Modification of Phosphonated Poly(pentafluorostyrene) toward Ductile Membranes for Electrochemical Applications

Abstract

In terms of proton exchange membranes, phosphonated poly(pentafluorostyrene) (PWN) is an auspicious material. It provides a high ion-exchange capacity, good thermal and chemical stabilities, as well as reduced anhydride formation and high proton dissociation degree due to low pKa. One major drawback preventing the utilization in fuel cells as pure material lies in its poor mechanical stability. Therefore, the need for modifying the polymer structure arises. Herein, we have investigated a rapid and scalable postfunctionalization of poly(pentafluorostyrene) to mechanically modified PWN. Alkyl chains of different lengths ranging from a chain of 6 carbons (-C06) to 18 carbons (-C18) were introduced via para-fluoro-thiol reaction, followed by substitution of the residual para-fluor atoms with phosphonic acid groups via Michaelis-Arbuzov reaction. For the first time, we were able to produce free-standing PWN membranes. Via differential scanning calorimetry measurements, molecular dynamics simulation, and tensile testing, we have proven that the plasticization effect ascends with increasing alkyl chain length. Thereby, the alkyl modification also turned out to be thermally stable (>332 °C), demonstrated with TGA-FTIR measurements. The membrane with 68% phosphonic acid groups and 28% C18 alkyl chains showed a proton conductivity of 93.6 ± 6.9 mS cm-1 and outperformed the Nafion membrane (N-212), which exhibited a proton conductivity of 69.2 ± 6.21 mS cm-1 at 105 °C and a relative humidity of approximately 90%.