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A Combinatorial study on hydrogen separation membranes

Pişkin, Fatih
Metallic membranes among the hydrogen separation membranes are quite attractive due to their very high hydrogen selectivity and hydrogen permeability. The efforts in metallic membranes generally concentrate on to identify membrane compositions which have a high hydrogen permeability with a reduced cost. Among the metallic membranes, Pd and Pd alloys, i.e. f.c.c. membranes are quite common as separation membranes. However, the high cost of Pd limits its widespread use in industrial applications. The efforts to address such problems have concentrated on two main approaches. One is reducing the Pd content of the membranes by alloying and producing membranes in the form of thin films. Another approach is to develop new alternative membrane compositions which are Pd-free. Of the Pd-free membranes, Nb, V, Ta and their alloys, i.e. b.c.c. membranes are particularly attractive due to their orders of magnitude higher hydrogen permeability as compared to Pd-based membranes. However, the b.c.c. membranes have an insufficient catalytic activity to dissociate hydrogen molecules into its atomic form, which is essential for hydrogen separation. The excessive hydrogen solubility resulting in embrittlement is another drawback for b.c.c. membranes. For this reason, the efforts on b.c.c. membranes have focused on identifying the alloying elements that would reduce the level of hydrogen solubility while maintaining a reasonable hydrogen permeability. However, developing alternative alloy compositions by alloying or finding completely new membrane compositions via traditional methods such as synthesizing one membrane composition at a time and testing it for the purpose are not always successful. These also require extended coordinated efforts that are time-consuming as well. Therefore, a combinatorial method that would allow the production of multiple material compositions in a single experiment and which may then be evaluated by an effective screening technique is a highly useful approach to develop new membrane compositions. The present work is an outcome of an effort to develop low-cost hydrogen separation membranes on the basis of combinatorial materials science. For this purpose, a total of three ternary systems, i.e. Nb-Pd-Ti, Pd-Ag-Ti, and Pd-Ag-Ni were investigated by a combinatorial method. Nb-Pd-Ti was selected aiming to produce dominantly b.c.c. membranes, while the Pd-Ag-Ti and the Pd-Ag-Ni ternary alloys were chosen to yield f.c.c. membranes. A total of 21 thin films, each with a different composition, covering a large compositional field in the ternary diagram were deposited in a single experiment via magnetron sputtering. The thin films were then screened by a four-probe resistivity measurement in terms of a reactivity index. The compositions identified as candidates as a hydrogen separation membrane were fabricated in the form of foils and then tested for the hydrogen permeability. The present work demonstrates that the adopted approach is a quite effective way of finding the suitable compositions for low-cost hydrogen separation membranes.