Mechanism Of Hydrogen Transport Research Articles

Large-eddy simulation on gas mixing induced by the high-buoyancy flow in the CIGMAfacility

The hydrogen behavior in a nuclear containment vessel is a significant issue when discussing the potential of hydrogen combustion during a severe accident. After the Fukushima-Daiichi accident in Japan, we have investigated in-depth the hydrogen transport mechanisms by utilizing experimental and numerical approaches. Computational fluid dynamics is a powerful tool for better understanding the transport behavior of gas mixtures, including hydrogen. This paper describes a Large-eddy simulation of gas mixing driven by a high-buoyancy flow. We focused on the interaction behavior of heat and mass transfers driven by the horizontal high-buoyant flow during density stratification. For validation, the experimental data of the Containment InteGral effects Measurement Apparatus (CIGMA) facility were used. With a high-power heater for the gas-injection line in the CIGMA facility, a high-temperature flow of approximately 390 °C was injected into the test vessel. By using the CIGMA facility, we can extend the experimental data to the high-temperature region. The phenomenological discussion in this paper helps understand the heat and mass transfer induced by the high-buoyancy flow in the containment vessel during a severe accident.

Hydrogen Crossover in PEM Water Electrolysis at Current Densities up to 10 A cm−2

Hydrogen crossover poses a critical issue in terms of the safe and efficient operation in polymer electrolyte membrane water electrolysis (PEMWE). The impact of key operating parameters such as temperature and pressure on crossover was investigated in the past. However, many recent studies suggest that the relation between the hydrogen crossover flux and the current density is not fully resolved. This study investigates the hydrogen crossover of PEMWE cells using a thin Nafion 212 membrane at current densities up to 10 A cm−2 and cathode pressures up to 10 bar, by analysing the anode product gas with gas chromatography. The results show that the hydrogen crossover flux generally increases over the entire current density range. However, the fluxes pass through regions with varying slopes and flatten in the high current regime. Only considering hydrogen diffusion as the single transport mechanism is insufficient to explain these data. Under the prevailing conditions, it is concluded that the electro-osmotic drag of water containing dissolved hydrogen should be considered additionally as a hydrogen transport mechanism. The drag of water acts opposite to hydrogen diffusion and has an attenuating effect on the hydrogen crossover in PEMWE cells with increasing current densities.

High temperature stability and transport characteristics of hydrogen in alumina via multiscale computation

The impact of hydrogen charge states on the stability and transport characteristics of hydrogen interstitials in alumina polymorphs is evaluated by multiscale computational methods including density functional theory (DFT), ab initio molecular dynamics (AIMD) and machine learned force fields. Thermodynamic calculations show that the protonic Hi+1 interstitial is the most stable defect species for most values of the electronic bandgap in both α and amorphous alumina (Al2O3). Further, active learned Gaussian approximation potentials (GAP) were developed using AIMD data to study temperature dependent long time proton diffusion in alumina. Diffusivity calculations from GAP-MD simulations are found to be comparable with of the AIMD data, while being ∼340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (Hi0 and Hi−1) and published experimental literature indicate that Hi+1 diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between Hi+1 diffusivity calculated in α-Al2O3 from DFT: 5.05 × 10−3 exp(-0.81 eV/kB/T) cm2/s and reported experiment: 9.7 ×10−4 exp(-0.83 eV/kB/T) cm2/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the pre-exponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of Hi+1 in amorphous Al2O3 in the 1000–2000 K range is calculated to be 2.53 × 10−2 exp(-0.89 eV/k B/T), just one order of magnitude higher than the corresponding value in α-Al2O3. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of Hi+1 in Al2O3. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.

