Platinum-based catalysts have long been used to accelerate the sluggish oxygen reduction reaction (ORR), a key cathodic step in fuel cells and zinc–air batteries. Iron–nitrogen–carbon catalysts are attractive low-cost alternatives, especially when iron atoms are isolated as single active sites. Yet many zeolitic imidazolate framework-8 (ZIF-8)-derived catalysts keep a compact three-dimensional microporous structure, leaving many iron–nitrogen (Fe-Nₓ) sites buried and poorly used. Their typical Fe-N₄ coordination can also bind reaction intermediates too strongly, limiting catalytic turnover. Two-dimensional carbon architectures and axial ligands are promising, but controllable formation remains difficult. Based on these challenges, there is a need to simultaneously open the catalyst structure and tune the local coordination of iron active sites.
Researchers from the School of Chemical Engineering at the University of Birmingham, the School of Science and Engineering at The Chinese University of Hong Kong-Shenzhen, and the Institute of New Energy and Low-Carbon Technology at Sichuan University report the work in eScience Energy, published (DOI: 10.1016/j.esen.2026.100068) online on May 13, 2026. The study describes how a potassium chloride/zinc chloride (KCl/ZnCl₂) eutectic salt mixture drives the formation of layered iron–nitrogen–carbon (Fe–N–C) catalysts with Fe-N₅ single-atom sites for efficient ORR and zinc–air battery operation.
The team began with Fe-doped ZIF-8 and mixed it with a KCl/ZnCl₂ eutectic before high-temperature pyrolysis. Zinc chloride (ZnCl₂) helped disrupt the pH-sensitive ZIF-8 framework, while the molten salt environment enabled fragments to disperse, rearrange and reconstruct into two-dimensional carbon nanosheets. Microscopy showed loosely stacked layered sheets, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) confirmed atomically dispersed iron rather than iron particles. X-ray diffraction (XRD) further showed no crystalline iron or zinc residues after leaching. The structural shift expanded the Brunauer–Emmett–Teller (BET) surface area from 302.6 m²/g in conventional Fe–N–C to 1474.2 m²/g in the layered catalyst, exposing more accessible reaction sites. X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), showed that the conventional catalyst mainly formed Fe-N₄ sites, whereas the layered catalyst reached a coordination number of about 5.2, consistent with Fe-N₅. Electrochemical tests in 0.1 molar potassium hydroxide (KOH) gave a half-wave potential of 0.874 volts versus reversible hydrogen electrode (RHE), 38 millivolts higher than commercial platinum on carbon (Pt/C). The catalyst also showed a low Tafel slope, four-electron ORR selectivity and strong stability after 10,000 cycles.
The authors said the work connects catalyst shape, atomic coordination and battery-level performance in one design under realistic device conditions. By using molten salts to reshape the carbon framework, they said, the study moves beyond simply adding more active sites and shows how to make those sites easier for oxygen and electrolyte to reach. They said the Fe-N₅ configuration gives the iron centers a more favorable reaction environment, while the layered structure shortens transport pathways. Together, these effects help explain why a non-precious-metal catalyst can exceed Pt/C in key alkaline ORR metrics.
In zinc–air battery testing, the layered catalyst delivered higher discharge voltages than Pt/C across current densities from 2 to 50 milliamperes per square centimeter and achieved a maximum power density of 0.20 watts per square centimeter, compared with 0.14 watts per square centimeter for Pt/C. It also reached a specific capacity of 718 milliampere-hours per gram of zinc, higher than 676 milliampere-hours per gram for Pt/C. These results suggest that molten-salt engineering could support scalable production of platinum-free air cathodes for metal–air batteries and related alkaline energy devices, while offering a broader strategy for designing single-atom catalysts with accessible, tunable active sites.
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References
DOI
10.1016/j.esen.2026.100068
Original Source URL
https://doi.org/10.1016/j.esen.2026.100068
About eScience Energy
eScience Energy is an open-access journal publishing cutting-edge scientific and technological research emerging from interdisciplinary fields related to advanced batteries, solar cells, fuel cells, redox flow cells, etc. Original, important or general interest contributions covering a diverse range of topics are considered. eScience Energy covers a broad spectrum of topics related to chemical and physical power sources.