Transition metal-based materials and their catalytic influence on MgH 2 hydrogen storage: A review

. The dependence on fossil fuels for energy has culminated in its gradual depletion and this has generated the need to seek alternative source that will be environmentally friendly and sustainable. Hydrogen stands to be promising in this regard as energy carrier which has been proven to be efficient. Magnesium hydride (MgH 2 ) can be used in storing hydrogen because of its availability, light weight and low cost. In this review, monoatomic, alloy, intermetallic and composite forms of Ti, Ni, V, Mo, Fe, Cr, Co, Zr and Nb as additives on MgH 2 are discussed. Through ball milling, additive reacts with MgH 2 to form compounds including TiH 2 , Mg 2 Ni, Mg 2 NiH 4 , V 2 O, VH 2 , MoSe, Mg 2 FeH 6 , NbH and Nb 2 O 5 which remain stable after certain de/hydrogenation cycles. Some monoatomic transition metals remain unreacted even after de/hydrogenation cycles. These formed compounds, including stable monoatomic transition metals, impart their catalytic effects by creating diffusion channels for hydrogen via weakening Mg - H bond strength. This reduces hydrogen de/sorption temperatures, activation energies and in turn, hastens hydrogen desorption kinetics of MgH 2 . Hydrogen storage output of MgH 2 /transition metal-based materials depend on additive type, ratio of MgH 2 /additive, ball milling time, ball – to combining materials ratio and de/hydrogenation cycle. There is a need for more investigations to be carried out on nanostructured binary and ternary transition metal-based materials as additives to enhance the hydrogen storage performance of MgH 2 . In addition, the already established compounds (listed above) formed after ball milling or dehydrogenation can be processed and directly doped into MgH 2 .


Introduction
Rapid growth in human productive engagements (aimed at improving standards of living) over the years have brought about urbanization.This has led to drastic population growth and placed a huge stress on energy generation and its utilization globally.The world community at large has been plagued with consequences of global warming owing to its large dependence on fossil fuels since the emergence of industrial revolution.These non-renewable resources have largely played a dominant role in energy production, maintenance and transportation.It has been reported by the United States Environmental Protection Agency (US EPA) that carbon dioxide (CO2) maintains the highest constituent (Figure 1a) among the greenhouse gases (US EPA, 2023).This gas is often released during burning of fossil fuels such as oil, coal, natural gas (Stephen, 2005), deforestation (Van der Werf et al. 2009) and other industrial/manufacturing activities (Figure 1b).It causes global warming and other health issues through hypercapnia (Yu et al. 2013;Shigemura et al. 2017;Taghizadeh-hesary et al. 2021).The basic source of energy generation for instance, as reported by Singh et al. (2021) entails the combustion of fossil fuels which yields the release of CO2 before (pre-combustion) or after (post-combustion) to the atmosphere.Globally, economic growth has been seen as a catalyst for greenhouse gas existence and its emission is mostly prevalent in China (Yang et al. 2020).African countries largely depend on fossil fuels for energy storage and use.In a model developed by Olubusoye and Musa, (2020), it was predicted that gradual economic growth rise in African countries will engender more greenhouse gas emissions.In South Africa for instance, Oladunni et al. (2022) reported that economic and population growths are the two major elements that have triggered the emission of greenhouse gas, most especially in the transport sector.
Efforts therefore are being made to produce renewable and sustainable energy that is friendly to the ecosystem.Hydrogen energy stands out as a renewable secondary energy which is clean, sustainable and can be used as an energy carrier and fuel cells.It is not like gas, coal or oil that serves as a primary energy source; hydrogen needs to be produced from another energy source (preferably from renewable sources in this case) such as biomass, wind, solar, hydro and geothermal.As the most abundant element in the universe, hydrogen is affirmed to be the ideal energy source in the 21st century that can be synthesized via water electrolysis with no pollutant released (Hou et al. 2021).

Hydrogen storage
Hydrogen is often stored as compressed gas, cryogenic liquid or in solid state.Liquid hydrogen is used as fuel for low temperature rockets and mobile applications (Al Ghafr et al.

Review Article
| 1142 ISSN: 2252-4940/© 2023.The Author(s).Published by CBIORE 2022; Jiang et al. 2023).It has been reported that cryogenic liquid hydrogen can be stored at -253 o C and 1 bar (Krishna et al. 2012); at this temperature, it maintains a density approximately 71 gL -1 (Edwards, et al. 2007).In the sphere of energy and space missions, the main tools to consider in liquid hydrogen storage are materials for transportation vessels and cryogenic storage.Alloys of titanium (Madina & Azkarate, 2009), steel (Krainz, et al. 2004) and aluminum-lined composites (Aceves et al. 2000;Aceves et al. 2010) have been used for the storage and transportation of cryogenic liquid hydrogen but drawbacks have been witnessed in their impact strength, ductility and toughness at low temperatures.Design and development of highly effective low temperature materials for cryogenic liquid hydrogen storage is being advocated for (Qiu et al. 2021).A methodical insulation needs to be devised in other to maintain hydrogen in its liquid phase because of its low boiling point in this cryogenic state.Storage in the compressed gaseous form is another common method to store hydrogen.Materials such as steel (Zhang et al. 2008) and aluminum-carbon fiber composites (Takeichi et al. 2003) have been used in designing vessels that can store and transport hydrogen compressed at high pressure.Unlike the liquid storage, compressed gas can be stored at room temperature (Zheng et al. 2012).The density of hydrogen stored in the compressed gaseous state is lower compared to that of cryogenic liquid; a lower energy per unit volume has also been affirmed to be witnessed in pressurized gas (Zhang et al. 2016).Although both storage methods allow easy accessibility of hydrogen, there are draw backs that have bedeviled its storage and transportation.Cryogenic liquid hydrogen for instance, can only be stored for a short period to avoid the risk of steady boiling (Prabhukhot et al. 2016;Edalati et al. 2018).The low specific gravity of hydrogen creates an issue for its storage in compressed form as it requires huge magnitude of energy to achieve this, though much energy is needed for the storage of hydrogen in the liquid form compared to when compressed in gaseous state (Krishna, et al. 2012).In the solid state form, hydrogen molecules combine with other materials; a little volume of material can store large 2.9% 6.2% 11.5% 79.4% hydrogen content than the two methods earlier discussed.A solid state hydrogen storage material easily adsorbs/absorbs and desorbs hydrogen at temperatures close or equal to room temperature.Storing hydrogen in the solid-state addresses issues of safety; boil off loss, heavy vessel weights and liquefaction energy associated with pressurized and liquid hydrogen storage.The volumetric and gravimetric densities of hydrogen storage methods are presented in Figure 2.
Hydrogen can physically or chemically combine with a solid material and when needed, it is made to desorb or dissociate from the material on involvement of thermal energy or any other means such as hydrolysis as discussed by Hou et al. (2021).In the physical process (physisorption), molecules of hydrogen adsorb on the material surface while a chemical bond is formed when hydrogen molecules chemically react with the material (also called chemisorption as material absorbs hydrogen molecules in this case).When hydrogen adsorbs on a material, there often exists weak van der Waals interactions between its molecules and the surface of the storage material.This implies that an appropriate material surface area will be required to achieve a remarkable hydrogen storage capacity.It is being reported that low activation energy is required to execute adsorption and desorption of hydrogen molecules in this case and this is responsible for fast ad/desorption kinetics (Prabhukhot et al. 2016).Materials with porous structures including activated carbon, grapheme, carbon nanotubes (CNTs), porous aromatic frame works (PAFs), metal organic frame works (MOFs) and zeolites have been used (see Table 1).
