High Diffusivity Of Hydrogen Research Articles

Microstructure and Electrochemical Performance of CeMg12/Ni/TiF3 Composites for Hydrogen Storage

Ball milling was used to prepare CeMg12/Ni/TiF3 composite alloys in this study. Microstructures of experimental alloys were analyzed by scanning electron microcopy (SEM), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) with electron diffraction (ED). The electrochemical and kinetic characteristics of the ball-milled composite alloys were further evaluated based on their galvanostatic charge–discharge measurement, high-rate dischargeability (HRD), hydrogen diffusion behavior, as well as electrochemical impedance spectrum (EIS). Results indicated that the ball-milled alloy with 3 wt.% TiF3 exhibited the best electrochemical discharge capacity, which was not only related to the formation of amorphous and/or nanocrystalline phase but to the formation of MgF2 because it can effectively reduce the thermodynamic stability of hydride. With increasing TiF3, the cycle degradation rates of the milled alloys were ameliorated remarkably. This improvement was attributed to the formation of an amorphous phase (which possesses strong anticorrosive and antioxidation abilities) as well as the formation of MgF2 and TiNi secondary phases. The ball-milled alloy with 3 wt.% TiF3 additive exhibited the strongest electrochemical kinetic properties, related to the highest hydrogen diffusion rate in the alloy (which was associated with multiple defects and grain boundary of amorphous and/or nanocrystalline phases) and the electrochemical reaction on the surface of alloy (which was related to the lowest apparent activation energy).

An experimental and theoretical study of the hydrogen resistance of Ti3SiC2 and Ti3AlC2

The hydrogen resistance of Ti3SiC2 and Ti3AlC2 is investigated by a combination of high temperature hydrogenation experiments and first-principles calculations. The hydrogen absorption rate in Ti3AlC2 powders is twice of that in Ti3SiC2. Hydrogenation leads to significant structure degradation on Ti3AlC2 by promoting the exudation of Al atoms which form Al bubbles on the powder surface, while the morphology of Ti3SiC2 is almost unchanged. The hydrogen-induced degradation on Ti3AlC2 is theoretically interpreted by the higher hydrogen diffusivity in Ti3AlC2 and the larger reduction of both formation energy and diffusion energy barrier of Al vacancy in Ti3AlC2 than that of Ti3SiC2.

Probing Colossal Ion Conductivity Hypotheses

Fuel cells have long promised to bring high efficiency to the production of energy. In practice, fuel cells have been limited to hydrogen systems, where the high diffusivity of hydrogen through membranes allows high current densities. Hydrogen based fuel cells pose three major limitations: the requirement of expensive catalysts, the expensive or dangerous shipping of hydrogen, and the limitation to the single fuel that is hydrogen. Alternatives are present in the form of solid oxide fuel cells (SOFCs), which are composed of earth abundant materials, and may utilize arbitrary fuels. Despite the prospects of SOFCs, their deployment has been limited by low current densities caused by the slow diffusion of oxygen through SOFC membranes. A scientific solution to the problem of low current density was found in the form of Colossal Ion Conductivity (CIC), where large changes in ion conductivity were observed in heterolayer environments. Both because of the complexity of this problem, and the importance of it as a technological solution, CIC immediately became hotly debated, with several researchers offered mechanistic hypotheses.

A new design concept for prevention of hydrogen-induced mechanical degradation: viewpoints of metastability and high entropy

