Palladium–ruthenium membranes for hydrogen separation fabricated by electroless co-deposition

Prior published research has claimed that the addition of 4.5 wt% ruthenium to palladium in dense, cold-worked membranes produces desirable properties for hydrogen separation, including greater mechanical strength and increased hydrogen permeability, particularly at higher temperature ranges. Electroless co-deposition onto porous media can be used to produce high-quality composite membranes which take advantage of both the mechanical strength of the support and the high flux of the thin film. The objectives of this investigation were to fabricate Pd–Ru alloy composite membranes, and to examine their properties.     

Origin of hydrogen embrittlement in vanadium-based hydrogen separation membranes

Hydrogen embrittlement in metals is a challenging technical issue in the proper use of hydrogen energy. Despite extensive investigations, the underlying mechanism has not been clearly understood. Using atomistic simulations, we focused on the hydrogen embrittlement in vanadium-based hydrogen separation membrane. We found that, contrary to the conventional reasoning for the embrittlement of vanadium, the hydrogen-enhanced localized plasticity (HELP) mechanism is the most promising mechanism. Hydrogen enhances the nucleation of dislocations near the crack tip, which leads to the localized plasticity, and eventually enhances the void nucleation that leads to the failure. Those results provide an insight into the complex atomic scale process of hydrogen embrittlement in vanadium and also help us design a new alloy for hydrogen separation membranes.     

First-principles study of hydrogen behavior in vanadium-based binary alloy membranes for hydrogen separation

Vanadium-based alloys are considered to be one of the most promising hydrogen separation membranes due to their high hydrogen permeability. In this study, we investigate the dissolution and diffusion behaviors of hydrogen in vanadium-based binary alloys, V₁₅M (where M = Al, Ti, Cr, Fe, Ni and Nb) alloys, using first-principles method based on density functional theory. The dissolution of hydrogen in V₁₅M alloys is affected by both the elastic and electronic properties, but the elastic effect is the main factor. The H solution energies in the alloys follow the sequence: VTi < VNb < VAl < VCr < VNi < VFe, and a smaller atom size increase the H solution energy. Therefore, the addition of alloying elements with smaller atomic sizes can reduce the solubility of hydrogen in vanadium and inhibit hydrogen embrittlement. For hydrogen diffusion, alloying elements Al, Ti and Nb can be good candidates because they have a higher diffusion coefficient. The VTi alloy has the highest hydrogen permeability, but will have serious hydrogen embrittlement due to the increased H solubility.     

Novel nanocomposite materials for oxygen and hydrogen separation membranes

Design of oxygen and hydrogen separation membranes is the point of current interest in producing syngas from biofuels. Nanocomposites with a high mixed ionic-electronic conductivity are known to be promising materials for these applications. This work aims at studying performance of oxygen and hydrogen separation membranes based on nanocomposites PrNi_0.5Co_0.5O_3-δ + Ce_0.9Y_0.1O_2-δ and Nd_5.5WO_11.25-δ + NiCu alloy, respectively. A high and stable performance promising for the practical application was demonstrated for these membranes. For oxygen separation membrane CH₄ conversion is up to 50% with H₂ content in the outlet feed being up to 25% at 900 °C. For reactor with hydrogen separation membrane complete EtOH conversion was achieved at T ∼ 700 °C even at the highest flow rate, and a high hydrogen permeation (≥1 ml H₂ cm⁻² min⁻¹) was revealed.     

FEM simulation supported evaluation of a hydrogen grain boundary diffusion coefficient in MgH2

The diffusion coefficient data of hydrogen in the Magnesium-hydrogen system shows a large scatter, their trends extrapolations vary at room temperature between 10^?12 m²/s and 10^?29 m²/s. At room temperature the hydrogen diffusion coefficient in MgH₂ is, thus, uncertain by about 17 orders of magnitude. This may be partially attributed to grain boundaries contributing to the measured diffusion coefficient. In this paper we use finite-element (FEM) simulations to evaluate the influence of the grain boundary diffusion on the measured total diffusion depending on the difference of the grain boundary (D_GB) and volume (D_V) diffusion coefficients, as well as on the grain size. These results will be compared to Harrisson's analytical solutions. When the diffusion coefficients differ by more than D_V < 10^?3·D_GB, Harrison's diffusion regime C becomes the best way to describe the total diffusion. The results are used to re-interpret literature data on hydrogen diffusion in MgH₂ from this grain boundary contribution point of view. At 300 K, a hydrogen grain boundary diffusion coefficient ranging from D_GB = 10^?17 m²/s to D_GB = 10^?20 m²/s, depending on the individual type of sample in MgH₂, results from the data evaluation.     

