Properties of modified Nafion® membranes with heavy amount of (3-mercaptopropyl) trimethoxysilane prepared by long-term infiltration

Abstract

Modified Nafion® membranes with a large amount of (3-mercaptopropyl) trimethoxysilane (MPTS) were fabricated by a long-term infiltration process. MPTS has been known as a strong surface modifier due to its ability to form inorganic phases having a high concentration of sulfonic acid ligands by proper oxidation of thiol-groups. In this work, the long-term infiltration of 3-mercaptopropyltrimethoxysilane (MPTS) monomers into the Nafion® matrix was performed to achieve the goal of reducing methanol crossover without sacrificing proton conductivity. Thiol (-SH) groups of MPTS molecules were converted into sulfonic acid groups by mild oxidation with hydrogen peroxide (H2O2). The fabricated membranes were characterized through microstructure observation, solvent uptake and expansion test, and proton conductivity measurement. The MPTS-derived inorganic phase formed in the ionic cluster pathways and the free volume in the Nafion® matrix could significantly reduce the methanol transfer rate, keeping high proton conductivity due to an increase in sulfonic acid concentration induced from the oxidation of the infiltrated MPTS.