Polyethyleneimine-confined halloysite nanotubes for poly(2,5-benzimidazole) composite membranes with phosphoric acid retention and proton conductivity via ion pairs for wide-temperature PEMFCs

To prevent phosphoric acid (PA) from leaching and harming traditional polybenzimidazole-based proton exchange membranes, polyethyleneimine-confined halloysite nanotubes (PEI@HNTs) were embedded in poly (2,5-benzimidazole) membranes to construct rich ion pairs that stably conducted protons. With the aid of supercritical carbon dioxide, PEI oligomers with basic groups were confined in HNTs. Then, through in-situ synthesis, highly-dispersed PEI@HNT enhanced the mechanical stability of composite membranes, whose basicity improved the absorption and retention capacity of PA. The PEI@HNTs/ABPBI composite membrane exhibited excellent mechanical strength and electrical performance over a temperature range of 40–180 °C. The through-plane proton conductivity of the 5%PEI@HNTs/ABPBI composite membrane was 0.061 S cm−1 at 98% relative humidity and 90 °C and 0.049 S cm−1 under anhydrous conditions at 180 °C. This performance was attributed to strong hydrogen bonding between PEI and PA, as well as the excellent retention of bound water within the HNTs. The peak power density of PA-doped 5%PEI@HNTs/ABPBI composite membrane reached 0.235 W cm−2 and 0.418 W cm−2 at 120 °C and 180 °C under anhydrous conditions, respectively, which were 1.68 and 1.96 times higher than those obtained with the pristine PA-ABPBI membrane. These results highlight the potential of this composite membrane for applications requiring a wide temperature range.