Sol–gel based sulfonic acid-functionalized silica proton conductive membrane

Sulfonic acid-functionalized glass membranes have been synthesized via sol–gel reactions for low-power direct methanol fuel cells (DMFCs). Minimizing the fuel loss due to methanol crossover is the most important issue for creating long-life, low-power DMFC sources. The inorganic glass membrane is of interest due to its low methanol permeability compared to polymer membranes. Three different alkoxy silane reactants were investigated in the sol–gel reaction: 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-mercaptopropyl trimethoxysilane (3MPS), and tetraethoxy orthosilicate (TEOS). The effect of oxidation time of the thiol group on the 3MPS, the mole fraction within the sol, and the water ratio in the reactant mixture were investigated. The ionic conductivity and methanol permeability has been characterized and optimized. The goal in this study was to find a balance between the ionic conductivity and methanol permeability, which determines the fuel conversion efficiency and device lifetime. The optimum glass membrane had a conductivity of 3.71 mS cm−1, and methanol permeability of 2.17 × 10−9 mol cm cm−2 day−1 Pa−1, which was significantly better than Nafion or other previously reported membranes for this application.