| The key factor to achieve the commercialization development of solid oxide fuel cells (SOFCs) is to decrease the operating temperarute of SOFCs down to intermediate-low range. To solve the ohmic loss caused by decreasing operating temperature, it is urgent to find SOFCs electrolytes with high ionic conductivity and reducing electrolyte thickness. Ionic-conducting materials such as doped CeO2, LaGaO3and stabilized Z1O2, and proton-conducting materials such as doped BaCeO3and BaZrO3are well developed SOFCs electrolyte materials. The thesis is focused on those electrolyte materials and discussed the electrolyte structure, fim fabrication techniques and electrochemical performance of SOFCs.Chapter1mainly described the research background, development status and working principles of SOFCs, including ionic-conducting and proton-conducting SOFCs, summarized SOFCs electrolyte materials and their problems, introducesd the concept of bilayer electrolytes and discussed the possibilities of conbine the structure with SOFCs electrolytes. Besides, common fabrication techniques of electrolyte membrane were included.In Chapter2, pulse laser deposition (PLD) technique was succesfully introduced in fabricating dense8mo1%Y2O3stabilized ZrO2(YSZ) electronic-blocking layer and Ceo.8Smo.202-5(SDC)buffer layer. As the deposition temperature is low, thus inerfacial reaction between the doped ceria and stabilized zirconia can avoided. After incorporating YSZ film as electrolyte layer, the open circuit voltage (OCV) of fuel cell improved significantly indicating the YSZ film deposited by PLD technique is dense enough to block the eletronic conduction caused by Ce4+reduction. With SDC buffer layer deposited on the bilayer electrolytes, thermal mismatch and chemical reaction between Smo.5Sro.5CoO3-δ (SSC) cathode and YSZ layer can be avoided, also the polarization loss was reduced and cell performance was improved. The results indicate that as an excellent film fabrication technique, PLD can be used in the researchment of SOFCs electrolyte films.Chapter3discussed the influence of substrate morphology on the quality of PLD deposited fims. The research results show that when electrolytes were directly deposited on anode substrate, the films could not cover anode pores due to the smaller size of film thickness than pores, and the interface between electrolyte and anode appeared cracks after reducing in H2, thus influenced the whole cell structure and electrochemical performance. With functional layer optimized anode pores, deposited YSZ/SDC bilayer electrolyte films were uniform without any cracks. The corresponding fuel cell has an excellent electrical performance, the maximum power density of1.19W cm-2and open circuit voltage (OCV) of0.959V was achieved at750℃which shows an improvement in SDC single electrolyte cell, indicating anode optimization has a great asistant on cell performance and structure stability.In Chapter4, the effect of deposition parameters on the quality of Lao.9Sro.1Gao.gMgo.203-δ (LSGM) thin film in PLD process were reserached. As LSGM has high ion conductivity and was incorporated as electronic-blocking layer onto SDC electrolyte. The oxygen pressure and post-annealing temperatures were investigated their influence on surface morphology, element component, roughness and phase structure. In terms of the quality, deposition rate and composition of the LSGM film deposited by PLD technique, the proper oxygen pressure seems to be0.67Pa. In order to improve the crystallinity of the deposited LSGM films, the suitable method is the one-step process which means keeping the cathode firing and the post-annealing procedure in the same process. The experiment results indicate that SDC/LSGM bilayer electrolyte structure has good mechanical stability, which not only blocks the electronic conduction, bue also prohibits the reaction between LSGM and Ni-base anode.In Chapter5, BaZro.8Yo.203-δ (BZY) films were incorporated onto BaCe0.8Yo.203-δ (BCY) electrolytes with PLD technique and the effect of BZY films thicknesses on chemical stability and electrochemical performance was investigated. Only when BZY film achieves to a certain thickness, BZY layer can protect BCY in CO2atmosphere. With increasing thickness of BZY layer, the ohmic resistance increases, besides the poor matching between BZY layer and cathode caused bad polarization loss, thus the corresponding fuel cell has a lower electrochemical performance. The research results indicate that BCY/BZY bilayer electrolyte fuel cell, with cetain thickness of BZY layer, has good chemical stability and considerable electrochemical performance.In Chapter6, based on the better conductivity of BaZr0.7Pr0.1Y0.2O3-δ (BZPY) than BZY, BZPY thin film was fabricated on BCY electrolyte with PLD technique. BCY/BZPY bilayer electrolyte structure shows an excellent chemical stability. Besides, the membrane conductivity and fuel cell performance improves a lot in BCY/BZPY bilayer electrolyte fuel cell, the maximum power density at700℃is250 mW cm-2, the membrane conductivity at700℃is6.02×10"3S cm-1, which are both comparable with tranditional rare earth doped BaCeO3electrolyte cell. The results indicate that BCY/BZPY bilayer electrolyte structure has good chemical stability and electrochemical performance and has good future as proton-condictiong SOFCs electrolytes.In Chaper7, the works presented in the thesis are concluded and future research on bilayer electrolyte SOFCs is proposed... |
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