Hydrogen permeation under high pressure conditions and the destruction of exposed polyethylene-property of polymeric materials for high-pressure hydrogen devices (2)-

Aiming to elucidate physical property affecting to hydrogen gas permeability of polymer materials used for liner materials of storage tanks or hoses and sealants under high-pressure environment, as model materials with different free volume fraction, five types of polyethylene were evaluated using two methods. A convenient non-steady state measurement of thermal desorption analysis (TDA), and steady-state high-pressure hydrogen gas permeation test (HPHP) were used both under up to 90 MPa of practical pressure. The limit of TDA method of evaluation for the specimens suffering fracture during decompression process after hydrogen exposure was found. Permeability coefficient decreased with the decrease of diffusion coefficient under higher pressure condition. Specific volume and degree of crystallinity under hydrostatic environment were measured. The results showed that the shrinkage in free volume caused by hydrostatic effects of the applied hydrogen gas pressure decreases diffusion coefficient, resulting in the decrease of permeability coefficient with the pressure rise.     

Optimization of electrochemical hydrogen compression through computational modeling

One of the main challenges in developing the hydrogen infrastructure is the distribution and storage of hydrogen. A common method to store hydrogen is as a compressed gas. Electrochemical compression (ECC) is a promising technology that can overcome some of the disadvantages of conventional mechanical compressors. ECC employs an externally powered electrochemical cell containing a polymer electrolyte membrane to compress the gas. This work presents a comprehensive 3D ECC model developed for a single cell using COMSOL Multiphysics 5.6 that incorporates all relevant physical and electrochemical processes, and examines the effect of key parameters on ECC performance. It also considers the important phenomenon of back diffusion resulting from the high-pressure differential between the cathode and anode during compression. Results from the current simulations were validated against experimental results obtained previously in our lab. Simulations were first conducted for the unpressurized cathode to understand the effect of membrane thickness, relative humidity of the anode hydrogen supply, temperature, and gas diffusion layer thickness on ECC performance. Next, simulations were conducted for the pressurized cathode, with and without considering back diffusion. In the absence of back diffusion, the pressure ratio reaches the value predicted by the Nernst equation. However, the presence of back diffusion greatly reduces the pressure ratio as was also observed in experiments. The study reveals that three parameters in particular viz. Membrane thickness, operating temperature, and voltage must be carefully selected to optimize ECC operation. These results also suggest that ECC is a viable alternative to conventional technologies for hydrogen compression. This work also provides a foundation for the modeling and analysis of full-scale ECC systems.     

Prediction of state property,flow parameter and jet flame size during transient releases from hydrogen storage systems

The accidental leakage of high-pressure gas storage systems including tank, pipe, etc. can lead to hazardous jet fires resulting in a serious of disastrous events. With the isentropic process assumption on the high-pressure gas leakage or release, the ideal gas equation of state is firstly used to solve the gas transfer problem, and then the Abel-Noble equation of state (AN-EOS) is adopted for the effect of gas molecule volume. Given both the molecule volume and intermolecular attraction should not be ignored for the high-pressure gas, this paper attempts to build the high-pressure gas leakage process model based on the van der Waals equation of state. Together with the available notional nozzle model and the flame size model, the gas leakage process model is used to calculate the gas state property and flow parameter of hydrogen tank leakage and its subsequent jet flame height. The predicted gas mass flow rate, flame height, and gas pressure and temperature are compared to the experimental measurements for validation and the predictions of the model based on ideal gas equation of state and AN-EOS. It is found that the proposed model can give more encouraging results compared to the previous models. The proposed theoretical model shows a great implication for the calculation of other gas tank leakage and can help to predict the thermal radiation field of jet fires.     

