>> Nickel based oxygen transfer materials supported on two different YSZs were tested to evaluate their performance in methane chemical-looping reforming. The oxygen transfer materials of YSZs were selected with different amount of the doped yittrium in the ZrO2 structure. The yittrium of 8 mol% stabilized the zirconia oxide to a cubic structure compare to the 3 mol% doping, which is known to be a good for oxygen transfer. Various nickel amounts (16wt.%, 32wt.%, 48wt.%) were loaded on the selected supports. The nickel amount of 32% shows the optimized catalyst structure with good physical properties and reducibility from the XRD, BET and H2-TPR analysis, especially when the support of 8YSZ was used. From the methane chemical-looping reforming, hydrogen was produced by methane decomposition catalyzed by Ni on both YSZs. Comparing two YSZ supports of 3YSZ and 8YSZ during the cycling tests, the catalyst with 8YSZ (Ni 32%) exhibits not only the higher methane conversion and hydrogen production but also a faster reaction rate reaching to the stable point.
Solid oxide fuel cells (SOFCs) are an attractive power generation due to their high energy efficiency and possibility of waste heat recovery with operating at high temperature. SOFCs were fabricated by commonly three steps with pre-sintering of anode, dip-coating electrolyte and screen printing cathode materials cause of different shrinkages. It is hard to commercialize with these high manufacturing costs of multi-step process. SOFCs have been focused on low-cost materials and process for commercializing with high efficiency and performance. 8 mol % yttria-stabilized zirconia (8 YSZ) has been considered as one of the most conventional material for SOFC electrolyte. The 8 YSZ electrolytes doped with Cu contents which used as a sintering aid leads to densify the electrolyte and increase the shrinkage and performance. So we attempted to fabricate with one step co-firing process with 8 YSZ electrolytes at low temperature as 1250 oC. First of all, we found addition of trace Cu is effective to improve the sinterability of the YSZ electrolytes. The YSZ electrolyte with 100ppm CuO shows better sinterability and ionic conductivity than the pure YSZ electrolyte. The YSZ electrolyte with 100ppm CuO was densified below 1300 oC and the increased oxygen vacancy concentration leads to have higher ionic conductivity. Furthermore, the performances of the single-layer YSZ cells with 100ppm CuO were about 1.5 times higher(0.51 W cm2) than the cell with the pure YSZ electrolyte (0.36 W cm2) at 800 oC. The GDC/YSZ (100 ppm CuO) bi-layer cells also showed higher cell performance at 700 oC which shows similar tendency to the single-layer YSZ cell tests. These show that addition of 100 ppm CuO into the YSZ electrolyte is greatly effective for improvement of SOFC performance with energy saving during sintering process. According to these results of enhancing the sinterability and ionic conductivity, co-firing process was developed by addition of 5000 ppm Cu to the YSZ electrolytes and anodes at 1250 oC which is seriously lower than conventional multi-step fabrications with the sintering temperature above 1500 oC. The sinterability of the YSZ electrolyte was significantly improved 1250 oC by the 5000 ppm Cu. The co-fired cells with one- and two-step exhibited reasonable OCV value of 1.016 V and 1.011 V, respectively. Maximum power density of two-step cell with 0.71 W cm-2 at 850 oC which was similar value compared to conventional cell but the one-step cell has low maximum power density with 0.14 W cm-2. Above 1200 oC as sintering temperatures, the LSM/YSZ cathode produces insulating phases at the interfaces and low porosity makes cathode polarization. As a result, the cathode should be improved to be stable at 1250 oC, or the co-firing temperatures should be reduced below 1200 oC.
Fuel cell is one of the most promising electrochemical energy conversion devices that has many advantages such as high efficiency, high power density, CO2 free, noise free, rapid start up, etc. Especially, polymer electrolyte membrane fuel cell (PEMFC) has been shown a lot of attention due to its high power density and relatively good portability. Generally, Pt supported on carbon black is used as an electrocatalyst in PEMFC. However, carbon corrosion is one of the main problem for electrode structure collapse and catalyst activity loss, which can be further accelerated at high temperature. Therefore, we need to find an alternative catalyst support material that is electrochemically stable and interacts strongly with the catalyst active sites. Recently, metal oxide supports have been reported as promising materials due to their excellent mechanical resistance and inherently higher stability. Among many candidates, titanium dioxide (TiO2) support has an extraordinary stability under severe acidic atmosphere, which provides the possibility to directly use as a support material. However, the low electrical conductivity, catalytic activity and surface area of TiO2hinder the direct apply to the PEMFC electrode. In order to solve these problems, we prepared TiO2 nanofibers by electrospinning method and platinum nanoparticles are deposited by microwave-assisted polyol method to synthesize an effective electrode catalyst. Then, CNT was winded around the catalyst surface to boost up the electrical conductivity. Furthermore, we found a modified Pt electronic structure that takes advantage of the strong synergetic interactions of TiO2 nanofibers, Pt nanoparticles and winded-CNT. This structure influences on a decrease of the d-band vacancy of Pt due to electron transfer from the support, resulting in an improved oxygen reduction reaction. Therefore, the cathode with the CNT modified Pt/TiO2 nanofiber composite shows higher catalytic activity due to the elimination of the drawback associated with conductivity and enhanced electronic active structure. As from various PEMFC tests, our CNT-Pt/TiO2 catalyst shows superior performance and durability, which is definitely distinguishable at high temperature condition of 120 oC and RH 40% compare to the commercial Pt/C.
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