Turning nitrate pollution into ammonia with smarter catalysts

Nitrate pollution has quietly become one of the most stubborn problems of the industrial age. Runoff from farms, discharge from sewage plants, and wastewater from metal plating and nuclear facilities have overloaded the natural nitrogen cycle, contaminating drinking water and damaging ecosystems. A new open-access review in the journal Catal, published on 17 November 2025, argues that this waste stream could instead become a resource: through electrocatalytic nitrate reduction, the nitrate ion can be converted into ammonia, the backbone of global fertilizer production and an increasingly important candidate as a carbon-free fuel.

The review, led by Joyjit Kundu, Toshali Bhoyar, Jeonghyeon Kim, Jinwon Choi and colleagues with senior author Sang-Il Choi, surveys the mechanistic insights and catalyst engineering strategies that have pushed the nitrate reduction reaction, known as NO3RR, toward practical application. The authors emphasize that the reaction is attractive precisely because it attacks two problems at once: it cleans nitrate-contaminated water while recovering nitrogen in a useful form, potentially reducing fertilizer costs in remote and underdeveloped regions when powered by renewable electricity.

The appeal becomes clearer when compared with the incumbent technology. Ammonia is currently made almost exclusively by the Haber-Bosch process, which cleaves the inert nitrogen-nitrogen triple bond under high temperatures of 300 to 600 degrees Celsius and pressures of 150 to 350 atmospheres, demanding roughly 485 kilojoules per mole of energy and emitting around 500 million tons of carbon dioxide annually. Electrochemical nitrogen reduction from air has been proposed as a greener route, but the 945 kilojoule per mole dissociation energy of the triple bond, competition from the hydrogen evolution reaction, and the poor solubility of nitrogen gas in water have kept it industrially impractical. Nitrate, by contrast, carries nitrogen already bonded to oxygen with a much weaker N-O bond energy of 204 kilojoules per mole, dissolves readily in water, and reduces faster at the liquid-solid interface. The review notes that nitrate reduction exhibits higher Faradaic efficiency than nitrogen reduction, with reported ammonia yield rates up to 3.45 millimoles per square centimeter per hour.

Still, the chemistry is demanding. Converting nitrate to ammonia is an eight-electron, nine-proton process that must thread through a sequence of adsorbed intermediates, from nitrate to nitrite, nitric oxide, nitroxyl species, and bare adsorbed nitrogen before final hydrogenation to ammonia. Along the way, the reaction competes with the two-electron hydrogen evolution reaction and can divert into by-products including nitrite, nitric oxide, nitrogen gas, hydrazine, and nitrous oxide. The balance of adsorbed hydrogen atoms is delicate: too few slow ammonia production, while an excess drives hydrogen atoms to pair off into wasteful hydrogen gas. Density functional theory calculations cited in the review show that on copper surfaces the rate-determining step shifts with pH, and that in alkaline media the barrier for nitrate reduction, 0.91 electronvolts, sits far below the 1.41 electronvolts required for hydrogen evolution, which helps explain why alkaline conditions dominate laboratory studies.

Much of the review is devoted to how catalyst designers have attacked these bottlenecks. Doping is the workhorse strategy. Copper-doped iron oxide achieved an ammonia yield of 179.55 milligrams per hour per milligram of catalyst with 100 percent Faradaic efficiency at minus 0.6 volts versus a reversible hydrogen electrode, with computations showing nitrate adsorption flipping from an unfavorable 1.50 electronvolts on pristine material to a favorable minus 1.11 electronvolts after doping. Copper-doped tungsten oxide reached 94 percent efficiency, while cobalt-doped copper oxide nanoarrays delivered 8.41 milligrams of ammonia per square centimeter per hour at 98.30 percent efficiency. Ruthenium atoms doped into copper nanowires produced one of the standout results: a current density of 1 ampere per square centimeter at minus 0.13 volts, 96 percent efficiency, and a generation rate of 76.5 milligrams per square centimeter per hour, with the two metals cooperatively suppressing hydrogen evolution.

