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

The electrochemical carbon dioxide (CO 2 ) reduction reaction (CO 2 RR) used for converting higher-value chemicals is a promising solution to mitigate CO 2 emissions. Nickel (Ni)-based catalysts have been identified as a potential candidate for CO 2 activation and conversion. However, in the CO 2 RR, the size effect of the Ni-based electrocatalysts has not been well explored. Herein, the single Ni atom and the Ni 4 cluster doped nitrogen-doped carbon nanotube (Ni@CNT and Ni 4 @CNT), and the Ni (1 1 0) facet were designed to explore the size effect in the CO 2 RR 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 CO 2 on the Ni 4 @CNT and Ni(1 1 0) facet is methane (CH 4 ) 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 Ni 4 @CNT (0.50 V). The rate-determining step (RDS) is the formation of *CHO on the Ni 4 @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 CH 4 among the three catalysts. The CO 2 reduction product changes from CO to CH 4 with the increasing size of the Ni-based catalysts. These results demonstrate that the catalytic activity and selectivity of CO 2 RR highly depend on the size of the Ni-based catalysts.