Thermodynamics and kinetics of hydriding and dehydriding reactions in Mg-based hydrogen storage materials

Mg-based materials are one of the most promising hydrogen storage candidates due to their high hydrogen storage capacity,environmental benignity,and high Clarke number characteristics.However,the limited thermodynamics and kinetic properties pose major challenges for their engineering applications.Herein,we review the recent progress in improving their thermodynamics and kinetics,with an emphasis on the models and the influence of various parameters in the calculated models.Subsequently,the impact of alloying,composite,and nanocrystallization on both thermodynamics and dynamics are discussed in detail.In particular,the correlation between various modification strategies and the hydrogen capacity,dehydrogenation enthalpy and temperature,hydriding/dehydriding rates are summarized.In addition,the mechanism of hydrogen storage processes of Mg-based materials is discussed from the aspect of classical kinetic theories and microscope hydrogen transferring behavior.This review concludes with an outlook on the remaining challenge issues and prospects.

Kinetics in Mg-based hydrogen storage materials:Enhancement and mechanism

Mg-based materials have been intensively studied for hydrogen storage applications due to their high energy density up to 2600 Wh/kg or 3700 Wh/L.However,the Mg-based materials with poor kinetics and the necessity for a high temperature to achieve 0.1 MPa hydrogen equilibrium pressure limit the applications in the onboard storage in Fuel cell vehicles(FCVs).Over the past decades,many methods have been applied to improve the hydriding/dehydriding(H/D)kinetics of Mg/MgH 2 by forming amorphous or nanosized particles,adding catalysts and employing external energy field,etc.However,which method is more effective and the intrinsic mechanism they work are widely differing versions.The hydrogenation and dehydrogenation behaviors of Mg-based alloys analyzing by kinetic models is an efficient way to reveal the H/D kinetic mechanism.However,some recently proposed models with physical meaning and simple analysis method are not known intimately by researchers.Therefore,this review focuses on the enhancement method of kinetics in Mg-based hydrogen storage materials and introduces the new kinetic models.

Mg-based materials for hydrogen storage

Over the last decade’s magnesium and magnesium based compounds have been intensively investigated as potential hydrogen storage as well as thermal energy storage materials due to their abundance and availability as well as their extraordinary high gravimetric and volumetric storage densities.This review work provides a broad overview of the most appealing systems and of their hydrogenation/dehydrogenation properties.Special emphasis is placed on reviewing the efforts made by the scientific community in improving the material’s thermodynamic and kinetic properties while maintaining a high hydrogen storage capacity.

Nanostructuring of Mg-Based Hydrogen Storage Materials:Recent Advances for Promoting Key Applications

With the depletion of fossil fuels and global warming,there is an urgent demand to seek green,low-cost,and high-efficiency energy resources.Hydrogen has been considered as a potential candidate to replace fossil fuels,due to its high gravimetric energy density(142 MJ kg^(-1)),high abundance(H_(2)O),and environmentalfriendliness.However,due to its low volume density,effective and safe hydrogen storage techniques are now becoming the bottleneck for the"hydrogen economy".Under such a circumstance,Mg-based hydrogen storage materials garnered tremendous interests due to their high hydrogen storage capacity(~7.6 wt%for MgH_(2)),low cost,and excellent reversibility.However,the high thermodynamic stability(ΔH=-74.7 kJ mol^(-1)H_(2))and sluggish kinetics result in a relatively high desorption temperature(>300℃),which severely restricts widespread applications of MgH_(2).Nano-structuring has been proven to be an effective strategy that can simultaneously enhance the ab/de-sorption thermodynamic and kinetic properties of MgH_(2),possibly meeting the demand for rapid hydrogen desorption,economic viability,and effective thermal management in practical applications.Herein,the fundamental theories,recent advances,and practical applications of the nanostructured Mg-based hydrogen storage materials are discussed.The synthetic strategies are classified into four categories:free-standing nano-sized Mg/MgH_(2)through electrochemical/vapor-transport/ultrasonic methods,nanostructured Mg-based composites via mechanical milling methods,construction of core-shell nano-structured Mg-based composites by chemical reduction approaches,and multi-dimensional nano-sized Mg-based heterostructure by nanoconfinement strategy.Through applying these strategies,near room temperature ab/de-sorption(<100℃)with considerable high capacity(>6 wt%)has been achieved in nano Mg/MgH_(2)systems.Some perspectives on the future research and development of nanostructured hydrogen storage materials are also provided.

