MgH2 System Research Articles

Synergistic Effects of In‐Situ‐Formed Multivalent Titanium and Mg2Ni for Improving the Hydrogen‐Storage Properties of MgH2

AbstractMagnesium hydride (MgH2) has gained significant consideration as a cost‐effective solid‐state hydrogen‐storage material with a substantial hydrogen capacity. However, its practical applications are impeded by its poor kinetics and high thermal stability. In this study, we employed a hydrothermal method and high‐temperature calcination to prepare well‐dispersed NiTiO3 nanoparticles smaller than 20 nm. Next, we investigated the effect of the NiTiO3 nanoparticles for educating the hydrogen storage performance of MgH2. The synergistic catalytic effect of multivalent titanium and Mg2Ni/Mg2NiH4 enhanced hydrogen storage performance of the MgH2 system. The addition of 5 wt % NiTiO3 lowered the initial hydrogen evolution temperature of MgH2 to 194 °C. After 12 minutes at 275 °C, the 5 wt % NiTiO3‐MgH2 composites released 6.1 wt % of hydrogen. The dehydrogenation activation energy (Ea) of the 5 wt % NiTiO3‐MgH2 composites decreased significantly to 38.78±2.1 kJ/mol. Furthermore, hydrogen uptake of 5 wt % NiTiO3‐MgH2 at 125 °C reaches 5.1 wt % in 300 s under hydrogen pressure of 20 bar. These findings introduce an innovative concept for preparing high active catalysts for improving the dehydrogenation/hydrogenation kinetics of MgH2.

A density functional theory study of defective and doped structures of MgB2 and their interaction with hydrogen

The LiBH4+MgH2 system exhibits promising potential for solid-state hydrogen storage, yet the sluggish rehydrogenation of MgB2 poses a significant challenge. In this study, we utilize density functional theory (DFT) simulations to investigate the energetics of hydrogen in pure, defective, and doped MgB2 in equilibrium with molecular hydrogen. Our findings reveal that the B-Frenkel defect process is the thermodynamically most favorable in MgB2. The calculated solution energy for hydrogen at the interstitial site indicates low hydrogen solubility, even at mild temperatures. Incorporating hydrogen into a pre-existing B vacancy enhances the process, facilitated by doping of O on the B site. Additionally, Li doping on the Mg site enhances incorporation by forming strong Li–H bonds, predicting lower activation barriers for hydrogen dissociation and diffusion. Doping MgB2 with O and F at the B site significantly enhances hydrogen solubility. These dopants make MgB2 a promising material for high-capacity and fast-kinetics hydrogen storage applications, and further research into these effects can lead to advancements in energy storage technology.

Solid-solution MAX phase TiVAlC assisted with impurity for enhancing hydrogen storage performance of magnesium hydride

Although MXene catalysts etched from precursor MAX have greatly improved the hydrogen storage performance of magnesium hydride (MgH2), the use of dangerous and polluting etchers (such as hydrofluoric acid) and the direct removal of potentially catalytically active A-layer substances (such as Al) present certain limitations. Here, solid-solution MAX phase TiVAlC catalyst without etching treatment has been directly introduced into MgH2 system to improve the hydrogen storage performance. The optimal MgH2-10 wt% TiVAlC can release about 6.00 wt% hydrogen at 300 °C within 378 s and absorb about 4.82 wt% hydrogen at 175 °C within 900 s. After 50 isothermal hydrogen ab/desorption cycles, the excellent cyclic stability and capacity retention (6.4 wt%, 99.6%) can be found for MgH2-10 wt% TiVAlC. The superb catalytic activity of TiVAlC catalyst can be explained by abundant electron transfer at external interfaces with MgH2/Mg, which can be further enhanced by impurity phase Ti3AlC2 due to strong H affinity brought from abundant electron transfer at internal interfaces (Ti3AlC2/TiVAlC). The influence of impurity phase which is common in MAX phase on the overall activity of catalysts has been firstly studied here, providing a unique method for designing composite catalyst to improve hydrogen storage performance of MgH2.

