Effects of organo-functionalization and sulfonation of MCM-41on the proton selectivities of MCM-41/Nafion composite membranes for DMFC

Abstract

Siliceous MCM-41 and various organo-functionalized MCM-41s have been successfully synthesized by co-condensation method with/without the addition of NaF. The results of XRD, FT-IR and TG/DTG analyses show that the organo-functional groups incorporated into silicate framework of MCM-41 have been stably anchored to the silicate framework even after surfactant extraction and subsequent sulfonation. It also shows that the addition of NaF to synthesis mixture could lead to well-resolved MCM-41 but the incorporation of organo-functional groups could be disturbed by F − anions, especially for vinyl group, enhancing the polycondensation than co-condensation. The proton conductivities of composite membranes fabricated by as-synthesized siliceous MCM-41s synthesized with/without the addition of NaF (M41F and M41) are lower than those of composite membranes fabricated by their calcined MCM-41s synthesized with/without the addition of NaF (CM41F and CM41) regardless of the addition of NaF while their methanol permeability are opposite, mainly due to pore blocking by surfactant. The results also clearly show that the higher and increasing methanol permeability of composite membrane fabricated with CM41 than fabricated with CM41F with powder content is ascribed to the combined effect of larger pore and broader pore size distribution and poorer adhesion between powder and Nafion dispersion. FT-IR and TGA results clearly show that the methanol permeability of composite membranes fabricated with various organo-functinalized- and sulfonated-MCM-41 strongly depends on the residual void space within the pore channel of MCM-41 resulting from the amount of incorporated organo-functional groups and hydrophilicity of sulfonic groups attached to organo-functional groups. It is also seen that the synthesis condition of MCM-41 such as the addition of NaF significantly affects the degree of organo-functionalization, especially for vinyl groups and thus sulfonation.