Metal-Free Catalysts for Electrochemical Nitrate Reduction to Ammonia

Electrochemical nitrate reduction (e-NO₃RR) to ammonia (NH₃) offers a promising route to simultaneously mitigate nitrate (NO₃⁻) pollution and enable NH₃ production under ambient conditions, providing an alternative to the energy- and carbon-intensive Haber−Bosch process. While NO₃⁻ reduction is thermodynamically more favorable than direct nitrogen activation, its practical implementation is hindered by complex multistep proton−electron transfer pathways, sluggish kinetics, and competition from the hydrogen evolution reaction, which together limit selectivity and stability. Metal-free catalysts have recently attracted increasing attention due to their earth abundance, chemical robustness, resistance to metal dissolution, and tunable electronic structures. This critical review examines the fundamental reaction mechanisms of e-NO₃RR and benchmarks this pathway against conventional NH₃ synthesis routes. Emphasis is placed on the role of the electrode, including surface−intermediate interactions, charge transfer characteristics, and reactor-level considerations. Key parameters governing NH₃ yield, Faradaic efficiency, and energy efficiency such as pH, electrolyte composition, applied potential, reactor architecture, and nitrate speciation are systematically analyzed. This review critically analyzes recent progress in metal-free carbon-based catalysts, from pristine and defect-engineered carbon to laser-induced graphene and nanotube architectures. Finally, key challenges and future opportunities are highlighted to guide the rational design of next-generation metal-free catalysts for scalable and efficient NH₃ synthesis.