Boosted thermopower in aqueous thermocells through additives-induced ionic regulation for low-grade heat harvesting

Aqueous thermocells are promising techniques for the conversion of low-grade waste heat into electricity. However, current improvement strategies are mainly focused on single redox ion and sacrifice the electrical conductivity due to concentrated molecular additives. Herein, we report a chemical additives-regulated thermocell that introduced two ionic additives, guanidine hydrochloride and cysteamine hydrochloride, into 0.4 M ferri/ferrocyanide ([Fe(CN)₆]^3-/4-) electrolyte to simultaneously exert the selective crystallization effect on [Fe(CN)₆]^4- and the chemical regulation effect for [Fe(CN)₆]^3-, synergistically inducing concentration gradients of both redox ions between two electrodes, thereby improving the thermoelectric performance. Our thermocell obtained a high thermopower of 4.34 mV K^-1 with comparable electrical conductivity and a Carnot-relative efficiency of 5.50% with minimal amounts of the two additives, showing adaptability to various cell orientations and thus different practical scenarios. A record-high thermopower of 9.06 mV K^-1 and a Carnot-relative efficiency of 12.65% were achieved adopting optimized concentrations of two additives under cold-over-hot orientation. A 20-unit module was developed to directly power various electronics, demonstrating its feasibility for low-grade heat harvesting.