Progress on Ion Exchange Membrane for VRB and Experience of Off-grid PV-VRB Power Supply System Design

Renewable energy becomes one of central topics of our times due to its dramatic contribution to reducing CO2 emissions. As estimated, renewable energy power will occupy 20% electricity generation in China by 2020. Accordingly new energy industry is regarded as top 7 strategic burgeoning industries in China. However the output of renewable energy such as wind and solar is not dispatchable. With large shares of such energy introducing to the grid, energy storage has to be combined to ensure the reliability and stability of the electricity. As one kind of energy storage technique, the vanadium redox flow battery (VRB) was well-suited for relatively large-scale utility applications due to its attractive features like long life, independence of energy and power ratings and easily SOC detection. As the key part of VRB, ion exchange membrane (IEM) will greatly determine the resulting performance of VRB as well as the total cost to a large extent. Different kinds of membranes were proposed and fabricated for VRB applications, among which non fluorinated polymer membranes were mostly investigated due to their low cost and high ion selectivity. One type of non-fluorinated membranes with excellent mechanical and oxidation stability were explored by DICP. The membranes show the Young’s modulus of 3500MPa and tensile strength of 120 MPa. The oxidation stability was detected by sinking the membranes into the Fenton reagent with temperature over 60 oC. Almost no weight loss was detected after 200 h soaking. A cell module with rated power of 500W was assembled with the home made membrane (Fig.1). The columbic efficiency for the VRB module developed by DICP was increased significantly to 99.5% (Nafion membrane is around 95%) and energy efficiency was about 80% which is similar to that of module using Nafion 115 membrane. The decreased crossover of vanadium ions across the membrane was very important to keep its stability and durability. Fig.2 shows that after 10,000 charge-discharge cycles there is no obvious degradation in columbic efficiency and energy efficiency in the cell employing such NF membrane.