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Review

Effect of Ti-Based Additives on the Hydrogen Storage Properties of MgH2: A Review

by
Mukesh Jangir
1,
Indra Prabh Jain
2 and
Daniele Mirabile Gattia
3,*
1
School of Applied Science and Technology, NIMS University, Jaipur 303121, India
2
Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur 302004, India
3
Department for Sustainability, ENEA Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy
*
Author to whom correspondence should be addressed.
Hydrogen 2023, 4(3), 523-541; https://doi.org/10.3390/hydrogen4030034
Submission received: 16 June 2023 / Revised: 29 July 2023 / Accepted: 1 August 2023 / Published: 5 August 2023
(This article belongs to the Special Issue Feature Papers in Hydrogen (Volume 2))
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Abstract

:
For the few past decades, study of new hydrogen storage materials has been captivating scientists worldwide. Magnesium hydride, MgH2, is considered one of the most promising materials due to its low cost, high hydrogen capacity, reversibility and the abundance of Mg. However, it requires further research to improve its hydrogen storage performance as it has some drawbacks such as poor dehydrogenation kinetic, high operational temperature, which limit its practical application. In this study, we introduce an overview of recent progress in improving the hydrogen storage performance of MgH2 by the addition of titanium-based additives, which are one of the important groups of additives. The role of Ti-based additive hydrides, oxides, halides, carbides and carbonitrides are overviewed. In addition, the existing challenges and future perspectives of Mg-based hydrides are also discussed.

1. Introduction

The rapidly increasing consumption of energy is depleting limited fossil fuel supplies and the world is facing the problem of energy resource scarcity and other environmental problems [1]. Moreover, burning of fossil fuels is one of the main causes of the greenhouse effect that is an actual environmental issue. In this scenario, sustainable energy resources and high-efficiency energy systems, which can meet the requirement of clean energy and sustainable growth, are required [2]. Hence, the development of renewable energy proceeds fast and new drivers are considered as an environmentally friendly solution, such as hydrogen energy. In the last few decades, the importance of hydrogen energy has been proposed, but still now researchers investigate how hydrogen can be used more significantly and lead to a more sustainable environment [3]. However, storing hydrogen remains challenging and some issues have to be taken into account such as safety, compactness, lightness, cycling and cost of the storage system [4].
Solid hydrogen storage is a suitable alternative to compressed gas storage and cryogenic liquid storage. There are two types of sorption mechanism to store hydrogen in materials, physisorption and chemisorption [5]. Earlier studies reveal that magnesium hydride, MgH2, is one of the most suitable candidates for storing hydrogen due to its low density, high hydrogen capacity (7.6 wt%), high abundance, low cost, good heat resistance, vibration absorption, reversibility and recyclability. Nevertheless, it suffers some limitations due to sluggish absorption/desorption kinetics and the elevated temperature required for hydrogen absorption and desorption [4].
Different approaches have been investigated by researchers to alleviate the above limitations, such as nano-structuring [6,7,8,9], alloying [10,11,12] and the addition of a catalyst [13,14,15]. The effect of high energy milling on MgH2 was investigated by TEM observations and analyses by M. Danaie et al. [16]. The authors found a reduction of the activation energy due to deformation of twinning in MgH2. Kinetics of MgH2 are notably enhanced by the addition of a transition metal, halides and oxides and carbon material by the reduction of the metal−hydrogen bond energy and by lowering of the decomposition temperature. Among them, because of the high affinity of the transition metal cation towards hydrogen, transition metal compounds are highly effective catalysts [17,18]. Transition metals promote the dissociation and re-combination of hydrogen and the kinetics are improved by the addition of small amounts of such material. In one of the studies, M. Danaie observed that ball milled MgH2 and Fe particles form Mg2FeH6 during the process of thermal hydrogenation. Fe particles behave as a catalyst and dissociate the molecular hydrogen to form atomic hydrogen [19]. The effect of few at% Fe on the hydrogenation of MgH2 thin film was also observed by some researchers and results showed the significant improvement of the hydride phase growth rate. Reduction in absorption/desorption entropy and enthalpy were also noticed [20].
B. Zahiri et al. studied the hydrogenation properties of Mg-15 at% Fe-15Ti, Mg10 at% Fe-10Ti and Mg-Ti-Cr thin films [21]. The authors observed that 5 wt% of hydrogen is absorbed by these thin films within seconds and desorbed in minutes at 250 °C. N. Hanada et al. [18] observed that 3d-transition metal (Fe, Co, Cu, Ni) doped MgH2, prepared by milling, presents improved hydrogen desorption properties with respect to pure ball-milled MgH2. Among these transition metals, Ni shows a better catalytic effect and MgH2 desorbs 6.5 wt% of hydrogen in the temperature range 150–250 °C. H. Gasan et al. [22] investigated the effect of the addition of 5 wt% of Ti, V, Nb and graphite on the dehydrogenation temperature and reported that these additives cause a reduction of the desorption temperature of about 40–50 °C compared with that of as-received MgH2.
Due to its properties, as a lightweight and low cost material, Ti captivates scientists’ attention. Ti acts as a catalyst and it exhibits a strong affinity for hydrogen even at moderate temperatures, improving the hydrogenation/dehydrogenation kinetics in several hydrogen storage systems [23]. The addition of catalysts provides a crucial track to enhance surface kinetics. In this paper, the use of Ti as an additive for hydrogenation/dehydrogenation of MgH2 is reviewed.

