Understanding of charge relaxation dynamics of Sr doped on Gd 2 Ti 2 O 7 pyrochlore system as Electrolyte for IT-SOFCs

Abstract

The pyrochlore-based system is gaining significant attention as a solid electrolyte in electrochemical energy devices, particularly solid oxide fuel cells (SOFC) due to its high oxygen-ion conductivity at the Intermediate temperature range (400−650 ^o C). In this study, we investigate the Gd _2 − x Sr _x Ti ₂ O ₇ , pyrochlore system doped with strontium (Sr), where, x = 0, 0.02 and 0.04, 0.06, 0.08 and 0.1 to develop an oxygen-ion conductor as an electrolyte for intermediate temperature SOFCs (IT-SOFCs). Structural information is collected using the X-ray diffraction technique and confirms the cubic pyrochlore phase with Fd−3m symmetry accompanied by superstructure peaks (111) (311) (511) and (111) planes across all compositions. The structural data are simulated using Rietveld Refinement. Microstructural features of as-calcined and sintered samples studied by Scanning Electron Microscopy; confirm non-spherical grains with high non-uniformity in particle size distribution. of as-calcined samples and highly dense sintered samples. Elemental composition is confirmed by EDAS. Raman spectroscopy reveals detailed insights into the dopant-induced local restructuring in the Gadolinium Titanate lattice. Few intense Raman modes related to E_g+F_2g and A_lg involve the modulation of crystal structure through the vibration of oxygen along < 100 > cubic axis. Ionic conductivity and activation energy data are extracted through AC impedance measurements. The electric modulus study reveals the ionic relaxation and ion hopping dynamics and their effect on ionic conductivity. M'' relaxation peak and its distribution in relaxation time are analyzed using the Kohlrausch-Williams-Watts (KWW) fit. The presence of dopants induced structural deformations and oxygen vacancies in the GTO host lattice. This led to the disordering of vacancies and modifications in the stretching exponent 'β' and activation energy. Cooperative hopping dynamics through ion-vacancy interactions are found to be a notable influence on ionic conductivity. The optimized dopant composition of GSTO−4 exhibits the highest conductivity peak (σ = 4.3 x 10 ^− 3 S/cm@650^oC). This suggests that apart from vacancy concentration and energy barriers for single-ion hopping, the cooperative dynamics of oxygen ions play a crucial role in determining the ionic conductivity values. Consequently, the GSTO−4 system demonstrates the potential for application as an electrolyte in intermediate temperature-SOFCs.