A new thermodynamic design links ion movement and concentration to energy output, creating soft polymer films that generate steady electrical power from tiny temperature differences such as body heat.
(Nanowerk Spotlight) Every surface that feels warm holds a trace of untapped energy. The warmth of a wristwatch against skin, the heat that seeps from a laptop, or the faint temperature rise on a phone’s casing all represent small, steady gradients between one side and another. In principle, those gradients could power sensors, medical patches, or communication chips without ever needing a battery. In practice, they mostly disappear into the air because converting gentle heat into electricity has proved surprisingly difficult.
Traditional thermoelectric materials can do this job, but only when heat differences are large, such as in engines or industrial pipes. They rely on the movement of electrons in rigid crystals like bismuth telluride, which conduct charge well but are brittle, costly, and poorly matched to soft surfaces. The dream of flexible devices that harvest warmth from the body or the environment has been limited by those physical constraints.
A younger approach tries something different. Instead of driving electrons through a solid, it lets ions, which are charged atoms or molecules, move through a soft polymer. When one side of the material is warmer, certain ions drift faster than others, creating an electrical potential. This effect can produce far larger voltages than conventional thermoelectrics, but it has brought its own puzzle.
Materials that give high voltage often carry current poorly, while materials that conduct well produce little or no voltage at all. Researchers have lacked a clear map showing how to balance these competing properties, and progress has depended on intuition more than theory.
By linking two basic quantities—the number of mobile ions and their speed of movement—to the voltage and power a material can produce, the authors demonstrate thin, flexible films that convert even a 1.5 degree temperature difference into usable electrical energy.
The study presents a thermodynamic map that connects ion concentration, meaning how many charge carriers are available, and diffusion coefficient, meaning how fast they move, to overall performance. By adjusting these parameters through chemical design, the team achieved record levels of voltage and power from polymer films that remain flexible and stable in ordinary air.
Characteristics of pristine PSS and ionic TE polymer complex. a) Image and principle of H+ dominated Soret effect in the pristine PSS.b) Schematic illustration of the molecular structure of the polymer complex, showing additional H+ dissociation from PEDOT+-PSS− complex formation. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Performance in ionic thermoelectrics is measured using the ionic figure of merit, written as ZTi. This value rises when the material has a high ionic Seebeck coefficient, noted as Si, a high ionic conductivity, noted as σi, and a low thermal conductivity.
The Seebeck coefficient is the voltage generated per unit of temperature difference. Conductivity reflects how easily ions move, and thermal conductivity shows how quickly heat itself flows. High voltage and high current together make strong power output, while low heat leakage preserves the temperature difference.
The study finds that both Si and σi depend on the same two levers, the ion concentration ni and the diffusion coefficient Di. With this realization, ionic thermoelectrics become a balance problem: adding more mobile ions increases conductivity but risks lowering voltage if both positive and negative ions move together. Reducing mobility can raise voltage but weakens current. The goal is to immobilize one ion while keeping the other mobile.
To demonstrate this principle, the study focuses on a polymer mixture built from poly(4 styrenesulfonic acid), known as PSS, a negatively charged polymer that naturally releases protons when humid. These protons serve as the positive carriers. PSS alone allows both protons and some mobile anions to drift, which limits voltage.
The team introduces poly(3,4 ethylenedioxythiophene), known as PEDOT, a conductive polymer carrying fixed positive charges. PEDOT binds tightly to the sulfonate groups on PSS. This binding releases extra protons but keeps the bulky PEDOT segments nearly immobile, ensuring that the protons dominate motion.
By adjusting the amount of PEDOT, the researchers trace how voltage and conductivity change. At 9.09 percent PEDOT by weight, the film produces an ionic Seebeck coefficient of 40.2 millivolts per kelvin and an ionic conductivity of 0.507 siemens per centimeter at 80 percent relative humidity. Those numbers yield an ionic power factor, the product of Si squared and σi, of 81.9 milliwatts per meter per kelvin squared, and a figure of merit ZTi of 49.5. These are the highest reported for a positive type ionic thermoelectric film.
