On the time-dependent electrolyte Seebeck effect

Abstract

Single-ion Soret coefficients αi 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 ≈ 2 mV K −1 for many electrolytes, roughly one order of magnitude larger than predictions based on literature αi. We fit our expression for S(t) to the experimental data, treating the αi as fit parameters, and also find larger-than-literature values, accordingly.

Simulation of a thermo-electrochemical cell with graphite rod electrodes

“…The transport of diluted ions including oxidized and reduced species in the TEC is described by following equations: 31 0 = −∇ N i + S i N i = − D i ∇ c i + c i u where N i , S i , D i and u are the molar flux vector, the source term, the diffusion coefficient and the velocity vector of electrolyte.…”

Thermodynamics of Ionic Thermoelectrics for Low-Grade Heat Harvesting

“…However, only a few works have considered finite thermal charging time when evaluating efficiency. 23,43 Since the ions in TICs are redox-inert, charges cannot transfer across the electrode−electrolyte interfaces.…”

“…Decreasing the thermal charging time is crucial for reducing unwanted heat conduction across the device and for improving efficiency. However, only a few works have considered finite thermal charging time when evaluating efficiency. , Since the ions in TICs are redox-inert, charges cannot transfer across the electrode–electrolyte interfaces. Heat is converted into electrostatic energy by ionic charges stored near the electrode surfaces of TICs.…”