Effective hydrogen diffusion coefficient for CoCrFeMnNi high-entropy alloy and microstructural behaviors after hydrogen permeation

Herein, the first observation of the effective hydrogen diffusion coefficient of CoCrFeMnNi high-entropy alloy (HEA) was performed using electrochemical hydrogen permeation; further, it was compared with those of stainless steels (SS) 304 and 316L. HEA and SS 316L showed similar effective hydrogen diffusion coefficient of 1.75 × 10⁻¹¹ m²/s and 1.91 × 10⁻¹¹ m²/s, respectively. SS 304 showed the smallest that of 0.58 × 10⁻¹¹ m²/s in the study. Hydrogen diffusion through the grain boundary was dominant in face-centered cubic metals. Hydrogen permeation resulted in no change in the microstructure of HEA and SS 316L; however, it caused a martensitic transformation in SS 304.     

Powering microbial electrolysis cells by electricity generation from simulated waste heat of anaerobic digesters using thermoelectric generators

Waste heat from anaerobic digesters can be converted to electricity by using thermoelectric generators (TEG). Herein, such energy was employed to power a microbial electrolysis cell (MEC) for producing hydrogen gas. Four TEG units could deliver a voltage of ~0.5 V, sufficient to drive the MEC that achieved a hydrogen production rate of 0.48 ± 0.13 m³ m⁻³ d⁻¹. This rate was further improved to 0.75 ± 0.05 m³ m⁻³ d⁻¹ when the temperature difference for TEG was increased from 18 to 28 °C. There was no significant difference between the TEG-powered MEC and power supply-supported MEC (at 0.6 V), in terms of current generation, hydrogen production, and organic removal. Ambient air was also studied as a cold-side source for TEG, although some challenges were encountered to maintain a large temperature difference. Those results will encourage further exploration of using TEG as a feasible power supply for sustainable MEC operation.     

Analysis and modelling of microwave plasma hydrogen production utilizing water vapor and tungsten electrodes

In this study, hydrogen production via microwave plasma is investigated, analyzed and simulated in a novel way for practical applications. The water vapor when in proximity of a tungsten electrode is modeled for the generation of hydrogen gas. A numerical simulation study is performed using plasma and electromagnetic wave COMSOL modules to analyze the plasmolysis of water vapor within a vacuum concealed reaction vessel entailing a tungsten electrode. A kinetic model is therefore developed to represent the reaction mechanisms and interactions between the species within the plasmolysis reactor. The dynamic results of electron density, electron temperature, plasma rate, and species interactions are extracted through the kinetic model. Within the time domain of 10⁻¹⁶ to 10⁻¹⁴ s, the hydrogen concentration is found to increase to 4.5815 $\times$ 10⁻¹¹ mol/m³ with a corresponding decrease in water vapor concentration of 1.782 $\times$ 10⁻⁸ mol/m³, respectively. The dynamic variations in the concentrations of other dissociated species are investigated across the geometry of the reaction domain studied, and it is therefore concluded that the tip of the electrode entails the highest species concentrations.     

CFD modeling and consequence analysis of an accidental hydrogen release in a large scale facility

In this study, the consequences of an accidental release of hydrogen within large scale, (>15,000 m³), facilities were modeled. To model the hydrogen release, an LES Navier–Stokes CFD solver, called fireFoam, was used to calculate the dispersion and mixing of hydrogen within a large scale facility. The performance of the CFD modeling technique was evaluated through a validation study using experimental results from a 1/6 scale hydrogen release from the literature and a grid sensitivity study. Using the model, a parametric study was performed varying release rates and enclosure sizes and examining the concentrations that develop. The hydrogen dispersion results were then used to calculate the corresponding pressure loads from hydrogen-air deflagrations in the facility.     

Pressure peaking phenomenon: Model validation against unignited release and jet fire experiments

