Aluminizing and oxidation treatments on the porous stainless steel substrate for preparation of H2-permeable composite palladium membranes

Composite palladium membranes based on porous stainless steel (PSS) substrate are idea hydrogen separators and purifiers for hydrogen energy systems, and the surface modification of the PSS is of key importance. In this work, the macroporous PSS tubes were aluminized through pack cementation at 850 °C in argon, followed by an oxidation with air at 600 °C. Palladium membranes were prepared by electroless plating. Their permeation performances were tested, and the hydrogen permeation kinetics was discussed. The substrate materials and the palladium membranes were characterized by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD). An Al₂O₃-enriched surface layer with small pore size was created through aluminizing and oxidation treatments, which greatly improves the membrane integrity. The intermetallic diffusion between the palladium membranes and the PSS substrate material was not observed after a heat-treatment at 500 °C under hydrogen for 200 h. However, the aluminizing and oxidation treatments still need to be further optimized in order to improve the membrane permeability and selectivity, and particularly, the high diffusion resistance of the substrate materials greatly limited the hydrogen flux.     

Fabrications and evaluations of hydrogen permeation on Al2O3/CeO2/graphene (ACG) composites membrane by Hot Press Sintering (HPS)

Ceramic membrane has high permeation rate of hydrogen and chemical stability. Al₂O₃ indicates stable at high temperature and a relatively large surface area. In addition, Al₂O₃ of porous is used as hydrogen separation membranes support, because of the high hydrogen permeability based on Knudsen diffusion mechanism.     

Preparation of thin Pd membrane on porous stainless steel tubes modified by a two-step method

Thin Pd membranes for hydrogen filtration were deposited on modified porous stainless steel (PSS) tubes using an electroless plating technique. Alumina oxide (Al₂O₃) particles of two different sizes were subsequently used to modify the non-uniform pore distribution and the surface roughness of the PSS tubes. The principle of the modification was to use large Al₂O₃ particles (∼10 μm) to fill larger pores on the surface, and leave the smaller pores intact. Small Al₂O₃ particles (∼1 μm) were then used to further decrease the surface roughness. The detailed manufacturing steps of the Al₂O₃ modification were investigated and optimized to achieve a continuous dense Pd membrane with a minimum thickness of 4.4 μm on the modified PSS tubes. The highest hydrogen permeance of the membrane was 2.94 × 10⁻³ mol/m²-s-kPa^0.5 at 773 K, with a selectivity coefficient (H₂/He) of 1124 under a pressure difference of 800 kPa. In comparison, the thickness and hydrogen permeance of a dense Pd membrane on unmodified PSS tubes were 31.5 μm and 5.97 × 10⁻⁴ mol/m²-s-kPa^0.5, respectively, at 773 K under an 800 kPa pressure difference. The stability of the membranes at high temperatures was also investigated. The hydrogen permeation flux at 773 K was stable during a test period of 500 h. These results demonstrate that the two-step method modifies the surface of PSS tubes in a relatively simple way and results in thin, dense Pd membranes with high hydrogen permeance and good thermal stability.     

Improved hydrogen permeation through thin Pd/Al2O3 composite membranes with graphene oxide as intermediate layer

Here we proposed the decreasing in the roughness of asymmetric alumina (Al₂O₃) hollow fibers by the deposition of a thin graphene oxide (GO) layer. GO coated substrates were then used for palladium (Pd) depositions and the composite membranes were evaluated for hydrogen permeation and hydrogen/nitrogen selectivity. Dip coating of alumina substrates for 45, 75 and 120 s under vacuum reduced the surface mean roughness from 112.6 to 94.0, 87.1 and 62.9 nm, respectively. However, the thicker GO layer (deposited for 120 s) caused membrane peel off from the substrate after Pd deposition. A single Pd layer was properly deposited on the GO coated substrates for 45 s with superior hydrogen permeance of 24 × 10⁻⁷ mol s⁻¹m⁻² Pa⁻¹ at 450 °C and infinite hydrogen/nitrogen selectivity. Activation energy for hydrogen permeation through the Al₂O₃/GO/Pd composite membrane was of 43 kJ mol⁻¹, evidencing predominance of surface rate-limiting mechanisms in hydrogen transport through the submicron-thick Pd membrane.     

