Thermogalvanic cells demonstrate inherent physiochemical limitations in redox-active electrolytes at water-in-salt concentrations

The majority of usable energy generated by humanity is lost as waste heat, but thermogalvanic systems (or thermocells) can address this problem by converting low-grade waste heat directly into electricity using redox chemistry. The concentration of the redox couple is a critical parameter; almost invariably, higher concentrations result in more power. This study exploits the simple synergy between Na⁺ and K⁺ counter ions to achieve—to the best of our knowledge—the most concentrated stable aqueous ferricyanide/ferrocyanide thermocell to date, at 1.6 m [Fe(CN)₆]^3−/4−. Despite increasing the concentration by 400% relative to the standard K₃/K₄[Fe(CN)₆] electrolyte (0.4 m), electrical power production increased only 166%. Pushing the system from conventional salt-in-water electrolytes into the quasi-stable water-in-salt region (up to 2.4 m) resulted in a decrease in power. Detailed characterization highlighted the various physicochemical hurdles introduced by these extremely concentrated electrolytes; the identified issues have direct relevance to other energy systems also seeking to use the highest possible concentration.