Rational Design of Nano and Atomically Dispersed Catalysts for Electrocatalytic Ammonia Synthesis from Nitrate

Ammonia is a vital industrial feedstock and a carbon-free hydrogen carrier, yet the conventional Haber–Bosch process requires extreme temperatures and pressures and produces substantial CO2 emissions, rendering its large-scale operation energy-intensive and environmentally unsustainable. Electrochemical nitrate reduction (NO3RR) has emerged as a promising alternative, benefiting from the relatively low N=O bond dissociation energy and the high solubility of nitrate ions in aqueous media. This approach not only enables ammonia production under ambient conditions but also provides a sustainable pathway for nitrate wastewater remediation, thereby addressing two pressing global challenges simultaneously. In this review, we present a comprehensive overview of recent advances in NO3RR catalyst development, with particular emphasis on two major classes: nanocrystalline catalysts and atomically dispersed catalysts. Nanocrystalline systems leverage tunable parameters such as crystal facet exposure, lattice strain, defect density, and heteroatom doping to regulate adsorption energies, enhance intermediate activation, and reconcile competing reaction pathways. Atomically dispersed catalysts—including single-atom, dual-atom, and sub-nanometer cluster architectures—achieve nearly complete atomic utilization while offering precisely defined active sites that serve as model platforms for probing structure–activity relationships. Together, these two categories represent complementary strategies: nanocrystals emphasize mesoscale control of morphology and electronic structure, whereas atomic-level catalysts advance the frontier of precision design and mechanistic understanding. We highlight the critical role of in situ and operando characterization in capturing dynamic transformations. Finally, this review discusses the challenges and opportunities that remain for translating these fundamental advances into scalable, efficient, and sustainable electrocatalytic ammonia synthesis.