Electrocatalytic reduction of CO₂ has been viewed as a promising approach to utilize CO₂ and reduce atmospheric greenhouse gas level. Transition metal complexes represent an important class of electrochemical CO₂ reduction (eCO₂RR) catalysts due to their atom-precise and tuneable active site structures.

The synthesis of binuclear complexes introduces potential synergistic effects between metal centres, and the construction of heterometallic sites allows electronic structure tuning of the active site, which may regulate reaction pathways and promote the formation of deep reduction products.

In a study published in Chemical Communications, Prof. ZHANG Teng and Prof. CAO Rong from the Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences, along with collaborators, synthesized a series of homometallic and heterometallic complexes, revealed their eCO₂RR performance, and elucidated the regulatory effect of heterometallic sites in eCO₂RR performance.

The researchers synthesized three binuclear complexes, Cu₂L²(ClO₄)₂, CuNiL²(ClO₄)₂, and CuCoL²(ClO₄)₂, and verified their structures through single crystal X-ray crystallography. All three complexes adopted in a planar binuclear coordination geometry with the moiety of CuMN₄O₂ (M = Cu, Ni, or Co). 

Electrochemical tests revealed that these complexes exhibited eCO₂RR activity in acetonitrile solution. The homometallic complex Cu₂L²(ClO₄)₂ produced formic acid selectively, while the heterometallic complexes exhibited moderate selectivity for hydrocarbon products. Faradaic efficiencies (FEs) for methane and ethylene with CuCoL²(ClO₄)₂ catalyst were found to be 8% and 9%, respectively.

Control experiments showed that the mononuclear analogue, CuL¹, exhibited eCO₂RR selectivity towards CO, indicating that the production of formic acid was promoted by the binuclear structure. Enhanced selectivity for hydrocarbon products was attributed to synergistic electronic effect on the bimetallic Cu-Co site. 

The combination of kinetic isotope effect measurements and density functional theory calculations revealed a possible mechanism of eCO₂RR. Differences of the CO adsorption energies on the reduced Cu site played a vital role in the higher hydrocarbon FEs on CuCoL²(ClO₄)₂.

This study provides an approach for the development of novel homogeneous eCO₂RR catalysts with the precise control of the active site composition, and it demonstrates that catalytic eCO₂RR performance of metal complexes can be systematically controlled through introduction of different metals.