Hydrogen transport and embrittlement in structural metals

The close relationship between hydrogen transport and embrittlement is indicated by evidence of hydrogen absorption preceding degradation of mechanical properties. Concentration of hydrogen at a crack or flaw by diffusion or by transport with moving dislocations is probably necessary also. Experimental studies show that hydrogen permeation is significantly influenced by surface conditions, particularly oxide films, and internal defects and impurities that trap diffusing hydrogen. The usual thermodynamic and diffusion relations, therefore, do not predict accurately the final distribution of hydrogen and the kinetics of the processes.Investigation of the effects of hydrogen on the mechanical properties of approximately fifty structural alloys at ambient temperature and pressures up to 69 MPa indicates that all alloys show evidence of susceptibility to hydrogen embrittlement. The degree of hydrogen embrittlement appears to be related to the amount of hydrogen absorbed and its distribution within the metal lattice. Surface condition, defect structure, hydrogen purity, and hydrogen pressure influence embrittlement. Of major importance is the transport of hydrogen with moving dislocations, a mechanism for concentration and redistribution of hydrogen that is operative at temperatures lower than for diffusion.     

氢气非预混燃烧的流场输运特性分析

采用高精度直接数值模拟的方法对氢气非预混燃烧流场进行了精细的预测.模拟所求解的控制方程为三维可压缩的无量纲形式的Navier-Stokes方程,采用六阶精度紧致差分格式,结合基于详细化学反应和输运过程的FGM化学反应机制,利用768个处理器核、共近4.53亿网格点进行了基于CPU的大规模高效并行计算,分析氢气非预混燃烧特性,并进一步探讨了浮力对氢气燃烧流场输运特性的影响.研究发现,由于氢气燃烧过程中产生不同扩散性质的化学组分,使燃烧过程中遵循优势扩散的行为.这将影响流场的输运特性和火焰不稳定性的形成.在浮力驱动的氢气优势扩散燃烧流场中,对流是质量、动量及热量输运行为的主要影响因素,而无浮力火焰中优势扩散主导着流场的输运特性.平均统计结果表明,有浮力和无浮力的燃烧流场中都可以捕捉到逆梯度输运现象,且浮力会促进逆梯度输运行为的发生.     

Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB₅-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB₅-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB₅-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB₅-type hydrogen storage alloy.     

Hydrogen delivery through liquid organic hydrides: Considerations for a potential technology

Carrying hydrogen in chemically bounded form as cycloalkanes and recovery of hydrogen via a subsequent dehydrogenation reaction is a potential option for hydrogen transport and delivery. We have earlier reported a novel method for transportation and delivery of hydrogen through liquid organic hydrides (LOH) such as cycloalkanes. The candidate cycloalkanes including cyclohexane, methylcyclohexane, decalin etc. contains 6 to 8 wt% hydrogen with volume basis capacity of hydrogen storage of 60–62 kg/m³. In view of several advantages of the system such as transportation by present infrastructure of lorries, no specific temperature pressure requirement and recyclable reactants/products, the LOH definitely pose for a potential technology for hydrogen delivery. A considerable development is reported in this field regarding various aspects of the catalytic dehydrogenation of the cycloalkanes for activity, selectivity and stability. We have earlier reported an account of development in chemical hydrides. This article reports a state-of-art in LOH as hydrogen carrier related to dehydrogenation catalysts, supports, reactors, kinetics, thermodynamic aspects, potential demand of technology in field, patent literature etc.     

Kinetic and thermodynamic studies of hydrogen adsorption on titanate nanotubes decorated with a Prussian blue analogue

In this paper, the kinetic and thermodynamic hydrogen adsorption characteristics of a novel composite comprising TiNT decorated with the Prussian blue analogue Cd₃Fe^III are investigated at high pressures and different temperatures. It is shown that boundary-layer (film) diffusion does not play a limiting role in the mass transport of hydrogen inside the composite material. The diffusion coefficient and time constant at different temperatures and pressures are calculated using an intra-particle diffusion model. The results suggest that molecular diffusion dominates Knudsen diffusion in the composite material. There are clear improvements in the mass transport characteristics compared to bulk Cd₃Fe^III. The Gibb's free energy is estimated by fitting isotherm equilibrium data to the Dubinin–Astakhov model and is used to calculate the enthalpy and the entropy of adsorption. The calculated value of enthalpy is characteristic of a physisorption process and is considerably higher than the activation energy for intraparticle diffusion, suggesting that the rate-limiting step of hydrogen is not mass transport to the adsorption sites.     