A chemical bond is formed when a material absorbs hydrogen molecules into its structure.Many metals fall into this category as they reversibly react with hydrogen to form hydrides which serve as a hydrogen storage material.The ease of hydride formation and dissociation is different among metal hydrides based on the strength of bond formed between the metal and hydrogen.For a metal hydride to be classified as suitable for hydrogen storage for energy use, it needs to possess High volumetric and gravimetric densities.Fast hydrogen release (during dissociation) at a reduced temperature and adequate pressure is also a key feature required of a metal hydride as this saves time and energy.Aluminum hydride (AlH3) for example, is affirmed to decompose into its constituent elements rapidly at room temperature and 700 bar with enthalpy of formation ranging between -6 to 7.6 kJ/mol (Zidan, 2010).This implies that it has fast dehydrogenation kinetics which qualifies it as a good storage material.Jieng et al. (2021) highlighted the high pressure (700 bar) as an issue when considering hydrogen storage on a large scale.Another class of hydride called complex hydride contains a counter ion with a coordination complex where hydrogen is covalently bonded.Electropositive Li + for instance, reacts with (BH) -to form Li(BH4). Prabhukhot et al. (2016) gave the complex hydride representation as AxMeyHz where elements in the first and second groups in the periodic table fit into "A" while "Me" is often occupied by aluminum or boron.Some other complex Fig. 2. Hydrogen storage methods with their volumetric and gravimetric densities (Edwards et al. 2007)

Table1
Hydrogen adsorption capacities of some activated carbon, metal organic frame works, porous aromatic frame works and zeolites  (Luo, 2004;Chen et al. 2006;Barison, et al. 2008) and Ca(BH4)2 (Muthukumar et al. 2005).These materials are affirmed to be safe to handle and do not decompose easily to their stable constituents as witnessed for some metal hydrides.Complex hydrides also possess high hydrogen storage capacity.In summary, choice of hydride for solid hydrogen storage is dependent on many factors such as availability, ease of processing, cyclic stability, hydrogen storage capacity, low cost of production, dehydrogenation and hydrogenation kinetics.Magnesium hydride (MgH2) has been used as a storage material for hydrogen owing to its availability, light weight, huge storage capacity (7.6 wt.%), low cost of processing, good reversibility and high volumetric capacity (109 g H2/L) (Shahi et al. 2015;hongtan et al. 2018).Low sorption kinetics has been recorded to be an issue with this material; Reaction of Mg with hydrogen to form MgH2 (absorption) is exothermic and occurs between 250-370 0 C (Figure 3); heat therefore needs to be supplied to the system for hydrogen desorption to occur.
To improve the hydrogen storage properties of the hydride, composites comprising combination of MgH2 with one or more of carbonaceous materials (Ródena et al. 2008), intermetallics (Lu et al. 2018) oxides and hydrides of metals (including complex hydrides) have been used as catalysts.

Alloys compounds and composites of transition metals as catalysts
Numerous transition metals in their single forms, compounds and alloys have been used as catalysts to improve the hydrogen storage properties of MgH2.Researches have also entailed doping MgH2 with other non-metallic materials such as grapheme, activated carbon, carbon nanotubes and metal organic frameworks to form composites.In this section, researches on the use of transition metals in the single or monometallic forms, alloys (binary, tertiary and multicomponent) and composites with non-metallic materials as catalysts are reviewed.

Titanium (Ti)
Having established that dehydrogenation of MgH2 will occur between 300 -350 o C (Jin et al. 2007a;Croston et al. 2010;Li et al. 2013), it is expected that MgH2-based composites should release hydrogen at a lower temperature and the activation energy required for this should reduce.Additives used impart their catalytic effects by remaining stable (unreacted) or form a new stable phase (owing to a chemical reaction) when exposed to de/hydrogenation temperatures.This boosts the hydrogen storage potency of MgH2.Lu et al. (2009) employed an ultra-energy-high pressure reactive milling on MgH2 and TiH2 powders for 4 h at room temperature.Nanostructured combination of MgH2 and TiH2 were maintained after milling as no additional phase was formed.Composites remained stable after 80 cycles of dehydrogenation and hydrogenation at 300 o C. Existence of TiH2 phase uniformly distributed in MgH2/TiH2 nanocomposite was reported to cause reduction in hydrogenation and dehydrogenation enthalpies.This was an additional claim to justify that asides grain size reduction, existence of well distributed stable additives will improve hydrogen storage performance of MgH2.Twenty hours continuous milling however, culminated in the transformation of Mg and Ti to MgH2 and TiH2 when Shao et al. (2011) doped 0.1 wt.% Ti into MgH2.Dehydrogenation of MgH2/TiH2 as determined by TGA/DTG curves was faster than as-milled MgH2; the former occurred at 342 o C which was 100 o C lower than the latter (as-milled MgH2).Unlike findings of Shao et al, (2011), Dehouch et al. (2003) earlier reported that Mg, MgO, Ti, V and Fe2O3 phases were formed after MgH2 was ball milled with Ti and V.Although the milling time and temperature were not stated, it could be deduced here that the milling process engendered the dissociation of Mg from H atoms which later got oxidized to MgO on reacting with oxygen in the air.Milling condition was also sufficient to yield the formation of Fe2O3, which was notified as an impurity that arose from the oxidation of the steel mill balls; Ti and V remained unreacted.On cycling (de/hydrogenation cycles) at 300 o C in the presence of hydrogen-containing moisture after milling, there was an increase in hydrogen storage capacity and absorption kinetics between 500 and 1000 cycles for the composite; after the 1000th cycle, an additional phase Mg (OH)2, was formed.The moisture acting as a contaminant in the hydrogen was affirmed to have engendered structural relaxation in the composite which led to its improved absorption kinetics.On the other hand, the structural modification retarded the hydrogen desorption kinetics after cycling.The investigators attributed this low kinetics to modifications that occurred on the surface of the composites.Although information regarding the mechanism involved in this surface modification was not detailed, one may suppose that Mg (OH)2 and Fe2O3 may have a role to play in this.To further elucidate on the Findings of Lu et al. (2009), Hao & Scholl, (2012) deduced the mechanism responsible for the reduction in MgH2/TiH2 composite's enthalpy of reaction compared to single MgH2 by adopting the first principles density functional calculations.This was used to distinguish the interfaces that could exist between MgH2 or Mg and TiH2.Calculations explained that strong equiaxial surfaces could exist between the low surface energy faces of MgH2 (or Mg) and TiH2.These interfaces induced strain on MgH2 and Mg which was identified to be responsible for the low dehydrogenation enthalpy.Addition of 0.05, 0.1, 0.25 and 0.5 wt.