T.A. Peters

Investigation of La1−xSrxCrO3−∂ (x ~ 0.1) as Membrane for Hydrogen Production

Various inorganic membranes have demonstrated good capability to separate hydrogen from other gases at elevated temperatures. Hydrogen-permeable, dense, mixed proton-electron conducting ceramic oxides offer superior selectivity and thermal stability, but chemically robust candidates with higher ambipolar protonic and electronic conductivity are needed. In this work, we present for the first time the results of various investigations of La1−xSrxCrO3−∂ membranes for hydrogen production. We aim in particular to elucidate the material’s complex transport properties, involving co-ionic transport of oxide ions and protons, in addition to electron holes. This opens some new possibilities for efficient heat and mass transfer management in the production of hydrogen. Conductivity measurements as a function of pH2 at constant pO2 exhibit changes that reveal a significant hydration and presence of protons. The flux and production of hydrogen have been measured under different chemical gradients. In particular, the effect of water vapor in the feed and permeate gas stream sides was investigated with the aim of quantifying the ratio of hydrogen production by hydrogen flux from feed to permeate and oxygen flux the opposite way (“water splitting”). Deuterium labeling was used to unambiguously prove flux of hydrogen species.

Chapter 11:Palladium-based Membranes in Hydrogen Production

Hydrogen is one of the most important chemicals used in industry today. In addition, hydrogen is prospected as an energy carrier for the future, since the energy contained in the molecule can be efficiently converted to electric energy in fuel cells for different applications. Different requirements exist to hydrogen purity or hydrogen gas mixture composition, depending on the application, and different separation technologies are therefore applied. Membranes with good permeation properties and high selectivity to hydrogen have been identified to represent potential production technology to give efficiency improvement and cost reduction, provided they have sufficiently high reliability and low cost. Pd and certain Pd alloy compositions are known to easily absorb and diffuse hydrogen in the solid matrix, and can hence be applied in membrane separation with 100 % selectivity. So far, few commercial applications have emerged due to cost and/or performance of the existing membranes, but continuous improvements have been made over more than 40 years of research and development.This chapter will give a review on the recent progress made in the Pd-based membrane and membrane reactor development, and describe applications in various hydrogen production processes. The intention is to give the reader insight about the status and to pin-point some trends and main challenges related to Pd-based membranes for hydrogen production.

Dual phase high-temperature membranes for CO2 separation – performance assessment in post- and pre-combustion processes

Dual phase membranes are highly CO2-selective membranes with an operating temperature above 400 °C. The focus of this work is to quantify the potential of dual phase membranes in pre- and post-combustion CO2 capture processes. The process evaluations show that the dual phase membranes integrated with an NGCC power plant for CO2 capture are not competitive with the MEA process for post-combustion capture. However, dual phase membrane concepts outperform the reference Selexol technology for pre-combustion CO2 capture in an IGCC process. The two processes evaluated in this work, post-combustion NGCC and pre-combustion IGCC, represent extremes in CO2 partial pressure fed to the separation unit. Based on the evaluations it is expected that dual phase membranes could be competitive for post-combustion capture from a pulverized coal fired power plant (PCC) and pre-combustion capture from an Integrated Reforming Cycle (IRCC).

New Insight to the Effects of Heat Treatment in Air on the Permeation Properties of Thin Pd77%Ag23% Membranes

Sputtered Pd77%Ag23% membranes of thickness 2.2–8.5 µm were subjected to a three-step heat treatment in air (HTA) to investigate the relation between thickness and the reported beneficial effects of HTA on hydrogen transport. The permeability experiments were complimented by volumetric hydrogen sorption measurements and atomic force microscopy (AFM) imaging in order to relate the observed effects to changes in hydrogen solubility and/or structure. The results show that the HTA—essentially an oxidation-reduction cycle—mainly affects the thinner membranes, with the hydrogen flux increasing stepwise upon HTA of each membrane side. The hydrogen solubility is found to remain constant upon HTA, and the change must therefore be attributed to improved transport kinetics. The HTA procedure appears to shift the transition from the surface to bulk-limited transport to lower thickness, roughly from ~5 to ≤2.2 µm under the conditions applied here. Although the surface topography results indicate that HTA influences the surface roughness and increases the effective membrane surface area, this cannot be the sole explanation for the observed hydrogen flux increase. This is because considerable surface roughening occurs during hydrogen permeation (no HTA) as well, but not accompanied by the same hydrogen flux enhancement. The latter effect is particularly pronounced for thinner membranes, implying that the structural changes may be dependent on the magnitude of the hydrogen flux.

CO2 Separation in Nanocomposite Membranes by the Addition of Amidine and Lactamide Functionalized POSS® Nanoparticles into a PVA Layer

In this article, we studied two different types of polyhedral oligomeric silsesquioxanes (POSS®) functionalized nanoparticles as additives for nanocomposite membranes for CO2 separation. One with amidine functionalization (Amidino POSS®) and the second with amine and lactamide groups functionalization (Lactamide POSS®). Composite membranes were produced by casting a polyvinyl alcohol (PVA) layer, containing either amidine or lactamide functionalized POSS® nanoparticles, on a polysulfone (PSf) porous support. FTIR characterization shows a good compatibility between the nanoparticles and the polymer. Differential scanning calorimetry (DSC) and the dynamic mechanical analysis (DMA) show an increment of the crystalline regions. Both the degree of crystallinity (Xc) and the alpha star transition, associated with the slippage between crystallites, increase with the content of nanoparticles in the PVA selective layer. These crystalline regions were affected by the conformation of the polymer chains, decreasing the gas separation performance. Moreover, lactamide POSS® shows a higher interaction with PVA, inducing lower values in the CO2 flux. We have concluded that the interaction of the POSS® nanoparticles increased the crystallinity of the composite membranes, thereby playing an important role in the gas separation performance. Moreover, these nanocomposite membranes did not show separation according to a facilitated transport mechanism as expected, based on their functionalized amino-groups, thus, solution-diffusion was the main mechanism responsible for the transport phenomena.

Grain Boundary Segregation in Pd-Cu-Ag Alloys for High Permeability Hydrogen Separation Membranes

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.

CHAPTER 6:Pd-based Membranes in Hydrogen Production: Long-term Stability and Contaminant Effects

H2 membrane separation technology has been identified as a key enabling technology for hydrogen as future energy carrier, in particular in conjunction with the capture and storage of CO2. Improved Pd-alloys and composite membrane structures are needed for this to become viable. In this chapter, the factors affecting the stability of Pd-based membranes, focusing particularly on the effect of structural changes and gaseous contaminants under long-term operation, are described. Approaches to enhance the stability and tolerance are introduced and discussed. Subsequently, an overview of the main application areas of Pd-based membranes is given, along with the stability demands of these applications. Finally, relevant long-term studies focusing on membrane stability are reviewed. It is concluded that considerable progress has been made with respect to the stability, with some applications close to commercialisation. Certain contaminants remain an issue, however, that are likely to require additional developments in gas cleaning technologies.