Vanadium Flow Battery Electrochemistry and Fluid Dynamics Model with in-Situ Current Distribution Validation

Abstract

It has been estimated that high penetration of renewable electricity generation (68%-73% from solar and wind) could be possible in the United States by 2050 [1]. To achieve this goal, large-scale energy storage technologies will play an important role. Energy storage technologies can improve electricity service stability, flexibility, reliability, and resilience on the grid. Among energy storage methods, vanadium redox flow batteries (VRFBs) have been a sustained focus of interest due to distinct advantages, e.g. flexibility and scalability, high columbic efficiency, and long cycle life. However, high system cost is a major obstacle for widespread commercialization of VRFBs; cell component costs (membranes and chemicals) tend to dominate overall system costs. It is possible to improve electrolyte utilization and system efficiency, leading to reductions in both component costs. This can be achieved through increasing vanadium solubility in the solvent, modifying membranes to be more ionically conductive, minimizing crossover, facilitating electrochemical kinetics, improving mass transport, and reducing parasitic pump losses. The VRFB is a complex system with multiple interrelated parameters (especially mass transport and electrochemical reactions) affecting electrochemical cell performance; these interrelated parameters are difficult to disentangle experimentally. Comprehensive and suitably validated mathematical simulations can both help to understand complex phenomena inside VRFBs and provide practical knowledge for controlling and optimizing VRFB systems. In this study, a three dimensional, steady-state multi-physics model is developed for VRFBs with strip cell architecture under the dilute solution theory assumption. The simulation domain includes a central membrane, both electrodes, flow plates, and current collectors. Continuum relationships including conservation of mass, momentum, species and charge coupled with Butler-Volmer kinetics are employed.In this study, the impacts of various electrochemical and transport parameters on the electrochemical performance and current distribution are investigated. It is found that the diffusion coefficient of the vanadium species is the most significant parameter affecting the current distribution along the channel. The model successfully predicts both the charge-discharge polarization curve and the current distribution with the fitted diffusion coefficient parameter. The diffusion coefficient of the vanadium species was found to be order of magnitude higher than the experimentally-measured values found in the literature [2].Polarization curve analysis and fully segmented, printed circuit board (PCB)-based, localized current distribution measurements are employed to validate the mathematical model. All tests are conducted with a simplified test bed with a segmented strip cell architecture, having only one straight channel and a total of 1 cm2 active area. Strip cell architecture effectively eliminates higher-dimensional behaviors (e.g. channel hopping, bypass at channel switchbacks, and potential fluid short circuits) and provide straightforward systems for phenomenological as well as comparative and detailed model validation studies [3].