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

The research, published in the journal Aerospace Systems, was carried out by Pavel Makeev of the Department of Aeronautical Engineering at the Moscow Aviation Institute. The work revisits and modernizes a concept first embodied in the tail rotor scheme of the TsAGI-1EA, an experimental helicopter built in the Soviet Union in the 1930s. The idea is deceptively simple: instead of mounting a single multi-bladed rotor on one side of the tail boom, the coaxial tail rotor places a pair of two-bladed rotors on the same axis but on opposite sides of the boom, one to the right and one to the left. This arrangement gives the rotors a substantial axial separation along the axis of rotation, and it is precisely that separation, the study finds, that unlocks an aerodynamic benefit familiar from coaxial main rotor systems.

To evaluate the concept, Makeev used a free-wake model, a computational approach that tracks the vortical wake shed by each blade as it moves through the air rather than imposing a prescribed wake geometry. The free-wake method is particularly valuable for coaxial configurations, where the wake of one rotor travels directly into the path of the other and the resulting blade-vortex interactions strongly influence both performance and loads. The simulations focused on hovering flight, the aerodynamically demanding condition in which a rotor must generate maximum thrust for the least possible power, and the rotors were analyzed in isolation, without the complicating influences of the main rotor wake or the helicopter airframe.

The coaxial tail rotor was benchmarked against two established alternatives: the conventional four-bladed tail rotor used on serial production helicopters and the so-called scissors tail rotor, a promising design in which blade pairs are mounted at an angle to one another on the same hub. The baseline axial spacing between the rotational planes of the coaxial blade pairs was set at 0.8 rotor radii, a value chosen because it is approximately equal to the doubled side spacing of a conventional tail rotor relative to the helicopter’s longitudinal axis. Both co-rotating and counter-rotating versions of the coaxial pair were examined, along with two different control strategies: one in which both rotors operate at the same blade pitch angle, and one in which both rotors are trimmed to produce the same thrust.

The headline result concerns hover efficiency, conventionally measured by the Figure of Merit, a dimensionless ratio that compares the ideal power required to produce a given thrust with the actual power consumed. For the coaxial counter-rotating configuration with 0.8R axial spacing operating under the equal-thrust strategy, the Figure of Merit increased by up to 12.2 percent relative to the conventional four-bladed rotor, while power consumption fell by approximately nine percent. That is a substantial gain for a component whose aerodynamic losses translate directly into fuel burn and payload capacity. The equal-thrust control strategy consistently outperformed the equal-pitch strategy, indicating that allowing each rotor of the pair to find its own pitch while matching thrust output extracts more of the available aerodynamic benefit.

The direction of rotation mattered as well. Counter-rotating pairs, in which the upper and lower rotors spin in opposite directions, delivered better performance than co-rotating pairs. This finding echoes the established behavior of coaxial main rotors, where counter-rotation allows the swirling wake of one rotor to partially cancel the swirl of the other, reducing rotational losses in the slipstream and improving the distribution of induced velocity across the rotor disk. The study also documented the shapes of the rotor wakes, the detailed flow patterns, and both distributed and total aerodynamic characteristics, providing a physical picture of how the separated blade planes interact through their vortex systems.

Perhaps the most intriguing result concerns how the axial spacing between the two rotors shapes the benefit. Makeev extended the analysis across a range of spacings from 0.6R to 2.0R and found that the maximum increase in Figure of Merit, reaching 15.8 to 16.2 percent, occurs at spacings of 1.4R to 1.8R. Beyond that range, the positive effect begins to decline. This non-monotonic behavior suggests there is an optimal window in which the wake of one rotor is far enough from the other to avoid destructive interference yet close enough for the two vortex systems to interact favorably. In practical terms, the axial separation inherent in the coaxial tail rotor arrangement makes it possible to maximize the positive aerodynamic effect that has long been recognized in coaxial main rotor research, where high Figure of Merit values at hover have been documented since the 1990s.

The implications extend beyond raw efficiency. Because the coaxial arrangement places rotors on both sides of the tail boom, it offers the potential for increased yaw control authority, which is directly relevant to flight safety. Loss of tail rotor effectiveness, whether caused by main rotor wake interference, crosswind conditions, or low-speed maneuvering, remains a serious hazard, and a tail rotor with higher aerodynamic efficiency and greater thrust capability for the same power budget could enlarge the safety margin in precisely the flight regimes where conventional designs are most vulnerable. The concept also fits into a broader research effort spanning variable-speed tail rotors, advanced blade tip shapes, and optimized twist distributions, all aimed at squeezing more performance from this hardworking component.

Makeev is careful to frame the findings as preliminary. All results were obtained for an isolated tail rotor, and real helicopters present a far messier aerodynamic environment. The wake of the main rotor sweeps past the tail, the airframe sheds its own vortices, and in forward flight the tail rotor operates in oblique flow rather than in the clean axial conditions of hover. Two open tasks stand out: quantifying how the main rotor wake and airframe modify the coaxial tail rotor’s performance, and assessing the drag penalty of the coaxial hub assembly at forward flight, since two hubs and their linkages inevitably create more parasite drag than a single conventional hub. The final choice of parameters, the study concludes, must account for the specifics of the helicopter on which the concept is to be installed.

Even with those caveats, the study makes a compelling case that a design idea nearly a century old deserves a second look with modern computational tools. A tail rotor that delivers more than twelve percent better hover efficiency and nine percent lower power consumption, with the potential for stronger yaw control and improved safety, would be a meaningful advance for an industry under pressure to reduce fuel consumption and emissions. As eVTOL aircraft and next-generation helicopters bring coaxial configurations back into the mainstream, the coaxial tail rotor may find itself moving from a 1930s curiosity to a standard feature of tomorrow’s rotorcraft.

Subject of Research: Numerical analysis of the hover aerodynamic performance of a coaxial helicopter tail rotor using a free-wake model

Article Title: Numerical study of the aerodynamic performance of the coaxial tail rotor concept in hover

Article References: Numerical study of the aerodynamic performance of the coaxial tail rotor concept in hover. (n.d.). https://doi.org/10.1007/s42401-026-00549-z

Image Credits: AI Generated

DOI: 10.1007/s42401-026-00549-z

Keywords: helicopter, coaxial tail rotor, hover, free-wake model, aerodynamic performance, Figure of Merit, rotorcraft, tail rotor efficiency, blade-vortex interaction, yaw control, computational fluid dynamics, aerospace engineering