Water, Not Salt, Causes Most of the Seebeck Effect of Nonisothermal Aqueous Electrolytes

A temperature difference between two electrolyte-immersed electrodes often yields a voltage $\mathrm{\Delta }\psi$ between them. This electrolyte Seebeck effect is usually explained by cations and anions flowing differently in thermal gradients. However, using molecular simulations, we found almost the same $\mathrm{\Delta }\psi$ for cells filled with pure water as with aqueous alkali halides. Water layering and orientation near polarizable electrodes cause a large temperature-dependent potential drop $\chi$ there. The difference in $\chi$ of hot and cold electrodes captures most of the thermovoltage, $\mathrm{\Delta }\psi \approx {\chi }_{\mathrm{hot}}-{\chi }_{\mathrm{cold}}$.

Figure 1

Simulation snapshot of 1 M LiI in water between graphene electrodes. Thermostats control the temperature in the blue and red regions. (${\mathrm{Li}}^{+}$, yellow; ${\mathrm{I}}^{-}$, purple; hydrogen, white; oxygen, red; carbon, cyan.)

Figure 2

Potential $\psi (z)$ in water-filled cells at 293 (a) and 373 K (b), and with a temperature gradient, where the left side is at 293 K and the right side is at 373 K (c).

Figure 3

Further analysis of the water-filled cell in Fig. 2 with temperature varying from 293 to 373 K. (a) Charge density and local potential, where the hot side at 373 K is mirrored around $z=0$ and its potential in vacuum shifted to 0 V. The vertical black line indicates the position of the first graphene layer. (b) Cumulative charge (lines) and oxygen number density (dotted lines). (c) Layer dipole moment of water in the first three neutral layers near the cold and hot sides of the cell, their sum, and the dipole moment of the bulk water in the remainder of the cell (black).