Continuous heat harvesting by an ionogel mixed with PEDOT:PSS under both fluctuated and steady temperature gradients

Abstract

Ionic thermoelectric (TE) materials have emerged as the next-generation TE materials mainly due to their high thermopower, which is higher than that of their electronic counterparts by 2–3 orders of magnitude. However, they cannot be used to continuously harvest heat because no electricity can be generated under a steady temperature gradient. Herein, we report a mixed ion-electron thermoelectric conductor that can be used to continuously harvest heat not only under fluctuating temperatures but also under a steady temperature gradient. It is made of an ionogel added with poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS, a conducting polymer), and it is a mixed ion-electron conductor because the ionic liquid is an ionic conductor, while PEDOT:PSS is an electronic conductor. MTEGs can continuously supply electricity to an external load even under a steady temperature gradient, which is similar to that of the conventional thermoelectric generators (TEGs) with electronic TE materials. Thermopower is related to the steady open-circuit voltage generated under a steady temperature gradient, and it is a metric calculated as the thermovoltage divided by the temperature gradient. It can reach a value of 13.87 mV K⁻¹, which is higher than the Seebeck coefficient of the best electronic TE materials by 2–3 orders of magnitude. The TE performance is attributed to the synergistic effects of hole tunneling across the PEDOT:PSS networks and the Soret effect of the ions, which involves the accumulation of cations and anions at the two ends of an ionogel under a temperature gradient.