% Ti, to MgH2 and milling for 2 h have produced Mg, MgH2 and Ti phases  (Pukazhselvan et al. 2020).Magnesium hydride, Ti and TiH2-x (nonstoichiometric hydride) were identified when MgH2 /0.25 wt.% Ti composite was milled for 15 h.When composites were dehydrogenated at 315 o C and 5 bars H2 pressure after 1, 5 and 10 cycles, Ti/TiH2-x was further hydrogenated to TiH2.Titanium additive remained unreacted under 2 h (mild milling condition as defined by the researchers) milling; it was partially hydrogenated when the milling was severe (15 h) and got completed after de/hydrogenation cycles.For the strong milled composite, XPS (X-ray Photoelectron Spectroscopy) indicated that Ti existed in mixed valence states (Ti 0 , Ti 2+ , Ti 3+ , Ti 4+ ).It was believed that Ti 4+ (oxidation state of Ti in TiO2) present in both samples (mild and strong miling) may have occurred as a result of surface oxidation on exposure to air.It was thus concluded that catalytic influence of Ti/TiH2 would only be effective if MgH2 would be milled long enough.Formation of TiH2 during strong milling reduced the dehydrogenation activation energy of MgH2 to 89.4 kJ/mol from 153 kJ/mol displayed by as-received MgH2.This further justified the findings of Hao & Scholl, (2012), who suggested that when there is an equiaxial contact between Mg/MgH2 and TiH2, the latter would induce strain on Mg/MgH2 contact surface which would end up lowering its dehydrogenation enthalpy.It has also been reported by Patelli et al. (2017) and Bhatnagar et al. (2018) where TiH2 was confirmed to improve the sorption kinetics of MgH2 by lowering the activation energy.This was also attributed to the imposition of lattice strain on MgH2 by TiH2.Malahayati et al. (2021) observed that 1 h ball milling may not be sufficient enough to induce reaction between MgH2 and Ti powders as no change of phase was noticed after the process.Agglomerations of combining powders with increased diameter only existed during this time.After 12 h of milling, both MgH2 and Ti still existed but this time, in nanosized form, which was attributed to the effect of energy induced by the milling process.The Ti phase disappeared after milling for 18 h while broad peaks of MgH2 remained.It was not mentioned in the report what happened to Ti but it could be assumed that Ti was oxidized to TiH2 which may have also led to the broadening of MgH2 peaks that was initially narrow.The composite (MgH2/Ti) absorbed hydrogen at 300 o C and 10 bar while desorption happened at 350 o C and 50 mbar.Both processes took place within 7 min and from previous findings of researchers, it can be deduced that TiH2 may be responsible for the much fast kinetics compared to additive -free MgH2.These temperatures however were outlined by the authors to be high for application purpose.Berezovets et al. (2022) observed that TiH4 phase was formed using Ti nanopowder additive which led to the increase in hydrogen storage capacity of MgH2 (6.7 wt.% H2) (Figure 4).Milling Mg and TiO2 nanopowders after 5 h, TiO2 remained unreacted and this led to a low storage capacity of 5.7 wt.% H2; it may be assumed here that the milling time was insufficient for TiO2 to get reduced.As illustrated in Figure 4, both additives facilitated hydrogenation compared to pure Mg which implied that they aided the crystallization of MgH2.More improved hydrogenation was realized on milling Mg powder with a suboxide of Ti4Fe2Ox (x= 0.3, 0.5) in the presence of hydrogen pressure.The suboxide additives promoted hydrogen dissociation and the Ti4Fe2OxHy phase formed after milling was responsible for its highest hydrogen storage capacity (6.76 wt.% H2).Presence of Ti, Fe and O in the suboxide created diffusion pathways for hydrogen to or from Mg/MgH2 system during de/hydrogenation, Titania (TiO2) has also been proven to be a good additive for MgH2.Titania.MgH2 and rock salt (Ti dissolved MgO) were reportedly formed when MgH2 was milled with 10 wt.% TiO2 for 5 h (Pukazhselvan et al. 2017a).A reduced phase, Ti2O3 was yielded when TiO2 was milled with 10 wt.% MgH2 for 30 h.A single phase rock salt was formed when MgH2 was milled with TiO2 in ratio 2:1.The Ti/Mg/O phase in the rock salt was confirmed to make the additive impact of TiO2 effective on hydrogen storage properties of MgH2.The single phase rock salt formed after milling 2MgH2 +TiO2 system for 30 h had the least dehydrogenation activation energy (110.9kJ/mol).Further works of Pukazhselvan et al. (2017b) established that TiO2 transformed as an inbuilt rock salt catalyst during dehydrogenation and its content depended on the variation of Mg/Ti.The proportion of Mg/Ti according to the researchers was assumed to cause e passivation of active rock salt.Shao et al. (2022) prepared three-dimensionally ordered macroporous (3DOM) TiO2 via colloidal crystal template technique.After 10 h ball milling with MgH2, the composite absorbed 4.17 wt.% H2 at 100 o C within 1800 s and released 5.75 wt.% H2 at 300 • C within 1000 s.Improvement in the hydrogen storage properties of MgH2 arose from the combined effect of 3DOM structure and electronic interactions as TiO2 was reduced by MgH2 and multiple valence Ti (Ti 0 , Ti 1+ and Ti 2+ ) were formed.These destabilized MgH2 and weakened Mg -H bonds.In addition, TiO2 nanoparticles were wrapped and uniformly distributed in carbon layer; this aided de/absorption of hydrogen in MgH2.Titanium and TiO2, each of 0.4 g and 0.2 g of nitrogen-doped graphene (XFNANO) was ball milled for 1 h to obtain TiO@N-C (Hong et al. 2023).The additive (0.1 g TiO@N-C) was mixed with 0.9 g of MgH2 and milled for 5 h.The composite completely desorbed hydrogen at 350 °C within 4 min and its dehydrogenated form absorbed 5.1 wt.% H2 in 4 min at 175 °C.There was a reversible reaction of Ti and TiH2 on Mg/MgH2 surface during de/hydrogenation which made hydrogen molecules dissociate and diffuse easily; the stability of MgH2 was also reduced by weakening of Mg -H bonds triggered by TiO2.Nitrogen-doped graphene was covered on MgH2 surface which impeded the agglomeration of MgH2 particles.In addition, carbon structural defects that existed in nitrogendoped grapheme acted as nucleation sites which promoted diffusion of hydrogen.These were responsible for the hydrogen storage performance of MgH2.Multi-phase interface comprising Ti, TiO2, Ti2O3 and MgH2 has been established to provide more diffusion paths for hydrogen and more nucleation sites for Mg/MgH2.This finding substantiates the investigations of Liu et al. (2021) who doped graphene-supported TiO2 nanoparticles (TiO2@rGO) into MgH2, Asides the fact that Mg was surrounded by the catalyst; partial reduction of Ti 4+ to Ti 2+ existed.This propelled charge transfer that advanced the de/hydrogenation kinetics of MgH2.In addition, Ren et al. (2022) recorded that multi-phase interfaces that comprised multi-valence Ti (Ti 2+ , Ti 3+ ) and MgH2 existed when flower-like MgH2/TiO2 heterostructure synthesized from 2D TiO2 nanosheets with oxygen vacancies.The multi-phases aided electron and hydrogen diffusion and created more nucleation sites for MgH2/Mg.Magnesium rod, Ni sheet and Ti pellets were melted in a vacuum induction levitation furnace to process MgNi-Ti and MgTiNi ingots (Li et al. (2018).The alloy ingots were crushed and mechanically ground to 200-mesh powder.When milled, Ni2Ti and NiTi phases were observed in both alloys.New phases-Mg2NiH4 and TiH2 were formed after 100 cycles of hydrogenation and dehydrogenation.These phases were responsible for 5.22 and 3.23 wt.