| With the continuous growth of global population,the depletion of fossil fuels and the increasingly serious environmental pollution,human beings pay more and more attention to energy and environmental problems.Besides,the effective development and utilization of new energy,that can replace traditional fossil fuels and other non renewable resources,has become more and more important in recent days.The solid oxide fuel cell（SOFC）is a kind of green energy in the 21st century with the advantages of clean,high efficiency and low noise.It can directly convert the chemical energy of various fuels into electrical energy,which is a new power generation technology.In the development and practical application of new energy research field,SOFC occupies an important position and is the current research hotspot.The operating temperature of traditional SOFC is very high（800～1000℃）,which causes many problems and hinders the commercialization of SOFC.Therefore,it is imperative to reduce the operating temperature of SOFC.The low operating temperature of SOFC can be realized by exploring electrolyte materials with high ionic conductivity at low temperatures,reducing the thickness of electrolyte layer and developing new fuel cell configurations.In order to develop low-temperature SOFC,reduce the cell cost and simplify the cell preparation process,a new fuel cell configuration,“Electrolyte layer-free fuel cell（EFFC）”or“Semiconductor-ionic membrane fuel cell（SIMFC）”,is adopted.The method not only simplifies the fuel cell preparation process,reduces the production cost,but also reduces the polarization loss of the cell electrode/electrolyte interface and the cell impedance loss（including the electrolyte and the electrode）at low operating temperatures,thereby the cell obtaining good performance output at low temperatures.It provides a new idea for the development of low-temperature SOFCs.In chapter 1,the types,characteristics and applications of fuel cells are roughly introduced,and the working principles and key materials（anode,cathode and electrolyte）of SOFC are briefly introduced.In addition,the development status of SOFC at home and abroad,especially the research and development of low temperature SOFC,are introduced,and then the research direction of low-temperature SOFC in this paper is established.Combined with the introduction of the research progress of EFFC（or SIMFC）,the specific research direction of this paper is further determined:to develop low-temperature SOFC with excellent cell performances based on the new fuel cell configuration EFFC（or SIMFC）.In chapter 2,the experimental chemicals and instruments used in this paper,as well as the methods used to characterize powder materials and bulk cells（XRD,XPS and SEM）are summarized.Moreover,the fabrication process and electrochemical performance test methods of new configuration fuel cell EFFC are introduced in detail,and then a comprehensive understanding of EFFC can be established.In view of the fact that Ce O2-based materials have high ionic conductivity at medium and low temperatures,they are considered as potential electrolyte materials for the development of medium and low temperature SOFCs.In this paper,Sm doped ceria oxide（Ce0.8Sm0.2O2-δ,SDC）was used as the ionic material for the intermediate semiconductor-ionic composite electrolyte membrane of EFFC.In addition,ABO3perovskite oxides with good catalytic activity,mixed ionic-electronic transport properties,chemical and structural stability and other characteristics,were used as semiconductor materials.SDC and perovskite materials are uniformly combined as a semiconductor-ionic electrolyte membrane,and the assembled EFFC is systematically studied.The main work contents（Chapters 3,4 and 5）are as follows:1.The SDC synthesized by the co-precipitation method and the La0.7Sr0.3Cr0.5Fe0.5O3-δcomposite（LSCr F’）synthesized by the sol-gel method are uniformly compounded to prepare the LSCr F’-SDC composite material,and then used as an electrolyte membrane to assemble EFFC.By testing the EFFC performances at different temperatures,we can find that the cell exhibits a good low-temperature operation feasibility,and the power output of 553～1059 m W cm-2 were obtained in the low temperature range from 470℃to 550℃.The effect of the weight ratio of SDC and LSCr F’in the electrolyte membrane on EFFC performances was also investigated,and found that when SDC:LSCr F’=6:4,a balance of ionic conduction and electronic conduction was reached,and the corresponding EFFC obtained the best performance.Compared with the traditional SOFC employed SDC as the electrolyte membrane,the EFFC using the LSCr F’-SDC composite electrolyte membrane obtains higher open circuit voltages and better cell performances.Combined with the impedance spectroscopy test,it is found that the semiconductor-ionic composite material formed by incorporating LSCr F’into SDC can reduce the electrode polarization,enhance the conductivity,and expand the triple-phase boundaries area,thereby accelerating the cell electrochemical reaction process and improving cell performances.2.The semiconductor La0.65Sr0.3Ce0.05Cr0.5Fe0.5O3-δ（CLSCr F）prepared by the sol-gel method is uniformly composited with the ionic conductor SDC,and the CLSCr F-SDC semiconductor-ion composite membrane with high ionic conductivity at low temperatures is obtained.By systematically studying the crystal structure,chemical compatibility,micro-morphology of CLSCr F and SDC materials,and the electrochemical performances of EFFCs which based on different weight ratios CLSCr F-SDC membranes,it is found that introducing an appropriate amount of semiconductor CLSCr F into the SDC ionic membrane can improve the electrolyte membrane ionic conductivity,reduce the cell polarization resistance and accelerate the electrode reaction in cathode zone.Among these CLSCr F-SDC cells,the cell based on3CLSCr F-7SDC membrane obtained the best performance,its maximum power density at 550℃is 837 m W cm-2,and the ionic conductivity is 0.15 S cm-1,which is nearly 2 times higher than that of the simplex ionic membrane SDC.Through the I-V characteristic test,we can find that the CLSCr F-SDC cell has formed a physical junction effect,which can prevent electrons from passing through internally and avoid the short circuit problem effectively.Moreover,a short-term stability test was performed and revealed that the developed CLSCr F-SDC cell is capable of stably operating for 18 h at 550℃under a fixed current density of 234 m A cm-2.3.The perovskite material Sr Fe O3-δ（SFO）synthesized by the solid state routine is composited with SDC,and then the obtained SFO-SDC is employed as the electrolyte membrane of EFFC.The EFFC based on two Ni0.8Co0.15Al0.05Li O2-δ（NCAL）electrodes and SFO-SDC electrolyte membranes through adjusting the weight ratio between semiconductor and ionic conductor in the membrane,concluded that the cell based on 3SFO-7SDC membrane obtaining the best output performance at 550℃,i.e.,open circuit voltage is 1.08 V and the maximum power density is 780m W cm-2.The EFFC performances in contrasted with the cell based on two NCAL electrodes and SDC membrane are much higher.Through the impedance spectroscopy analysis,it is found that replacing the SDC membrane with SFO-SDC can expand the triple-phase boundary and provide more active sites to accelerate the fuel cell reaction,thus enhancing cell performances.In addition,by using SFO as the cathode material to replace the original NCAL cathode,the cell performance can be further improved,and a higher power output of 907 m W cm-2 can be obtained.Through the analysis of the microscopic morphology of the cell cross section,it is found that the SFO-SDC membrane has good thermal compatibility with two electrode materials NCAL and SFO,which provides a guarantee for EFFCs obtaining good output performances.In summary,this paper employs a new fuel cell configuration EFFC（or SIMFC）to greatly eliminate the limitation of the traditional SOFC ionic electrolyte/electrode interfaces,promote the transfer of electrons and ions in the electrolyte membrane/electrode connection area,and then reduce the cell ohmic losses.And by incorporating the perovskite material into the ion electrolyte membrane,the electrode reaction is accelerated,the cell polarization loss is reduced,the ionic conductivity of the electrolyte membrane is improved,and then the purpose of improving the SOFC electrochemical performances at low temperatures is achieved.It provides a new idea for exploring high-performance low-temperature SOFCs,which is expected to be applied in the industrial production of SOFC. |
