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Open AccessArticle

Role of Heat Expansion with a Series of Ionic Liquids: The Case for Isochoric Thermoelectric Generators and Minimal Steric Repulsion

1
Department of Chemistry, Queens College of the City University of New York, Flushing, NY 11367, USA
2
Department of Physics, Queens College of the City University of New York, Flushing, NY 11367, USA
3
Department of Mechanical Engineering, University of California at Riverside, Riverside, CA 92521, USA
*
Author to whom correspondence should be addressed.
Entropy 2019, 21(11), 1086; https://doi.org/10.3390/e21111086
Received: 8 October 2019 / Revised: 31 October 2019 / Accepted: 1 November 2019 / Published: 6 November 2019
(This article belongs to the Special Issue Thermodynamics of Thermoelectric Devices and Applications)
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. View Full-Text
Keywords: thermoelectric; Seebeck coefficient; convective thermoelectric generator; complex ionic-liquid systems; steric forces thermoelectric; Seebeck coefficient; convective thermoelectric generator; complex ionic-liquid systems; steric forces
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MDPI and ACS Style

Jackson, M.; Engel, R.R.; Vuong, L.T. Role of Heat Expansion with a Series of Ionic Liquids: The Case for Isochoric Thermoelectric Generators and Minimal Steric Repulsion. Entropy 2019, 21, 1086.

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