Composition Optimization And Microstructure Modification Of Sm_xSr_1-xMnO₃Perovskites As Cathode Material For Intermediate Temperature Solid Oxide Fuel Cell

Solid oxide fuel cell is an efficient electrochemical device with low noise and zero emission. The high price of the materials in the SOFC systems and the performance degradation in the high temperature range prohibits its large-scale application. It is now developing towards the intermediate temperature range (IT-SOFC). However, the most conventional Lao.8Sro.2MnO3(LSM82) cathode is not competent for its deteriorated performance within650～800℃. Many perovskites are polupar cathode materials for their low cost, high catalytic activity towards the oxygen reduction reaction (ORR) and high phase stability above800℃. Recently, some perovskites, such as, Sm0.5Sr0.5CoO3, La1-xSrxCo1-yFeyO3-δ, Ba0.5Sr0.5Co0.8Fe0.2O3, exhibit excellent electro-catalytic activities towards ORR, however, their poor chemical and thermal compatibility with the traditional YSZ electrolyte restricts their application as cathode materials for IT-SOFCs.Strontium-doped samarium manganite perovskites (SmxSr1-xMnO3) are investigated as cathode materials. SmxSr1-xMnO3perovskites with different Sm content are prepared via sol-gel method (x=0.3,0.5,0.8, denoted as SSM37, SSM55and SSM82). Their electrochemical performances are compared to the commercial LSM82cathode by studying their surface element valence distribution and surface oxygen exchange kinetics. It can be concluded that the electrochemical performances of SmxSr1-xMnO3are superior to the commercial LSM82, and Sm0.5Sr0.5MnO3is proved to be the optimal composition. To further investigate the feasibilities of Sm0.5Sr0.5MnO3as cathode material for IT-SOFCs, its physical properties, such as, electrical conductivity, thermal expansion coefficient, sintering properties and thermal stability are examined. The sintering temperature effect on the morphorlogy and electrochemical performance is studied. The ORR mechanism for SSM55cathode is proposed by evaluating the dependance of the electrochemical performance on oxygen partial pressures. Then SSM55is mixed mechanical with YSZ to fabricate SSM55-YSZ composite cathode to further improve the catalytic activity towards ORR for the SSM55cathode. The research on the ORR mechanism for the SSM55-YSZ composite cathode is conducted. Solution infiltration method is also applied to fabricate nano-structured SSM55+YSZ composite cathode. The catalytic activity towards the ORR for SSM55+YSZ composite cathode with various SSM55loadings is evaluated. The influence of the cathodic current treatment on the nano-structured SSM55+YSZ composite cathode is provided. The conclusions are summarized as below:(1) SSM perovskites exhibit better electrocatalytic activity towards ORR compared to LSM82due to their faster oxygen surface exchange rate. The SSM55cathode has the best electrocatalytic activity towards ORR for its high surface Mn4+/Mn3+and Oad/Oiattice concentration ratios. High Mn4+/Mn3+ratio is related with fast charge transfer process in the high-frequency range and high Oad/Olattice ratio is beneficial for the low-frequency oxygen adsorption and dissociation processes involved in the ORR.(2) SSM55is a promising cathode materials within650～800℃, with high electrical conductivity of138S cm-1at800℃and a matching thermal expansion coefficient (10.7×10-6K-1) with YSZ and other components of SOFCs. The SSM55cathode sintered at1020℃exhibits the lowest polarization resistance, which is1.57Ω· cm2at800℃. It is also indicated that the oxygen transfer process could be responsible for the ORR rate-determining step.(3) The SSM55-YSZ composite cathode was fabricated by mechanical mixing of the SSM55and YSZ powders. The electrochemical performances of the composite cathode are enhanced due to their improved oxygen ion transfer capability. The optimal ratio between SSM55and YSZ phase is70:30, with area specific resistance of0.38Ω·cm2at800℃. With increasing temperature, the rate-determing step for the SSM55-YSZ composite cathode transforms from charge transfer process to the combination process of absorbed oxygen with electron.(4) Solution infiltration is proved to be an effective method to fabricate nano-structured SSM55+YSZ composite cathodes. The porous YSZ ionic conducting substrate is well sintered with uniform electrical conducting phase distribution. The optimal SSM55loading is18wt.%, with polarization resistance of0.17Ω· cm2at800℃, its polarization resistance decreased by38.7%after passing600mA·cm-2cathodic current for12h. The peak power density for Ni-YSZ anode supported single cell with18-SSM55+YSZ cathode is341mW·cm2,492mW·cm-2,645mW·cm2and763mW·cm-2at650℃,700℃,750℃and800℃, respectively.