Synthesis And Properties Of Sm³⁺-deficient Sm_1-xBaCo₂O_5+δLayered Perovskite Oxides As Cathode Materials Of IT-SOFC

Solid oxide fuel cell (SOFC) are all-solid electrochemical devices that convert chemical energy stored in the fuel and oxidizer directly into electrical energy with high efficiency and low emission of pollutants. After for cogeneration, fuel efficiency is as high as70%-80%. But traditional SOFC using yttria stabilized zirconia (YSZ) as electrolyte operate at high temperature (800℃-1000℃), which can result in a series of problems such as the electrode sintering, interface reaction between cell components, and high material and manufacture. With the commercialization of SOFC, reducing the operation temperature of SOFC to the intermediate-temperature (IT) range of600℃-800℃is an important development trend. Reducing temperature can bring several benefits such as extending the materials’selection, prolonging lifetime and reducing manufacture cost. However, the electrolyte ohmic resistance and the cathode polarization resistance will increase rapidly with the temperature reduction, which can result in poor electrochemical performance. Thus it is significant to develop new cathodes with low polarization losses and high catalytic activity.In recent years, mixed ionic and electronic conducting (MIEC) LnBaCo2O5+δs (LnBCO, Ln=Lanthanide) oxides with double-layered perovskite structures have received increasing attention. The LnBCO oxides have the Ln-O and Ba-O layers alternating stacking along the c-axis direction, which are advantageous for oxygen ionic conduction and electrochemical reaction activities, and as a result the LnBCO oxides are considered as potential cathode materials of IT-SOFC. The available results have demonstrated that with increase in radius of the Ln3+ions at A-sites (La3+>Pr3+>Nd3+>Sm3+>Gd3+>Y3+), electrochemical performance of LnBCO improves while the thermal expansion coefficient (TEC) becomes larger, and the large TEC doesn’t match with the electrolyte materials and then can lead to structural cracking of the SOFC during its high-temperature working conditions. As a result, it is significant to find an effective way of improving electrochemical performance and meanwhile decreasing TEC values of the LnBCO oxides. SmBaCo2O5+δ (SBCO) is one of the LnBCO oxides with the medium sized Sm3+ions, which has the compatible TEC value with the electrolyte materials, while its electrochemical performance needs to be improved.Electrochemical reaction activities of the cathode materials are closely related to chemical compositions, chemical defects and phase structures. This work aims at improving electrochemical performance of SBCO by introduction of A-site Sm3+-deficienies. In this work, Sm1-xBaCo2O5+δ (S1-xBCO) oxides with various A-site Sm3+-deficiencies (x=0.00-0.08) were synthesized and evaluated as cathode materials of IT-SOFC. The x-ray diffraction (XRD) results have shown that pure phase of Pmmm space group was obtained for the S1-xBCO oxides with x=0.00-0.05, while x=0.08caused formation of impurity phase. Introduction of Sm3+-deficiency has caused structural expansion of S1-xBCO. S1-xBCO oxides were chemically stable with GDC electrolyte at1050℃and below. Oxygen contents of the samples were measured with iodomatric titration method at room temperature. The results indicate that the S1-xBCO oxides with x=0.03and x=0.05have much lower oxygen contents than the parent oxide of x=0.00, suggesting that formation of oxygen vacancies (V0") is the dominant charge compensation mechanism in Sm3+-deficient S1-xBCO oxides. Electrical conductivities of S1-xBCO were measured at various temperatures in air with a four-electrode method. The conductivities gradually decrease with the higher temperatures while at the same temperature the Sm3+-deficient samples have lower conductivities than the x=0.00parent oxide. Electrochemical performance of S1-xBCO cathodes were characterized by impedance spectra measurement based on S1-xBCO/GDC/S1-xBCO symmetric cells. Higher Sm3+deficiency content has resulted in decreased area specific resistances (ASR), i.e. enhanced electrochemical reaction reactivity for the S1-xBCO cathodes. For example, the ASR values at650℃are0.214Ω·cm2at x=0.00,0.146Ω·cm2at x=0.03and0.137Ω·cm2at x=0.05. Among the studied samples, the S0.95BCO (x=0.05) oxide showed the best electrochemical performance with ASR values of0.316Ω·cm2at600℃,0.137Ω·cm2at650℃,0.068Ω·cm2at700℃and0.038Ω·cm2at750℃respectively, thus it’s a promising cathode material of IT-SOFC.