Performance And Optimization Of La_1.4Ca_0.6CoMnO_5+? Double Perovskite Cathode Material For Intermediate-temperature Solid Oxide Fuel Cells

Fuel cells are a type of conversion devices which can convert chemical energy directly into electrical energy with high efficiency and low polution.Solid oxide fuel cell?SOFC?is the optimal choice to combine environmental friendliness and fuel flexibility among the various types of fuel cells.Traditional SOFCs are operated at high temperatures up to approximately 1000 °C.Such a high operation temperature causes many practical problems,such as interface reactions between components,high cost,limitation of materials,thermal expansion mismatch,degradation of the components and short lifetime.Reducing the operating temperature can significantly alleviate these problems.However,the reduction of operation temperature results in significant increases in the ohm polarization of the electrolyte and activition polarization losses of electrodes?especially cathode?,which seriously affects the performance of the cells.For example,the interface resistance of the conventional SOFC cathode La_0.8Sr_0.2MnO_3-??LSM?was 0.39 ?cm² at 900 °C,while it reached 55.7 ?cm² when the temperature was reduced to 700 °C.Thus,lots of researches have been done on the development of novel cathode materials with high performance of oxygen reduction within the intermediate-temperatures?600?800 °C?for IT-SOFCs.In this paper,based on the La₂CoMnO₆ parent oxide,we partly replace La³⁺by Ca²⁺ to obtain the La_1.4Ca_0.6CoMnO_5+??LCCM?and to produce oxygen vacancy,which is correlated with the oxygen adsorption and conduction properties,so as to promote its electrochemical performance as cathode material of IT-SOFCs.Subsequently,we optimized the electrochemical and thermal properties of LCCM through these two ways,introducing Sm_0.2Ce_0.8O_1.9?SDC?electrolyte or double perovskite cathode SmBaCo₂O_3-??SBC?to form composite cathode materials,in order to improve the cathode performance for IT-SOFCs.We synthesized the A-site Ca-doped double perovskite LCCM by sol-gel method.The phase composition of the samples separately sintered at 1250,1300 and 1350 °C for 10 h in air is the same.They can be well indexed based on a monoclinic perovskite structure with space group P1 21/n 1.We take samples sintered at 1300 °C for the subsequently thermal and electrochemical analysis because of the best crystallization and electrical conductivity?We use LCCM to represent the sample sintered at 1300 °C unless otherwise specified?.The in situ HTXRD confirmed that the LCCM possesses highly structural stability within a wide temperature range from RT to 900 °C without any apparent phase transition.The oxygen content of LCCM is 5.71 at RT,so the concentration of oxygen vacancy is very low.The LCCM sample shows good chemical compatibility with the common electrolytes,such as La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85?LSGM?and Sm_0.2Ce_0.8O_1.9?SDC?,after calcining at 1000 °C for 10 h.XPS study indicates that mixed valence states of Co^2+/3+ and Mn^3+/4+ coexist in the LCCM material.The average thermal expansion coefficient?TEC?of LCCM is 15.13×10^-6 K^-1 within 30?1000 °C temperature range,which is significantly lower than those of other cobalt-containing cathodes,such as LnBaCo₂O_5+?,showing better thermal expansion compatibility with the most common electrolytes.In the temperature range of 300?850 °C,these LCCM samples sintered at three different temperatures all exhibit a typical p-type semiconducting behavior due to the small polaron hopping.The sample sintered at 1300 °C exhibits the highest conductivity of 228 Scm^-1 at 850 °C.The polarization resistance of LCCM cathode on LSGM electrolyte is still very large in the intermediate-temperature range,which is 0.9345 ?cm² at 850 °C.However,it is much smaller than that of conventional SOFC cathode material LSM.Both the equivalent circuit analysis and test results indicate that the low frequency parts related to oxygen adsorption and diffusion are the main limiting factors within the temperature range studied.The maximum power densities of NiO-SDC/SDC/LSGM/LCCM single-cell with H2 as fuel,air and high-purity oxygen as oxidant are 460 and 513 mWcm–2 at 850 °C,respectively.After long term stability test at 700 °C for 20 h,the performance of single cells with LCCM cathode in air declines 10.76%.As mentioned above,the electrochemical performances of LCCM as cathode of IT-SOFC are not so good due to the limited oxygen vacancy concentration.The oxygen content of LCCM is 5.71 at RT.It's still relatively large that will lead to insufficient oxygen vacancy concentration for oxygen reduction.In view of the good chemical compatibility of the LCCM cathode with SDC electrolyte,we introduce SDC with high oxygen ionic conductivity into LCCM cathode to form LCCM-SDC composite cathodes and optimize the electrochemical properties of the LCCM.The average TEC of the LCCM-SDC40 composite cathode is decreased to 14.0×10-6 K-1 within 30?1000 °C temperature range.At 850 °C,the ASR of the optimum composition LCCM-SDC30 composite cathode is decreased to 0.2256 ?cm²,which is only 24% of LCCM cathode;the maximum power densities are 611.3 and 611.3 mWcm^-2 at 750 °C in air and oxygen atmospheres,respectively,which are increased by 25% and 21%,respectively.The single-cell with LCCM-SDC30 composite material shows relatively high electrochemical stability within a 20 h test period,which there is only a minor decline.These results show that the introduction of SDC into LCCM not only can improve the oxygen adsorption and conduction performance,but also can reduce the TEC of LCCM,thus enhancing the electrochemical properties and durability of the cells.To further optimize the performance of the LCCM as cathode of IT-SOFCs,we introduced SBC double perovskite cathode into LCCM,to form LCCM-SBC double cathode materials.The SBC material with high ion and electron conductivity was also synthesized by the sol-gel method and followed by sintering at 1050 °C for 12 h.The XRD analysis indicats that the LCCM has a good chemical compatibility with SBC after calcining at 1000 °C for 10 h.The TECs of composite double cathode increase with the increasing SBC within 30?850 °C temperature range.The average TEC of LCCM-SBC10 reveals the lowest value of 14.8 ×10^-6K^-1.The polarization resistance of the best composite proportion LCCM-SBC40 decreased to 0.073 ?cm² at 700 °C,which was only 0.98% of LCCM cathode?the polarization resistance of LCCM was 7.472 ?cm²?.This value is significantly lower than the desirable target value of 0.15 ?cm² as IT-SOFCs cathode.The maximum power densities of single-cell with LCCM-SBC40 cathode were 706.2 and 742.1 mWcm^-2 at 850 °C in air and oxygen atmospheres,respectively,which increased 246.2 and 230.1 mWcm^-2,respectively,compared to LCCM cathode.Negligible degeneration of cell performance was observed for the LCCM-SBC40 cathode within a 20 h testing duration at 750 °C.These results indicate that the introduction of SBC into LCCM not only improved the ionic conductivity but also enhanced the oxygen adsorption and diffusion properties.The LCCM-SBC40 composite cathode exhibits better thermal expansion compatibility with LSGM,more excellent electrochemical performance and durability as compared to the LCCM and LCCM-SDC cathodes.Therefore,the LCCM-SBC40 composite double cathode is a more promising cathode for use in IT-SOFCs.