Materials for Improving Vanadium Redox Flow Battery Performance
Creators: name: Pahlevaninezhad, Maedeh
Publisher: name: Schulich School of Engineering
Year: 2023
Subjects: subject: Vanadium redox flow battery
subject: Energy
subject: Engineering--Environmental

description: Redox flow batteries are emerging as a promising grid storage solution due to their long lifetime, independent power and energy, high round trip efficiency, scalability, and design flexibility. Among the different chemistry of redox flow batteries, the vanadium redox flow battery (VRFB) is the most promising and commercially developed technology due to its use of a single element (vanadium) with different oxidation states on both sides of the battery. The electrolyte, membrane, and electrode are the three key components of VRFBs, and the battery performance is largely dependent on their properties. This thesis is focused on improving the battery performance in parallel with cost reduction as the key parameters to facilitate widespread commercialization of VRFBs. The effect of Al3+, Fe2+, Mn2+ impurities on the electrochemical activity, VRFB performance and durability is studied by cyclic voltammetry, flow battery charge/discharge, in-situ hydrogen evolution measurement, and material characterization. Since a small increase in the electrolyte purity higher than 98.5% can have a significant impact on the electrolyte costs, understanding the effects of impurities on VRFB performance is essential. The presence of Mn2+ ions at concentrations of up to 0.1 M is found to have a negligible impact on charge-discharge efficiencies over 200 cycles. With Al3+ or Fe2+ ions in the electrolyte, severe detrimental effects on battery performance and durability are observed. The presence of Fe2+ decreases the VRFB discharge capacity by 36% after 200 cycles. In the presence of Al3+ an alumina containing precipitate is formed on the electrodes, severely affecting the battery performance. Degradation analysis of the electrode, membrane and electrolyte showed that the presence of Al3+ had a severe impact on the electrodes. With Al3+ in the electrolyte, a precipitate was observed to block the electrode, especially on the fibers of the carbon paper. This led to the lowest capacity retention over cycling (less than 10% over 190 cycles), and the battery failed after the 190 cycles number, while in the case of other impurities, the capacity retention was around 85-90% over the 200 cycles number. Although the lowest battery efficiency occurs in the presence of a mixture of the three impurities, the precipitation and capacity decay are less severe than with Al3+ alone. This suggests that the presence of Mn2+ and/or Fe2+ helps to stabilize the electrolyte and mitigate the precipitation. In order to increase the time between regeneration cycles and to improve the overall efficiency of VRFBs, the use of a thin, graphene coating on the surface of various Nafion membranes was investigated. Electrochemically exfoliated graphene (EEG) was dispersed at the air-water interface of a Langmuir-Blodgett trough, compressed, and transferred to Nafion 117 (180 μm thickness) and Nafion 115 (127 μm) membranes. Single-cell vanadium redox flow batteries assembled with the coated membranes had significantly higher energy efficiency (13%), increased power density (by 67%) and discharge capacity (by 17.5%) compared to uncoated Nafion. The graphene layer is stable during cycling, and electrochemical impedance spectroscopy and self-discharge experiments indicated that the improved battery performance is due to a combination of reduced vanadium crossover, and enhanced electrochemical activity provided by the graphene at the electrode surface. Lastly, the development of an electro-catalyst for the negative electrode with high performance and long–term stability of VRFBs was explored. NH4HF2-etched MXene was synthesized and applied as an electrocatalyst for the negative electrode of a VRFB for the V2+/V3+ redox reaction. This study compared the performance of three electrode materials in VRFB: pristine and thermally treated carbon paper, and MXene modified carbon paper. The EE at a current density of 60 mA cm-2 for the MXene modified, thermally treated, and pristine electrodes were 81.5%, 69%, and 56%, respectively. The charge-discharge EE of the VRFB operated at 60 mA cm-2 improved by 25.5% and 12.5 % when compared with the pristine and thermally treated carbon paper electrodes, respectively. In addition, the maximum current density for VRFB charge-discharge increased from 80 mA cm-2 to 120 mA cm-2 when MXene was decorated on the thermally treated carbon paper negative electrode. Increasing the operation current density in parallel with higher voltage efficiency enhanced the maximum power density of the battery by 41%. Moreover, an 85% capacity retention after 150 cycles was recorded at the highest current density of 120 mA cm-2. The MXene led to significantly improved VRFB performance due to the increased electrochemical activity, electrical conductivity, and wettability of the negative electrode. These factors provide extra active sites for enhanced V2+/V3+ redox reaction and lower overpotentials. Consequently, a higher battery power density was achieved due to increased operating current density and VE that could significantly reduce the battery stack size for a higher target power output.