Alloying effects of Ru and W on hydrogen diffusivity during hydrogen permeation through Nb-based hydrogen permeable membranes

The hydrogen permeability have been measured for pure niobium and Nb-5 mol%X (X = Ru and W) alloys in order to investigate the alloying effects of ruthenium and tungsten on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, J·d, and the difference of hydrogen concentration, ΔC, between the inlet and the outlet sides of the membrane. It is found that the addition of ruthenium or tungsten into niobium increases the hydrogen diffusion coefficient during the hydrogen permeation. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion is decreased by alloying of ruthenium or tungsten into niobium.     

Improving hydrogen permeability and sustainability of Nb30Ti35Co35 eutectic alloy membrane by substituting Co using Fe

The microstructure and hydrogen permeation performance of Nb₃₀Ti₃₅Co_35-xFe_x (x = 0, 5, 10, 15, 20) alloys have been investigated. With Fe less than 15 at%, the as-cast Nb₃₀Ti₃₅Co_35-xFe_x ingots exhibit fully eutectic structure. When the Fe content is higher than 15 at%, primary bcc-(Nb, Ti) phase appears in combination with eutectic structure. Substituting Co using Fe leads to slightly increased hydrogen solubility but highly enhanced hydrogen permeability, which comes mainly from the increased hydrogen concentration-independent diffusion coefficient D1. With Fe content up to 10 at%, Nb₃₀Ti₃₅Co_35-xFe_x membranes exhibit stable and higher hydrogen permeation flux than Nb₃₀Ti₃₅Co₃₅ at 673 K during hydrogen permeation test up to 72 h. Nb₃₀Ti₃₅Co_35-xFe_x alloys of fully eutectic structures exhibit no hydrogen-induced failure when cooled down to room temperature under hydrogen atmosphere, indicating the potential application of the membrane at lower temperature range. An optimal combination of hydrogen permeability and hydrogen embrittlement resistance is achieved at 10 at% Fe, since further increase of Fe leads to comparable D1 as that of Nb₃₀Ti₃₅Co₂₅Fe₁₀ but higher hydrogen solubility.     

Time and frequency domain analysis of hydrogen permeation across PdCu metallic membranes for hydrogen purification

Hydrogen permeation is a process used in the industry for purification purposes. Palladium alloys (PdAg and PdCu) are commonly used as membrane material. In this communication, we report on the kinetics of hydrogen permeation across Pd_0.47Cu_0.53 metallic membranes which can be used in catalytic crackers of biofuels. The permeation mechanism is a multi-step process including surface chemisorption of molecular hydrogen (upstream side of the membrane), hydrogen diffusion across bulk regions, hydrogen recombination (downstream side of the membrane) and evolution. The role of different operating parameters (temperature, surface state, sample microstructure) is analyzed and discussed using both time and frequency domain experiments. Experimental pneumato-chemical impedance diagrams show that there is no significant rate-limitation at surfaces, except at low temperatures close to room temperature. Diffusion-controlled transport of hydrogen across the membrane is rate-determining. However, the value of the hydrogen diffusion coefficient does not rise exponentially with operating temperature in the 40–400 °C temperature range under investigation, as expected for a thermally activated diffusion process. At temperatures as low as 300 °C, new rate-limitations appear. They can be attributed to recrystallization and/or phase transformation processes induced by temperature and the presence of hydrogen.     