Fabrication of Pd/ceramic membranes for hydrogen separation based on low-cost macroporous ceramics with pencil coating

Increasing hydrogen energy utilization has greatly stimulated the development of the hydrogen-permeable palladium membrane, which is comprised of a thin layer of palladium or palladium alloy on a porous substrate. This work chose the low-cost macroporous Al₂O₃ as the substrate material, and the surface modification was carried out with a conventional 2B pencil, the lead of which is composed of graphite and clay. Based on the modified substrate, a highly permeable and selective Pd/pencil/Al₂O₃ composite membrane was successfully fabricated via electroless plating. The membrane was characterized by SEM (scanning electron microscopy), field-emission SEM and metallographic microscopy. The hydrogen flux and H₂/N₂ selectivity of the membrane (with a palladium thickness of 5 μm) under 1 bar at 723 K were 25 m³/(m² h) and 3700, respectively; the membrane was found to be stable during a time-on-stream of 330 h at 723 K.     

Hydrogen permeation on Al2O3-based nickel/cobalt composite membranes

Al₂O₃ was synthesized using the sol-gel process with aluminum isopropoxide as the precursor and primary distilled water as the solvent. Nickel and cobalt metal powders were used to increase the strength of the membranes. The Al₂O₃-based membranes were prepared using HPS following a mechanical alloying process. The phase transformation, thermal evolution, surface and cross-section morphology of Al₂O₃ and Al₂O₃-based membranes were characterized by XRD, TG-DTA and FE-SEM. The hydrogen permeation of Al₂O₃-based membranes was examined at 300–473 K under increasing pressure. Hydrogen permeation flux through an Al₂O₃-20wt%Co membrane was obtained to 2.36 mol m⁻² s⁻¹. Reaction enthalpy was calculated to 4.5 kJ/mol using a Van’t Hoff’s plot.     

Toward low-cost Pd/ceramic composite membranes for hydrogen separation: A case study on reuse of the recycled porous Al2O3 substrates in membrane fabrication

The development of hydrogen energy systems has placed a high demand on hydrogen-permeable membranes as compact hydrogen separators and purifiers. Although Pd/Ceramic composite membranes are particularly effective in this role, the high cost of these membranes has greatly limited their applications; this high cost stems largely from the use of expensive substrate material. This problem may be solved by substrate recycling and the use of lower cost substrates. As a case study, we employed expensive asymmetric microporous Al₂O₃ and low-cost macroporous symmetric Al₂O₃ as membrane substrates (average pore sizes are 0.2 and 3.3 μm, respectively). The palladium membranes were fabricated by electroless plating, and substrate recycling was carried out by palladium dissolution with a hot HNO₃ solution. The functional surface layer of the microporous Al₂O₃ was damaged during substrate recycling, and the reuse of the substrate led to poor membrane selectivity. With the assistance of pencil coating as a facile and environmentally benign surface treatment, the macroporous Al₂O₃ can be successfully utilized. Furthermore, the macroporous Al₂O₃ can be also recycled and reused as membrane substrate, yielding highly permeable, selective and stable palladium membranes. Consequently, the substrate cost can be further decreased, and the applications of this kind of membranes would expand.     

Pool Boiling of Water–Al2O3 and Water–Cu Nanofluids Outside Porous Coated Tubes

This paper deals with pool boiling of water–Al₂O₃ and water–Cu nanofluids on porous coated, horizontal tubes. Commercially available stainless-steel tubes having 10 mm outside diameter and 0.6 mm wall thickness were used to fabricate a test heater. Aluminum porous coatings 0.15 mm thick with porosity of about 40% were produced by plasma spraying. A smooth tube served as a reference tube. The experiments were conducted under different absolute operating pressures of 200 kPa, 100 kPa, and 10 kPa. Nanoparticles were tested at concentrations of 0.01%, 0.1%, and 1% by weight. In all cases tested, enhancement heat transfer was always observed during boiling of water–Al₂O₃ and water–Cu nanofluids on smooth tubes compared to boiling of distilled water. Contrary to smooth tubes, addition of even a small amount of nanoparticles resulted in deterioration of heat transfer during pool boiling of water–Al₂O₃ and water–Cu nanofluids on porous coated tubes in comparison with boiling of distilled water.     