Ball-milling MoS_2/carbon black hybrid material for catalyzing hydrogen evolution reaction in acidic medium

Replacing platinum for catalyzing hydrogen evolution reaction （HER） in acidic medium remains great chal- lenges. Herein, we prepared few-layered MoS2 by ball milling as an efficient catalyst for HER in acidic medium, The activity of as-prepared MoS2 had a strong dependence on the ball milling time, Furthermore, Ketjen Black EC 300J was added into the ball-milled MoS2 followed by a second ball milling, and the resultant MoS2/carbon black hybrid material showed a much higher HER activity than MoS2 and carbon black alone. The enhanced activity of the MoS2/carbon black hybrid material was attributed to the increased abundance of catalytic edge sites of MoS） and excellent electrical coupling to the underlving carbon network.

In situ formation of multiple catalysts for enhancing the hydrogen storage of MgH_(2) by adding porous Ni_(3)ZnC_(0.7)/Ni loaded carbon nanotubes microspheres

MgH_(2) is considered one of the most promising hydrogen storage materials because of its safety,high efficiency,high hydrogen storage quantity and low cost characteristics.But some shortcomings are still existed:high operating temperature and poor hydrogen absorption dynamics,which limit its application.Porous Ni_(3)ZnC_(0.7)/Ni loaded carbon nanotubes microspheres(NZC/Ni@CNT)is prepared by facile filtration and calcination method.Then the different amount of NZC/Ni@CNT(2.5,5.0 and 7.5 wt%)is added to the MgH_(2) by ball milling.Among the three samples with different amount of NZC/Ni@CNT(2.5,5.0 and 7.5 wt%),the MgH_(2)-5 wt%NZC/Ni@CNT composite exhibits the best hydrogen storage performances.After testing,the MgH_(2)-5 wt%NZC/Ni@CNT begins to release hydrogen at around 110℃ and hydrogen absorption capacity reaches 2.34 wt%H_(2) at 80℃ within 60 min.Moreover,the composite can release about 5.36 wt%H_(2) at 300℃.In addition,hydrogen absorption and desorption activation energies of the MgH_(2)-5 wt%NZC/Ni@CNT composite are reduced to 37.28 and 84.22 KJ/mol H_(2),respectively.The in situ generated Mg_(2)NiH_(4)/Mg_(2)Ni can serve as a"hydrogen pump"that plays the main role in providing more activation sites and hydrogen diffusion channels which promotes H_(2) dissociation during hydrogen absorption process.In addition,the evenly dispersed Zn and MgZn2 in Mg and MgH_(2) could provide sites for Mg/MgH_(2) nucleation and hydrogen diffusion channel.This attempt clearly proved that the bimetallic carbide Ni_(3)ZnC_(0.7) is a effective additive for the hydrogen storage performances modification of MgH_(2),and the facile synthesis of the Ni_(3)ZnC_(0.7)/Ni@CNT can provide directions of better designing high performance carbide catalysts for improving MgH_(2).

Vermiform Ni@CNT derived from one-pot calcination of Ni-MOF precursor for improving hydrogen storage of MgH_(2)

The Ni-coated carbon nanotubes(Ni@CNT)composite was synthesized by the facile“filtration+calcination”of Ni-based metal−organic framework(MOF)precursor and the obtained composite was used as a catalyst for MgH_(2).MgH_(2)was mixed evenly with different amounts of Ni@CNT(2.5,5.0 and 7.5,wt.%)through ball milling.The MgH_(2)−5wt.%Ni@CNT can absorb 5.2 wt.%H_(2)at 423 K in 200 s and release about 3.75 wt.%H_(2)at 573 K in 1000 s.And its dehydrogenation and rehydrogenation activation energies are reduced to 87.63 and 45.28 kJ/mol(H_(2)).The in-situ generated Mg_(2)Ni/Mg_(2)NiH4 exhibits a good catalytic effect due to the provided more diffusion channels that can be used as“hydrogen pump”.And the presence of carbon nanotubes improves the properties of MgH_(2)to some extent.

Understanding the dehydrogenation properties of Mg(0001)/MgH_(2)(110)interface from first principles

Magnesium hydride is one of the most promising solid-state hydrogen storage materials for on-board application.Hydrogen desorption from MgH_(2) is accompanied by the formation of the Mg/MgH_(2) interfaces,which may play a key role in the further dehydrogenation process.In this work,first-principles calculations have been used to understand the dehydrogenation properties of the Mg(0001)/MgH_(2)(110) interface.It is found that the Mg(0001)/MgH_(2)(110) interface can weaken the Mg-H bond.The removal energies for hydrogen atoms in the interface zone are significantly lower compared to those of bulk MgH_(2).In terms of H mobility,hydrogen diffusion within the interface as well as into the Mg matrix is considered.The calculated energy barriers reveal that the migration of hydrogen atoms in the interface zone is easier than that in the bulk MgH_(2).Based on the hydrogen removal energies and diffusion barriers,we conclude that the formation of the Mg(0001)/MgH_(2)(110) interface facilitates the dehydrogenation process of magnesium hydride.

