Activation Energy For Hydrogen Diffusion Research Articles

Hydrogen dynamics features in BaZr1 − x Sc x O3 − x/2(OH) y : high-temperature 1H NMR studies

The 1H nuclear magnetic resonance measurements were carried out to study proton dynamics features in the hydrated scandium-doped barium zirconate, BaZr1 − xScxO3 − x/2(OH)y, with x = 0.2 and 0.4 in the temperature range 300–600 K. The obtained data evidence a fast proton motion in both samples, which is characterized by the jump frequency of about 108 s−1 at 450 K. However, the microscopic nature of hydrogen diffusion in these two samples is quite different. For the sample with x = 0.2, hydrogen motion mechanisms are defined by a rapid chemical exchange between Sc–OH–Zr and Zr–OH–Zr positions. The estimated value of activation energy for hydrogen diffusion of about 0.5 eV is determined by the energy barrier produced by Sc3+ ion. The increase of Sc concentration to x = 0.4 leads to the drastic changes of sample properties. Experimental results allow to assume the formation of nanoscaled Sc-rich domains and the decomposition of hydrogen sublattice with formation of two proton subsystems: the protons in Zr–OH–Zr coordinations and those concentrated in Sc–OH–Sc environments. The proton motion in both these subsystems is rather fast, but the chemical exchange between them is highly suppressed. Hydrogen motion inside Sc-rich environments has most likely localized nature. Our estimates yield the energy values of about 0.25 and 0.55 eV for hydrogen motion in “free lattice” and in Sc-rich clusters, respectively. The Sc-rich domains can retain hydrogen up to 600 K.

Effect of dissolved oxygen on hydrogenation of vanadium and hydrogen diffusion in the monohydride phase

We elucidate the effect of dissolved oxygen on the phase transformation and the diffusion of hydrogen in the V–H system. V forms monohydride (β) and dihydride (γ) phases by hydrogenation but a small amount of dissolved oxygen (O/V ≈ 0.01) leads to formation of another β phase expanded along the c-axis in a body centered tetragonal structure (BCT) rather than the γ phase. This change in the phase transformation is due to trapping hydrogen atoms by oxygen atoms, resulting in the shift of site occupation of hydrogen from octahedral (O) sites to tetrahedral (T) sites. Proton nuclear magnetic resonance (1H NMR) demonstrates that the activation energy for hydrogen diffusion is increased by the trapping effect. Oxygen dissolved in V suppresses not only the formation of the γ phase but also the diffusion of hydrogen.

Influence of Ta and Nb on the hydrogen absorption kinetics in Zr-based alloys

The kinetics of hydrogen absorption in Zr alloys containing Nb and Ta admixtures (10 wt%Nb, 12 wt%Ta and 10 wt%Nb&12 wt%Ta) is addressed. Hydrogen absorption is measured in the temperature range 400–700 °C at hydrogen pressure 1 bar using the volumetric method, and a kinetic analysis is performed to determine the mechanisms and rates of hydrogen absorption. To get further insight into hydrogen diffusivity in zirconium hydride, 1H NMR spectroscopy is used to provide the activation energies for proton jumping in the studied samples. The tantalum is found to dominantly influence the surface reactivity, while niobium influences large decrease of the activation energy for hydrogen diffusion in the bulk of zirconium hydride. This behavior is linked to the information on the structure of the alloys obtained by XRD.

Kinetics of Hydrogen Diffusion in Ti-6Al-4V Alloy

Abstract The diffusion kinetics of hydrogen in Ti-6Al-4V alloy was investigated by the gaseous hydrogen charging at the initial hydrogen pressures of 0.012, 0.04 and 0.07 MPa and the temperatures of 800, 850 and 900 °C. The results show the variation of the average hydrogen concentration in the alloy with hydrogen charging time exhibits an exponential characteristic relationship at different initial hydrogen pressures or hydrogen charging temperatures. The hydrogen charging process is controlled by the diffusion of hydrogen atoms in Ti-6Al-4V alloy. Based on the solution of diffusion equation, the apparent activation energy of hydrogen diffusion in Ti-6Al-4V alloy is estimated to be 32.80 kJ/mol. This energy is the activation energy of hydrogen diffusion in β phase of Ti-6Al-4V alloy.