Effects of alloy compositions on hydrogen behaviors at a nickel grain boundary and a coherent twin boundary

The effects of several common metallic and nonmetal alloy compositions (i.e., Cr, Mo, B, and P) on the energetics and kinetics of hydrogen behaviors at a nickel grain boundary (GB) and a coherent twin boundary (CTB) were systematically investigated by first-principles calculations. H, Cr, Mo, B, and P have a stronger segregation into Ni GB than Ni CTB due to the presence of a cavity in GB. Cr, Mo, B, and P all act as obstacles for H segregation and diffusion in both GB and CTB, but the physical mechanisms are different: In Ni GB, Cr and Mo result in the shrinkage of isosurfaces of optimal charge densities for H, and B and P provide a strong competitive tendency to accumulate into the GB; in Ni CTB, Cr and Mo induce charge accumulation, and B and P result in a repulsive interaction to H. The present study provides the microscopic images of H compositions in Ni GB and CTB under the effects of alloy compositions; this is essential for understanding the mechanism of hydrogen embrittlement (HE) and improving the ability of alloys against HE.     

Extraction of ultrapure hydrogen with V-alloy membranes: From laboratory studies to practical applications

The membranes of vanadium alloys can be substantially more productive and much cheaper than the palladium alloy membranes which are widely used for hydrogen separation. However, the insufficient workability of vanadium alloys makes difficult producing the membranes of any shape, except for small thick flat samples, suitable only for laboratory studies, while membranes in the form of thin-walled tubes are most preferable for practical use. Such seamless thin-walled self-maintained membranes of tubular shape, made of V-Pd and V-Fe alloys with a thin Pd-coating on their inner and outer sides and with welded joints to stainless steel on both ends, were fabricated and tested for their throughput, service life, mechanical stability, and capability of hydrogen recovering from WGS mixtures. In view of reliable and highly productive operation of tubular V-alloy membranes as well as of their absolute selectiveness, the assembly from 18 membranes was fabricated to use it in the multi-fuel processor feeding a 1 kW PEM FC and providing up to 16 slpm of ultrapure hydrogen by the steam conversion of light and heavy hydrocarbons including diesel “Euro 5”. A similar assembly made of V-Fe alloy membranes was estimated for hydrogen purification in MOCVD applications.     

Carbon molecular sieve membranes supported on non-modified ceramic tubes for hydrogen separation in membrane reactors

Supported carbon membranes have been regarded as more competitive than traditional gas separation materials due to the versatile combination of different pyrolyzable polymers and supports which in turn leads to high separation factors and mechanical stability. In order to determine the extent to which supported carbon membranes are more competitive, the transport mechanism of supported carbon membranes was investigated in the range 32–150 °C and 1–2.5 bar. Polyimide (Matrimid 5218) material was coated and pyrolyzed under N₂ atmosphere on TiO₂-ZrO₂ macroporous tubes (Tami) that had not been structurally modified in any way. The supported carbon membrane was studied to determine its permeation for low molecular weight gases such as H₂, CH₄, CO, N₂ and CO₂. For these gases, the permeance of the composite supported carbon membranes obtained after pyrolysis at 550 °C increased with inverse square root of molecular weight. The temperature dependence of the permeance was described using an Arrhenius law with the negative activation energies for hydrogen, carbon dioxide and nitrogen providing evidence of a non-activated process. The ideal separation selectivity computed from single gas measurements leads to values slightly lower than the Knudsen because of the influence of viscous flow. The coexistence of more than one transport mechanism in the composite membrane was confirmed. After plugging the possible defects with Polydimethylsiloxane (PDMS), the supported carbon membranes obtained at a pyrolysis temperature of 650 °C showed evidence of a molecular sieving mechanism. This paper shows the separation properties of a crack-free supported carbon membrane obtained using a simple fabrication method that does not require modification of the mesoporous support. The permeance and selectivity values were compared with those of other hydrogen selective materials. Finally, the membranes were applied to methanol steam reforming (MSR).     