2. Fundamentals of the MgH2 System

2.1. Crystal Structure of MgH2 System

MgH2 is a hexagonal-close packed and stoichiometric compound with a H/Mg atomic ratio of 0.99 ± 0.01 [24] and density of 1.74 g/cm3. Magnesium reacts with hydrogen under different conditions and forms different types of structures. During absorption, firstly the hydrogen molecule dissociates on the Mg particles surface. Hydrogen atoms initially occupy tetrahedral interstitial sites to form an α phase. The structure of α-MgH2 is a tetragonal Rutile type. Under high pressure, α-MgH2 undergoes polymorphic transformation to form two modifications: γ-MgH2 and β-MgH2 having an orthorhombic [25] and a hexagonal structure, respectively [26]. According to Varin et al., high energy ball milling of MgH2 results in the formation of both γ- and β-MgH2 phases, and the presence of these metastable phases lowers the desorption temperature. In Figure 1 the crystal structure of MgH2 is reported [27].
In Table 1, different crystalline MgH2 phases and their crystal parameters are reported [28].

2.2. Thermodynamics of the MgH2 System

Stampfer et al. [24] for the first time evaluated the thermodynamic properties of MgH2 and reported its enthalpy (−74.5 kJ/mol·H2) and entropy (136 J/K·mol·H2) of formation. Thermodynamic behaviour of a hydride can be explained by pressure composition isotherms, representing the pressure vs. hydrogen to metal ratio at a constant temperature. Reporting the plateau pressures in the Van’t Hoff plot, the enthalpy and the entropy are respectively determined by the slope and the intercept of the linear fit (Figure 2 [29]).
In terms of thermodynamic properties, suitable materials for solid hydrogen storage should have a bonding enthalpy in the range 25–45 kJ/mol·H2 [30]. Hence, many efforts have been done to remove constraints and lower the ∆H, which are important for on-board application. Some methods were suggested such as alloying, downsizing and the stress effect. In Figure 3, the alloying formation of stable Mg compounds with other elements, such as Mg2Si, Mg2Ca, Mg2NiH4, Mg2FeH6 is reported [31,32,33,34]. Mg-Ti-H systems have been investigated but, due to immiscibility, no compounds can be formed between Mg and Ti. K. Asano et al. [35] reported hexagonal close-packed, face-centred cubic and body-centred cubic MgxTi100−x (35 ≤ x ≤ 80) alloys, obtained by ball milling. P. Vermeulen et al. [36] investigated the extremely low plateau pressure (≈10−6 bar at room temperature) in the Mg-Ti-H system. When it forms ternary composite with Al or Si a higher plateau pressure can be observed and its hydrogen capacity increases until 6 wt%.

2.3. Kinetics of MgH2 System

In order to store hydrogen in metals in the atomic form or through chemical reactions involving hydrogen, molecular hydrogen must first be split into hydrogen moieties during chemical storage. Hydrogen reacts with metal and forms stronger bonds whereas, in physisorption, hydrogen is weakly adsorbed on the metal surface. When atomic hydrogen reacts with other atoms, or molecules, it either gains or loses an electron, depending on the electronegativity of the other metal atom or molecule. In the case of electropositive metals, such as alkali or alkaline earth metals, atomic hydrogen gains an electron and it forms an ionic hydride; on the other hand, in the case of an electronegative atom or molecule (p-block elements, such as boron and nitrogen), the hydrogen atom exists as a proton. Hydrogen absorption reaction consists of several different steps [38,39].
(i)
Surface assimilation of molecular hydrogen:
Physisorption of an H2 molecule on a metal surface requires extremely little activation energy, it is not commonly seen as a limiting step. The remaining steps may be rate limiting, which is worth debating.
(ii)
Hydrogen molecules convert into atoms:
Both theoretical calculations and experimental results indicate that a significant energy barrier must be overcome when hydrogen dissociates on a pure Mg surface. According to Du et al. [40], the hydrogen dissociation activation barrier decreases from 1.051 eV for a pure Mg(0001) surface to 0.103 eV and 0.305 eV for Ti-doped and Pd-doped Mg(0001) surfaces, respectively.
Once the hydrogen molecules dissociate into atoms on the surface, an obstacle may still exist to prevent transfer of hydrogen atoms from catalytic sites into the bulk. The so-called “hydrogen spillover” mechanism may play the role in Mg-TM catalysed systems [41].
(iii)
Penetration of hydrogen atoms on the surface:
Surface modification is required to increase dehydrogenation and hydrogenation kinetics due to the presence of a surface oxide layer, which prevents hydrogen atoms from penetrating into the bulk. Even in an inert gas with a trace amount of O2/H2O, the continuous passive MgO/Mg(OH)2 layer would easily cover the Mg/MgH2 surface [42].
Recent research on the effect of air exposure on TiMn2 catalysed MgH2 [43] found that direct air exposure reduces the hydrogen storage capacity while only causing modest kinetic degradation. Further surface characterisation revealed that MgH2 forms a layer with Mg(OH)2 and MgO. The layer may shatter during hydrogen cycling, however, the nanocomposite can be reactivated in the presence of a catalyst. The doped catalyst particles on hydride surfaces can act as pathways for hydrogen to be transferred from the surface to the bulk, or from MgH2 to the outside. As asserted in MgH2-Nb [44] and MgH2-Pt [45] catalysed systems, this mechanism is known as the “hydrogen gateway” effect.
(iv)
Diffusion of atomic hydrogen:
The use of a catalyst could help to accelerate the rate of hydrogen diffusion in the matrix. Because MgH2 has a sluggish diffusion rate, it is expected that the reaction will switch to diffusion control when the developing hydride covers the particles during hydrogenation. Nano-doped catalytic species are thought to expedite hydrogen diffusion in both hydrogenation and dehydrogenation processes. This mechanism is commonly referred to as the “hydrogen pathway” effect.
(v)
Formation of hydride at the metal/hydride interface:
The final step in the hydrogenation of Mg is the nucleation and development of the MgH2 phase. Because of the crystal structural difference between Mg metal and its developing hydride, the nucleation and growth of a hydride phase will result in significant interfacial energy shifts. It is still debated whether the rate-limiting stage is controlled by nucleation and growth or hydrogen diffusion.
Desorption takes place via a reverse reaction. In most cases, hydride nucleation and growth or diffusion are rate determining steps for desorption kinetics [39]. In general, H2 molecule dissociation requires an additional amount of energy, known as the activation energy. Experiments for kinetic studies of metal hydride are carried out under isothermal and non-isothermal conditions.
Generally, a Sievert-type apparatus is used for isothermal kinetic study, which permits us to find a precise volumetric measurement of the amount of hydrogen released at appropriate pressure and temperature. In order to obtain comprehensive information about the dehydrogenation/hydrogenation kinetics, the activation energy (Ea) for a reaction is calculated using the Arrhenius equation, which combines the concept of activation energy with a Boltzmann distribution law [31].
k(T) = A exp(−Ea/RT)
where k(T) is the reaction-rate constant; A is a pre-exponential factor; Ea is the activation energy for the reaction; R is the ideal gas constant; T is the absolute temperature.
Under non-isothermal conditions, thermogravimetric analysis is used. It allows us determine the weight loss as a function of temperature. The changes in thermal characteristics are represented by a deflection or peak in the curve. The fluctuation in peak temperature can be used to calculate the activation energy of first order processes. The activation energies are evaluated by plotting a curve between lnK and 1/RTP using the following equation:
lnk = −Ea/RTp + a
where
  • k = β/Tp2;
  • β = heating rate;
  • Tp = peak temperature;
  • Ea = activation energy of desorption;
  • R = gas constant.