The aim of this study is validation of pressure peaking phenomenon models for unignited and ignited releases of hydrogen in enclosures with limited ventilation, e.g. residential garages. The existence of “unexpected” peak in the pressure transient during release of a lighter than air gas in a vented enclosure was observed by Brennan et al. (2010) by carrying out theoretical and numerical research. The amplitude and duration of this pressure peak vary depending on the enclosure volume, vent size and leak flow rate. The peak can significantly exceed the steady-state overpressure, which is reached when the enclosure is fully occupied by leaking with a constant rate gas. The pressure peaking phenomenon can jeopardise a civil structure integrity in the case of accident if it is ignored at the design stage of hydrogen-powered vehicles. This could cause serious life safety and property protection issues that requires development of prevention and mitigation strategies and innovative safety engineering solutions. The experimental validation of the phenomenon was absent up to this work. The previous model for unignited release and developed in this study model for ignited release (jet fire) have been validated against experiments performed in a vented enclosure of 1 m³ volume with three different gases: air, helium, and hydrogen. The model for unignited release reproduces closely the experimental pressure peak and the pressure dynamics within the enclosure. The model for ignited release reproduces the pressure peak with acceptable engineering accuracy, and the simulation of pressure dynamics after the peak requires the increase of the discharge coefficient due to the change of vent flow from heavier air at the start to lighter hot combustion products afterwards and ultimately hydrogen. The methodology to calculate the pressure peaking phenomenon in two steps is described in detail. Examples of pressure peaking phenomenon calculation for typical hydrogen applications are presented. The phenomenon is relevant to most of indoor applications, when release of lighter than air gas is possible in an enclosure with limited ventilation. It must be considered when performing safety engineering design of inherently safer hydrogen systems and infrastructure.     

Improved hydrogen permeation through thin Pd/Al2O3 composite membranes with graphene oxide as intermediate layer

Here we proposed the decreasing in the roughness of asymmetric alumina (Al₂O₃) hollow fibers by the deposition of a thin graphene oxide (GO) layer. GO coated substrates were then used for palladium (Pd) depositions and the composite membranes were evaluated for hydrogen permeation and hydrogen/nitrogen selectivity. Dip coating of alumina substrates for 45, 75 and 120 s under vacuum reduced the surface mean roughness from 112.6 to 94.0, 87.1 and 62.9 nm, respectively. However, the thicker GO layer (deposited for 120 s) caused membrane peel off from the substrate after Pd deposition. A single Pd layer was properly deposited on the GO coated substrates for 45 s with superior hydrogen permeance of 24 × 10⁻⁷ mol s⁻¹m⁻² Pa⁻¹ at 450 °C and infinite hydrogen/nitrogen selectivity. Activation energy for hydrogen permeation through the Al₂O₃/GO/Pd composite membrane was of 43 kJ mol⁻¹, evidencing predominance of surface rate-limiting mechanisms in hydrogen transport through the submicron-thick Pd membrane.     

Transport of hydrogen and deuterium in 316LN stainless steel over a wide temperature range for nuclear hydrogen and nuclear fusion applications

The permeation of hydrogen and deuterium through 316LN stainless steel (316LN SS) was investigated over a wide temperature range of 300–850 °C for nuclear hydrogen and nuclear fusion applications. We presented the first complete datasets of permeability Φ, diffusivity D, and solubility S for both hydrogen (H) and deuterium (D) in 316LN SS. Φ_H and Φ_D were 3.47 × 10⁻⁷exp(−66.6 × 10³/RT) and 2.71 × 10⁻⁷exp(−67.5 × 10³/RT) mol·m⁻¹ s⁻¹ Pa^−0.5, respectively. D_H and D_D were 15.9 × 10⁻⁷exp(−56.5 × 10³/RT) and 13.8 × 10⁻⁷exp(−56.8 × 10³/RT) m²∙s⁻¹, respectively. The estimated isotope effect ratios of Φ_H/Φ_D, D_H/D_D, and S_H/S_D were ~1.4, ~1.2, and ~1.2, respectively. The previously reported results for 316LN SS were extrapolated to the temperature range used herein and were compared with the results of this study. Although some discrepancies were observed between the results of this study and previous studies, they were within the acceptable scattering range.     

Hydrogen production and storage analysis of a system by using TRNSYS

In this study, hydrogen production and storage were investigated. The Transient System Simulation Program (TRNSYS) and Generic Optimization Program (GenOpt) packages were combined for the design and optimization of a system that produces hydrogen from water and stores the hydrogen it produced in the compressed gas tank. The system design is based on the electricity grid. Electrical energy produced in photovoltaic (PV) panels was used to electrolyze water. The systems for Izmir, Istanbul and Ankara provinces which are in different climate zones of Turkey were optimized and the annual system performances based on the optimum angles were analyzed. For the mentioned provinces, the PV tilt angles which minimize electricity drawn from the grid at the electrolyzer are also investigated. The electrical energy produced in the photovoltaic panels, the hydrogen and oxygen amounts produced, the efficiency of the electrolyzer, the gas and pressure levels in the hydrogen tank were compared. According to the results of the analysis, the annual total power produced in photovoltaic panels is 42803.66 kW in İzmir, 42573.74 kW in Istanbul and 44613.95 kW in Ankara. Hydrogen levels produced in the system are calculated as 10488.39 m³ year⁻¹ in Izmir, 9824.70 m³ year⁻¹ in Istanbul, and 10368.65 m³ year⁻¹ in Ankara.     

