Energy storage has become a critical and enabling technology in the use of many renewable energy sources that are intermittent in nature, such as solar and wind power. Among emerging technologies, the all-vanadium redox flow battery (VRFB) is a promising candidate for large-scale stationary storage applications because of its unique features, including tolerance to deep discharge without any risk of damage, long lifetime, no crossover contamination, and simple structure. Although appealing, the VRFB is still hindered by several critical challenging issues before widespread commercialization. For given electrolyte and electrode materials, the performance of the VRFB is basically determined by the mass and charge transport characteristics in the porous electrode. A better understanding of...[ Read more ]

Energy storage has become a critical and enabling technology in the use of many renewable energy sources that are intermittent in nature, such as solar and wind power. Among emerging technologies, the all-vanadium redox flow battery (VRFB) is a promising candidate for large-scale stationary storage applications because of its unique features, including tolerance to deep discharge without any risk of damage, long lifetime, no crossover contamination, and simple structure. Although appealing, the VRFB is still hindered by several critical challenging issues before widespread commercialization. For given electrolyte and electrode materials, the performance of the VRFB is basically determined by the mass and charge transport characteristics in the porous electrode. A better understanding of these coupled characteristics thus becomes essential. However, the complex trans-scale transports in liquid electrolyte flowing through the porous electrode with redox reactions at solid-electrolyte interfaces make it difficult to reveal the real-world details.

This thesis starts with the design, fabrication, and test of the VRFBs with different architectures. It is demonstrated that as opposed to the conventional cell architecture with a flow-through arrangement, adding a flow field onto the electrode results in better cell performance, but a higher pressure drop through the cell; the reversed trends of the cell performance and the pressure drop with an increase in the flow rate lead to a maximum power efficiency. The thesis work is then focused on the determination of two critical constitutive properties for the mass transport of ions through porous electrodes saturated with a liquid electrolyte: the effective diffusivity at the representative element volume (REV) level of the porous electrode and the pore-level mass transfer coefficient for modeling the transport from the REV level to the solid surfaces of pores. With the obtained constitutive transport properties, a two-dimensional mass/charge transport and electrochemical model for the VRFB is developed. As compared with conventional models, the present model takes account of the effects of flow dispersion through the porous electrode and the mass transport process from the REV level to the pore surfaces. It is shown that increasing the flow rate enhances the dispersion effect; but compared with the increase rate of the dispersion effect, the increase rate of the convection effect with the flow rate is faster, making the convective transport predominate at high flow rates. The numerical results also suggest that if the intrinsic diffusivities of vanadium ions are several folds larger while the electrolyte flow rate remains unchanged, the cell performance will be remarkably improved. Among various operating parameters, the electrolyte concentration shows the most significant effect: An increase in the electrolyte concentration leads to a significant increase in both the storage capacity and the round-trip efficiency (RTE), while not evidently increasing the pumping power. The model is then applied to the study of the effect of flow field designs on VRFB performance. Results show that a cell with the serpentine flow field at the optimal flow rate shows a higher energy-based system efficiency than the parallel flow field does. Finally, the effect of SOC-dependent electrolyte viscosity on VRFB performance is numerically investigated. It is shown that the SOC-dependent electrolyte viscosity leads to a higher pressure drop through the cell, especially in the positive half-cell, and more uneven distributions of overpotential and local current density along the electrode thickness. Moreover, the calculated energy-based system efficiency suggests that the assumption of a constant viscosity in conventional models causes the overestimation of the cell performance.

Keywords: Vanadium redox flow battery (VRFB); mass transport properties; numerical modeling; flow field design; SOC-dependent electrolyte viscosity