3. Effect of Catalyst

To improve the kinetics of hydrogen absorption and desorption reactions of magnesium-based materials, the addition of catalysts or additives and nano-structuring are two important approaches.

3.1. Transition Metal Catalyst or Additives

Among all the additives, the transition metal (TM)-based catalyst shows significant improvement in the kinetics without sacrificing the hydrogen storage capacity. The effect of various TM-based catalysts on the hydrogenation/dehydrogenation properties of MgH2 have been intensively investigated.
The addition of a small amount of these catalysts improved the kinetics of MgH2 as these facilitate the dissociation and recombination of hydrogen. Liang et al. [17] studied the effect of TM (as Ti, V, Mn, Fe and Ni) on the hydrogenation properties of ball milled MgH2. No change in entropy and enthalpy but a reduction in activation energy of desorption for magnesium hydride were reported. The activation energies (Ea) of MgH2-V, MgH2-Ti, MgH2-Ni, MgH2-Fe and MgH2-Mn are evaluated to be 62.3. kJ/mol, 71.1 kJ/mol, 88.1 kJ/mol, 67.6 kJ/mol and 104.6 kJ/mol, respectively, which are significantly reduced compared to that of the ball-milled pure MgH2 (120 kJ/mol).
Among them, MgH2–V and MgH2–Ti show the fastest desorption at low temperatures. Study of mechanically milled MgH2 with 5 wt.% of additives (V, Nb, Ti and graphite) showed a reduction of the hydrogen desorption temperature by about ∼40–50 °C compared with that of as-received MgH2 [22]. In another study, Zhou et al. found that elements of the IV-A and V-A groups are the most effective additives. Elements of VII-A (Mn), VIII-A (Fe, Co, Ni) have a moderate effect and Mo, Mo oxides and W oxide Yttrium (III-A group) and Yttrium oxide showed no improvement in the kinetics. MgH2 with Ti has the lowest onset temperature followed by V, Mn, Zr, Nb, Fe, Ni, Cr and Co. In Figure 4, the effect of different additives on the desorption temperature of MgH2 is reported, as reprinted from [46], while in Figure 5 the desorption curves of MgH2–5 at%Tm (Tm = Ti, V, Mn, Fe and Ni) composites under a hydrogen pressure of 0.015MPa and different temperatures are reported [17].
In another study, Rizo-Acosta et al. performed a comparative study of early transition metal doped (Sc, Y, Ti, Zr, V, and Nb) MgH2 and found that Y and V allowed for the highest absorption performances. The sequence of hydrogen absorption was Y < V < Ti < Nb < Sc < Zr. Nevertheless, with an increasing number of cycles, a slight degradation was observed but this effect was less evident for the Ti doped system, which revealed better hydrogenation properties in terms of cycling and wt% [47]. All of the studies indicate that the Ti-based additives or catalysts play a crucial role in the improvement of the hydrogenation properties of MgH2.

3.2. Titanium-Based Additives

Effects of various Ti-based additives or catalyst such as Ti hydride, halide and oxide have been studied. Researchers observed a dramatic improvement in the hydrogen kinetics by addition of Ti [48,49,50,51].

3.2.1. Titanium or Titanium Hydride

Choi et al. observed that MgH2-TiH2 shows significant enhancement in the kinetics compared to pure MgH2 ball milled at the same conditions, with onset of the desorption temperature at 400 K at a heating rate of 5 K/min [52].
Kinetic energy of ball milled MgH2 is 96 kJ/mol and after adding TiH2 the kinetic energy is reduced by 15 kJ/mol. This shows the positive effect of TiH2 on hydrogen storage of MgH2. Desorption enthalpy also reduces in a MgH2-TiH2 sample by the effect of TiH2. According to experimental results, the dehydrogenation properties of the MgH2 can be improved by TiH2 addition, which also weakens the Mg−H bond [53]. These findings encourage additional research into how TiH2 affects the hydrogen absorption and desorption capabilities of MgH2 [54,55]. N. Patelli et al. [56] synthesised Mg-Ti-H nanoparticles with different Ti contents, from 14 to 63 at%, by gas phase condensation and investigated the sorption kinetics in the temperature range 100–150 °C. Results reveal an increase of the activation energy with Ti content in the range 43−52 kJ/mol and an increase of the rate constant (at 150 °C) from 27×10 s−1 to 92×10 s−1. Choi et al. [57] studied the hydrogenation properties of Ti-added MgH2 composite (MgH2 + 15 Ti). They observed that Ti enhances the kinetics of MgH2. Samples absorbed 2.96 wt% H in 2.5 min and 5.51 wt% H in 60 min at 593. K.M. Lototskyy [58] observed that the introduction of a small amount of graphite improves the hydrogen storage performance of MgH2. In Table 2, the hydrogen storage properties of Mg/MgH2 with elemental Ti or TiH2 additive for different authors are reported.

