Boosting N‐Type Ferrous/Ferric Thermocells by a Ternary Electrolyte Leveraging Dual‐Ligand Coordination

ABSTRACT

Thermocells (TECs) offer a promising route for directly converting low‐grade heat into electricity, while n‐type TECs historically lag behind their p‐type counterparts due to their sluggish reaction kinetics and thermodynamic limitations. Herein, a ternary electrolyte with synergistic coordination effect is proposed to uniquely address these challenges within the ferrous/ferric n‐type TECs. In this electrolyte, 15‐crown‐5 selectively encapsulates Fe ³⁺ while p‐toluenesulfonic acid provides additional coordination sites, effectively suppressing detrimental hydrolysis and enhancing the electrochemical reversibility that is often compromised by the crown ether used alone. Such selective and synergistic coordination directly enlarges entropy change and facilitates charge transfer, significantly contributing to the overall thermoelectrochemical performance. The resulting TECs based on the optimized ternary electrolyte exhibit a notably enhanced thermopower of −2.41 mV K ⁻¹ and a markedly increased normalized power density of 1.348 mW m ⁻²  K ⁻² , representing one of the highest values reported for n‐type TECs. The substantial enhancement in both thermopower and power density facilitates an excellent match with the classic p‐type ferricyanide/ferrocyanide TECs, and a device integrated with 14 p‐n TEC pairs is further demonstrated, showing high potential toward practical low‐grade heat harvesting applications.