Ionic Liquid Trick Boosts Polyaniline Thermoelectric Power 400-Fold

Waste heat is everywhere — radiating from car engines, industrial pipes, laptops, and even the human body — and for decades researchers have dreamed of materials that could capture it and turn it directly into electricity. A team of materials scientists from Algeria and France now reports a striking step toward that goal using one of chemistry’s most humble workhorses: polyaniline, a cheap and flexible conducting polymer. Writing in the Journal of Materials Science, the group describes an ionic liquid-assisted synthesis strategy that boosted the thermoelectric performance of polyaniline by roughly a factor of four hundred compared with the polymer made under conventional conditions, a result that could reshape how engineers think about organic energy-harvesting materials.

Thermoelectric materials sit at the intersection of heat and electricity: when one side of such a material is hotter than the other, charge carriers diffuse from the hot region to the cold one, generating a voltage. The efficiency of this conversion is governed by a dimensionless figure of merit, ZT, which depends on the Seebeck coefficient, the electrical conductivity, and the thermal conductivity of the material. The problem, famously, is that these three quantities are entangled in awkward ways. Improving one often degrades another, which is why the best inorganic thermoelectrics — bismuth telluride and its relatives — are expensive, brittle, and often built from scarce or toxic elements such as tellurium, antimony, and lead.

Conducting polymers offer an appealing escape route. Polyaniline in particular is inexpensive, mechanically flexible, lightweight, and far less toxic than its inorganic rivals, and it can be synthesized by simple oxidative polymerization of aniline in water. Its weakness has always been modest thermoelectric performance: its electrical conductivity and Seebeck coefficient are individually unremarkable, and the power factor — the product of the Seebeck coefficient squared and the conductivity — has remained low. The new study attacks that limitation not by adding metal nanoparticles or carbon fillers, but by redesigning the polymerization chemistry itself with tailor-made ionic liquids.

Ionic liquids are salts that are liquid at or near room temperature, composed of bulky organic cations paired with organic or inorganic anions. Their defining virtues are negligible vapor pressure, high thermal stability, and an extraordinary capacity for structural tuning. In polymer synthesis they can play several roles at once: as solvents, as soft templates that steer the growing polymer into particular morphologies, and as dopants that insert counter-ions into the polymer backbone. Earlier work has shown that ionic liquids can drive polyaniline to form nanofibers and other nanostructures, and that adding ionic liquids to conducting polymer films can raise conductivity while preserving or even enhancing the Seebeck coefficient.

The research team, led by Mohamed Ali Mokrani of the Ecole Militaire Polytechnique in Algiers, together with colleagues at INSA Lyon and Université Paris-Est, took this idea further by employing peroxydisulfate-based ionic liquids as oxidants. Two variants were tested: EMImS₂O₈, built on the 1-ethyl-3-methylimidazolium cation, and BMImS₂O₈, based on the 1-butyl-3-methylimidazolium cation. Because the peroxydisulfate anion is itself a strong oxidizing agent, these ionic liquids could initiate the polymerization of aniline while simultaneously acting as structural modifiers, their bulky imidazolium cations interfering with the way polymer chains aggregate. A third ionic liquid, the Brønsted acidic HMImHSO₄, served as the dopant, protonating the emeraldine form of polyaniline to create mobile charge carriers.

The resulting composites were subjected to a battery of characterization techniques. Fourier-transform infrared spectroscopy and Raman spectroscopy confirmed the formation of the emeraldine salt phase and revealed interactions between the polymer and the imidazolium species, including hydrogen bonding of the kind previously documented between polyaniline and imidazolium salts. UV-Vis spectroscopy tracked the doping state of the polymer, X-ray diffraction probed its crystallinity and chain ordering, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy mapped the morphology and elemental composition of the composites. Together these measurements showed that the ionic liquids had substantially reorganized the polymer’s microstructure relative to polyaniline synthesized without them.

The thermoelectric measurements told the most compelling story. The composite prepared with the ethyl-substituted oxidant, PANI–EMImS₂O₈–HMImHSO₄, delivered an electrical conductivity of 106.97 S per meter at room temperature together with a Seebeck coefficient of 33.21 microvolts per kelvin. While the Seebeck coefficient remained moderate — as is typical for heavily doped conducting polymers — the combination produced a markedly improved power factor, the quantity that determines how much electrical power a thermoelectric leg of given geometry can deliver. The dimensionless figure of merit reached 5.76 × 10⁻⁴ at room temperature, approximately four hundred times the value measured for neat polyaniline synthesized under otherwise similar conditions.

The comparison with the butyl-substituted variant proved equally instructive. Changing only the alkyl chain length on the imidazolium cation altered the final thermoelectric performance, underscoring that the cation is not a passive spectator but an active participant in shaping chain packing, doping efficiency, and charge transport. This sensitivity to molecular architecture gives chemists a genuine design lever: by selecting cation size, anion chemistry, and dopant identity, they can tune the microstructure of the polymer at the nanoscale without ever changing its fundamental composition. The finding echoes the group’s earlier work on imidazolium ionic liquids in polypyrrole, in which the anionic moiety of the ionic liquid was shown to influence thermoelectric properties, and it aligns with broader evidence that ionic-liquid processing can simultaneously raise conductivity and Seebeck coefficient in polymers such as PEDOT:PSS.

It is important to keep the absolute numbers in perspective. A ZT of 5.76 × 10⁻⁴ is still far below the values of one to two that define state-of-the-art inorganic thermoelectrics, so nobody should expect polyaniline films to compete with bismuth telluride modules in high-power applications any time soon. But the relevant comparison for many emerging applications is not with power stations; it is with the alternatives for low-grade, distributed energy harvesting, where flexibility, processability, low cost, and mechanical compliance matter as much as raw efficiency. Flexible thermoelectric generators for wearable electronics, self-powered sensors, and coating of curved surfaces are exactly the niches where organic materials hold structural advantages that inorganic crystals cannot match.

The study also carries a broader message about how materials chemistry advances. Rather than searching for an entirely new polymer, the researchers showed that a rational, molecularly precise modification of a well-known synthesis route — swapping a conventional oxidant for an ionic liquid that doubles as a structural template — can multiply performance by orders of magnitude. The work was supported by the Ecole Militaire Polytechnique and the Ecole Nationale Préparatoire aux Etudes d’Ingéniorat, and its authors suggest that rationally designed ionic liquid systems offer a general route to tailoring the microstructure and optimizing the thermoelectric properties of conductive polymers. If subsequent studies can push the power factor further, perhaps by combining ionic-liquid templating with nanostructuring or hybrid fillers, the dream of wrapping waste-heat harvesters around engines, pipes, and even skin may come one flexible step closer to reality.

Subject of Research: Ionic liquid-assisted synthesis of polyaniline composites for enhanced thermoelectric performance

Article Title: Evaluation of thermoelectric performance of polyaniline-based peroxydisulfate ionic liquid composites

Article References: Mokrani, M. A., Bekkar Djelloul Sayah, Z., Chabane, H., Mekki, A., Bourenane Cherif, Y., Nedjar, L., Kaufa, M., Slimani, Z. Y., Livi, S., & Durastanti, J. F. (2026). Evaluation of thermoelectric performance of polyaniline-based peroxydisulfate ionic liquid composites. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13899-y

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13899-y

Keywords: thermoelectrics, polyaniline, ionic liquids, peroxydisulfate, conducting polymers, waste heat recovery, power factor, Seebeck coefficient, doping, energy materials, imidazolium, figure of merit