% H2 recorded for MgNi-Ti and MgTiNi alloys, respectively.Amorphous TiMgVNi3, produced via Induction melting of Ti, Ni and V powders, has been used as a catalyst on MgH2 (Hu et al. 2022).After 100 h of milling, of cast TiMgVNi3, 10 wt.% was further milled with MgH2 for 10 h under 5 MPa H2 pressure.Magnesium hydride and (Ti,Mg,V,Ni)Hx were formed after milling.When MgH2/ TiMgVNi3composite underwent 2 cycles of hydrogenation and dehydrogenation, a homogenous distribution of (Ti,V)H2 and Mg2NiH4 nanoparticles formed on the surface of MgH2; these were responsible for its fast hydrogen desorption Magnetic levitation melting has been used in preparing TiV based BCC alloy (Ti0.4Cr0.15Mn0.15V0.3)which was mechanically pulverized into particles as an additive for MgH2 (Yu et al. 2010).Some alloy powders were water quenched while others were hydrogenated at 20 bars H2 for 2 h at room temperature.Alpha -MgH2, γ-MgH2 and HBCC were formed after milling.The BCC contributed to the improvement of atomic diffusivity of hydrogen as well as its ease of dissociation and recombination.Hydrogenated BCC appeared to impart the most effective followed by quenched BCC.Solid BCC (ingot) offered the least effect.El-Eskandarany et al. (2019) milled MgH2 with 10 wt.% TiMn2 master alloy powders for 50 h under 70 bar H2 pressure.The composite formed was further consolidated (compaction) into circular buttons of .1.2and 8e2.0 cm diameter and thickness respectively.The consolidation enabled the TiMn2 nanopowders got embedded in the micro/nanopores of MgH2 grains which acted as a good hydrogen diffusion path during hydrogenation and dehydrogenation.The buttons could absorb and desorb 5.8 wt.% H2 at 225 o C within short periods of 150 s and 500 s, respectively Titanium carbide (TiC) nanoparticles was formed on the grain boundaries of MgH2 when both underwent cryo-milling (using N2 to enact freezing) for 8 h followed by high energy ball milling (at room temperature ) for 16 h.(Tan & Shang, 2012).The formation of the additive on the grain boundary shortened the diffusion length and weakened the Mg-H bond which lowered the desorption temperature and activation energy to 190 °C and 235 to 104kJ/mol respectively.Sandwich-like Ti3C2/TiO2(A)-C was processed via facile gas-solid approach and doped into MgH2 by 10 h ball milling (Gao et al. 2020).X-Ray diffraction results showed that MgH2, with few contents of Mg and MgO were formed after milling.From their investigation, incomplete hydrogenation or dehydrogenation of MgH2 was suggested to have culminated in the formation of Mg while the reaction between MgH2 and TiO2resulted in the formation of MgO.The composite could absorb 5 wt.%H2 at 250 o C within 1700 s (42.32 kJ/ moll) and within 800 s, 4 wt.%H2 was desorbed at 125 o C (77.69 kJ/ mol).Multiple valence Ti compounds of Ti 4+ , Ti 3+ , Ti 2+ and Ti 0 as observed by XPS and synergetic effects between the layered structure were reported to be the mechanism of the catalytic influence of Ti3C2/TiO2(A)-C.A modified wet chemical method used in fabricating sandwich-like Ni/Ti3C2 catalysts, was introduced to MgH2 to improve its hydrogen storage performance (Gao et al. 2021).A strong electronic interaction existed between nanoparticles of Ni and Ti3C2.This was affirmed to be responsible for the improved hydrogen absorption feature of MgH2.Catalytic effect of NiTi3C2 was influenced by the electron transfer in the multiple valences of Ti (Ti 4+ , Ti 3+ , Ti 2+ and Ti 0 ).Works of Gao et al. (2021) justify earlier works (Cui et al. 2013) where it was claimed that Ti3C2 derived its catalytic influence from electron transfer in multi-valence Ti which triggered the transformation of Mg +2 and Mg, H -and H.

3.2. Nickel (Ni)
A common Ni-based intermetallic compound -Mg2Ni has been synthesized and reported over the years to be a good hydrogen storage material (Zaluski et al. 1995a;Zaluski et al. 1995b).Recently, Baroutaji et al. (2022) summarized that another Mg-Ni based compound -Mg2NH4 can be realized (together with Mg) when Mg2Ni directly interacts with MgH2.It can boost MgH2 hydrogen storage feature when both materials interact (Lu et al. 2022).These two Mg-Ni based compounds have played important roles on MgH2 hydrogen storage features and they can also be formed on reacting pure Ni or Ni-based compounds with MgH2.Hanada et al. (2005) concluded that observing de/hydrogenation cycle of 2% mol Ni nanoparticle -doped MgH2 at 200 o C, hydrogen desorption properties of the composite degraded as a result of Mg2Ni which formed at that temperature.Here, the Ni content and/or the composite processing method (15 h ball milling under1 MPa H2 pressure) may have been responsible for the limiting impact of Mg2Ni.Using Ni in uncombined form could have also been responsible.It was reported by Liang et al. (2000) that milling Mg and 30 wt.% LaNi5 intermetallic mechanically for as long as 40 h would not yield chemical reaction unless when hydrogenated.At this point, the intermetallic decomposed and a composite comprising MgH2, LaH3 and Mg2NiH4 phases was formed.When MgH2 and 30 wt.% LaNi5 were ball milled prior to hydrogenation, part of MgH2 and LaNi5 decomposed to form Mg2NiH4 while some part was reduced by La to form stable LaH3 and Mg.After hydrogenation, MgH2/ LaH3/ Mg2NiH4 system similar to that observed on milling Mg powder with LaNi5 was formed.Milling LaNi5 with MgH2 was preferable to Mg because the former facilitated ease of powder size reduction, which enhanced absorption properties of MgH2 and not desorption.Magnesium absorption kinetics was improved by the presence of LaH3; beyond 373 K, Mg2Ni imparted a better catalytic effect.Hydrogen storage kinetics has been improved by doping MgH2 with 5 wt.%SiC (Ranjbar et al. 2009a) but addition of 10 wt.% Ni further enhanced this property because it improved composite's surface area and reduced the concentrations of defects (Ranjbar et al. (2009b).In addition, hydrogen desorption reaction was influenced by bulk nucleation and 3D growth of the existing Mg nuclei; finely dispersed Ni nanoparticles increased the amount of nucleation sites.Mao et al. (2010) ball milled MgH2 with 10 wt.% NiCl2 for 2 h.Magnesium hydride phase was observed after milling without a trace of Ni; this could be attributed to the little milling time employed or content of additive used.After hydrogenation and dehydrogenation cycles, more phases were formed which followed the suggested reaction: In their work, MgH2 was ball milled with Ni nanoparticles which were uniformly dispersed and anchored on reduced graphene oxide (Ni@rGO) for 2, 5, 10 and 20 h under 1MPa H2 pressure.
