Quasi‐solid‐State Electrolytes for Low‐Grade Thermal Energy Harvesting using a Cobalt Redox Couple

Thermoelectrochemical cells, also known as thermocells, are electrochemical devices for the conversion of thermal energy directly into electricity. They are a promising method for harvesting low‐grade waste heat from a variety of different natural and manmade sources. The development of solid‐ or quasi‐solid‐state electrolytes for thermocells could address the possible leakage problems of liquid electrolytes and make this technology more applicable for wearable devices. Here, we report the gelation of an organic‐solvent‐based electrolyte system containing a redox couple for application in thermocell technologies. The effect of gelation of the liquid electrolyte, comprising a cobalt bipyridyl redox couple dissolved in 3‐methoxypropionitrile (MPN), on the performance of thermocells was investigated. Polyvinylidene difluoride (PVDF) and poly(vinylidene fluoride‐co‐hexafluoropropene) (PVDF‐HFP) were used for gelation of the electrolyte, and the influence of the different polymers on the mechanical properties was studied. The Seebeck coefficient and diffusivity of the cobalt redox couple were measured in both liquid and gelled electrolytes, and the effect of gelation on the thermocell performance is reported. Finally, the cell performance was further improved by optimizing the concentration of the redox couple and the separation between the hot and cold electrodes, and the stability of the device over 25 h of operation is demonstrated. Calling on cobalt: A quasi‐solid‐state redox‐active electrolyte is developed to enable a leak‐free, durable, and flexible device for harvesting low‐grade waste heat. The optimized device is able to achieve a power density up to 23 mW m−2 (0.0144 mW m−2 K−2) with a temperature gradient of 40 °C and is stable over 25 h of operation.