A non-isothermal water formation cell for electrochemical heat recovery†

Low-grade heat harvesting has emerged as a promising strategy to recover waste heat into usable energy. However, most of the thermo-electrochemical approaches are limited to redox reactions involving metal ion complexes and halide species, which often exhibit low heat-to-electricity conversion efficiencies. We demonstrate a heat harvesting approach based on a non-redox reaction; water formation driven by a net-zero hydrogen redox process. Under standard conditions, its positive entropy change enables the interconversion of nearly 30% of surrounding heat into electrical energy, resulting in a thermodynamic efficiency greater than unity. This water formation-based galvanic–thermogalvanic device demonstrated a temperature-insensitive maximum power density as high as ∼33.55 mW m⁻² K⁻². Notably, this figure of merit is ∼70 times higher than the state-of-the-art ferrocyanide–ferricyanide-based thermogalvanic devices, thereby extending the scope of electrochemical heat harvesting beyond conventional redox processes.