The vanadium redox flow battery (VRFB) is attracting burgeoning attention for large-scale applications to stabilize the electricity obtained from fluctuated and intermittent renewables, such as solar and wind energies, attributed to its excellent design flexibility, high efficiency, long lifetime, minimized cross-contamination and site independence. However, the critical issues, including low power density, low electrolyte utilization and high capital cost, have hindered the widespread commercialization of VRFBs. The resolution of these issues requires improving the battery performance by simultaneously reducing the activation loss, ohmic loss and concentration loss with cost-effective methods. In this thesis, the primary objective is to fabricate high-performance VRFBs by cost-...[ Read more ]

The vanadium redox flow battery (VRFB) is attracting burgeoning attention for large-scale applications to stabilize the electricity obtained from fluctuated and intermittent renewables, such as solar and wind energies, attributed to its excellent design flexibility, high efficiency, long lifetime, minimized cross-contamination and site independence. However, the critical issues, including low power density, low electrolyte utilization and high capital cost, have hindered the widespread commercialization of VRFBs. The resolution of these issues requires improving the battery performance by simultaneously reducing the activation loss, ohmic loss and concentration loss with cost-effective methods. In this thesis, the primary objective is to fabricate high-performance VRFBs by cost-effectively enhancing the transport and electrochemical properties of electrodes, which is the core component of VRFBs.

This thesis begins with the design of gradient structured electrodes to enhance the transport properties for the flow-field structured VRFBs. The proposed electrodes have a gradient pore structure with the porosity gradually decreasing from the flow field side to the membrane side, which not only enhances the diffusion and under-rib convection of electrolyte, but also avoids the loss of active surface area at the main reaction region. The battery with the gradient structured electrodes shows a 7.00% increased energy efficiency and a 16.12% increased electrolyte utilization than that with the traditional electrodes at 240 mA cm^-2.

To decouple the link between specific surface area and hydraulic permeability, thus enhancing the electrodes’ transport and electrochemical properties at the same time, this thesis then focuses on increasing the surface area of carbon fibers without reducing the size of large pores between them. The first approach to achieve this is to deposit nanomaterials on carbon fibers. The bi-functional and metal-free B₄C nanoparticles are adopted to modify the electrodes by an immersion method, leading to the enhanced surface area and surface activity. In addition, with a combined first-principle and experimental study, binder-free electrodes are fabricated by uniformly electrodepositing bismuth nanoparticles in small size with larger numbers, achieving an energy efficiency of 80.10% at 320 mA cm^-2 in the battery test. The second approach to increase the specific surface area is to etch the carbon fibers. The bi-porous electrodes, which has primary pores as macroscopic pathways for electrolyte flow and secondary pores to increase active surface area, are thus fabricated by a simple yet effective catalytic etching method in the ambient air. The VRFB with the prepared electrodes achieves an energy efficiency of 82.47% at 300 mA cm^-2. Apart from this, to reduce the fabrication cost, room-temperature activated electrodes are prepared for VRFBs, exhibiting even higher battery performance than the conventional thermally treated electrodes which consume a large amount of energy during fabrication.

In addition to the charge-discharge performance, the performance of VRFB is also determined by the cycling stability. Therefore, ultra-stable boron-doped electrodes are rationally designed, fabricated and tested for VRFBs via a bottom-to-up strategy. The assembled battery with boron-doped electrodes achieves an energy efficiency of 82.51% at 240 mA cm^-2, which is 19.5% higher than that with original electrodes. More remarkably, the battery can be stably cycled for more than 2,000 cycles with a high capacity retention rate of 99.972% per cycle and a high energy efficiency retention rate of 99.9998% per cycle.

Finally, a rational cell design is conducted to systematically optimize the membrane, electrode, cell structure and operating condition for VRFBs. With these efforts, the optimized VRFB achieves energy efficiencies of 80.83% and 70.40% at the current densities of 600 and 1000 mA cm^-2, representing the recorded charge-discharge performance for flow batteries in the open literature. More remarkably, the battery can deliver a peak power density of 2.75 W cm^-2 at room temperature, and be stably cycled for more than 15,000 cycles.

Keywords: Vanadium redox flow battery; transport properties; electrochemical properties; porous electrode design; rational cell design; capital cost