A Numerical Investigation into the Relationship between Fluid Flow and Electrodeposition in a Silver Thermally Regenerative Ammonia Battery

摘要

As the need for a sharp reduction in carbon emissions continues to grow, many researchers are looking to electrochemical technologies to provide sustainable power to the grid. One promising technology is the silver thermally regenerative ammonia battery (Ag-TRAB), which takes advantage of low-grade waste heat (<100℃) to create electrical power. Although the Ag-TRAB operates in a manner similar to a conventional flow battery, it is not charged by electrical power; instead the source of energy is (waste) heat. The Ag-TRAB uses silver as the active metal ion for deposition and dissolution on each half of the battery and porous carbon as the electrode material, driven by ammonia added into only one chamber. Following a discharge cycle, ammonia is removed using waste heat, and a new cycle is initiated using the ammonia added to the opposite electrolyte chamber. This technology can cycle hundreds of times without a decrease in performance, but its commercial implementation is currently limited by poor power and energy densities. In many electrodeposition technologies, porous flow-through electrode materials have been shown to have superior performance because of factors such as less dendrite growth and lower overpotentials. However, the relationship between the fluid flow and the deposition process within the porous electrode has not yet been investigated. As the Ag-TRAB is either charged or discharged, the deposition or dissolution of material can alter the porosity of the electrode, clog pores, and create preferential flow paths. Because the fluid flow directly governs convective mass transfer within the system, this flow evolution can have a substantial effect on performance. A numerical model was created using Comsol Multiphysics to study the relationship between fluid flow and electrodeposition in porous media. For each half-reaction, rotating disk electrode experiments were used to obtain the kinetic data for this specific chemistry. The impacts of porosity and Reynolds number on the mass transfer of the electroactive ions are presented. How these parameters affect electrodeposition uniformity, alter the flow through the electrode, and ultimately impact the peak performance of the cell is also discussed. It is shown that with increased porosity comes increased mass transfer as a result of reduced pore-clogging and increased unsteadiness within the fluid.