Thermoelectric energy conversion in nanochannels filled with ionic liquids*

In the past few years, thermoelectric energy conversion in electrolyte-filled nanochannels has received increasing attention. The thermovoltage created with corresponding devices often exceeds that of solid-state thermoelectric devices. Usually, significant thermovoltages can only be generated in the case of overlapping electric double layers (EDLs) from opposing channel walls. Ionic liquids (ILs) can be regarded as molten salts and are fundamentally promising electrolytes because of their high density of primary charge carriers. For ILs, the standard scheme of computing the EDL thickness results in values in the range of Angstroms, which precludes EDL overlap for all practical purposes. However, recent experimental results [1] indicate that the effective charge carriers in confined ILs are actually pseudoparticles, i.e. clusters of many primary charge carriers that can partially dissociate in a thermally activated process. Based on the coupled Poisson-Nernst-Planck, Navier-Stokes and heat transport equations, we study thermovoltage generation in an IL-filled nanochannel with different temperatures applied to the two ends of the channel. In that context, the number density of effective charge carriers is obtained from an Arrhenius equation. Next to the numerical model, we have developed an analytical model based on the long-wavelength approximation. The results indicate that for the same degree of EDL overlap, the thermovoltage obtained with ILs is significantly higher than that obtained with aqueous electrolytes, by about a factor of five at a wall zeta potential of 25 mV. The predictions of the analytical model agree very well with the numerical results. In total, our theoretical studies indicate that confined ILs bear a significant potential for thermoelectric energy conversion.

[1] Gebbie, M. A., Dobbs, H. A., Valtiner, M. Israelachvili, J. N. Long-range electrostatic screening in ionic liquids, PNAS, 2015, 112, 7432–7437.