Palladium composite membrane fabricated on rough porous alumina tube without intermediate layer for hydrogen separation

A thin palladium composite membrane without any modified layer was successfully obtained on a rough porous alumina substrate. Prior to the fabrication of palladium membrane, a poly(vinyl) alcohol (PVA) layer was first coated onto the porous substrate by dip-coating technique to improve its surface roughness and pore size. After deposition of palladium membrane on the PVA modified substrate, the polymer layer can be completely removed from the composite membrane by heat treatment. The microstructure of the palladium composite membrane was characterized in detail using SEM, EDXS and XRD analysis. Permeation measurements were carried out using H₂ and N₂ at temperatures of 623 K, 673 K, 723 K and 773 K. The results indicated that the hydrogen permeation flux of 0.238 mol m^?2 s^?1 with H₂ separation factor α(H₂/N₂) of 956 for the as-prepared palladium membrane was obtained at 773 K and 100 kPa. Furthermore, the good membrane stability was proven during the total operation time of 160 h at the temperature range of 623 K–773 K and gas exchange cycles of 30 between hydrogen and nitrogen at 723 K.     

An improvement of the hydrogen permeability of C/Al2O3 membranes by palladium deposition into the pores

The development of compact hydrogen separator based on membrane technology is of key importance for hydrogen energy utilization, and the Pd-modified carbon membranes with enhanced hydrogen permeability were investigated in this work. The C/Al₂O₃ membranes were prepared by coating and carbonization of polyfurfuryl alcohol, then the palladium was introduced through impregnation–precipitation and colloid impregnation methods with a PdCl₂/HCl solution and a Pd(OH)₂ colloid as the palladium resources, and the reduction was carried out with a N₂H₄ solution. The resulting Pd/C/Al₂O₃ membranes were characterized by means of SEM, EDX, XRD, XPS and TEM, and their permeation performances were tested with H₂, CO₂, N₂ and CH₄ at 25 °C. Compared with the colloid impregnation method, the impregnation–precipitation is more effective in deposition of palladium clusters inside of the carbon layer, and this kind of Pd/C/Al₂O₃ membranes exhibits excellent hydrogen permeability and permselectivity. Best hydrogen permeance, 1.9 × 10⁻⁷ mol/m² s Pa, is observed at Pd/C = 0.1 wt/wt, and the corresponding H₂/N₂, H₂/CO₂ and H₂/CH₄ permselectivities are 275, 15 and 317, respectively.     

Influence of the type of siliceous material used as intermediate layer in the preparation of hydrogen selective palladium composite membranes over a porous stainless steel support

In this work, several composite membranes were prepared by Pd electroless plating over modified porous stainless steel tubes (PSS). The influence of different siliceous materials used as intermediate layers was analyzed in their hydrogen permeation properties. The addition of three intermediate siliceous layers over the external surface of PSS (amorphous silica, silicalite-1 and HMS) was employed to reduce both roughness and pore size of the commercial PSS supports. These modifications allow the deposition of a thinner and continuous layer of palladium by electroless plating deposition. The technique used to prepare these silica layers on the porous stainless steel tubes is based on a controlled dip-coating process starting from the precursor gel of each silica material. The composite membranes were characterized by SEM, AFM, XRD and FT-IR. Moreover they were tested in a gas permeation set-up to determine the hydrogen and nitrogen permeability and selectivity. Roughness and porosity of original PSS supports were reduced after the incorporation of all types of silica layers, mainly for silicalite-1. As a consequence, the palladium deposition by electroless plating was clearly influenced by the feature of the intermediate layer incorporated. A defect free thin palladium layer with a thickness of ca. 5 μm over the support modified with silicalite-1 was obtained, showing a permeance of 1.423·10⁻⁴ mol m⁻² s⁻¹ Pa^−0.5 and a complete ideal permselectivity of hydrogen.     