From the perspective of new technology of blending hydrogen into natural gas pipelines transmission: Mechanism,experimental study,and suggestions for further work of hydrogen embrittlement in high-strength pipeline steels

Blending hydrogen into high-strength pipeline steels for high-pressure transmission may cause materials' hydrogen embrittlement (HE) failure. Although the hydrogen-induced failure of metallic materials has been studied for a long time, the process of hydrogen into the materials, hydrogen-induced delayed failure, and dynamic mechanisms of high-strength pipeline steels under high pressure have not been fully understood. This paper aims to provide a detailed review of the latest research on the hydrogen-induced failure of high-strength pipeline steels in hydrogen-blended natural gas transmission. First, introduced the typical hydrogen blending natural gas pipeline transmission projects and their associated research conclusions. Then, described the physical process of the HE in high-strength pipeline steels and the principle, development, and latest research progress of typical hydrogen embrittlement mechanisms in detail. Third, reviewed the research methods and progress of experimental and theoretical simulations for the HE in steels, including hydrogen permeation (HP) experiments, hydrogen content measurements, hydrogen distribution detection, mechanical property tests, and molecular dynamics simulations. The shortcomings of existing experimental and theoretical simulation methods in the hydrogen-induced analysis of high-strength natural gas pipeline steels under high pressure are discussed. Finally, the future research directions and challenges of this problem are proposed from three aspects: the multimechanism synergy mechanism, the improvement of experimental methods, and the establishment of a new interatomic multiscale model.     

Analysis of hydrogen diffusion coefficient during hydrogen permeation through pure niobium

The hydrogen solubility and the hydrogen permeability of pure niobium at high temperature are investigated in order to analyze the hydrogen diffusion coefficient during the hydrogen permeation. It is shown that the hydrogen dissolution reaction into niobium metal does not follow the Sieverts' law at the practical hydrogen permeation pressures. The hydrogen diffusion coefficient during the hydrogen permeation through pure niobium at high temperature is evaluated from the linear relationship between the normalized hydrogen flux, J·d, and the hydrogen concentration difference, ΔC. It is found that the hydrogen diffusion coefficient under the practical condition is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd–Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773 K during the hydrogen permeation.     

Multi-criteria analysis for screening of reversible metal hydrides in hydrogen gas storage and high pressure delivery applications

Metals and alloys forming reversible hydrides with hydrogen gas are potential building blocks for compact, solid state hydrogen storage systems. Based on the materials’ thermodynamic characteristics, their use as temperature-swing gas compression and delivery systems in the hydrogen economy is also possible. Given the wide variety of materials developed and tested at laboratory and pilot scales, a harmonized method of selecting the feasible material(s) for a particular real-life application is required. This study proposes a system selection framework based on a normalized, multi-criteria metric. Using calculated values of multi-criteria metric, multi-criteria screening and ranking of potential materials has been demonstrated for a particular use case. It is found that the alloy TiMn_1.52 having value of additive metric between 0.25 and 0.35 represents the best material for a single stage system. The alloy pair CaNi₅–Ti_1.5CrMn represents the best alternative for a two-stage system with additive metric values between 0.63 and 0.82. Energy and economic characteristics of the metal hydride gas compression and delivery systems are evaluated and compared with an equivalent mechanical compression system producing the same final effect (i.e., delivery of a given quantity of gas at a defined pressure).     

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

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

Mechanical property and hydrogen permeability of ultrafine-grained Pd–Ag alloy processed by high-pressure torsion

A process of high-pressure torsion (HPT) was used to produce an ultrafine-grained Pd–Ag alloy, and to improve mechanical property and hydrogen permeability simultaneously. Hardness values of the HPT-processed sample were much higher than those of a cold-rolled sample which is strengthened by dislocation accumulation. Additionally, in contrast to the degradation of hydrogen permeability in the cold-rolled sample due to a high density of dislocations which act as trapping sites of hydrogen atoms, the hydrogen permeability in the HPT-processed sample was improved due to a high density of high-angle grain boundaries which act as a fast diffusion path. The ultrafine-grained structure in the Pd–Ag alloy was retained during permeation testing at 300 °C due to the addition of silver.     

Water permeation through gas diffusion layers of proton exchange membrane fuel cells

Water transport through gas diffusion layer of proton exchange membrane fuels cells is investigated experimentally. A filtration cell is designed and the permeation threshold and the apparent water permeability of several carbon papers are investigated. Similar carbon paper with different thicknesses and different Teflon loadings are tested to study the effects of geometrical and surface properties on the water transport. Permeation threshold increases with both GDL thickness and Teflon loading. In addition, a hysteresis effect exists in GDLs and the permeation threshold reduces as the samples are retested. Moreover, several compressed GDLs are tested and the results show that compression does not affect the breakthrough pressure significantly. The measured values of apparent permeability indicate that the majority of pores in GDLs are not filled with water and the reactant access to the catalyst layer is not hindered.     