A high surface area of 161.4 m 2 /g possessed by Ni@C nanorods mixed with MgH2 was investigated to absorb 6.4 wt.% H2 within 10 min and 300 o C (An et al. 2014).Here, it was suggested that Ni@C composites had the capacity to create interface with MgH2 to form catalytic site for hydrogen diffusion.When the combination of SrTiO3 and Ni were used as additive for MgH2 (Yanya & Ismail, 2018), Mg2Ni and Mg2NiH4 were formed after dehydrogenation and hydrogenation respectively.The two phases were concluded to be active in improving the hydrogen storage properties of MgH2.The phase SrTiO3 remained unreacted throughout the process but its catalytic influence was imparted in the modification of the composite's microstructure.This created an additional advantage ahead of using only SrTiO3.Nano Ni particles were dispersed in nanoporous carbon material (CMk-3) prepared by impregnation reduction and 10 wt.% of the combination was added to MgH2 (Chen et al. 2018).Under 3 MPa H2 pressure and 150 0 C, the composite MgH2/Ni/CMk-3 absorbed 3.1 wt.% H2 after 360 s while 5.7 wt.% H2 was absorbed with 2400 s.At 328 K, and 3 MPa, H2 pressure, 3.9 wt.% H2 was absorbed.Nickel nanoparticles played an active role in lowering the decomposition enthalpy of MgH2 by forming Mg2Ni and Mg6Ni.Combined effect of activation and destabilization from Ni was responsible for the enhanced performance of MgH2.Ma et al. (2018) employed carbonization process to synthesize carbon supported nano-Ni (Ni@C) additive for MgH2.Inclusion of 5 wt.% of the additive promoted MgH2 the hydrogen storage display.After 10 cycles, average grain size of MgH2 grew to 35.5 nm and this was reported to be responsible for its reduced storage capacity at that instant.Furthermore, Mg2NiH4 also appeared after 10 cycles and was suggested to have a negative impact on the composite's hydrogen performance.Increase in milling time up to 10 h has led to gradual reduction in crystallite and grain sizes of MgH2/ nano Ni anchored on reduced graphene oxide (Ni@rGO) composite (Yao et al. (2020).On the other hand, prolong milling up to 20 h led to the welding and agglomeration of particles which made them bigger.Catalytic effect of the additive was influenced by the formation of Mg2Ni/ Mg2NiH4 phase which was formed after rehydrogenation.Milling for 5 h offered the best result as the composite could absorb 5 wt.%H2 in 20 min at 100 o C and within 15 min, 6.1 wt.% H2 was released at 300 o C. It was easier for Mg2NiH4 to release H2 with ease.The rGO created hydrogen diffusion channels and active catalytic site, which was responsible for the lowering of dehydrogenation temperature and kinetics.Solid solution of Ni-Cu powders has created a platform for enhanced MgH2 nucleation and de/hydrogenation by reducing the bond strength of Mg-H (Zhang et al. 2020).At 300 0 C, the composite could eject 5.14 wt.% H2 after 15 min while within 30 min, 4.37 wt.% H2 was absorbed at 250 0 C Magnesium hydride hydrogen storage display was elevated by the formation of Mg2Ni(Cu), which allowed hydrogen molecules to dissociate and recombine to MgH2.Ball milling of Mg powder and MgNi2 alloy followed by hydrogen combustion synthesis technique was devised by Fu et al. (2020)  enhanced the hydrogen storage properties of MgH2 as they were finely dispersed in its matrix.

Vanadium (V)
Mechanically ball milling MgH2 with V for 20 h was observed to yield βMgH2, MgH2 and VH0.81 after hydrogenation (Liang et al. 1999).Hydrogen was completely desorbed after 2000 s at 800 K with MgH2/5 wt.% V.The nanocomposite absorbed 2 wt.%H2 within 1000 s at 10 MPa H2 pressure and 302 K; at 373 K, 4 wt.%H2 was absorbed after 100 s and at 473 K, 6.5 wt.% H2 was absorbed in 250 s.The microstructure of composite with V inclusion improved the hydrogenation kinetics.Vanadium (5 wt.%) was added to MgH2 powders and mechanically ballmilled for 100 h to nano scale (Rivoirard et al. 2003).At 253 K MgH2 absorbed hydrogen slowly while that activated with V was faster.Fine grains of MgH2 formed were also responsible for its enhanced hydrogen absorption kinetics.At 603 K the absorption kinetics was reduced because it was noticed that at that temperature,  VHx which was formed after ball milling became unstable.They concluded that nature of combining materials would not only contribute to hydrogen absorption properties of MgH2; particle size reduction, distribution and agglomeration would also play key roles.After mechanically milling Mg and V powders for 20 h, MgV0.05 was formed and this improved the hydrogen storage property of Mg (Schimmel, et al. (2005).At the onset of hydrogenation, MgH1<x<2 phase was formed and there was much hydrogen vacancies which enabled the phase have higher diffusion coefficient.This was responsible for the improved hydrogenation kinetics of the nanocomposite.Conceição et al. (2014) compared the effects of pure V, vanadium chloride (VCl3) and vanadium carbide (VC) catalysts on the hydrogen storage properties of MgH2.Adding 5 wt.% separately of each additive to MgH2, VCl3 showed the best catalytic effect in terms of hydrogen storage capacity and de/hydrogenation kinetics.It was reported that considering the same content for all additives (wt.%), the amount of V in VCl3 was the least compared to that in VC and pure V. Vanadium carbide could enhance desorption of MgH2 but its high stability contributed to its retarded desorption kinetics.According to Kadri et al. 2(015), catalyzed V synthesized from vanadium hydride, VH2 could act as a hydrogen splitting agent which could hasten dissociation of hydrogen from MgH2.Catalytic influence of bismuth vanadate (BiVO4) on MgH2 hydrogen storage properties via ball milling has been investigated (Xu et al. 2017).At 150 o C, and 3 MPa H2 pressure, the composite composed of MgH2/16.7 wt.% BiVO4 had the capacity to absorb 1.99 wt.% H2 while additive-free MgH2 had 0.94 wt.% H2 absorbed under the same conditions of temperature and pressure (Figure 5a).At 400 o C, 1.1 wt.% H2 was desorbed within 1200 s (Figure 5b).Catalytic influence of BiVO4 was attributed to the formation of V-containing compounds (Mg2V2O7 and V2O3) that were formed during dehydrogenation at 400 o C.
