A Multifunctional MXene/PVA Hydrogel as a Continuous Ionic Thermoelectric Generator and a Strain/Temperature Sensor

This research reports a continuous output ionic thermoelectric (i-TE) system based on MXene/PVA (polyvinyl alcohol) hydrogel, by utilizing thermo-diffusion of Cu²⁺ and Cl^- ions and the redox reaction involving Cu/Cu²⁺ at the electrode interfaces. The thermopower of the i-TE system can be independently tuned to a value of -3.13 mVK^-1 by adjusting the ion diffusivity via MXene (Ti₃C₂T_x). The i-TE system demonstrates a rapid response time of less than 100 s, outperforming any other polyelectrolyte-based system. Crucially, the i-TE system achieves continuous current output when equipped with copper electrodes, facilitated by the redox reaction involving Cu/Cu²⁺, and maintains stable long-term outputs across a range of resistances from 1 kΩ to 1 MΩ. A three-serial-connected i-TE module demonstrates an output voltage of 26 mV with 6 °C temperature difference, confirming the feasibility of creating an array of i-TE devices for substantial energy output. Beyond energy harvesting, the MXene/PVA hydrogel serves as multifunctional strain/temperature sensors, capable of detecting mechanical strains via the piezoresistive effect and locating finger contact points via the ionic thermoelectric effect.