Hydrazine, a molecule consisting of nothing more than two nitrogen atoms and four hydrogen atoms, has long fascinated fuel-cell engineers. Its electrochemical oxidation in alkaline media liberates up to four electrons per molecule, producing only nitrogen gas and water as byproducts, and it can be stored as a liquid at room temperature—properties that make it an attractive anode fuel for direct hydrazine fuel cells, or DHFCs. The concept is not new; hydrazine-air fuel cells were already emerging from the laboratory in the 1960s, with early work reported in Science in 1967. Yet the technology has remained tethered to noble-metal catalysts, primarily platinum and palladium, whose scarcity and cost have impeded commercial deployment. Compounding the challenge, recent mechanistic studies have revealed that hydrazine oxidation is a self-inhibiting reaction on certain electrode surfaces, with intermediates poisoning active sites, making the design of robust, inexpensive electrocatalysts all the more critical.

The new research, led by Katia Mohand Saidi and Smail Khelili of the University of Mohamed Seddik Ben Yahia in Jijel, Algeria, in collaboration with colleagues at Eskişehir Osmangazi University in Turkey and several Algerian institutions, took a deliberately different path. Rather than relying on transition metals such as nickel, cobalt, or copper—though these too have been extensively explored—the team turned to magnesium oxide, a wide-bandgap alkaline earth oxide whose catalytic potential for hydrazine had been largely overlooked despite earlier demonstrations of nanocubic MgO for hydrazine sensing. The choice carries obvious advantages: magnesium is among the most abundant elements in the Earth’s crust, MgO is non-toxic, thermally stable, and its production carries a fraction of the environmental and financial burden associated with precious metals.

What distinguishes the synthesis is its adherence to green-chemistry principles. The researchers dissolved a magnesium precursor in the presence of chitosan, a naturally derived polysaccharide obtained from the deacetylation of chitin in crustacean shells. Chitosan’s amine and hydroxyl groups coordinate magnesium ions and, crucially, organize them into a supramolecular framework during solution processing. When the composite is subsequently calcined, the chitosan burns away as a sacrificial template, leaving behind magnesium oxide that has inherited the template’s one-dimensional morphology. The result is a population of MgO nanorods—elongated, rod-like particles whose geometry maximizes exposed surface area while providing continuous pathways for electronic conduction along their axes. Sacrificial templating of this kind is an elegant workaround to a persistent problem in nanomaterials synthesis: untemplated oxide particles formed by simple decomposition of magnesium hydroxide tend to agglomerate severely, destroying the nanoscale features that make them useful.

Structural characterization confirmed the formation of the rock-salt MgO phase, with crystallite sizes estimated through X-ray diffraction analysis using the Scherrer equation and Williamson–Hall methods, which separate the contributions of crystallite size and lattice strain to peak broadening. Transmission and scanning electron microscopy revealed the distinctive nanorod morphology, though the authors are candid that some regions exhibit partial agglomeration—an honest acknowledgment of the trade-offs inherent in scalable green synthesis. More importantly, the calcination process left the material riddled with mesopores and crystallographic defects. Far from being flaws, these imperfections are the secret to the catalyst’s performance: defect sites and undercoordinated surface ions generate additional Mg²⁺ active centers where hydrazine molecules can adsorb and react, while the porous architecture shortens diffusion distances and facilitates rapid charge transfer across the electrode-electrolyte interface.

The electrochemical results are striking. When the MgO-modified electrode was tested in a 1.0 M potassium hydroxide electrolyte containing 0.5 M hydrazine, it delivered a peak anodic current density of 37.63 milliamperes per square centimeter at an applied potential of 0.80 volts versus an Ag/AgCl reference electrode. The onset potential for hydrazine oxidation—a key metric, since lower onsets mean less electrical energy wasted driving the reaction—was measured at +0.389 volts, and Tafel analysis yielded a slope of 76.90 millivolts per decade, values that the team shows are highly competitive against copper nanoparticle composites supported on functionalized multiwalled carbon nanotubes and against numerous benchmarks established for transition-metal-oxide-based hydrazine oxidation catalysts. For a noble-metal-free catalyst built from one of the cheapest oxides available, the comparison is remarkable.

Kinetic investigation added further depth to the picture. Analysis of how the oxidation current varies with scan rate in cyclic voltammetry revealed a diffusion-controlled mechanism, meaning the reaction proceeds so rapidly at the electrode surface that the overall current is limited only by how quickly hydrazine molecules can migrate from the bulk electrolyte to the active sites. This is the hallmark of a fast catalyst, and the researchers quantified it through a high catalytic turnover rate—indicating that each active site processes hydrazine molecules at an impressive pace. The diffusion-controlled regime also bodes well for sensing applications, since it underpins the linear current-concentration relationships that amperometric hydrazine sensors depend on for quantifying this toxic pollutant in environmental samples.

