The Seebeck effect lies at the heart of thermoelectric textiles. When one side of a material is hotter than the other, charge carriers diffuse from the hot region to the cold one, generating a measurable voltage. The ratio of that voltage to the temperature gradient defines the Seebeck coefficient, the central parameter for any thermoelectric material. Because the human body maintains a temperature of roughly 37 degrees Celsius against an ambient surroundings that is often cooler, clothing that exploits this gradient could, in principle, trickle-charge sensors, health monitors, and other smart textile devices without batteries. Cotton is a natural substrate for such applications: it is the most widely used fabric in the fashion industry, comfortable against the skin, and compatible with established coating techniques.

The challenge has always been how to make cotton conductive without destroying its textile character. Carbon nanomaterials such as carbon nanotubes and carbon nanofibers offer promising electronic properties, but they are typically produced as solid powders that resist integration into fabrics. The standard workaround is to disperse them into inks, which can then be applied by screen printing, inkjet printing, spray coating, or dip coating. In this study, the researchers formulated aqueous inks containing two Pyrograf III carbon nanofiber grades, PR 24 LHT XT and PR 25 PS XT, both synthesized by chemical vapor deposition at 1,100 degrees Celsius but subjected to different post-synthesis thermal treatments: 1,500 degrees Celsius for the PR 24 grade and 700 degrees Celsius for the PR 25 grade. These treatments strip a polyaromatic carbon layer from the nanofiber surfaces, and they leave the two grades with distinct structural signatures.

Transmission electron microscopy revealed that both nanofibers share a double-walled structure enclosing a hollow tubular core, with an inner layer of compacted parallel graphene sheets oriented at an angle to the core axis and a more disordered outer layer. The PR 24 fibers, however, have smaller total diameters of 80 to 100 nanometers compared with 125 to 150 nanometers for PR 25, and Raman spectroscopy confirmed that PR 24 possesses higher crystallinity, showing narrower D and G bands and a detectable 2D mode. The fabrics were functionalized by repeated immersion in inks containing 1.6, 3.2, or 6.4 milligrams of nanofibers per milliliter of distilled water, stabilized with the surfactant sodium dodecybenzenesulfonate, followed by drying cycles and rinsing steps to remove surfactant residues. Scanning electron microscopy showed that at the lowest concentration the cotton weave remained largely visible, while at 6.4 milligrams per milliliter an entangled mat of nanofibers completely covered the fabric surface, with the fibers concentrated near the surface rather than penetrating the full weave.

Thermogravimetric analysis under nitrogen confirmed that all coated fabrics remained thermally stable up to 300 degrees Celsius, matching the behavior of uncoated cotton, and revealed that the fabrics coated with the most concentrated inks carried approximately 8 to 9 percent nanofiber content by weight. At the highest concentration, the residue percentages at 600 degrees Celsius converged to about 18 to 19 percent for both nanofiber types, indicating that the coating had reached saturation and become homogeneous. This saturation point proved decisive for the electrical performance of the resulting conductive textiles.

The thermoelectric measurements at 30 degrees Celsius told a nuanced story. Both nanofiber types, measured in agglomerated solid form, showed similar electrical conductivities of roughly 133 to 136 siemens per meter, values that represent a practical ceiling for textile applications using simple dispersion methods. The coated fabrics, however, diverged depending on ink concentration: at the lowest loading, fabrics coated with the thicker PR 25 fibers actually conducted better, reaching 5.1 siemens per meter, while at the highest loading the PR 24 fabrics took the lead, achieving 35.4 siemens per meter against 23.2 for PR 25. Both fabric types exhibited negative Seebeck coefficients, confirming n-type conduction, with PR 24-based textiles reaching about minus 6 microvolts per kelvin compared with minus 1 for PR 25. Intriguingly, the absolute Seebeck values of the fabrics exceeded those of the corresponding nanofibers, a phenomenon the researchers attribute to slight electron doping from adsorption between the graphitic planes and cellulose molecules, and possibly from residual surfactant.

Combining these parameters, the best fabric, coated with the highest concentration of PR 24 nanofibers, achieved a power factor of approximately 1.2 times 10 to the minus 3 microwatts per meter per square kelvin. Using a novel null-point scanning thermal microscopy method, the team measured the thermal conductivity of the coated fabrics at just 0.06 watts per meter per kelvin, only twice that of uncoated cotton and far below the values typical of carbon nanotube-filled polymer composites. This low thermal conductivity is a genuine advantage for thermoelectric applications, since it keeps heat from leaking across the gradient. The resulting figure of merit, zT, reached approximately 6.3 times 10 to the minus 6 at 30 degrees Celsius, modest compared with inorganic thermoelectrics but notable for a flexible textile and higher than values previously reported for the same nanofibers in melt-mixed polymer composites.

The temperature dependence of the electrical conductivity between 30 and 100 degrees Celsius revealed perhaps the most striking result. While the nanofibers themselves showed a positive temperature coefficient, with conductivity decreasing as temperature rose, the coated fabrics behaved in the opposite way, exhibiting a negative temperature coefficient. The best PR 24 fabric increased from 35.4 to 45.0 siemens per meter across the range, and the PR 25 fabric rose from 23.2 to 29.5. Fitting the data to a three-dimensional variable range hopping model showed that the fabrics conduct through thermally activated hopping of charge carriers across a random network of potential wells, with mean energy potentials of roughly 15 electronvolts, whereas the nanofibers alone conduct through thermally enhanced backscattering from impurities or structural defects. In other words, coating the fibers onto cotton fundamentally changes the transport mechanism.

The Seebeck coefficients of the fabrics also grew in absolute value with temperature, from minus 6.1 to minus 7.5 microvolts per kelvin for the lowest-concentration PR 24 fabric. Modeling this nonlinear behavior with an approach developed for doped multi-walled carbon nanotube mats suggested that the PR 24 fabrics are more likely to be negatively doped, consistent with electron donation from the cellulose fibers or the surfactant used in the ink formulation. The PR 25 fabrics, by contrast, retained a positive metallic term in the model, indicating a different doping character despite their similar structure.

Overall, the study demonstrates that despite meaningful structural differences between the two Pyrograf III grades, the resulting conductive textiles are remarkably similar in their thermoelectric properties, with the smaller-diameter, more crystalline PR 24 fibers holding a slight edge at high ink concentrations. The researchers suggest these n-type carbon nanofiber textiles could serve as reference materials for future studies combining them with other n-type organic or inorganic components, moving toward a new generation of flexible, wearable thermoelectric generators that draw power directly from the temperature difference between our bodies and the world around us.

Subject of Research: thermoelectric cotton textiles coated with Pyrograf III carbon nanofibers

Article Title: Comparative study of thermoelectricity in cotton fabrics coated by immersion with Pyrograf® III carbon nanofibers-based inks

Article References: Paleo, A. J., Abreu, J., Krause, B., González-Casal, S., Thomas, L., Cerqueira, M. F., Muñoz, E., & Pötschke, P. (2026). Comparative study of thermoelectricity in cotton fabrics coated by immersion with Pyrograf® III carbon nanofibers-based inks. Frontiers in Materials, 13, Article 1933448. https://doi.org/10.3389/fmats.2026.1933448

Image Credits: AI Generated

DOI: 10.3389/fmats.2026.1933448

Keywords: thermoelectric textiles, carbon nanofibers, cotton fabrics, n-type conduction, smart textiles, wearable energy harvesting, dip coating, Seebeck effect, Comparative, thermoelectricity, cotton, fabrics