Determining single-ion Soret coefficients from the transient electrolyte Seebeck effect.

Single-ion Soret coefficients 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 but also readily 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. Here, we derive $S(t)$ generated by a $z_{+}:z_{-}$ electrolyte subject to a thermal gradient, without enforcing local electroneutrality. Next, we experimentally measure $S(t)$ for twelve acids, bases, and salts near stainless steel and titanium electrodes. At steady state, we find $S\approx2~\mathrm{mV~K}^{-1}$ for many electrolyte-electrode combinations, much higher than predictions based on previous literature. We fit our expression for $S(t)$ to the experimental data, treating the single-ion Soret coefficients as fit parameters, and also find larger-than-literature values, accordingly.