Effects of grafting parameters on the properties of proton exchange membranes based on sulfo-functionalized porous silicon for micro direct methanol fuel cells

3-Mercaptopropyltrimethoxysilane [MPTMS, (CH3O)3SiCH2CH2CH2SH] can be grafted to porous silicon via a simple chemical grafting method and then sulfonated to prepare a proton exchange membrane (PEM) for application in micro direct methanol fuel cells (μDMFCs). The concentration of MPTMS and the pH of the solvent, which is varied by adding glacial acetic acid (GAA), play important roles in the chemical grafting process. In this work, PEMs based on sulfo-functionalized porous silicon were prepared at MPTMS concentrations ranging from 10 wt.% to 90 wt.% and GAA concentrations ranging from 0 wt.% to 10 wt.% to study the effects of grafting parameters on the proton transport properties of PEMs. Electrochemical impedance spectroscopy shows that the proton conductivity of the PEMs can be tuned by changing the MPTMS and GAA concentrations, and it reaches a maximum of 0.082 S/cm at an MPTMS concentration of 30 wt.% and a GAA concentration of 5 wt.%. The effects of MPTMS and GAA concentrations on the properties of PEMs are discussed in the context of two competitive reaction pathways of MPTMS molecules: dehydration condensation with silanols on the walls of porous silicon and self-polymerization.