Hydrogen effect on fracture toughness of pipeline steel welds, with in situ hydrogen charging

The API 5L X70 and X52 pipeline steel weld fracture toughness parameters are measured in a hydrogen environment and compared to the ones in air. The hydrogen environment is created by in situ hydrogen charging, using as an electrolyte a simulated soil solution, with three current densities, namely 1, 5 and 10 mA/cm². A specially designed electrolytic cell mounted onto a three-point bending arrangement is used and hydrogen charging is performed during the monotonic loading of the specimens. Ductility is measured in terms of the J₀ integral. In all cases a slight change in toughness was measured in terms of K_Q. Reduction of ductility in the base metal is observed, which increases with increasing current density. A more complex phenomenon is observed in the heat affected zone metal, where a small reduction in ductility is observed for the two current densities (1 and 5 mA/cm²) and a larger reduction for the third case (10 mA/cm²). Regarding microstructure of tested X70 and X52 base and HAZ metal, it is observed that the hydrogen degradation effect is enhanced in banded ferrite-pearlite formations. The aforementioned procedure is used for calculating the fracture toughness parameters of a through-thickness pipeline crack.     

Fracture toughness properties of HIC susceptible carbon steels in sour service conditions

Hydrogen Induced Cracking (HIC) in carbon steels is a well-studied mechanism, where diffusing hydrogen atoms accumulates at the steel imperfections/laminations to create gaseous hydrogen with very high pressure, leading to initiation and growth of internal cavities, so-called HIC. Measurements of relevant fracture toughness properties of non-HIC resistant steels in hydrogen environment is critical to predict and assess the initiation and growth of HIC. The present work attempts to quantify the effect of hydrogen on the fracture toughness properties (K_Q and CTOD) of an API X42 pipeline steel under simulated H₂S in-service conditions. The fracture toughness properties are measured in TL and SL directions: perpendicular and parallel to the pipeline wall thickness, respectively, following ASTM E1820, standard. Since the X42 is a non-HIC resistant steel, the measurement of the fracture toughness properties in the SL direction is more relevant in terms of HIC initiation and growth than fracture toughness properties in the TL direction. Indeed, parallel to the thickness of the pipeline wall, X42 steel shows microstructural features prone to HIC formation and growth. Steady state H₂S in-service conditions were simulated by charging the specimen for 48 h using a special electrolytic solution and then tested (ex-situ) to evaluate the fracture toughness properties. The steady state H₂S environment was obtained by measuring the Hydrogen Concentration (CH) in the bulk of the specimen, using Thermal desorption Spectroscopy at three levels of CH. It was observed that the K_Q was not affected in the SL direction, while it was reduced in the TL direction for 1.5 ppmw of CH. The CTOD showed mixed results in the TL direction while it was significantly reduced in the SL direction reaching a saturation at 1 ppmw of CH. Besides, microstructural analyses showed that the presence of inclusions coalescence in form of dimples promote the early failure, which is more pronounced in the hydrogen environment especially at higher levels of CH.     

Microstructure and properties of intercritically reheated coarse-grained heat affected zone in pipeline steel after secondary thermal cycle

As a kind of high strength microalloyed steel, pipeline steel has high strength and toughness. However, the formation of the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) during the welding process is an important limitation affecting its performance. Herein, microstructural transformation in the ICCGHAZ of X100 pipeline steel after a secondary thermal cycle and hydrogen sulfide stress corrosion cracking (SSCC) resistance of the transformed microstructure are investigated. The microstructure of X100 pipeline steel after the secondary thermal cycle is similar to that after the primary thermal cycle, comprising lath bainite and granular bainite. With an increase in t_8/5 (time required for the material to cool from 800 °C to 500 °C), the M-A constituents that continuously precipitate along the prior austenite grain boundaries in the second thermal cycle are coarsened and form a necklace-type structure. A variation in t_8/5 does not significantly affect the crystallographic characteristics of the ICCGHAZ. The stress corrosion test shows that the resistance to SSCC decreases and cleavage fracture characteristics become more noticeable with an increase in t_8/5.     

