Study On The Solid Oxide Fuel Cell Materials And Ni-based Catalysts For Dry Reforming Of Methane

The present work studies the solid oxide fuel cell (SOFC) materials and Ni-based catalysts for dry reforming of methane (DRM). SOFC is an electrochemical device which converts chemical energy in fuel into electrical energy directly. It possesses many advantages such as low pollution, high efficiency, solid design and modularity. Recently, the researches focus on reducing the cost, enhancing the performance and stability of SOFC. DRM is a catalytic reaction which produces the important syngas (H2 and CO) by feeding two greenhouse gases (CH4 and CO2). It is important to increase the utilization of CH4, decrease environmental pollution, and benefits chemical industry. Many efforts have been made on the synthesis of stable catalysts with high activity and carbon resistance.The main contents of this paper include:(1) develop a low cost technology which is suitable for mass production with short production cycles to fabricate SOFC, and the key parameters are optimized. (2) optimize the anode materials by adding Al2O3 into anode support layer (ASL) which can tailor the anode sintering properties so as to improve the efficiency and stability of SOFC. (3) synthesize catalysts for DRM by wet impregnation method. Through the characterization and catalytic activity tests, the mechanism of carbon deposition on catalysts is analyzed, and a favored catalyst is obtained. The main findings are listed as follows:The main work in the second chapter is developing a technology to fabricate SOFC half cells. The YSZ electrolyte layer (EL) and NiO-YSZ anode functional layer (AFL) are prepared by wet powder spraying (WPS), while the NiO-YSZ anode support layer (ASL) is prepared by tape casting (TC). The parameters in the fabrication process are optimized. It is found that when preparing YSZ film by WPS, the temperature of the substrate, the spraying rate and the concentration of PVB in the paste affect the dispersion and initial packing density of YSZ powders in the green tape. We optimize the spraying parameters and the composition of the paste. Flat and dense YSZ films with uniform thickness are successfully obtained by an auto-WPS equipment. This method is also applied to prepare NiO-YSZ AFL. On the other hand, the effects of initial NiO powders and PVB contents in the ASL paste on the sintering properties of ASL prepared by TC are studied, and the preparation of ASL by this method is improved. On the basis of these results, we combine the WPS with TC and (?)sintering technique to produce half cell green tape with ASL/AFL/EL at (?)95%. The output power of a single cell with an active area of 4×4 cm2 is (?) and the open circuit voltage is over 1.00 V at 750 ℃ when using LSM-YSZ as cathode, H2 as the fuel and air as the oxidant.The main work in the third chapter mainly focuses on the effect of Al2O3 on the sintering properties of the anode. In this chapter we report a two-step sintering method to fabrice flat three-layer half cells. The first sintering step is a freestanding ng process at a low temperature (1280 ℃). The second sintering step is a constrained sintering process at 1400 ℃. The shrinkage of the ASL and the curvature of the half-cell can be adjusted by adding Al2O3 into the ASL in the first sintering step. Effects of Al2O3 addition on the NiO-YSZ anode material are also studied. We find that NiO reacts with Al2O3 to form NiAl2O4 spinel at the early sintering stage. This reaction transiently promotes the grain growth of NiO. Once the reaction terminates and the NiAl2O4 spinel is formed, the grain growth of NiO will be suppressed, even at higher sintering temperatures. Our results indicate that by a proper amount (approximately 0.2 wt%) of Al2O3 addition, smaller NiO grains can be obtained while the side effects of NiAl2O4 are negligible, which is favorable to increase the conductivity and stability of the ASL, and can enhance the performance of SOFC. The output power of the optimized single cell with an active area of 4×4 cm2 is over 6.0 W at the output voltage of 0.7 V and 750 ℃, and the open circuit voltage is 1.05 V when using LSM-YSZ as cathode, H2 as the fuel and air as the oxidant.The main work in the fourth chapter is the preparation and characterization of catalysts for DRM. We studied the carbon deposition mechanism and improved the carbon resistance of the catalysts. For the Ni-Al2O3 catalysts for DRM, the interaction between Ni and γ-Al2O3 is stronger than that between Ni and α-Al2O3. This is because NiO is easier to react with γ-Al2O3 and forms Nil2O4 spinel structure in the precursor. A strong interaction is benefit for the reaction between CO2 absorbed on the support and the carbon absorbed on the surface of Ni, which increases the CO2 conversion. However, the Ni particles still sinter and agglomerate during the DRM reaction and leads to growth of Ni particles, resulting in the deactivation of the catalyst and large amount of carbon deposition. Improving the carbon resistance of Ni-based catalysts for the DRM through the perovskite precursor is an attractive strategy. Ni-based perovskite precursors with the nominal compositions of La2NiO4 and LaNiO3, as well as their Fe partially substituted counterparts (La2Ni0.5Fe0.5O4 and LaNi0.5Fe0.5O3), were prepared by a wet impregnation method. Perovskite structures in the samples without Fe partial substitution are unstable and completely reduced daring the DRM test, forming catalysts composed of Ni as the active component and La2O3 as the support. The stability of the perovskite structure is significantly enhanced by the Fe partial substitution, and improved carbon resistance are observed in these catalysts, which is attributed to the smaller particle size and better dispersion of Ni resulted from the stronger metal-support interaction. The LaNixFe1-xO3 perovskite plays an important role in the structural stability of mixed perovskite catalysts in reducing atmosphere and the enhancement of metal-support interaction. Our results indicate that the LaNi0.5Fe0.5O3 precursor synthesized by wet impregnation method is feasible to obtain stable Ni-based catalysts with high carbon resistance for DRM. The CH4 and CO2 conversions are both over 60% when the mole ratio of feeding CH4:CO2 is 1:1 at a space velocity of 1.2×104 ml/gcat.h,750 ℃. The content of carbon deposition over this catalyst after 8 h DRM test is less than 0.03 gc/gcat..