Confinement Effect on Thermopower of Electrolytes

TL;DR

This paper develops a theoretical model using Poisson-Nernst-Planck equations to explain the giant thermopower observed in confined electrolytes, clarifying the role of confinement and ionic diffusivities in the ionic Seebeck effect.

Contribution

It provides an analytical and numerical solution to the PNP equations for confined electrolytes, elucidating the confinement effects on thermopower and resolving theoretical ambiguities.

Findings

Analytical solution matches numerical results across various parameters.

Confinement perpendicular to temperature gradient enhances thermopower for mismatched diffusivities.

Giant thermopower arises from confinement effects explained by the model.

Abstract

Ionic Seebeck effect of electrolytes has shown promising applications in harvesting energy from low-grade waste-heat sources with small temperature difference from the environment, which can power sensors and Internet-of-Things devices. Recent experiments have demonstrated giant thermopower (~ 10 mV/K) of electrolytes under confinement due to the overlapping of electric double layer (EDL). Nonetheless, there has been no consensus on the theory of the ionic Seebeck effect, especially whether the thermopower depends on ionic diffusivities, imposing confusion on the theoretical interpretation of experimental discovery on giant thermopower of confined electrolytes. This article presents a linear perturbative solution of Poisson-Nernst-Planck (PNP) equations to describe the ionic Seebeck effect of confined liquid electrolytes. We provide both analytical and numerical solutions to the PNP equations for closed systems and open systems connected to reservoirs of electrolytes. The analytical solution captured the confinement effect both along and perpendicular to the temperature gradient, and showed excellent agreement with numerically solved PNP equations for a wide range of EDL potentials, channel widths, and lengths. Finally, we show that for polyelectrolytes with largely mismatched diffusivities, thermopower can only be enhanced for closed system through confinement perpendicular to the temperature gradient.