Hydrogen embrittlement behavior in the nugget zone of friction stir welded X100 pipeline steel

Hydrogen-induced damage is an inevitable challenge in pipeline safety applications, especially, the fusion welded joints owing to microstructure heterogeneity caused by welding process. In this work, X100 pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 300–600 rpm under water cooling, and the relationship among the microstructure, hydrogen diffusivity, and hydrogen embrittlement (HE) behavior of the nugget zone (NZ) were studied. The NZ at 600 rpm had the highest effective hydrogen diffusion coefficient (D_eff) of 2.1 × 10^?10 m²/s because of the highest dislocation density and lowest ratio of effective grain boundary. The D_eff decreased with decreasing rotation rate due to the decrease of dislocation density and the increase of ratio of effective grain boundary, and the lowest D_eff of 1.32 × 10^?10 m²/s was obtained at 300 rpm. After hydrogen charging, the tensile strength of all specimens decreased slightly, while the elongation decreased significantly. As the rotation rate decreased, the elongation loss was obviously inhibited, and ultimately a lowest elongation loss of 31.8% was obtained at 300 rpm. The abovementioned excellent mechanical properties were attributed to the fine ferrite/martensite structure, low D_eff, and strong {111}//ND texture dramatically inhibiting hydrogen-induced cracking initiation and propagation.     

Hydrogen diffusion and hydrogen influenced critical stress intensity in an API X70 pipeline steel welded joint – Experiments and FE simulations

An API X70 pipeline steel has been investigated with respect to hydrogen diffusion and fracture mechanics properties. A finite element cohesive element approach has been applied to simulate the onset of hydrogen-induced fracture. Base metal, weld simulated heat affected zone and weld metal have been investigated. The electrochemical permeation technique was used to study hydrogen diffusion properties, while in situ fracture mechanics testing was performed in order to establish the hydrogen influenced threshold stress intensity. The average effective diffusion coefficient at room temperature was 7.60 × 10⁻¹¹ m²/s for the base metal, 4.01 × 10⁻¹¹ m²/s for weld metal and 1.26 × 10⁻¹¹ m²/s for the weld simulated heat affected zone. Hydrogen susceptibility was proved to be pronounced for the heat affected zone samples. Fracture toughness samples failed at a net section stress level of 0.65 times the yield strength; whereas the base metal samples did not fail at net section stresses lower than the ultimate tensile strength. The initial cohesive parameters which best fitted the experimental results were σ_c = 1500 MPa (3.1·σ_y) for the base metal, σ_c = 1800 MPa (3.0·σ_y) for weld metal and σ_c = 1840 MPa (2.3·σ_y) for heat affected zone. Threshold stress intensities K_Ic,HE were in the range 143–149 MPa√m.     

Effect of hydrogen on fracture toughness properties of a pipeline steel under simulated sour service conditions

The effect of hydrogen on the fracture toughness properties of an API X65 pipeline steel is studied under simulated H₂S in-service conditions. The fracture toughness properties are measured in LT and SL directions (perpendicular and parallel to the pipeline wall thickness, respectively), following ASTM E1820. Due to size restrictions of standard single edge notch bending (SEB) specimens at the direction parallel to the thickness of the pipeline wall, an experimental protocol (see the patent) was developed to carry out the fracture toughness tests, while complying with ASTM standard 1820. This approach is especially useful in situations where hydrogen induced cracking (HIC) and in a broader sense, stepwise cracking takes place, since these cracks initiate and grow primarily in planes parallel to the pipeline rolling plane. Such values of fracture toughness are often different from those commonly measured in planes perpendicular to the rolling plane. Hydrogen might not have the same effect on fracture toughness properties as measured in different directions, due to microstructural features which are inherent from steel manufacturing process. The steady state H₂S in-service conditions are simulated by electrolytically charging the specimen, for 48 h and then testing (ex-situ) the specimen for evaluating the fracture toughness properties. The steady state H₂S environment charging was obtained by measuring the hydrogen concentration in the bulk of the specimen through thermal desorption spectroscopy (TDS) at three levels of hydrogen concentration. It was observed that the K_Q was moderately decreased with increasing hydrogen concentration in the bulk of the steel, while CTOD₀ showed a significant reduction with increasing hydrogen concentration.     

Effect of low partial hydrogen in a mixture with methane on the mechanical properties of X70 pipeline steel

In this study, the effect of a low partial hydrogen in a mixture with natural gas on the tensile, notched tensile properties, and fracture toughness of pipeline steel X70 is investigated. An artificial HE aging is simulated by exposing the tested sample to the mixture gas condition for 720 h. In addition, a series of tests is conducted in ambient air and 10 MPa of 100% He and H₂. Overall, 10 MPa of 100% H₂ significantly degrades the mechanical properties of an X70 pipeline steel. However, it is observed that the 10 MPa gas mixture with 1% H₂ does not affect the mechanical properties when tested with a smooth tensile specimen. In the notched tensile test, a significant reduction in loss in the area is observed when tested with a notched specimen with a notch radius of 0.083 mm. It is also confirmed that a 10-MPa gas mixture with 1% H₂ causes a remarkable reduction in the toughness. The influence of the exposure time to 1% hydrogen in a mixture with natural gas was found to be minor.     

