(383k) Fabrication of Pd/Ta Composite Metallic Membranes By Sputtering: The Impact of Pd Layer Thickness on Permeation Behavior

Hydrogen is one of the key factors in mitigating global environmental problems caused by greenhouse gas emissions. It offers a pathway to reduce fossil fuel consumption and has been gaining great attention as an alternative energy carrier because it does not generate any pollutants during energy conversion. However, the majority of hydrogen used in industries is ‘grey hydrogen’, produced with impurities such as CO₂ and CO. Therefore, it is necessary to purify hydrogen to fuel cell-grade quality, ensuring it meets the ISO 14,687 standard; H₂>99.97%, CO₂<2ppm, CO<0.2ppm.

Hydrogen purification using palladium(Pd)-based metallic membranes represents a viable method for producing fuel-cell grade, high-purity hydrogen, thanks to the unique permeation behavior of hydrogen through the metallic layers. Due to the high cost of Pd, conventional Pd-based metallic membranes have been fabricated by coating thin Pd layers on both sides of bulk supporting layers, such as porous stainless steel and BCC (body-centered cubic) metals. From our previous works, we have observed that the permeation behavior is significantly influenced by the thickness of Pd thin layer, due to the surface reactions associated with hydrogen’s dissociation and association.

In this study, we prepared metallic membranes with varying thicknesses on the feed and permeate sides to investigate the trends in permeability permeation flux associated with these differences and to determine the optimal thickness for each side. The thickness of Pd layer was carefully controlled using vacuum sputtering method, with thicknesses ranging from 10 to 200 nm for the feed and permeated sides, respectively. From the permeation test, we determined the optimized thickness for the Pd thin layer on bulk Ta metallic layer. The resulting composite membrane exhibited exceptionally high permeability, exceeding 3.6x10^-8 mol/m²âˆ™s∙Pa^0.5, and demonstrated robust mechanical strength; capable of withstanding up to 6 bar in the feed stream. Furthermore, we found that the rates of hydrogen dissociation and association are notably affected by the thickness of the Pd catalytic layer. This insight would be pivotal in developing cost-efficient Pd-based metallic membranes with high permeability. Additionally, the Pd/Ta composite membranes showed significant potential as a separation material for producing fuel-cell grade hydrogen from gas mixtures containing H₂, CO₂, CO, and CH₄. These components are typically found in the reformate gas from the fossil-fuel based hydrocarbon reforming processes.