Cmt4204

The Fourier-transform infrared (FTIR) spectra of the membranes were recorded using a Nicolet 6700 spectrometer (Nicolet, Waltham, MA, USA). The thermal stabilities of the polymers were measured with a Q50 thermogravimetric analyzer (TA Instruments, New Castle, DE, USA) at a heating rate of 10 °C min⁻¹ under an N₂ flow of 50 mL min⁻¹. The mechanical properties of the membranes were evaluated at room temperature on an electromechanical universal test machine (CMT4204, MTS systems, Jinan, China) at a strain rate of 2 mm min⁻¹. The Br content of the membranes was determined by X-ray fluorescence spectroscopy (S4 Explorer, Bruker, Karlsruhe, Germany). Typical measurements of the properties of the membranes are described in detail in the supplementary data, including the gel fraction, phosphoric acid uptake, mechanical properties, oxidative stability, and single-cell performance.

The thermal gravimetric analysis (TGA) of SPIs were measured with Q50 (TA Instruments, NewCastle, DE, USA) at a heating rate of 10 °C min⁻¹ under the protection of N₂. The Fourier-transform infrared spectra (FTIR) of the SPIs were recorded using a Nicolet 6700 spectrometer (Thermo Scientific, Nicolet, Waltham, MA, USA). ¹H NMR of the B₃ and SPIs were recorded using a Bruker AVANCE III 500 MHz NMR instrument. The microstructure of the membranes of SPI-BFDA was tested by Field emission scanning electron microscopy (FESEM, S-3400N). The mechanical property of SPIs was measured with an electromechanical universal testing machine (CMT 4204, MTS systems, Jinan, China) at a stretch rate of 5 mm min⁻¹ (the relative humidity was 85%). The SPIs test samples were cut into (15 mm × 4 mm) dumbbell samples.