Chemical measurements support the design logic. As PEDOT content rises, the solution’s acidity increases, showing that more protons are freed. When PEDOT concentration grows too high, it aggregates into larger domains that hinder ion flow, reducing the diffusion coefficient. The synthesis also involves careful control of the oxidant ammonium persulfate. Too much oxidant leaves mobile bisulfate anions that travel easily and cancel the electric field created by protons. The data illustrate how tuning ion concentration and mobility together determines the direction and magnitude of voltage.
The same strategy yields a negative type material by adding copper chloride, written chemically as CuCl₂, to the optimized PEDOT PSS mixture. Chloride ions are small and fast. Copper ions, in contrast, form strong bonds with sulfonate groups and PEDOT, becoming effectively stationary. This asymmetry leaves chloride as the main carrier.
At 50 percent CuCl₂ by weight, the Seebeck coefficient reaches minus 64.2 millivolts per kelvin and conductivity 0.129 siemens per centimeter, with a thermal conductivity of 0.488 watts per meter per kelvin. The resulting ZTi equals 32.2, again the highest value reported for a negative type ionic thermoelectric.
Spectroscopic analysis confirms the chemistry. A shift in a characteristic PEDOT Raman peak shows that copper ions reduce the polymer’s level of positive charge, locking it in place and suppressing unwanted proton motion. Beyond the optimum CuCl₂ content, extra ions crowd the channels, increasing concentration but lowering mobility and power output. The interplay of concentration and diffusion again explains the trend, validating the thermodynamic design model.
With both polarities available, the researchers build ionic thermoelectric capacitors, devices that store charge created by a temperature difference. Each consists of two electrodes separated by the ionic polymer film. When one side is warmer, ions migrate and the device charges. Connecting it to an external circuit releases the energy.
Even with a temperature difference as small as 1.5 kelvin, the positive type device delivers a normalized power density of 46.7 milliwatts per square meter per kelvin squared, while the negative type device reaches 79.0 in the same units. These outputs exceed previous records for such small gradients.
The team then assembles flexible modules by linking multiple units on thin plastic films coated with gold. A module combining ten positive negative pairs generates about 1.03 volts per kelvin and a power density near 981 milliwatts per square meter per kelvin squared. A module with nineteen negative type units alone achieves 1.15 volts per kelvin and 654 milliwatts per square meter per kelvin squared. Both remain stable after two months of storage in air without protective coatings and retain over 95 percent of their initial performance. A temperature difference of only 1.5 kelvin across the module is enough to light a standard light emitting diode directly, with no external amplifier or energy storage.
These results demonstrate ionic thermoelectrics working under conditions that resemble real world surfaces such as skin, clothing, or pipelines. The films are thin, flexible, and made from water based polymers and common salts, suggesting a straightforward route to scaling. Their performance depends on humidity, which enhances ion motion by opening water rich channels within the polymer, a factor that designers must consider for applications in drier environments.
Beyond the numbers, the study’s lasting contribution is methodological. Instead of exploring materials by trial, it constructs a thermodynamic design map that links molecular interactions to measurable parameters. Strong binding between the polymer and one ion species immobilizes it and allows the opposite ion to carry charge efficiently. Ion concentration and diffusion can be adjusted through chemical composition and synthesis conditions, giving clear guidance for future optimization.
By translating these principles into design rules, the researchers supply a framework for soft thermoelectric devices that can harvest gentle temperature gradients without rigid materials or large heat differences. The work moves ionic thermoelectrics from empirical curiosity toward predictable engineering, a necessary step for any technology meant to function quietly on flexible surfaces and powered only by the warmth that surrounds it.
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Sung-Yeon Jang (Ulsan National Institute of Science and Technology)
, 0000-0002-4225-3167 corresponding author
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