SPEEK-based composite proton exchange membrane regulated by local semi-interpenetrating network structure for vanadium flow battery

To solve the trade-off between proton conduction and ion permeation of proton exchange membrane, we develop an amphoteric composite membrane composed of sulfonated poly(ether ether ketone) containing imidazole chains (SPEEK-IM) and side-chain sulfonated polyethersulfone having covalently cross-linked perfluoroalkyl chains (CSPF). Through the local semi-interpenetrated network regulated by imidazole and sulfonic acid groups in the CSPF, a synergistic proton transport channel is constructed in the SPEEK-IM/CSPF membrane. The proton conductivity (31.2 mS cm-1) and ion selectivity (49.5 x 103 S min cm-3) of SPEEK-IM/CSPF-10 show an outstanding improvement against Nafion 212 (28.0 mS cm-1 and 3.0 x 103 S min cm-3). SPEEK-IM/CSPF-10 demonstrates a superior energy efficiency, 74.2%, at 200 mA cm-2, which is much higher than that of SPEEK (58.7%), SPEEK-IM (64.7%), and Nafion 212 (64.0%). Meanwhile, a 48.6% charge capacity retention after 200 charge-discharge cycles at 150 mA cm-2 and 600-time cycle stability further support the formed local cross -linking network and acid-base interaction guarantee the mechanical and chemical stability of the composite membrane. This study demonstrates that synergistic proton conduction channels regulated by covalently crosslinked structure and semi-interpenetrating network can explore a promising high-performance composite membrane for the VFB application.