Damage evolution in polymer due to exposure to high-pressure hydrogen gas

The use of hydrogen as a fuel is increasing exponentially, and the most economical way to store and transport hydrogen for fuel use is as a high-pressure gas. Polymers are widely used for hydrogen distribution and storage systems because they are chemically inert towards hydrogen. However, when exposed to high-pressure hydrogen, some hydrogen diffuses through polymers and occupies the preexisting cavities inside the material. Upon depressurization, the hydrogen trapped inside polymer cavities can cause blistering or cracking by expanding these cavities. A continuum mechanics–based deformation model was deployed to predict the stress distribution and damage propagation while the polymer undergoes depressurization after high-pressure hydrogen exposure. The effects of cavity size, cavity location, and pressure inside the cavity on damage initiation and evolution inside the polymer were studied. The stress and damage evolution in the presence of multiple cavities was also studied, because interaction among cavities alters the damage and stress field. It was found that all these factors significantly change the stress state in the polymer, resulting in different paths for damage propagation. The effect of adding carbon black filler particles and plasticizer on the damage was also studied. It was found that damage tolerance of the polymer increases drastically with the addition of carbon black fillers, but decreases with the addition of the plasticizer.     

Research on two‐phase flow considering hydrogen crossover in the membrane for a polymer electrolyte membrane fuel cell

Hydrogen crossover has an important effect on the performance and durability of the polymer electrolyte membrane fuel cell (PEMFC). Severe hydrogen crossover can accelerate the degradation of membrane and thus increase the possibility of explosion. In this study, a two‐phase, two‐dimensional, and multiphysics field coupling model considering hydrogen crossover in the membrane for PEMFC is developed. The model describes the distributions of reactant gases, current density, water content in membrane, and liquid water saturation in cathode electrodes of PEMFC with intrinsic hydrogen permeability, which is usually neglected in most PEMFC models. The conversion processes of water between gas phase, liquid phase, and dissolved water in PEMFC are simulated. The effects of changes in hydrogen permeability on PEMFC output performance and distributions of reactant gases and water saturation are analyzed. Results showed that hydrogen permeability has a marked effect on PEMFC operating under low current density conditions, especially on the open circuit voltage (OCV) with the increase of hydrogen permeability. On the contrary, the effect of hydrogen permeability on PEMFC at high current density is negligible within the variation range of hydrogen permeability in this study. The nonlinear relations of OCV with hydrogen diffusion rate are regressed.     

Quantitative contribution of non-ideal permeability under diffusion-controlled hydrogen permeation through Pd-membranes

In this paper, a powerful and simple method to evaluate quantitatively the influence of the non-ideal contribution of the hydrogen permeation under diffusion control is provided. For this purpose, a Non-Ideality Index (α) is defined as the ratio of the non-ideal permeability term and the overall permeability, this leading to a direct quantification of the non-ideal effect in membrane.     

Flow characteristics of hydrogen gas through a critical nozzle

Critical nozzles are widely used in the flow measurement and can be used for mass flow-rate measurement of hydrogen gas. The effect of real gas state equation on discharge coefficient of hydrogen gas flow through a critical nozzle was investigated. The real gas critical flow factor was introduced which considers the effect of the real gas on discharge coefficient. An analytic solution of real gas critical flow factor of hydrogen gas calculated from the modern equations of state based on Helmholtz energy, over a wider range of temperature 150–600 K and pressure up to 100 MPa was presented. An accurate empirical equation for real gas critical flow factor was determined by the nonlinear regression analysis. The equation was in good agreement with the high-pressure hydrogen gas experimental data by Morioka and CFD solutions by Nagao and Kim. Using this equation, the discharge coefficient can be directly and accurately calculated. It indicates that the discharge coefficient of hydrogen gas should be comprehensively taken into consideration with stagnation temperature, stagnation pressure and nozzle throat diameter. A lot of detailed results about the effect of real gas state equation were obtained.     