Improved hydrogen storage kinetics of MgH_(2) using TiFe_(0.92)Mn_(0.04)Co_(0.04) with in-situ generated α-Fe as catalyst

While TiFe alloy has recently attracted attention as the efficient catalyst to enhance de/hydrogenation rates of Mg/MgH_(2),the difficulty of its activation characteristics has hindered further improvement of reaction kinetics.Herein,we report that the TiFe_(0.92)Mn_(0.04)Co_(0.04) catalyst can overcome the abovementioned challenges.The synthesized MgH_(2)-30 wt% TiFe_(0.92)Mn_(0.04)Co_(0.04) can release 4.5 wt%of hydrogen in 16 min at 250℃,three times as fast as MgH_(2).The activation energy of dehydrogenation was as low as 84.6 kJ mol^(-1),which is 46.8%reduced from pure MgH_(2).No clear degradation of reaction rates and hydrogen storage capacity was observed for at least 30 cycles.Structural studies reveal that TiFe_(0.92)Mn_(0.04)Co_(0.04) partially decomposes to in-situ generatedα-Fe particles dispersed on TiFe_(0.92)Mn_(0.04)Co_(0.04).The presence ofα-Fe reduces the formation of an oxide layer on TiFe_(0.92)Mn_(0.04)Co_(0.04),enabling the activation processes.At the same time,the hydrogen incorporation capabilities of TiFe_(0.92)Mn_(0.04)Co_(0.04) can provide more hydrogen diffusion paths,which promote hydrogen dissociation and diffusion.These discoveries demonstrate the advanced nature and importance of combining the in-situ generatedα-Fe with TiFe_(0.92)Mn_(0.04)Co_(0.04).It provides a new strategy for designing highly efficient and stable catalysts for Mg-based hydrogen storage materials.

Novel progress in the development of hydrogen storage materials

A new dehydrogenation mechanism for LiBH4, a new hydrogen storage material, has recently been

Microstructural evolution of melt-spun Mg-10Ni-2Mm hydrogen storage alloy

The microstructural evolution of a Mg-10Ni-2Mm（molar fraction,%）（Mm=Ce,La-rich mischmetal） hydrogen storage alloys applied with various solidification rates was studied.The results show that the grain size of melt-spun ribbon is remarkably reduced by increasing the solidification rate.The microcrystalline,nanocrystalline and amorphous microstructures are obtained by applying the surface velocities of the graphite wheel of 3.1,10.5 and 20.9 m/s,respectively.By applying the surface velocity of the graphite wheel of 3.1 m/s,the melt-spun specimen obtains full crystalline with a considerable amount of coarse microcrystalline Mg and Mg2Ni except for some Mm-rich particles.The amount of nanocrystalline phases significantly increases with increasing the surface velocity of the wheel to 10.5 m/s,and the microstructure is composed of a large amount of nanocrystalline phases of Mg and Mg2Ni particles.A mixed microstructure containing amorphous and nanocrystalline phases is obtained at a surface velocity of the wheel of 20.9 m/s.The optimal microstructure with a considerable amount of nanocrystalline Mg and Mg2Ni in an amorphous matrix is expected to have the maximum hydrogen absorption capacity and excellent hydrogenation kinetics.

Catalytic effect of Ni@rGO on the hydrogen storage properties of MgH2

Uniform-uispersed Ni nanoparticics(NPs)anchored on reduced graphene oxide(Ni@rGO)catalyzed MgH2(MH-Ni@rGO)has been fabricated by mechanical milling.The effects of milling time and Ni loading amount on the hydrogen storage properties of MgH2 have been investigated.The initial hydrogen desorption temperature of MgH2 catalyzed by 10 wt.%Ni4@rGO6 for milling 5 h is significantly decreased from 251℃ to 190℃.The composite can absorb 5.0 wt.%hydrogen in 20 min at 100℃,while it can desorb 6.1 wt.%within 15 min at 300℃.Through the investigation of the phase transformation and dehydrogenation kinetics during hydrogen ab/desorption cycles,we found that the in-situ formed Mg2Ni/Mg2NiH4 exhibited better catalytic effect than Ni.When Ni loading amount is 45 wt.%,the rGO in Ni@rGO catalysts can prevent the reaction of Ni and Mg due to the strong interaction between rGO and Ni NPs.

