Proton conductivity of Al(H2PO4)(3)-H3PO4 composites at intermediate temperature

Composites of Al(H2PO4)(3) and H3PO4 were synthesised by soft chemical methods with different Al/P ratios. The Al(H2PO4)(3) obtained was found to have a hexagonal symmetry with parameter a = 13.687(3)angstrom, c = 9.1328(1)angstrom. The conductivity of this material was measured by a.c. impedance spectroscopy between 100 degrees C and 200 degrees C in different atmospheres. The conductivity of pure Al(H2PO4)(3) in air is in the order 10(-6)-10(-7) S/cm between 100 and 200 degrees C. For samples containing small excess of H3PO4, much higher conductivity was observed. The impedance responses of the composites were found to be similar with AlH2P3O10 center dot nH(2)O under different relative humidity. The conductivity of Al(H2PO4)(3)-H3PO4 composite with Al/P = 1/3.5 reached 6.6 mS/cm at 200 degrees C in wet 5% H-2. The extra acid is found to play a key role in enhancing the conductivity of Al(H2PO4)(3)-H3PO4 composite at the surface region of the Al(H2PO4)(3) in a core shell type behaviour. 0.7% excess of H3PO4 can increase the conductivity by three orders of magnitude. These composites might be alternative electrolytes for intermediate temperature fuel cells and other electrochemical devices. Conductivity (9.5 mS/cm) changed little, when the sample was held at 175 degrees C for over 100 h as the conductivity stabilised. (C) 2008 Elsevier B.V. All rights reserved.