Electrocatalytic nitrate reduction (NO₃RR) emerges as a promising strategy for simultaneously addressing nitrogen pollution and enabling renewable energy conversion, thereby underpinning the emerging concept of an electrochemical nitrogen cycle. As a fundamental biogeochemical process, the nitrogen cycle is profoundly perturbed by anthropogenic activities, particularly excessive fertilizer application and fossil fuel combustion, resulting in the widespread accumulation of nitrate species in aquatic and terrestrial environments. This imbalance poses severe ecological and human health risks, rendering nitrate remediation on urgent global priority. Conventional nitrate removal technologies, including biological denitrification and physicochemical treatments, are extensively developed, but remain constrained by high energy consumption, stringent operating conditions, and the potential for secondary pollution. In contrast, NO₃RR employs electrons as clean reducing agents and water as a proton source to drive nitrate conversion under mild conditions, enabling seamless integration with a renewable electricity. Beyond pollutant removal, this process also facilitates the synthesis of value-added chemicals, such as ammonia, thereby offering a sustainable route for energy storage and chemical production. Reflecting its strategic significance, the electrochemical nitrogen cycle is identified as a key emerging direction in modern chemistry.

Despite rapid progress, the NO₃RR field remains fragmented and lacks a unifying framework. This challenge primarily stems from the intrinsic complexity of NO₃RR as a multistep proton-coupled electron transfer process involving numerous nitrogen-containing intermediates, leading to convoluted reaction pathways and ambiguous rate-determining steps. In addition, competing reactions such as hydrogen evolution also complicate the evaluation of catalytic selectivity. The existing studies are organized according to material categories, where catalytic performance typically originates from multi-site synergistic effects. Although such systems can deliver a high activity, they obscure the identification of true active centers and impede the establishment of clear relationships among structure, adsorption behavior, and catalytic performance, thereby limiting mechanistic insight and cross-study comparability. This lack of structural clarity also hinders the development of generalizable descriptors capable of linking electronic structure to reaction energetics. To overcome these limitations, increasing attention is directed toward atomically ordered catalysts as an emerging research paradigm. These systems allow for the construction of structurally well-defined and identifiable active centers via enabling precise control over atomic species, spatial distribution, and coordination environments. Such an atomic-level determinacy facilitates direct correlations among electronic structure, adsorption energetics, and reaction pathways, providing a foundation for descriptor-based understanding of catalytic behavior. In this context, representative systems, including single-atom alloys, intermetallic compounds, demonstrate an enhanced catalytic performance and an improved mechanistic clarity relative to conventional materials. Moreover, these model systems offer opportunities to examine the validity and limitations of scaling relationships and to explore strategies for breaking conventional activity and selectivity constraints.

This review proposes a unified classification framework for atomically ordered NO₃RR catalysts based on three key dimensions, i.e., active site scale, local bonding and coordination environment, and atomic arrangement order. This multidimensional perspective enables a systematic comparison across diverse catalyst systems and supports the extraction of fundamental design principles. Furthermore, recent progress on rational catalyst design, descriptor development, and device-level integration is summarized, with an emphasis on bridging the gap between fundamental understanding and practical implementation. Collectively, this framework aims to provide a basis for the development of more predictive and transferable strategies in NO₃RR catalyst design.