Configuration Design And Electrode Stability For Single Chamber Solid Oxide Fuel Cell

Single chamber solid oxide fuel cell （SC-SOFC） is one special type for Solid oxidefuel cells （SOFC）, which consists of one chamber and the anode and the cathode areexposed to unique fuel–oxidant mixtures. This configuration characterized by aneliminating sealing, has several advantages, such as increased thermomechanical stability,more compact and simplified designs and so on. Unfortunately, there are still somechallenges for SC-SOFCs. For example, an important type of SC-SOFCs is the coplanarcell, in which the electrodes are located on the same side of an electrolyte. However, thecell performance is significantly restrained due to its large ohmic resistance. Ni/YSZcermet is the most commonly used anode material in SC-SOFCs. While, cyclic oxidationand reduction processes will occur on the Ni metal surface when the anode was exposedto the fuel–oxidant mixtures, leading to the oscillation behaviors of SC-SOFC. Carbondeposition could also occur when the content of hydrocarbon is high, leading to cellperformance degradation. They are three key factors for SC-SOFC. Therefore，thisdissertation provides a detail stdudy for cell design and electorde stability to solve thoseproblems. In addition, we proposed a single phase BaPr_1-xIn_xO_3-δ（x=0.1，0.2，0.3，BPI）cathode and study the application of protonic SOFC based on BaPr0.8In0.2O3-δ（BPI2）cathode and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ（BZCYYb） electrolyte to SC-SOFC.The coplanar design has several advatages such as new cell designs and easier stackassembly and so on. However, the ohmic resistance of the electrolyte plays a dominantrole in limiting the cell performance. In general, some researchers have focused onreducing the gap between the electrodes, reducing the width of electrodes and increasingthe thickness of electrolyte to reduce the ohmic resistance of coplanar design. However,they will bring some new problems. In this dissertation, we provide a groovesconfiguration and right-angular configuration, which provide an effective way todecrease the ohmic resistance of the coplanar SC-SOFC by changing the electrolytestructure and electrodes arrangement, respectively. The obtained results indicate that thegrooves configuration and right-angular configuration configuration obviously reduce theconductive distance of oxygen ion transport and form a gas separator between theelectrodes, leading to a remarkable reduction in ohmic resistance, an elevation ofopen-circuit voltage, and, ultimately, an improved performance.The in situ measurements of the local anode resistances （R_s） associated with thevoltages （V）, the actual temperatures of the cell （T_c）, electrochemical impedance spectraand discharge curves give a direct evidence that and all oscillations are attributed to thecyclic oxidation and reduction processes occurring on the Ni metal surface and furtherreveal the regime of the Ni/NiO redox. By varying the operating conditions, therelationships between the redox and the operating parameters, methane-to-oxygen ratios （M）, current density （J） and anode thickness, are investigated. The obtained resultsindicate that M and anode thickness are key parameters related to redox of Ni inSC-SOFC. A low M value and a very thin anode could lead to unstable cell operationwith intense oscillations. Under these conditions, a latent oxidation of Nianodesaccompanying the redox process may be a main failure mechanism for the SC-SOFC.SEM micrographs further show that the repeated redox of Ni/YSZ lead to irreversiblegrain refinement and redistribution of Ni particles, and microstructure damage, which arethe main reason for irreversible degradation of SC-SOFCBaCO₃modfied NiO/YSZ （BaCO₃-NiO-YSZ） greatly enhance sulfur and cokingtolerance of Ni/YSZ, which has been proved to be a simple and cost-effective anodecatalyst for SOFC. X-ray diffraction spectra （XRD）, Post-micro-Raman scattering（Raman）, Thermogravimetric analysis （TG） and Scanning electron microscope （SEM）are used to study in details the phase/composition of the “reaction production” of theBaCO₃-modified Ni-YSZ anodes during fabrication and testing of SOFC. We reveal theformed reason of “selected microstructure”, which BaPr_1-xIn_xO_3-δwas coated on thesurface of YSZ and nano-islands of BaO was coated on the Ni surface. Furthermore, thecoking tolerance of BZY-Ni-YSZ anode under CH₄/O₂mixture and the correspondingperforamance of the cell based on BZY-Ni-YSZ anode in single chamber conditions areevaluated and studied. The results indicate that BZY-Ni-YSZ is a promising simple andcost-effective anode for SC-SOFC to enhance its coking tolerance.Finally, a mixed ionic and electronic conductor, BaPr0.8In0.2O3δ（BPI2）, wassynthesized and examined as a cathode material for proton-conducting solid oxide fuelcells （H-SOFCs）. The conductivity and chemical stability of BPI2are systematicallyevaluated and the obtained resultas indicate that the relatively smaller ionic radius andlarger electro-negativity of In greatly enhance the stability of the BaPrO3structure. BPI2shows satisfactory tolerance to CO₂and H₂O and good chemical compatibility withBaZr0.1Ce0.7Y0.1Yb0.1O3δ（BZCYYb） electrolyte. However, the cell based on BPI2cathode and BZCYYb electrolyte shows poor performance and stability under singlechamber condition, which is mainly due to due to the full reduction of Pr （IV）. Therelative mechanism related to the application of protonic electrolyte to SC-SOFC isanalysed and discussed.