均相电催化氮气还原反应（NRR）可在温和条件下探索分子固氮路径，并通过调控金属中心、配体骨架与反应介质构建和优化催化体系，从而在分子层面捕获关键固氮中间体。然而，该领域仍受制于所需还原电势较负、析氢反应竞争强、可持续周转频率受限及长期稳定性不足等瓶颈。本综述以“电子如何递送至分子活性中心”为主线，对均相电化学氮气活化/转化体系进行分类：一类为直接电子传递（DET），电极将电子传递给可循环的介导体“mediator”，再由介导体在均相体系中传递给催化循环。介导体系进一步分为仅传递电子的电子转移（ET）介导体与实现质子-电子耦合递送的PCET介导体。在DET部分，我们梳理从早期低价钛、钨物种到近年来广泛研究的钼、铁、铼分子配合物的结构与反应过程中的关键中间体，并将机理归纳为两条主路径：其一为“cleavage-first”断键路径，通过N≡N裂解生成可分离/可表征的金属氮化物（M≡N）；其二为“PCET-first”路径，在保持N-N键的前提下逐步氢化形成N_xH_y中间体并进一步生成氨。上述归纳为建立配体电子效应、二级配位层、多金属协同及溶剂/电解质微环境与选择性之间的构效关系提供参考。在介导部分，ET介导体可在一定程度上将强还原需求从催化剂本体“外移”，缓冲过强的还原环境并降低失活风险；PCET介导体则依托可调的氧化还原电位与介体-H键强度（BDFE），以更可控的方式进行电子/质子转移，从而促进中间体氢化并提升有效活性与周转数。最后，我们强调均相电催化氮气还原的研究仍需坚持严格的污染控制与定量产物鉴定标准，并指出有必要通过更合理的分子与体系层面设计提升催化反应在长时间运行下的稳定性。进一步地，我们展望了可将介导体策略、分子催化剂设计与电解槽工程相结合，推动均相体系由机理模型迈向具备规模化潜力的电化学合成氨器件。

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 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 electrochemical 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.