A new catalyst made from iron, nitrogen and carbon has achieved a performance milestone that could help reduce the dependence of fuel cells and zinc–air batteries on platinum. Researchers from the University of Birmingham, The Chinese University of Hong Kong-Shenzhen and Sichuan University have developed a molten-salt method for converting an iron-doped molecular framework into thin, layered carbon nanosheets. The resulting material contains a high concentration of exposed, atomically dispersed iron sites and an unusual Fe–N₅ coordination structure. In alkaline oxygen-reduction tests, the catalyst exceeded the performance of commercial platinum-on-carbon, while zinc–air batteries equipped with it delivered higher power and longer discharge performance.
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