3.2.2. Titanium Oxide

Metal oxides have a catalytic effect as well as they act as a milling aid by creating defects in the MgH2 structure. Among various used catalysts, TiO2 cost is low, it presents large availability with respect to other oxides and it consists of numerous interesting catalytic characteristics [71,72,73,74]. M. Polanski et al. [72] investigated the effect of several oxides (Cr2O3, TiO2, Fe3O4, Fe2O3, In2O3 and ZnO) on ball-milled MgH2. Experimental results reveal that Cr2O3 and TiO2 are superior catalysts because they enhance MgH2 absorption and desorption properties. S. K. Pandey et al. [75] observed that particles of 50 nm diameter allowed for the lowest activation energy, confirming that particle size of TiO2 is an important factor in the sorption properties of magnesium hydride. Further, M. Daryani et al. [76] investigated that, by the addition of Ti-based catalysts, the activation energy of MgH2 is reduced by up to 20–30 kJ/mol. They also observed a significant improvement in desorption kinetics during cyclic dehydrogenation of MgH2 due to the presence of micro-cracks at the particle surfaces. D. Mirabile Gattia et al. synthesised MgH2-5%TiO2-5%ENG pellets by ball milling followed by cold pressing at various compaction loads. The pellets fabricated at the highest pressure showed improved performance [77,78]. Y. C. Pan et al. [42] prepared the Mg-TiO2 composite by the arc plasma method and found that TiO2 can act as a catalyst to improve the hydrogenation properties of Mg. K.S. Jung et al. [79] and R. Vujasin et al. [80] studied the effect of different phases of TiO2, such as anatase and rutile, on the sorption properties of magnesium hydride. X. Zhang et al. [80] studied the effect of carbon-supported nanocrystalline TiO2 (TiO2@C) on the hydrogenation properties of MgH2 and found that MgH2-10 wt%TiO2@C started to desorb and absorb hydrogen at 205 °C and room temperature, respectively, revealing the catalytic activity of carbon-supported nanocrystalline TiO2. Mg-5 mol% Ti4Fe2Ox also absorbs hydrogen at room temperature after desorption and after the addition of 3 wt% graphite the cyclic stability was sustained, as reported by V. Berezovets et al. [81]. In Table 3, the hydrogen sorption properties of MgH2 and titanium oxides are reported.

3.2.3. Titanium Halide

S.-A. Jin et al. [87] performed a comparative study of the effect of VF4, NbF5, NiF2, TiF3, CrF2, CuF2, FeF2, ZrF2 and YF3 on the hydrogenation properties of MgH2 and observed that, among various transition metal fluorides, NbF5 and TiF3 showed a better catalytic effect. The formation of a MgF2 layer has been observed, due to the reaction of MgH2 with metal halides during the process of milling and releasing of hydrogen. Moreover, MgF2 contains an F- anion having a high affinity towards hydrogen and weakening the Mg−H bonding [88,89,90,91,92,93]. In another study S.-A. Jin et al. reported that, when MgH2 reacts with TiF3, the formation of TiH2 also takes place, following the formula: 3MgH2 + 2 TiF3 → 3MgF2 + 2TiH2 + H2.
Further remarkable effect of magnesium fluoride has been reported by I.E. Malka et al. [93]. MgF2 acts as a catalyst and it enhances the sorption kinetics of MgH2. Halides having the highest oxidation state reduce the magnesium decomposition temperature more efficiently than their counterparts with a lower oxidation state [93]. Study of the effect of various halide additives reveals that fluorides are better catalysts than chlorides. S.-K. Peng et al. [94] investigated the hydrogenation properties of TiF3 and TiF4 doped Mg (AlH4)2 and they observed that TiF4 shows superior catalytic abilities due to the higher oxidation state of Ti. M. Jangir et al. [95] studied the effect of TiF4 on MgH2 and found that hydrogen properties of MgH2 markedly improved by TiF4 addition by lowering the onset desorption temperature compared to as-milled MgH2. In Table 4, the sorption properties of MgH2 with different halides for different authors are reported.
Furthermore, a comparative study of various Ti-based additive such as elemental Ti, TiO2, TiN and TiF3 has been carried out by Y. Wang et al. [51]. Different Ti-based materials are added to MgH2 and their catalytic effects are studied thoroughly. The findings revealed that all of these Ti-based compounds may significantly increase MgH2 dehydrogenation performance. TiF3 has the best catalytic properties, followed by elemental Ti, oxide TiO2 and covalent TiN in that order.
The initial desorption temperature drops from 308 °C for pure MgH2 to 280 °C for MgH2-TiN, then to 257 and 216 °C for MgH2-Ti and MgH2-TiO2 composites and ultimately to 173 °C for the MgH2-TiF3 sample. The reduced Ea also indicated the favourable impacts of these additives, which were 10.3%, 26.3%, 35.8% and 53.3% lower following the addition of TiN, TiO2, Ti and TiF3, respectively. Mechanism analyses revealed that the covalent TiN was stable throughout the dehydrogenation process. During ball milling, elemental Ti and ionic TiF3 react with MgH2 to create active TiH1.971 and TiH2, which operate as active species throughout the desorption. The degradation of the MgH2-TiO2 sample occurred in two steps. As a result, the diverse reaction mechanisms and active species can explain the various effects of these Ti-based materials on the dehydrogenation capabilities of MgH2 [95].

