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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.