Build‐up of hydrogen up‐flow model in consideration of air resistance and entrainment

Hydrogen explosion is one of the most dangerous accidents in nuclear power plants, which has drawn experts' attention since Fukushima nuclear accident. Plenty of research has been carried out on hydrogen distribution, such as AIHMS program in India, SPARC program in Korea, and CIGMA program in Japan recently, for the purpose of figuring out the hydrogen transport mechanisms and developing simulation codes. Hydrogen up-flow model is one of the important models that initiates the hydrogen source and describes the hydrogen distribution in the centerline of the containment. However, the original model cannot predict the mass fraction of hydrogen precisely and efficiently. In this paper, a one-dimensional up-flow (along with the vertical direction) model has been proposed, in consideration of air resistance and entrainment, which has been inferred from AIHMS test results. This new model is verified by the experimental data from Sandia National Laboratory (SNL) and compared with literature findings, which shows good predictions compared with the original one. Based on the typical Froude numbers, the present optimized model is applied to conditions of typical injection characters. With the help of analyzing the gas velocity, characteristic length of B, nondimensional parameter of z/ls, and mass fraction of hydrogen, a definition of hydrogen expansion process has been given, which can be quantified and detailed by an proposed explosive expansion criterion. This work can help understand the dominated regime of either buoyancy or momentum and can be employed in simulation codes in the future, for the purpose of quick evaluation in severe accident management and accidents emergency decision.

Innovative N2-selective metallic membranes for the potential use of CO2 capture

The permeability of nitrogen in Group V metal membranes having an approximate thickness of 40 μm was determined for the first time at conditions of relatively low temperature (400–600 °C) and pressure (300–600 kPa). Vanadium, niobium and tantalum membranes were investigated to evaluate the permeation characteristics towards nitrogen permeation. Analogous to the hydrogen transport mechanism through palladium-based membranes, the nitrogen permeating flux through the metallic membranes was determined to take place through a solution-diffusion mechanism. In particular, at 400 °C, the nitrogen permeability was found to be 1.15 × 10-12, 1.05 × 10-12, and 6.54 × 10-13 mol/(m s Pa0.5) for vanadium, niobium and tantalum, respectively.The optimal nitrogen partial pressure exponent value was found to be 0.5 for all the membranes investigated. The nitrogen permeability for the vanadium membrane was described by an Arrhenius-type relation where the pre-exponential factor and apparent activation energy for this correlation were 3.55 × 10-15 mol/(m s Pa0.5) and 31.8 kJ/mol, respectively. For niobium and tantalum membranes, it was not possible to perform any permeation tests due to cracks, which formed at temperatures higher than 500 °C.SEM and EDS analyses revealed the formation of metal oxides on the surface of V, Nb, Ta membranes, which may benefit the transport of nitrogen through the membrane as it may aid in preserving the structured nature of the bulk phase thereby preventing the formation of defects and/or grain migration at high temperature. Although the nitrogen permeability through metallic membranes is low and metal oxides form on the surface, this work describes the performance of metallic membranes toward nitrogen permeation, and it can be considered as the benchmark for future work related to this field.

Finite Element Analysis of Hydrogen Transport in Steel Pressure Vessel at Room Temperature

Hydrogen embrittlement is commonly considered as an important failure mechanism for some typical steel pressure vessels and pipes made of such as Cr–Mo and 4130X steels at high-pressure hydrogen environment. In previous work, we investigated the hydrogen transport mechanisms of Crmo steel pressure vessels at room temperature. Furthermore, high temperature environment may affect the hydrogen transport mechanisms and hydrogen-induced crack behaviors in these structures to a large extent. In this paper, we study the hydrogen transport mechanisms in 2.25Cr–1Mo steel pressure vessel at high temperature under the support of National Key Fundamental Research and Development Project of China (2015.1-2019.12). The main work is to explore the effects of temperature, hydrogen concentration, and structural sizes on the transient hydrogen diffusion and distribution behaviors in Crmo steel pressure vessels using finite element analysis. Numerical results show that elevated high temperature accelerates the hydrogen embrittlement sensitivity, especially at structural discontinuities.