The dual utility of the material is one of the study’s most compelling aspects. Hydrazine is not only a prospective fuel; it is also a hazardous industrial chemical used in water treatment, pharmaceutical synthesis, and aerospace propulsion, and its detection in drinking water is a genuine environmental concern. The same mesoporous, defect-rich nanorod architecture that accelerates fuel-cell anode chemistry also provides the sensitivity required for electrochemical detection, positioning the material at the intersection of energy conversion and environmental monitoring. Meanwhile, the green synthesis itself—water-compatible processing, a biopolymer template derived from seafood waste, and no toxic reagents—embodies the sustainability ethos that the field has been preaching, demonstrating that high-performance electrocatalysis and environmentally responsible manufacturing need not be opposing goals.

The implications for direct hydrazine fuel cells are particularly noteworthy. DHFCs operating in alkaline media with anion-exchange membranes have attracted renewed attention because hydrazine’s liquid handling sidesteps the storage and infrastructure challenges that plague hydrogen, and because alkaline conditions permit the use of non-precious-metal catalysts at the anode. If a catalyst as inexpensive as MgO can sustain hydrazine oxidation currents comparable to those of composite systems involving carbon nanotubes and multiple metal components, the anode cost barrier that has limited DHFC commercialization becomes substantially lower. The findings also add magnesium oxide to the growing roster of metal oxides—alongside nickel oxide, cobalt oxides, manganese oxides, and zinc oxide—that are being engineered for hydrazine electrooxidation, expanding the design space available to catalyst developers.

The study, which received no dedicated funding and was carried out through institutional collaboration, is candid about its limitations and future directions. The partial agglomeration observed in some regions suggests that optimizing calcination conditions or template loading could push performance further, and translating laboratory-scale half-cell measurements into full fuel-cell devices remains the essential next step. Nevertheless, the work offers a consistent and economical alternative to noble-metal catalysts and substantiates the potential of chitosan-templated MgO nanorods for high-efficiency direct hydrazine fuel cells and sophisticated electrochemical sensors. In a field where progress is often measured in marginal gains from increasingly complex multicomponent nanocomposites, the demonstration that a single, abundant, green-synthesized oxide can hold its own against engineered benchmark systems is a reminder that sometimes the most powerful advances come from the simplest materials, viewed through a newly engineered lens.

The advance addresses one of the central bottlenecks in clean electrochemical energy technologies. Fuel cells and metal–air batteries rely on the oxygen reduction reaction, or ORR, at the cathode. During ORR, oxygen molecules are converted into water or hydroxide ions, depending on the electrolyte and device chemistry. Although the reaction is essential, it is intrinsically slow and usually requires a catalyst. Platinum remains one of the most effective ORR catalysts, but its scarcity, high price and vulnerability to supply-chain constraints make large-scale deployment more difficult. Researchers have therefore spent years developing iron–nitrogen–carbon, or Fe–N–C, catalysts that can reproduce platinum-like activity using abundant elements.

Fe–N–C materials typically contain isolated iron atoms embedded in a carbon matrix and coordinated by nitrogen atoms. These iron centers can bind and activate oxygen, allowing the reaction to proceed more rapidly. However, conventional Fe–N–C catalysts often inherit a compact three-dimensional structure from their precursor materials. Many of their active sites become trapped inside micropores, where oxygen and electrolyte cannot easily reach them. In addition, the iron centers commonly adopt an Fe–N₄ configuration that can hold oxygen-reduction intermediates too strongly. The combination of buried active sites, restricted mass transport and unfavorable reaction energetics limits the amount of catalytic activity that can be used in a working device.

The new approach begins with iron-doped zeolitic imidazolate framework-8, or Fe-ZIF-8. ZIF-8 is a porous crystalline material containing zinc ions connected by organic imidazolate linkers. It has long been investigated as a precursor for carbon catalysts because its framework can be transformed into nitrogen-containing carbon during heating. In this study, the researchers mixed Fe-ZIF-8 with a eutectic salt composed of potassium chloride and zinc chloride before subjecting the mixture to high-temperature pyrolysis. A eutectic mixture melts at a lower temperature than either of its individual components, creating a liquid environment in which the precursor can reorganize during thermal treatment.

The molten salt played several roles at once. Zinc chloride helped destabilize and disrupt the pH-sensitive ZIF-8 framework, while the liquid salt phase separated and dispersed carbon-containing fragments as the precursor decomposed. Instead of collapsing into a dense, three-dimensional carbon particle, the material was reconstructed into loosely stacked two-dimensional nanosheets. After pyrolysis, the salts and remaining inorganic residues were removed by leaching, leaving behind a layered Fe–N–C architecture with substantially greater exposure to the surrounding electrolyte. The process effectively used the salt as a temporary chemical and structural medium for reshaping the catalyst at high temperature.

Microscopy revealed the difference between the conventional and molten-salt-derived materials. The new catalyst consisted of thin carbon sheets arranged in an open, layered structure rather than tightly packed particles. High-angle annular dark-field scanning transmission electron microscopy showed bright individual iron atoms distributed throughout the carbon matrix, with no evidence of large iron nanoparticles. X-ray diffraction likewise detected no crystalline iron or zinc residues after the purification process. This atomic dispersion is important because isolated iron sites can provide more uniform catalytic environments, whereas iron clusters may promote unwanted side reactions, accelerate degradation or reduce the fraction of metal that participates in ORR.