Feasibility study on the controlled hydriding combustion synthesis of Mg–La–Ni ternary hydrogen storage composite

The feasibility of using the controlled hydriding combustion synthesis (CHCS) under a high magnetic field to prepare the Mg–La–Ni ternary hydrogen storage alloys was studied. Comparison was made between the conventional hydriding combustion synthesis (HCS) and the CHCS. The influence of a high magnetic field on the physicochemical properties (thermodynamic and kinetic characteristics, hydrogen absorption/desorption (A/D) properties, thermal behavior, phase composition and morphology) of the Mg–La–Ni composite was analyzed and the results suggested that a high magnetic field can change the microstructure and phase compositions, decrease the hydriding/dehydriding (H/D) temperature and the particle size of the composite, and increase the H/D rates. Based on this study, the lower-temperature metal hydrides prepared by CHCS seem to be technically feasible for lab-scale and it could represent a possible and attractive alternative to prepare the hydrogen storage materials.     

The hydrogen transport through the metal alloy membranes with a spatial variation of the alloy composition: Potential diffusion and enhanced permeation

It is feasible to make hydrogen separation membranes of an alloy whose elemental or phase composition controllably varies at least in one direction. However the problem of hydrogen transport through such membranes is found not to be solved with the standard equation of diffusion in heterogeneous media. That is because the effect of heterogeneity on diffusion phenomena is considered to be caused by only spatial variations of diffusion coefficient while the spatial difference in the potential energy of diffusing particles due to their interactions with the inhomogeneous medium is not taken into consideration. The corrected equation shows the existence of an additional driving force making possible the diffusion in the direction opposite to that prescribed by Fick's law even in the isothermal medium when there is no any external field of force. The solution for the hydrogen transport through the membrane made of the alloy of variable composition indicates the possibility of great increase in the hydrogen permeation flux due to the optimization of spatial distribution of the alloy composition.     

Molecular Dynamics Simulation on Thermodynamic Properties and Transport Coefficients

ＭｏｌｅｃｕｌａｒＤｙｎａｍｉｃｓＳｉｍｕｌａｔｉｏｎｏｎＴｈｅｒｍｏｄｙｎａｍｉｃＰｒｏｐｅｒｔｉｅｓａｎｄＴｒａｎｓｐｏｒｔＣｏｅｆｆｉｃｉｅｎｔｓＤ．Ｘ．Ｘｉｏｎｇ；Ｙ．Ｓ．Ｘｕ，Ｚ．Ｙ．Ｇｕｏ（ＤｅｐａｒｔｍｅｎｔｏｆＥｎｇｉｎｅｅｒｉｎｇＭ...     

Analysis of composite hydrogen storage cylinders subjected to localized flame impingements

A comprehensive non-linear finite element model is developed for predicting the behavior of composite hydrogen storage cylinders subjected to high pressure and localized flame impingements. The model is formulated in an axi-symmetric coordinate system and incorporates with various sub-models to describe the behavior of the composite cylinder under extreme thermo-mechanical loadings. A heat transfer sub-model is employed to predict the temperature evolution of the composite cylinder wall and accounts for heat transport due to decomposition and mass loss. A composite decomposition sub-model described by Arrhenius's law is implemented to predict the residual resin content of thermal damaged area. A sub-model for material degradation is implemented to account for the loss of mechanical properties. A progressive failure model is adopted to detect various types of mechanical failure. These sub-models are implemented in ABAQUS commercial finite element code using user subroutines. Numerical results are presented for thermal damage, residual properties and profile of resin content in the cylinder. The developed model provides a useful tool for safe design and structural assessment of high pressure composite hydrogen storage cylinders.     

