Investigation of Sr doping effect on oxygen ion de-localization in Gd2Ti2O7 pyrochlore system and its influence on charge relaxation dynamics and ionic conductivity: as electrolyte for IT-SOFCs

Abstract

In the present attempt, we explore Gd_2-xSr_xTi₂O₇, pyrochlore system, where x = 0, 0.02 and 0.04, 0.06, 0.08 and 0.1 as an electrolyte for intermediate temperature (500–650 °C) solid oxide fuel cell (IT-SOFCs). Structural information is collected using X-ray diffraction and confirmed by Rietveld Refinement as an anion-deficient pyrochlore phase with Fd-3 m symmetry. Microstructural features of as-calcined and sintered samples are studied by using scanning electron microscopy (SEM); Sr existence in the GTO matrix is verified by the EDAX study. Raman spectroscopy study reveals (1) the suppression of F_2g near 481 cm⁻¹ associated with Gd-O(1) stretching vibration with increasing Sr²⁺ doping level and (2) the disappearance of F_2g modes near 610 cm⁻¹ associated with Gd-O(2) stretching vibration with the emergence of new vibration modes near 796 cm⁻¹ in GSTO compositions having x ≥ 4 related to the de-localization of oxygen ion from 48f to vacant 8a site. Ionic conductivity and activation energy data are extracted through AC impedance measurement, and conductivity maximum is obtained for composition GSTO-4. The electric modulus study is explored to reveal ion hopping dynamics. Sr doping in GTO exhibits dispersion in oxygen-ion relaxation frequency. To quantify the relaxation mechanism, M″ relaxation peak and its dispersion are mapped using the Kohlrausch–Williams–Watts (KWW) fit; the stretching exponent “β” is extracted. Cooperative hopping dynamics is measured in terms of oxygen ion-vacancy interactions which is notably influenced on ionic conductivity. The optimized dopant composition of GSTO-4 exhibits the highest conductivity (σ = 4.3 × 10⁻³ S/cm@650 °C). Button-shape SOFCs are fabricated using GTO and GSTO-4 as electrolytes and NiO-GSTO-4 as anode and Dy_0.9Sr_0.1Co_0.8Fe_0.2O_3-δ:GSTO-4 as cathode and depict maximum power densities of the cells 41, 33, and 25 mW cm⁻² at 650 °C, 600 °C, and 550 °C, respectively, for the GSTO-4 system. In contrast, the pristine GTO system exhibits power densities of 32, 23, and 19 mW cm⁻² at the same temperatures. The study demonstrates the potential of GSTO-4 as a prospective system for IT-SOFCs.