Molecular Catalysts for Electrochemical Nitrogen Activation toward Sustainable Ammonia Synthesis

Homogeneous electrocatalytic nitrogen reduction reaction (NRR) provides a powerful framework to interrogate molecular nitrogen-fixation pathways under mild conditions. By tuning the metal center, ligand architecture, and reaction medium, these systems enable capturing key intermediates and delivering mechanistic insight at the molecular-level resolution. Nevertheless, advances remain constrained by highly reduced operating potentials, intense competition from the hydrogen evolution reaction (HER), limited durability in turnover, and inadequate long-term stability.

In this review, we take electron delivery to the molecular active site as the guiding principle for organizing homogeneous electrochemica N₂ activation and transformation. We classify reported systems into direct electron transfer (DET), in which the electrode reduces the molecular catalyst directly, and mediated electron transfer (MET), in which the electrode reduces a redox mediator, and the reduced mediator subsequently transfers electrons to the molecular catalyst to access the active states that drive N₂ conversion. Mediated systems are further divided into electron-transfer (ET) mediators, which shuttle electrons only, and proton-coupled electron transfer (PCET) mediators, which deliver coupled proton-electron equivalents. For DET systems, we chart progress from early low-valent Ti and W species to widely studied Mo, Fe, and Re complexes, highlighting structurally defined intermediates identified along the reaction pathway. Mechanistically, DET reactivity commonly falls into two routes: a cleavage-first pathway that splits N≡N to form isolable, characterizable metal nitride (M≡N), and a PCET-first pathway that preserves the N−N bond, with stepwise hydrogenation generating N_xH_y intermediates before NH₃ release. This perspective clarifies how ligand electronics, secondary-sphere design, multimetal cooperativity, and solvent/electrolyte microenvironments together control activity and selectivity in NRR. In mediated electrocatalysis, ET mediators can partly shift the burden of extreme reducing conditions away from the catalyst, shielding it from over-reduction and deactivation. PCET mediators, enabled by tunable redox potentials and mediator−H bond strengths, offer a more controlled route for coupled proton/electron transfer, thereby accelerating intermediate hydrogenation, and improving effective activity and turnover of electrocatalytic NRR. Finally, we emphasize that homogeneous NRR still demands stringent contamination control and quantitative product identification, and we highlight the need for more rational molecular- and system-level design strategies to enhance stability and durability under extended operation. Looking ahead, integrating mediator strategies, molecular catalyst design, and electrolyzer engineering could help move homogeneous platforms from mechanistic models toward scalable electrochemical ammonia devices.