Single-Atom Catalysts for the Electro-Reduction of CO2 to Syngas with a Tunable CO/H2 Ratio: A Review

Nowadays, transition towards green chemistry is becoming imperative. In this scenario, an attractive perspective consists in the generation of CO through the electrochemical reduction of CO2 under ambient conditions. This approach allows storage of the electrical energy from intermittent renewable sources in the form of chemical bonds, and simultaneously reduces greenhouse gas emissions, giving carbon a second chance of life. However, most catalysts adopted for this process, i.e., noble metal-based nanoparticles, still have several issues (high costs, low current densities, high overpotentials), and in the view of generating syngas through co-electrolysis of H2O and CO2, do not enable a widely tunable CO/H-2 ratio. Single-atom catalysts with N-doped carbon supports have been recently introduced to face these challenges. The following review aims to answer the demand for an extended and exhaustive analysis of the metal single-atom catalysts thus far explored for the electro-reduction of CO2 in aqueous electrolyte solution. Moreover, focus will be placed on the objective of generating a syngas with a tunable CO/H-2 ratio. Eventually, the advantages of single-atom catalysts over their noble metal-based nano-sized counterparts will be identified along with future perspectives, also in the view of a rapid and feasible scaling-up.