Cobalt catalysts turn nitrate pollution into ammonia, review finds

Ammonia sits at the heart of modern civilization. It feeds roughly half the world’s population through nitrogen fertilizers, and it is increasingly being touted as a zero-carbon energy carrier that could ferry renewable power across oceans and seasons. Yet almost every molecule of ammonia produced today comes from the Haber-Bosch process, a century-old industrial method that consumes vast amounts of energy derived from fossil fuels and releases hundreds of millions of tonnes of carbon dioxide each year. At the same time, another nitrogen problem is spreading quietly across the planet: nitrate pollution. Agricultural runoff and industrial discharge have loaded groundwater and waterways with nitrate at concentrations that threaten ecosystems and human health, from algal blooms that suffocate lakes to drinking water contamination linked to serious health risks. A new review published in the journal Environmental Surfaces and Interfaces argues that these two problems could be solved together, using a single electrochemical reaction that converts nitrate waste into valuable ammonia using renewable electricity.

The reaction in question, known as electrocatalytic nitrate reduction to ammonia, or NO3RR, has been attracting intense research attention in recent years. The concept is elegantly simple: apply an electric current to a catalyst immersed in nitrate-containing water, and the nitrate ions are progressively reduced, ultimately yielding ammonia that can be harvested from the electrolyte. Because the process can run on electricity from solar panels or wind turbines, it offers a route to ammonia synthesis that is decoupled from fossil fuels. Even better, the feedstock is a pollutant that society currently pays to remove. The challenge lies in the chemistry. Nitrate is a remarkably stable molecule, and converting it to ammonia requires a sequence of eight electron transfers and multiple protonation steps. If the reaction takes a wrong turn along the way, it can stall at nitrite, which is itself toxic, or end as nitrogen gas, which wastes the nitrogen entirely. Controlling that pathway is the central problem the review addresses.

The authors, researchers from Guizhou University in China and the Bioeconomy Science Institute in New Zealand, focus their analysis on cobalt-based electrocatalysts, a family of materials they argue offers an unusually compelling combination of performance and practicality. Corresponding author Professor Hu Li explains that cobalt stands out among candidate metals because it is abundant and inexpensive compared with precious metal alternatives, while possessing a unique electronic structure that strongly adsorbs nitrate ions. According to Li, the sp3 orbitals of cobalt atoms can form stable bonds with nitrate, which facilitates the initial and often rate-limiting reduction step that strips an oxygen atom from the nitrate molecule. That initial activation is where many catalysts falter, so a metal that grips nitrate tightly has an inherent advantage in driving the reaction forward.

The performance numbers reported for some cobalt-based systems are striking. Li notes that certain cobalt catalysts have achieved close to 100 percent Faradaic efficiency, a measure of how much of the electrical current flowing through the cell is directed into producing ammonia rather than into side reactions. The chief competitor for electrons is the hydrogen evolution reaction, in which water is split into hydrogen gas instead of ammonia being formed. In dilute nitrate solutions especially, hydrogen evolution can dominate and render the process uneconomical. A catalyst that channels nearly all the current into ammonia therefore represents a major step toward practical devices. Faradaic efficiency alone is not enough, however; the review emphasizes that ammonia production rate, current density, stability over long operating periods, and the energy cost per kilogram of ammonia all matter when evaluating whether a laboratory result can scale to industrial relevance.

Because the reaction pathway involves so many intermediates, the selectivity of the catalyst depends on how strongly each of those transient species binds to its surface. The review describes how researchers can fine-tune these binding energies through deliberate structural design. Alloying cobalt with other metals shifts the electronic environment of the active sites, introducing defects creates undercoordinated atoms that bind intermediates differently, and engineering the crystal structure changes which crystal facets are exposed to the electrolyte. By adjusting these variables, Li says, the binding of intermediates can be steered so that the reaction follows the eight-electron route to ammonia with high selectivity rather than diverting to nitrite or nitrogen gas. This principle of intermediate-binding control, borrowed in part from the broader field of electrocatalysis, is the conceptual thread that runs through the material families surveyed in the review.