3.2.4. Ti-Based Intermetallics

In recent years Ti-based intermetallics as catalysts have captivated the interest of researchers. TiFe [100,101,102], (Fe0.8Mn0.2)Ti [103], Ti2Ni [104] and TiMn1.5 [105] additives showed good catalytic effects in improving the hydrogen storage properties of MgH2. C. Zhou et al. [64] investigated the effect of a series of intermetallic additives (i.e., TiAl, Ti3Al, TiNi, TiFe, TiNb, TiMn2 and TiVMn) and observed that TiMn2-doped Mg was capable of absorbing hydrogen at room temperature. They also reported that the apparent activation energy is 20.59 kJ/mol·H2. These results have been confirmed by El-Eskandarany et al. [106,107] and also by the first principles calculation performed by J.H. Dai et al. [108]. V.V. Berezovets et. al. investigated the hydrogen absorption and desorption capabilities of nanostructured MgH2 modified with Ti-based materials (nano-Ti, nano-TiO2 and Ti4Fe2Ox) and produced by reactive ball milling in hydrogen. In the case of nano-Ti and Ti4Fe2Ox additions, a greater hydrogen storage capacity was attained [81]. In Table 5 hydrogen sorption properties of Mg-based compounds with Ti intermetallics and alloys are reported.

3.2.5. Titanium Carbides and Carbonitrides

MXene is a newly developed, two-dimensional transition metal carbide or carbonitride and is structurally comparable to graphene. One of the oldest and most extensively used MXenes for improving the hydrogen storage properties of MgH2 is Ti3C2 [111]. The 2D Ti2C MXene has a good catalytic impact on MgH2 dehydrogenation. MgH2-5 wt%Ti2C has a lower onset dehydrogenation temperature, reduced to 37 °C, the apparent activation energy, Ea, is 36.5% lower and the total enthalpy changes (ΔH) are higher than those of pure MgH2 (11%) [112]. Further, Y. Liu et al. used ball milling to incorporate a two-dimensional Nb4C3Tx (MXene) produced through chemical exfoliation into MgH2. The activated MgH2-5 wt%Nb4C3Tx composite has excellent hydrogen storage kinetics [113]. According to the findings, the use of a novel two-dimensional Ti3C2 MXene-based catalyst (Ni@Ti-MX) improved the MgH2 hydrogen sorption significantly [114].
Microstructural investigations demonstrated that the as-prepared Ni@TiMX, which has a uniform dispersion of self-assembled Ni nanoparticles on its surface, reacts with MgH2 during de/hydrogenation cycles, resulting in the creation of multiphase components (Mg2Ni, TiO2, metallic Ti or amorphous C) [115].
W. Zhu et al. demonstrated a method for using delaminated Ti3C2 folded nanosheets (MXenes) as a support material for nanoconfinement of MgH2/Mg NPs with a high loading capacity. Microstructural and property tests demonstrated that the Ti-MX support material not only acted as a scaffold to anchor the ultradispersed MgH2/Mg NPs (15 nm), but also had a substantial catalytic effect in increasing the hydrogen storage performances of MgH2 NPs without the use of a doping catalyst [116].
By alkali treating Ti3C2 MXene, an unusual Hamamelis-like structure of K2Ti6O13, with branches 1020 nm in length, is synthesised. It has an excellent catalytic activity for hydrogen desorption from MgH2 [117]. Y. Wang studied the investigated the remarkable enhancing impact of PrF3 on Ti3C2 MXene using a hydrothermal technique, and it demonstrated a superior catalytic effect on hydrogen storage in MgH2 [118].
Sheng et al. [119] attempt to use (Ti0.5V0.5)3C2 to lower the initial temperature of the hydrogen desorption of MgH2 to 210 °C. It was demonstrated that MgH2 reacted with (Ti0.5V0.5)3C2 to form Ti and V metals, which were suggested to act as active catalysts for the hydrogen sorption process.
In order to create a sandwich-like Ti3C2/TiO2, Gao et al. [120] partially oxidised Ti3C2 MXene. At a constant temperature of 250 °C, the MgH2 + 5 wt% of Ti3C2/TiO2 can desorb 5.0 wt% of hydrogen while it absorbs 4.0 wt% at a temperature of 125 °C. The enhancement of MgH2 by Ti3C2/TiO2 was thought to be caused by the layered structures and the various valence Ti-containing compounds.
By exfoliating V2AlC and Ti3AlC2, Liu et al. [121] created V2C and Ti3C2 MXenes. At about 180 °C, MgH2 +10 wt% of 2V2C/Ti3C2 started the hydrogen desorption process, and after 60 min at 225 °C, 5.1 wt% of the hydrogen was desorbed. During the desorption process, hydrogen atoms or molecules may transfer preferentially through the MgH2/V2C/Ti3C/Ti3C2 triple-grain boundaries, and during the absorption process, through the Mg/Ti3C2 interfaces. Specifically, V2C and Ti3C2 simultaneously function as effective catalysts for MgH2. Chen et al. [122] added Ti3C2 to a 4MgH2-LiAlH4 composite; the dehydrogenation onset temperature of the 4MgH2-LiAlH4-Ti3C2 composite was reduced by 64 K and 274 K, respectively, with 4MgH2-LiAlH4 and as-milled MgH2. The Ti produced in situ from the MXene Ti3C2 is responsible for the destabilisation of 4MgH2-LiAlH4. Few-layer Ti3C2 can only exist in solution as a film and is easily agglomerated, reducing the number of active hydrogen absorption and desorption sites. Multilayer Ti3C2 (ML-Ti3C2), on the other hand, can exist as a solid powder, making it easier to form a composite with MgH2. According to the experimental findings, the initial desorption temperature of MgH2-6 wt% ML-Ti3C2 is lowered to 142 °C with a 6.56 wt% capacity. In the MgH2-6 wt% ML-Ti3C2 hydrogen storage system, the Ea of hydrogen desorption is around 99 kJ/mol, which is 35.3% less than that of pure MgH2. Two complementary effects, one in which Ti facilitates the easier dissociation or recombination of hydrogen molecules and the other in which multivalent Ti generates electron transfer that facilitates the simpler conversion of hydrogen, are responsible for the enhancement of kinetics in hydrogen absorption and desorption by ML-Ti3C2 [120].
X. Huang et al. [123] studied the effect of Ti3CN MXene on the hydrogenation properties of MgH2. They observed that Ti3CN MXene lowers the operating temperature and improves the kinetics of MgH2. Desorption enthalpy changes for MgH2 and MgH2 + 7.5 wt%Ti3CN were 79.3 and 78.8 kJ mol−1, respectively. Using Ti3CN does not affect the MgH2′s thermal stability.
El-Eskandarany et. al. ball milled MgH2 with titanium carbide and studied its sorption properties at 275 °C under a hydrogen gas pressure in the range 100 mbar to 8 bar [124]. R.M.A. Khalil et al. performed an ab initio study on the properties of MgH2 and TiC systems [125].
Z. Tian et al. synthesised transition metal carbides (Ti3C2, Ni3C, Mo2C, Cr3C2 and NbC) to improve the hydrogen storage behaviours of magnesium-based materials. Mechanical ball milling was used to incorporate carbides with a weight ratio of 5% into MgH2, and the microstructure, phase composition and hydrogen storage capabilities of the composites were thoroughly investigated. All of these metal carbides can improve the MgH2′s hydrogen absorption and desorption kinetics. Ti3C2 demonstrated the best catalytic effect on MgH2 dehydrogenation kinetic characteristics, followed by Ni3C, NbC, Mo2C and Cr [126].