Stabilization of the ordered structure of thin condensed foil of the Pd-Cu solid solution in a hydrogen environment

Structural transformations of thin (≈4 µm) Pd-Cu alloy foil fabricated by magnetic sputtering of a target with the composition corresponding to the highest temperature of the existence of the ordered structure (β phase, a CsCl lattice) have been studied in the course of heating and cooling in vacuum or in a hydro� gen atmosphere. Initial foil has an ordered structure (β phase), a gradient grain substructure, and domi� nant 〈211〉 texture. In vacuum, phase disordering (the β → α transi� tion) occurs within the range 773-873 K, i.e., within the boundaries of the equilibrium phase diagram of the system. This is accompanied by collective recrystalli� zation that eliminates the gradient of the grain sub� structure and by appearance of 〈110〉 texture of the β phase. Annealing in a hydrogen environment led to an increase of the existence range of a singlephase structure (β phase) by nearly 200 K: at 1123 K, the intensity of the 111α reflections was ≈60% of the 110β intensity. In this paper, we report for that first time that annealing of the Pd-Cu alloy in a hydrogen environ� ment extends the temperature range of the existence of the ordered structure with a CsCltype lattice. The Pd-Cu solid solutions are among alloys rec� ommended for fabrication of selective membranes for deep purification of hydrogen (1, 2). In this aspect, the possibility of solid solution ordering to form the β phase (CsCl lattice), shown by the phase diagram (3), is of special interest since the activation energy of hydrogen diffusion in the ordered Pd-Cu alloy (0.035 eV) is almost one order of magnitude lower than in the disordered solid solution (0.325 eV) and in palladium (0.23 eV) (4). As follows from the studies of kinetics of phase transformations that accompany the formation of the ordered structure of the Cu-49 at % Pd alloy (5-7), the degree of ordering essentially depends on the preliminary deformation of the samples and their subsequent heat treatment. Longterm heat treatment after intensive plastic deformation afforded a nearly singlephase ordered structure (7). It has been demonstrated that singlephase ( β phase) condensed foil can be fabricated by magnetic sputtering of a Pd- Cu alloy target of the corresponding composition (8, 9). Studying the hydrogen permeability of Pd-Cu foil has confirmed the expected result of ordering: specific hydrogen permeability of ordered alloy membranes is ten or more times higher than that of membranes made of the alloy with a disordered structure (10). In this context, it is of interest to study the effect of the major component of the membrane environment (hydrogen gas) on the temperature range in which the ordered structure of a thin Pd-Cu alloy foil persists.

The Effect of Stoichiometry on Hydrogen Embrittlement of Ordered Ni3Fe Intermetallics

The effects of Fe stoichiometry on hydrogen embrittlement and hydrogen diffusion in ordered Ni3Fe intermetallics were investigated. The experimental results show that the ordered Ni3Fe alloy with the normal stoichiometry has the lowest mechanical property, the highest susceptibility to hydrogen, and the highest ability of catalytic reaction. The mechanical properties, the susceptibility to hydrogen embrittlement, and the amount of adsorbed hydrogen of the ordered Ni3Fe alloy are dependent of degree of order of the alloy. The apparent hydrogen diffusion coefficient of the ordered Ni3Fe alloy is independent on degree of order of the alloy but depends on Fe stoichiometry. The activation energy of hydrogen diffusion decreased linearly with Fe stoichiometry for the ordered Ni3Fe alloy.

Enhancement of hydrogen diffusion in the body-centered tetragonal monohydride phase of the V–H system by substitutional Al studied by proton nuclear magnetic resonance

The diffusion and site occupation of hydrogen in the monohydride phase (β phase) of the V1−xAlx–H (x⩽0.1) system was studied by means of proton nuclear magnetic resonance (1H NMR). Hydrogen atoms which occupied the octahedral (Oz) sites along the c-axis in a body-centered tetragonal structure shifted to the tetrahedral (T) sites with the Al content. V0.90Al0.10H0.68 had a body-centered cubic structure and the component observed in the temperature dependence of 1H spin-lattice relaxation time was due to the contribution of hydrogen on the T sites. The addition of Al reduced the site radius of the Oz sites and resulted in reducing the activation energy for hydrogen diffusion, EH, for the Oz sites. Moreover, the diffusion of hydrogen was enhanced by a shift of site occupation from the Oz sites to the T sites, which had lower EH than the Oz sites. The addition of larger substitutional Al atoms than V enhanced the diffusion of hydrogen in the β phase of V–H.

