Abstract:

Objective With the rapid development of new energy industry, research on different types of various redox flow batteries has become a hot topic nowadays. Among them, iron-chromium redox flow batteries, with abundant and inexpensive raw materials such as iron ions and chromium ions as the active substances, can significantly reduce the battery manufacturing cost and have higher safety, demonstrating better prospects for industrialization and applications. The membrane is an important part of the iron-chromium redox flow batteries, not only blocking the active substances in the positive and negative electrolyte cascade, but also to providing a channel for the proton transfer in the electrolyte to balance the charge. The ideal membrane for iron-chromium flow battery is expected to have high proton conductivity, high ion selectivity, excellent electrochemical stability, low cost and other characteristics.

Method In order to reduce the cost of iron-chromium redox flow battery membranes, polyether ether ke-tone(PEEK) was modified by sulfonation, and a series of sulfonated poly (ether ether ketone) (SPEEK) membranes were obtained by modulating the sulfonation process to explore the effect of the sulfonation process on the SPEEK electrospinning. Then, MXene was introduced during the spinning process to improve the water absorption, swelling, and pore distribution of SPEEK nanofiber membranes to enhance the performance of the membranes. The battery cycling performance of MXene/SPEEK nanofiber membranes was thoroughly investigated.

Results With 25% mass fraction of spinning solution, 22 kV spinning voltage, and 1 mL/h flow rate of spinning solution, the morphology of nanofiber battery membranes appeared uniform. With prelonged the sulfonation reaction time, the degree of sulfonation was increased, while the diameter of the electrospinning fibers tended to decrease. The average diameter of the SPEEK-10 was 107.5 nm when the sulfonation reaction time reached 10 h. The water contact angles of SPEEK nanofiber battery membranes with different degrees of sulfonation were less than 90°, and all SPEEK nanofiber battery membranes showed hydrophilicity, except that the water contact angle of the SPEEK-10 was only 8.1°. The elongation at the break of the SPEEK-10 increased to approximately 135.4%, and the tensile stress was slightly higher (16.3 MPa) than that of other SPEEK nanofiber battery membranes. SPEEK-10 illustrated excellent hydrophilicity and mechanical properties, meeting the basic performance requirements as a battery membrane of iron-chromium redox flow battery. However, an increase in the swelling radio of the pure SPEEK nanofiber battery membranes after excessive water uptake destroyed the dimensional stability of the nanofiber battery membranes, resulting in an increase in ionic permeability and a decrease in battery performance. This suggests that low swelling radio should be a prerequisite for the use of high-performance iron-chromium redox flow batteries. Using the ability to form hydrogen bonds between MXene and SPEEK, the mechanical stability of the nanofiber battery membranes was increased by electrospinning with MXene doping. By adjusting the MXene doping level, the water absorption and swelling rates of the nanofiber battery membranes were optimised. When the MXene doping amount was 15%, the water uptake and swell radio of SPEEK nanofiber battery membranes were only 12.7% and 14.3%. MXene/SPEEK nanofiber battery membranes show improved physical properties. In addition, its specific surface area and pore size distribution were better than those of the pure SPEEK nanofiber battery membranes. Notably, the MXene/SPEEK-15% shows excellent performance in the cycling test of iron-chromium redox flow battery, with a Coulombic efficiency of 97.7% and an energy efficiency of over 70%.

Conclusion The prepared MXene/SPEEK nanofiber battery membranes demonstrate high hydrophilicity, ion selectivity and good mechanical properties, which satisfy the basic requirements for being used as iron-chromium redox flow battery membranes. The high conductivity and high specific surface area of MXene effectively reduced the water absorption and swelling of the SPEEK-based nanofiber battery membranes, while inhibiting the permeation of iron-chromium ions. The results show that by optimizing the sulfonation process and MXene doping, the MXene/SPEEK nanofiber battery membrane exhibits high ion selectivity and excellent electrochemical stability while maintaining high proton conductivity. This study is expected to provide a new low-cost and high-performance solution for iron-chromium redox flow battery membranes.

Key words: iron-chromium redox flow battery, sulfonated poly(ether ether ketone), MXene, electrospinning, nanofiber, battery membrane, electro-chemistry stability