3.3. Other Catalysts and Additives

In this section, some recent developments on catalysts for MgH2 are reported, which consider different approaches based on the use of perovskites, ternary metal oxides, metal alloys and core-shell particles.
Hydrogen storage performance can be improved when cobalt lanthanum oxide (LaCoO3) is added via the ball milling process. When compared to pure MgH2 and milled MgH2, the results demonstrate that adding 10 weight per cent LaCoO3 significantly reduces the beginning hydrogen release. However, when compared to milled MgH2 under the same conditions, quicker ab/desorption was seen with the addition of 10 weight per cent LaCoO3. Additionally, when compared to milled MgH2, the apparent activation energy for MgH2-10 wt% LaCoO3 was significantly lower [127].
X. Yang et al. [128] proved that adding cubic K2MoO4 to magnesium hydride could change its kinetic characteristics. Its distinctive cubic structure produces novel species during the absorption and desorption of hydrogen, and this produces exceptional catalytic activity during the storage of hydrogen in MgH2. The quantity of K2MoO4 affects the duration of hydrogen dissociation and adsorption. The kinetic performance is faster and the time used is shorter with an increasing amount of K2MoO4. Studies on the catalytic process reveal that K2MoO4 is evenly spread across the MgH2 substrate’s surface. The reaction resulted in the formation of three new compounds, KH, MgO and MgMo2O7, which collectively enhanced the hydrogen storage capabilities of MgH2. They increase the number of catalytically active sites, which increases the speed of the hydrogenation processes and shortens the hydrogen pathways, which lowers the energy barriers for H dissociation and absorption.
M.S. El-Eskandarany et al. [129] observed that the dehydrogenation kinetics of the Zr2Ni-milled samples were promising, as indicated by the short time (700 s) required to release their full storage capacity (6.2 wt%H2). The powders also demonstrated remarkable cyclability, as it was possible to reach 646 cycles in 985.5 h without experiencing significant deterioration.
The exact impacts of the Ni@C material addition on the hydrogen desorption properties of the MgH2-x wt% Ni@C (x = 0, 1, 2, 4 and 6) composites have been discussed by the C. An et al. According to the experimental results, MgH2-Ni@C composites’ dehydrogenation kinetics can be positively improved by the addition of Ni@C components [130].

4. Discussion

The discovery and identification of suitable catalysts is critical for the development of Mg-based hydrogen storage systems. Numerous attempts at catalyst optimisation have resulted in significant improvements in the hydrogen absorption/desorption kinetics of MgH2. The multivalence transition metals as well as their oxides, hydrides, halides, carbides and carbonitrides and intermetallics have been shown to outperform other additions in catalysis.
Transition metal-based additions showed improved hydrogen storage characteristics of MgH2. Ti and its compounds are recognised as a potential group of transition metal-based additives that have been extensively researched in many various aspects, including catalytic effects, catalysis mechanism, nano- and microstructures and synthesis methods. This group of additives has received a lot of attention and effort over the last few decades, and progress has been made. We seek to present overviews of specific fundamental concepts and clear catalytic mechanisms for Mg-based materials, with a special emphasis on Ti-based catalysts.
Furthermore, the hydrogen storage properties of the MgH2-TiF3 composite outperform those of the MgH2-TiH2 composite. As the Ti species is introduced into magnesium, TiF3 aids in the dissociation of the hydrogen molecule at low temperatures. A study of TiF3 and TiCl3 additives revealed that TiF3 had a superior catalytic effect to TiCl3. This result revealed that the F- anion has a particular catalytic role. It was discovered that titanium chloride and fluoride reacted similarly with MgH2 during milling or hydrogen cycling operations, creating TiH2 and MgCl2 or MgF2. As a result, it was believed that the fluoride anion would produce catalytically active species, but the chloride anion would not.
Although a complete knowledge of the role of Ti-based catalysts is still lacking, evidence suggests that catalysts do play key roles in encouraging certain of the processes. The doped catalyst species might lower the hydrogen molecule’s dissociation energy barrier while simultaneously facilitating hydrogen transport in the Mg/MgH2 matrix. Kinetic modelling could become a more valuable tool for interpreting reaction control processes. Future mechanism research should focus on observing catalytic activity and microstructure evolution under carefully regulated reaction circumstances in order to give more accurate comparisons with boundary conditions for the different models used for interpretation.

