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Tuning Electronic and Ionic Transport by Carbon‐Based Additives in Polymer Electrolytes for Thermoelectric Applications

Thermoelectric materials utilizing ionic transport open‐up entirely new possibilities for the recuperation of waste heat. Remarkably, solid state electrolytes which have entered the focus of battery research in recent years turn‐out to be promising candidates also for ionic thermoelectrics. Here, the dynamics of ionic transport and thermoelectric properties of a methacrylate based polymer blend in combination with a lithium salt is analyzed. Impedance spectroscopy data indicates the presence of just one transport mechanism irrespective of lithium salt concentration. In contrast, the temperature dependent ionic conductivity increases with salt concentration and can be ascribed to a Vogel–Fulcher–Tammann (VFT) behavior. The obtained Seebeck coefficients of 2 mV K−1 allow for high power outputs while the polymer matrix maintains the temperature gradient by its low thermal conductivity. Adding multi‐walled carbon nanotubes to the polymer matrix allows for variation of the Seebeck coefficient as well as the ionic and electronic conductivities. As a result, a transition between a high temperature VFT regime and a low temperature Arrhenius regime appears at a critical temperature, Tc, shifting upon addition of salt. The observed polarity change in Seebeck voltage at Tc suggests a new mode of thermoelectric operation, which is demonstrated by a proof‐of‐concept mixed electronic‐ionic‐thermoelectric generator.