Effects of Cr/Ti co-doping on the electrical and thermal properties of tantalum-based electrolyte materials for solid oxide fuel cells

New triclinic oxide material Ta_1-x-yTi_xCr_yO_2.5-δ (x = 0.077, y = 0-0.053) was synthesized via an oxalate co-precipitation method and tested as a novel electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Structural, electrical, and thermal properties of tantalum-based electrolyte co-doping with Cr³⁺/Ti⁴⁺ content were studied. X-ray powder diffraction showed that the crystal structure changed from orthorhombic to triclinic due to co-doped, and its high-temperature triclinic (pseudo-tetragonal) phase of tantalum pentoxide (H-Ta₂O₅) structure was stabilized to room temperature by doping. The crystalline phase facilitates the generation of more oxygen vacancies, which increases the electrical conductivity. The appearance of oxygen vacancies in the crystal with triclinic structure was confirmed with Raman spectroscopy and X-ray photoelectron spectroscopy. Scanning electron microscopy results exhibited the grain size decreases gradually with doping. Impedance curves showed high total ionic conductivity (1.48 × 10^-1 S/cm at 700 °C) and low activation energy (E_a = 0.857 eV) for Ta_0.9Ti_0.067Cr_0.033O_2.5-δ. Thermal expansion coefficients (3.09 × 10^-6 K^-1) for co-doped samples were much lower in comparison to other electrolytes. Based on the results reported in this work, Ta_1-x-yTi_xCr_yO_2.5-δ can be an excellent oxygen conductor and recommended as solid electrolytes for IT-SOFC applications.