Recent advances in Mo-based electrocatalysts: correlating electronic spin configurations with electrochemical nitrogen reduction reaction performance

Electrocatalytic nitrogen reduction reaction (NRR) offers a green alternative for ambient NH 3 synthesis. However, its efficiency remains constrained by the extremely high N N bond energy, the competitive hydrogen evolution reaction (HER), and the difficulty in synergistically regulating the adsorption energy barriers of intermediates. Generally, the NRR performance is strongly correlated with the oxidation state of active site, local coordination environment, and its spin configuration. The Mo-based catalyst is a promising category due to the unique empty/occupied d orbital structure and the synergistic activation mechanism involving d-π* electron feedback. Consequently, this paper provides a systematic review of relationships among the formal oxidation states, localized coordination environments, and spin states. The study clarifies the fundamental differences between formal oxidation states and d electron distribution and localized spin polarization. Meanwhile, it elucidates the mechanisms by which different coordination fields regulate d orbital occupancy and spin configurations. Additionally, the mechanism of spin polarization suppresses the HER, analyzed using multidimensional descriptors derived from thermodynamics, electronic structure, and structural parameters. Building on this, we summarize precise strategies for controlling the spin configuration and valence state evolution through defect engineering and element doping, emphasizing the synergistic effects of coordination unsaturation, charge redistribution, and spin polarization. Finally, this research proposes critical reflections and prospects regarding the causal identification of Mo 5+ active sites, suggesting the long-term electrocatalytic stability and standardized testing of isotope labeling to avoid false positives. This work summarizes the rigorous theoretical guidance for breaking the constraints of scaling relationships and designing superior active and selective catalysts.