Liquid-state thermocells: Opportunities and challenges for low-grade heat harvesting

Low-grade heat (<100°C) is abundant and ubiquitous but is generally discarded owing to a lack of cost-effective recovery technologies. Emerging liquid-state thermocells (thermogalvanic cells or thermo-electrochemical cells), with advantages such as low cost, scalability, flexibility, and intrinsic high thermopower, may be a promising thermoelectric technology for low-grade heat harvesting. In the past decade, extensive efforts have been directed toward the performance enhancement of thermocells and their modules; however, they still suffer from challenges such as a low conversion efficiency and power density of single cells and stability issues (e.g., leakage, freezing or boiling of electrolytes) in devices working over a wide temperature window. From this perspective, we first summarize the advances in thermocells regarding their electrodes, electrolytes, and devices. Then, we highlight the critical challenges and determine future research directions toward more cost-effective thermocells with higher robustness, simpler fabrication, and wider applications in low-grade heat harvesting, temperature detection, cooling, and beyond.