Tailoring phase stability and electrical conductivity of Sr _0.02La _0.98Nb _1-xTa _xO ₄ for intermediate temperature fuel cell proton conducting electrolytes

Sr _0.02La _0.98Nb _{1 - x}Ta _xO ₄ (SLNT, with x = 0.1, 0.2, and 0.4) proton conducting oxides were synthesized by solid state reaction for application as electrolyte in solid oxide fuel cells operating below 600 °C. Dense pellets were obtained after sintering at 1600 °C for 5 h achieving a larger average grain size with increasing the tantalum content. Dilatometric measurements were used to obtain the SLNT expansion coefficient as a function of tantalum content (x), and it was found that the phase transition temperature increased with increasing the tantalum content, being T = 561, 634, and 802 °C for x = 0.1, 0.2, and 0.4, respectively. The electrical conductivity of SLNT was measured by electrochemical impedance spectroscopy as a function of temperature and tantalum concentration under wet (p _H2O of about 0.03 atm) Ar atmosphere. At each temperature, the conductivity decreased with increasing the tantalum content, at 600 °C being 2.68 × 10 ^-4, 3.14 × 10 ^-5, and 5.41 × 10 ^-6 Scm ^-1 for the x = 0.1, 0.2, and 0.4 compositions, respectively. SLNT with x = 0.2 shows a good compromise between proton conductivity and the requirement of avoiding detrimental phase transitions for application as a thin-film electrolyte below 600 °C.