5. Future Prospects and Challenges

Storage of hydrogen in safe and operable conditions remains a key challenge. Current literature proposes extensive possibilities of storing hydrogen in metal hydrides, however, to achieve suitable hydrogen absorption/desorption kinetics at ambient conditions, further rigorous research would be required. Various catalysts or additives have been studied to enhance the hydrogen absorption/desorption kinetics in solid hydrogen storage materials. Catalysts significantly improve the hydrogen sorption kinetics in metal hydride, providing fast and effective dissociation of hydrogen molecules. The addition of the additive to MgH2 reduced the activation energy for dehydrogenation, therefore, lowering the decomposition temperature and improving the desorption properties of MgH2. However, it is still difficult to assess the effectiveness of different catalysts. The mechanism of catalysis consists of several steps. For example, in the process of hydrogenation, these steps include hydrogen dissociation, surface penetration, diffusion, hydride nucleation and growth. In spite of many efforts by researchers to understand the role of the catalyst in improving the kinetics of solid-state hydrogen storage materials, some aspects remain unclear. Several findings suggested that the use of an additive or of a catalyst can reduce the dissociation energy barrier of the hydrogen molecule, and it can promote the hydrogen diffusion in the Mg/MgH2 matrix. Interpreting the controlling steps of the reactions kinetic modelling could become a crucial tool. Future mechanism studies should look on the development of the microstructure and catalytic activity under tightly controlled reaction conditions. Moreover, the development of effective catalysts for hydrogen storage at the solid state requires further research. As reviewed in this work, among the different types of catalysts, Ti-based additive was revealed to be highly effective in enhancing the hydrogen properties of MgH2. Effect of various types of Ti-based catalysts including hydrides, oxides, halides, carbides, carbonitrides and intermetallics on the dehydrogenation and hydrogenation kinetics have been studied by researchers in the last few decades. These promising results could be at the base of suitable hydrogen storage systems in view of a future hydrogen-based economy.

