Study of various types of membranes for vanadium redox flow battery application

The increase in renewable energy penetration to grids and micro-grids resulted in increase in use of battery storage. Today, the energy supplied from solar PV and battery has become cheaper than conventional electricity in many countries. Redox flow batteries are popular mainly due to their independent power and energy capacity sizing, among which the all vanadium redox flow battery (VRFB) is one of the mature and commercially available technology. VRFBs uses vanadium ions mixed in sulfuric acid as electrolytes in both tanks to store and generate the electricity. This is made possible due to Vanadium’s ability to exist in four oxidation states. VRFBs have numerous advantages, such as long life, stable capacity over the lifetime, low maintenance, flexibility of increasing the energy capacity just by increasing the size of the electrolyte storage tanks [1]. Ion exchange membranes are used as a separator in VRFB. In the market, anion and cation exchange type membranes are popular. In VRFB application, selection of a good membrane is crucial for securing the long life of the battery and stable performance. The thickness of the membrane and type of counter ion determines the ion selectivity and ion conduction. In addition, stability of the membrane in the corrosive electrolyte environment is very important. Vanadium ion crossover through the membrane is an undesired phenomenon that can lead to capacity loss and affect the battery performance. Research has been performed to reduce such losses, by developing new membrane materials and various techniques that lower the crossover of the vanadium ions. However, it is a pain to developers to find the best membrane in the market, as there is no systematic study that compared a wide variety of membranes available in the market for ion selectivity, ionic conduction and long-term stability in acidic environment. In this Final Year Project (FYP), a wide range of commercial membranes were examined to determine how they affect VRFB’s performance. The study carried out through VRFB cell cycling test using different membranes and stability test in acidic media for different time intervals. Electrochemical Impedance Spectroscopy (EIS) was used to determine the ionic conduction and long-term stability of the membranes. Through this study, it was found that Anionic Exchange Membranes (AEM) are not stable in V5+ solutions if exposed for long time. Long exposure to V5+ solution caused resistance increment, which lowered the performance of the battery. It was also found that AEM causes faster electrolyte imbalance and faster capacity decay compared to Cationic Exchange Membranes (CEM). For specific categorization of the membranes, the thickness of the membrane also altered the performance of the battery. This study concludes that CEM are superior for VRFB application despite their high cost compared to AEM.Bachelor of Engineering (Materials Engineering