Thermocells-enabled low-grade heat harvesting: challenge, progress, and prospects

Low-grade heat is abundant and ubiquitous, but it is generally discarded due to the lack of cost-effective recovery technologies. Emerging thermocells (TECs) based on temperature-sensitive redox pairs demonstrate advantages of low cost, scalability, flexibility, and inherent high thermopower, and thus are promising for low-grade heat harvesting. According to the temperature gradient-potential relationship, TECs can be divided into p-type (Vhot−VcoldThot−Tcold<0) and n-type (Vhot−VcoldThot−Tcold>0). To obtain higher output voltages and avoid thermal shorts, Π-integration (electrically in series but thermally in parallel) of single p-type and n-type TECs is required. This paper first summarizes the thermo-electric performances of p-type and n-type TECs. Although the state-of-the-art p-type (Fe(CN)64−/3-) TECs possess a commercializable cost-performance metric, the performance of n-type TECs, such as thermopower and power density, lags significantly, hindering the potential of integrated p-n TECs in heat recovery. Subsequently, this paper analyzes the thermodynamics, kinetics, and bottlenecks of TECs and identifies several potential p-type and n-type redox pairs with high absolute value of temperature coefficient (dE0/dT) and low absolute value of electrode potential (E0) for high-performance TECs. Then, this paper reviews the recent advances in TECs, especially n-type, from the aspects of electrolytes, electrodes, and integrated devices. Finally, this paper proposes potential research directions for high-performance TECs.