On a Continuous Aqueous Thermogalvanic Redox Agent with Anomalous Thermopower.

Achieving stable and sustained power output remains a major challenge in the development of ionic thermoelectric devices (such as thermogalvanic cells and thermoionic capacitors) for low-grade heat harvesting. Many such systems exhibit strong performance in the initial throes of operation but degrade rapidly over time, limiting their utility. Here, we report an aqueous thermogalvanic redox agent ([Ni(bpy)₃]^2+/3+) with a Seebeck coefficient approximately double that of the canonical [Fe(CN)₆]^4-/3-, which can be utilized continuously for multiple hours under large temperature gradients (ΔT ≥ 60 K) without significant change in electrical performance. Molecular dynamics simulations suggest that significant differences in hydration shell behavior between the oxidant and reductant, which in turn provide a significant (here configurational) entropy difference, drive the observed thermopower. This new redox pair demonstrates stability, cyclability, and tunability in both liquid- and gel-based electrolytes, and provides a novel redox platform from which to build next-generation high-thermopower waste-heat recovery cells.