Supramolecular Thermocells Based on Thermo-Responsiveness of Host-Guest Chemistry

Molecular machines leverage sub-nanometer level intermolecular forces and host-guest interactions to perform useful work observable at the macroscopic level. The development of molecular machines for the past three decades has resulted in successful applications from molecular switches, chemical sensing, to actuators. However, the application of molecular machines and supramolecular chemistry in energy production is rare and has been highly anticipated. This review introduces the advancement of supramolecular thermocells, initially proposed by our research group, which use thermo-responsive host-guest interaction to regenerate electrochemical energy from low-grade heat sources. The selective transport of a redox species carried by the host molecule from the cooled to the heated electrode creates a concentration gradient of the guest redox species and provides an additional voltage to the thermocell. The key properties of useful host molecules for the supramolecular thermocells are: (1) high selectivity of the host molecule to capture either the oxidized or reduced species as the guest, (2) inhibition of the redox activity after the encapsulation, (3) large entropy change at the release of the guest molecule in response to the temperature changes. Design principles and promising candidates of molecular machines for the future development of supramolecular thermocells are presented here.