Effect of bimetallic nitride NiCoN on the hydrogen absorption and desorption properties of MgH2 and the catalytic effect of in situ formed Mg2Ni and Mg2Co phases

Magnesium hydroxide has attracted extensive attention in the past few years due to its excellent hydrogen storage capacity, good reversibility, and low cost, but its high thermodynamic stability and low rate of kinetic activity hinder its application. Therefore, in order to improve the hydrogen storage capacity of MgH2, we prepared ternary transition metal nitride NiCoN to catalyze MgH2 by hydrothermal and calcination techniques. Owing to the addition of 6 wt% NiCoN, the dehydrogenation process of MgH2 started at 164.4 °C (ball-milled MgH2 started dehydrogenation at 287.6 °C). At 285 °C, 4.95 wt% of hydrogen gas was desorption within 30 min, while 4.6 wt% of hydrogen gas was absorbed at 125 °C. And calculated by the Kissinger method, the activation energy required for the dehydrogenation reaction of MgH2-6NiCoN is reduced to 56.5 kJ/mol (ball-milled MgH2 is 127.3 kJ/mol). It is found that MgH2-6NiCoN forms Mg2Co/Mg2Ni in situ during the first dehydrogenation process, and then transforms into Mg2CoH5/Mg2NiH4 after rehydrogenation, implying a reversible phase transition between the Mg2Co/Mg2CoH5 and Mg2Ni/Mg2NiH4 phases. Due to the formation of intermediate products, it still maintains a good hydrogen desorption capacity after 40 cycles. The dehydrogenation retention rate is 94 %. The excellent behavior of MgH2 is attributed to the formation of Mg2Ni/Mg2NiH4 and Mg2Co/Mg2CoH5, which are responsible for the improved hydrogen absorption/desorption behavior of MgH2. These findings provide new insights into the hydrogen storage behavior of the catalytic MgH2 system.

Effect of MOF-derived carbon–nitrogen nanosheets co-doped with nickel and titanium dioxide nanoparticles on hydrogen storage performance of MgH2

Magnesium hydride (MgH2) is considered one of the most ideal materials for hydrogen storage. However, the practical applications of MgH2 are greatly hindered by its high operating temperature and poor hydrogen storage kinetics. In the present work, carbon–nitrogen nanosheets co-doped with disk-like nickel and titanium dioxide nanoparticles ((Ni, TiO2)CN) were prepared using metal–organic frameworks (MOFs) as precursors to improve the hydrogen storage performance of MgH2. The as-synthesized ((Ni, TiO2)CN)-doped MgH2 system manifested excellent hydrogen storage properties at low temperatures and could absorb 5.08 wt% of hydrogen in 100 min at 40 °C and 6.17 wt% of hydrogen in 10 min at 125 °C. The dehydrogenation activation energy of the ((Ni, TiO2)CN)-doped MgH2 system was obtained as 83.1 kJ∙mol−1, which was considerably lower than that of pure MgH2. The mechanism analysis revealed that Ni reacted with Mg to form Mg2Ni coating around Mg, the in situ Mg2Ni and its hydride (Mg2NiH4) acted as a “hydrogen pump” to drive hydrogen diffusion and dissociation. And the mutual reversible change in polyvalent Ti ions could promote the transfer of charge, which was conducive to accelerating the formation of Mg/MgH2. Additionally, stable CN layers not only could inhibit the fragmentation and agglomeration of MgH2 particles, but also could make the active substance dispersed more evenly; thus, the cycle stability and hydrogen absorption and desorption kinetics of MgH2 was greatly enhanced under the synergistic catalysis of in situ formed Mg2Ni/Mg2NiH4, polyvalent titanium, and N-C nanosheets. The findings of this work could provide a new strategy to improve the properties of Mg-based hydrogen storage materials.

The influence of grain size and catalytic doping on magnesium hydride nanocrystals for hydrogen storage

Magnesium hydride (MgH2) nanocrystals of different sizes were constructed by a facile hydrogenated ball milling method. X-ray diffraction (XRD) analysis showes that the size of MgH2 nanocrystals decreased gradually with the increasing milling time. The as-prepared MgH2 nanocrystals started to release and take up hydrogen from 281 to 119 °C, respectively. By doping the prepared Nb2O5@MOF into MgH2 nanocrystals, the initial hydrogen desorption and absorption temperatures were further reduced to 228 and 25 °C. Microstructure analysis revealed that the uniform distribution of Nb2O5 on MgH2 nanocrystals contributed to the greatly enhanced de/hydrogenation kinetics of modified MgH2 system.