Study of the plasma and heating effect on hydrogen permeation through Pd0.60-Cu0.40 membrane in a micro-channel plate reactor

The diffusion of hydrogen through palladium and palladium-copper alloys membrane have been provided the highest hydrogen selectivity and permeance. In this study the composite Pd_0.60-Cu_0.40 wt% membrane foil with thickness 20 μm was measured in the micro-channel plate reactor (MPR) with gap length 4.5 mm. The hydrogen permeation flux was measured at atmospheric feeding pressure for 100% H₂ concentration in the temperatures range of 423–573 K under heating only and plasma-heating experiments. The plasma firing high voltage source ranges of 10–18 kV are tested. The hydrogen permeation flux and hydrogen permeability have been calculated according to Fick's and Sieverts combining laws with power exponent n-value 0.5. It was found that the maximum hydrogen flux, hydrogen permeability and Permeation rate percent of the heating only experiment at MPR heating temperature of 573 K and flow rate 0.1 l/min. In the plasma heating experiment, it was observed that the maximum hydrogen flux, hydrogen permeability, and permeation rate percent at MPR heating temperature of 573 K and plasma firing voltage of 14 kV. Also, the hydrogen permeation rate percent decreased due to the hydrogen reverse reaction even though the plasma firing voltage increased to 16 kV and 18 kV. The results also reveal that the activation energy and Pre-exponential constant factor decreased with increasing the feeding H₂ flow rate while the linear regression R² decreased with increasing H₂ feeding flow rate that in the heating only experiment, in contrast, the plasma-heating experiment showed non-linearity values. A comparison between both experiments showed the hydrogen permeation flux of the plasma-heating experiment is higher than that obtained from the heating only experiment, additionally; the plasma effect increased at low hydrogen flow rates. In contrast, the energy efficiency of heating only experiment was higher than that obtained from the plasma-heating experiment due to the total energy consumption of plasma experiment is high.     

Role of microstructure on electrochemical hydrogen permeation properties in advanced high strength steels

The hydrogen permeation process in steels is closely associated with the microstructure of steels that greatly affect hydrogen trapping and hydrogen diffusion behaviors. In this study, the electrochemical hydrogen permeation experiment using a modified Devanathan-Stachurski (D-S) cells was employed to evaluate the hydrogen permeation properties in advanced high strength steels with four types of microstructures (from single phase, dual phase to complex phase). Results showed that both phase interfaces and retained austenite (RA) could act as the trapping sites for hydrogen and consequently reduced the hydrogen diffusion coefficient in steels. Furthermore, it was suggested that the role of RA on hydrogen trapping behaviors depended on its morphology. Finally, the lattice diffusion coefficient (D_L) in each steel was determined and the correlations between the microstructure in steels and hydrogen evolution reaction (HER) kinetics were also investigated.     

Computational analysis of geometrical factors affecting experimental data extracted from hydrogen permeation tests: I - Consequences of trapping

Electrochemical permeation tests enable the experimental determination of the diffusion coefficient of a metal. To get a better understanding and a correction of experimental measures, we investigated the effects of hydrogen trapping on the diffusion of hydrogen through a metallic membrane by simulating a FEM model. The trap binding energy ΔE_T ranges from −0.1 to −0.32 eV, the density of traps ranges between 10⁻⁴ and 100 mol/m³, and the thickness of the membrane fluctuates from 100 μm to 1 mm. It appears that the effective diffusion coefficient extracted from desorption flux data of a single membrane is not influenced by its geometry and depends on both the density of trapped hydrogen and the trap binding energy such as the apparent diffusion coefficient implemented in the code. Thus we do not detect any scale effect. In the other hand, the effective subsurface concentration evaluation using usually Fick’s laws doesn’t correspond directly to hydrogen concentration in the membrane. Analytical equations to solve the problem to extract erroneous data (diffusion coefficient and hydrogen concentration) to the experimental measurements of the flux vs time curves have been proposed.     

