Multiple redox system for energy harvesting from ubiquitous temperature fluctuations

Thermogalvanic cells, which convert heat to electricity, typically require complex thermal and electrical designs to maintain a sufficient spatial temperature difference with low electrical impedance, thereby limiting their practical applications. Herein, we present a multiple redox system (MRS) designed to harvest energy from ubiquitous temporal temperature fluctuations rather than fixed thermal gradients. The MRS employs two thermogalvanic half cells containing an electrolyte composed of a redox pair with reaction entropies of opposite signs, separated by a cation-selective membrane. Cyclic voltammetry and open-circuit voltage measurements demonstrated the fundamental principle of this device, confirming that the electromotive force generated (with a temperature coefficient of ~1.2 mV K −1 ) was consistent with the difference in the temperature-dependent redox potential of each half cell (~1.4 mV K −1 ), with the base temperature located near room temperature. Loading experiments revealed the repeated generation of an alternating current power under cyclic temperature modulations above and below a central base temperature at which the redox potentials of the half cells were identical. Enhanced energy conversion efficiency, with a cumulative energy output of 3.9 mJ from a total temperature-change history of 60 K, was achieved by reducing the electrode gap and modularizing several MRS devices. As a proof-of-concept, the MRS was successfully used to convert ambient air-temperature changes into sufficient electrical energy and activate a direct current (DC)-to-DC converter. Our device-engineering investigations highlight the potential applicability of the MRS beyond a single thermogalvanic cell, with downscaling of the device size leading to marked improvements in power and response speed by suppressing the impedance and heat capacity. This study contributes to the realization of clean, ubiquitous, and self-sustaining power supply systems utilizing ambient thermal energy. • Energy is harvested from temperature fluctuations by a multiple redox system (MRS). • The MRS comprises two galvanic half cells with reaction entropies of opposite signs. • K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ] and Fe 4 [Fe(CN) 6 ] 3 serve as ideal redox pairs. • Device engineering for maximizing energy conversion is proposed. • The MRS can drive a DC-to-DC converter.