Atomically Precise Copper Nanoclusters for CO2-to-C2 Electroreduction: Coupling-Site Topology, Reaction Pathways, and Active-State Evolution

Electrochemical CO2 reduction to multicarbon products is a promising route for carbon valorization and renewable-energy storage, yet the structural heterogeneity and potential-induced reconstruction of conventional Cu catalysts obscure relationships between local motifs, C–C coupling, and product selectivity. Atomically precise Cu nanoclusters provide a structurally defined platform in which nuclearity, metal arrangement, coordination environment, and ligand shell can be systematically varied. This review organizes recent advances according to the elementary sequence of C1-intermediate generation, partial hydrogenation, dual-intermediate adsorption, C–C coupling, and post-coupling product branching. We focus on how accessible adjacent-site topology, including intersite geometry, coordination asymmetry, electronic differentiation, and ligand-shell exposure, regulates intermediate supply and coupling pathways. Emerging links between site symmetry, oxygenated-intermediate stabilization, and ethylene–ethanol selectivity are assessed together with the effects of electrode potential, proton-transfer environment, and structural reconstruction. Intact clusters, partially reconstructed clusters, and cluster-derived phases are further distinguished to clarify the evidence required for active-state assignment. These insights provide design principles for Cu nanoclusters with persistent multicenter sites and selective CO2-to-C2 conversion.