Size-Dependency of Electrochemically Grown Copper Nanoclusters Derived from Single Copper Atoms for the CO Reduction Reaction.

Electrochemically grown copper nanoclusters (CuNCs: <3 nm) from single-atom catalysts have recently attracted intensive attention as electrocatalysts for CO and CO reduction reaction (CORR/CORR) because they exhibit distinct product selectivity compared with conventional Cu nanoparticles (typically larger than 10nm). Herein, we conducted a detailed investigation into the size dependence of CuNCs on selectivity for multicarbon (C) production in CORR. These nanoclusters were electrochemically grown from single Cu atoms dispersed on covalent triazine frameworks (Cu-CTFs). Operando X-ray absorption fine structure analysis revealed that Cu-CTFs containing 1.21 wt % and 0.41 wt % Cu (Cu(h)-CTFs and Cu(l)-CTFs, respectively) formed CuNCs of 2.0 and 1.1 nm, respectively, at -1.0 V vs. RHE. The selectivity for CORR products was particularly dependent on the size of CuNCs, with the Faraday efficiencies of C products being 52.3 % and 32.7 % at -1.0 V vs. RHE with Cu(h)-CTFs and Cu(l)-CTFs, respectively. Spherical CuNCs modeling revealed that larger cluster sizes led to a greater proportion of a surface coordination number (SCN) of 8 or 9. Density functional calculations revealed that the CO dimerization reaction was more likely to proceed at SCNs of 8 or 9 compared to SCN of 7 because of the stability of the *OCCO intermediate.