Abstract
Synthesis Optimization of a Nonaqueous Redox Flow Battery Active Material: A major obstacle to incorporation of renewable and carbon-neutral energy sources, such as wind and solar, into the electrical grid is their inherent intermittency. As a result, grid-scale energy storage has been identified as a useful component in balancing the variability of these sources, with hopes to transition global energy utilization away from fossil fuels. Research on this topic has transitioned towards vanadium redox flow batteries (RFBs) that make use of solvents with wide electrochemical windows, low vapor pressure, low melting points, and high dielectric constants. Compared to aqueous RFBs, nonaqueous RFBs (NRFBs) have the added advantage of a higher operating voltage per cell due to the wider potential windows of organic solvents and ionic liquids. Despite how promising NRFBs appear, advancement has been severely hampered by instability of active materials. To address this, research in the Cappillino lab is focused on a bio-inspired active material known as calcium (II) vanadium (IV) bis-hydroxyiminodiacetate, or CVBH. This compound is a close analogue to a molecule isolated from mushrooms known as amavadin. It is a naturally occurring and highly stable vanadium-chelating molecule that can be used and modified to create a stable RFB active material. Optimization of CVBH Synthesis Substitution of Ca(OH)2 for NaOH in a procedure from the literature for the synthesis of N-hydroxy-iminodiacetic acid (H3HIDA), led to the creation of a proposed one-pot synthesis mechanism for CVBH. Ca(OH)2 eliminates the need for zinc acetate dihydrate to precipitate a zinc salt of the ligand and allows direct metalation in solution with vanadyl acetylacetonate, VO(acac)2. In addition, utilization of chloroacetic acid reduces cost, when compared to the literature procedure, by 75%. The one-pot allows for scalable production of the active material and the obviation of the zinc reagent, as well as the substitution of chloroacetic acid, facilitates this one-pot preparation to be cost-efficient. Evaluation of the Structure-Property Relationship in NRFBs: The current ion exchange membranes (IEMs) used in NRFB systems do not demonstrate high capacity and low crossover rates at the same time. As essential components, IEMs should have low active-material permeability to efficiently provide ionic conductance from one electrode to the other. Nafion, a perfluorosulfonated polymer cation exchange membrane (CEM), is commonly used in these battery systems because of its high ion conductivity, chemical stability, and commercial availability; however, the characteristically high active-material permeability of Nafion membranes causes an undesirable drop in Coulombic efficiency and rapid capacity decay. Reported here is a rationally designed siliconized Nafion membrane that can mitigate the crossover of VBH ions. Ultraviolet/visible (UV-Vis) spectroscopy was used to monitor the concentration of VBH during H cell operation. Infrared spectroscopy was utilized to study the effect of varying time on a silicon modification of Nafion membranes. Of the Nafion 212, 115, and 117 (NX, X = 212, 115, 117) membranes available, N117 membranes provided less crossover, both when modified with a silicon derivative and when non-modified, as compared to N115 and N212. N115 had less crossover than N212. Overall, exposing the membrane to tetraethyl orthosilicate (TEOS), our modifying silicone reagent, caused a decrease in crossover of VBH active-material in the system over time. Full cell cycling research inspired by this data, conducted by another member of the Cappillino lab, confirms that membrane thickness does not affect cell performance. In conclusion, thickness of IEMs does not affect cell performance but greatly determines active material crossover. The systematic study of transport properties and CEM composition provides valuable insight that can aid in elucidating the structure-property relationship of IEMs and in establishing design criteria for the development of high-performance membranes.