thermo-electrochemical cells using the temperature-dependent electrochemical potential of redox materials such as ferricyanide/ferrocyanide in liquid electrolyte. Recently, the temperature dependence of redox materials and electrostatic potential were utilized in a thermally regenerative electrochemical cycle to establish thermal energy harvesting systems. [ 14,[16][17][18] Differing from these effects, the Soret effect denotes a difference in molecule/ ion concentrations within a mixture as a result of molecule/ion migration driven by a temperature gradient, and thus research related to transport behaviors has been mostly focused on gas and liquid phases. [ 7,8 ] In previous report, Bonetti et al. showed very high thermally induced voltage (7 mV K −1 ) in nonaqueous electrolyte. [ 9 ] Moreover, Zhao et al. reported over 10 mV K −1 of so-called ionic Seebeck coeffi cient in liquid state polymeric electrolyte and also demonstrated the thermoelectric charging behavior of electric double layer capacitor. [ 10 ] However, electrical conductivities of these liquid-type devices were reported to be undesirably very low (1 × 10 −3 -8 × 10 −3 S m −1 ). The liquid electrolytes typically necessitate bulky packaging to prevent leakage problem, which are not favorable to wearable and portable devices. Moreover, liquid electrolytes are subject to heat convection, which makes it diffi cult to maintain temperature gradient.Here our novel approach employed a "solid-state" ionic conductor to readily maintain the temperature gradient along with a high ionic electrical so as to fully utilize the Soret effect. This paper reports a high thermo-induced voltage (8 mV K −1 ) at an outstanding electrical conductivity (9 S m −1 ), for the fi rst time, to the best of our knowledge. Furthermore, the large output voltage produced by the Soret effect was further electrochemically stored in integrated redox electrodes like a supercapacitor. Energy storing unit is important since small electrical power/ energy from this type of energy harvesting devices need to be accumulated for practical use. Here our integrated energy storing harvester can be used as a stand-alone device that can be charged simply imposing temperature gradients.

Thermoelectric energy conversion is very effective in capturing low-grade waste heat to supply electricity particularly to small devices such as sensors, wireless communication units, and wearable electronics. Conventional thermoelectric materials, however, are often inadequately brittle, expensive, toxic, and heavy. We developed both p- and n-type fabric-like flexible lightweight materials by functionalizing the large surfaces and junctions in carbon nanotube (CNT) mats. The poor thermopower and only p-type characteristics of typical CNTs have been converted into both p- and n-type with high thermopower. The changes in the electronic band diagrams of the CNTs were experimentally investigated, elucidating the carrier type and relatively large thermopower values. With our optimized device design to maximally utilize temperature gradients, an electrochromic glucose sensor was successfully operated without batteries or external power supplies, demonstrating self-powering capability. While our fundamental study provides a method of tailoring electronic transport properties, our device-level integration shows the feasibility of harvesting electrical energy by attaching the device to even curved surfaces like human bodies.

Hybrids of carbon nanotubes (CNTs) and poly(3,4-ethylenedioxythiophene) (PEDOT) treated by tetrakis(dimethylamino)ethylene (TDAE) have large n-type voltages in response to temperature differences. The reduced carrier concentration by TDAE reduction and partially percolated CNT networks embedded in the PEDOT matrix result in high thermopower and low thermal conductivity. The high electron mobility in the CNTs helps to minimally reduce the electrical conductivity of the hybrid, resulting in a large figure-of-merit.

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