Influence of microstructure on hydrogen trapping and diffusion in a pre-deformed TRIP steel

Austenitic steels are known to exhibit a low hydrogen diffusion coefficient and hence a good resistance to hydrogen embrittlement. Therefore, it is an experimental challenge to investigate their hydrogen diffusion properties. In this study, the electrochemical permeation technique is used to determine the hydrogen diffusion coefficients in different pre-deformed states (φ = 0, 0.32, 0.39, 0.49) of the high-alloy austenitic TRIP steel X3CrMnNiMoN17-8-4 in a temperature range of 323 K–353 K. In combination with microstructural analysis, a correlation between phase transformation from γ-austenite to α′-martensite and dislocation density is shown. As a result of the lattice transformation from fcc to bcc, the diffusion rate of hydrogen is significantly increased (D_{app, φ = 0} = 3.6 × 10^?12 cm² s^?1, D_{app, φ = 0.32} = 1.6 × 10^?11 cm² s^?1at 323 K). With higher degrees of deformation, the dislocation density also increased in the martensite islands, resulting in a degressive growth of the diffusion coefficient (D_{app, φ = 0.39} = 5.3 × 10^?11 cm² s^?1, D_{app, φ = 0.49} = 1.1 × 10^?10 cm² s^?1at 323 K). Moreover, detailed calculations are performed to describe the way of hydrogen trapping and to give a possible mechanism of diffusion.     

Superior hydrogen absorption and desorption behavior of Mg thin films

Pd-capped Mg films prepared by magnetron sputtering achieved complete dehydrogenation in air at room temperature and behaved as favorable gasochromic switchable mirrors. Their cyclic hydrogen absorption and desorption kinetics in air were investigated by using the Bruggeman effective medium approximation. The overall activation energy was 80 kJ mol⁻¹, while the reaction orders controlling desorption were deduced to be n = 2 at 328 K and n = 1 at lower temperatures by analyzing the transmittance data. The hydrogen diffusion coefficient and the corresponding activation energy were calculated by electrochemical measurements. Mg thin films exhibited the smaller activation energy and remarkable diffusion kinetics at room temperature which implied potential applications in smart windows.     

Novel non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation

This study presents a new non-alloy Ru/Pd composite membrane fabricated by electroless plating for hydrogen separation. It shows that palladium and ruthenium can be deposited on an aluminum-oxide-modified porous Hastalloy by using our new EDTA-free plating bath at room temperature and 358 K, respectively. A 6.8 μm thick non-alloy Ru/Pd membrane film could be plated and helium leak test confirmed that the membrane was free of defects. Hydrogen permeation test showed that the membrane had a hydrogen permeation flux of 4.5 × 10⁻¹ mol m⁻² s⁻¹ at a temperature of 773 K and a pressure difference of 100 kPa. The hydrogen permeability normalized value with thickness of the membrane was 1.4 times higher than our pure Pd membrane having similar structure. The EDX profiles of the front and back side membrane, cross-sectional EDX line scanning and XRD profile show that there was no alloying progress between the palladium and ruthenium layer after hydrogen permeation test at 773 K.     

Estimation of hydrogen crossover through Nafion membranes in PEMFCs

It is well known that the membrane electrode assembly (MEA) of proton exchange membrane fuel cells (PEMFCs) can undergo deterioration, during long term operation, of both the electrode materials and the membrane. Hydrogen crossover, i.e., the undesired diffusion of the gas from the anode to the cathode through the membrane, has been ascribed as one of the main causes of deterioration of perfluorinated ionomer membranes, normally employed in PEMFCs. One of the effects of the hydrogen permeation across the membrane is the decrease of the cell's open circuit voltage (OCV), due to the reaction between the fuel and the oxidant at the cathode surface. Such reaction can lead to the production of peroxide radicals, causing the degradation of both the PEM and the catalyst layer. Hydrogen crossover increases when temperature, pressure and humidity of the cell rise. The hydrogen permeation rate through a very thin PEM is typically lower than 1 mA cm⁻² for a new MEA, but it can exceed 10-20 mA cm⁻² after long term operation. Various methods have been proposed to measure the rate of hydrogen crossover, mainly based on electrochemical tests on a single FC with a flow of nitrogen at the cathode, so that the steady state current corresponds to the oxidation of crossed hydrogen. Hydrogen crossover has been also determined indirectly by assuming that the changes in the OCV values are due to the passage of fuel from the anode to the cathode.In this paper, a simplified mathematical model for the direct determination of hydrogen crossover permeation rate is presented. Such a model is based on analytical expressions of the polarization terms and it is employed to determine the hydrogen crossover rate. The main results show that the hydrogen crossover current densities increased from 0.12 to 0.32 mA cm⁻², by decreasing the thickness of the membranes and increasing the operating cell temperature. Moreover, the hydrogen crossover determined for a fresh MEA was compared with that of a degraded one, exposed to repetitive freezing/thawing cycles. It was found that the hydrogen crossover for the degraded MEA was more than twice the value obtained with the fresh one at the same temperature.     