‟How crack growth is prevented” is key to improve both fatigue and monotonic fracture resistances under an influence of hydrogen. Specifically, the key points for the crack growth resistance are hydrogen diffusivity and local ductility. For instance, type 304 austenitic steels show high hydrogen embrittlement susceptibility because of the high hydrogen diffusivity of bcc (α´) martensite. In contrast, metastability in specific austenitic steels enables fcc (γ) to hcp (ε) martensitic transformation, which decreases hydrogen diffusivity and increases strength simultaneously. As a result, even if hydrogen-assisted cracking occurs during monotonic tensile deformation, the ε-martensite acts to arrest micro-damage evolution when the amount of ε-martensite is limited. Thus, the formation of ε-martensite can decrease hydrogen embrittlement susceptibility in austenitic steels. However, a considerable amount of ε-martensite is required when we attempt to have drastic improvements of work hardening capability and strength level with respect to transformation-induced plasticity effect. Since the hcp structure contains a less number of slip systems than fcc and bcc, the less stress accommodation capacity often causes brittle-like failure when the ε-martensite fraction is large. Therefore, ductility of ε-martensite is another key when we maximize the positive effect of ε-martensitic transformation. In fact, ε-martensite in a high entropy alloy was recently found to be extraordinary ductile. Consequently, the metastable high entropy alloys showed low fatigue crack growth rates in a hydrogen atmosphere compared with conventional metastable austenitic steels with α´-martensitic transformation. We here present effects of metastability to ε-phase and configurational entropy on hydrogen-induced mechanical degradation including monotonic tension properties and fatigue crack growth resistance.

Performance assessment of catalytic combustion-driven thermophotovoltaic platinum tubular reactor

This study is aimed to enhance the overall efficiency of micro-thermophotovoltaic (micro-TPV) reactor by collecting radiations from emitter and combustion chamber. However, the proper fuel deployment and fluid design for the micro-TPV reactor are strongly associated with combustion stability and radiant intensity of the micro-TPV reactor. Therefore, the system performance of the micro-TPV reactor was investigated with regard to combustion, thermal radiation, and electrical output. A platinum tube with a ring of perforated holes was utilised with specific fuel deployment, that is, hydrogen employed in the inner chamber for facilitating induction of methane catalytic combustion in the outer chamber. Because of the inherently high diffusivity of hydrogen, the heat and radicals could be delivered to the other chamber through the perforated holes; in this manner, the methane catalytic combustion could be successfully initiated. The flame-stabilizing mechanism of micro-TPV platinum tubular reactor was addressed and interpreted through the simplified simulation of segmented platinum tubular reactor with a gap. The effective power efficiency of the TPV system was 3.24% when ERin-H2 = 0.7 and ERout-CH4 = 0.9. With a mirror and a recirculating tube, effective power efficiency was enhanced to 6.32%.

Fast lateral hydrogen diffusion in magnesium-hydride films on sapphire substrates studied by electrochemical hydrogenography

Hydrogen diffusion in thin magnesium films in lateral film direction is studied by stepwise electrochemical loading via the change in optical light transmission (‘electrochemical hydrogenography’) during the dihydride formation. During the first loading step, the dihydride front propagation kinetics allows determining the lateral hydrogen diffusion coefficient in the film, by applying an analytical model and, alternatively, by comparison with finite-element simulations. Subsequent loading steps show a time lag behavior in the dihydride front propagation. Therefore, they can be regarded as permeation experiments. These subsequent loading steps can be explained by hydrogen diffusion along grain boundaries and by taking the Gaussian shape of the hydrogen site energy distribution in the grain boundaries into account. In average, an effective hydrogen diffusion coefficient of 3−2+12⋅10−12m2s was found in lateral film direction. This value is the highest value known for hydrogen diffusion in Mg-dihydride, at room temperature. Possible sources for this high lateral hydrogen diffusivity are discussed including preferential hydrogen diffusion along the Mg/Mg-oxide interface, anisotropy of diffusion coefficients and lowering of diffusion energy barrier upon anisotropic film expansion.

Deuterium retention removal in China reduced activation ferritic-martensitic steels through thermal desorption and hydrogen isotope exchange

Effective removal of deuterium and tritium trapping in fusion reactor materials is of great importance both to recycle fuel and prevent the performance degradation. Experiments on deuterium removal in China Low Activation Martensitic (CLAM) steel and China Low-activation Ferritic (CLF-1) steel through thermal desorption and hydrogen isotope exchange have been performed. The results indicate that CLAM steel has only one desorption peak at 578K with an activation energy of 21kJ/mol, while CLF-1 steel has two desorption peaks, whose corresponding activation energy is 31kJ/mol at 563K and 94kJ/mol at 797K respectively. After 2h thermal desorption, deuterium removal efficiency for CLAM is more than 90% above 373K. Yet for CLF-1, deuterium removal efficiency is less than 90% below 573K. Hydrogen isotope exchange can slightly enhance deuterium release. Overall, Deuterium retention in both materials can be removed effectively at relatively low temperature (<673K) through thermal desorption due to the high hydrogen diffusivity, especially for CLAM steels.