Ductility loss of 7075 aluminum alloys affected by interaction of hydrogen, fatigue deformation, and strain rate

The ductility loss phenomenon of the 7075 alloys affected by the interaction of the hydrogen, pre-fatigue deformation, and strain rate was elucidated. In order to investigate the tensile properties of pre-fatigued material in varying humidity environment, quasi-static and impact tensile tests on pre-fatigued materials were conducted, which were prepared in environments with different humidities. The flow stress obtained by tensile test was not affected by pre-fatigue deformation with varying humidity and frequency, regardless of the strain rate. In contrast, pre-fatigue deformation resulted in a loss of ductility. The ductility loss phenomenon, owing to pre-fatigue deformation, was most significant for a combination of long pre-fatigue time, high humidity, and low strain rate. Therefore, the microstructure of the materials was observed in order to determine the main factor. The microstructure of pre-fatigued materials using TEM indicated that dislocations accumulated in some regions inside the crystal grains near the Al7Cu2Fe (local dislocation field). Thus, the ductility loss phenomenon is believed to occur as this local dislocation field develops during tensile deformation, thereby promoting the formation and growth of voids, and in turn promoting ductile fractures. According to the hydrogen-enhanced localized plasticity theory, accumulated hydrogen promotes the formation and multiplication of dislocations near Al7Cu2Fe, and increases the density of the local dislocation field, which in turn results in the progression of the ductility loss. Furthermore, the hydrogen transport mechanism owing to dislocations operates at low strain rates; thus, hydrogen affects the dislocation activities during tensile deformation. This results in a faster density increase of the local dislocation field, which in turn promotes a loss of ductility. This phenomenon can therefore be described as a type of hydrogen-induced ductility loss.

Multiscale Approaches to Hydrogen-Assisted Degradation of Metals

Hydrogen embrittlement (HE) is a serious and costly industrial problem that affects many commonly used structural metals. Given the wide range of service environments in which hydrogen may occur or be produced, this represents a very serious threat to the structural integrity of machinery and infrastructure in many industries. Despite having been studied for several decades, there is still little consensus regarding the underlying mechanisms for HE. Recently developed theoretical and experimental methods, which enable the evaluation of the influences of hydrogen on the mechanical behavior of metals at the nano-scale, are helping to elucidate new aspects of these mechanisms. However, an urgent need remains to develop tools for the prediction of the reliability and lifetime of materials and components affected by hydrogen. Critical to achieving this goal is the development of accurate descriptions of hydrogen–microstructure interactions under conditions relevant to those occurring in service. As these interactions occur at all length scales, this poses a true multiscale challenge (Fig. 1). The International Symposium on Multiscale Approaches to Hydrogen-Assisted Degradation of Metals at the TMS2014 Annual Meeting & Exhibition in San Diego, California organized with the aim of promoting the exchange of ideas and information regarding the application of cutting-edge theoretical and experimental methods to industrial problems involving hydrogen-assisted degradation of metals. A particular focus of the symposium was multiscale modeling (e.g., coupled atomistic, mesoscopic, and macroscopic descriptions of hydrogen transport and damage mechanisms) as well as integrated theoretical and experimental approaches to hydrogen– microstructure interactions (e.g., the application of experimental methods for model validation and determination of modeling parameters). The symposium brought together leading researchers in the field of HE, as well as representatives from a broad range of industries that provided vital insights into the impact of HE. A total of 32 presentations were given during the 4-day meeting. The following five articles are based on presentations that were selected to show the diversity of topics covered at the symposium. In the article ‘‘Hydrogen Embrittlement of PulsePlated Nickel,’’ Reese et al. provide an overview of an experimental program at Airbus Group, which is aimed at evaluating the susceptibility of pulse-plated (PP) Ni to HE. This material is used in the fabrication of the components for the ARIANE 5 launcher. Due to the nature of the PP process, some hydrogen may be incorporated into the material during deposition. Depending on the process parameters used, this hydrogen, in combination with residual stresses resulting from welding, may result in HE. Although this does not pose an immediate problem, there is a need to better understand the influences of microstructure and process parameters on the susceptibility of PP-Ni to HE. The study is part of a wider EU-funded program called ‘‘MultiHy’’ (Multiscale modelling of hydrogen embrittlement), which aims to develop advanced multiscale models to assist companies to better understand how HE occurs during manufacture and service. ‘‘Hydrogen Embrittlement of Ferritic Steels: Deformation and Failure Mechanisms and Challenges in the Oil and Gas Industry’’ by Srinivasan and Neeraj reviews recent and forthcoming publications in which the HE of two commonly used pipeline steels, X65 and X80, was investigated using advanced analytical techniques. Analysis of the deformation substructures under the fracture surfaces using transmission electron microscopy (TEM) and the observation of nanovoids on the fracture surfaces using high-resolution scanning electron microscopy (SEM) led the authors to propose that the mechanism for HE involves vacancy-induced nanovoid nucleation and growth. The results are JOM, Vol. 66, No. 8, 2014

