A novel strategy for high conductivity and dimensional stability sulfonated polyimide proton exchange membranes for direct methanol fuel cells

A strategy based on the combined regulation of the DS and FFV of membranes. • The prepared membrane exhibits high proton conductivity and dimensional stability. • The BSPI-50 membrane exhibited a peak power density of 72.97 mW cm −2 . • The membrane exhibited good long-term stability in single-cell tests. The trade-off between proton conductivity and dimensional stability of sulfonated polyimide (SPI) proton exchange membranes (PEMs) greatly limits their practical application in direct methanol fuel cells (DMFCs). Herein, a strategy for regulating the proton conductivity and dimensional stability of copolymer membranes through the coupled regulation of the degree of sulfonation (DS) and fractional free volume (FFV) is proposed. By introducing bulky fluorene groups as side chains to regulate the FFV, a series of SPI PEMs with different DS and FFV were prepared. It is found that an appropriate balance between DS and FFV is critical for achieving high proton conductivity and dimensional stability. In particular, the BSPI-50 membrane achieves a proton conductivity of 113 mS·cm −1 at 80 ℃ while maintaining a low swelling ratio of 35.2 %. Molecular dynamics simulations reveal that FFV can be tuned by adjusting the content of fluorene units, as increased side-group content weakens intermolecular interactions between polymer chains. An appropriate balance between DS and FFV facilitates the formation of well-connected hydrophilic nanochannels, thereby promoting proton transport while maintaining dimensional stability. The BSPI-50 membrane delivers a peak power density of 72.97 mW·cm −2 in single cells and exhibits durability over 100 h of continuous operation, with a minimal voltage decay rate of 0.7 mV·h −1 . This work demonstrates that the combined regulation of DS and FFV provides an effective approach to balancing proton transport and dimensional stability, enabling the development of SPI PEMs with excellent overall performance for DMFCs. Download: Download high-res image (134KB) Download: Download full-size image