Abstract
Low-grade waste heat is an abundant and underutilised energy source. In this context, thermo-electrochemical cells (i.e., systems able to harvest heat to generate electricity) are being intensively studied to deliver the promises of efficient and cost-effective energy harvesting and electricity generation. However, despite the advances in performance disclosed in recent years, understanding the internal processes occurring within these devices is challenging. In order to shed light on these mechanisms, here we report an operando magnetic resonance imaging approach that can provide quantitative spatial maps of the electrolyte temperature and redox ion concentrations in functioning thermo-electrochemical cells. Time-resolved images are obtained from liquid and gel electrolytes, allowing the observation of the effects of redox reactions and competing mass transfer processes such as thermophoresis and diffusion. We also correlate the physicochemical properties of the system with the device performance via simultaneous electrochemical measurements.
Highlights
Low-grade waste heat is an abundant and underutilised energy source
Under an applied thermal gradient, either oxidation or reduction will dominate at the hot electrode depending on the sign of the reaction entropy[4], with the opposite process occurring at the cold electrode. This generates an electric potential across the cell, and when an external load resistance is connected to the electrodes the drawn current will drive further redox reactions, resulting in the continuous generation of power with no fuel consumption or emissions for as long as the temperature gradient is maintained
Despite the simplicity of their design, there are a large number of distinct physical processes that can occur within the thermocell electrolyte. These include the redox reactions and accompanying changes in ion clustering and solvation, the establishment of local concentration gradients and resulting diffusion along these gradients, self-diffusion, thermophoresis, electrophoresis and convection. All of these processes will be highly inter-dependent, for example convection can be caused by the applied temperature gradient and by variations in the electrolyte density resulting from the redox reactions and concentration gradients
Summary
Low-grade waste heat is an abundant and underutilised energy source. In this context, thermo-electrochemical cells (i.e., systems able to harvest heat to generate electricity) are being intensively studied to deliver the promises of efficient and cost-effective energy harvesting and electricity generation. Additional temperature maps obtained from the liquid electrolyte under various applied temperature differentials, including with different image slice orientations (parallel and perpendicular to the electrodes), are shown in Supplementary Figs.
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