Quasi-solid n-type thermogalvanic thermocells with enhanced ionic conductivity for continuous low-grade heat harvesting

Quasi-solid thermocells show great potential to save power terminals from periodic charging but still face the grand challenge of low thermoelectric efficiency. Despite many efforts devoted to improve thermopower, few studies have been reported to address the trade-off between thermopower and ionic conductivity encountered by n-type quasi-solid thermocells. Herein, a directional freeze-thawing method is developed to fabricate high-performance n-type quasi-solid thermocells with hierarchically anisotropic networks, enabling the decoupling of thermopower and ionic conductivity. The n-type thermopower is up to 0.74 mV/K, and the ionic conductivity is independently improved to be about 9.3 S/m. Thus, the output power density reaches ∼200 mW/m2, which is the same level among the quasi-solid n-type thermocells. Meanwhile, benefiting from the crystalline domains and alignment structures of the solid network, the thermocells achieve the strength of ∼380 kPa and an elongation at break of ∼320%. Moreover, the thermocells work stably when being pressed, bent, and stretched in practical uses. We believe this work not only demonstrates a particularly important example for fabricating high-performance n-type quasi-solid thermocells but also inspires the development of thermocell devices to achieve large-scale low-grade heat harvesting in wearable systems.