The adaptable effect of Ru on hydrogen sorption characteristics of the MgH2 system

In this study, a MgH2–Ru composite is prepared by mechanical ball milling of MgH2 and Ru powders. The microstructural analysis revealed the uniform distribution of Ru nanoparticles in the MgH2 matrix and the formation of intermetallic compounds (i.e. Mg6.2Ru and Mg1.5Ru) after dehydrogenation of the composite. The substantial improvements in the de/hydrogenation properties of MgH2 are due to the adjustable catalytic role of Ru under low temperature and pressure conditions. MgH2–Ru composite started to release hydrogen at an onset temperature of 300 °C, much lower than the mechanically milled pure MgH2 (350 °C). Hydrogen absorption of the MgH2–Ru composite is relatively fast at 350 °C and stored 6.1 wt% in 5 min. At the same temperature, the hydrogenated MgH2–Ru composite released all stored hydrogen in less than 5 min. Compared with the catalyst-free MgH2, the hydrogenation and dehydrogenation activation energies of the MgH2–Ru composite are reduced to 65.12 ± 2.4 kJ/mol H2, and 97.46 ± 4.15 kJ/mol H2, respectively. In addition, it further demonstrated how the reversible transformation of Mg–Ru intermetallic phases induces the thermodynamic destabilization of the MgH2 system. The presence of Ru not only facilitated the breaking of the Mg–H bond but also provided very good catalytic interfaces for enhanced de/hydrogenation properties of MgH2.

On the Catalytic Mechanism of 3d and 4d Transition-Metal-Based Materials on the Hydrogen Sorption Properties of Mg/MgH2

The slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability of the Mg–H bond are the two major limitations for the large-scale utilization of MgH2. In this review, we introduce the catalytic mechanism of 3d and 4d transition metal (TM) on the hydrogen sorption properties of Mg/MgH2. The relative contribution of interatomic interactions to the thermodynamic stability of the TM-substituted MgH2 system is discussed. A synergy effect between the electronegativity and the radius of the TM element is proposed to explain the charge transfer process between TM and H in the TM-substituted MgH2 system. The catalytic mechanism of TM nearby the surface of Mg is more complicated than that in the volume of Mg, as the surface-doped TM can experience more options for doping sites, leading to the hindrance effect and causing various contributions of the d band center to the dissociation of hydrogen molecules and the diffusion of hydrogen atoms nearby the surface of Mg. In terms of the catalytic mechanism of TM for hydrogen sorption kinetics of Mg/MgH2, we particularly focused on the “hydrogen pump” effect existing in the Mg–TM–H system. Other mechanisms, such as a possible catalytic mechanism of TM for the hydrogen sorption properties of nano-sized freestanding Mg/MgH2, were also presented.

First-principles study on the hydrogen storage properties of MgH2(1 0 1) surface by CuNi co-doping

Exploring the hydrogen storage properties on the surface of transition metal-modified MgH2 systems is beneficial to develop new hydrogen storage regions. The first-principles approach was used to study the hydrogen storage mechanism on the CuNi co-doped MgH2(101) surface. The most stable energy adsorption site for hydrogen on the surface of CuNi co-doped MgH2(101) is above the Ni atom, and the corresponding chemisorption energy is −0.51 eV. The distribution of charge densities indicates substantial charge exchange occurs between the hydrogen molecule and Ni atom, while partial density of state analysis reveals orbital hybridization between the H-s (H2) and Ni-d orbits. Furthermore, molecular dynamics simulations indicate that the temperature range (473 to 573 K) of hydrogen desorption on the surface is similar to previous experimental results. Therefore, the (101) surface of CuNi co-doped MgH2 can be well matched with itself to obtain the additional hydrogen storage region.

Effects of metal-based additives on dehydrogenation process of 2NaBH4 + MgH2 system

We report a systematic investigation of the effect that selected metal-based additives have on the dehydrogenation properties of the reactive hydride composite (RHC) model system 2NaBH4+MgH2. Compared to the pristine system, the material doped with 3TiCl3·AlCl3 exhibits superior dehydrogenation kinetics. The addition of 3TiCl3·AlCl3 alters the controlling mechanism of the second dehydrogenation step making it change from a two-dimensional interface controlled process to a two-dimensional nucleation and growth controlled process. The microstructural investigation of the dehydrogenated 2NaBH4+MgH2via high-resolution transmission electron microscopy (HRTEM) shows significant differences in the MgB2 morphology formed in the doped and undoped systems. The MgB2 has a needle-like structure in the sample doped with 3TiCl3·AlCl3, which is different from the plate-like MgB2 structure in the undoped sample. Moreover, nanostructured metal-based phases, such as TiB2/AlB2 particles, acting as heterogeneous nucleation sites for MgB2 are also identified for the sample doped with 3TiCl3·AlCl3.

