Construction of high-density proton transport channels in phosphoric acid doped polybenzimidazole membranes using ionic liquids and metal-organic frameworks

Abstract

Introducing metal-organic frameworks (MOFs) into phosphoric acid (PA) doped polybenzimidazole (PBI) membranes to form proton transfer channels is an effective strategy to reduce the dependence of proton conductivity on PA. However, excessive MOF loading inevitably leads to reduced mechanical properties and damaged membrane structure. Ionic liquids (ILs) are reported as good plasticizers that can obviously improve the interface performance. Along this line, IL is introducing into the branched poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (BOPBI) cross-linked by KH-560. The interface performance between the MOF and PBI in the cross-linked BOPBI (CBOPBI)@MOF composite system is improved. The high MOF loading (50 wt%) composite membranes are prepared with acceptable mechanical properties for the first time. High density proton transport channels are constructed based on the high MOF loading in the PA doped CBOPBI@MOF membranes. For instance, the CBOPBI@MOF50%-IL30 shows an excellent proton conductivity (0.135 S cm−1), and a power density about a 26.9% increase (736 mW cm−2) compared with that of the CBOPBI@MOF40% membrane. Furthermore, the load voltage of the membrane with 50 wt% MOF is also well maintained. The membranes with high density proton transport channels exhibit high potential application prospects as high-temperature proton exchange membranes (HT-PEMs).