Molecular Dynamics Study of Hydrogen in α-Zirconium

Molecular dynamics approach is used to simulate hydrogen (H) diffusion in zirconium. Zirconium alloys are used in fuel channels of many nuclear reactors. Previously developed embedded atom method (EAM) and modified embedded atom method (MEAM) are tested and a good agreement with experimental data for lattice parameters, cohesive energy, and mechanical properties is obtained. Both EAM and MEAM are used to calculate hydrogen diffusion in zirconium. At higher temperatures and in the presence of hydrogen, MEAM calculation predicts an unstable zirconium structure and low diffusion coefficients. Mean square displacement (MSD) of hydrogen in bulk zirconium is calculated at a temperature range of 500–1200 K with diffusion coefficient at 500 K equals 1.92 * 10−7 cm2/sec and at 1200 K has a value 1.47 * 10−4 cm2/sec. Activation energy of hydrogen diffusion calculated using Arrhenius plot was found to be 11.3 kcal/mol which is in agreement with published experimental results. Hydrogen diffusion is the highest along basal planes of hexagonal close packed zirconium.

Formation of thin foil of the ordered Pd-Cu solid solution with a CsCl-type lattice during magnetron sputtering

Among Pdbased membrane alloys (1), the Pd-Cu system has long attracted attention for deep hydrogen purification. This system is distinguished by the for� mation of a CsCltype ordered solid solution ( β phase) (2) in a rather narrow composition range close to Pd- 40 wt % Cu. It is accepted (3) that the CsCltype lattice, as compared to the less dense fcc lattice of the disordered solid solution ( α) phase), and, respectively, the shorter distance between octahedral voids (involved in hydro� gen diffusion) are responsible for a lower barrier for diffusion and a manyfold enhancement of hydrogen permeability. According to the results of numerous experimental studies summarized in (3), the hydrogen diffusion activation energy is 0.035 eV in the ordered solid solution, 0.325 eV in the disordered solid solu� tion, and 0.23 eV in palladium. The hydrogen diffu� sion coefficient at 300 K in the β phase is almost four orders of magnitude higher than in the α phase and two orders of magnitude higher than in Pd (3). Thus, the first, major, way of improving the perfor� mance of a Pd-Cu alloy membrane is to increase hydrogen permeability through ordering of the solid solution. The kinetics and mechanism of solid solu� tion ordering have been studied for more than three

Design and Development of Nb-W-Mo Alloy Membrane for Hydrogen Separation and Purification

The concept for alloy design of Nbbased hydrogen permeable membrane is applied to NbWMo ternary system. The alloying effects of tungsten and molybdenum on the solubility of hydrogen, the resistance to hydrogen embrittlement, the hydrogen permeability and diffusivity are investigated in a fundamental manner. It is found that the addition of tungsten and molybdenum into niobium decreases the hydrogen solubility. As a result, the resistance to hydrogen embrittlement improves and higher hydrogen pressures can be applied to the NbWMo alloy membrane. It is shown that the designed Nb5mol%W5mol%Mo alloy membrane with single solid solution phase exhibits excellent hydrogen permeability together with strong resistance to hydrogen embrittlement. In addition, it is found that the alloying of tungsten and molybdenum with niobium enhances the hydrogen diffusivity. In fact, the activation energy for hydrogen diffusion decreases in the order, pure Nb > Nb5mol%W > Nb5mol%W5mol%Mo.

Effect of substitutional Cr on hydrogen diffusion and thermal stability for the BCT monohydride phase of the V–H system studied by 1H NMR

The diffusion of hydrogen in the monohydride phase (β phase) of the V1−xCrx–H (x≤0.1) system was studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in the hydrides occupied the octahedral (O) sites in a body centered tetragonal (BCT) structure. The BCT lattice was contracted by addition of Cr because Cr has a smaller atomic radius than V. The activation energy for hydrogen diffusion, EH, and the Korringa constant, T1eT, were estimated from the temperature and frequency dependence of 1H spin–lattice relaxation times, T1. The value of EH increased with increase in the Cr content. The contraction of the BCT lattice by addition of Cr suppressed the diffusion of hydrogen which occupied the interstitial sites. Addition of Cr also increased the T1eT, which was accompanied by reducing in the stability of the monohydride phase of the V–H system.