Effect of microstructure inhomogeneity on hydrogen embrittlement susceptibility of X80 welding HAZ under pressurized gaseous hydrogen

Hydrogen embrittlement (HE) of high-grade pipeline welded joint is a threat to hydrogen gas transport. In this research, slow strain rate tension (SSRT) tests in high-pressure hydrogen gas, combined with hydrogen permeation tests and microstructure analysis were conducted on X80 steel, intercritical heated-affected zone (ICHAZ), fine-grained heat-affected zone (FGHAZ) and coarse-grained heat-affected zone (CGHAZ). The change of HE susceptibility from high to low was CGHAZ, FGHAZ, ICHAZ, and base metal. Microstructure was the important factor influencing hydrogen permeation and susceptibility to HE. Susceptibility to HE was increased in the order of “fine-grained massive ferrite (MF) and acicular ferrite (AF)”, “fine-grained granular bainite (GB) and MF”, “coarse-grained GB and bainite ferrite (BF) embedded with martensite-austenite (M-A) constitute”. The fine-grained MF and AF in base metal with lower hydrogen diffusivity can impede the embrittlement behaviour, while the coarse-grained GB and BF with higher hydrogen diffusivity in CGHAZ increased its susceptibility to HE.

Preliminary study on lean premixed combustion of ammonia-hydrogen for swirling gas turbine combustors

Hydrogen has been considered one of the most promising materials for energy storage during the last decade with considerable research having been undertaken to demonstrate the use of the molecule in power production systems. However, hydrogen presents drawbacks in terms of global commercialisation and deployment since its distribution is only feasible with significant dedicated infrastructure investment including liquefaction or if it is combined with other gases such as methane. The latter will still produce carbon emissions, whilst the former is not economically viable with current technologies. Therefore, an alternative is to use ammonia as a hydrogen storage vector. Ammonia, a molecule that has been used for more than a century, is a well-known material distributed across the world. Moreover, its properties allow its liquefaction at a relatively low pressure under atmospheric temperature compared to hydrogen, serving as a compound that can be used from fertilising to industrial processes. For power generation, ammonia has demonstrated to have a very slow reaction hence flame speeds, thus one option is to dope the fuel with a more reactive molecule such as hydrogen, which conveniently can be obtained from cracking ammonia. Hence, this paper presents the results of a numerical and experimental campaign where a 50:50 (vol%) ammonia-hydrogen blend was used for lean premixed combustion in a generic swirl combustor used in gas turbine studies. The results show that whilst the mixture can produce a good flame velocity similar to methane with the mixture having near equivalent laminar flame speed characteristics, the high diffusivity of hydrogen under these conditions leads to a narrow operational envelope with the potential for boundary layer flashback. High NOx emissions are produced due to the excess production of OH and O radicals. Recommendations for further studies and future developments are also discussed.

Tunable Nanoporous Metallic Glasses Fabricated by Selective Phase Dissolution and Passivation for Ultrafast Hydrogen Uptake

Realizing a large specific area in disordered metallic glasses is of great scientific and technological importance. Here we report a nanoporous multicomponent metallic glass fabricated by the combination of selective phase dissolution and passivation of a spinodally decomposed glassy precursor. The nanoporous metallic glass shows superior hydrogen uptake performance by taking advantage of the large specific surface area of the nanoporous structure and the high diffusivity of hydrogen in metallic glasses. The facile route of selective corrosion and passivation, decoupling the galvanic corrosion and alloy stability, opens a new avenue for functionalizing metallic glasses as a large-surface area and lightweight material for various structural and functional applications.

