Proton-Conductive Membranes Doped with Orthophosphoric Acid Based on Inorganic–Organic Hybrid Materials

A series of proton-conductive inorganic–organic hybrid membranes doped with phosphoric acid (H₃PO₄) have been prepared by the sol–gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-aminopropyltriethoxysilane (APTES), and tetraethoxysilane (TEOS) as precursors. High proton conductivity of 3.0 × 10^?3 S/cm with a composition of 50TEOS–30GPTMS–20APTES–50H₃PO₄ was obtained at 120°C under 50% relative humidity (RH). The differential thermal analysis curve showed that thermal stability of membrane is significantly enhanced by the presence of an SiO₂ framework up to 250°C. X-ray ray diffraction revealed that the gels were amorphous. Infrared spectra showed a good complexation of H₃PO₄ in the matrix. The porous hybrid membrane, characterized by scanning electron microscopy, shows humidity-dependent conduction, and the conductivity under 75% relative humidity was significantly improved by addition of APTES due to the increase in the concentration of the defective site in the hybrid matrix.