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Hydrogen Solubility and Permeability of Ni-Nb-Zr based Amorphous Alloy Membranes
AuthorPal, Narendra K.
Chemical and Materials Engineering
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The demand of hydrogen, which is currently among the top industrial gases produced yearly in the world, will further increase due to the additional need for upgrading the low-grade crude feedstock and our endeavor to bring hydrogen economy a reality in the twenty-first century.Hydrogen produced at cheaper price would be essential for realizing hydrogen economy in real sense on a sustainable basis. Hydrogen if produced from coal-gasification would be available at very cheap price at least for medium-term on sustainable basis as countries like USA, Australia etc have huge coal reserves which can last for hundreds years. For producing hydrogen from coal-gasification, we should have efficient, cost effective and robust purification membrane which can separate hydrogen from other gases like CO etc. Conventionally, Pd and Pd alloy membranes are being used for filtration of hydrogen from other mixed gases in coal-gasification produce. But high cost of Pd have forced scientific community to explore the possibility of using alternate to Pd as separation membrane and amorphous alloy membranes have edge over crystalline one as the former posses higher solubility and less chances of embrittlement in presence of hydrogen. Under similar conditions of temperature and pressure, some metallic glass alloys can absorb more hydrogen than their crystalline counterpart. This fact and the ductility of the amorphous alloys, suggests that they may be more promising than crystalline inter-metallic compounds as a medium for hydrogen storage. Under the present circumstances, Ni-Nb-Zr amorphous alloy membranes are considered to be prospective candidate for separation membranes as they posses' permeability close to Pd. Apart from better performance, these amorphous alloy membranes also provide considerable cost benefits compared to Pd and Pd-alloy based membranes. In this study, Ni-Nb-Zr based amorphous alloy membranes series of (Ni0.6Nb0.4)100-xZrx, (Ni0.6Nb0.3Ta0.1)100-xZrx and (Ni0.42Nb0.28Zr0.30)100-xBx were successfully fabricated and subsequently coated with thin layer of Pd on both sides. As permeation of hydrogen through membranes is governed by the hydrogen solubility and diffusion of the alloy membranes, I analyzed these amorphous alloy membranes for hydrogen solubility, permeability and durability. Hydrogen permeability of (Ni0.6Nb0.4)70Zr30 alloy membrane was found to be 2.2410-8 mol H m / m2 s Pa0.5 at 200OC, which is comparable with Pd and Pd alloy membranes. This alloy also gave the highest hydrogen solubility of 0.65 (H/M) at 200OC. Using pressure-concentration isotherms, the diffusion coefficients were calculated and (Ni0.6Nb0.4)70Zr30 amorphous alloy membrane gave highest diffusion coefficient of 4.9110-10 m2/s followed by (Ni0.6Nb0.3Ta0.1)70xZr30 amorphous alloy membrane which gave a value of 3.9910-10 m2/s for diffusion coefficient. It was found that Ta addition resulted in overall increase in durability but at the cost of permeation, that means there exists a trade-off between permeation and durability which needs to be addressed to arrive on final solution.