Tuning the Solvation Environment of Polyoxometalate Redox-Active Species to Boost Thermo-Electrochemical Cell Performance

Abstract

In modern industrial societies, large amounts of waste heat are inevitably generated from sources such as manufacturing, power generation, and transportation. In particular, low-grade waste heat below 200 °C constitutes a major portion of the total industrial waste heat generated worldwide. The thermal energy loss significantly compromises the overall efficiency of primary energy utilisation. Recovering and reusing waste heat not only improves energy efficiency but also plays a crucial role in reducing CO2 emissions and advancing sustainable energy systems. However, low-grade waste heat remains largely unexploited because of its low energy density and poor conversion efficiency. Thermo-electrochemical cells (TECs)—which convert thermal energy into electricity via temperature-dependent redox reactions—have emerged as a low-cost, eco-friendly technology for harvesting low-temperature heat. The use of redox-active molecules in TECs enables molecular-level tunability, structural flexibility, and temperature responsiveness, features that are not achievable with conventional thermoelectrics. Nonetheless, improving the thermoelectric performance of TECs remains a key challenge for practical application. In this study, we systematically investigate the effect of organic solvents on the thermoelectric properties of Keggin-type polyoxometalates as a redox system. By modifying solvation environments, we enhanced the Seebeck coefficient and output power density through increased solvation entropy changes during the redox reactions. Our findings elucidate the contributions of redox species and the solvation structure, providing design principles for advanced TEC materials. Tailoring solvation environments via the appropriate selection and composition of organic solvents in aqueous media offers a promising route toward optimising TEC performance for future energy applications.