Vanadium oxide supported on cubic carbon nanoboxes (nano-V2O3@C) has been ball milled with MgH2 (Wang et al. 2018).Within 10 min, MgH2/-9 wt.% V2O3@C composite could release 6.0 wt.% H2 while additive -free MgH2 desorbed 0.4 wt.% H2 within this time.The metallic V formed from V2O3 during milling and at the initial stage of heating was responsible for the fast dehydrogenation kinetics of MgH2; it elongated Mg -H bond length and weakened its strength.Vanadium chloride was reduced to metallic V when milled with MgH2 Kumar et al. (2018).The metallic V imparted a good catalytic effect on MgH2 hydrogenation and dehydrogenation kinetics.This was achieved by MgH2 grain refinement and crystallite size reduction that eventually created the diffusion path for hydrogen.In the investigations of Liu et al. (2021), 7 wt.% of V nanoparticles was added to MgH2.Within 10 min, 6.5 wt.% H2 was released at 300 0 C (MgH2 could not achieve this at this time).Fully dehydrogenated composite had the potency of absorbing hydrogen at room temperature and 5.6 wt.% H2 at 150 0 C. Vanadium remained stable all through hydrogenation and dehydrogenation processes.Well dispersed Ni and vanadium trioxide nanoparticles in nanoporous carbon ((Ni-V2O3)@C) has been used as catalyst on MgH2 (Lan et al. 2022).There was a partial transformation of V2O3 to VO during milling while MgH2, V2O3, VO, and V remained unreacted during hydrogenation and dehydrogenation.In contrast, Ni reacted with Mg to form Mg2Ni and this further reacted with hydrogen to form Mg2NiH4.The Mg2Ni/ Mg2NiH4 particles acted as hydrogen pump as it was observed to be coated on Mg/MgH2; this aided diffusion and dissociation of hydrogen The presence of carbon (C) enhanced the catalytic effect, promoted MgH2/Mg lattice expansion and held up their crumbling during hydrogen de/absorption, which ended improving MgH2 cyclic stability.It was concluded that m a multicomponent catalyst comprising V, VO, V2O3, C, and Mg2Ni/Mg2NiH4 will improve the hydrogen  performance of MgH2.Two dimensional canadium carbide (V2C) MXene has been added to MgH2 to improve its hydrogen desorption kinetics (Lu et al. 2022).Improved cyclic stability was not only caused by the additive; low hydrogen desorption temperature (from 318 0 C in MgH2 to 198 0 C in MgH2/V2C composite) was also enhanced.The V2C played a role of reducing Mg H bond length to hasten desorption kinetics of MgH2.Tian et al. (2023) ball milled hydrothermally synthesized V-based catalysts (V2O5, FeVO4and NiV2O6) with MgH2. to improve its hydrogen storage properties.Dehydrogenation behaviour of MgH2/ FeVO4 displayed the best performance followed by MgH2/V2O5.During this process, the Fe-V complex oxide reduced elemental Fe and V which eventually lowered Mg-H bond strength.This hastened the absorption and desorption of MgH2.It was concluded that Fe would improve the catalytic effect of V2O5 while Ni will not.Structured V-based MOFs (MOFs-V) synthesized by facile hydrothermal reaction and calcination has been doped with MgH2 via ball milling to modify its hydrogen storage properties (Lu et al. 2023).It was observed by Scanning Electron Microscope (SEM) that the MOFs composed of bullet-like V2O3.The desorption temperature of composite containing 5 wt.%MOFs was 199.8 o C which was 142 o C lower than catalyst-free MgH2; increasing the content of MOFs to 7 and 9 wt.% further lowered the desorption temperatures to 190 and 186 o C respectively.During ball milling, MOFs-V was reduced to metallic V which created a catalytic effect.Catalytic effect of vanadium disulphide (VS2) on MgH2 was investigated by Verma et al. (2023).The composite began to release hydrogen at 289 o C which was 87 o C lower than MgH2.Hydrogen desorption activation energy barrier required to reduce MgH2 to Mg in the presence of VS2 was lower (98.09kJ/mol) compared to42 kJ/mol in catalyst-free MgH2.Du et al. 1(997) had earlier reported that V in VS2 could be reversibly converted from V 4+ to V 5+ during de/hydrogenation.The existence of V in its variable oxidation form would weaken Mg-H bond and thus trigger fast de/hydrogenation kinetics.

Molybdenum (Mo)
Jia et al. (2013) reported that molybdenum disulphide (MoS2) had more effect in improving the absorption and desorption kinetics of MgH2 than molybdenum oxide (MoO2).During ball milling, the following reactions were investigated to have taken place as indicated from XRD patterns: Considering similar ball milling conditions, reaction (3) was observed to be faster than reaction (4).As-milled MgH2 could absorb 90% of its hydrogen storage capacity within 72 min; MgH2/MoO2 could attain this within 31 min while it took 13 min for MgH2/MoS2 to achieve this.Formation of MgS/ Mo and MgO/Mo in each reaction was suggested to have been responsible for the absorption/desorption kinetics of MgH2.Addition of 10 wt.% MoS2 to Mg particles according to Han et al. (2016) would be enough to prevent agglomeration and cold welding of particles as it acted as a dispersant and lubricant.The milling process reduced crystallite size of MgH2 to a little below 38.8 nm and this was sustained all through milling because MoS2 confined its growth.The reduction in crystallite size was responsible for the reduced dehydrogenation temperature of MgH2.During MgH2 decomposition, crystal of Mg was reported to grow by three dimensions controlled by interface transformation.The researchers concluded that MoS2 had a weak catalytic influence on the decomposition of MgH2.Rather than use bulk MoS2, Setijadi et al. (2016) synthesized MgH2 nanoparticles using delaminated MoS2 through a simple hydrogenolysis route which involved the decomposition of din-butylmagnesium.The delaminated additive led to the formation of Mg worm-like structures that collapsed and recrystallized during hydrogen cycling.Thermodynamic features of Mg/MgH2 reaction was strongly influenced by the additive through destabilization of the Mg-H bond.Han et al. (2017) observed that after ball milling Mg/C (combination of magnesium and crystalline carbon) with Mo for 3 h under 1 MPa H2 pressure, MgH2 was formed.Molybdenum and crystalline carbon offered a synergistic effect on improving the hydrogenation kinetics of MgH2 in the reactive ball milling process.Enhanced dehydrogenation rate of MgH2 was attributed to the conductive capacity of Mo The use of 2% mol.MoO3 was researched to have a positive impact on hydrogen storage performance of MgH2 (Dan et al. 2019).During hydrogenation and dehydrogenation, MoO3 was an active site for hydrogen absorption and desorption; the oxide was observed to also create a fast diffusion pathway for hydrogen atoms.Formation of MoO2 occurred during hydrogenation (reduction of MnO3 to MnO2), which was affirmed to reduce the catalytic effect of MoO3 on the long run.Synthesized nanosheets of NiMoO4 were ball milled with MgH2 (Chen et al. 2020).After activation, MoNi and Mg2Ni nanoparticles were formed, which created reaction surfaces and hydrogen diffusion channels.Synergic effects of MoNi on MgH2 increased hydrogenation and dehydrogenation kinetics than the use of mono atomic Mo and Ni on MgH2.The MoNi possessed high hydrogen absorption capacity which was able to dissociate hydrogen from MgH2 by breaking Mg-H bonds.
Magnesium hydride has been separately milled with 10 wt.% MoSe2@FeNi3 hollow nanospheres, FeNi3, and MoSe2 particles (Gao et al. 2020).All additives showed improved catalytic influence on the hydrogenation and dehydrogenation reactions of MgH2 but MoSe2@FeNi3 offered the best performance.The combination of FeNi3, and MoSe2 was responsible to its excellent catalytic performance.Dehydrogenation of 10 wt.% MoSe2@FeNi3doped MgH2 composite commenced from 194 o C; it could absorb 5.8 wt.% H2 within 0.5 min at 150 o C. The combined additive propitiated the formation of active MgSe, Fe, Mg2Ni and Mo species that were uniformly distributed on the surface of MgH2.They were reported to have engendered the de/hydrogenation stability of MgH2.Furthermore, Mg2Ni turned MgH2 to an effective pathway for hydrogen absorption and desorption.Molybdenum flakes have been used in improving the hydrogen storage capacity of MgH2 (Cheng et al. 2023).Adding 7 wt.%Mo flakes to MgH2 powder, hydrogen desorption commenced at 250 0 C, which was 100 0 C lower than ordinary MgH2 (350 0 C).The composites released 6.5 wt.% hydrogen for 20 min at 325 0 C. At room temperature, the composite began to absorb hydrogen and 6 wt.% was absorbed at 250 0 C within 10 min.Lamellar surfaces possessed by the flakes provided more diffusion pathways and contact surfaces which hastened diffusion of hydrogen at Mg/MgH2 interfaces.Molybdenum remained stable during de/hydrogenation cycles and this made it impart an active catalytic effect on MgH2.