Effect of alkaline earth oxides on nickel catalysts supported over γ-alumina for butanol steam reforming: Coke formation and deactivation process

Nickel catalysts were synthesized by the wet impregnation of three different supports: γ-Al₂O₃ and alumina promoted with either 10 wt % of MgO or 10 wt% of CaO. The catalysts were evaluated in butanol steam reforming at 500 °C, atmospheric pressure, GHSV of 500,000 h⁻¹ and 10% v/v butanol in the feed. Both promoters decreased catalyst acidity and increased basicity. The catalyst promoted with MgO exhibited the lowest acidity (1.1 μmolNH₃ m⁻²), whereas that promoted with CaO, the highest basicity (870.7 μmolCO₂ m⁻²). The promotion with MgO led to the highest hydrogen yield (66%) and stability, associated with its highest nickel dispersion (3.4%), lowest acidity and lowest coke formation normalized by carbon converted (3.0 mmol L mol⁻¹). The catalyst promoted with CaO presented the most severe deactivation, associated with its lowest dispersion (1.0%) and the highest amount of encapsulated coke (3.5 mmol L mol⁻¹).     

Kinetics of hydrogen desorption from Zircaloy-4: Experimental and modelling

Under Pressurized Water Reactor normal operating conditions, the external surface of zirconium alloys cladding absorbs a fraction of the hydrogen produced by water reduction. During spent fuel transport, hydrogen may desorb from the cladding. The study aims to identify and quantify the rate-limiting step in the hydrogen desorption process initially present in the alloy. To better understand this process, the Thermal Desorption Spectrometry (TDS) was used in association with X-ray Photoelectron Spectroscopy analysis. TDS results were analysed with finite elements simulations using the Cast3M code. The optimization of the kinetic constants of hydrogen desorption was performed with CEA (Alternative Energies and Atomic Energy Commission)-tool URANIE. Results showed that hydrogen desorption kinetics from the metal is limited by the surface molecular recombination. Arrhenius-type temperature dependence of kinetic constants allowed to simulate experimental data with a good agreement. The optimized activation energy and the pre-exponential factor for desorption processes were in the range of 290 ± 10 kJ mol⁻¹ and 3 × 10⁷ m⁴ mol⁻¹ s⁻¹ respectively.     

One-step electroplating of palladium–copper alloy layers on a vanadium membrane for hydrogen separation: Quick,easy, and low-cost preparation

Palladium–copper (PdCu) alloy layers with 150 nm thickness were prepared by one-step electroplating on both sides of a vanadium foil. The entire process, including pretreatment, washing, and electroplating, was complete in <5 min. The PdCu layer had a high surface coverage and almost completely covered the vanadium foil. The composition difference between the center and the edges was within 3 at%, and the alloy composition was controlled to nearly the target composition (50:50). Peel tests revealed that the as-electroplated layer had sufficient adhesion for practical applications. XRD analysis confirmed the successful formation of a face-centered cubic (FCC) PdCu alloy. In-situ XRD analysis performed under a hydrogen atmosphere revealed the structural change of PdCu from FCC to body-centered cubic (BCC) immediately after the temperature reached 300 °C. The hydrogen permeability of electroplated PdCu/V/PdCu at 200–300 °C (1–3 × 10⁻⁸ mol m⁻¹ s⁻¹ Pa^−1/2) was considerably higher than that of the 10 μm-thick electroplated BCC PdCu.     

Rapid preparation of hydrogen barrier films by a novel ultrasonic atomization-assisted layer-by-layer self-assembly method

Layer-by-layer (LBL) self-assembled films of polyethyleneimine (PEI)/graphene oxide (GO) based on non-covalent force exhibit superior hydrogen barrier properties. Nevertheless, immersing the film in the self-assembly solution brings the problem of cross-contamination during the experiment. The deionized water rinsing step not only consumes time, but also increases the tediousness of membrane production and generates plenty of waste liquid. Herein, a PEI/GO hydrogen barrier film was prepared by ultrasonic atomization-assisted technology. Relying on stable and continuous spraying process and tiny droplets to achieve a unified and ordered surface of the film. The spray time, the dispersion concentration of GO, the pH of PEI solution, and the number of assembled layers were investigated in depth. The study shows that the hydrogen barrier film with excellent performance can be prepared by the atomization-assisted method. When the concentration of GO was 0.9 mg ml⁻¹, the spray time of PEI and GO were 8 s and 20 s, the pH of PEI and GO were 10 and 4, respectively, the hydrogen transmission rate of 25-layer PEI/GO composite films was 21.343 cm³ m⁻²·day⁻¹·0.1 MPa⁻¹, which was 92.38% lower than that of polyethylene terephthalate (PET) substrates film. Furthermore, this method has high raw material utilization and is easy to scale.     

