Effect of nickel-based electrocatalyst size on electrochemical carbon dioxide reduction: A density functional theory study

The electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) used for converting higher-value chemicals is a promising solution to mitigate CO2 emissions. Nickel (Ni)-based catalysts have been identified as a potential candidate for CO2 activation and conversion. However, in the CO2RR, the size effect of the Ni-based electrocatalysts has not been well explored. Herein, the single Ni atom and the Ni4 cluster doped nitrogen-doped carbon nanotube (Ni@CNT and Ni4@CNT), and the Ni (110) facet were designed to explore the size effect in the CO2RR by using density functional theory (DFT) calculations. The results show that carbon monoxide (CO) is produced on the Ni@CNT with a free energy barrier of 0.51 eV. The reduction product of CO2 on the Ni4@CNT and Ni(1 1 0) facet is methane (CH4) in both cases, via different reaction pathways, and the Ni(1 1 0) facet is a more efficient electrocatalyst with a low overpotential of 0.27 V when compared to Ni4@CNT (0.50 V). The rate-determining step (RDS) is the formation of *CHO on the Ni4@CNT (The "*" represents the catalytic surface), while the *COH formation is the RDS on the Ni(1 1 0) facet. Meanwhile, the Ni(1 1 0) facet also has the highest selectivity of CH4 among the three catalysts. The CO2 reduction product changes from CO to CH4 with the increasing size of the Ni-based cat-alysts. These results demonstrate that the catalytic activity and selectivity of CO2RR highly depend on the size of the Ni-based catalysts.