Iron (Fe)
Iron is regarded as an inexpensive and the most abundant transition metal on earth (Du et al. 2015).Several studies have proven that Fe can upgrade the hydrogen storage properties of MgH2 in its pure form, as an alloy/compound and in composite with other materials.Yavari et al. (2005) ball milled MgH2 with 3 wt.%iron (III) fluoride (FeF3) nanoparticles.There was a partial transfer of fluorine (F) which formed protective intergranular MgF2 with fine dispersed Fe nanoparticles in Mg or MgH2 according to equation ( 5).The catalytic effect could have come from the Fe formed.
Optimum microstructure that showed a uniform distribution of micron and submicron-sized Fe particles in the MgH2 matrix was achieved at high milling energy (BPR of 10:1 and 20:1).Also, 10 wt.% Fe was the optimum catalyst concentration because contents lower than this led to the formation of poorly catalyzed regions; concentrations beyond this value yielded no improvement either.Ten weight percent of Fe and Iron (III) oxide (Fe2O3) were separately ball milled with MgH2 for 3 h under 0.3 MPa H2 pressure (Baum et al. 2007).Although mechanism of the improved ad/desorption properties of MgH2 was not fully established, Fe acted as an active site for hydrogen sorption (Fe2O3 was also reduced to Fe).Iron (III) oxide displayed a better catalytic effect.It could be assumed here that a more reduced crystallite size offered by Fe2O3 may be responsible for this.Santos et al. (2014) discovered that using elemental Fe nanoparticles would give a better catalytic effect on MggH2 hydrogen sorption performance than nanoparticles of FeNb (ferroniobium) alloy.During milling, it was observed that nanointerfaces comprising Mg (MgH2)/Fe and Mg (MgH2)/FeNb alloy were formed which acted as diffusion paths for hydrogen into the bulk particle.These interface according to the researchers, possessed high chemical interfacial energies.Formation of NbO2 and Nb2O5 when milled with the alloy may have been responsible for its lower sorption kinetics.The catalytic effect of as-synthesized graphene sheet templated Fe3O4 nanoparticles (Fe3O4@GS) on MgH2 was examined by (Bhatnagar et al. 2016).The structure of as milled composite (MgH2/ Fe3O4@GS) contained MgH2, Fe, Fe2O3, MgO and Mg1-xFexO.
Graphene sheet prevented the agglomeration of Fe nanoparticles (formed by reduction of Fe3O4), increased the surface area, durability and cycle stability (after 25 cycles of de/hydrogenation) of MgH2.It was investigated that the layer of MgO was punctured by MgxFexO which created hydrogen diffusion pathway through its layer.In addition, electron transfer between Mg + and H -during de/hydrogenation was engendered by the multiple valence of Fe.Surfactant-assisted solvothermal method was used in preparing FeS2 micro-spheress which was ball milled with MgH2 (Zhang et al. 2018).Adding 16.7 wt.% of the additive, the following reactions were confirmed to have taken place after milling: 4. After hydrogenation at 350 o C, MgH2 and Mg2FeH6 appeared as the major phases while their decomposition yielded metallic Mg and Fe after dehydrogenation at the same temperature.The composite could release 1.24 wt.% H2 at 350 o C within 1400 s while pure MgH2 could release 0.18 wt.% H2 under similar condition.The composite absorbed 3.71 wt.% H2 within 1400 s at 250 o C, compared with 1.03 wt.% H2 of the as-milled pure MgH2.Mg2FeH6 and MgS created diffusion pathway for H2 diffusion.Sazelee et al. (2018) confirmed that MgO, Fe and Ba were formed after milling MgH2 with 10 wt.% BaFe12O19 and these imparted synergic effects on hydrogen storage properties of MgH2.Onset decomposition temperature for composite was 270 o C while as-milled MgH2 was 340 o C. At 150 o C, its absorption capacity was 4.5 wt.% H2 after 10 min while for the additive-free MgH2, it was 3.5 wt.% H2.The composite could release 4.2 wt.% H2 in 30 min while additive-free MgH2 could do that 3.4 wt.% H2within the same time.Iron based MOFs has been synthesized and introduced to MgH2 by ball milling.(Ma et al. 2019).The improved hydrogen storage of the composite was ascribed to the formation of nano α-Fe particles which was uniformly distributed on de/hydrogenated Mg/MgH2 surface.Ball milling was employed in creating homogenous dispersion of the individual catalyst: Fe, Fe2O3 and Fe3O4 in MgH2 powders (Gattia et al. 2019).Activation energy for decomposition as calculated by Kissinger plots showed that 10 h ball milled MgH2/5 wt.% Fe possessed the least magnitude of 220.69 kJ/mol.Vales recorded for MgH2/5 wt.% Fe2O3 and MgH2/5 wt.% Fe3O4 were 231.90 and 304.45 kJ/mol respectively.Low activation energies maintained using Fe and Fe2O3 compared to that of uncatalyzed MgH2 (241.46 kJ/mol) indicated that these two materials gave good catalytic effects on desorption kinetics of MgH2.Gao et al. (2019) doped MgH2 with 10 wt.% iron boride (FeB) by dry milling and wet milling (with cyclohexane) at room temperature.Both milling techniques improved the hydrogenation and dehydrogenation performance of MgH2 compared to additive-free MgH2 as the in situ formed Fe and B served as active species during the process.Wet milling yielded smaller particles than dry milling and this was responsible for its better performance.Iron nanosheets have been synthesized to act as catalystt on MgH2 (Zhang et al. 2019).Adding 5 wt.%Fe nanosheets made the activation energy for the dehydrogenation reaction to be 40.7 kJ/mol, which was 85.2 kJ/mol lower than the catalyst-free MgH2.Within 10 min, 6 wt.% H2 was absorbed by the composite at 200 o C while 2.3 wt.% H2 was taken up after 45 min at the same temperature.It was noticed that during the first hydrogenation and dehydrogenation processes, the Fe nanosheets became ultrafine nanoparticles on MgH2; this created more active sites in the cycles that followed.At the onset of adding Fe nanosheets, Mg-H bond was broken.Catalytic influence of nanostructured Fe7S8 (pyrhotite) on hydrogen sorption properties of MgH2 was studies by Cheng et al. (2021).Doping the parent hydride with 16.7 wt.% Fe7S8 and ball milling, 4 wt.%H2 was absorbed at 200 o C within 1800 s; undoped MgH2 had the capacity to absorb 1.847 wt.% H2 at the same temperature and time.Also, within 1800 s and 350 0 C, 4.403 and 2.479 wt.% H2 were released by MgH2/Fe7S8 composite and MgH2 respectively.The Fe7S8 catalyzed MgH2 composite began to desorb hydrogen at a much lower temperature (147 o C) compared to as milled MgH2 (437 o C), Improvements on the hydrogen storage performance of MgH2 by the catalyst was credited to the formations of MgS and Fe from the reacting materials that occurred during ball milling.Synthesized Fe nanoparticles were ball milled with MgH2 to tailor its hydrogen storage performance (Song et al. (2022).Magnesium hydride remained dominant after ball milling and hydrogenation while Mg phase was formed in the dehydrogenated phase.The existence of stable Fe in the three stages was responsible for the enhanced absorption and desorption of Mg/MgH2 system as it acted as an active catalytic site during these processes.The composite retained 93.4% of its hydrogen capacity after the 20th cycle.At this point, grain growth in MgH2 and Fe catalyst occurred which was responsible for capacity loss and kinetics reduction.Soni et al. (2023) reported effect of Fe nanoparticles and hollow carbon spheres composite on the hydrogen storage properties of MgH2.During hydrogenation and dehydration cycles, the valence state of Fe was converted from +3 to +2 and this was responsible for the improved hydrogen storage properties of MgH2.

