Influence of transition or lanthanide metal doping on the properties of Sr0.6Ba0.4Ce0.9M0.1O3-δ (M = In, Pr or Ga) electrolytes for proton-conducting solid oxide fuel cells

Proton-conducting electrolytes offer an alternative electrolyte to replace the oxygen-ion conducting conventional electrolyte for solid oxide fuel cells (SOFCs). In this study, proton-conducting Sr0.6Ba0.4Ce0.9M0.1O3-δ (M = In, Pr, Ga) electrolytes without Zr were synthesized by the glycine-nitrate process for intermediate-temperature SOFC applications. The thermal decomposition and crystalline structure of the electrolyte powders were analysed by thermogravimetry (TG) analyses and X-ray diffraction (XRD), respectively. The morphological structure and chemical stability of the electrolyte pellets were examined by field-emission scanning electron microscopy (FESEM) and XRD. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the proton conductivity of each electrolyte pellet at different operating temperatures (500–800 °C) in a gas mixture composed of hydrogen (10%) and nitrogen (90%) humidified at room temperature (wet H2/N2), air humidified at room temperature (wet air) and dry air. The ideal calcination temperature for the electrolyte is determined to be 1000 °C based on TGA and XRD analyses, which indicate a high degree of crystallization without any formation of secondary phases. Furthermore, the relative density of all sintered electrolyte pellets is found to lie within an acceptable range (>90%) for good ion conduction. The Sr0.6Ba0.4Ce0.9Ga0.1O3-δ electrolyte displays the highest relative density (99%). However, the chemical stability analysis shows that Sr0.6Ba0.4Ce0.9Pr0.1O3-δ is the most stable electrolyte with a small additional secondary phase. The EIS results indicate that the Sr0.6Ba0.4Ce0.9In0.1O3-δ electrolyte shows the highest ionic conductivities of 1.80 × 10-3, 2.26 × 10-3 and 1.28 × 10-3 S/cm at 700 °C in wet H2/N2, wet air and dry air, respectively. Overall, Sr0.6Ba0.4Ce0.9M0.1O3-δ doped with transition elements (In and Ga) are considered potential electrolytes for proton-conducting SOFCs. In terms of sinterability, chemical stability and proton conductivity, the Sr0.6Ba0.4Ce0.9In0.1O3-δ electrolyte doped with indium (In) is considered the best electrolyte in this work.