Effect of Mn on the long-term cycling performance of AB5-type hydrogen storage alloy

Rare-earth AB₅-type hydrogen storage alloys are widely studied due to their extensive application potentials in hydrogen compressors, heat pump, Ni–MH batteries etc. However, their shortcomings such as plateau splitting and capacity degradation during hydrogen absorption/desorption hinder their practical applications. In this paper, we study the effect of Mn partial substitution for Ni on the plateau characteristics and long-term cycling performance of LaNi_5-xMn_x alloys. It is found that Mn addition expands the lattice interstitial for hydrogen accommodation, thus prohibiting the plateau splitting phenomenon. In addition, the substitution of Mn for Ni stabilizes the crystal structure of the alloys against hydrogen absorption/desorption, thus relieving the capacity degradation. The capacity retention of the alloys at the 1000th cycle (S₁₀₀₀) increases from 83.2% (x = 0) to 94.0% (x = 0.75). But when x reaches 1, the hydrogen desorption reversibility is reduced due to the low plateau pressure, resulting in a slight decrease in capacity retention.     

Effective hydrogen diffusion coefficient for CoCrFeMnNi high-entropy alloy and microstructural behaviors after hydrogen permeation

Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of stainless steels (SS) 304 and 316L. HEA and SS 316L showed similar effective hydrogen diffusion coefficient of 1.75 × 10⁻¹¹ m²/s and 1.91 × 10⁻¹¹ m²/s, respectively. SS 304 showed the smallest that of 0.58 × 10⁻¹¹ m²/s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and SS 316L; however, it caused a martensitic transformation in SS 304.     

The effects of hydrogen-induced lattice distortion and Nb-D formation on its permeation through niobium

Abnormal permeation behavior of hydrogen through niobium has been investigated in this paper, i.e. the permeation flux saturated with long-term decrease after reaching a maximum. The diffusivity and permeability have been deduced from the decay edge of permeation transient. Three kinds of polycrystalline niobium foils with different annealing temperature have been compared, to verify the effect of defects and grain properties on the permeability and diffusivity. In the temperature range of (773–1023) K, the heat treatment along with the permeation cycles could either reduce or increase the permeability and diffusivity depending sensitively on temperature and showing a temperature threshold around 950 K. The permeation flux is proportional to square root of pressure, revealing that the abnormal permeation was still bulk diffusion-limited. The diffusivity gradually decreased with permeation cycles, and became more and more sensitive to pressure. The niobium foil expanded macroscopically along the gradient of hydrogen concentration, which reveals the strong and unrecoverable lattice distortion in this temperature and pressure range. The X-ray diffraction studies showed that splitting of all the Nb peaks and shifting of Nb-D peaks along with hydrogen loadings. The phase transition was expected to eliminate the lattice strain during hydrogen loading and which in turn acted as a diffusion barrier.     

