Optimization of Catalytic Layer for Pd/Ta Composite Metallic Membranes and Their Application to Hydrogen Purification

다국어 초록 (Multilingual Abstract)

Amid the escalating global energy crisis and the worsening impacts of climate change, reducing greenhouse gas emissions and transitioning to clean energy systems have emerged as critical challenges for humanity. Hydrogen, in particular, has attracted significant attention as a next- generation clean energy carrier because it produces no carbon dioxide upon combustion and possesses a remarkably high gravimetric energy density (140 MJ kg-1). Industrial hydrogen is currently produced through processes such as steam methane reforming (SMR), coal gasification, purification of refinery off-gases, and water electrolysis. However, hydrogen generated from these processes typically contains various impurities—including CO, CO2, CH4, H2O, and H2S—which necessitates downstream purification to obtain high-purity hydrogen (> 99.97%) suitable for fuel cell applications. Representative hydrogen purification technologies include pressure swing adsorption (PSA), cryogenic distillation, polymeric membranes, and metal membranes. Among these, metal membranes are regarded as the most ideal technology because they can operate stably under high-temperature and high-pressure conditions while producing ultra-high-purity hydrogen (> 99.999%) in a single step. Metal membranes separate hydrogen via the Gorsky mechanism, in which hydrogen molecules dissociate and are absorbed on the metal surface, diffuse through the metallic lattice as atomic hydrogen, and subsequently recombine on the permeate side. During this process, the metal selectively allows only hydrogen to permeate. Palladium (Pd), in particular, is the most widely used membrane material due to its excellent hydrogen selectivity as well as its superior thermal and corrosion resistance, making it a key material for high-purity hydrogen purification in fuel cell applications and for high-temperature reactor systems such as steam reforming and water-gas shift processes. This study aims to develop a Pd/Ta composite metallic membrane while maintaining and enhancing hydrogen permeation performance. To achieve this, Pd films of various thicknesses were fabricated on BCC metal supports using electroless plating method and sputtering deposition method, and the effects of Pd layer thickness on hydrogen flux and permeability were systematically analyzed and compared by calculating flux and permeability of each samples. As a result, the highest permeation flux and permeability of the Pd/Ta membrane having 50 nm thickness of palladium synthesized by sputtering vacuum deposition method were measured as 10.72 cm3 cm-2 min-1 and 7.29 × 10-8 mol m-1 s-1 Pa-0.5, respectively. It was confirmed that when the Pd film was too thin, the hydrogen dissociation reaction on the Pd surface was significantly hindered, while an excessively thick film resulted in dominant internal diffusion resistance due to the inherently low hydrogen diffusivity of palladium. Consequently, it was observed that an intermediate, properly balanced Pd thickness exhibited the most optimized hydrogen permeation performance. A comparative analysis with electroless plating-based membranes of similar hydrogen permeation performance (10.91 cm3 cm-2 min-1 at flux and 8.67 × 10-8 mol m-1 s-1 Pa-0.5 at permeability) demonstrated that sputtered membranes could achieve equivalent permeability while extremely reducing Pd consumption compared with electroless plating method. Moreover, the sputtering process is a dry, environmentally friendly technique that requires no chemical plating solutions, reducing agents, or wastewater treatment. It also offers shorter processing times and precise control of film thickness, making it a highly efficient and sustainable fabrication method. In addition, asymmetric Pd configurations were designed by varying the Pd thickness between the feed and permeate sides while maintaining the same total Pd loading, to investigate the effect of each side’s Pd layer on overall hydrogen permeation performance. The results revealed that the Pd thickness on the feed side played a dominant role in determining hydrogen flux and permeability, whereas variations in the permeate-side thickness had only a minor effect. Samples which have different thickness of Pd on feed side show wide range of flux (1.5 ~ 12.08 cm3 cm-2 min-1) and permeability (0.82 ~ 8.59 × 10-8 mol m-1 s-1 Pa-0.5), but samples with different thickness of Pd on permeate side show narrow range of flux (3.8 ~ 4.5 cm3 cm-2 min-1) and permeability (2.3 ~ 2.6 × 10-8 mol m-1 s-1 Pa-0.5). Similar to the previous experiment, membranes with a properly optimized Pd thickness on the feed side exhibited the highest permeation performance, while changes in the permeate-side thickness had negligible influence when the feed-side thickness remained constant. This finding indicates that concentrating Pd on the feed side is a more efficient design strategy, enabling improved permeation efficiency while further reducing material costs. In conclusion, this study demonstrates that optimizing the Pd layer thickness (50 nm) and adopting a feed-side–focused asymmetric configuration are key design strategies for simultaneously achieving high performance and cost efficiency in Pd/Ta composite metallic membranes. In practical applications, these strategies could significantly reduce the overall cost of metallic membranes used in membrane-based processes while maintaining sufficient performance. Therefore, the findings of this study provide concrete design guidelines for Pd-based membrane cost reduction and structural optimization, offering a practical technological foundation for applying Pd-based membranes to hydrogen production and purification processes such as SMR, water gas shift (WGS), and reverse WGS (rWGS).

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Overview 1
• 1.2 Renewable primary energy with negligible CO2 emissions 4
• 1.3 Hydrogen as an energy carrier 8
• 1.3.1 Significance and potential of hydrogen energy 8
• 1.3.2 Importance and types of hydrogen separation technologies 15
• 1.4 Hydrogen separation based on metallic membranes technology 18
• 1.4.1 Principle and characteristics of metallic membranes 18
• 1.4.2 Differences between metallic membranes and other types of membranes 19
• 1.4.3 Research challenges and perspective 23
• 1.4.4 Technical solution for Pd/Ta membrane design 25
• 1.5 Research objectives 32
• Chapter 2. Experimental method and characteristics 34
• 2.1 Fabrication of Pd/Ta composite metallic membranes 34
• 2.1.1 Substrate information 34
• 2.1.2 Electroless plating method 36
• 2.1.3 Sputtering vacuum deposition 37
• 2.2 Characterization techniques 40
• 2.2.1 Surface and structural analysis 40
• 2.2.2 Gas permeation test setup 41
• 2.2.3 Flux and permeability calculation methods 43
• Chapter 3. H2 permeation characteristics of Pd/Ta composite metallic membrane 45
• 3.1 Overview 45
• 3.2 Results and discussion 46
• 3.2.1 Fabrication results of Pd films on membrane 46
• 3.2.2 Flux and permeability performance 51
• 3.2.3 Differences and performance comparison between the two methods 59
• 3.3 Summary 62
• Chapter 4. Effect of catalytic layer thickness of Pd/Ta composite metallic membranes on H2 permeation behavior 63
• 4.1 Overview 63
• 4.2 Results and discussion 64
• 4.2.1 Effect of asymmetry film structure on hydrogen flux and permeability of membrane 64
• 4.2.2 Cost reduction by asymmetric Pd configuration 71
• 4.3 Summary 72
• Chapter 5. Conclusion 73
• Reference 76