Single-atom and dual-atom catalysts take the precision further. Copper single atoms on nitrogen-doped carbon reached 100 percent Faradaic efficiency, and breaking the coordination symmetry of copper-nitrogen sites, by substituting oxygen atoms into the first coordination shell, unlocked an industrially relevant current density of 366 milliamperes per square centimeter with an ammonia production rate of 27.84 milligrams per square centimeter per hour and stability lasting 2000 hours. Dual-atom designs exploit synergy between neighboring metals: a copper-nickel pair embedded in nitrogen-doped carbon achieved 97.28 percent efficiency, while a molybdenum-iron pair on palladium metallenes reached 94.6 percent efficiency with a yield of 13.4 moles per gram of catalyst per hour, using a spillover mechanism in which nitrogen intermediates hop back and forth between the two metal sites as they are progressively hydrogenated.

Heterointerfaces and alloy architectures add another layer of control. A nickel cobaltate-copper oxide interface with plasma-induced oxygen vacancies achieved 95.07 percent efficiency, and a cobalt phosphide-nickel phosphide nanoarray pushed current density to 2.18 amperes per square centimeter with a yield of 172.64 milligrams per square centimeter per hour, aided by facile hydrogen spillover across the junction. High-entropy alloys composed of iron, copper, cobalt, chromium, zirconium, nickel and bismuth reached 92.3 percent efficiency, with each element contributing a distinct role, from catalysis to intermediate stabilization to conductivity. Facet engineering matters too: on copper nanosheets, the (100) facet preferentially adsorbs and activates nitrate while the (111) facet handles the hydrogenation steps, a division of labor that mirrors the multi-site cooperation seen across the entire catalyst design landscape.

The most forward-looking section addresses single-cell systems, the membrane electrode assembly electrolyzers needed to move beyond laboratory H-cells, whose distant electrodes and high resistance limit energy efficiency. In MEA devices, a ruthenium-copper cocatalyst system sustained a current of 10 amperes at 4.2 volts with near-100 percent Faradaic efficiency and only 50 kilowatt-hours of electricity per kilogram of ammonia in concentrated nitrate solution, remaining stable up to near-saturated 3.0 molar conditions. Comparisons with conventional water electrolyzers show nitrate electrolysis reaching a given current density at roughly 80 millivolts lower overpotential, while also producing valuable ammonia instead of merely hydrogen. The review is candid about the caveats: low nitrate concentrations, typical of municipal sewage at parts-per-million levels, cause mass transport limitations and efficiency collapse, and real wastewaters contain contaminants such as hexavalent chromium and high chemical oxygen demand that hinder the reaction.

Membrane engineering, pulse electrolysis, flow rate control, and electrodialysis coupling round out the systems-level toolkit, from covalently interlocked bipolar membranes that ran at 1 ampere per square centimeter at just 3.89 volts for over 1100 hours, to a membrane-free design that recovered 99 percent of the ammonia produced. The authors close with a sober assessment of what remains: only a limited number of catalysts meet industrial requirements for current density and long-term stability, most studies use synthetic solutions rather than real wastewater, standardized performance metrics are lacking, and stack components must resist ammonia stress corrosion cracking and hydrogen embrittlement. Their verdict is measured but optimistic: with continued integration of materials innovation, device engineering, and standardized evaluation, electrocatalytic nitrate reduction could evolve into a viable route for future ammonia production, turning a pollutant into a product along the way.

Subject of Research: Electrocatalytic reduction of nitrate to ammonia through catalyst design and single-cell electrolysis systems

Article Title: Electrocatalytic nitrate to ammonia conversion: from mechanistic insights to catalyst engineering for practical applications

Article References: Kundu, J., Bhoyar, T., Kim, J., Choi, J., Kim, D., Kwon, T., Kim, J. Y., Lee, K., & Choi, S.-I. (2025). Electrocatalytic nitrate to ammonia conversion: from mechanistic insights to catalyst engineering for practical applications. Catal, 1(1), Article 10. https://doi.org/10.1007/s44422-025-00010-w

Image Credits: AI Generated

DOI: 10.1007/s44422-025-00010-w

Keywords: electrocatalysis, nitrate reduction, ammonia, nitrogen cycle, Haber-Bosch, single-atom catalysts, heterointerfaces, membrane electrode assembly, wastewater treatment, green chemistry, Faradaic efficiency, catalyst engineering