Investigations into the Governing Factors of Performance for Redox Targeting Flow Batteries with Redox Active Polymers

Flow batteries are a high potential technology for long duration energy storage through the use of modular design and low cost active materials. To date technology has seen moderate success in the commercial space but is limited in development. One hurdle flow batteries have faced is that of energy density due to solubility limits of active materials in the flowing solutions. One proposed solution to overcome this limitation is that of redox targeting flow batteries (RTFBs). These systems use soluble redox mediators to transfer charge between the electrode and a solid storage material (the redox target). By using a solid active material, there is potential to increase the energy density over the solubility limits of alternatives. This thesis looks to create a model system for polymeric based redox targeting flow batteries to study the design parameters and begin to highlight the necessary conditions to be met along the technological roadmap. To do this, in chapter 2 we first design, synthesize, and test a polymer based RTFB with a series of mediator systems. Next, a thermodynamic model is refined to provide insight into the most important variables for success in achieving high accessibility in an RTFB system: formal potential difference and mediator accessibility. The model system is next used to test the impact of lower flow rates and higher charge rates on the accessibility of the redox target in chapter 3. At both 2 mL/min and 20 mL/min the study finds that mediator accessibility is the dominant factor determining performance suggesting that the mediator-target kinetics in the storage tank are fast relative to electrochemical rates. To get an idea of the rate capabilities of the system a technique was developed to isolate the reaction rate between the redox mediator and solids in the storage tank suggesting the ability to reach near equilibrium in as little as 20 minutes. Chapter 4 takes a turn to look at alternative benefits that can be found from the open system concept of RTFBs in contrast to other architectures. By developing a reprocessing loop, we show that the redox targeting material can be isolated, recovered, and reused in 5 different flow cells with minimal change in performance. Finally, Chapter 5 investigates the performance of the same system using a different solvent: water. Water is attractive because of its high conductivity, low cost, and low flammability. In the chapter it is discovered that the rate capabilities of the water-based system are significantly reduced. After investigating multiple charge rates it is also found that very unique and repeatable behavior is seen in the voltage curves during initialization cycling. These odd profiles eventually go away to show a system capable of high accessibility, though at very low rates.