Dielectric Relaxation and Protonic Conductivity of Substitutional Solid Solutions and Composites Based on Polyantimonic Acid and Containing Niobium (5+) Ions

Abstract

Substitutional solid solutions based on polyantimonic acid H₂Sb_{2 – x}Nb_xO₆·nH₂O \(\left( {n \geqslant 1} \right)\) and their composites H₂Sb_{2 – x}Nb_xO₆·0.5yNb₂O₅ nH₂O \(\left( {n \geqslant 1} \right)\) the promising components for low-temperature fuel cell membranes, are synthesized by coprecipitation. The Nb⁵⁺ ions which have the similar radius and electronegativity as the Sb⁵⁺ ions are used as the dopant. The elemental composition of samples (the Nb⁵⁺/Sb⁵⁺ ratio) is refined using X-ray fluorescence analysis. The solid solution composition, limiting as regards the dopant concentration, i.e., H₂Sb_1.6Nb_0.4O₆·nH₂O ions increase the lattice parameter for the samples with 0 < x ≤ 0.4. By analyzing the dielectric properties and the protonic conductivity of samples at a temperature of 25°C and a relative humidity of 58%, it is shown that the sample with x = 0.6 has the best transport properties. Its protonic conductivity is 11.5 × 10^–3 S/m. The protonic conductivity of polyantimonic acid and the solid solution with the limiting dopant concentration (x = 0.4) is 7.2 × 10^–3 and 5.0 × 10^–3 S/m, respectively. A possible mechanism of protonic conduction is proposed, namely, the correlated proton transport along a certain chain of hydrogen bonds through channels in the pyrochlore structure and through the intergranular space that contains adsorbed water and niobic acid.