Influence of fluorine doping on the electrical and optical properties of La0.9Sr0.1Ga0.8Mg0.2O3-δ solid electrolyte

Abstract

Anion doping offers a promising approach to enhance the ionic conductivity of solid electrolytes at intermediate temperatures, a key factor hindering the widespread commercialization process of solid oxide fuel cells (SOFCs). This study, for the first time, explores the influence of fluorine doping at the concentrations of 0, 5, and 10 mol% in La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM) perovskite structure, synthesized using the glycine-nitrate combustion method. X-ray diffraction (XRD) analysis revealed a transition from orthorhombic to monoclinic phase upon increasing the fluorine incorporation, while maintaining the tolerance factor near unity, indicating a minimal structural distortion within the GaO₆ octahedra. X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of fluorine ions, with an associated enhancement in oxygen vacancy that contributed to improved ionic conductivity. Field-emission scanning electron microscopy (FE-SEM) studies revealed that the 10 mol% fluorine-doped LSGM (LSGMF10) exhibited the largest grain size which facilitated faster oxygen vacancy mobility. The optical measurements indicated a reduced bandgap for LSGMF10 due to the increase in oxygen vacancy concentration. Electrochemical impedance spectroscopy (EIS) demonstrated a remarkable conductivity of 3.8 mS/cm at 600 °C for LSGMF10 (0.24 mS/cm for LSGM) that can be attributed to the synergistic effects of minimal lattice distortion, reduced bandgap energy, and improved grain growth induced by fluorine doping. These findings establish fluorine doping as a promising approach for developing high-performance SOFC electrolytes at intermediate temperatures.