An analytical model for hydrogen and nitrogen crossover rates in proton exchange membrane fuel cells

In this paper, the hydrogen and nitrogen crossover through the membrane in proton exchange membrane fuel cells, are investigated by developing a semi-empirical analytical model. Different factors that affect the gas crossover rates were considered including pressure drop in gas diffusion layer (GDL) and catalyst layer (CL), operating temperature, relative humidity (RH) of the reactants, GDL compression, and the current density effect on the membrane temperature. The model is validated by published experimental data. It is found that RH is the most important parameter, followed by temperature. The hydrogen pressure drop through GDL and CL greatly depends on the GDL substrate properties, microporous layer (MPL) and CL. When permeability is low, an increase in current density reduces gas crossover. GDL compression, when MPL is used, was found to have a low impact on gas crossover. Gas crossover is improved with current density due to an increase in membrane temperature.     

A thermodynamic analysis of refueling of a hydrogen tank

A thermodynamic analysis of refueling of a gaseous hydrogen fuel tank is described. This study may lend itself to the applications of refueling a hydrogen storage tank onboard a hydrogen fuel-cell vehicle. The gaseous hydrogen is treated as an ideal or a non-ideal gas. The refueling process is analyzed based on adiabatic, isothermal, or diathermal condition of the tank. A constant feed-rate is assumed in the analysis. The thermodynamic state of the feed stream also remains constant during refueling. Ideal-gas assumption results in simple closed-form expressions for tank temperature, pressure, and other parameters. The non-ideal behavior of high-pressure gaseous hydrogen is addressed using the newly developed equation of state for normal hydrogen, which is based on the reduced Helmholtz free energy formulation. Sample calculations are presented using initial tank and feed stream conditions commensurate to practical vehicular applications. Comparing to the non-ideal analysis, the ideal-gas assumption always results in under-prediction of the tank temperature and pressure irrespective of the filling condition. For a given target tank pressure, the refueling time is the shortest under adiabatic condition and is the longest under isothermal condition with the tank being maintained at the initial tank temperature. The adiabatic and isothermal conditions can be viewed, respectively, as the lower and upper bounds of the refueling time for a given final target tank pressure.     

Effect of sweep gas on hydrogen permeation of supported Pd membranes: Experimental and modeling

Membrane reactor processes are being increasingly proposed as an attractive solution for pure hydrogen production due to the possibility to integrate production and separation inside a single reactor vessel. High hydrogen purity can be obtained through dense metallic membranes, especially palladium and its alloys, which are highly selective to hydrogen. The use of thin membranes seems to be a good industrial solution in order to increase the hydrogen flux while reducing the cost of materials. Typically, the diffusion through the membrane layer is the rate-limiting step and the hydrogen permeation through the membrane can be described by the Sieverts’ law but, when the membrane becomes thinner, the diffusion through the membrane bulk becomes less determinant and other mass transfer limitations might limit the permeation rate. Another way to increase the hydrogen flux at a given feed pressure, is to increase the driving force of the process by feeding a sweep gas in the permeate side. This effect can however be significantly reduced if mass transfer limitations in the permeate side exist. The aim of this work is to study the mass transfer limitation that occurs in the permeate side in presence of sweep gas. A complete model for the hydrogen permeation through PdAg membranes has been developed, adding the effects of concentration polarization in retentate and permeate side and the presence of the porous support using the dusty gas model equation, which combines Knudsen diffusion, viscous flow and binary diffusion. By studying the influence of the sweep gas it has been observed that the reduction of the driving force is due to the stagnant sweep gas in the support pores while the concentration polarization in the permeate is negligible.     

