Branched polybenzimidazole/polymeric ionic liquid crosslinked membranes with high proton conductivity and oxidation stability for HT-PEMFCs

For further enhance the proton conductivity and long-term performance of the BOPBI (branching ratio of 6%) with rigid triazine structure with large free volume, a large volume of positive polymeric ionic liquid that can be crosslinked is introduce into the system, and then KH-560 crosslinker with 3% quality fraction is introduced into the system. When the CBOPBI-PIL-X membranes are soaked in KOH solution, The proton conductivity of CBOPBI-PIL-X membranes (X = 0%–40%) was great enhanced due to the formation of ion-pairs (X+- H2PO4−), which can be used as both a proton donor and receptor, providing a binding site for proton transmission. Since the high free volume provided by the branched structure can help the membrane absorb more phosphoric acid, and the unique mechanism of action of PIL can provide new proton transport channels, among these membranes, the ultimate tensile strength (4.68 MPa) of PA-doped CBOPBI-PIL-30% film was great and the membrane can also gain excellent proton conductivity (155.8 mS cm−1 at 180 °C). Due to the addition of KH-560, the Si-O-Si crosslinking network can resist the oxidative free radical attack effectively, which substantially improves the oxidation stability of membranes. After 144 h of testing, all CBOPBI-PIL-X membranes are not broken. And after 240 h of acid retention testing at 160 °C/0% RH condition, the best PA retention ratio of CBOPBI-PIL-X membranes can still reach 83.7%, which can prove that the membranes have excellent performance. Therefore, CBOPBI-PIL-X membranes are expected to be used in high-temperature proton exchange membranes (HT-PEMs).