Vanadium redox flow batteries are a promising technology for large-scale energy storage to eliminate the mismatch between renewable energy sources and power consumption due to their merits of high safety, long lifespan, and flexible design. However, the widespread application of this technology is hindered by the high capital cost. Improving the battery performance is regarded as the solution to reduction in the cost, which requires minimizing the activation overpotential, ohmic overpotential, and concentration overpotential. To achieve this, in addition to optimizing the key components and operating conditions, accurate battery simulations are also crucial to guide the optimization process. The primary objective of the thesis is to develop highly precise numerical models and high-perfo...[ Read more ]

Vanadium redox flow batteries are a promising technology for large-scale energy storage to eliminate the mismatch between renewable energy sources and power consumption due to their merits of high safety, long lifespan, and flexible design. However, the widespread application of this technology is hindered by the high capital cost. Improving the battery performance is regarded as the solution to reduction in the cost, which requires minimizing the activation overpotential, ohmic overpotential, and concentration overpotential. To achieve this, in addition to optimizing the key components and operating conditions, accurate battery simulations are also crucial to guide the optimization process. The primary objective of the thesis is to develop highly precise numerical models and high-performance vanadium redox flow batteries by optimizing the electrodes and flow fields, as well as increasing the operating temperature.

The thesis begins with the reduction in the activation overpotential by in-situ electrodepositing uniform and dense bismuth particles (58 nm) onto anodes, which is achieved by a new in-situ electrodeposition strategy that uses a catholyte with a low concentration of vanadium ions (33 mM V³⁺). Compared with the conventional method using a catholyte with a high concentration of vanadium ions (1700 mM VO²⁺), the new strategy renders Bi nanoparticles not being oxidized by VO²⁺ and VO₂+ transported across the membrane from the catholyte. As a result, the battery with the anode treated by the new strategy achieves an energy efficiency of 76.3% even at a current density of 300 mA cm⁻², which is higher than that of the battery with an anode treated by the conventional method (74.9%), and the untreated anode (73.3%). The concentration overpotential of VRFBs is then reduced by the optimization of ribs of serpentine flow fields, which is inspired by the simulation results of a newly developed model. This model incorporates solid mechanics to precisely simulate the electrochemical reaction in electrodes with uneven compression caused by ribs of fields. From the simulation results, we found that there are minor increases in ohmic resistance but an obvious enhancement of convection mass transfer when decreasing the compression ratio from 68% to 30%. Thus, we change the flat ribs of the serpentine flow field to ramped ribs, leading to enhanced convection in the electrodes of the under-rib region. As a result, the energy efficiency of the battery with ramped ribs reaches 80.3%, which is higher than the battery with conventional flat ribs (77.6%) at the current density of 200 mA cm^-2. Finally, activation overpotential, ohmic overpotential, and concentration overpotential are simultaneously reduced by increasing the temperature of electrolytes. To accurately control the temperature of the electrolytes, an electrochemical-thermal coupled model is built to predict the temperature of different components of the battery system. The simulation results show that the temperature of electrolytes and graphite bipolar plates is at least 3.0 °C higher than that of other key components. Accordingly, the temperature of electrolytes in the battery system is accurately increased from 25 to 50 °C by a temperature controller, leading to the energy efficiency of the battery system being increased from 82.1% to 84.8% at 200 mA cm^-2.

Keywords: Vanadium Redox flow battery; Bismuth electrodeposition; Floe field design; Operation temperature; Thermal model; Electrode deformation.