Graphite coating on alumina substrate for the fabrication of hydrogen selective membranes

Development of composite membranes is a suitable alternative to improve the hydrogen flux through palladium membranes. The porous substrate should not represent a barrier to gas permeation, but the roughness of its surface should be sufficiently smooth for the deposition of a thin and defect-free metal layer. In this study, the performances of the modification of the outer surface of an asymmetric alumina hollow fibre substrate by the deposition of a graphite layer were evaluated. The roughness of the substrate outer surface was reduced from 120 to 37 nm after graphite coating. After graphite coating, the hydrogen permeance through the composite membrane produced with 2 Pd plating cycles was of 1.02 × 10^?3 mol s^?1 m^?2 kPa^?1 at 450 °C and with infinite H₂/N₂ selectivity. Similar hydrogen permeance was obtained with the composite membrane without graphite coating, also at infinite H₂/N₂ selectivity, but 3 Pd plating cycles were necessary. Thus, graphite coating on asymmetric alumina hollow fibres is a suitable alternative to reduce the required palladium amount to produce hydrogen selective membranes.     

Preparation and characterization of palladium ceramic alumina membrane for hydrogen permeation

In this study, a tubular palladium membrane has been prepared by an electroless plating method using palladium II chloride as a precursor with the intent of not having a completely dense film since its application does not require high hydrogen selectivity. The support used was a 15 nm pore sized tubular ceramic alumina material that comprised of 77% alumina and 33% titania. It has dimensions of 7 mm inner and 10 mm outer diameters respectively. The catalyst was deposited on the outside tube surface using the electroless deposition process. The membrane was morphologically characterized using scanning electron microscopy/energy dispersive x-ray analysis (SEM/EDXA) and liquid nitrogen adsorption/desorption analysis (BET) to study the shape and nature of the palladium plating on the membrane. The catalytic membrane was then inserted into a tubular stainless-steel holder which was wrapped in heating tapes so as to enable the heating of the membrane in the reactor. The gases used for permeation tests comprised H₂, N₂, O₂ and He. Permeation tests were out at 573 K and at pressure range between 0.05 and 1 barg. The results showed that hydrogen displayed a higher permeation when compared to other gases that permeated through the membrane and its diffusion is also thought to include solution diffusion through the dense portions of the palladium in addition to Knudsen, convective and molecular sieving mechanisms occurring through cracks and voids along the grain boundaries. While high hydrogen selectivity is critically important in connection with hydrogen purification for fuel cells and in catalytic membrane reactors used to increase the yield of thermodynamically limited reactions such as methane steam reforming and water–gas shift reactions whereby the effective and selective removal of the H₂ produced from the reaction zone shifts the equilibrium, it is not so important in situations in which the membrane has catalytic activity such that it is possible to carryout the reaction in situations where the premixed reactants are forced-through the membrane on which the catalysts is attached. This type of catalytically active membranes is novel and has not been tested in gas-solid-liquid reactions and liquid-solid reactions before. With such a reactor configuration, it is possible to achieve good feed stream distribution and an optimal usage of the catalytic material. The preparation and characterization of such membrane catalysts has gained increased interest in the process industries because it can be adapted to carryout the chemical reactions if one of the reactants is present in low concentration and an optimal reactant distribution results in a better utilization of the active catalytic material. However, there are concerns in terms of the high cost of palladium membranes and research on how to fabricate membranes with a very low content of the palladium catalyst is still ongoing. Work is currently underway to deploy the Pd/Al₂O₃ membrane catalysts for the deoxygenating of water for downhole injection for pressure maintenance and in process applications.     

