Strong thermoelectric response of weak electrolytes*

Recently, there has been increasing attention to materials that yield large thermoelectric responses. We have analyzed the potential of weak electrolytes for thermoelectric energy conversion. Weak electrolytes only partially dissociate into ions when dissolved. For these electrolytes, the equilibrium between the dissociated and the charge-neutral solute is defined by the dissociation constant. The dissociation constant depends on the temperature of the electrolyte solution. We have computed the thermovoltage generated when a nanochannel is filled with a weak electrolyte and subjected to different temperatures at each end. When the EDLs of two opposing channel walls overlap, the channel is mainly filled with counterions. A counterion concentration gradient forms along the channel, which is caused by the temperature-dependent partial dissociation of ions. This concentration gradient drives an electric current. We have modified the Nernst-Planck equation to take partial ion dissociation into account and numerically solved it along with the Poisson and heat transport equations. The results show that, for the same degree of EDL overlap, a thermal voltage is achieved with weak electrolytes that can be 3.5 times higher than for fully dissociated electrolytes. This makes confined solutions of weak electrolytes promising candidates for thermoelectric energy conversion.