Influence of grain boundary misorientation on hydrogen embrittlement in bi-crystal nickel

Computational techniques and tools have been developed to understand hydrogen embrittlement and hydrogen induced intergranular cracking based on grain boundary (GB) engineering with the help of computational materials engineering. This study can help to optimize GB misorientation configurations by identifying the cases that would improve the material properties increasing resistance to hydrogen embrittlement. In order to understand and optimize, it is important to understand the influence of misorientation angle on the atomic clustered hydrogen distribution under the impact of dilatational stress distributions. In this study, a number of bi-crystal models with tilt grain boundary (TGB) misorientation angles (θ) ranging between 0°≤ θ ≤ 90° were developed, with rotation performed about the [001] axis, using numerical microstructural finite element analysis. Subsequently, local stress and strain concentrations generated along the TGB (due to the difference in individual neighbouring crystals elastic anisotropy response as functions of misorientation angles) were evaluated when bi-crystals were subjected to overall uniform applied traction. Finally, the hydrogen distribution and segregations as a function of misorientation angles were studied. In real nickel, as opposed to the numerical model, geometrically necessary dislocations are generated due to GB misorientation. The generated dislocation motion along TGBs in response to dilatational mismatch varies depending on the misorientation angles. These generated dislocation motions affect the stress, strain and hydrogen distribution. Hydrogen segregates along these dislocations acting as traps and since the dislocation distribution varies depending on misorientation angles the hydrogen traps are also influenced by misorientation angles. From the results of numerical modelling it has been observed that the local stress, strain and hydrogen distributions are inhomogeneous, affected by the misorientation angles, orientations of neighbouring crystal and boundary conditions. In real material, as opposed to the numerical model, the clustered atomic hydrogens are segregated in traps near to the TGB due to the influence of dislocations developed under the effects of applied mechanical stress. The numerical model predicts maximum hydrogen concentrations are accumulated on the TGB with misorientation angles ranging between 15°< θ < 45°. This investigation reinforces the importance of GB engineering for designing and optimizing these materials to decrease hydrogen segregation arising from TGB misorientation angles.     

Novel process for preparation of metal-polymer composite membranes for hydrogen separation

In the present work, a new preparation method for metal-polymer composite materials for hydrogen separation which consist of hydride-forming intermetallic compound LaNi₅ and polyethylene was developed. According to this technique, the mechanical activation of the initial powder mixtures was employed to provide good interface between the phases. A series of composite membranes with various filler concentrations was synthesized and characterized by X-ray diffraction, scanning electron microscopy and differential scanning calorimetry. The gas transport properties of the obtained materials in relation to H₂, O₂, N₂, CO₂ and CH₄ were tested. The results indicate that the addition of the hydride-forming intermetallic compound to the barrier polymer leads to significantly improved selectivity with respect to hydrogen. The proposed method can be considered as a promising approach to producing of high performance composite membranes for hydrogen separation.     

Susceptibility of different crystal orientations and grain boundaries of polycrystalline Ni to hydrogen blister formation

The effects of crystallographic orientations, grain boundaries (GBs) and the possible contribution of dislocations to the diffusion of hydrogen were studied. Our experimental results show that the (001) crystal orientation has the minimum number of hydrogen induced blisters compared to other crystal orientations. We observed formation of blisters along the slip traces after plastic deformation and along special GBs. It shows that the interaction between dissolved hydrogen and lattice defects (e.g. dislocations and GBs) could cause void formation and ultimately induce intergranular and/or transgranular cracks in nickel. In this work we analyzed hundreds of GBs mostly with a misorientation of less than 60°. It was observed that the random GBs with a misorientation between 30 and 40° have the fastest hydrogen diffusion rate and are very sensitive to hydrogen segregation. In contrast, random GBs with a misorientation below 25°, low angle GBs and the coincidence site lattice (CSL) GBs are much less prone to blister formation.     