Fundamental Investigation of Hydrogen Transport in Ferritic Stainless Steel for SOFC Interconnect Applications

Ferritic stainless steel (FSS) is widely used to separate different gaseous environments in many energy conversion systems such as turbines, heat exchangers, and intermediate temperature solid oxide fuel cell (SOFC) stacks. This so called “dual atmosphere exposure” at elevated temperatures (650-800°C) is commonly believed to facilitate accelerated and/or anomalous corrosion. In SOFC stacks, FSS interconnects (ICs) separate individual cells in a stack and are simultaneously exposed to the fuel (H2) on one side and the oxidant (air) on the other. This study focuses on the effects of SOFC operating conditions on FSS AISI 441, which has been observed to exhibit significant dual atmosphere effects on air-side corrosion. Subsequent 100 hours of dual atmosphere exposure (separating moist H2 from moist air at 800°C) FSS 441 sample surfaces and polished cross sections were analyzed using a field-emission scanning electron microscope (FE-SEM) equipped with energy dispersive x-ray spectroscopy (EDX) for elemental analysis. The study confirms the results of previous studies exploring secondary phase precipitation at grain boundaries and within the grains of FSS 441. Additionally, large Ti-rich nodules were observed on the cross-section surfaces. Ongoing efforts to investigate phase composition and texture using electron backscatter diffraction (EBSD) are discussed with the aim to better understand hydrogen transport mechanisms and attendant dual atmosphere corrosion of FSS 441.

(Invited) Negative-Bias Temperature Instability – Insight from Recent Dynamic Stress Experiments

The purpose of this paper is to summarize key experimental evidences on the important role of hole trapping on negative-bias temperature instability (NBTI). For a long time, the focus of this research topic had been on interface degradation driven by hydrogen transport and hole trapping was regarded as a side effect arising out of fast measurement techniques proposed to mitigate the effect of recovery on measurement data. In recent studies, we showed that the threshold voltage (V t) fluctuations one typically observed under dynamic NBTI were mainly the result of hole trapping and not hydrogen-transport driven interface-state generation/passivation proposed earlier. In particular, the cyclical V t shifts and constant V t recovery are inconsistent with the basic principle of the hydrogen transport model. Such behaviors are better described in terms of hole trapping/detrapping at pre-existing oxide defects. We have also shown that interface degradation during NBTI stressing has no apparent impact on bulk (oxide) trap generation, i.e. interface trap generation does not lead to bulk trap generation. This result raises further questions on the validity of the hydrogen transport mechanism and the long standing hypothesis on hydrogen-induced bulk trap generation and gate oxide breakdown. Finally, it is shown that the transient hole trapping responsible for the V t shift fluctuations could be transformed into more permanent trapped holes under NBTI stressing. The extent of transformation is accelerated by a high oxide field and temperature. An excellent correlation with stress induced leakage current indicates that such transformation underlie the generation of bulk traps reported by earlier studies.

Hydrogen migration in metals under the combined action of acoustic and ionizing radiation

The hydrogen migration in metals under the action of ionizing and acoustic radiation is considered. The joint action of ionizing and acoustic radiation is found to enhance the hydrogen migration in metals (“interference” effect). The hydrogen transport mechanism is determined by the interaction of ionizing radiation with the hydrogen subsystem of a metal and a vibrodiffusion effect.

Hydrogen species within the metals: Role of molecular hydrogen ion H2+

AbstractNovel mechanism of hydrogen interaction with transition metals via stepwise reversible dissociative ionization of H2 molecule is proposed instead of a commonly accepted dissociative adsorption. It involves ionization of H2 to molecular ion (H2+)ad on the outer surface of metal phase, its subsequent absorption and dissociation within the metal phase into (H+)ab ions, i.e., absorbed protons, as described by: H2⇄(H2+)ad+e− and (H2+)ad⇄(H2+)ab⇄2(H+)ab+e−. Absorption here is treated as adsorption on the inner surface of the tetrahedral and octahedral voids within metal lattice. The mechanism is based on the first principles and explains consistently the dependence of mechanical properties of metals on the amount of absorbed hydrogen as well as the mechanism of hydrogenation and hydrogen transport through the metals. The proposed dissociative ionization mechanism is well supported by thermodynamic and steric arguments. In the case of noble metals the presented mechanism carries versatile character as it is valid for both gaseous phase and aqueous solutions.

