Synthesis and properties of phosphonic acid-grafted hybrid inorganic–organic polymer membranes

Abstract

A class of phosphonic acid-grafted hybrid inorganic–organic polymer membranes was synthesized using a sol–gel process. Their thermal stability, water uptake, and proton conductivity were investigated. TGA–DSC analysis indicated that these membranes are thermally stable up to at least 220 °C in dry air. The proton conductivities of the new membranes increase with –PO₃H₂ group content and relative humidity, reaching 6.2 × 10⁻² S cm⁻¹ at 100 °C with ∼100% relative humidity, comparable to those of Nafion^® under similar conditions. These new membranes have high proton conductivity at low relative humidity and thus have great potential to be used as electrolytes for high-temperature, low-humidity PEM fuel cells and other electrochemical applications. The proton conductivity of the membranes in the anhydrous state was enhanced by substitution of –CH₂–PO₃H₂ groups with –CF₂–PO₃H₂ groups owing to the large electron-withdrawing effect of C–F bonds. However, it was found that the concentration of –PO₃H₂ groups and the molecular structures of the new membranes are the key factors for the proton transport process in a humidified environment.