Hydrogen effect on a low carbon ferritic-bainitic pipeline steel

Hydrogen effect on an API 5L X65 low carbon ferritic-bainitic steel is investigated, by evaluating the fracture toughness parameters in air and in hydrogen environment. The hydrogen environment is manifested by in situ hydrogen charging of the X65 steel, using the electrolytic solution NS4, which simulates the electrolyte trapped between the pipeline steel and the coating in a buried pipeline. The fracture toughness results of the X65 are compared to two other pipeline steels with different microstructures, namely an X52 and an X70, possessing a banded ferritic-pearlitic and banded ferritic-mixed bainitic-pearlitic microstructure, respectively. The X65 steel exhibits significant reduction of fracture toughness parameter J₀ integral due to hydrogen charging and insignificant variation of fracture toughness parameter K_Q. Comparing the three steels, the lowest reduction of J₀ integral due to hydrogen charging, is met on the X52 and the highest in the X65.     

Hydrogen permeation and distribution at a high-strength X80 steel weld under stressing conditions and the implication on pipeline failure

Hydrogen permeation and distribution at pipeline welds is critical to integrity maintenance of the pipelines, especially for those made of high-strength steels. The situation becomes even more important under stressing conditions. In this work, metallographic characterization and micro-hardness measurements were conducted at an X80 steel weld. Potentiodynamic polarization and electrochemical hydrogen permeation testing were performance at various zones at the weld, along with numerical modeling of hydrogen distribution at the zones. The X80 steel contains a microstructure of bainite bundles and polygonal ferrite. There are more polygonal ferrite, fewer bainite and some segregated cementite at heat-affected zone (HAZ). The weld metal is featured with acicular ferrite and some grain boundary ferrite. HAZ softening occurs at the weld. The hardness of the weld metal, HAZ and base steel is about 290, 248 and 261 HV_0.2, respectively. There is the greatest corrosion current density, i.e., corrosion rate, at HAZ under both elastic and plastic stresses. An applied stress further increases the corrosion current density. Under the plastic stress of 1.1σ_ys (σ_ys is yield strength), the corrosion current densities of HAZ, base steel and weld metal are 41.04, 17.03 and 25.49 μA/cm², respectively. There are always the greatest hydrogen trapping density and the smallest hydrogen diffusivity at HAZ. Hydrogen, once penetrating the welded steel, tends to accumulate at the HAZ, compared with other two zones. When the welded steel is under stresses, especially a plastic stress (i.e., 1.1σ_ys), the hydrogen diffusivity and permeability decrease, while the subsurface hydrogen concentration and hydrogen trapping density increase remarkably. Plastic deformation favors the hydrogen permeation and trapping at weld, especially the HAZ, to elevate the susceptibility to hydrogen damage. The hydrogen distribution at different welding zones can be evaluated and determined by a developed modeling method.     

Room temperature hydrogen generation from hydrolysis of ammonia–borane over an efficient NiAgPd/C catalyst

NiAgPd nanoparticles are successfully synthesized by in-situ reduction of Ni, Ag and Pd salts on the surface of carbon. Their catalytic activity was examined in ammonia borane (NH₃BH₃) hydrolysis to generate hydrogen gas. This nanomaterial exhibits a higher catalytic activity than those of monometallic and bimetallic counterparts and a stoichiometric amount of hydrogen was produced at a high generation rate. Hydrogen production rates were investigated in different concentrations of NH₃BH₃ solutions, including in the borates saturated solution, showing little influence of the concentrations on the reaction rates. The hydrogen production rate can reach 3.6–3.8 mol H₂ mol_cat⁻¹ min⁻¹ at room temperature (21 °C). The activation energy and TOF value are 38.36 kJ/mol and 93.8 mol H₂ mol_cat⁻¹ min⁻¹, respectively, comparable to those of Pt based catalysts. This nanomaterial catalyst also exhibits excellent chemical stability, and no significant morphology change was observed from TEM after the reaction. Using this catalyst for continuously hydrogen generation, the hydrogen production rate can be kept after generating 6.2 L hydrogen with over 10,000 turnovers and a TOF value of 90.3 mol H₂ mol_cat⁻¹ min⁻¹.     

