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

Abstract

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/H₂ 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.