Hydrogen transport properties of several vanadium-based binary alloys

Vanadium-based alloys are an emerging alternative to palladium alloys for use in hydrogen-selective alloy membranes. The tendency of vanadium to embrittle, due to its high hydrogen absorption, means it lacks the robustness required for industrial hydrogen separation applications. Alloying vanadium with certain elements reduces hydrogen absorption, but also influences the diffusivity of hydrogen through the bulk material. Consequently, diffusivity and absorption data must be decoupled in order to fully evaluate the influence of various alloying additions on the hydrogen transport properties of vanadium alloys. To address this need, the hydrogen transport properties of V–Al (V₉₅Al₅, V₉₀Al₁₀, V₈₅Al₁₅, V₈₀Al₂₀, V₇₅Al₂₅, expressed as atom%) and V–Cr (V₉₅Cr₅, V₉₀Cr₁₀, V₈₅Cr₁₅) alloys have been compared through a series of absorption and flux measurements. Pd-coated alloy disks were formed from arc melted and sectioned ingots, and each alloy was subjected to a microstructural analyses and a detailed examination of hydrogen absorption and permeation properties. Additions of Al and Cr reduce the hydrogen absorption and diffusivity of vanadium, with V–Cr alloys exhibiting the greatest hydrogen diffusivity for a given hydrogen feed pressure. The diffusivity of each alloy showed strong concentration dependence. Diffusivity-concentration results have been overlayed with an isoflux curve corresponding to a target flux of 1.0 mol m⁻² s⁻¹, enabling prediction of the thickness and pressure required to achieve this target flux target for a given alloy.     

Effect of alumina on thermodynamic performance of palladium-H2 (D2) system

Palladium (Pd) has a strong hydrogen isotope effect, widely used in the Thermal Cycling Absorption Process (TCAP) separation column. However, due to pulverization and high gas resistance, Pd was usually dispersed in backing materials, such as alumina (Al₂O₃), to improve the service performance. In this paper, the microstructure of sponge palladium and Pd–Al₂O₃ pellet was observed by Scanning Electron Microscopy (SEM), and the crystalline structure before and after hydrogeneration was characterized by X-Ray Diffractometer (XRD). To study the support effect of backing material Al₂O₃ on the hydrogen/deuterium thermal absorption performance of palladium, the pressure-composition-temperature (P–C-T) curves were measured by series experiments. The result shows that Al₂O₃ acts as a support material between palladium particles to suppress the effect of the pellet expansion-shrinking during H₂ (D₂) absorption/desorption cycling. Alumina could provide free position for hydrogen atoms. And the inflection points of α phase to α+β phase shift from 0.02 to 0.09 for Pd–Al₂O₃ pellet/hydrogen system. The entropy and enthalpy variation of Pd/H (Pd/D) system in α+β phase are respectively ?18.34 (?16.83) kJ/mol and ?45.65 (?42.44) J/mol/K, which show no correlation with alumina. The H-D separation factor positively correlates with the quantity of hydrogen in the solid phase, and alumina could restrain the separation effect.     

Hydrogen isotope permeation through yttria coatings on Eurofer in the diffusion limited regime

In fusion power plants a tritium permeation barrier is required in order to prevent the loss of the fuel. Moreover, the tritium permeation barrier is necessary to avoid that the radioactive tritium accumulates in the first wall, the cooling system, and other parts of the power plant. Oxide thin films, e.g. Al₂O₃, Er₂O₃ and Y₂O₃, are promising candidates as tritium permeation barrier layers. With regard to the application, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y₂O₃ layers with thicknesses from 100 nm to 500 nm are deposited on both sides of Eurofer substrates by RF magnetron sputter deposition. Some of the samples are additionally deposited with palladium thin films to analyse the limited regime. During the annealing in the experiments the palladium layers do not show any crack formation or delamination, verified by scanning electron microscopy. After annealing the cubic crystal structure of the Y₂O₃ layers is verified by X-ray diffraction. The cubic phase contains a small amount of a monoclinic phase, which is eliminated after the permeation measurements. The permeation reduction factors of the samples are determined in gas-driven deuterium permeation experiments. A permeation reduction of 5000 of the yttria thin film is verified. The diffusion limited regime is identified by the pressure dependence of the permeation measurement and by permeation experiments with the palladium top layers on the Y₂O₃ thin films. Furthermore, the activation energy of the permeation through the yttria thin films is determined. Pre-annealing times for more than 70 h of the Y₂O₃ thin films and permeation measurements with temperature cycles for 20 days are performed to show the stability of the permeation flux and hence the microstructure of the barrier layers. Measurement times at each constant temperature level of more than 25 h are required for the stabilization of each permeation flux to a constant value. The permeation measurement setup is enhanced to enable a continuously running equipment for these measurement times.     

