On The New Oxide Ion Conductor Solid Electrolytes Based On Na_0.5Bi_0.5TiO₃

Solid oxide fuel cells?SOFCs?have several advantages,such as high energy conversion efficiency,environmentally friendly and fuel flexibility,which is known as the green energy in 21st century.The huge challenge in improving SOFC technology is to reduce the working temperature to 500–700°C,then reduce the high costs of component materials and increase energy conversion efficiency.To lower the working temperature of SOFCs,the key is to discover and develop solid electrolytes with high ionic conductivity,negligible electronic conduction and thermodynamic stability over a wide range of temperature and oxygen partial pressure.As a new perovskite-structured oxide-ion conductor,Na_0.5Bi_0.5TiO₃ can be substituted in A and/or B sites with different cations over a wide range.Lower valence cations doping can improve the ionic conductivity of Na_0.5Bi_0.5TiO₃-based oxide-ion conductor,which shows a great potential application in intermediate-temperature SOFCs.In this paper,nonstoichiometric Na_0.5Bi_0.5TiO₃ doped with Al³⁺and Mg²⁺were synthesized by solid-state reaction,the ionic conductivity was measured by AC impedance spectroscopy method and the microstructures were characterized with XRD,SEM.Then microstructures,oxide-ion conductivity properties and mechanisms were systematically investigated.Firstly,Al³⁺was chosen to improve the oxygen ionic conductivity at intermediate temperature of Na_0.5Bi_0.49TiO_3-?.With increasing Al³⁺dopant,the ionic conductivity increases gradually and subsequently decreases.Sample Na_0.5Bi_0.49Ti_0.96Al_0.04O_3-?shows the highest oxygen ionic conductivity 2.71×10^-3S/cm at 600°C in air atmosphere,which is lower than other intermediate-temperature electrolytes.In order to further improve the oxygen ionic conductivity,Na_0.5Bi_0.49TiO_3-?was doped with Mg²⁺.The conductivity increases with increasing Mg²⁺dopant content?0-3mol%?and subsequently decreases.Sample doped with 3mol%Mg²⁺exhibits the highest oxygen ionic conductivity(?=6.29×10^-3S/cm)at 600°C in air atmosphere.Compared with other conventional electrolytes,Na_0.5Bi_0.49Ti_0.97Mg_0.03O_3-?shows a great potential application in intermediate-temperature SOFCs.Finally,we successfully synthesize Na_0.5Bi_0.49Ti_0.97Mg_0.03O_3-?with high density and excellent oxide-ion conductivity,and explore the relationship between microstructures and oxide-ion conductivity.The relative density is higher,the oxygen ionic conductivity is better.