Advances in hydrogen selective membranes based on palladium ternary alloys

Hydrogen containing a minimum amount of contaminants is required for its application in fuel-cell technology. For this purpose, palladium and palladium binary alloy membranes have been widely studied in the last decades due to their ability to selectively permeate hydrogen. The scope of this review is to provide an in-depth analysis of the research on Pd-based ternary alloys and their application as hydrogen separation membranes with a special focus on the PdAgAu, PdCuAg, and PdCuAu systems. The combination of these particular elements - Cu, Au, Ag - can improve hydrogen permeability and chemical resistance. Correlations between structural, surface and permeation properties of the ternary alloys under pure hydrogen and gas mixtures are extensively discussed. A general correlation between hydrogen permeability and the lattice parameter is proposed. In particular, the surface segregation behavior is analyzed for these ternary alloys even after being exposed to CO, CO₂, and H₂S. Further research is needed to develop membranes with improved long-term stability.     

Effect of aluminium acetyl acetonate on the hydrogen and nitrogen permeation of carbon molecular sieves membranes

With a growing interest in hydrogen as energy carrier, the efficient purification of hydrogen from gaseous mixtures is very important. This paper addresses the separation of hydrogen using Carbon Molecular Sieves Membranes (CMSM), which show an attractive combination of high permeability, selectivity and stability. Supported CMSM containing various amounts of aluminium have been prepared from novolac and aluminium acetyl acetonate (Al(acac)₃) as carbon and alumina precursors. The thickness of the CMSM layers depend on the content of Al(acac)₃ in the dipping solution, which also has influence in the pore size and pore size distribution of the membranes. The permeation properties of the membranes against the Al content in the membrane follows a volcano shape, where the membrane containing 4 wt (%) of Al(acac)₃ has the best properties and was stable during 720 h for hydrogen at 150 °C and 6 bar pressure difference. All the CMSM have permeation properties well above the Robeson Upper limit.     

Modelling of surface segregation for palladium alloys in vacuum and gas environments

Surface segregation of a series of forty Palladium-based binary alloys has been investigated using a thermodynamic model based on an atom exchange approach. Their surface segregation behaviour, both in vacuum and in gas environments, were comprehensively estimated. The calculated results are in good agreement with the available experimental and computational data reported in literatures. Effects of mixing enthalpy, temperature, crystal orientation on the surface, elastic strain energy, adsorption and absorption of gases like H₂, O₂, CO have been discussed in detail. These results can be considered as basic guidelines to design novel Pd alloys for hydrogen separation membranes, sensors or catalysts. The model itself also offers a convenient and accurate routine to predict the surface segregation of other than Pd-based binary alloys in different gas atmospheres.     

Molecular dynamics studies of the grain-size dependent hydrogen diffusion coefficient of nanograined Fe

Nanograined materials have much denser grain boundary (GB) networks than their coarse-grained counterparts, thereby the hydrogen (H) diffusion and trapping behaviors in nanograined materials, which are strongly influenced by GBs, may differ greatly from those in coarse-grained materials. In the present research, the grain-size dependent hydrogen diffusion coefficient in nanograined Fe is studied by theoretical analysis and molecular dynamics (MD) simulations. A theoretical model based on thermodynamics is developed. The GB-related material parameters required by the model are then obtained by fitting the MD simulation results. Finally, the grain-size dependent diffusion coefficients are compared with model predictions to evaluate the validity of the model. It is found that the trapping effect of triple junctions that usually ignored in coarse-grained materials becomes increasing important as grain size and temperature decrease. Due to the strong trapping effect of GBs in nanograined Fe, H diffusion is slowed down by the GBs.     