Heterogeneous electrolyte (YSZ–Al2O3) based direct oxidation solid oxide fuel cell

Bilayers comprised of dense and porous YSZ–Al₂O₃ (20 wt%) composite were tape cast, processed, and then fabricated into working solid oxide fuel cells (SOFCs). The porous part of the bilayer was converted into anode for direct oxidation of fuels by infiltrating CeO₂ and Cu. The cathode side of the bilayer was coated with an interlayer [YSZ–Al₂O₃ (20 wt%)]: LSM (1:1) and LSM as cathode. Several button cells were evaluated under hydrogen/air and propane/air atmospheres in intermediate temperature range and their performance data were analyzed. For the first time the feasibility of using YSZ–Al₂O₃ material for fabricating working SOFCs with high open circuit voltage (OCV) and power density is demonstrated. AC impedance spectroscopy and scanning electron microscopy (SEM) techniques were used to characterize the membrane and cell.     

Hydrogen permeation measurements of Pd and Pd-Cu membranes using dynamic pressure difference method

Hydrogen permeation across membranes is measured using a dynamic pressure difference method. In the method, a transient system for continuously monitoring hydrogen flux of a membrane is conducted. Three different membranes, consisting of two pure palladium (Pd) membranes with different thicknesses and one palladium-copper (Pd-Cu) membrane supported by porous stainless steel (PSS) tubes, are taken into account. Three different operating temperatures of 320, 350 and 380 °C as well as two different initial pressure differences of 5 and 10 atm are considered to evaluate the effects of the operating parameters upon the hydrogen permeation. The results suggest that a threshold of pressure difference is always exhibited at the end of the permeation process, regardless of which membrane is tested. The hydrogen permeation rate can be predicted well for the pressure exponent in the range of 0.1-1.0; however, the optimal pressure exponent is located between 0.5 and 0.8. The theoretical analysis indicates that the characteristic time of hydrogen permeation in the present system ranges from 245 to 460 s and the entire permeation period is longer than the characteristic time by an order of magnitude. In regard to the effect of membrane temperature on the permeation, the activation energies of the three membranes range from 11 to 18 kJ mol⁻¹.     

Hydrogen separation from blended natural gas and hydrogen by Pd-based membranes

Hydrogen separation membranes based on a heated metal foil of a palladium alloy, offer excellent permeability for hydrogen as a result of the solution-diffusion mechanism. Here, the possibility to separate hydrogen from the mixture of Natural Gas (NG) and hydrogen (NG+H₂) with various NG concentrations using Pd, PdCu₅₃ and PdAg₂₄ hydrogen purification membranes is demonstrated. Hydrogen concentrations above ∼25% (for Pd and PdCu₅₃) and ∼15% (for PdAg₂₄) were required for the hydrogen separation to proceed at 400 °C and 5 bar pressure differential. Hydrogen permeability of the studied alloys could be almost fully recovered after switching the feed gas to pure hydrogen, indicating no significant interaction between the natural gas components and the membranes surface at the current experimental condition. Hydrogen flux of the membranes at various pressure differential was measured and no changes in the hydrogen permeation mechanism could be noticed under (NG 50%+H₂) mixture. The hydrogen separation capability of the membranes is suggested to be mainly controlled by the operating temperature and the hydrogen partial pressure.