Evaluation of a thermally-driven metal-hydride-based hydrogen compressor

Currently, the hydrogen storage method used aboard fuel cell electric vehicles utilizes pressures up to 70 MPa. Attaining such high pressures requires mechanical gas compression or hydrogen liquefaction followed by heating to form a high-pressure gas, and these processes add to the cost and reduce the energy efficiency of a hydrogen fueling system. In previous work we have evaluated the use of high-pressure electrolysis, in which hydrogen is generated from water and the electrolyzer boosts the hydrogen pressure to values from 13 to 45 MPa. While electrolytic compression is a novel and energy efficient method to produce high-pressure hydrogen, it has several limitations at present and will require more development work. Another concept is to use hydrogen absorbing alloys that form metal hydrides, in combination with a heat engine (hot and cold reservoirs), to drive a cyclic process in which hydrogen gas is absorbed and desorbed to compress hydrogen. Furthermore, by using a thermally-driven compressor, the hot and cold reservoirs can be obtained using renewable energy such as sunlight for heating together with ambient air or water for cooling. In this work we evaluated the thermodynamics and kinetics of a prototype metal hydride hydrogen compressor (MHHC) built for us by a research group in China. The compressor utilized a hydrogen input pressure of approximately 14 MPa, and, operating between an initial temperature of approximately 300 K and a final temperature of 400 K, a pressure of approximately 41 MPa was attained. In a series of experiments with those conditions the average compression ratio for a single-stage compression was approximately three. In the initial compression cycles, up to 300 g of hydrogen was compressed for each 100 K temperature cycle. The enthalpy of the metallic-alloy-hydriding reaction was found to be approximately 20.5 kJ per mole of H₂, determined by measuring the pressure composition isotherm at three temperatures and using a Van't Hoff plot. The thermodynamic efficiency of the compressor, as measured by the value of the compression work performed divided by the heat energy added and removed in one complete cycle, was determined via first and second law analyses. The Carnot efficiency was approximately 25%, the first law efficiency was approximately 3–5%, and the second law efficiency was approximately 12–20%, depending on the idealized compression cycle used to assign a value to the compression work, as well as other assumptions. These efficiencies compare favorably with values reported for other thermally-driven compressors.     

A mixed electronic and protonic conducting hydrogen separation membrane with asymmetric structure

In this work, highly doped ceria with lanthanum, La_0.5Ce_0.5O_2−δ (LDC), are developed as hydrogen separation membrane material. LDC presents a mixed electronic and protonic conductivity in reducing atmosphere and good stability in moist CO₂ environment. LDC separation membranes with asymmetrical structure are fabricated by a cost-saving co-pressing method, using NiO + LDC + corn starch mixture as substrate and LDC as top membrane layer. Hydrogen permeation properties are systemically studied, including the influence of operating temperature, hydrogen partial pressure in feed stream and water vapor in both sides of the membrane on hydrogen permeating fluxes. Hydrogen permeability increases as the increasing of temperature and hydrogen partial pressure in feed gas. Using 20% H₂/N₂ (with 3% of H₂O) as feed gas and dry high purity argon as sweep gas, an acceptable flux of 2.6 × 10⁻⁸ mol cm⁻² s⁻¹ is achieved at 900 °C. The existing of water in both sides of membrane has significant effect on hydrogen permeation and the corresponding reasons are analyzed and discussed.     

Hydrogen separation from blended natural gas and hydrogen by Pd-based membranes

Hydrogen separation membranes based on a heated metal foil of a palladium alloy, offer excellent permeability for hydrogen as a result of the solution-diffusion mechanism. Here, the possibility to separate hydrogen from the mixture of Natural Gas (NG) and hydrogen (NG+H₂) with various NG concentrations using Pd, PdCu₅₃ and PdAg₂₄ hydrogen purification membranes is demonstrated. Hydrogen concentrations above ∼25% (for Pd and PdCu₅₃) and ∼15% (for PdAg₂₄) were required for the hydrogen separation to proceed at 400 °C and 5 bar pressure differential. Hydrogen permeability of the studied alloys could be almost fully recovered after switching the feed gas to pure hydrogen, indicating no significant interaction between the natural gas components and the membranes surface at the current experimental condition. Hydrogen flux of the membranes at various pressure differential was measured and no changes in the hydrogen permeation mechanism could be noticed under (NG 50%+H₂) mixture. The hydrogen separation capability of the membranes is suggested to be mainly controlled by the operating temperature and the hydrogen partial pressure.