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.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

Design and fabrication of C–Mn0.5Cd0.5S/Cu3P ternary heterojunction catalyst for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution from water splitting is a promising strategy to solve the energy demand of human beings. Here, we first designed a C–Mn_0.5Cd_0.5S/Cu₃P ternary heterojunction catalyst for photocatalytic hydrogen production. The results show that the combination of C and Cu₃P can effectively improve the photocatalytic activity of Mn_0.5Cd_0.5S. C–Mn_0.5Cd_0.5S loading with 5 wt% Cu₃P exhibits the highest hydrogen evolution rate (44.1 mmol g⁻¹ h⁻¹), which is 3.2 and 2.8 times higher than that of pure Mn_0.5Cd_0.5S (13.7 mmol g⁻¹ h⁻¹) and Mn_0.5Cd_0.5S/3 wt%Pt (15.6 mmol g⁻¹ h⁻¹), respectively. In addition, it shows a high hydrogen evolution rate (19.6 mmol g⁻¹ h⁻¹) under visible light (≥420 nm) irritation and the apparent quantum efficiency (AQE) is detected to be 3.2% at 420 nm. The enhanced photocatalytic activity can be attributed to the good conductivity of C and the formation of p-n heterojunction, which is beneficial for light harvesting and the separation and transportation of charge carriers. Besides, a possible mechanism is proposed. This work provides an effective way to improve the photocatalytic activity of Mn_0.5Cd_0.5S by using non noble metal co-catalysts.     

Examination of the catalytic effect of Ni,NiCr, and NiV catalysts prepared as thin films by magnetron sputtering process in the hydrolysis of sodium borohydride

In this study, nickel, nickel-chromium alloy, and nickel-vanadium alloy were coated to form a thin film on the slides prepared by magnetron sputtering process, which were used as a catalyst for the hydrolysis of alkaline sodium borohydride. Factors, such as the temperature of the solution, amount of the catalyst, initial pH of the solution and the performance of these catalysts on hydrogen generation rate were investigated using response surface methodology. Moreover, the catalysts were characterized using XRD and FE-SEM/EDS analyses. Utilizing the obtained optimum conditions of the response surface methodology estimation, the maximum hydrogen generation rate was 35,071 mL min⁻¹ g_NiV⁻¹ from NiV catalyst at 60 °C, pH 6, and 1.75 g catalyst conditions. Under the same experiment conditions, the maximum hydrogen generation rates of Ni and NiCr catalyst systems are 28,362 mL min⁻¹ g_Ni⁻¹, and 30,608 mL min⁻¹ gNiCr⁻¹, respectively.     

Investigating the influence mechanism of hydrogen partial pressure on fracture toughness and fatigue life by in-situ hydrogen permeation

In this work, we investigate the influence mechanism of hydrogen partial pressure on fracture toughness and fatigue life of a high strength pipeline steel. Both fracture toughness test and fatigue life test are carried out under different hydrogen partial pressure. The experimental results show that with the increasing of hydrogen partial pressure, fracture toughness and fatigue life decrease and the decrease trends gradually flatten out. Hydrogen has a larger effect on fatigue life than fracture toughness. Only 3% hydrogen gas can cause a 67.7% decrease of fatigue life. The in-situ hydrogen permeation test is performed respectively in 2 MPa, 5 MPa and 8 MPa hydrogen partial pressure. With the increasing of hydrogen partial pressure, the increase trend of hydrogen permeation current gradually tends to be gentle, which indicates that the hydrogen atoms entering into the material gradually become saturated. This result can be used to clarify the influence mechanism of hydrogen partial pressure on fracture toughness and fatigue life.     

Inhibition by the ionic strength of hydrogen production from the organic fraction of municipal solid waste

