Design Of Bi-layer Electrolyte And Related Electrode Materials For Low Temperature Solid Oxide Fuel Cells

As a clean and efficient energy conversion device,solid oxide fuel cell(SOFC)occupies an important position in the energy field.However,the high operating temperature of traditional SOFC limits its commercial application.The main approach to achieve the low temperature of SOFC include two aspects:On the one hand,develop electrolyte materials with high ionic conductivity or thin electrolyte to reduce ohmic resistance.On the other hand,since the reduction in operating temperature brings significant polarization resistance problems,the development of high-performance low-temperature cathode materials is also crucial.As the material with the highest oxygen ion conductivity in the currently developed electrolyte system,doped bismuth oxide(DBO)has attracted much attention of the researchers,but DBO materials are easily decomposed into metallic bismuth in the reducing atmosphere.In addition,the melting point of DBO is as low as 824?,which makes the preparation of electrolyte membrane difficult.The current application of DBO mainly focuse on the research as the electrolyte layer on the cathode side of the bi-layer electrolyte.Proton conducting materials have low activation energy and are widely used electrolyte materials in recent years.At present,the high-performance cathode for proton based SOFCs is limited,and the composite cathode of traditional perovskite cathode material and electrolyte is the most widely used.The research of this paper includes(1)the high conductivity DBO materials are applied as electrolyte additive to improve the sinterability and electrochemical properties of ceria-based LT-SOFCs;Structural design,process optimization and electrochemical performance studies on YSZ | DBO double-layer electrolyte system.(2)Develop high-performance low-temperature cathode materials to improve the electrochemical performance of SOFC.In Chapter 1,the relevant background and development overview of the SOFCs are briefly introduced,than we expounds the latest development of the electrolyte system and the cathode system,focuses on the application of high-performance Bi-based electrolyte materials and the technology of bi-layer electrolyte.Chapter 2 investigates the effect of ESB as an additive on the electrical propertie s and sintering activity of ESB-SNDC composite electrolytes.The structure of NiCuO-SNDCIESB-SNDCIESB-LSM was innovatively prepared and the electrochemical performance was tested.The results showed that adding a small amount of ESB can significantly improve the sintering performance of the electrolyte,and the conductivity of the composite electrolyte also showed that the ESB-SNDC composite electrolyte can be used as the electrolyte choice of LT-SOFC.The electrochemical performance was tested using humidified hydrogen as the fuel gas,the result showed that the addition of 1%ESB can increase the output power of the cell at 600? from 417 mW cm-2 to 510 mW cm-2.However,as the amount of ESB added increased,the problem that ESB-SNDC cannot tolerate reducing gas becomes more prominent,and the output performance decreased instead.By increasing the thickness of the electrolyte,the chemical stability of the electrolyte structure can be improved.When the thickness of the 5%ESB-SNDC electrolyte was increased from 13 ?m to 20 ?m,the maximum power density(MPD)at 450? was increased from 41 mW cm-2 to 129 mW cm-2.Compared with SNDC electrolyte,the cell performance of 5%ESB-SNDC(20?m)has been significantly improved.In Chapter 3,YSB and DWSB powders were successfully prepared by solid-state method,and the 5-layer cell structure NiO-YSZ|NiO-YSZ functional layer |YSZ |DBO|DBO-LSM was prepared by layer-by-layer coating-sintering process.The performance of the YSZ|DBO bi-layer electrolyte cell was systematically studied and the electrochemical behavior and rate-limiting steps of the cell at different temperatures were analyzed.Under the condition of hydrogen as fuel gas,the use of YSB and DWSB double-layer electrolyte layers can improve the output power of the original YSZ single-layer electrolyte in a higher temperature range,but at operating temperatures below 600?,the YSZ|YSB double-layer The electrolyte performance was slightly worse than the YSZ monolayer electrolyte.When the higher ionic conductivity material DWSB is used,the temperature dependence of the bi-layer electrolyte performance can be alleviated.The YSZ|DWSB cell exhibited higher performance in the test temperature range of 500-700? compared to YSZ-based single cell.Although the long-term stability of the YSZ|DWSB cell was not satisfactory,the output performance of the YSZ|DWSB cell(1491mW cm-2 at 700 0C)showed that the YSZ|DBO structure had great advantages as an LT-SOFC electrolyte system.In Chapter 4,microwave sintering technology was used to prepare the ESB electrolyte layer of YSZ|ESB-based SOFC.The effect of microwave sintering temperature on the performance of ESB and the output of YSZ|ESB single cell were studied.The structure of YSZ|ESB bi-layer electrolyte was optimized as well.The results showed that microwave sintering can achieve fast and dense sintering of the electrolyte layer at low temperature,and the obtained ESB monolith had higher conductivity performance.The YSZ|ESB cell obtained by microwave sintering at 700? for 3 hours showed the best microscopic morphology and performance output.The MPD at 700? reached 1449 mW cm-2 for YSZ|ESB-700 single cell and which showed good stability of 140h at 600? 0.7V tesed.YSZ|ESB was an excellent LT-SOFC electrolyte structure for practical application.In Chapter 5,this work explored the application of Ruddlesden-Popper(R-P type)layered materials LaxSr2-xMnO4 as a proton conductor-based SOFC(H-SOFC)cathode material and compares the performance of Ruddlesden-Popper type(R-P type)layered materials and traditional perovskite LSM cathodes.The effect of radio of La/Sr on the electrochemical performance was also analyzed.The R-P type material exhibited a more excellent cathode performance compared with the traditional perovskite LSM.When single-phase La0.5Sr1.5MnO4 applied to the BaZr0.1Ce0.7Y0.2O3-? electrolyte structure,the cell achieved a maximum power density of 743.6 mW cm-2 and a polarization resistance of 0.089 ? cm2 at 700 ? with hydrogen as fuel gas.Even at a lower temperature of 500?,the MPD of the La0.5Sr1.5MnO4 cell remained at 173.1 mW cm-2.This shows that R-P cathode material is an effective high-performance low-temperature cathode material choice.In Chapter 6,a new cathode Lai.2Sr0.8Ni1-xFexO4 was developed for H-SOFC.The optimized La1.2Sr0.8Ni0.6Fe0.4O4 cathode had good electrical conductivity and good thermal expansion performance.when used as a cobalt-free single-phase cathode in cell structure NiO-BZCY| NiO-BZCY functional layer|BZCY|La1.2Sr0.8Ni0.6Fe0.4O4,the performance of the cell at 700? was shown as a maximum power density of 922 mW cm-2 and a polarization resistance of 0.043 ? cm2.When test the long-term stability of the cell under 0.7V at 600?,the cell can maintain 310 mW cm-2 without decay within 100 hours.This showed that R-P type La1.2Sr0.8Ni0.6Fe0.4O4 is an excellent cathode material for low-temperature H-SOFC.Chapter 7 summarizes and prospects this paper.