Thermodynamic analysis of two‐step solar water splitting with mixed iron oxides

A two‐step thermochemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, the present work describes a thermodynamic study aiming at the improvement of process conditions and at the evaluation of the theoretical potential of the process. In order to evaluate the maximum hydrogen production potential of a coating material, theoretical considerations based on thermodynamic laws and properties are useful and faster than actual tests. Through thermodynamic calculations it is possible to predict the theoretical maximum output of H₂ from a specific redox‐material under certain conditions. Calculations were focussed on the two mixed iron oxides nickel–iron‐oxide and zinc–iron‐oxide. In the simulation the amount of oxygen in the redox‐material is calculated before and after the water‐splitting step on the basis of laws of thermodynamics and available material properties for the chosen mixed iron oxides. For the simulation the commercial Software FactSage and available databases for the required material properties were used. The analysis showed that a maximum hydrogen yield is achieved if the reduction temperature is raised to the limits of the operation range, if the temperature for the water splitting is lowered below 800°C and if the partial pressure of oxygen during reduction is decreased to the lower limits of the operational range. The predicted effects of reduction temperature and partial pressure of oxygen could be confirmed in experimental studies. The increased hydrogen yield at lower splitting temperatures of about 800°C could not be confirmed in experimental results, where a higher splitting temperature led to a higher hydrogen yield. As a consequence it can be stated that kinetics must play an important role especially in the splitting step. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of magnetic and optic field in water electrolysis

Hydrogen production via water electrolysis was studied under the effect of magnetic and optical field. A diode solid state laser at blue, green and red were utilized as optical field source. Magnetic bar was employed as external magnetic field. The green laser has shown a greatest effect in hydrogen production due to its non-absorbance properties in the water. Thus its intensity of electrical field is high enough to dissociation of hydronium and hydroxide ions during orientation toward polarization of water. The potential to break the autoprotolysis and generate the auto-ionization is the mechanism of optical field to reveal the hydrogen production in water electrolysis. The magnetic field effect is more dominant to enhance the hydrogen production. The diamagnetic property of water has repelled the present of magnetic in water. Consequently the water splitting occurs due to the repulsive force induced by the external magnetic field. The magnetic distributed more homogenous in the water to involve more density of water molecule. As a result hydrogen production due to magnetic field is higher in comparison to optical field. However the combination both fields have generated superior effect whereby the hydrogen yields nine times higher in comparison to conventional water electrolysis.     

Mg-based metastable nano alloys for hydrogen storage

Mg-based materials have been widely researched for hydrogen storage development due to the low price of Mg, abundant resources of Mg element in the earth's crust and the high hydrogen capacity (ca. 7.7 mass% for MgH₂). However, the challenges of poor kinetics, unsuitable thermodynamic properties, large volume change during hydrogen sorption cycles have greatly hindered the practical applications. Here in this review, our recent achievements of a new research direction on Mg-based metastable nano alloys with a Body-Centered Cubic (BCC) lattice structure are summarized. Different with other metals/alloys/complex hydrides etc. which involve significant lattice structure and volume change from hydrogen introduction and release, one unique nature of this kind of metastable nano alloys is that the lattice structure does not change obviously with hydrogen absorption and desorption, which brings interesting phenomenon in microstructure properties and hydrogen storage performances (outstanding kinetics at low temperature and super high hydrogen capacity potential). The synthesis results, morphology and microstructure characterization, formation evolution mechanisms, hydrogen storage performances and geometrical effect of these metastable nano alloys are discussed. The nanostructure, fresh surface from ball milling process and fast hydrogen diffusion rate in BCC lattice structure, as well as the unique nature of maintaining original BCC metal lattice during hydrogenation result in outstanding hydrogen storage performances for Mg-based metastable nano alloys. This work may open a new sight to develop new generation hydrogen storage materials.     

Stress/strain effects on thermodynamic properties of magnesium hydride: A brief review

Magnesium hydride MgH₂ is an attractive hydrogen storage candidate due to its high reversible hydrogen mass capacity of 7.6 wt%, abundant resources of Mg and low cost. Unfortunately, its stubborn thermodynamic stability results in a high temperature of 573 K for hydrogen desorption, which is still far from the target for practical applications. In this article, we highlight the recent advances in stress/strain effects on the de/rehydrogenation thermodynamics of MgH₂, which sheds a new light on tuning the thermodynamic properties of magnesium and other metal hydrides for hydrogen storage.     

