Proton transportation in an organic-inorganic hybrid polymer electrolyte based on a polysiloxane/poly(allylamine) network

A new class of proton-conducting polymer was developed via the sol-gel process from amino-containing organic-inorganic hybrids by the treatment of poly (allylamine) with 3-glycidoxypropyltrimethoxysilane doped with ortho-phosphoric acid. The polymer matrix contains many hydrophilic sites and consists of a double-cross-linked framework of polysiloxane and amine/epoxide. Differential scanning calorimetry results suggest that hydrogen bonding or electrostatic forces are present between H₃PO₄ and the amine nitrogen, resulting in an increase in the glass-transition temperature of the poly(allylamine) chain with an increasing P/N ratio. The ³¹P magic-angle spinning NMR spectra indicate that three types of phosphate species are involved in the proton conduction, and the motional freedom of H ₃PO₄ is increased with increasing P/N ratios. The conductivity above 80 °C does not drop off but increases instead. Under a dry atmosphere, a high conductivity of 10^-3 S/cm at temperatures up to 130 °C has been achieved. The maximum activation energy obtained at P/N = 0.5 suggests that a transition of proton-conducting behavior exits between Grotthus- and vehicle-type mechanisms. The dependence of conductivity on relative humidity (RH) above 50% is smaller for H₃PO ₄-doped membranes compared with H₃PO₄-free ones. These hybrid polymers have characteristics of low water content (23 wt %) and high conductivity (10^-2 S/cm at 95% RH), making them promising candidates as electrolytes for fuel cells.