Hydrogen adsorption, absorption and diffusion on and in transition metal surfaces: A DFT study

Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydrogen with different facets of seventeen transition metals—the (100) and (111) facets of face-centered cubic (fcc) metals, the (0001) facet of hexagonal-close packed (hcp) metals, and the (100) and (110) facets of body-centered cubic (bcc) metals. Calculated geometries and binding energies for surface and subsurface hydrogen are reported and are, in general, in good agreement with both previous modeling studies and experimental data. There are significant differences between the binding on the close-packed and more open (100) facets of the same metal. Geometries of subsurface hydrogen on different facets of the same metal are generally similar; however, binding energies of hydrogen in the subsurface of the different facets studied showed significant variation. Formation of surface hydrogen is exothermic with respect to gas-phase H2 on all metals studied with the exception of Ag and Au. For each metal studied, hydrogen in its preferred subsurface state is always less stable than its preferred surface state. The magnitude of the activation energy for hydrogen diffusion from the surface layer into the first subsurface layer is dominated by the difference in the thermodynamic stability of these two states. Diffusion from the first subsurface layer to one layer further into the bulk does not generally have a large thermodynamic barrier but still has a moderate kinetic barrier. Despite the proximity to the metal surface, the activation energy for hydrogen diffusion from the first to the second subsurface layer is generally similar to experimentally-determined activation energies for bulk diffusion found in the literature. There are also some significant differences in the activation energy for hydrogen diffusion into the bulk through different facets of the same metal.

Electrochemical performances of Mm 0.7Mg xNi 2.58Co 0.5Mn 0.3Al 0.12 ( x = 0, 0.3) hydrogen storage alloys in the temperature range from 238 to 303 K

A series of experiments have been performed to investigate electrochemical properties of Mm 0.7Mg x Ni 2.58Co 0.5Mn 0.3Al 0.12 ( x = 0, 0.3) alloy at various temperatures (238 K, 273 K and 303 K). The results indicate that both alloy electrodes exhibit high dischargeabilities after elemental substitution, above 320 mAh g −1 even at 238 K. The capacity degradation of the two alloys are primarily ascribed to serious pulverization, other than the oxidation of active components at the initial stage. Moreover, the electrochemical performances of Mm 0.7Mg x Ni 2.58Co 0.5Mn 0.3Al 0.12 ( x = 0, 0.3) alloy electrodes depend on the alloy type and testing temperature. Mm 0.7Mg 0.3Ni 2.58Co 0.5Mn 0.3Al 0.12 alloy, consisting of LaNi 5-phase and La 2Ni 7-phase, shows better properties of discharge capacity, cyclic stability, self-discharge and pulverization resistance at the three temperatures than those of single LaNi 5-phase Mm 0.7Ni 2.58Co 0.5Mn 0.3Al 0.12 alloy. The electrochemical kinetics studies indicate that the activation energy of hydrogen diffusion and exchange current density ( I 0) of Mm 0.7Mg 0.3Ni 2.58Co 0.5Mn 0.3Al 0.12 alloy are lower than those of Mm 0.7Ni 2.58Co 0.5Mn 0.3Al 0.12 alloy. When the temperature increases from 238 to 303 K, the capacity loss, high-rate dischargeability, exchange current density I 0 and hydrogen diffusion coefficient ( D/ a 2) of the two alloys increases, while capacity retention decreases. Further analysis of kinetics suggests that bulk hydrogen diffusion is the rate-determining step of the battery reaction at low temperature 238 K, and charge-transfer reaction on alloy surface is the rate-determining step when tested at 273 K and 303 K for both alloys. The perfect low temperature discharge capacities of the two alloys can mainly attribute to the decrease of activation energy for hydrogen diffusion after elemental substitution.

Hydrogen diffusivity in B-doped and B-free ordered Ni 3Fe alloys

The hydrogen diffusion coefficient of the ordered Ni 3Fe–B alloys with and without boron additions was measured by a method of the cathodical precharging with hydrogen. The apparent hydrogen diffusion coefficient decreases with increasing the boron concentration doped in the ordered Ni 3Fe alloy. Comparing with the B-free ordered Ni 3Fe alloy, the activation energy of hydrogen diffusion for the ordered B-doped Ni 3Fe alloy increases by as high as 42% when the boron content is sufficient. The doping boron in the Ni 3Fe alloy is effective in reducing the hydrogen diffusion at the grain boundary.