Among the material classes examined are monometallic cobalt, cobalt alloys, oxides, sulfides, phosphides, and metal-organic frameworks, along with derivatives formed from those precursors during operation. The review highlights several standout examples that illustrate what careful design can achieve. Cobalt-nickel alloys have delivered industrial-level current densities while maintaining 100 percent Faradaic efficiency in neutral media, a combination that matters because real nitrate waste streams are typically neither strongly acidic nor strongly alkaline. Phosphorus-doped cobalt catalysts have achieved ultra-high ammonia production rates exceeding 200 milligrams per hour per square centimeter of catalyst, placing them among the most productive nitrate reduction systems reported. High-entropy alloys containing cobalt, in which multiple metal elements share a single disordered lattice in ordered atomic arrangements, have been shown to suppress the competing hydrogen evolution reaction, simultaneously boosting both activity and stability.

Each of these material families brings its own trade-offs. Oxides and sulfides tend to be easy to synthesize and robust, but their surfaces may restructure under the reducing potentials required for nitrate conversion, sometimes creating new active phases and sometimes degrading. Phosphides and phosphorus-doped materials benefit from the electronic modulation that phosphorus imparts, enhancing nitrate adsorption and hydrogenation kinetics. Metal-organic frameworks offer extraordinarily high surface areas and precisely tunable metal centers, but their electrical conductivity and long-term durability under operating conditions remain hurdles. The review’s comparative treatment of these classes gives researchers a map of the design space, showing which strategies have demonstrably improved selectivity, which have improved throughput, and which remain promising but unproven at scale.

Beyond the catalysts themselves, the review surveys methodological advances that are accelerating the field. Machine learning is increasingly being used for high-throughput catalyst screening, allowing computational models to predict which compositions and structures are most likely to bind nitrate and its intermediates with the right strength before any material is synthesized in the laboratory. In-situ and operando characterization techniques, which probe the catalyst while it is actually working, are revealing how surfaces evolve during the reaction, exposing dynamic changes such as surface reconstruction, oxidation state shifts, and intermediate adsorption that static ex-situ measurements miss entirely. These tools are shifting the field from trial-and-error discovery toward rational design, and they are helping to explain why certain cobalt configurations outperform others rather than simply documenting that they do.

The authors also look past the catalyst to the reactor and the process. They envision integrated systems that combine nitrate reduction with ammonia separation, so that the ammonia formed in the electrolyte is continuously extracted rather than accumulating and potentially being re-oxidized or lost. Coupling the electrochemical cell with upstream nitrate concentration steps and downstream product recovery will be essential if the technology is to treat real wastewater economically. Durability remains a central concern: catalysts that perform brilliantly for hours in the laboratory must survive months or years in industrial service, resisting corrosion, poisoning, and structural degradation. Li frames the review as a roadmap for developing next-generation cobalt-based electrocatalysts that can turn a global pollutant into a valuable resource, and suggests that with continued progress on catalyst durability and system engineering, nitrate-to-ammonia conversion could become a cornerstone of sustainable chemistry and the circular economy.

The broader significance of this line of research is hard to overstate. If electrocatalytic nitrate reduction matures into a commercial technology, it would simultaneously address water pollution, reduce dependence on the fossil-fuel-intensive Haber-Bosch process, and create a distributed source of ammonia that could be sited wherever nitrate waste and renewable electricity coexist. Farms, wastewater treatment plants, and industrial facilities could become ammonia producers rather than nitrogen polluters, closing the loop on a nutrient cycle that industrial civilization has so far run in one direction. Cobalt, abundant and affordable, may prove to be the metal that makes that vision practical. The review makes clear that the fundamental chemistry is increasingly well understood; what remains is the engineering discipline of turning high-performing laboratory catalysts into durable, economical, large-scale systems.

Subject of Research: Cobalt-based electrocatalysts for electrochemical nitrate reduction to ammonia

Article Title: Recent progress on cobalt-based electrocatalysts for nitrate reduction to ammonia

Article References: Recent progress on cobalt-based electrocatalysts for nitrate reduction to ammonia. (n.d.). Original publication

Image Credits: AI Generated

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Keywords: ammonia synthesis, nitrate reduction, electrocatalysis, cobalt catalysts, Haber-Bosch, water pollution, renewable energy, Faradaic efficiency, high-entropy alloys, metal-organic frameworks, machine learning, circular economy