Review on Magnesium Hydride and Sodium Borohydride Hydrolysis for Hydrogen Production

Metal hydrides such as MgH2 and NaBH4 are among the materials for with the highest potential solid-state hydrogen storage. However, unlike gas and liquid storage, a dehydrogenation process has to be done prior to hydrogen utilization. In this context, the hydrolysis method is one of the possible methods to extract or generate hydrogen from the materials. However, problems like the MgH2 passivation layer, high cost and sluggish self-hydrolysis of NaBH4 are the known limiting factors for this process, but they can be overcome with the help of catalysts. In this works, selected studies have been reviewed on the performance of catalysts like chloride, oxide, fluoride, platinum, ruthenium, cobalt and nickel-based on the MgH2 and NaBH4 system. These studies show a significant enhancement in the amount of hydrogen released as compared to the hydrolysis of the pure MgH2 and NaBH4. Therefore, the addition of catalysts is proven as one of the options in improving hydrogen generation via the hydrolysis of MgH2 and NaBH4.

Influence of Nanosized CoTiO3 Synthesized via a Solid-State Method on the Hydrogen Storage Behavior of MgH2.

Magnesium hydride (MgH2) has received outstanding attention as a safe and efficient material to store hydrogen because of its 7.6 wt.% hydrogen content and excellent reversibility. Nevertheless, the application of MgH2 is obstructed by its unfavorable thermodynamic stability and sluggish sorption kinetic. To overcome these drawbacks, ball milling MgH2 is vital in reducing the particle size that contribute to the reduction of the decomposition temperature. However, the milling process would become inefficient in reducing particle sizes when equilibrium between cold-welding and fracturing is achieved. Therefore, to further ameliorate the performance of MgH2, nanosized cobalt titanate (CoTiO3) has been synthesized using a solid-state method and was introduced to the MgH2 system. The different weight percentages of CoTiO3 were doped to the MgH2 system, and their catalytic function on the performance of MgH2 was scrutinized in this study. The MgH2 + 10 wt.% CoTiO3 composite presents the most outstanding performance, where the initial decomposition temperature of MgH2 can be downshifted to 275 °C. Moreover, the MgH2 + 10 wt.% CoTiO3 absorbed 6.4 wt.% H2 at low temperature (200 °C) in only 10 min and rapidly releases 2.3 wt.% H2 in the first 10 min, demonstrating a 23-times-faster desorption rate than as-milled MgH2 at 300 °C. The desorption activation energy of the 10 wt.% CoTiO3-doped MgH2 sample was dramatically lowered by 30.4 kJ/mol compared to undoped MgH2. The enhanced performance of the MgH2–CoTiO3 system is believed to be due to the in situ formation of MgTiO3, CoMg2, CoTi2, and MgO during the heating process, which offer a notable impact on the behavior of MgH2.

Microwave Plasma Enhancing Mg-Based Hydrogen Storage: Thermodynamics Evaluation and Economic Analysis of Coupling SOFC for Heat and Power Generation

Hydrogen, as a kind of green and efficient energy, plays an increasingly important role in current social development. Hydrogen storage technology is considered to be one of the main bottlenecks in limiting the large-scale application of hydrogen energy. The solid-state hydrogen storage technology based on Mg-based materials has received extensive attention due to its advantages of high hydrogen capacity, good reversibility, and low cost, but there are still shortcomings such as high reaction temperature, large energy consumption, and slow reaction kinetics. In order to solve these problems, this article proposes a new method of using microwave plasma to ionize hydrogen into H− ion. The possible activation mechanism of microwave plasma to improve the hydrogen storage properties is put forward. Based on the activation mechanism, the thermodynamic performance of Mg-based hydrogen storage is evaluated using density functional theory. It is concluded that the reaction temperature is significantly reduced from 339°C to 109°C with the help of microwave plasma. In addition, the comparison between the conventional heating hydrogen storage process based on MgH2 and microwave enhanced advanced hydrogen storage process based on MgH2 systems coupled with solid oxide fuel cells for heat and power generation is conducted to evaluate the economic feasibility. The results show that the energy consumption cost of the proposed microwave plasma enhancing hydrogen storage system is approximately 1.71 $/kgH2, which is about 50% of the energy consumption cost of the conventional system.