Author Contributions

M.J., I.P.J. and D.M.G. contributed in the same manner to the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Crystal structure of MgH2 [27].
Figure 1. Crystal structure of MgH2 [27].
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Figure 2. Schematic pressure composition isotherm and Van’t Hoff plot [29].
Figure 2. Schematic pressure composition isotherm and Van’t Hoff plot [29].
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Figure 3. Schematic representation of destabilisation mechanism kinetics (Copyright Elsevier 2016 [37]).
Figure 3. Schematic representation of destabilisation mechanism kinetics (Copyright Elsevier 2016 [37]).
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Figure 4. Effect of different additives on desorption temperature of MgH2 (Copyright Elsevier 2015 [46]).
Figure 4. Effect of different additives on desorption temperature of MgH2 (Copyright Elsevier 2015 [46]).
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Figure 5. Desorption curves of MgH2-5 at%Tm (Tm = Ti, V, Mn, Fe and Ni) composites under a hydrogen pressure of 0.015MPa at: (a) 573 K; (b) 523 K; (c) 508 K. (Copyright Elsevier 1999 [17].)
Figure 5. Desorption curves of MgH2-5 at%Tm (Tm = Ti, V, Mn, Fe and Ni) composites under a hydrogen pressure of 0.015MPa at: (a) 573 K; (b) 523 K; (c) 508 K. (Copyright Elsevier 1999 [17].)
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Table 1. Structural parameters for MgH2 in ambient and high-pressure phases. (Reprinted with permission from ref. [28]. Copyright 2006 American Physical Society.)
Table 1. Structural parameters for MgH2 in ambient and high-pressure phases. (Reprinted with permission from ref. [28]. Copyright 2006 American Physical Society.)
Structure TypeUnit Cell (Å)Positional
Parameters		BO (GPa)Bo
ABC
β-MgH24.51764.51763.0206Mg(2a), 0, 0, 045.00 ± 23.35 ± 0.3
γ-MgH24.66554.66554.6655Mg(4a), 0, 0, 047.41 ± 43.39 ± 0.4
α-MgH24.52485.4424.9285Mg(4c), 0, 0, 044.03 ± 23.17 ± 0.4
δ-MgH28.80694.68384.3699Mg(4c), 0.8823, 0.271, 0.279049.83 ± 53.49 ± 0.6
Table 2. Hydrogen storage properties of Mg/MgH2 with elemental Ti or TiH2 additive.
Table 2. Hydrogen storage properties of Mg/MgH2 with elemental Ti or TiH2 additive.
MaterialsSynthetic MethodTdesHydrogen
Desorption Capacity (wt%)		Activation
Energy (kJ/mol H2)		Reference
7MgH2/TiH2Ball milling126 °C5.579[53]
10MgH2/TiH2Ball milling101 °C571[53]
MgH2-1 at%TiBall milling278 °C4.9208[49]
MgH2-5 at%TiBall milling274 °C4.5156[49]
MgH2-Ni/TiBall milling256 °C2.981[49]
MgH2-Ti5Fe5Ni5Ball milling270 °C5.345.63[50]
MgH2-Ti2Ball milling257 °C6.18103.9[51]
MgH2-0.1TiH2Ultrahigh-energy−high pressure
(UHEHP) ball milling		290 °C6.2058.4[54]
0.7MgH2–0.3TiH2Reactive ball milling573 K less than 100 s- [59]
MgH2/0.1TiH2High pressure ball milling269–301-77.4[60]
Mg-2% TiInert gas condensation320 °C4.50 [61]
MgH2 + 2 at%TiCold rolling (5 times, air)623 K6.00 [62]
MgH2-4 mol%TiBall Milling573 K1.10 [63]
MgH2-5 at%TiBall Milling235.6 °C 70.11[64]
MgH2-5 at%TiBall Milling523 K5.5071.1[64]
MgH2-5 at%Ti 573 K5.20 [64]
Mg-5%TiChemical Vapor Synthesis 104[65]
Mg-14 at%TiGas phase condensation 35[56]
MgH2-15%TiBall Milling573 K0.12 [57]
Mg0.9Ti0.1Ball Milling 76[58]
Mg0.75Ti0.25Ball Milling 88[58]
Mg0.5Ti0.5Ball Milling 91[58]
MgH2-20%TiBall Milling 72 ± 3[66]
MgH2-coated TiBall Milling250 °C5.00 [66]
Mg83.5Ti16.5Ball Milling300 °C2.50 [67]
15Mg-TiChemical Method [68]
MgH2-5 at%TiChemical Method270 °C5.8067.24[63]
4MgH2-TiH2Ball Milling 68[52]
MgH2 + 10 mol%TiH2Ball Milling 16.24[55]
Mg-9.2% TiH1.971-3.7%TiH1.5Ball Milling573 K4.1046.2[69]
Mg0.65Ti0.35D1.2Ball Milling 17[70]
Table 3. Hydrogen storage properties of Mg/MgH2 with elemental Ti or TiO2 additive.
Table 3. Hydrogen storage properties of Mg/MgH2 with elemental Ti or TiO2 additive.
MaterialsSynthetic MethodTdes.Hydrogen
Desorption
Capacity (wt%)		Eact(kJ/mol H2)Reference
Mg-TiO2Arc evaporation300 °C6.3477.2[42]
MgH2-5 mol%TiO2 (rutile)Ball milling300 °C4.40 [78]
MgH2-5 mol%TiO2 (anatase)Ball milling300 °C1.9552.7[78]
MgH2-10% TiO2Ball Milling300 °C6.00 [82]
MgH2-20% TiO2Ball Milling350 °C4.40 [83]
MgH2 +5 wt%TiO2(np) (>50 nm)Ball Milling335 °C [75]
MgH2 + 5 wt%TiO2 (np) (<50 nm)Ball milling310 °C 57[75]
MgH2 + 6% TiO2Ball Milling 145.8[84]
MgH2 +10% TiO2Ball Milling200 °C 75.50[85]
MgH2-x wt%TiO2@CBall Milling195 °C6.2106[79]
MgH2-TiO2@rGO-EABall Milling265 °C4.286.7 ± 8.0[86]
MgH2-40TiO2@rGO-EGBall Milling261 °C5.9 [86]
Table 4. Hydrogen sorption properties for Mg-based compounds with halides.
Table 4. Hydrogen sorption properties for Mg-based compounds with halides.
MaterialsSynthetic MethodTdes.Hydrogen
Desorption
Capacity (wt%)		Eact
(kJ/mol H2)		Reference
MgH2 + 4 mol%TiF3Ball milling173 °C6.14 [51]
MgH2 + 4 mol%TiF3Ball milling300 °C5 [96]
MgH2 + 5 wt%TiF3Ball milling300 °C5.5 [97]
MgH2 + 10 wt%K2TiF6Ball Milling245 °C6.5 [98]
MgH2 + 7 wt%TiCl3Ball Milling225 °C 79[93]
MgH2-10% TiF4Ball Milling216 °C6.671[99]
MgH2-10% TiF4Ball milling154 °C 70[95]
MgH2 + 4 mol%TiF3Ball Milling573 °C4.5 [63]
MgH2 + 4 mol%TiCl3Ball Milling573 °C 3.70[63]
Table 5. Hydrogen sorption properties of Mg-based compounds with Ti intermetallics and alloys.
Table 5. Hydrogen sorption properties of Mg-based compounds with Ti intermetallics and alloys.
MaterialsSynthetic MethodTdes.Hydrogen
Desorption
Capacity (wt%)		Eact
(kJ/mol H2)		Reference
MgH2-5 at%TiAlBall milling270 °C4.9065.08[64]
MgH2-5 at%Ti3AlBall milling232.3 70.61[64]
Mg85Al7.5Ti7.5DC-Magnetron Co-Sputtering200 °C5.30 [109]
Mg0.63Ti0.27Si0.10D1.1Ball Milling 27[70]
MgH2-5 at%TiNiBall Milling242.4 °C 73.09[64]
15Mg-Ti-0.75NiChemical method 63[68]
MgH2-5 at%TiNbBall Milling27 °C5.9071.72[64]
MgH2-5 at%Cr-5 at%TiFilm200 °C6.003.70[110]
MgH2-7 at%Cr-13 at%TiFilm200 °C5.00 [110]
MgH2-5 at%TiFeBall Milling270 °C5.2072.63[64]
MgH2-5 at%TiMn2Ball Milling270 °C4.6074.22[64]
Mg87.5Ti9.6V2.9Hydrogen plasma metal reaction300 °C4.0073.08[69]
MgH2-5 at%TiVMnBall Milling270 °C5.7085.20[64]
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Jangir, M.; Jain, I.P.; Mirabile Gattia, D. Effect of Ti-Based Additives on the Hydrogen Storage Properties of MgH2: A Review. Hydrogen 2023, 4, 523-541. https://doi.org/10.3390/hydrogen4030034

AMA Style

Jangir M, Jain IP, Mirabile Gattia D. Effect of Ti-Based Additives on the Hydrogen Storage Properties of MgH2: A Review. Hydrogen. 2023; 4(3):523-541. https://doi.org/10.3390/hydrogen4030034

Chicago/Turabian Style

Jangir, Mukesh, Indra Prabh Jain, and Daniele Mirabile Gattia. 2023. "Effect of Ti-Based Additives on the Hydrogen Storage Properties of MgH2: A Review" Hydrogen 4, no. 3: 523-541. https://doi.org/10.3390/hydrogen4030034

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