Spinodal-type phase separation and proton conductivity of Al2O3-doped porous glasses

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Porous glasses with the surface area of ≈300 m2/g were prepared utilizing the spinodal-type phase separation of Al2O3-doped Na2O–B2O3–SiO2 glasses. The proton conductivity under humidified conditions and their relations to the amount of doped Al2O3 were investigated. The proton conductivity of the glass doped with 2 mol % of Al2O3 was about 6 times higher than that of the undoped glass. Fourier transform infrared (FT–IR) measurements revealed that the absorption by the strongly hydrogen-bonded OH groups increased with increasing the amount of Al2O3. Bond overlap populations (BOPs), which are directly related to the strength of the covalent bond, between the surface OH groups and an absorbed water molecule were calculated based on a first-principles theory (DVXα). The BOP values between O and H of the surface OH groups decreased by the Al2O3 doping. Substitution of Si atoms of the Si–O–Si silica networks with Al atoms, and formation of the Al–OH–Si bonds effectively improved the proton conductivity under humidified conditions.