Preparation And Properties Of Cross-Linked Ionexchange Membranes Based On Polybenzimidazole/Functional Ionic Liquids

Proton exchange membranes?PEMs?are one type of ion exchange membranes.As a core component,its performance directly determines the properties and service life of the fuel cell.Polybenzimidazole?PBI?has good thermal stability and mechanical strength and low gas and methanol permeability.However,since the proton dissociation degree in PBI is very low,the proton conductivity is only 10^-7 to 10^-6 S/cm resulting in cannot be directly used for a fuel cell.Researchers typically dope PBI with phosphoric acid to achieve higher proton conductivity.Nevertheless,phosphoric acid is easily leaking out as the reaction proceeds because phosphoric acid is not fixed in the main chain and is easily soluble in water,leading to a significant decrease in fuel cell performance.This is a major problem faced by inorganic acid doped polymer electrolyte membranes.Studies have shown that polymerized ionic liquids can effectively increase the proton conductivity of the membrane,while having the characteristics of ionic liquids without leaching out.In this work,polycationic ionic liquid?PIL?was synthesized and introduced into a polybenzimidazole-based polymer system.The cross-linking agent KH560 was added to hydrolyze under acidic conditions forming covalent cross-linking network.Therefore,the PBI/PIL cross-linked composite high-temperature proton exchange membrane was prepared.Compared with pure membrane,the composite membrane has excellent mechanical properties and proton conductivity,especially the conductivity of cPBI-BF₄-40membrane was up to 0.117 S/cm under anhydrous conditions at 170? and tensile strength was more than 6 MPa.It is promising for application in high temperature proton exchange membrane fuel cells.We introduce PBI into the AEMs and simultaneously form a covalent cross-linking network to solve the contradiction between mechanical properties and ionic conductivity.The PBIOH-AESP series composite membranes were synthesized by grafting the anion exchange silica precursor?AESP?based on hydroxyl polybenzimidazole?PBIOH?.Cross-linked membranes possessed better mechanical properties,thermal and chemical stability than pure membranes.For example,the tensile strength was still above 20 MPa after alkali doping and the thermal decomposition temperature was above 300?.The PBIOH-AESP 20 membrane showed the highest ionic conductivity about 0.077 S/cm at90?,which was higher than that of the pure membrane about 148%.The conductivity still had 0.043 S/cm even after immersing in 3.5 M KOH for 216 hours,which basically meets the needs of fuel cells.