Size effect of nickel from nanoparticles to clusters to single atoms for electrochemical CO2 reduction

Electrochemical CO2 reduction to value-added chemicals is a promising solution for alleviating environmental issues. Nickel-nitrogen-carbon catalysts have been considered as potential candidates for CO2 reduction. However, the size effect of Ni-based catalysts in the electrochemical CO2 reduction reaction (CO2RR) remains elusive. Here, we precisely controlled the size of nitrogen-doped-carbon anchored Ni from nanoparticles to clusters to single atoms and systematically investigated the size effect of Ni on the CO2 electroreduction performance for the first time. The faradaic efficiency of CO varies from 59% over Ni nanoparticles to 85% over single atoms at -0.8 V vs. RHE, along with an excellent turnover frequency for CO production (3554.5 h(-1)) and a current density of 12 mA cm-2, surpassing most other state-of-the-art catalysts. The formation of H* for the competitive hydrogen evolution reaction is facile on Ni nanoparticles and clusters, which enable syngas production with a wide range of CO/H-2 ratios (1.1-6.3). The characterization results reveal that the electronic state of Ni species can be tuned by varying the particle sizes. The unique electronic nature of single-atom Ni optimizes the adsorption of CO2 and the formation of *COOH intermediates, thus promoting CO2 electroreduction. This work elucidates the importance of the particle size on the electrochemical CO2 reduction performance.