Analysis of Exhaust Gas Recirculation (EGR) effects on exergy terms in an engine operating with diesel oil and hydrogen

Despite the fact that the high diffusivity of hydrogen in air leads to the fast mixing and homogeneous premixed mixture, it is not suitable for operation as a single fuel because it needs a high temperature to reach self-ignition; moreover, its operation results in knocking. In this study, a Deutz dual fuel (operation with diesel oil and hydrogen) was used and from the view point of second law the effects of EGR introduction was investigated. The fuel injection amount and gas fuel-air ratio were kept constant in 6.48 mg/cycle and øH2 = 0.3, respectively. For combustion simulation and exergetic analyses, the Extend Coherent Flame Model-Three Zone model (ECFM-3Z) and a FORTRAN-based code were used, respectively. For four EGR mass fraction cases (i.e. 0%, 10%, 20%, and 30%) the rate and accumulation of various exergy terms were calculated. The results showed that as EGR introduction increases from 0% to 30%, the exergy efficiency decreases from 42.4% to 14.1%. Furthermore, the value of exhaust loss exergy increases from 14.9% to 56.7% of the mixture fuel chemical exergy. The results of the current study were compared with those existing in the relevant literature, and an acceptable behavior was observed.

(Henry B. Linford Award for Distinguished Teaching) Determination of Local Hydrogen Concentrations in High Performance Alloys Using Local Probe Methods

Introduction: Techniques for the measurement of hydrogen in high strength metallic alloys and effective hydrogen diffusivity (DH,eff) lack micrometer scale spatial resolution. This is of great importance since the hydrogen-metal interactions and processes occurring at the micrometer to nanometer length scales control hydrogen embrittlement (HE). This presentation discusses three methods which enable spatial resolution and mapping of absorbed hydrogen at sub-millimeter length scales in ultra-high strength steels (UHSS) and stainless steels. The scanning Kelvin probe (SKP), scanning Kelvin probe force microscope (SKPFM), and SKP combined with secondary ion mass spectrometry have recently been developed as useful techniques for spatial H detection with depth (using cross-sectional analysis), lateral detection, and mapping.[1-7] In terms of H resolution, the SKP can detect concentrations as low as 0.01 atomic ppm.[8] This presentation explores three novel techniques for the spatial detection of hydrogen concentrations. These include the SKP, the scanning electrochemical microscope (SECM) and re-scaling methods to produce mm scale cracks or crevices that are electrochemically equivalent to small scale occluded sites [5, 9-11]. Experimental and Results: Two secondary age hardened martensitic steels were examined, UNS K92580 and UNS S46500. The SKP voltage (Φ) dependence on dissolved H concentration was optimized by mapping under conditions producing a low corrosion rate; an essential aspect of successful hydrogen mapping. A calibration curve was developed relating absorbed hydrogen concentration to Φ. High strength steels were pre-charged and exposed to MgCl2 and NaCl droplets, which produced natural acidic pit sites that enabled local hydrogen uptake[12]. The factors controlling calibration of SKP potential and hydrogen concentration as well as those limiting spatial resolution are discussed. Moreover, DH,eff coefficients were determined.[5] Results were corroborated with previous studies of DH,eff.[13] The SECM also was optimized to enable detection of corrosion processes with spatial resolution and utilized to measure spatially a pre-dissolved concentration of hydrogen across pre-charged and uncharged zones [10]. The talk will close with comparison of the length scales of various physical processes occurring during hydrogen embrittlement compared to those accessed by these techniques. Acknowledgement: Research was sponsored by the US Air Force Academy under agreement number FA7000-13-2-0020 and ONR under PROJ0007990. The authors would like to acknowledge the Army Research Laboratories, as well as Monash University, in particular Dr. Sebastian Thomas and Prof. Nick Birbilis. The research was partially supported by a 2014 Australia Endeavour Award Research Fellowship with support from Dr. Kishore Venkatesan and Dr. Ivan Cole at CSIRO.

