Recent Advances and Perspectives of Metal Clusters for Electrochemical Nitrogen Reduction

The rapid intensification of anthropogenic nitrogen cycle perturbations and the energy-intensive nature of Haber-Bosch process have motivated the pursuit of sustainable routes for ammonia synthesis. Electrocatalytic nitrogen reduction reaction (NRR) represents a compelling strategy to convert inert N₂ into NH₃, yet its practical realization remains constrained by formidable activation barriers, sluggish kinetics, and strong competition from hydrogen evolution reaction. Well-defined metal clusters have emerged as a distinctive catalytic platform, offering tunable electronic structures and active motifs that bridge the gap between single-atom catalysts and nanoparticles. Our review establishes a unified framework for understanding cluster-enabled NRR, beginning with the physicochemical principles of N₂ activation, including orbital interactions, reaction energetics, and key activity descriptors, while considering electrolyte and proton-coupled effects. We then analyze cluster characteristics such as size-dependent electronic behavior, geometric configurations, and support interactions, highlighting their implications for catalytic selectivity. A systematic overview of electrocatalytic platforms is presented, including metal clusters, bimetallic systems, heterostructures, and nitrogenase-inspired architectures, followed by critical perspectives on reproducibility, durability, and future development pathways. Looking ahead, the integration of operando characterization, theoretical modeling, and scalable synthesis is expected to accelerate the rational development of efficient and reliable cluster-based NRR systems.