Ligand and support engineering of metal nanoclusters for electrochemical CO₂ reduction

Electrochemical CO₂ reduction reaction (CO₂RR) provides an attractive route for converting CO₂ into value-added fuels and chemicals using renewable electricity. However, its practical development is still hindered by thermodynamic stability and kinetic inertness of CO₂, the competing hydrogen evolution reaction, and the complex multielectron-proton reaction pathways. Atomically precise metal nanoclusters (NCs) have emerged as a distinctive class of CO₂RR electrocatalysts owing to their well-defined compositions, discrete electronic structures, abundant low-coordination metal sites, and highly tunable surface microenvironments. In this review, we summarize recent advances in ligand-protected and supported metal NCs for CO₂ electroreduction with emphasis on reaction mechanisms and structure-performance relationships. We first discuss reaction mechanisms obtained from atomically precise NCs as model systems for identifying active sites, key intermediates, and selectivity-determining steps. Then, ligand engineering for regulating the catalytic performance of NCs is highlighted, including conventional ligand protection, functional ligand engineering, and controlled ligand removal or reconstruction, which regulate electronic structures, cation accumulation, interfacial solvation, and local reaction environments. Subsequently, supported metal NCs are discussed by classifying support effects into conductive or defective carbon interfaces, heteroatom-doped carbon, and metal oxide/zeolite-based confined interfaces. Finally, we outline current challenges and future opportunities in precise synthesis, operando characterization, theoretical modeling, dynamic structural stability, and device-level evaluation, aiming to guide the rational design of next-generation nanocluster electrocatalysts for CO₂RR and beyond.