Study on the hydrogen barrier performance of the SiOC coating

SiOC coatings were prepared on X70 pipeline steel substrate by a simple dipping method at low temperatures, and their performance of hindering hydrogen penetration was studied through electrochemical hydrogen permeation experiment. The sample thermal-treated at 120 °C achieved a low diffusion coefficient of hydrogen of 8.20 × 10^?9 cm² s^?1, which was nearly three orders of magnitude lower than 3.58 × 10^?6 cm² s^?1 for the X70 steel. This was due to that the amorphous coating did not provide a stable hydrogen diffusion channel, thus limiting hydrogen diffusion. Density functional theory (DFT) calculation further proved that hydrogen moleculars were difficult to be adsorbed at different sites on the surface of the coating.     

Preparation and hydrogen penetration performance of TiO2/TiCx composite coatings

Titanium carbide is a good candidate for tritium permeation barrier in a fusion reactor. However, its oxidation susceptibility and the mismatch between the ceramic coating and substrate are still a challenge. In this study, a promising candidate as a hydrogen permeation barrier, comprising a titanium-based ceramic TiO₂/TiC_x composite coating, was proposed. The preparation process of this TiO₂/TiC_x composite coating involves two steps of carbon ion implantation and oxidation under ultra-low oxygen partial pressure. According to the results, the optimal oxidation temperature for TiO₂ coating is 550 °C, with the increase of the oxidation temperature, the particles on the surface of the oxide layer become coarse and loosely arranged, and the protective performance of the oxide layer is greatly reduced. The hydrogen barrier permeation behavior of the composite coating in a fusion reactor was simulated via hydrogen plasma discharge environment, the results show that the hydrogen barrier permeation performance of the composite is significantly better than that of a single TiO₂ coating. In addition, the coatings treated with hydrogen plasma showed a certain self-repairing performance through the diffusion growth of the TiC_x layer. These findings illustrate a novel method for preparing composite coatings to restrain hydrogen permeation, providing insight into the development of hydrogen permeation barrier materials.     

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.     

Hydrogen microscopy – Distribution of hydrogen in buckled niobium hydrogen thin films

Hydrogen absorption in thin metal films clamped to rigid substrates results in mechanical stress that changes the hydrogen's chemical potential by Δμ_H(σ) = −1.124σ kJ/mol_H for σ measured in [GPa]. In this paper we show that local stress relaxation by the detachment of niobium hydrogen thin films from the substrate affects the chemical potential on the local scale: using coincident proton–proton scattering at a proton microprobe, the hydrogen concentration is determined with μm resolution, revealing that hydrogen is not homogenously distributed in the film. The local hydrogen solubility of the film changes with its local stress state, mapping the buckled film fraction. In niobium hydrogen thin films loaded up to nominal concentrations in the two-phase coexistence region, the clamped film fraction remains in the solid solution phase, while the buckles represent the hydride phase. These results are compared to a simple model taking the stress impact on the chemical potential into account.     

Effect of single atomic layer graphene film on the thermal stability and hydrogen permeation of Pd-coated Nb composite membrane

Pd coated Nb-base composite membranes are preferable in the fields of hydrogen permeation. However, the rapid reduction of hydrogen permeability caused by high-temperature interfacial diffusion of Pd and Nb atoms hinders their large-scale application. In this paper, a single atomic layer graphene film was used for improving the thermal stability of a hydrogen-permeable composite membrane comprising a Pd coating on the Nb substrate. First, the graphene film was transferred onto the surface of the “clean” niobium substrate. Then a thin palladium coating was deposited on it by magnetron sputtering to form the niobium/graphene/palladium (Nb/Gr/Pd) composite membrane. The interfacial stability was evaluated in the temperature range of 673–973 K under vacuum, and the hydrogen permeation behavior was studied by gas-driven permeation method at 573–823 K. The results show that the single atomic layer graphene film can effectively compress the interdiffusion of Pd coating and Nb substrate and achieve a good hydrogen permeability below 823 K. However, it would be broken due to the micro-deformation of Nb substrate, the high mobility of Pd atoms, and the grain growth at a higher temperature. Therefore, it is concluded that the single atomic layer graphene film is unsuitable as an intermediate hindering layer for Nb-based hydrogen-permeable membranes.