Abstract
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/H2 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/H2 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.
Highlights
Since the last century, the global average atmospheric concentration of CO2 has been dramatically increasing at an exponential rate, reaching, at present, over 417 ppm, which is about 50% higher than the level at the beginning of the Industrial Revolution
A cost/power analysis performed on a CO2 plant fed with 13,750 ton/day of CO2 and producing syngas revealed that, for instance, when adopting a Ni–N–C catalyst rather than an Ag-based catalyst in a flow-cell, less power is required for the reactions to occur [99]
They are highly active towards CO2RR due to their unique characteristics, which distinguish them from their nanoparticle-based counterparts: lowcoordination state and homogeneity of catalytically active sites with maximum metal utilization efficiency, i.e., all the catalytic active sites of the metal are exposed to the reactants
Summary
The global average atmospheric concentration of CO2 has been dramatically increasing at an exponential rate, reaching, at present, over 417 ppm, which is about 50% higher than the level at the beginning of the Industrial Revolution. By virtue of the strong quantum confinement effects at this size, these catalysts exhibit unique properties, such as several valence states and discrete electronic configurations, which strongly depend on the number of atoms constituting the cluster These properties, along with the increased number of under-coordinated metal atoms, i.e., the increased amount of active catalytic sites, make nanocluster catalysts highly active towards a wide range of chemical reactions, such as CO2RR and HER [22–25]. The interaction between nanoparticles and support can lead to a modification of the electronic environment of the metal particles, increasing the interactions between catalyst surface and reaction intermediates and changing the catalytic activity of the former [27]. The following review aims to respond to the necessity of an extended and exhaustive analysis of the metal single-atom catalysts far adopted for the electro-reduction of carbon dioxide in aqueous solutions to produce CO2RR.
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