A theoretical study on hydrogen transport mechanism in SrTio3 perovskite

AbstractHybrid‐DFT calculations were performed to clarify the hydrogen transport mechanism in SrTiO3 perovskite. From a molecular‐orbital analysis, we discussed the changes of the chemical bonding between conductive hydrogen and others, and the proton conduction paths. It is concluded that the conductive hydrogen is almost neutral, with a diagonal conduction path favored inside of Ti4O4 square. We considered the nitrogen doping at the oxygen site to enhance the proton conductivity. Doped nitrogen exists as the part of NH2− ion, enhancing proton conductivity. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Hydrogen selective thin palladium–copper composite membranes on alumina supports

Abstract Thin and defect-free Pd–Cu composite membranes with high hydrogen permeances and selectivities were prepared by electroless plating of palladium and copper on porous alumina supports with pore sizes of 5 and 100 nm coated with intermediate layers. The intermediate layers on the 100 nm supports were prepared by the deposition of boehmite sols of different particle sizes, and provided a graded, uniform substrate for the formation of defect-free, ultra-thin palladium composite layers. The dependence of hydrogen flux on pressure difference was studied to understand the dominant mechanism of hydrogen transport through a Pd–Cu composite membrane plated on an alumina support with a pore size of 5 nm. The order in hydrogen pressure was 0.98, and indicated that bulk diffusion through the Pd–Cu layer was fast and the overall process was limited by external mass-transfer or a surface process. Scanning electron microscopy (SEM) images of the Pd–Cu composite membrane showed a uniform substrate created after depositing one intermediate layer on top of the alumina support and a dense Pd–Cu composite layer with no visible defects. Cross-sectional views of the membrane showed that the Pd–Cu composite layer had a top layer thickness of 160 nm (0.16 μm), which is much thinner than previously reported.

Effects of surface dynamic behavior on hydrogen storage properties of sputter-deposited MgNi films

We have studied experimentally the distribution profiles of accommodated oxygen and hydrogen atoms in Mg–Ni films after different stages of hydrogenation at 8 bar pressure in the range of temperatures 210–250 °C by Elastic Recoil Detection Analysis (ERDA) and Nuclear Reaction Analysis (NRA) techniques and performed X-ray diffraction (XRD) structural and surface topography analysis. In agreement with previous publications, it is shown that even small impurity levels of uncontrollable oxygen during hydrogenation result in the formation of the oxide barrier on the surface changing hydrogen permeation properties. It is registered that the quantity of stored hydrogen during the first 3 h of hydrogenation increases up to 40 at.%, decreases up to 20 at.% after 6 h and increases again up to 40 at.% after 72 h. The quantity of the released hydrogen after 3 h hydrogenation increases with the increase in hydrogenation temperature. The observed experimental results are explained assuming that properties of the surface barrier layer changes during hydrogenation and modify hydrogen transport mechanism. The growing hydride phase in the bulk generates stresses that induce cracks and holes in the oxide barrier formed during the initial stages of hydrogenation, and hydrogen release becomes possible. At steady state, the hydrogenation properties adjust to dynamic processes on the surface and in the bulk.

Influence of electroless coatings of Cu, Ni–P and Co–P on MmNi 3.25Al 0.35Mn 0.25Co 0.66 alloy used as anodes in Ni–MH batteries