The structural transformation also produced a dramatic increase in accessible surface area. The conventional Fe–N–C catalyst had a Brunauer–Emmett–Teller surface area of 302.6 square meters per gram, while the layered material reached 1474.2 square meters per gram. A larger surface area does not automatically guarantee better catalysis, but in this case it provides more interfaces where oxygen, hydroxide and reaction intermediates can interact with the active sites. The open nanosheets also shorten diffusion pathways through the catalyst layer. These changes can improve mass transport, reduce the accumulation of reaction products and make a greater proportion of the iron centers available under realistic operating conditions.

Spectroscopic measurements indicated that the molten-salt treatment altered the local atomic structure of iron as well as the overall shape of the catalyst. X-ray absorption spectroscopy showed that the conventional material was dominated by iron sites coordinated by approximately four nitrogen atoms, consistent with Fe–N₄ structures. In the layered catalyst, the measured coordination number increased to about 5.2, supporting the formation of axially coordinated Fe–N₅ sites. The additional axial ligand changes the electronic environment around iron and can tune how strongly it binds oxygen-reduction intermediates such as oxygenated species and hydroxide. This balance is crucial: intermediates must bind strongly enough to be activated but weakly enough to leave the surface during the catalytic cycle.

In electrochemical tests conducted in 0.1-molar potassium hydroxide, the layered catalyst reached a half-wave potential of 0.874 volts versus the reversible hydrogen electrode. This value was 38 millivolts higher than that of commercial platinum-on-carbon under the reported conditions, indicating a more favorable ORR potential in the alkaline electrolyte. The catalyst also exhibited a low Tafel slope, consistent with efficient reaction kinetics, and favored the four-electron oxygen-reduction pathway. In this pathway, oxygen is reduced efficiently rather than producing large quantities of peroxide, an unwanted intermediate that can damage catalyst layers and reduce device efficiency. After 10,000 electrochemical cycles, the material retained strong activity, suggesting that the combination of atomic iron dispersion and a robust carbon framework can provide useful durability.

The most important test took place inside zinc–air batteries, where laboratory electrochemical advantages must translate into performance at the device level. Across current densities from 2 to 50 milliamperes per square centimeter, batteries using the layered Fe–N–C air cathode delivered higher discharge voltages than batteries using Pt/C. The catalyst achieved a maximum power density of 0.20 watts per square centimeter, compared with 0.14 watts per square centimeter for the platinum-based reference. It also produced a specific capacity of 718 milliampere-hours per gram of zinc, exceeding the 676 milliampere-hours per gram measured for Pt/C. These results suggest that the catalyst can support both rapid power delivery and efficient use of the zinc anode.

The researchers say the significance of the work lies in combining three forms of catalyst engineering that are often studied separately: controlling the shape of the carbon framework, exposing more active sites and tuning the coordination environment of the metal atoms. The molten-salt process addresses all three in a single synthesis. Its open nanosheets improve access and transport, while the Fe–N₅ configuration modifies the electronic behavior of the iron centers. Together, these effects help explain why an inexpensive, platinum-free material can outperform Pt/C in key alkaline ORR measurements. The method could also be adaptable to other single-atom catalysts in which the accessibility and coordination of active sites are as important as their total number.

The findings point toward a broader strategy for designing air electrodes for zinc–air batteries, alkaline fuel cells and related energy systems. Zinc–air batteries are attractive because zinc is relatively abundant, inexpensive and capable of storing substantial energy, but their practical performance depends heavily on the cathode reaction. Replacing platinum with a catalyst made primarily from iron, nitrogen and carbon could lower costs and reduce reliance on scarce precious metals. The use of a salt-assisted process may also offer manufacturing advantages because molten salts can act as heat-transfer media, dispersants and temporary templates during pyrolysis. Further work will need to determine how consistently the catalyst can be produced at larger scales and how it performs during extended cycling under commercial operating conditions. Even so, the study demonstrates that reshaping a catalyst around its active atoms—not merely adding more of them—can unlock a powerful route toward more affordable clean-energy technologies.

Subject of Research: Molten-salt engineering of layered iron–nitrogen–carbon single-atom catalysts for the oxygen reduction reaction and zinc–air batteries

Article Title: Layered Fe–N–C catalysts with axially coordinated single-atom sites induced by molten salts for oxygen reduction reaction

News Publication Date: May 13, 2026

Web References: eScience Energy: https://www.sciencedirect.com/journal/escience-energy; Article DOI: https://doi.org/10.1016/j.esen.2026.100068

References: Layered Fe–N–C catalysts with axially coordinated single-atom sites induced by molten salts for oxygen reduction reaction, eScience Energy, DOI: 10.1016/j.esen.2026.100068

Image Credits: Liqiu Liu, et al.

Keywords

Iron–nitrogen–carbon catalysts, single-atom catalysts, molten salts, oxygen reduction reaction, zinc–air batteries, fuel cells, Fe–N₅ sites, platinum alternatives, energy storage, electrochemistry