Colossal thermo-hydro-electrochemical voltage generation for self-sustainable operation of electronics

Thermoelectricity, the direct conversion of heat into electricity or vice versa, has applications in thermocouples and Peltier devices. A major challenge is achieving high voltage per temperature difference (thermopower or Seebeck coefficient). Traditional solid-state inorganic and organic thermoelectric materials exhibit limited thermopower (0.01–0.1 mV K⁻¹). Recent research has explored the Soret effect, the thermodiffusion of ions, achieving thermopower up to 8 mV K⁻¹. However, exceeding several mV K⁻¹ remains difficult. Studies have demonstrated larger thermopower values through different mechanisms, including the thermodiffusion of electron/ion mixtures and temperature-dependent redox reactions. Solid-state polyelectrolytes utilizing the Soret effect show promise, but high water uptake often leads to stability issues. This paper focuses on achieving a high thermal-to-electrical conversion (TtoE) factor, which accounts for various principles generating thermally induced voltage. To power typical wearable electronics, many thermoelectric legs are needed to achieve sufficient voltage. This research introduces a novel approach based on the variation of potentials caused by a temperature difference to achieve a significantly higher TtoE factor.

The literature extensively documents research on enhancing thermoelectric energy conversion. Previous studies have explored various mechanisms to increase thermopower, focusing on thermodiffusion of ions (Soret effect) and temperature-dependent redox reactions. Solid-state polyelectrolytes have shown potential, but their performance is often limited by water uptake and stability issues. While some studies reported significant progress in increasing thermopower through various ion migration mechanisms, these mostly relied on high humidity levels, leading to stability concerns. Moisture-powered generators (MPGs) utilizing ion movement are distinct from thermally induced voltage generation, responding only to humidity changes. The authors aim to improve upon these limitations by exploring a thermo-hydro-electrochemical approach.

The device consists of polyaniline and polystyrene sulfonate (PANI:PSS) as a solid-state electrolyte and carbon steel foils as electrodes. PANI:PSS powders were synthesized, dissolved in deionized water with hydrochloric acid, and drop-casted onto the carbon steel electrodes. The assembly was left in a fume hood to allow for corrosion of the carbon steel surface, forming a porous layer composed of β-FeOOH nanorods. Water uptake in PANI:PSS was characterized as a function of relative humidity (RH). Thermally induced voltage was measured by varying the temperature difference between the electrodes and recording voltage as a function of time. Electrochemical impedance spectroscopy (EIS) was used to characterize the impedance of the oxidation layer. Experiments were conducted with graphite foils instead of carbon steel to study the role of thermodiffusion of ions. Moisture exposure experiments were performed to compare the device's response to heat and humidity. A 3-electrode configuration was used to study corrosion behavior with different proton concentrations. In situ attenuated total reflectance (ATR) Fourier transform-infrared spectroscopy (FTIR) was employed to observe water thermodiffusion in PANI:PSS. An Evans diagram is used to explain the voltage generation mechanism. The functionality of the device as an energy harvester was demonstrated by connecting it to a capacitor and a load resistor, and by connecting multiple devices in series and parallel. A self-sustainable fever detector was developed by integrating the device with an electrochromic display.

The researchers achieved a colossal TtoE factor of -87 mV K⁻¹ at 22 °C and 50% RH using carbon steel electrodes and a PANI:PSS solid-state electrolyte. This value is significantly higher than values reported in the literature. The increase in the TtoE factor is attributed to the variation in corrosion potentials due to thermodiffusion of water. The study found that the thermodiffusion of protons in PANI:PSS is not the major contributor to voltage generation in this system, unlike previous high-TtoE factor studies. Experiments with graphite electrodes produced a much smaller TtoE factor. The optimum performance at 50% RH indicates a distinct working principle compared to previous studies which showed monotonic increases in TtoE factor with RH. The increase in impedance of the oxidation layer, caused by the corrosion of the carbon steel, is key to the improved TtoE factor. In situ ATR-FTIR spectroscopy confirmed water thermodiffusion from the hotter to the colder side. The Evans diagram illustrates how the potential difference is developed due to the change in corrosion overpotential resulting from the altered water concentration on each electrode. Connecting multiple devices in series linearly increased the output voltage, demonstrating the scalability of the approach. A proof-of-concept fever detection device was created by integrating the thermo-hydro-electrochemical energy harvester with an electrochromic display. The device showed distinct voltage responses under simulated normal and fever conditions.

The results demonstrate a novel mechanism for generating a large TtoE factor, significantly surpassing previous achievements. The thermo-hydro-electrochemical approach overcomes limitations of existing methods by utilizing readily available and low-cost materials while operating under typical ambient conditions. The findings show the importance of considering corrosion effects and water thermodiffusion in the development of highly efficient thermoelectric devices. The ability to power electronic devices, including a functional fever detector, highlights the practical applications of this technology. This approach presents a new direction for developing self-sustainable electronics and sensors.

This research demonstrates a novel thermo-hydro-electrochemical method for generating a high thermal-to-electrical energy conversion factor (-87 mV K⁻¹), significantly exceeding previous results. The system leverages the change in corrosion potential caused by water thermodiffusion using readily available carbon steel and PANI:PSS. The practical applications are demonstrated through powering a self-sustainable fever detection device. Future research should focus on optimizing material properties and exploring different electrode and electrolyte combinations to further enhance the TtoE factor and expand applications.

The current study focuses on a specific electrode and electrolyte combination. Further investigation is needed to explore the performance of different materials and their influence on the TtoE factor. The long-term stability of the device under continuous operation needs more investigation. While the corrosion of the carbon steel was shown to be minimal over six months, further studies on the lifetime and durability of the device are required.