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 K-3/K-4[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.

Automated summary

The majority of usable energy generated by humanity is lost as waste heat, but thermogalvanic systems can convert low-grade waste heat directly into electricity. The concentration of the redox couple is critical for power generation, with higher concentrations usually resulting in more power. This study demonstrates the use of Na+ and K+ counter ions to achieve the most concentrated stable aqueous ferricyanide/ferrocyanide thermocell to date.