Engineering asymmetric solvation structures for synergistically boosted quasi-solid thermocells

Quasi solid-state thermocells (QTECs) based on the thermogalvanic effect offer a promising route for directly converting abundant low-grade heat into electricity. Introducing a single solvent into hydrogel electrolytes, a common strategy to enhance thermopower, often yields a marginal solvation entropy difference between redox ions and provides limited gains in ion transport. To break this longstanding trade-off, we present a simple yet highly effective co-solvent strategy that employs trimethyl phosphate and ethylene glycol to construct hybrid hydrogel electrolytes. This approach synergistically enlarges solvation entropy differences of redox ions, amplifies the concentration gradient across the thermocell, and enhances redox ion transport through the hydrogel network. The resulting hydrogel electrolyte achieves superior thermoelectrochemical performance and demonstrates efficient harvesting of low-grade heat even at sub-zero temperatures. Advanced characterization techniques, integrated with molecular simulations, elucidate that the enhanced thermoelectrochemical performance originates from co-solvent engineered asymmetric solvation structures. This work demonstrates targeted, additive-free modulation of the solvation environment in thermogalvanic hydrogels as a practical strategy to significantly enhance thermoelectrochemical performance.