Ion/electron thermoelectric capacitance

Ionic thermoelectric (TE) materials are considered as the next-generation TE materials mainly because of their high thermopower, which is two to three orders of magnitude higher than that of their electronic counterparts. They can be used in an ionic TE capacitor (ITEC) to harvest heat into electricity. TE conversion depends not only on the TE properties of the ionic materials but also on the capacitance of the ITECs. It is of great significance to deeply understand the capacitance of ITECs. Here, we report that TE capacitance mainly arises from the interface between the electrodes and the ionic TE layer instead of the capacitance due to the dielectric constant of the ionic TE material. The interfacial capacitance (Ci) is higher than that due to the bulk (Cb) ionic TE material by several orders in magnitude. Cations and anions accumulate at the two ends of an ionic TE material under temperature gradient. The accumulation of the cations (or anions) attracts electrons (or holes) from the external load to the cathode (or anode). This can give rise to very high capacitance because of the short separation between the cations (or anions) and the electrons (or holes) at the interface. This article is part of the discussion meeting issue 'Ionic liquids and the future of soft materials'.