Novel process of grain boundary metallisation on mc Si solar cells

The electronic properties of multicrystalline silicon are heavily influenced by impurities concentrated along the grain boundaries that increase the recombination activity near the crystallite borders. Dopants can also diffuse preferentially down the grain boundaries, which leads to a low resistance path down the grain. These and other effects decrease the efficiency of multicrystalline silicon solar cells. Additionally, the efficiency is lowered by the shading of areas of silicon by metallisation lines due to the reduction of the active conversion area of the cell. We present a new way to combine the grain boundaries and the front contact grid with the aim to improve the efficiency of multicrystalline silicon solar cells. A first approach has been developed to produce multicrystalline silicon solar cells with a front contact metallisation following the grain boundaries: The different grain boundaries of a multicrystalline silicon wafer are detected by optical scanning of the wafer surface. Together with the emitter sheet resistivity this image serves as an input to calculate a net of finger lines that follow the grain boundaries wherever possible. Onto these detected grain boundaries the metallisation is performed by evaporative deposition of copper and photolithography. We report on the successful implementation of such a grid on 100×100 mm² wafers.     

A comparison of the performance and stability of Pd/BCC metal composite membranes for hydrogen purification

Composite membranes were fabricated by sputtering 100 nm of Pd on to both sides of dense BCC metal foils (V, Ta, Nb). Under pure H₂ gas testing at 500 °C the maximum permeability of all three metals exceeds previously reported values and closely approach theoretical limits. However, the stability of each membrane varied significantly due to unique failure mechanisms. Pd/V membranes failed quickly (<20 h) due to a combination of Pd–V interdiffusion and high susceptibility to oxidation as shown through microscopy and compositional analysis. The Pd/Ta membranes were the most resilient to oxygen, but their mechanical integrity was relatively poor and they failed within 48 h due to Pd–Ta interdiffusion. In contrast, Pd/Nb membranes exhibited high permeability throughout the 168 h of testing, with no Pd–Nb interdiffusion observed. The decline in permeability observed during testing was attributed to partial Pd delamination as a result of membrane deformation. These results provide pathways for further development of these membranes.     

Experimental evaluation of graphene oxide/TiO2-alumina nanocomposite membranes performance for hydrogen separation

In recent years, graphene oxide membranes showed interesting performances in terms of high permeating flux and perm-selectivity in several applications of gas separation because of their inherent properties combined to a low energy consumption. In this paper, a graphene oxide layer is coated on modified TiO₂-alumina tubular substrate in order to prepare graphene oxide nanocomposite membranes useful for hydrogen separation. Nanocomposite graphene oxide membrane samples were obtained by using vacuum deep coating method, depositing the graphene oxide solution as single layers on TiO₂-alumina substrate. Temperature and pressure variations were evaluated to achieve high H₂ permeance, high H₂/CO₂ selectivity and membrane performance stability during the experimental tests. Furthermore, it was found that the temperature increase causes a perm-selectivity (H₂/N₂ and H₂/CO₂) decrease, while the transmembrane pressure increase involves a general improvement of the perm-selectivity.     

Numerical study on effect of inclusions on hydrogen segregation in steel under stress conditions

It is generally believed that hydrogen embrittlement (HE) of metallic material is closely related to the presence of hydrogen traps such as inclusions inside. Therefore, the study of hydrogen segregation in the inclusions is essential for the further understanding of the mechanism of HE phenomena. In this study, a numerical model for exploring the hydrogen transport and hydrogen segregation in inclusions under stress conditions was developed, and the effect of inclusion's shape and orientation, micro-cracks and stress magnitude on hydrogen segregation was investigated. Results showed that, inclusion's shape made a great effect on the hydrogen concentration at the inclusions/matrix interface, which became higher as inclusion closed to a globular shape. The hydrogen concentration inside the inclusions was affected by both the shape of inclusions and the angle between the orientation of inclusions and the hydrogen diffusion direction. The hydrogen segregation in inclusions with micro-cracks was much greater than that without micro-cracks, which caused 3–4 times local hydrogen concentration increase in inclusions, leading to the enhancement of HE susceptibility of the steel. The magnitude of stress did not change the regions of hydrogen segregation, but the hydrogen concentration at each region increased with the increasing stress.