate

“…Therefore, utilizing a gas diffusion electrode (GDE), which was originally developed for fuel cell applications, for direct CO 2 delivery to the catalyst surface using water vapor as a carrier was proposed [84]. An article by Delafontaine et al reported that in a conventional H-type cell, the concentration of CO 2 in aqueous solution was 0.038 M. Direct delivery of humidified CO 2 to the flow cell resulted in a modest increase in the relative saturated CO 2 concentration to 0.041 M. In the same article, the diffusion coefficient of aqueous CO 2 was reported to be as low as 0.0016 mm 2 s −1 in CO 2 -saturated 0.1 M KHCO 3 [85,86]. Delivering humidified gaseous CO 2 with a GDE dramatically increased the CO 2 diffusion coefficient 10,000-fold to 16 mm 2 s −1 (Figure 10a) [86].…”

“…An article by Delafontaine et al reported that in a conventional H-type cell, the concentration of CO 2 in aqueous solution was 0.038 M. Direct delivery of humidified CO 2 to the flow cell resulted in a modest increase in the relative saturated CO 2 concentration to 0.041 M. In the same article, the diffusion coefficient of aqueous CO 2 was reported to be as low as 0.0016 mm 2 s −1 in CO 2 -saturated 0.1 M KHCO 3 [85,86]. Delivering humidified gaseous CO 2 with a GDE dramatically increased the CO 2 diffusion coefficient 10,000-fold to 16 mm 2 s −1 (Figure 10a) [86]. The ease with which gaseous CO 2 reached the catalyst resulted in a high CO 2 availability and a subsequent increase in the CO 2 RR partial current density.…”

The inchoate horizon of electrolyzer designs, membranes and catalysts towards highly efficient electrochemical reduction of CO2 to formic acid

“…A more gaseous CO 2 has been reached at the catalyst, resulting in a large CO 2 availability and a consequent raise in the CO 2 R PCD. 24,47 Therefore, the usage of GDEs is the prominent strategy to increase the HCOO À PCD but it is very much challenging to avail a quick HCOO À production rate and a low overpotential with a high FE HCOOH .…”

Towards the Large-Scale Electrochemical Reduction of Carbon Dioxide

“…This is because the configuration can convert CO2 into products without the need for a liquid catholyte, so the cell resistance is very low, and a liquid catholyte circulation loop is not required [40]. Moreover, this minimizes any losses due to the dissolution of CO2 in the catholyte and results in an approximately 50% decrease in the power required to generate the same amount of CO with the same family of electrocatalysts as the flow cell [41,42]. This makes the zero-gap electrolyzer a promising configuration.…”