Rational Design of Thermocells Driven by the Volume Phase Transition of Hydrogel Nanoparticles

Thermocells are thermoelectrochemical conversion systems for harvesting low-temperature thermal energy. Liquid-state thermocells are particularly desirable because of low cost and their high conversion efficiency at temperatures around physiological temperature, and they have, thus, been extensively studied. However, the performance of the thermocells has to be improved to utilize them as energy chargers and/or batteries. Recently, we reported that a liquid-state thermocell driven by the volume phase transition of hydrogel nanoparticles showed highly efficient thermoelectric conversion with Seebeck coefficient (S_e) of -6.7 mV K^-1. Here, we report the design rationale of the thermocells driven by the phase transition. A high S_e of -9.5 mV K^-1 was achieved at temperature between 36 and 40 °C by optimizing choice and amount of redox chemical species. The figure of merit (ZT) of the thermocell was improved by selecting appropriate electrolyte salt to increase the ionic conductivity and prevent the precipitation of nanoparticles. Furthermore, screening of nanoparticles revealed the high correlation between S_e and the pH shift generated as a result of phase transition of the nanoparticles. After optimization, the maximum ZT of 8.0 × 10^-2 was achieved at a temperature between 20 and 70 °C.