Iron-Nitrogen Carbon Nanosheets Match Platinum in Microbial Fuel Cell Electrodes

Microbial fuel cells promise a rare double win in the energy transition: they generate electricity while bacteria break down organic pollutants in wastewater, all under mild conditions and with non-toxic metabolic end products. Yet turning that promise into practical devices has long hinged on a stubborn bottleneck at the cathode, where the oxygen reduction reaction governs how efficiently electrons flowing out of the microbial community are converted into usable current. A new study published in Discover Catalysis reports a low-cost iron-based catalyst that, in laboratory tests, performs on par with the commercial platinum benchmark that researchers have been trying to replace for decades.

The material, designated Fe3C/FeN−X-CNS, is a family of stacked two-dimensional carbon nanosheets containing iron carbide and iron nitride phases. It was created by a team led by Decheng Lu, Hua Song and Chunlei Zhang, who built the catalyst around a zeolitic imidazolate framework, a porous metal-organic scaffold prized for its structural tunability and high surface area. The synthesis route is deliberately simple: ferric nitrate and dicyandiamide are complexed in water, coated onto the zinc-based framework, and then pyrolyzed at 900 degrees Celsius. During that high-temperature treatment, the zinc species volatilize, leaving behind a porous architecture riddled with pathways for electron transport.

The variable X in the catalyst’s name encodes the molar ratio of iron to dicyandiamide, and tuning that ratio proved decisive. Scanning and transmission electron microscopy showed that without dicyandiamide the product formed irregular rods and dispersed particles, while adding the nitrogen source produced curved, sheet-like flocculent structures whose dimensions grew as nitrogen content increased. At the highest dicyandiamide level, corresponding to an iron-to-nitrogen ratio of 1:10, the sheets agglomerated, a sign that excess nitrogen introduces defects that degrade activity. The intermediate composition, Fe3C/FeN−5-CNS, emerged as the sweet spot.

Spectroscopic analysis explained why. X-ray diffraction confirmed the coexistence of FeN and Fe3C crystalline phases, with no residual zinc detected, supporting the volatilization mechanism. X-ray photoelectron spectroscopy identified pyridinic nitrogen, iron nitride, pyrrolic nitrogen, graphitic nitrogen and oxidized nitrogen species on the catalyst surface. Pyridinic nitrogen and FeN sites are considered active centers for oxygen reduction, and the Fe3C/FeN−5-CNS sample contained more of them than the other compositions in the series. The authors attribute the enhanced catalysis to a synergistic effect between the FeN and Fe3C phases embedded in the graphitized carbon lattice.

Electrochemical testing in oxygen-saturated potassium hydroxide put the material through its paces. Using a rotating disk electrode, the team measured an onset potential of 0.10 volts for Fe3C/FeN−5-CNS, essentially matching the 0.11 volts recorded for commercial 20-weight-percent platinum on carbon. The half-wave potential was −0.11 volts and the limiting current density reached −6.35 milliamperes per square centimeter, slightly exceeding the platinum reference’s −6.05. Koutecky-Levich analysis yielded an electron transfer number of 3.89, close to the ideal four-electron pathway that defines an efficient oxygen reduction catalyst, and a Tafel slope of 27 millivolts per decade sat near platinum’s 24.

Impedance measurements added a further dimension. The charge transfer resistance of Fe3C/FeN−5-CNS was 22 ohms, roughly 9 percent higher than platinum on carbon but 68 percent lower than the same material prepared without dicyandiamide modulation. The authors link this low internal resistance to more efficient charge transfer at the electrode-electrolyte interface, which reduces oxygen reduction overpotential and supports higher kinetic performance. Gas sorption analysis showed that introducing dicyandiamide reshaped the pore structure, increasing the average pore size to about 1.21 nanometers, a change the researchers say helps expose active sites and facilitate electron movement.

The decisive test came in a working device. The team constructed single-chamber microbial fuel cells with a 30-cubic-centimeter chamber, carbon fiber brush anodes and catalyst-coated carbon cloth cathodes, inoculated with domestic sewage from the Daqing City Drainage Company in China and operated at 37 degrees Celsius. Over 60 days of operation, the cell equipped with Fe3C/FeN−5-CNS started up in 8 days, the same start-up period as platinum, and delivered a stable output voltage of 0.61 plus or minus 0.03 volts with no significant decay, compared with 0.58 volts for the platinum cell.

Power density told a similar story. Polarization measurements after the cells reached stable output gave a maximum power density of 688 plus or minus 7 milliwatts per square meter for Fe3C/FeN−5-CNS, about 7 percent higher than the 642 plus or minus 3 milliwatts per square meter of the platinum reference. The other compositions ranked lower: 568 for the 1:10 variant, 528 for the 1:2 variant and 511 for the 1:1 variant. Durability favored the iron catalyst as well. After 40 additional days of operation, platinum’s power density had fallen 12 percent to 605 milliwatts per square meter, while Fe3C/FeN−5-CNS declined only 3 percent, to 621.

Cost is the argument that could carry the finding beyond the laboratory. The authors note that the laboratory-grade platinum catalyst used for comparison is priced roughly 200 times higher than their prepared iron-based material. Because microbial fuel cells are conceived as clean, low-cost and renewable power generation devices, replacing precious metal cathodes with abundant iron and nitrogen chemistry directly addresses the economic barrier that has limited scaling. The researchers conclude that Fe3C/FeN−5-CNS, a readily prepared non-precious metal catalyst, is a promising alternative to platinum-based materials for sustainable energy applications in microbial fuel cells.

The study also leaves open questions typical of early-stage electrode research. All results come from small laboratory reactors with 18-milliliter internal volumes, and long-term performance under real wastewater conditions, with their fluctuating organic loads and competing chemical reactions, remains to be demonstrated. The precise identity and density of the active sites in the stacked Fe3C/FeN structure, which the authors acknowledge as a persistent challenge in the field of nitrogen-doped iron catalysts, will need further work to pin down. Still, the combination of platinum-level activity, superior stability in a live fuel cell and a two-hundred-fold cost advantage makes these stacked carbon nanosheets one of the more compelling entries yet in the search for affordable oxygen reduction catalysts.

Subject of Research: Synthesis of iron carbide and iron nitride carbon nanosheet catalysts for the oxygen reduction reaction in microbial fuel cells

Article Title: Preparation of stacked 2D Fe3C/FeN-X carbon nanosheets and their catalytic performances for oxygen reduction reaction in microbial fuel cell

Article References: Lu, D., Song, H., Yang, L., & Zhang, C. (2026). Preparation of stacked 2D Fe3C/FeN-X carbon nanosheets and their catalytic performances for oxygen reduction reaction in microbial fuel cell. Discover Catalysis, 3(1), Article 3. https://doi.org/10.1007/s44344-026-00033-y

Image Credits: AI Generated

DOI: 10.1007/s44344-026-00033-y

Keywords: microbial fuel cell, oxygen reduction reaction, iron carbide, iron nitride, carbon nanosheets, zeolitic imidazolate framework, electrocatalysis, platinum-free catalyst, wastewater, power density, pyrolysis, nitrogen doping