Recent progress of nanotechnology in enhancing hydrogen storage performance of magnesium-based materials: A review

Hydrogen has attracted wide attention in the field of new energy, triggering a comprehensive study of hydrogen production, storage and application. This paper mainly studies the hydrogen storage capacity of magnesium-based materials with nanostructure. The reversible hydrogen capacity of Mg-based hydrogen storage materials can reach 7.6 wt%, but due to its poor kinetic and thermodynamic properties, its hydrogen storage performance is not as good as other hydrogen storage materials. In order to reduce the desorption temperature of materials, many studies have been carried out. Alloying, nanostructure and adding catalyst are feasible methods to improve the properties of Mg-based hydrogen storage alloys. By adding catalyst and alloy with other transition elements, the dehydrogenation temperature of magnesium-based materials has been reduced to less than 200 °C. The hydrogen storage of magnesium-based alloys has been practically applied.     

Improved thermodynamic properties of doped LiBH4 for hydrogen storage: First-principal calculation

First-principles calculations have been performed on lithium borohydride LiBH₄ using the ultrasoft pseudopotential method, which is a potential candidate for hydrogen-storage materials due to its extremely large gravimetric capacity of 18 mass % hydrogen. We focus on an orthorhombic phase observed at ambient conditions and predict its fundamental properties; De-hydrogenation and electronic properties of doped Li_1+xB_1−xH₄ by Li (with 0 < x < 0.75); to be used as a material for hydrogen-storage; are studied from density-functional theory based first-principles calculations. The results suggest that the substitution of B by Li decrease the desorption enthalpy of hydrogen from 75 kJ/mol.l to 40. Our calculation results show the function of Li in improving thermodynamics, which provides a favorable thermodynamic modification.     

Numerical simulation of transcritical strained laminar flames

We investigate reactive and non-reactive strained flows associated with high pressure cryogenic rocket engines. A detailed high pressure fluid model based on thermodynamics of irreversible processes, statistical mechanics as well as kinetic theory of dense gases is used. This model insures the positivity of chemical entropy production and of molecular transport related entropy production. We first investigate a mixing layer between cold hydrogen and oxygen and the dramatic influence of nonideal transport near thermodynamic instabilities. Diffusion and partially premixed H₂–O₂ transcritical flame structures are then studied as well as strain extinction limits and dilution extinction limits.     

Models of thermodynamic and transport properties of POE VG68 and R410A/POE VG68 mixture

The thermodynamic properties of a refrigerant-oil mixture are the foundation to predict the performance of air-conditioning and refrigeration systems and to evaluate the influence of oil on heat transfer and pressure drop. Models of the thermodynamic and transport properties of POE VG68 and R410A/POE VG68 mixture were provided based on the analysis of state-of-the-art correlations. New models were developed by modifying the coefficients in existing correlations with multiple regression method according to experimental data. The maximum deviation of the predicted values of these models to the experimental data is within 5%. These models can be used for R410A/POE VG68 to obtain accurate and reliable thermodynamic and transport parameters to evaluate the influence of POE VG68 on the performance of an R410A air-conditioning and refrigeration system.     

Accurate evaluation of hydrogen crossover in water electrolysis systems for wetted membranes

In fuel cell and electrolysis systems, hydrogen crossover is a phenomenon where hydrogen molecules (H₂) permeate through a membrane, lowering the overall process efficiency and generating a potential safety risk. Many works have been reported to mitigate this undesired phenomenon, but it is yet difficult to accurately measure the rate of hydrogen crossover, particularly when the membrane is fully wetted in water. In this work, we investigated the pressure decay method as a simple, convenient, and low-cost method to characterize hydrogen crossover through wetted membranes for water electrolysis systems. Three different ion exchange membranes were analyzed: Nafion 212, Nafion 115, and in-house sulfonated poly(arylene ether sulfone). We rigorously confirmed our method and data by comparing it to the ANSI dataset with the current state-of-the-art equations of state (EOS) to account for the nonideality of high pressure hydrogen systems. The error from the gas non-ideality was less than 0.03%. As expected, the rate of hydrogen crossover showed high dependency on the temperature; more importantly, hydrogen crossover increased significantly when the membrane was fully soaked in water. For dry membranes, the proposed pressure decay method corroborated well with the literature data measured using other known methods. Moreover, for wetted membranes, the obtained data showed high similarity compared to the GC method which is currently the most reliable method in the literature. We attempted to predict the hydrogen permeability of wetted membranes using the solution diffusion model. The model based on the given thermodynamic parameters overestimated the hydrogen permeability, which can be used to estimate the ion channel tortuosity.