An n-Type Ionic Thermoelectric Device Enabled by Synergistic Interactions Between Electrodes and PVA Hydrogel

Ionic thermoelectric (i-TE) materials have attracted increasing attention for low-grade heat harvesting owing to their high thermovoltage output under small temperature gradients. However, the development of n-type i-TE materials remains challenging. Electrode-enabled polarity regulation provides a promising alternative to material-design strategies for developing n-type i-TE devices. In this work, a poly(vinyl alcohol) (PVA)-based ionic hydrogel was prepared with dimethyl sulfoxide (DMSO) and potassium chloride (KCl) through a freeze-thaw process, and its thermoelectric behavior was regulated by electrodes. While the i-TE hydrogel device with typical Cu electrodes exhibited p-type behavior, replacing the electrodes with graphite paper (GP) electrodes converted the device response from p-type to n-type. Morphological and spectroscopic analyses suggest that the GP surface selectively adsorbed K⁺ ions through cation-π interactions, suppressing cation thermodiffusion and enabling Cl^--dominated ion migration under a temperature gradient. As a result, the PVA-GP device achieved a maximum S_i of -4.36 ± 0.26 mV K^-1. In addition, the device exhibited favorable thermoelectric output, with a maximum PF_i of 57.668 μW m^-1 K^-2, a room-temperature ZT of 0.0864, and a peak transient power density of 2.33 mW m^-2 during short-time discharge. Owing to the large interfacial area of the GP electrodes, the device could also function as an ionic thermoelectric supercapacitor with appreciable energy-storage capability. This work demonstrates an effective electrode-engineering strategy for constructing n-type i-TE devices and provides a feasible route for simultaneous low-grade heat harvesting and transient energy storage.