Researchers have turned industrial lignin, the aromatic polymer that pulp mills discard by the ton, into fluorescent carbonized polymer dots and then locked those dots into an iron-cobalt layered double hydroxide to build a supercapacitor electrode that stores an unusually large amount of charge. The hybrid electrode delivered a specific capacitance of 1842.0 F g−1 at a current density of 1 A g−1 in a three-electrode test, and a symmetrical supercapacitor assembled from two of these electrodes reached an energy density of 33.79 Wh kg−1 at a power density of 375 W kg−1. When the device reached the end of its life, the authors did not throw the electrode away. They disassembled the spent cell, washed the recovered material, and showed that it still catalyzes the destruction of tetracycline antibiotics in water, closing a loop that links energy storage with wastewater treatment.
The work, published as an open-access research article in the journal Collagen and Leather on 1 August 2026, sits at the intersection of biomass valorization and electrochemistry. Carbonized polymer dots, or CPDs, are quasi-spherical carbon-based fluorescent nanoparticles generally smaller than 10 nm, with a graphitized carbon core and a shell of polymer chains. The team chose lignin as the carbon precursor because it contains at least 60 percent carbon, carries aromatic conjugated structures, and holds reversible redox-active functional groups. Converting lignin into CPDs, the authors note, could also reduce the environmental burden of industrial pulping black liquor while providing a route to upcycle the waste stream.
The synthesis itself is straightforward. One gram of crude industrial lignin, first rinsed with acidic water at pH 2 and vacuum-dried, was mixed with two grams of m-phenylenediamine, a nitrogen-doping additive, in 100 mL of deionized water. The mixture was sealed in a Teflon-lined autoclave and heated at 200 °C for 10 hours. After filtration through a 0.22 μm membrane, centrifugation, and dialysis, the solid CPDs were collected by vacuum freeze-drying at −35 °C for 72 hours. Dynamic light scattering showed a narrow size distribution between 2.1 and 2.5 nm, with an average diameter of 2.32 nm, and a zeta potential of −20.21 mV, a signature of the negatively charged, heteroatom-decorated surface.
Spectroscopy confirmed the chemical picture. Raman spectra showed D and G bands at 1359 and 1587 cm−1 with an IG/ID ratio of about 1.48, indicating a mix of disordered and graphitic carbon. Fourier transform infrared spectra revealed O−H and N−H stretching at 3441 cm−1, C−H at 2927 cm−1, and bands assigned to C=O, C−N, and C−O−C, while X-ray photoelectron spectroscopy measured elemental compositions of 63.24 percent carbon, 2.61 percent nitrogen, and 34.15 percent oxygen. The dots fluoresced at two emission centers, near 431 and 500 nm, depending on the excitation wavelength, and glowed bright green under 365 nm ultraviolet light. This dual emission turned out to matter for the next step, because the fluorescence dims predictably when certain metal ions bind to the dot surface.
That quenching behavior doubled as a sensing test and as evidence for the binding chemistry that anchors the dots to the hydroxide. The CPDs responded selectively to Fe3+ at both emission wavelengths and to Cu2+, Ag+, Hg2+, and Co2+ at the 500 nm channel. Calibration curves gave limits of detection of 0.61 μM for Fe3+ at 431 nm emission, 0.82 μM for Fe3+ at 500 nm, and 0.76 μM for Cu2+, with correlation coefficients as high as 0.9992. The authors attribute the quenching to stable complexes formed between the metal ions and the N- and O-containing functional groups on the dot shells, the same groups that let the dots chelate iron and cobalt during electrode assembly.
To build the electrode, the researchers mixed iron nitrate, cobalt nitrate, ammonium fluoride, and 4 mg of CPDs in water, then grew the composite hydrothermally on nickel foam at about 120 °C for 12 hours. Electron microscopy revealed a 0D/2D architecture in which the roughly 2 nm dots insert between and onto the surfaces of the layered double hydroxide nanoflakes, forming a rough, hierarchically porous network. Lattice fringes at 0.21 nm and 0.77 nm matched the graphitized (100) plane of the CPDs and the (003) plane of the FeCo-LDH, and X-ray diffraction confirmed the layered double hydroxide crystal structure alongside a broad amorphous carbon peak at 20.9°. X-ray photoelectron spectra showed that the Fe 2p and Co 2p peaks shifted by 0.15 and 0.05 eV after dot incorporation, evidence of electronic interaction between the two phases.
