steel plate.
2011年5月4日 by admin
Fuel Cell is an electrochemical device which converts chemical energy into electrochemical energy directly with theoretical conversion up to 90%. It is the fourth power generation following as thermal power, water power and nuclear power. Owing to high efficiency, less emission, safety and variety, fuel cell is regarded as the most important power source in future. Direct methanol fuel cell（DMFC） uses liquid methanol as fuel whose energy density is 34 times higher than that of liquid hydrogen. The storage and transporting of methanol is more convenient and safer than hydrogen. Due to the low cost, small size and light weight,DMFC is regarded as the promising candidate power for portable electronic products.Proton exchange membrane （PEM） is the core component of DMFC which directly influences the DMFC’s performance, energy conversion and operating life. PEM plays a role in two aspects as follow: （1） Separating fuel and oxidants （2） Conducting the proton efficiently. Currently, the perfluorosulfonic acid polymer such as Nafion is used as the commercial PEM materials dues to its high proton conductivity. However, Nafion is suffering from low operation temperature （<80℃） and high methanol permeability which limits the DMFC for application and it is urgent to find an alternative proton exchange membrane with a low cost and excellent performance.Sulfonated poly （ether ether ketone） （SPEEK） was a new kind of PEM materials with good mechanical and thermal properties. SPEEK exhibits excellent methanol resistance and proton conductivity with much lower cost than Nafion and it has been studied extensively in the field of proton exchange membrane. However, there is a common problem existing in the SPEEK polymer that when the sulfonation degree is too high, the dimensional stability and mechanical strength as well as methanol resistance declined significantly. These shortcomings hindered its application for fuel cell. In order to meet the demands of fuel cell, it is essential to modify the SPEEK properly. This work is aiming to maintain the proton conductivity of membrane within the appropriate level, while reducing the methanol permeability and increasing the dimensional stability as well as mechanical strength. SPEEK based composite membrane was prepared to obtain the PEM materials with improving overall performance.Firstly, we synthesized sulfonated poly（ether ether ketone） with a high sulfonation degree （DS=1.2） using 3,3’,5,5’-tetramethyldiphenyl-4.4’-diol, 4,4’-difluorobenzophenone and sodium 5,5’-carbonylbis（2-fluorobenzene sulfonate） via nucleophilic substitution reaction. Then three measures such as organic-inorganic blend, organic cross-linking and blending-surface modification were used to modify thesteel plate. as following:Inorganic-organic blending is a main method of modifying polymer material which introduces inorganic into organic via impregnation or sol-gel. There are many kinds of additives, such as silica dioxide, ZrP and HPA, in which silica dioxide was researched with a long history. There are a large number of hydroxyl groups in silica dioxide which can improve the membrane’s water retention and proton conductivity especially sulfonated silica dioxide. Firstly, we prepared a sulfonated silica precursor with a long pendant sulfonate group via the reaction between KH550 and 1,4 butane sultone, and then a series of silica/SPEEK composite membranes were prepared by sol-gel. The IEC and water uptake of SPEEK-S-Si composite membranes both increased with the increasing of the content of sulfonated silica. The swelling ratio of composite membranes was lowed because of the three-dimensional network structure formed by cross-linked sulfonated silica. With the increasing of water uptake and IEC , the proton conductivity increased. Moreover the long flexible pendant chain bringing with the sulfonate group helps to form a wide proton transport channel which could transport protons efficiently, as a result, the proton conductivity of composite membranes is higher than that of pure steel plate. membrane. In addition, the methanol permeability decreased slightly and the selectivity of composite membrane was improved. The mechanical and thermal properties were still satisfying with the requirements of proton exchange membrane materials.Organic-organic modification is also an important method to enhance material performance and there are many reports about steel plate.modified by Nafion. However, due to the different polarity between Nafion and SPEEK, an agglomeration of Nafion always formed in SPEEK complex. In order to solve the non-homogeneous in SPEEK-Nafion composite materials, we prepared a series of cross-linked SPEEK-Nafion composite membrane using sulfonated diamine as the cross-linking agent. The results showed that: IEC water uptake and swelling ratio of the composite membrane decreased remarkably with the increasing content of Nafion cross-linking. The dimensional stability of SPEEK-Nafion composite membrane has been effectively improved. Although the proton conductivity deceased slightly, it still met the demands of fuel cell. The methanol permeability of the composite membrane was lowed obviously because of the introduction the cross-linked structure. The selectivity of the composite membrane was higher than the pure SPEEK membrane. The mechanical and thermal properties had been improved at the same time which helps to enlong the life of fuel cell.At last, we prepared SPEEK/HPW membrane firstly followed by modifying it with polypyrrole aiming to improve the stability of HPW in membrane and evaluate the methanol resistance. The SPEEK/HPW/Ppy membrane was constructed by surface polymerization of pyrrole and showed a sandwich structure. The properties were researched in details. Comparing with SPEEK/HPW membrane, the IEC value, water uptake and swelling ratio of SPEEK/HPW/Ppy composite membranes showed a remarkable decline. This was because Ppy was lack of sulfonate group and exhibited a hydrophobic property. Due to the decrease of IEC and water uptake, the proton conductivity of membrane reduced too, but it still remained in the available range. We investigated the stability of HPW in the composite membrane by water immersion test, the result showed that Ppy modified composite membranes demonstrated more effect on immobilizing HPW, the composite membrane with the highest content of Ppy showed a weight loss of HPW with only 6.4%. In addition, the methanol permeability of composite membranes was much lower than that of substrate membrane due to the Ppy layers with a high dense structure and the selectivity of composite membranes was evaluated significantly. The mechanical and thermal properties of the composite membrane modified by Ppy were all improved. It was worth mentioning that the Ppy in the surface could play a role in catalysis methanol oxidation in the MEA. So the SPEEK/HPW/Ppy composite membrane was promising material in the direct methanol fuel cell
Tags: steel plate. steel sheet
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