Beneficial effects of Mg-excess in La1-xSrxGa1-yMgy + zO3-δ as solid electrolyte

One of the paramount obstacles to the commercialization of La1-xSrxGa1-yMgyO3-δ (LSGM)-based intermediate-temperature solid oxide fuel cells is the high cost and low mechanical strength associated with the use of Ga2O3. We demonstrate in this study the use of excess Mg-doping on Ga-site as a means of lowering cost and potentially increasing mechanical strength of the final LSGM product. Conventional techniques such as XRD, SEM and AC impedance spectroscopy are applied to characterize the synthesized Mg-excess La0.8Sr0.2Ga0.83Mg0.17 + zO3-δ. The microstructures revealed by SEM indicate an increased appearance of the Mg-rich second phase along the grain-boundaries with increasing Mg-excess level even though regular XRD could not discern the presence of this second phase. Surprisingly, the total oxide-ion conductivity of Mg-excess LSGM becomes progressively higher than the stoichiometric LSGM as the temperature increases, which is also accompanied by a gradual disappearance of the grain-boundary effect. Mechanisms of re-dissolution of the Mg-rich grain-boundary phase at elevated temperatures and formation of dopant-vacancy associates are proposed to interpret the observed compositional as well as the grain-boundary effect on the total ionic conductivity. Overall, no alarming adversary effect on oxide-ion conductivity of LSGM from Mg-excess doping (up to 10 mol%) has been found in this study.

► Mg-excess LSGM (La0.80Sr0.20Ga0.83Mg0.17 + zO3-δ) has been for the first time studied as a promising solid electrolyte.
► Mg-excess LSGM is better oxide-ion conductor than the baseline stoichiometric LSGM.
► Mechanisms of re-dissolution of the Mg-rich grain-boundary phase at elevated temperatures is proposed.