Hydrogen jet fires in a passively ventilated enclosure

This paper describes a combined experimental, analytical and numerical modelling investigation into hydrogen jet fires in a passively ventilated enclosure. The work was funded by the EU Fuel Cells and Hydrogen Joint Undertaking project Hyindoor. It is relevant to situations where hydrogen is stored or used indoors. In such situations passive ventilation can be used to prevent the formation of a flammable atmosphere following a release of hydrogen. Whilst a significant amount of work has been reported on unignited releases in passively ventilated enclosures and on outdoor hydrogen jet fires, very little is known about the behaviour of hydrogen jet fires in passively ventilated enclosures. This paper considers the effects of passive ventilation openings on the behaviour of hydrogen jet fires. A series of hydrogen jet fire experiments were carried out using a 31 m³ passively ventilated enclosure. The test programme included subsonic and chocked flow releases with varying hydrogen release rates and vent configurations. In most of the tests the hydrogen release rate was sufficiently low and the vent area sufficiently large to lead to a well-ventilated jet fire. In a limited number of tests the vent area was reduced, allowing under-ventilated conditions to be investigated. The behaviour of a jet fire in a passively ventilated enclosure depends on the hydrogen release rate, the vent area and the thermal properties of the enclosure. An analytical model was used to quantify the relative importance of the hydrogen release rate and vent area, whilst the influence of the thermal properties of the enclosure were investigated using a CFD model. Overall, the results indicate that passive ventilation openings that are sufficiently large to safely ventilate an unignited release will tend to be large enough to prevent a jet fire from becoming under-ventilated.     

Experimental investigations of AB5-type alloys for hydrogen separation from biological gas streams

AB₅-type intermetallic compounds are suitable materials for hydrogen separation due to their ability selectively absorb hydrogen from different gas streams, including biologically produced ones. Recent studies show that metal hydride-based purification systems can effectively extract hydrogen from biogas with high CO₂ concentration. Alloys LaNi_5-xM_x (M = Fe, Al, Mn, Sn) are prepared and activated during several cycles of H₂ sorption/desorption and their PCT properties are measured in Sievert's type apparatus. Two compositions LaNi_4.4Fe_0.3Al_0.3 and LaNi_4.6Mn_0.2Al_0.2 are chosen for further investigations because they meet the requirements for biohydrogen separation system. After PCT measurements of 50-g samples, metal hydride powders are investigated by means of Quantochrome Nova 1200 and scanning electron microscopy to determine porosity, average particle size, specific surface area and permeability of metal hydride bed. Powder bed permeabilities are defined as 9.08 × 10⁻¹³ m² for LaNi_4.4Fe_0.3Al_0.3 and 6.86 × 10⁻¹³ m² for LaNi_4.6Mn_0.2Al_0.2 by Kozeny-Carman equation. AB₅ type LaNi_4.4Fe_0.3Al_0.3 and LaNi_4.6Mn_0.2Al_0.2 alloys show good characteristics: low equilibrium pressures 0.025–0.03 MPa and acceptable reversible hydrogen capacity 1.1 %wt. for stationary hydrogen separation system.     

Hydrogen dispersion in a closed environment

The highly combustible nature of hydrogen poses a great hazard, creating a number of problems with its safety and handling. As a part of safety studies related to the use of hydrogen in a confined environment, it is extremely important to have a good knowledge of the dispersion mechanism.The present work investigates the concentration field and flammability envelope from a small scale leak. The hydrogen is released into a 0.47 m × 0.33 m x 0.20 m enclosure designed as a 1/15 – scale model of a room in a nuclear facility. The performed tests evaluates the influence of the initial conditions at the leakage source on the dispersion and mixing characteristics in a confined environment. The role of the leak location and the presence of obstacles, are also analyzed. Throughout the test, during the release and the subsequent dispersion phase, temporal profiles of hydrogen concentration are measured using thermal conductivity gauges within the enclosure. In addition, the BOS (Background Oriented Schlieren) technique is used to visualise the cloud evolution inside the enclosure. These instruments allow the observation and quantification of the stratification effects.     

