thermogalvanic

Thermogalvanic refers to phenomena and devices that convert thermal energy into electrical energy through the thermogalvanic effect. In a thermogalvanic cell, two electrodes containing a redox couple are kept at different temperatures. The temperature dependence of the redox potentials establishes a potential difference between the electrodes, which drives current through an external circuit when the circuit is closed.

Mechanism and performance: The open-circuit voltage of a thermogalvanic cell is approximately proportional to the temperature difference across the cell, with the proportionality defined by the Seebeck coefficient of the chosen redox couple (V ≈ S × ΔT). The Seebeck coefficient for thermogalvanic systems is typically on the order of a few millivolts per kelvin, varying with the redox couple, electrolyte, and solvent. The overall performance depends on the electrochemical kinetics, internal resistance, and the stability of the redox couple under the operating temperatures.

Materials and systems: Common redox couples include ferri/ferrocyanide, iodine/iodide, quinone/hydroquinone pairs, and various transition-metal complexes. Both aqueous and non-aqueous electrolytes are used, and electrodes are often carbon-based or metallic. Temperature gradients can be established by external heating, by exploiting body heat, or by ambient thermal differences. Liquid electrolytes can enable large ΔT but introduce challenges such as convection, chemical stability, and long-term durability.

Applications and challenges: Thermogalvanic systems offer a path to harvest low-grade heat from waste streams or body heat, potentially enabling flexible and low-cost energy sources. Challenges include achieving higher Seebeck coefficients, reducing internal resistance, ensuring chemical stability across operating ranges, and scaling the technology for practical power output. Ongoing research explores new redox couples, solvents, and device architectures to improve efficiency and durability.