Renewable energy such as solar and wind energy is regarded as the most attractive alternatives to traditional fossil fuels due to its merits including clean, pollution-free and renewable. However, its intermittent and fluctuated nature makes it mismatch to the demand for humankind. At this point, an energy storage device is needed to balance the mismatch. Among various energy storage devices, flow batteries become a promising candidate because of their site-independence, safety and decoupled nature of capacity and power. However, the most critical issue preventing the technology from widespread applications is its high capital cost. The capital cost of flow batteries is mainly affected by three major issues, including the cost of materials (mainly stack components and electroly...[ Read more ]

Renewable energy such as solar and wind energy is regarded as the most attractive alternatives to traditional fossil fuels due to its merits including clean, pollution-free and renewable. However, its intermittent and fluctuated nature makes it mismatch to the demand for humankind. At this point, an energy storage device is needed to balance the mismatch. Among various energy storage devices, flow batteries become a promising candidate because of their site-independence, safety and decoupled nature of capacity and power. However, the most critical issue preventing the technology from widespread applications is its high capital cost. The capital cost of flow batteries is mainly affected by three major issues, including the cost of materials (mainly stack components and electrolytes), low power densities of stacks, and low utilization rate of active species. This thesis aims to address these issues through both system and material designs.

Beginning with a search for a cheap alternative to Nafion membranes in all-vanadium flow battery systems, an acid-resistant PES based nanofiltration membrane that is capable of conducting protons and blocking the crossover of vanadium ions through the size-exclusion mechanism is adopted. We experimentally tested a series of nanofiltration membranes with different pore sizes and finally selected the optimized type. Although this type of membrane achieves performance slightly lower than Nafion membranes in conventional flow-through vanadium flow cells, its price is only one-hundredth that of Nafion membranes, which can significantly decrease the capital cost of flow batteries.

To increase the power density of the vanadium flow battery, thereby reducing the size and the cost of the stack, we test the effect of operating temperature on vanadium flow battery performance. The voltage efficiency of the VRFB is found to increase from 86.5% to 90.5% at 40 mA cm^-2 when the operating temperature is increased from 15 ℃ to 55 ℃. More remarkably, the peak power density also increases from 259.5 mW cm^-2 to 349.8 mW cm^-2 at the same temperature increment.

To further improve the power density of vanadium flow batteries, we firstly adopt a multi-walled carbon nanotube thin film as a high-performance electrode. The MWCNT thin film has a much higher specific surface area compared than conventional carbon paper electrodes composed of carbon fibers do. The peak power density of the battery reaches 516 mWcm^-2, which is 65% higher than that of a battery equipped with a carbon paper electrode at a similar thickness. More importantly, the electrolyte utilization rate increases from only 54.6% to 85.3% at a current density of 40 mA cm^-2, which is a significant improvement in reducing the VRFB system cost.

While the capital cost that can be efficiently reduced by enhancing the battery performance, the system cost of VRFB is still highly restricted by the cost of vanadium salts. As the active material, the cost of vanadium salts already counts for 77% of DOE's target price on energy storage systems, leaving only limited space for system optimization. Further exploration of developing low-cost flow batteries in this thesis focuses on searching for alternative active species. Among all alternative redox couples, organic redox species are promising for realizing massive electrical energy storage at significantly reduced cost. The organic species can be synthesized from inexpensive commodity chemicals or even natural abundant such as quinones. Although organic flow batteries have attracted burgeoning attentions, their developments are still hindered by two critical issues: one is the low solubility, the other is the severe irreversible crossover.

To increase the solubility of quinones, we study a non-aqueous electrolyte that adopting propylene carbonate as the supporting electrolyte and para-benzoquinone as the redox species. The solubility of p-BQ in PC reaches as high as 1.9 M, resulting in a volumetric capacity of 101.8 Ah L^-1. The proposed catholyte was tested with a lithium anode, and this battery has an average discharge voltage of 2.8 V, together with an energy efficiency of 81% and an electrolyte utilization rate of 78%.

To address the severe crossover issue, we propose and develop a symmetric organic flow battery. A coupling reaction is adopted to connect two redox-active molecules with a suitable potential difference, and then form an artificial bipolar combi-molecule VIO-Ferrocene. The synthesized VIO-ferrocene combi-molecule has been firstly tested with a lithium anode, in which the VIO-ferrocene demonstrated a stable electrochemical behavior over 220 cycles. A proof-of-concept symmetric battery has been successfully demonstrated over 40 cycles with the energy efficiency keep above 70%.

Finally, to verify the practical feasibility of the flow battery technology on mobile applications, we have designed and built two flow battery prototypes. Compared with current electric vehicles, the concept of flow battery car shines by eliminating lengthy charging time. The prototypes are designed with lab-scale battery performance data and then fabricated and integrated into prototypes with typical machining methods.

Keywords: Redox flow batteries; costs; membranes; electrodes; organic electrolytes; symmetric flow batteries