Electroless coatings of Ni–P, Co–P and Cu were applied on the surface of non-stoichiometric MmNi 3.25Al 0.35Mn 0.25Co 0.66 (Mm: misch metal) metal hydride alloy. Elemental analysis was made with Energy Dispersive X-ray Analysis (EDAX). The structural analysis of bare and coated alloys was done by X-ray diffraction (XRD) whereas surface morphology was examined with scanning electron microscope (SEM) and transmission electron microscope (TEM). The electrode characteristics inclusive of electrochemical capacity and cycle life were studied at C/5 rate. Superior performance is obtained with copper coated alloy. Microstructure observations indicate that the observed excellent performance could be attributed to uniform and efficient surface coverage with copper. Also, lanthanum surface enrichment in samples during Cu coating leads to improvement in performance. It is inferred from electro analytical investigations that copper coatings act as microcurrent collectors with alterations in hydrogen transport mechanism and facilitate charge transfer reaction on the alloy surface without altering battery properties. Moreover, supportive first time TEM evidence of existence of such copper nano current collectors (about 8 nm in diameter and length about 20 nm) is reported.

Movement of hydrogen molecules in pristine, hydrogenated and nitrogen-doped single-walled carbon nanotubes

Carbon nanotubes (CNT) are considered promising nano-scale materials because of their unique structural, mechanical and electronic properties. Due to their long seamless cylindrical shaped structures they could be applied as effective nano-channels for mass transfer and relevant storages for hydrogen molecules. We study hydrogen transport mechanisms in CNTs for various chiral indices and different peculiarities, using the molecular dynamics simulation and quantum mechanical approach. Various CNT models such as pristine, hydrogenated and doped by nitrogen atoms of zigzag (10,0), chiral (7,5) and armchair (6,6) types with hydrogen molecules diffusing inside are simulated at 300 K. The behaviour of hydrogen molecules inside CNTs is analysed using mean-square displacements and velocity autocorrelation functions. From the quantum mechanical approach, the electronic density distribution of CNT is calculated to verify the smooth characteristics of inner surfaces of nanotubes.

Dense Metal Membranes for the Production of High-Purity Hydrogen

AbstractDense metal membranes are a well-developed technology for the production of high-purity hydrogen. The physical mechanism of hydrogen transport across metal films—dissociation of molecular hydrogen, diffusion of interstitial atomic hydrogen, and subsequent recombinative desorption of molecular hydrogen—means that metal membranes can have extremely high selectivities for hydrogen transport relative to other gases. We describe current experimental and theoretical trends in the development of metal alloy membranes for hydrogen purification in practical, chemically robust processes.

Pure nickel coating on a mesoporous alumina membrane: Preparation by electroless plating and characterization

This work concerns the preparation and characterization of a composite inorganic membrane based on nickel supported on a tubular porous ceramic for use in high temperature hydrogen separation. The characteristics required of this material for a high hydrogen permselectivity are a defect-free coating and thermal stability. Thus, electroless plating of nickel using hydrazine as reducing agent enables a metallic nickel coating to be deposited directly on an asymmetric alumina support having a mesoporous top layer composed of γ-alumina. To optimize conditions for reduction of the metal ion, variables including the temperature, the nature of the nickel salt, the choice of stabilizer and the hydrazine concentrations in the plating bath were studied. Metallic nickel was found to be dispersed both in and on the γ-alumina. After heat treatment at high temperature, a thin and uniform pure Ni˙ layer (thickness ≈1–1.5 μm), characterized by scanning electron microscopy (SEM) and X-ray Diffraction (XRD), was formed. The separation performance of the Ni/ceramic membrane was investigated using single gases (hydrogen, nitrogen, carbon dioxide and methane) at room temperature. The results indicate that the mechanism of hydrogen transport is related to the Knudsen diffusion.

Chemical exchange in KHSeO4 and NH4HSeO4 studied by two-dimensional NMR

The microscopic mechanism of proton transport in partially deuterated potassium hydrogen selenate (KHSe) and in partially deuterated ammonium hydrogen selenate (AHSe) were studied by means of one-dimensional Fourier transform2H nuclear magnetic resonance (NMR), two-dimensional2H NMR and dielectric measurements over a wide temperature range. In both systems, KHSe and AHSe, the slow chemical exchange processes of deuterons between different hydrogen bridges occur. It was established that the rates of exchange between deuteron sites, which are involved in infinite chains of hydrogen bonds, are approximately the same for both crystals. The rates of exchange between these positions and the deuterons in the dimer groups of KHSe are approximately hundred times more slowly. On the basis of our findings, we discuss the models of the microscopic mechanism of hydrogen transport for both substances.