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As the bridge between nanocrystal catalyst and single-atom catalyst, single-unit-cell catalyst seems to be unavailable for catalysis due to the quantum effect and synthetic difficulties. Here, we synthesize 24 nm Cu9S5 nanowires via the LaMer pathway. Interestingly, when polyoxometalate (POM) clusters are introduced during the nucleation process, the 0.9 nm Cu9S5 nanowires are finally formed via covalent co-assembly, analogue to A-B-A-B type block copolymerization in polymer field (“A” and “B” represent Cu9S5 unit cells and POM clusters, respectively). Multiple characterizations prove that Cu9S5 exists as single-unit-cell structure. Therefore, each unit cell can work as an isolated active site. The single-unit-cell structure exhibits higher electrocatalytic activity and faradaic efficiency (FE) of formic acid (82.0% at -0.8 V vs. RHE) during CO2 electroreduction, while nanocrystal structure generates HCOO-, methanol and ethanol with low FEs. Our study suggests that the single-unit-cell catalyst displays a great potential for precise catalysis by the finite size effect.