Composition of the Organic Fraction of Municipal Solid Waste (OFMSW) in organic compounds and inorganic ions is highly variable and might impact the microbial activity in dark fermentation processes. In this study, the effect of the total amount of inorganic ions on fermentative hydrogen production was investigated. Batch experiments were carried out at pH 6 and under a temperature of 37 °C. A freshly reconstituted organic fraction of municipal solid waste (OFMSW) was used as model substrate. At low concentrations in ammonium or chloride ions (2.9–5.1 g L⁻¹, respectively), the hydrogen yield reached a maximum of 40.8 ± 0.5. mLH₂.gVS⁻¹ and 25.1 ± 5.6 mLH₂.gVS⁻¹. In contrast, at high total ionic concentrations of ammonium and chloride (11.1–35.5 g L⁻¹ respectively), a strong inhibition of the fermentative microbial activity and more particularly hydrogen production, was observed. When considering the ionic strength of each ion, the effects of ammonia, chloride or a mixture of different ions (Na⁺, K⁺, H⁺, Li⁺, NH₄⁺, Mn²⁺, NH₄⁺, Mg²⁺, Cl⁻, PO₄³⁻, Br⁻, I⁻, SO₄²⁻) showed very similar inhibitory trends regardless the type of ion or the composition of the ionic mixture. A threshold inhibitory value of the ionic strength was estimated at 0.75 ± 0.13 M with a substantial impact on the fermentative activity from 0.81 ± 0.12 M, with hydrogen yields of 18.1 ± 3.3 and 6.2 ± 4.1 mLH₂.gVS⁻¹, respectively. Microbial community composition was also significantly impacted with a specific decrease in relative abundance of hydrogen-producing bacteria from the genus Clostridium sp. at high ionic strength.     

Thermodynamic and kinetic properties of calcium hydride

Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH₂), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH₂. In this work, a literature review of the wide-ranging thermodynamic properties of CaH₂ is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH₂. The thermodynamic values of hydrogen release from both molten and solid CaH₂ were determined as ΔH_des (molten CaH₂) = 216 ± 10 kJ mol⁻¹.H₂, ΔS_des (molten CaH₂) = 177 ± 9 J K⁻¹ mol⁻¹.H_2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH₂ of 947 ± 65 °C. Similarly, in the solid-state: ΔH_des (solid CaH₂) = 172 ± 12 kJ mol⁻¹.H₂, ΔS_des (solid CaH₂) = 144 ± 10 J K⁻¹ mol⁻¹.H₂. Moreover, the activation energy of hydrogen release from CaH₂ was also calculated using DSC analysis as E_a = 203 ± 12 kJ mol⁻¹. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material.     

High efficient biohydrogen production from palm oil mill effluent by two-stage dark fermentation and microbial electrolysis under thermophilic condition

Biohydrogen production from palm oil mill effluent by two-stage dark fermentation and microbial electrolysis was investigated under thermophilic condition. The optimum chemical oxygen demand (COD) concentration and pH for dark fermentation were 66 g·L⁻¹ and 6.5 with a hydrogen yield of 73 mL-H₂·gCOD⁻¹. The dark fermentation effluent consisted of mainly acetate and butyrate. The optimum voltage for microbial electrolysis was 0.7 V with a hydrogen yield of 163 mL-H₂·gCOD⁻¹. The hydrogen yield of continuous two-stage dark fermentation and microbial electrolysis was 236 mL-H₂·gCOD⁻¹ with a hydrogen production rate of 7.81 L·L⁻¹·d⁻¹. The hydrogen yield was 3 times increased when compared with dark fermentation alone. Thermoanaerobacterium sp. was dominated in the dark fermentation stage while Geobacter sp. and Desulfovibrio sp. dominated in the microbial electrolysis cell stage. Two-stage dark fermentation and microbial electrolysis under thermophilic condition is a highly promising option to maximize the conversion of palm oil mill effluent into biohydrogen.     

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.     

Study on catalytic effect and mechanism of MOF (MOF = ZIF-8, ZIF-67, MOF-74) on hydrogen storage properties of magnesium

Using a deposition-reduction method, Mg/MOF nanocomposites were prepared as composites of Mg and metal-organic framework materials (MOFs = ZIF-8, ZIF-67 and MOF-74). The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, within 5000 s, 0.6 wt%, 1.2 wt%, 2.7 wt%, 3.7 wt% of hydrogen were released from Mg, Mg/MOF-74, Mg/ZIF-8, Mg/ZIF-67, respectively. Activation energy values of 198.9 kJ mol⁻¹ H₂, 161.7 kJ mol⁻¹ H₂, 192.1 kJ mol⁻¹ H₂ were determined for the Mg/ZIF-8, Mg/ZIF-67, Mg/MOF-74 hydrides, which are 6 kJ mol⁻¹ H₂, 43.2 kJ mol⁻¹ H₂, and 12.8 kJ mol⁻¹ H₂ lower than that of Mg hydride, respectively. Moreover, the cyclic stability characterizing Mg hydride was significantly improved when adding ZIF-67. The hydrogen storage capacity of the Mg/ZIF-67 nanocomposite remained unchanged, even after 100 cycles of hydrogenation/dehydrogenation. This excellent cyclic stability may have resulted from the core-shell structure of the Mg/ZIF-67 nanocomposite.