André Luiz Sehnem | Searcharxiv

arXiv2022

Dynamic response of a thermoelectric cell induced by ion thermodiffusion

Aqueous electrolyte solutions under the influence of a temperature gradient can generate thermoelectric fields, which arise from different responses of the positive and negative charges. This is related to the thermo diffusion effect which is quantified by the ionic heat of transport and the thermal diffusion coefficient. When a solution is confined between two metallic electrodes with different temperatures, it is expected to measure the electrostatic potential difference due to the thermoelectric field. We performed experiments in aqueous electrolyte solutions, and our results show that a thermoelectric field appears instantly in the solution as the temperature difference between the electrodes stabilizes. This field arises from the trend to separate both ionic charges in the temperature gradient. The experimental results also show that a slow change in the potential difference is observed, which is related to thermodiffusion in the entire cell. These results are important to understand how the thermoelectric response can be optimized, given the broad interest in the literature to generate thermoelectric energy from thermoelectric cells.

cond-mat.soft↗

arXiv2021

On the time-dependent electrolyte Seebeck effect

Single-ion Soret coefficients $α_{i}$ characterize the tendency of ions in an electrolyte solution to move in a thermal gradient. When these coefficients differ between cations and anions, an electric field can be generated. For this so-called electrolyte Seebeck effect to occur, the different thermodiffusive fluxes need to be blocked by boundaries -- electrodes, for example. Local charge neutrality is then broken in the Debye-length vicinity of the electrodes. Confusingly, many authors point to these regions as the source of the thermoelectric field yet ignore them in derivations of the time-dependent Seebeck coefficient $S(t)$, giving a false impression that the electrolyte Seebeck effect is purely a bulk phenomenon. Without enforcing local electroneutrality, we derive $S(t)$ generated by a binary electrolyte with arbitrary ionic valencies subject to a time-dependent thermal gradient. Next, we experimentally measure $S(t)$ for five acids, bases, and salts near titanium electrodes. For the steady state we find $S\approx2~\mathrm{mV~K}^{-1}$ for many electrolytes, roughly one order of magnitude larger than predictions based on literature $α_{i}$. We fit our expression for $S(t)$ to the experimental data, treating the $α_{i}$ as fit parameters, and also find larger-than-literature values, accordingly.

physics.chem-ph↗