Improved hydrogen desorption properties of exfoliated graphite and graphene nanoballs modified MgH2

Magnesium hydride is a leading hydrogen storage material with high hydrogen content, however, suffers with sluggish kinetics. Several methods have been adopted to improve its kinetics, out of which, the addition of catalyst is an impressive way. Carbon materials have shown their promises as catalyst for several hydrogen storage materials. The present work is devoted to investigating the catalytic effects of exfoliated graphite and graphene nanoballs on dehydrogenation kinetics of MgH2. The lowest onset temperature of 282 °C is observed for graphene nanoballs modified MgH2 system. Exfoliated graphite mixed MgH2 desorbed hydrogen at onset temperature 301 °C which is also less than the dehydrogenation temperature of pure MgH2 (410 °C). The dehydrogenation kinetics has significantly improved by the addition of these catalysts as compared to the pure MgH2. The activation energy for the hydrogen desorption of MgH2 was reduced from 170 (pure MgH2) to 136 ± 2 and 140 ± 2 kJ/mol by the addition of exfoliated graphite and graphene nanoballs, respectively. The XRD results confirmed the presence of MgH2 after milling with exfoliated graphite and graphene nanoballs that indicates that there are no reactions during the milling thus both the additives are effective to improve the dehydrogenation as a catalyst.

Improved MgH2 kinetics and cyclic stability by fibrous spherical NiMoO4 and rGO

BackgroundThe synergistic catalysis has a good improvement on the hydrogen storage properties of MgH2. MethodsIn this work, fibrous spherical NiMoO4 bimetallic catalysts were prepared by hydrothermal and sintering methods, and introduced into the MgH2 system by ball milling to improve their kinetic performance. In addition, rGO (reduced graphene oxide) was also introduced to improve the cycling stability of MgH2. Significant findingsIt is observed that NiMoO4 has a significant improvement in the kinetics of MgH2 absorption and dehydrogenation. MgH2 + 10 wt% NiMoO4 can start dehydrogenation at 191 °C. After complete dehydrogenation, it starts to absorb hydrogen below 50 °C. The apparent activation energy of dehydrogenation and hydrogen absorption of MgH2 + 10 wt% NiMoO4 composites were reduced by 35.43 kJ mol−1 and 30.10 kJ mol−1, respectively, compared with pure MgH2. The results of characterization indicated that the uniformly dispersed NiMoO4 produced Mg2Ni and elemental Mo in situ after the first hydrogen release reaction. It is believed that the synergistic effect of Mo and Ni elements accelerates the hydrogen absorption and desorption performance of hydrogen storage materials. Cycling performance tests showed that the addition of rGO improved the cycling stability of the hydrogen storage material.

Exergo-economic analysis for screening of metal hydride pairs for thermochemical energy storage for solar baking system

The novelty of this research work is the exergo-economic analysis (including the cost of exergy destruction and exergy loss) of metal hydride based thermal energy storage system coupled with a solar bakery unit for the screening of metal hydride pairs (Case 1: pure MgH2/LaNi5& Case 2: V2O5 doped MgH2/LaNi5) for thermochemical energy storage. Firstly, a numerical simulation is performed by using COMSOL Multiphysics 5.3a software. Secondly, an economic and exergo-economic model is developed to calculate the annual levelized cost of the thermal energy storage system. The life-cycle economic assessment findings indicate that the Levelized thermal energy storage cost of the pure MgH2 based system (32.28 $ /kWhth) is 8.2 times higher than that of the V2O5 doped MgH2 system (3.954 $/kWhth). Moreover, an 87.75% decrease in cost was observed in Case 2 (V2O5 doped MgH2). Furthermore, Case 2 (V2O5 doped MgH2) can save 92.58 % of hydrogenation reaction time as compared to Case 1 (Pure MgH2). Ultimately, the selection of V2O5 based MgH2 as a thermal heat-storing medium is then assessed as a better option for the MHTES for the solar bakery unit (SBU). The findings of this research provide a clear insight into the mechanism of cost formation in the system.

