Development of Ionic-thermoelectric Material and Device for Converting Low-grade Heat to Electricity

Description

Low-grade heat (<100°C) is abundantly available in industrial processes, the environment, living things, and solar-thermal and geothermal energy. However, the efficient conversion of low-grade heat into electricity remains a great technological challenge. Existing solid-state thermoelectric (TE) semiconductors rely on electrons as charge carriers, which have thermopower or Seebeck coefficients on the scale of 100 to 200 μV K−1. The generation of a useful voltage of 1–5 V from a small temperature difference requires the challenging integration of thousands of tiny TE elements or the use of a DC–DC voltage booster. Ionic thermoelectric (i-TE) materials, in contrast, use ions as energy carriers and provides an effective alternative approach for consolidating widely distributed waste heat into electricity. i-TE systems can reach a high thermopower on the scale of mV K−1 and offer opportunity to engineer entropy changes and thermal and electrical transport, which are inherently coupled in solid-state TEs. Interest in the exploration of i-TE materials for energy harvesting can be traced back to the last decade, particularly to research in the thermodiffusion effect (Soret effect) and thermogalvanic effect in liquid or gel systems. The recently discovered phenomenon of giant i-TE thermopower portends great promise in the use of quasi-solid-state i-TE materials for future low-grade heat energy-harvesting applications.In this project, we aim to build a game-changing i-TE technology with a synergistic combination of thermodiffusion and thermogalvanic effects, which will require a coherent i- TE material design and device optimization. A hydrogel material will be chosen as the host skeleton, as such hydrogels not only swell upon absorbing large amounts of water and dissolve salts but also provide abundant polymer chains and functional groups for ion transport regulation. Strategies to engineer the Eastman entropy of ion transfer and redox entropy change will involve tailoring the interactions between solvents (e.g., polymer chains, water-structure breaking molecules, nonaqueous solvents, thermosensitive materials) and solutes (e.g., nonredox/ redox ions, bi-redox ions), with the aim of ensuring that each species can perform its function separately and synergistically in the hydrogel matrix. Device design is equally important for a high power output, and key concerns in this area include the electrode design and effective device configuration. We will also construct a theoretical model to methodically engineer i-TE systems. This proposed model will require advanced characterization techniques to provide basic measurements for model verification. The device efficiency and cycle performance will be carefully studied to achieve a reliable i-TE system.