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

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.