High Proton Selectivity Sulfonated Polyimides Ion Exchange Membranes for Vanadium Flow Batteries

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Chemistry

Background:

• Ion exchange membranes (IEMs) are crucial for electrochemical devices like vanadium flow batteries (VFBs).
• Achieving high proton selectivity in IEMs is essential for efficient energy conversion and storage.
• Current membranes, such as Nafion115, face limitations in proton selectivity and performance.

Purpose of the Study:

• To synthesize and characterize novel sulfonated polyimide (SPI) based IEMs.
• To evaluate the performance of these SPI membranes in VFBs, focusing on proton selectivity and energy efficiency.
• To compare the properties of non-fluorinated and fluorine-containing SPI membranes.

Main Methods:

• One-step high-temperature polymerization was used to synthesize SPIs with approximately 40% sulfonation degree.
• Characterization included chemical structure analysis, physicochemical property assessment, and single-cell performance testing in VFBs.
• Proton selectivity, mechanical stability, oxidative stability, and energy efficiency were key performance metrics.

Main Results:

• High molecular weight SPIs demonstrated good mechanical and oxidative stability.
• Fluorine-containing SPI membranes exhibited five times higher proton selectivity compared to Nafion115.
• Single cells with SPI membranes achieved 84.8% energy efficiency, outperforming Nafion115 by 4.6% at 100 mA·cm⁻².
• Excellent capacity retention was observed over 500 charge-discharge cycles.

Conclusions:

• Sulfonated polyimide membranes, especially fluorinated variants, are promising alternatives for high-performance IEMs.
• The enhanced proton selectivity and stability of SPIs contribute to superior VFB performance.
• Fluorinated SPIs hold significant potential for applications in the new energy sector.