The vanadium redox flow battery (VRFB) has been widely recognized as a leading technology for large-scale energy storage because of several attractive features including excellent system scalability, long lifetime, high efficiency and site independence. To make this technology commercially viable, however, technical and economic barriers, such as high capital cost, low power density, and rapid vanadium ion crossover, need to be addressed. Removal of these barriers requires fundamental understanding of coupled electrochemical and transport characteristics in VRFBs. In this thesis, the electrochemical and mass/ion/electron transport characteristics in two key cell components (porous electrode and membrane) are investigated, based on which corresponding rational cell component design strat...[ Read more ]

The vanadium redox flow battery (VRFB) has been widely recognized as a leading technology for large-scale energy storage because of several attractive features including excellent system scalability, long lifetime, high efficiency and site independence. To make this technology commercially viable, however, technical and economic barriers, such as high capital cost, low power density, and rapid vanadium ion crossover, need to be addressed. Removal of these barriers requires fundamental understanding of coupled electrochemical and transport characteristics in VRFBs. In this thesis, the electrochemical and mass/ion/electron transport characteristics in two key cell components (porous electrode and membrane) are investigated, based on which corresponding rational cell component design strategies are proposed and proved experimentally.

To identify key electrode design parameters, this thesis begins with the development of a two dimensional improved VRFB transport model, which incorporated the effects of ion concentrations on the ion mobility, resulting in a more accurate estimation of ion transport resistance. It is demonstrated that porous electrodes with high surface area and thin thickness can lead to a high battery performance due to the reduced ion transport and charge-transfer resistances. Hence, the thesis then focuses on the design of porous electrodes with high surface area and excellent transport properties. Based on the conventional carbon paper electrode, we propose and fabricate a dual-scale porous carbon electrode via KOH activation method. It is shown that the specific surface area of the carbon paper is increased by a factor of 16 while maintaining the same hydraulic permeability. The dual-scale electrode base-VRFB demonstrates an energy efficiency ranging from 82% to 88% at current densities of 200-400 mA cm^-2, which is record breaking as the highest performance of VRFB in the open literature. To further improve the electrode transport properties, we replace the carbon paper with the carbon cloth as the skeleton of the dual-scale electrode. It is demonstrated that the electrolyte utilization at the current density of 400 mA cm^-2 is increased from 61.1% to 76.4% and a peak power density of 2.18 W cm^-2 is also achieved.

To investigate the ion crossover mechanism through the membrane, we further consider the vanadium ion transport through the porous membrane in the transport model previously developed. It is found that the transport mechanism of vanadium-ion crossover through available separators with the mean pore size of 45 nm is predominated by convection. Reducing the pore size below 15 nm effectively minimizes the convection-driven vanadium-ion crossover while further reduction in diffusion-driven vanadium-ion crossover can be achieved only when the pore size is reduced to the level close to the sizes of vanadium ions due to the size-exclusion effects. Based on these findings, we propose to use the polybenzimidazole (PBI) membrane, instead of conventional Nafion membranes, as the membrane of VRFB, which features formation of pores with sizes ranging from 0.5-2 nm, resulting in a high columbic efficiency of 99% at the current densities of 20-80 mA cm^-2. Apart from this, we also test a subnanosized channel membrane with mean pore size of 0.5 nm to further reduce ion crossover. It is found that this membrane maintains extremely low vanadium-ion permeability that is four orders of magnitude lower than that of Nafion membranes. The battery with the subnanosized channel membrane can deliver a columbic efficiency of 98.4% and energy efficiency of 82.9% at the current density of 160 mA cm^-2. Finally, a commercial composite membrane, which consists of a thin Nafion layer and a porous layer, is evaluated in VRFBs, demonstrating an excellent conductivity and acceptable columbic efficiency at high current densities. It is suggested that this type of membrane is suitable for the high-power VRFB design considering its low cost and high conductivity.

Keywords: Vanadium redox flow battery; porous electrode design; hydraulic permeability; porous membrane; ion crossover