FE analysis of hydrogen diffusion around a crack tip in an austenitic stainless steel

Hydrogen-assisted cracking (HAC) is a coupled phenomenon between crack growth rate and hydrogen diffusion to the region ahead of crack tip. To understand the cracking mechanism and to control or predict the HAC rate, knowledge of hydrogen diffusion around a crack tip is essential. Metastable austenitic stainless steels can be severely embrittled by hydrogen due to strain-induced α′ martensite transformation around crack tip, as the hydrogen diffusivity in α′ martensite is much higher than in γ austenite while the solubility is lower. However, the effect of induced α′ martensite on local diffusion of hydrogen around a loaded crack tip has not been intuitively demonstrated. This study first reveals the quantitative relations of hydrogen diffusivity and solubility with induced α′ martensite amount, and then performs FE analysis of hydrogen diffusion around a crack tip in a 304L ASS cylinder storing high pressure gaseous hydrogen with considering the combined effect of α′ martensite transformation and hydrostatic stress on diffusion. The hydrostatic stress exhibits a peak value at a distance away from crack tip, while the peak α′ martensite volume fraction locates at the crack tip. Compared to the analysis only considering stress effect, the presence of induced α′ martensite can not only significantly accelerate the hydrogen transport to the critical region ahead of crack tip, but also markedly elevate the level that the peak hydrogen concentration reaches. The peak hydrogen concentration doesn't locate at the peak hydrostatic stress site as generally expected, but at the site in the γ austenite-rich region abutting the α′ martensite-rich region at crack tip vicinity, because this site possesses very high hydrogen solubility and close to the high hydrogen diffusivity region, consequently accommodating the hydrogen getting off the α′ martensite diffusion “highway”.

パラジウム中の水素拡散に及ぼす転位の影響

Understanding the diffusion properties of hydrogen in metals, especially interactions between hydrogen and lattice defects such as dislocations, is essential in order to improve the performance and reliability of equipment associated with hydrogen. We investigate the effect of edge dislocations on hydrogen diffusion in face-centered-cubic (fcc) metals using molecular dynamics simulations of palladium-hydrogen as a model system. We find that the hydrogen diffusion in a perfect crystal without the dislocations is isotropic, whereas that in a system with the dislocations has anisotropy. In addition, our results predict high hydrogen diffusivity along the dislocation lines in high hydrostatic stress region with high hydrogen density after hydrogen accumulation caused by the interactions between hydrogen atoms and the dislocations. The activation energy of the hydrogen diffusivity in the system with the dislocations is estimated to be 0.10 eV, which is 38 % smaller than that in the perfect crystal.

Substantial enhancement of hydrogen permeability and embrittlement resistance of Nb30Ti25Hf10Co35 eutectic alloy membranes by directional solidification

As-cast Nb30Ti35−xHfxCo35 (x=0, 10, and 17.5) alloys consist of a fully lamellar eutectic structure, where the substitution of Ti by Hf induces higher hydrogen solubility and permeability, but poorer embrittlement resistance. Different as-cast alloys were found to be subject to embrittlement failure after hydrogen permeation for 88.3h (x=0), 71.1h (x=10) and 16.2h (x=17.5) at 673K. In contrast, directionally solidified (DS) Nb30Ti25Hf10Co35 exhibits a substantial enhancement of hydrogen permeability and embrittlement resistance. Typically, DS samples solidified at 1μm/s show the highest permeability of 4.83×10−8molH2m−1s−1Pa−0.5 at 673K, which is 1.72 times that of its as-cast counterpart and more than three times that of pure Pd. This sample does not fail during hydrogen permeation for 120h at 673K. The high permeability and large embrittlement resistance of DS samples are attributed to their modified eutectic microstructure, which induces a high overall hydrogen diffusivity and a high tolerance to lattice expansion. The present work demonstrates that the directional solidification technique has a high potential to produce high-performance eutectic alloy membranes for hydrogen separation.

Studies of premixed and non-premixed hydrogen flames

The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p=1 and 7atm. The experiments were designed to sensitize the three-body H+O2+M→HO2+M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H2O and CO2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. The measured extinction strain rates exhibit both a positive and negative dependence on pressure with and without weighting with the density, and this non-monotonic behavior is caused by the competition between the H+O2→O+OH and H+O2+M→HO2+M reactions as well as HO2 kinetic pathways as pressure increases. The various kinetic models considered in this investigation did not reproduce equally well the non-premixed flame extinction data with added H2O. On the other hand, the predicted extinction strain rates were consistent between the various models in the case of added CO2. Finally, it was shown that the formulation of binary diffusion coefficient pairs including H–N2 and H2–N2 has a first order effect on the prediction of extinction strain rates of non-premixed H2 flames.