Those interactions translated into measurably better charge transport. The band gap of the hybrid, calculated from Tauc plots, was 1.68 eV, lower than the 1.82 eV of the bare FeCo-LDH, and the charge transfer resistance dropped from 19.17 Ω to 17.92 Ω. In 6 M potassium hydroxide, the hybrid electrode sustained a capacitance retention of about 50.19 percent, corresponding to 924 F g−1, at 10 A g−1, compared with about 32.73 percent, or 534 F g−1, for the undoped hydroxide. Kinetic analysis of cyclic voltammetry gave b-values of 0.54 for the hybrid and 0.51 for the control, indicating battery-type redox control, and the capacitive contribution reached 66.06 percent at a scan rate of 30 mV s−1. Over 20,000 charge-discharge cycles at 30 A g−1, the hybrid retained roughly 99.68 percent of its capacitance with near-unity coulombic efficiency.
The full device told a similar story. Two 10 mm electrodes were pressed into a CR2032 button cell with a Whatman separator and 6 M KOH electrolyte. The cell operated optimally at 1.5 V, delivered a specific capacitance of 108.13 F g−1 at 0.5 A g−1, and produced 33.79 Wh kg−1 at 375 W kg−1, still holding about 10 Wh kg−1 at 7500 W kg−1. At a punishing 20 A g−1, the device kept approximately 100 percent of its capacitance over 10,000 cycles with a coulombic efficiency of about 101.6 percent. The authors credit the dots with providing electron transport pathways, enlarging the interlayer spacing of the hydroxide to open ion channels, improving wettability through their hydrophilic groups, and cushioning the volume changes that normally degrade layered hydroxide electrodes at high current densities.
The recycling step is where the study departs from most supercapacitor reports. After mechanical disassembly of the spent cells, the recovered FeCo-LDH@CPDs electrodes were tested as Fenton-like catalysts for activating peroxymonosulfate, a common oxidant in advanced oxidation processes. With only PMS or nickel foam alone, less than 27.1 percent of tetracycline was removed, but the recovered catalyst removed about 90 percent within 30 minutes, fitting a pseudo-first-order rate constant of 0.05 min−1. The performance held up in seawater and tap water and in the presence of common anions such as sulfate, bicarbonate, and dihydrogen phosphate, and after five consecutive catalytic cycles the removal rate still reached about 72 percent.
Quenching experiments and electron paramagnetic resonance identified the reactive species involved. Furfuryl alcohol, a singlet oxygen scavenger, suppressed tetracycline degradation to just 3.4 percent with a rate constant of 0.001 min−1, marking singlet oxygen as the dominant reactive oxygen species, followed by contributions from superoxide, sulfate radicals, and hydroxyl radicals. Electrochemical measurements showed current spikes and open-circuit potential shifts upon adding PMS and tetracycline, supporting a direct electron transfer pathway in which adsorbed PMS forms a surface complex that accepts electrons from the antibiotic. The authors note that the same hybrid material could serve as a nanofiller in collagen-based materials and could catalytically degrade dyes and other pollutants typical of industrial leather wastewater when activating oxidants such as persulfate, though those applications remain proposals rather than demonstrated results. The reported figures come from laboratory-scale button cells and batch degradation tests, so translating the energy densities and catalytic lifetimes to practical devices and real effluents remains the next hurdle for this lignin-to-electrode-to-catalyst pipeline.
Subject of Research: Recyclable lignin-based carbon dot hybrid electrodes for supercapacitor energy storage and Fenton-like antibiotic degradation
Article Title: Immobilizing lignin-based carbonized polymer dots onto layered double hydroxides for high-performance supercapacitors with recyclable Fenton-like reactivity
Article References: Zhou, P., Guo, J., Chen, S., Yuan, S., Zhu, Y., Xiao, X., & Xu, J. (2026). Immobilizing lignin-based carbonized polymer dots onto layered double hydroxides for high-performance supercapacitors with recyclable Fenton-like reactivity. Collagen and Leather, 8(1), Article 30. https://doi.org/10.1186/s42825-026-00256-7
Image Credits: AI Generated
DOI: 10.1186/s42825-026-00256-7
Keywords: lignin, carbonized polymer dots, layered double hydroxides, supercapacitors, energy density, Fenton-like catalysis, tetracycline degradation, singlet oxygen, biomass valorization, water purification, electrode recycling, hydrothermal synthesis
News Source: Russell Cooper. (October 29, 2026). Lignin-derived carbon dots boost supercapacitors and recycle into pollutant-eating catalysts. Scienmag.