Effect of the position and the area of the vent on the hydrogen dispersion in a naturally ventilated cubiod space with one vent on the side wall

The design of ventilation system has implications for the safety of life and property, and the development of regulations and standards in the space with the hydrogen storage equipment. The impact of both the position and the area of a single vent on the dispersion of hydrogen in a cuboid space (with dimensions L x W x H = 2.90 × 0.74 × 1.22 m) is investigated with Computational Fluid Dynamics (CFD) in this study. Nine positions of the vent were compared for the leakage taking place at the floor to understand the gas dispersion. It was shown a cloud of 1% mole fraction has been formed near the ceiling of the space in less than 40 s for different positions of the vent, which can activate hydrogen sensors. The models show that the hydrogen is removed more effectively when the vent is closer to the leakage position in the horizontal direction. The study demonstrates that the vent height of 1.00 m is safer for the particular scenario considered. The area of the vent has little effect on the hydrogen concentration for all vent positions when the area of the vent is less than 0.045 m² and the height of the vent is less than 0.61 m.     

Mass transfer coefficients associated with the mixing of a confined gas volume by the random motion of loose spheres

This investigation quantifies the change in mass transfer within a confined gas volume subjected to mixing by loose spheres. A cylindrical vessel containing between 1 and 50 Teflon spheres in a tracer gas is vigorously shaken. Extractive sampling provides time histories of tracer gas concentrations extracted from the vessel. Fitting the results from a simple 1-D mass transfer model to the experimental data yields an effective mass transfer coefficient k′ for each experimental condition. Compared to diffusive mass transfer where k′ = D_ab = 7.58 × 10⁻⁶ m²/s, k′ exhibits a cubic dependency on the number of spheres with a maximum at 17 spheres where k′ = 3.5 × 10⁻³ m²/s.     

Thermodynamics analysis of hydrogen storage based on compressed gaseous hydrogen,liquid hydrogen and cryo-compressed hydrogen

Safe, reliable, and economic hydrogen storage is a bottleneck for large-scale hydrogen utilization. In this paper, hydrogen storage methods based on the ambient temperature compressed gaseous hydrogen (CGH₂), liquid hydrogen (LH₂) and cryo-compressed hydrogen (CcH₂) are analyzed. There exists the optimal states, defined by temperature and pressure, for hydrogen storage in CcH₂ method. The ratio of the hydrogen density obtained to the electrical energy consumed exhibits a maximum value at the pressures above 15 MPa. The electrical energy consumed consists of compression and cooling down processes from 0.1 MPa at 300 K to the optimal states. The recommended parameters for hydrogen storage are at 35–110 K and 5–70 MPa regardless of ortho-to parahydrogen conversion. The corresponding hydrogen density at the optimal states range from 60.0 to 71.5 kg m⁻³ and the ratio of the hydrogen density obtained to the electrical energy consumed ranges from 1.50 to 2.30 kg m⁻³ kW⁻¹. While the ortho-to para-hydrogen conversion is considered, the optimal states move to a slightly higher temperatures comparing to calculations without ortho-to para-hydrogen conversion.     

Preparation of palladium membrane on Pd/silicalite-1 zeolite particles modified macroporous alumina substrate for hydrogen separation

A palladium composite membrane was successfully fabricated by electroless plating on a macroporous alumina tube. Pd/silicalite-1 zeolite particles were employed to reduce the pore size of the alumina support and improve its surface roughness. Moreover, the Pd⁰ existed in the Sil-1 particle can avoid the time consuming sensitization and activation steps for palladium seeding. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction (XRD) analysis were conducted for analyzing the detailed microstructure of the palladium composite membrane. The hydrogen permeation performance of the resulting palladium membrane was investigated at temperatures of 623 K, 673 K, 723 K and 773 K. The hydrogen permeance of 1.95 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with an H₂/N₂ ideal selectivity of 1165 for the palladium membrane was obtained at 773 K. Furthermore, the resulting palladium membrane was stable for a long-term operation of 15 days at 673 K.     

Hydrogen diffusivity in different microstructural components in martensite matrix with retained austenite

We elucidate the hydrogen diffusivity in martensite matrix with retained austenite (RA). Two aspects are focused: effect of microstructure on hydrogen diffusion behavior; hydrogen diffusivity calculation for different microstructural components. Quenched martensite (QM) had the highest effective hydrogen diffusion coefficient because of high dislocation density. Effective hydrogen diffusion coefficient decreased with the increase of intercritical annealing temperature because of decrease in dislocation density and increase of RA. According to the principle of Maxwell-Garnett equation, the hydrogen diffusion coefficient for grain boundary (GB) was 7.99 × 10^?8 m²/s and hydrogen diffusion coefficient of tempered martensite (TM) was 7.84 × 10^?11 m²/s.