Study of Phase Stability and Hydride Diffusion Mechanism of BaTiO3 Oxyhydride from First-Principles

First-principles calculations were performed to study structural, electronic and hydride diffusion properties of BaTiO3 oxyhydride. In agreement with experiment (Nat. Mater. 2012, 11, 507 and J. Am. Chem. Soc. 2012, 134, 8782), we find that the incoming H species occupy the anion vacancy sites left by oxygen, forming the stable hydride anions H–1. As a result of the electron doping introduced by H species, both interstitial H and hydride anion H–1 can induce metallicity and eliminate ferroelectricity in BaTiO3. We further clarify the role of the migration of the interstitial H in determining the hydrogen diffusion properties of the oxyhydrides. A low diffusion barrier was predicted, responsible for high hydrogen diffusion mobility observed in experiment. Based on our results, we demonstrate that BaTiO3 oxyhydride can be used as a mixed electron/hydride conductor, displaying the promising applications as the electrolytes for solid-oxide fuel cells.

Hydrogen permeation behavior of Nb30Ti35Ni35−xCox (x = 0…35) alloys containing high fractions of eutectic

The hydrogen permeability of cast Nb30Ti35Ni35−xCox (x = 0…35) alloys is found to increase with the Co content, induced by the tailored microstructures containing high fractions of eutectic and less primary phase. Among the cast alloys, Nb30Ti35Co35 consisting entirely of eutectic exhibits the highest permeability, particularly 2.65 × 10−8 mol H2 m−1 s−1 Pa−0.5 at 673 K. This permeability is further increased to 3.58 × 10−8 mol H2 m−1 s−1 Pa−0.5 by aligning eutectic grains perpendicularly to the membrane surface using directional solidification. The high permeability is attributed to the high hydrogen solubility and diffusivity in alloys. Our work demonstrates that hydrogen permeable alloys containing high fractions of eutectic may exhibit high permeability by adequately optimizing morphology, volume fraction and alignment of the bcc-Nb phase in the eutectic as the pathways for hydrogen permeation.

Vanadium alloy membranes for high hydrogen permeability and suppressed hydrogen embrittlement

The structural properties and hydrogen permeation characteristics of ternary vanadium–iron–aluminum (V–Fe–Al) alloy were investigated. To achieve not only high hydrogen permeability but also strong resistance to hydrogen embrittlement, the alloy composition was modulated to show high hydrogen diffusivity but reduced hydrogen solubility. We demonstrated that matching the lattice constant to the value of pure V by co-alloying lattice-contracting and lattice-expanding elements was quite effective in maintaining high hydrogen diffusivity of pure V.

Water formation at low temperatures by surface O2 hydrogenation III: Monte Carlo simulation

Water is the most abundant molecule found in interstellar icy mantles. In space it is thought to be efficiently formed on the surfaces of dust grains through successive hydrogenation of O, O2 and O3. The underlying physico-chemical mechanisms have been studied experimentally in the past decade and in this paper we extend this work theoretically, using Continuous-Time Random-Walk Monte Carlo simulations to disentangle the different processes at play during hydrogenation of molecular oxygen. CTRW-MC offers a kinetic approach to compare simulated surface abundances of different species to the experimental values. For this purpose, the results of four key experiments-sequential hydrogenation as well as co-deposition experiments at 15 and 25 K-are selected that serve as a reference throughout the modeling stage. The aim is to reproduce all four experiments with a single set of parameters. Input for the simulations consists of binding energies as well as reaction barriers (activation energies). In order to understand the influence of the parameters separately, we vary a single process rate at a time. Our main findings are: (i) The key reactions for the hydrogenation route starting from O2 are H + O2, H + HO2, OH + OH, H + H2O2, H + OH. (ii) The relatively high experimental abundance of H2O2 is due to its slow destruction. (iii) The large consumption of O2 at a temperature of 25 K is due to a high hydrogen diffusion rate. (iv) The diffusion of radicals plays an important role in the full reaction network. The resulting set of 'best fit' parameters is presented and discussed for use in future astrochemical modeling.