Tailoring MgH_(2) for hydrogen storage through nanoengineering and catalysis

Hydrogen energy has been recognized as “Ultimate Power Source” in the 21st century, which could be the best solution to the looming energy crisis and climate degeneration in the near future. Due to its high safety, low price, abundant resources and decent hydrogen storage density, magnesium based solid-state hydrogen storage materials are becoming the leading candidate for onboard hydrogen storage. However,the high operation temperature and slow reaction rate of MgH_(2), as a result of the large formation enthalpy and high reaction activation energy,respectively, are the first and most difficult problems we need to face and overcome to realize its industrialization. Herein, a state-of-the-art review on tailoring the stable thermodynamics and sluggish kinetics of hydrogen storage in MgH_(2), particularly through nanoengnieering and catalysis is presented, aiming to provide references and solutions for its promotion and application. Promising methods to overcome the challenges faced by MgH_(2)/Mg, such as bidirectional catalysts and nanoconfinement with in-situ catalysis are compared and the required improvements are discussed to stimulate further discussions and ideas in the rational design of MgH_(2)/Mg systems with ability for hydrogen release/uptake at lower temperatures and cycle stability in the near future.

Mn nanoparticles enhanced dehydrogenation and hydrogenation kinetics of MgH_(2) for hydrogen storage

Mn nanoparticles(nano-Mn)were successfully synthesized and doped into MgH_(2) to improve its de/hydrogenation properties.Compared with MgH_(2),the onset desorption temperature of 10 wt.%nano-Mn modified MgH_(2) was decreased to 175℃ and 6.7,6.5 and 6.1 wt.%hydrogen could be released within 5,10 and 25 min at 300,275 and 250℃,respectively.Besides,the composite started to take up hydrogen at room temperature and absorbed 2.0 wt.%hydrogen within 30 min at low temperature of 50℃.The hydrogenation activation energy of MgH_(2) was reduced from(72.5±2.7)to(18.8±0.2)kJ/mol after doping with 10 wt.%nano-Mn.In addition,the MgH_(2)+10 wt.%nano-Mn composite exhibited superior cyclic property,maintaining 92%initial capacity after 20 cycles.

Structural and Electronic Properties of Li2Mg（NH）2 for Hydrogen Storage： First-principles Study

The structural and electronic properties of Li2Mg（NH）2 for hydrogen storage have been studied by first-principles calculation. The optimal unit cell parameters and the distance of N-H are determined, which are in good agreement with the experimental data. The bulk modules and the energies of zero pressure are obtained by using Murnaghan equation of states. The results show that the α-Li2Mg（NH）2 is a ground state configuration. The overlap population analysis shows that the N-Li/Mg ionic characteristics and N-H interaction of αphase are weaker than those of βphase. The valence band is dominated by the presence of N s and p states, hybridized with the H s state.

Application of Nanocrystalline LaNi_5-type Hydrogen Absorbing Alloys in Ni-MH_x Batteries

The structure and electrochemical properties of nanocrystalline LaNi_5-type alloys were studied. These materials were prepared by mechanical alloying (MA) followed by annealing. The properties of hydrogen host materials can be modified substantially by alloying to obtain the desired storage characteristics. It was found that the partial substitution of Ni by Al or Mn in LaNi_(5-x)M_x alloy leads to an increase in discharge capacity. The alloying elements such as Al, Mn and Co greatly improved the cycle life of LaNi_5 material. For example, in the nanocrystalline LaNi_(3.75)Mn_(0.75)Al_(0.25)Co_(0.25) powder, discharge capacity up to 258 mAh·g^(-1) was measured (at 40 mA·g^(-1) discharge current). Furthermore, the effect of the graphite coating on the structure of some nanocrystalline alloys and the electrodes characteristics were investigated. The mechanical coating with graphite effectively reduced the degradation rate of the studied electrode materials. The combination of a nanocrystalline LaNi_5-type hydride electrodes and a nickel positive electrode to form a Ni-MH battery, was successful.

Effect of cerium content on microstructure and hydrogen storage performance of Ti_(24)Cr_(17.5)V_(50)Fe_(8.5)Ce_x(x=0-1.0) alloys

Effect of Ce addition on microstructure and hydrogen storage performance of Ti24Cr17.5V50Fe8.5Cex（x=0, 0.5at.%, 0.8at.% and 1.0at.%） alloys was studied by X-ray diffraction, scanning electron microscopy and P-C-isotherm measurements.The results indicated that Ce addition was a useful way to improve the flatness of the plateau and increase hydrogen storage capacity of Ti24Cr17.5V50Fe8.5 alloy.It was indicated that both homogenization of composition and increase of hydrogen diffusion coefficient were the main reasons for improving the hydrogen storage performance of Ti24Cr17.5V50Fe8.5Cex alloys.