Effect of substitutional Mo on diffusion and site occupation of hydrogen in the BCT monohydride phase of V–H system studied by 1H NMR

The diffusion and site occupation of hydrogen in the monohydride phase of V 1− x Mo x –H (0 ≤ x ≤ 0.1) were studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in VH 0.68 occupied the octahedral ( O) sites in a body centered tetragonal (BCT) structure. The addition of Mo to V reduced the activation energy for hydrogen diffusion, E H, for the O sites. In V 0.9Mo 0.1H 0.68 hydrogen atoms occupied both the O and tetrahedral ( T) sites, which was demonstrated by two components observed in the temperature dependence of the 1H spin-lattice relaxation time. The value of E H for the T sites was lower than that for the O sites. Hydrogen atoms in V 0.9Mo 0.1H 0.68 diffused faster than those in VH 0.68.

Alloying Effects on the Hydrogen Diffusivity during Hydrogen Permeation through Nb-Based Hydrogen Permeable Membranes

The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects 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, , and the difference of hydrogen concentration, C, between the inlet and the outlet sides of the membrane. It is found that the hydrogen diffusion coefficient during the hydrogen permeation is increased by alloying ruthenium or tungsten into niobium. 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 decreases by the addition of ruthenium or tungsten into niobium.

Structural evolution and electrochemical hydrogenation behavior of Ti 2Ni alloy

The structure, morphology, and electrochemical hydrogenation behavior of Ti 2Ni alloy, prepared by an induction melting method, have been systematically investigated. The results of X-ray diffraction showed that hydrogen absorption led to a significant inelastic lattice expansion of the alloy. The alloy had a maximum discharge capacity of 235.5 mAh/g at a current density of 60 mA/g and a high rate dischargeability of 73.3% at 300 mA/g. The increase in the electrolyte temperature induced a decrease in discharge capacity, a decay of cycle life, an increase and subsequently a decrease in exchange current density, and an enhancement of hydrogen diffusion. Moreover, the alloy with a smaller particle size showed a higher discharge capacity. The mechanism for the capacity loss of the alloy during charge and discharge cycling was studied. The apparent activation energy for hydrogen diffusion within the bulk alloy was 10.7 kJ/mol.

HYDROGEN ADSORPTION IN ORDERED MESOPOROUS CARBON SYNTHESIZED BY A SOFT-TEMPLATE APPROACH

An ordered mesoporous carbon was synthesized with soft-template approach as a potential adsorbent for hydrogen storage through physical adsorption. The carbon adsorbent prepared in this work has a Brunauer-Emmett-Teller (BET) specific surface area of 798 m2/g, uniform pore size distribution with an average pore diameter of 6.26 nm, and pore volume of 0.87 cm3/g. Hydrogen adsorption equilibrium and kinetics in the carbon adsorbent were measured in a volumetric adsorption apparatus at 77,194.5, and 298 K. A hydrogen adsorption capacity of 1.27 wt % was observed at 1.05 bar and 77 K, hydrogen diffusivity in the carbon is estimated to be 1 × 10−6 cm2/s at 194.5 K, and activation energy for hydrogen diffusion in the carbon is 0.236 kJ/mol. The isosteric heat of adsorption for hydrogen in the carbon adsorbent is between 3 and 6 kJ/mol, which is in the same range of the heat of adsorption of hydrogen measured on other carbonaceous materials.

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, Δ C, 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.

Anodic Polarisation Characterisation of Absorbed Hydrogen in Modified Mm(B<sub>5</sub>)<sub>x</sub> Alloys

Anodic polarisation curves of Mm(Ni3.6Co0.7Mn0.4Al0.3)x (Mm = mischmetal, 0.85  x  1.15) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), potential sweeping rates (v), temperatures (T), and amounts of reducing agent (y = [NaBH4]) in alkaline solution. Anodic peak current (Ip) at the Mm(B5)x electrodes increased with an increase in T, x and y values. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential sweeping rate, irrespective of T, x and y values. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in x and y values. From these results, it is considered that the surface reduction treatment of the Mm(B5)x alloys, performed in alkaline solution with sodium borohydride, the nonstoichiometry and the initial concentration of absorbed hydrogen, are important factors for improving the charge-discharge performance of negative electrodes for metal-hydrogen energy systems.