Abstract
Dense metal membranes that are based on palladium (Pd) are promising for hydrogen separation and production due to their high selectivity and permeability. Optimization of alloy composition has normally focused on bulk properties, but there is growing evidence that grain boundaries (GBs) play a crucial role in the overall performance of membranes. The present study provides parameters and analyses of GBs in the ternary Pd-Ag-Cu system, based on first-principles electronic structure calculations. The segregation tendency of Cu, Ag, and vacancies towards 12 different coherent ∑ GBs in Pd was quantified using three different procedures for relaxation of supercell lattice constants, representing the outer bounds of infinitely elastic and stiff lattice around the GBs. This demonstrated a clear linear correlation between the excess volume and the GB energy when volume relaxation was allowed for. The point defects were attracted by most of the GBs that were investigated. Realistic atomic-scale models of binary Pd-Cu and ternary Pd-Cu-Ag alloys were created for the ∑5(210) boundary, in which the strong GB segregation tendency was affirmed. This is a starting point for more targeted engineering of alloys and grain structure in dense metal membranes and related systems.
Highlights
Cost-effective production of ultra-pure hydrogen can facilitate the widespread implementation of fuel cells and is one of the remaining bottlenecks before hydrogen can be introduced as an energy carrier on a large scale [1]
grain boundaries (GBs)? This may be relevant for hydrogen solubility and diffusivity since both depend strongly on the GB models depicted in Figure are by no means representing the possible
Atomic-scale calculations based on density functional theory were used to investigate various properties of low-number coherent ∑ grain boundaries (GBs) in fcc Pd, Pd-Cu, and Pd-Cu-Ag alloys
Summary
Cost-effective production of ultra-pure hydrogen can facilitate the widespread implementation of fuel cells and is one of the remaining bottlenecks before hydrogen can be introduced as an energy carrier on a large scale [1]. 100% selectivity and high permeability, and allow for direct production of high purity hydrogen for use in fuel cells [2,3,4,5] Combining these membranes with appropriate catalysts in membrane reactors to produce hydrogen from different sources has been described in numerous studies [1,6,7,8]. It appears that the potential of binary Pd-based membranes has been exhausted in the literature, and several groups have recently started working on ternary compounds as the generation membrane material [9,10,11,12,13,14,15,16,17,18,19,20]. The challenge with this approach is that ternary compounds are difficult to engineer when using plating, rolling, etc. as processing techniques [14]
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