Effect of ternary transition metal sulfide FeNi_(2)S_(4)on hydrogen storage performance of MgH_(2)

Hydrogen storage is a key link in hydrogen economy,where solid-state hydrogen storage is considered as the most promising approach because it can meet the requirement of high density and safety.Thereinto,magnesium-based materials(MgH_(2))are currently deemed as an attractive candidate due to the potentially high hydrogen storage density(7.6 wt%),however,the stable thermodynamics and slow kinetics limit the practical application.In this study,we design a ternary transition metal sulfide FeNi_(2)S_(4)with a hollow balloon structure as a catalyst of MgH_(2)to address the above issues by constructing a MgH_(2)/Mg_(2)NiH_(4)-MgS/Fe system.Notably,the dehydrogenation/hydrogenation of MgH_(2)has been significantly improved due to the synergistic catalysis of active species of Mg_(2)Ni/Mg_(2)NiH_(4),MgS and Fe originated from the MgH_(2)-FeNi_(2)S_(4)composite.The hydrogen absorption capacity of the MgH_(2)-FeNi_(2)S_(4)composite reaches to 4.02 wt%at 373 K for 1 h,a sharp contrast to the milled-MgH_(2)(0.67 wt%).In terms of dehydrogenation process,the initial dehydrogenation temperature of the composite is 80 K lower than that of the milled-MgH_(2),and the dehydrogenation activation energy decreases by 95.7 kJ·mol-1 compared with the milled-MgH_(2)(161.2 kJ·mol^(-1)).This method provides a new strategy for improving the dehydrogenation/hydrogenation performance of the MgH_(2)material.

Microstructure and electrochemical properties of LaNi_(4-x)FeMn_x (x=0-0.8) hydrogen storage alloys

In order to reduce the cost of LaNi5 based hydrogen storage alloys, effect of substitution of Mn for Ni on structural and electrochemical properties of LaNi4-xFeMnx (x=0-0.8) hydrogen storage alloys was studied systematically. X-ray diffraction (XRD) and scanning electron microscope (SEM) showed that LaNi5 and La2Ni7 phases were invariably present in all alloy samples, and when x >= 0.4, (Fe, Ni) phase was observed. Electrochemical studies revealed that the discharge capacity reached a maximum value of 306.4 mAh/g when x=0.2 and the cycling stability decreased with the increase of x. With the increase of Mn content, hydrogen diffusion coefficient decreased, whereas high rate discharge-ability (HRD) and exchange current density first increased slowly when x <= 0.2 and then decreased markedly when x=0.8, indicating that electrochemical reaction on the surface of alloy electrodes had strong influence on kinetic property.

High-loading, ultrafine Ni nanoparticles dispersed on porous hollow carbon nanospheres for fast (de)hydrogenation kinetics of MgH_(2)

Magnesium hydride(MgH2) is one of the most promising hydrogen storage materials for practical application due to its favorable reversibility, low cost and environmental benign;however, it suffers from high dehydrogenation temperature and slow sorption kinetics.Exploring proper catalysts with high and sustainable activity is extremely desired for substantially improving the hydrogen storage properties of MgH2. In this work, a composite catalyst with high-loading of ultrafine Ni nanoparticles(NPs) uniformly dispersed on porous hollow carbon nanospheres is developed, which shows superior catalytic activity towards the de-/hydrogenation of MgH2. With an addition of 5wt% of the composite, which contains 90 wt% Ni NPs, the onset and peak dehydrogenation temperatures of MgH2are lowered to 190 and 242 ℃, respectively. 6.2 wt% H2is rapidly released within 30 min at 250 ℃. The amount of H2that the dehydrogenation product can absorb at a low temperature of 150 ℃ in only 250 s is very close to the initial dehydrogenation value. A dehydrogenation capacity of 6.4wt% remains after 50 cycles at a moderate cyclic regime, corresponding to a capacity retention of 94.1%. The Ni NPs are highly active,reacting with MgH2and forming nanosized Mg2Ni/Mg2NiH4. They act as catalysts during hydrogen sorption cycling, and maintain a high dispersibility with the help of the dispersive role of the carbon substrate, leading to sustainably catalytic activity. The present work provides new insight into designing stable and highly active catalysts for promoting the(de)hydrogenation kinetics of MgH2.