Double-site catalysts have attracted widespread attention in the field of electrocatalysis due to their high metal loading, adjustable active centres, and electronic valence states. However, the development of bimetallic sites catalysts that coordinate with definite atoms is still in the exploratory stage. Here, we designed and synthesized a bimetallic palladium complex (BPB-Pd₂) with conjugated backbone. The supported BPB-Pd₂ was applied to electrochemical CO₂ reduction reaction (CO₂RR) for the first time. The as-obtained BPB-Pd₂ gives an exceptional Faradaic efficiency of CO (FE_CO) of 94.4% at −0.80 V vs. reversible hydrogen electrode (RHE), which is significantly superior to monoatomic palladium catalyst (BPB-Pd₁). The density functional theory (DFT) calculations revealed that the essential reason for the outstanding activity of BPB-Pd₂ toward CO₂RR was that the electronic effect between diatomic palladium reduces the free energy change for CO₂RR process. Thus, BPB-Pd₂ exhibits moderate free energy change to form COOH* intermediate, which was beneficial for the generation of CO in CO₂RR.

Reducing the size of heterogeneous nanocatalysts is generally conducive to improving their atomic utilization and activities in various catalytic reactions. However, this strategy has proven less effective for Cu-based electrocatalysts for the reduction of CO₂ to multicarbon (C₂₊) products, owing to the overly strong binding of intermediates on small-sized (< 15 nm) Cu nanoparticles (NPs). Herein, by incorporating pyrenyl-graphdiyne (Pyr-GDY), we successfully endowed ultrafine (~ 2 nm) Cu NPs with a significantly elevated selectivity for CO₂-to-C₂₊ conversion. The Pyr-GDY can not only help to relax the overly strong binding between adsorbed H^* and CO^* intermediates on Cu NPs by tailoring the d-band center of the catalyst, but also stabilize the ultrafine Cu NPs through the high affinity between alkyne moieties and Cu NPs. The resulting Pyr-GDY-Cu composite catalyst gave a Faradic efficiency (FE) for C₂₊ products up to 74%, significantly higher than those of support-free Cu NPs (C₂₊ FE, ~ 2%), carbon nanotube-supported Cu NPs (CNT-Cu, C₂₊ FE, ~ 18%), graphene oxide-supported Cu NPs (GO-Cu, C₂₊ FE, ~ 8%), and other reported ultrafine Cu NPs. Our results demonstrate the critical influence of graphdiyne on the selectivity of Cu-catalyzed CO₂ electroreduction, and showcase the prospect for ultrafine Cu NPs catalysts to convert CO₂ into value-added C₂₊ products.