High-performance aromatic proton exchange membranes bearing multiple flexible pendant sulfonate groups: Exploring side chain length and main chain polarity

Fabrication of aromatic proton exchange membrane (PEM) with excellent conductivity and stability is a long-standing challenge. Here, we propose an aromatic PEM containing multiple sulfonated alkyl side chains (SAFPAEK-x and SAFPAEF-x), and then implement an ideal strategy to address the trade-off between conductivity and stability by optimizing the side chain length and main chain polarity. Specifically, the synthesized PEMs achieve an efficient proton transport (proton conductivity >0.124 S cm−1 at 80 °C) under the synergistic effect of multiple pendant sulfonate groups and a pronounced microphase separation structure. Meanwhile, regulating the polarity of the hydrophobic domain via introducing fluorinated structures can effectively suppress the swelling behavior, so that SAFPAEF-x display excellent dimensional stability (swelling ratio <10% at 80 °C). The theoretical calculations of molecular models and ex-situ experiments demonstrate that appropriate increases in side chain length and main chain polarity lead to noticeably improved radical stability of aromatic PEMs. Finally, the laboratory-assembled direct borohydride fuel cell using our rationally designed PEMs reaches the highest peak power density of 191.17 mW m−2 at room temperature, outperforming the Nafion 115 (95.87 mW m−2) under the same conditions. Therefore, this membrane design strategy stimulates great potential in designing high-performance aromatic PEMs for fuel cells.