A Buffered Hexacyanoferrate Electrolyte for Thermogalvanic Heat Harvesting across Symmetric and Asymmetric Electrode Configurations.

Abstract

Thermogalvanic waste-heat harvesting offers a promising route to the productive utilization of low-grade thermal energy, but current cells remain limited by low thermopower and power output. Here, we report a thermogalvanic electrolyte that inserts the canonical Fe(CN)₆^3-/4- redox cycle within an optimized NH₄OH-CH₃COOH alkaline environment. Our results show that the optimal mixture establishes differential formation energies between the hexacyanoferrate redox pair, thereby amplifying the configurational entropy difference between redox states and boosting the thermopower from 1.48 to 2.64 mV K^-1 in a symmetric graphite cell, with a normalized power density of ∼1.5 mW m^-2 K^-2 sustained across Δ<i>T</i> = 10-50 K. We show that placing the optimized electrolyte within an asymmetric copper-graphite electrode setup leads to a hybrid mechanism of galvanic-thermogalvanic (gTg), which produces a total output substantially exceeding the sum of isothermal galvanic and electrode-analogous thermogalvanic cell outputs, while also demonstrating significantly extended operational longevity over isothermal discharge. A 50-cell proof-of-concept device mounted on a PC tower produces 42 V and 160 mW total output, of which 7 V and 37.6 mW are derived from the thermal contribution.