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The role of convection in liquid thermoelectric cells may be difficult to predict because the inter- and intramolecular interactions are not currently incorporated into thermodynamic models. Here, we study the thermoelectric response of a series of five anhydrous 1-methyl-3- alkylimidazolium halide ionic liquids with varied chain length and counterion in a high-aspect-ratio, horizontal-temperature-gradient geometry, where convection is minimal. While a canonical constant-volume thermodynamic model predicts that the longer aliphatic groups exhibit larger Seebeck coefficients, we instead measure the opposite: Longer aliphatic chains correlate with lower densities and greater heat expansion, stronger intermolecular associations, stronger steric repulsion, and lower Seebeck coefficients. As evidence of the critical role of thermal expansion, we measure that the Seebeck effect is nonlinear: Values of −2.8 mV/K with a 10 K temperature difference and −1.8 mV/K with a 50 K difference are measured with ether ion. Our results indicate that steric repulsion and heat expansion are important considerations in ionic liquid design; with large temperature differences, the Seebeck coefficient correlates negatively with heat expansion. Our results suggest that Seebeck values will improve if thermal expansion is limited in a pressurized, isochoric, convection-free design.
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Converting Heat to Electrical Energy Using Highly Charged Polyoxometalate Electrolytes
Thermally regenerative electrochemical cycles and thermogalvanic cells harness redox entropy changes (ΔSrc) to interconvert heat and electricity with applications in heat harvesting and energy storage. Their efficiencies depend on ΔSrc because it relates directly to the Seebeck coefficient, yet few approaches exist for controlling the reaction entropy. Here, we demonstrate the design principle of using highly charged molecular species as electrolytes in thermogalvanic devices. As a proof-of-concept, the highly charged Wells-Dawson ion [P2W18O62]6– exhibits a large ΔSrc (−195 J mol–1 K–1) and a Seebeck coefficient comparable to state-of-the-art electrolytes (−1.7 mV K–1), demonstrating the potential of linking the rich chemistry of polyoxometalates to thermogalvanic technologies.
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- PAR ID:
- 10506903
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- ACS Applied Energy Materials
- ISSN:
- 2574-0962
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsaem.4c00036