A Unique Nanoflake‐Shape Bimetallic Ti–Nb Oxide of Superior Catalytic Effect for Hydrogen Storage of MgH2

MgH2 is one of the most promising solid hydrogen storage materials due to its high capacity, excellent reversibility, and low cost. However, its operation temperature needs to be greatly reduced to realize its practical applications, especially in the highly desired fuel cell fields. This work synthesizes a 2D nanoflake-shape bimetallic Ti-Nb oxide of TiNb2 O7 , which has high surface area and shows superior catalytic effect for the hydrogen storage of MgH2 . Incorporated with the TiNb2 O7 nanoflakes as low as 3 wt%, MgH2 shows a low onset dehydrogenation temperature of 178 °C, which is lowered by 100 °C compared with the pristine one. A dehydrogenation capacity as high as 7.0 wt% H2 is achieved upon heating to 300 °C. The capacity retention is as high as 96% after 30 cycles. The mechanism of the improved hydrogen storage properties is analyzed by density functional theory (DFT) calculation and the microstructural evolution during dehydrogenation and hydrogenation. This work provides an MgH2 system with high available capacity and low operation temperature by a unique structural design of the catalyst. The high surface area feature of the TiNb2 O7 nanoflakes and the synthesis method hopefully can develop the application of TiNb2 O7 .

A new permutation-symmetry-adapted machine learning diabatization procedure and its application in MgH2 system.

This work introduces a new permutation-symmetry-adapted machine learning diabatization procedure, termed the diabatization by equivariant neural network (DENN). In this approach, the permutation symmetric and anti-symmetric elements in diabatic potential energy metrics (DPEMs) were simultaneously simulated by the equivariant neural network. The diabatization by deep neural network scheme was adopted for machine learning diabatization, and non-zero diabatic coupling was included to increase accuracy in the near degenerate region. Based on DENN, the global DPEMs for 11A' and 21A' states of MgH2 have been constructed. To the best of our knowledge, these are the first global DPEMs for the MgH2 system. The root-mean-square-errors (RMSEs) for diagonal elements (H11 and H22) and the off-diagonal element (H12) around the conical intersection region were 5.824, 5.307, and 5.796meV, respectively. The RMSEs of global adiabatic energies for two adiabatic states were 4.613 and 12.755meV, respectively. The spectroscopic calculations of the 11A' state in the linear HMgH region are in good agreement with the experiment and previous theoretical results. The differences between calculated frequencies and corresponding experiment values are 1.38 and 1.08 cm-1 for anti-symmetric stretching fundamental vibrational frequency and first overtone. The global DPEMs obtained in this work should be useful for further quantum mechanics dynamic simulations on the MgH2 system.

One-step self-assembly of TiO2/MXene heterostructures for improving the hydrogen storage performance of magnesium hydride

Self-assembly of TiO2 nanoparticles (M-TiO2, 15–20 nm in size) on few layer Ti3C2Tx (FL-Ti3C2Tx) are conducted via simple one-step ultrasound method at room temperature, in which van der Waals interactions act as the driving force. After introduced into MgH2 system, M-TiO2/FL-Ti3C2Tx heterostructure with mass ratio of 60: 40 (6 M-TiO2/FL-Ti3C2Tx) shows the best catalytic activity. The one-step self-assembly preparation can alleviate the restacking of FL-Ti3C2Tx and the agglomeration of M-TiO2 nanoparticles to a certain extent, which can generate lots of interfaces between M-TiO2 and FL-Ti3C2Tx. The rich interfaces can not only act as hydrogen diffusion channels, but also the electron transfer on the interfaces can enhance the catalytic activity of the whole heterostructure. Besides, abundant electrons from multiple valence Ti (Ti4+, Ti3+, Ti2+, Ti0) can effectively promote the reversible reaction between MgH2 and Mg, H2. The facile preparation method facilitates the understanding of MXene heterostructure catalysts, without the influence of particle growth and impurities. These findings provide new insights to design heterostructure with high catalytic activity and guide the understanding of its catalytic mechanism.

Effect of driving force on the activation energies for dehydrogenation and hydrogenation of catalyzed MgH2

Catalyst doping is a critical approach to improve the kinetics of MgH2. However, reported activation energy (Ea) values of catalyzed MgH2 systems show a large discrepancy. This study investigates the effect of the driving force on Ea for (de)hydrogenation reactions. The kinetic rate constant is calculated using Johnson-Mehl-Avrami-Kolomogorov (JMAK) model. Ea is obtained according to a series of kinetic measurements under a nearly constant driving force. Under a low driving force (PIn/Peq=1.5), the Ea for dehydrogenation and hydrogenation are calculated to be 102.3±5.0 and 41.6±7.5 kJ/mol H2, respectively. Under a high driving force (PIn/Peq=10), the Ea for dehydrogenation and hydrogenation are calculated to be 101.4±25.8 and 57.4±5.0 kJ/mol H2, respectively. The result shows that the high driving force leads to higher hydrogenation Ea over the Ea obtained the low driving force, while dehydrogenation Ea remains unchanged under different driving forces.