Structural Design And Electrochemical Study Of Ceria Based Solid Oxide Fuel Cells

Solid oxide fuel cells(SOFCs),which can directly convert the chemical energy of fuel into electrical energy,are a high efficiency,environmentally friendly power generation device.After decades of development,reducing operating temperature to moderate and low temperatures is one of the most important topics in the field of SOFCs.Doped ceria materials possess high ionic conductivity at reduced temperatures.Therefore,doped ceria(DCO)-based SOFCs are considered to be very promising low/intermediate-temperature SOFCs.In this thesis,all the works are focused on developing new materials and architectures according to the problems of DCO-based SOFCs,and the electrochemical performance and the mechanism of experiment were measured and investigated.In Chapter 1,the research background and working priciple of SOFCs were reviewed.In addition,electrolytes as the key materials for SOFCs were intensively discussed.In Chapter 2,an in situ reaction method was proposed to prepare BaCe1-xSmxO3-?decorated Ce0.8Sm0.2O1.9 composite electrolyte.A series of BaCe1-xSmxO3-? decorated Ce0.8Sm0.2O1.9 composite electrolytes were synthesized via the reaction of BaO and SDC during firing process.The synthesized composite electrolytes were characterized,and the electrochemical performance of different composite electrolyte-based fuel cells were evaluated.The experimental results indicating that BaCe1-xSmxO3-? was generated in-situ and coated on the surface of Ce0.8Sm0.2O1.9 particles,forming BaCe1-xSmxO3-? decorated Ce0.8Sm0.2O1.9 composite electrolyte.The open circuit voltage,ohmic resistance,and polarization resistance of the single cell increase with increasing BaCe1.xSmxO3-? content in the composite electrolyte.The composite electrolyte with small amount of BaCe1-xSmxO3-? is beneficial to enhance power performance of a single cell compared with conventional SDC-based SOFCs,which is also superior to the reported similar composite electrolytes.The results show that the in-situ solid state reaction may be a potential strategy to design composite electrolyte.In Chapter 3,a single cell with the structure of NiO-SDC anode support,NiO-BZCY anode functional layer,SDC electrolyte and SSC-SDC cathode layer were developed.The catalytic performance of NiO-SDC and NiO-BZCY as anode support for SDC-based fuel cells was compared.Besides,the relationship between the Ba-containing layer thickness,electron-blocking layer thickness,and the cell performance,including open circuit voltage and power performance were investigated.It was found that the electrochemical performance of the NiO-SDC anode-supported fuel cell was significantly better than that of NiO-BZCY anode-supported fuel cell.In addition,the thickness of the electron-blocking layer increases as the thickness of the Ba-containing anode functional layer increases.When the thickness of the anode functional layer is greater than or equal to 50 ?m,the electron-blocking layer can protect SDC electrolyte and effectively eliminate the internal short circuit behavior.The fuel cell with a 50 ?m thick anode functional layer also outputed the highest electrochemical performance.At 650?,the maximum power density and open circuit voltage of the fuel cell were achieved as high as 1068 mWcm-2 and 1.016 V.It should be noted that the fuel cell also showed excellent performance at low temperature,for example,at 450?,the cell still had a maximum power density of 254 mWcm-2.No matter under working condition at the voltage of 0.7 V or open circuit,the cell operated at 600 0C about 50 h and 80 h,respectively,displays good stability and almost no performance decay can be observed.The results demonstrated that this kind of fuel cell with optimized structures is a good alternative for low-temperature SOFCs.In Chapter 4,utilizing Sr elements diffused from anode to react with SDC electrolyte forming electron-blocking layer in situ and consequently improving the open circuit voltages were proposed and verified for the first time.Raman spectroscopy and high resolution transmission electron microscopy(HRTEM)results indicating that the diffused Sr elements reacted with SDC forming SrCe1-x(Sm,Yb)xO3-? perovskite phase with an SDC@SrCe1-x(Sm,Yb)xO3-? core/shell structure.The thickness of the in situ generated electron-blocking layer is highly depended on the sintering temperature of the half cell,and increases with increasing the sintering temperature.In addition,Sr elements diffusing into the SDC electrolyte can effecticely reduce the sintering temperature of the half cell.For example,dense SDC electrolytes can be obtained when the sintering temperature is as low as 1100?,which remarkable lower than that of the typical NiO-SDC anode-supported SDC-based fuel cell,decreased by nearly 300? without using additional sintering aids.The results show that applying Sr-containing anode materials is a simple and effective method to reduce the sintering temperature and improve the open circuit voltage of SDC-based fuel cells.In Chapter 5,a novel SDC-based SOFCs with low sintering temperature and free from internal short circuit was designed and investigated.The diffusion behavior of Ba and Sr elements in the SDC electrolyte membrane was researched by designing experiments.A series of Ba and Sr containing anode(NiO-Ba1-xSrxCe0.7Zr0.1Y0.2O3-?,0?x?0.3)were prepared by controling the proportion of Ba and Sr in A site.The presence of Ba ensures the formation of BaCeO3-based electron-blocking layer,which has very high ion transport number and completely eliminates the internal short-circuit current across SDC electrolyte consequently improved the open circuit voltage,while Sr incorporation can substantially promote the sintering activity of the anode and electrolyte,and hence reduces the sintering temperature of the half cells.The results indicate that Sr elements diffused more easily into the SDC electrolyte than Ba elements.The electrochemical performance of the SDC-based cells varies significantly with the anode composition.Ni-Ba0.9Sr0.1Ce0.7Zr0.1Y0.2O3-? is demonstrated to be the optimal anode composition showing high open circuit voltages(1.038 V at 650?)and peak power densities(677 mW cm-2 at 650?).The excellent electrochemical performance suggests that the Ba-and Sr-containing composites are promising electrodes for DCO-based SOFCs and SOECs with high working efficiency.In Chapter 6,a summary of the thesis and some recommendations and future work concerning for the development of doped ceria-based solid oxide fuel cells are presented.