Thermosensitive slurry electrolyte design for efficient electrochemical heat harvesting†

Efficient and cost-effective recovery technologies are needed to harvest the abundant energy stored in low-grade heat sources (<100 °C). A thermally regenerative electrochemical cycle (TREC) is a promising approach for low-grade heat harvesting with high energy conversion efficiency. Here, we achieved co-optimization of the temperature coefficient (−3.96 mV K⁻¹), specific charge capacity (theoretical, 97.36 A h L⁻¹) and specific heat capacity in a TREC by applying a thermosensitive slurry electrolyte system, in which Fe(CN)₆⁴⁻-based thermosensitive crystallization was incorporated into the Fe(CN)₆^3−/4− solution. We demonstrated an electrically assisted TREC system with a Fe(CN)₆^3−/4− catholyte and an Ag/AgCl anode, and a charging-free TREC system with a Fe(CN)₆^3−/4− catholyte and an I₃⁻/I⁻ anolyte. Both systems exhibit high absolute heat-to-electricity energy conversion efficiencies of 4.42% and 2.51%, respectively, in the absence of heat recuperation. This study provides a general approach to electrolyte design aimed at enhancing the temperature coefficient and specific charge capacity, while simultaneously optimizing specific heat capacity, thereby facilitating the development of more efficient TREC systems.