Cobalt (Co)
The modification of MgH2 hydrogen sorption potency has been achieved by doping with combined oxides of Co and Ni (Cabo et al. 2011).Addition of 5 wt.% mesoporous NiO increased the desorption rate 7 times greater than MgH2 with reduced sorption activation energy.Addition of nanoporous Co3O4 showed a minimal improvement while nanoporous NiCo2O4 imparted property that was in between MgH2/NiO and MgH2/ Co3O4.The role of CoFe2O4 nanoparticles on the dehydrogenation of MgH2 was demonstrated by Shan et al. (2014).Ball milled MgH2/ 7 mol% CoFe2O4 composite began to desorb hydrogen at 160 o C and this was 200 o C less than the onset desorption temperature of additive-free MgH2.It was observed that during dehydrogenation, CoFe2O4 reacted with MgH2 to form a ternary combination of Co3Fe7, MgO and Co; these were affirmed to catalyze the decomposition of MgH2.Hierarchical Co@C nanoflowers have been reported to create more hydrogen diffusion channels and active catalytic sites that aided the reduction of hydrogen desorption temperature of MgH2 (Li et al. 2017).The hierarchical Co@C nanoflowers were synthesized by employing a simple route that was based on a low temperature solid-phase reaction; it was milled with MgH2 for 5 h under 1 MPa H2.Onset desorption temperature of the composite (201 o C) was 99 o C lower when compared with asmilled MgH2.Within 30 min, the composite released 5.74 wt.% H2 within 30 min and at 300 o C , 6.08 wt.% H2 was released in 60 min.On the other hand, as-milled MgH2 could release 0.37 wt.% H2 within 30 min; at 300 o C, 1.20 wt.% H2 was released in 60 min.Gao et al. (2020) used 10 wt.% CoFeB/CNTs as an additive to improve de/hydrogenation behaviours of MgH2.This was actualized by in situ formed stable Co3MgC, Fe, CoFe and B which created active nucleation sites for de/hydrogenation reactions.In addition to the formation of these phases, uniform distributions of Co, B, Fe and C in the composite contributed to its good cyclic stability.Hydrolysis of MgH2 in the presence of 2.5 -10 wt.% CoCl2 to produce hydrogen was executed by Filiz (2021).It was concluded that the best performance in terms of kinetics of hydrogen generation was displayed using CoCl2 solution with a concentration of 6.55 wt.%.Core-shell CoNi@C bimetallic nanoparticles (MOFs) were introduced to MhH2 to improve its hydrogen storage properties (Zhao et al. 2021).During dehydrogenation, Mg2Co and Mg2Ni were formed as the composite desorbed 5.83 wt.% H2 at 275 o C within 1800 s.During hydrogenation, Mg2Co and Mg2Ni were transformed to Mg2CoH5 and Mg2NiH4.The composite could absorb 4.83 wt.% H2 within 1800 s at 100 o C. Hydrogen dissociation and recombination were hastened as a result of the reversible phase transitions of Mg2Co/ Mg2CoH5 and Mg2Ni/Mg2NiH4.Heat conduction during the thermal cycles was facilitated by the good thermal conductive feature of carbon and this hindered agglomeration of nanoparticles.Carbon also provided a confinement effect which also aided the stability of MgH2 during de/hydrogenation cycles.Ali et al. (2022) doped MgH2 with 10 wt.% CoTiO3.Hydrogen was desorbed at 270 o C, which was lower than that of MgH2 that occurred at 340 o C. Within the first 10 min, 6.4 wt.% H2 was adsorbed.Activation energy of MgH2 was measured to be 135 kJ/mol while on adding CoTiO3, it reduced to 104.6 kJ/mol.In situ formation of MgTiO3, CoMg2, CoTi2, and MgO formed during heating elevated the hydrogen storage performance of MgH2.Clusters of Mg2NiH4/Mg2CoH5 interfaces were reportedly formed after mechanically milling MOF-derived bimetallic Co@NiO catalyst with MgH2 for 6 h (Zhang et al. 2022a).The interfaces were formed on the surface of MgH2 and they were confirmed to create low energy barrier hydrogen diffusion channels which culminated in rapid release and uptake of hydrogen.Zhang et al. (2022b) doped Co particles into MgH2 via 2 h ball milling.Cobalt particles were uniformly distributed on the surface of MgH2 and this created active sites and paths for hydrogen diffusion.De/hydrogenation kinetics of MgH2 was hastened as a result of Mg2Co/Mg2CoH5 phase change that existed during hydrogenation and dehydrogenation.When 10 wt.% CoMoO4/rGO nanosheets was milled with MgH2 for 4 h (Zhang et al. (2022c), these three components -Mo, Co7Mo6 and MgO were formed.They had a synergic catalytic effect on improving the hydrogen storage capacity of MgH2.The composite began to release hydrogen at 204 o C, while as-milled MgH2 commenced desorption at 330 o C. The combined catalytic effect of the generated components was also responsible for accelerated hydrogen diffusion.

Zirconium (Zr)
Hydrogen storage features of MgH2 separately catalyzed with 7 wt.%ZrF4 and NbF5 after series of cyclic loading were researched by Malka et al. (2011).At 325 o C, MgH2/ZrF4 maintained 5 wt.%H2 after 30 cycles while MgH2/NbF5 could hold 4.5 wt.% H2; thus implied that the former composite possessed a better hydrogen sorption stability at this temperature.Existence of stable ZrF4 nanoparticles in the structure of MgH2 was found responsible for a better hydrogen storage capacity.Reduction in the stability of MgH2/ZrF4 however was attributed to the gradual grain size increase by virtue of extended number of cycling.At this point, it was reported that more stable MgH2/Mg phases were formed and contributed to formation of large grain sizes.Shahi et al. (2015) introduced ZrFe2 and its hydride (ZrFe2Hx) to investigate hydrogenation features of MgH2 by producing MgH2/ZrFe2 and MgH2/ ZrFe2Hx nanocomposites after milling.Intermetallic ZrFe2 was converted to fine powders of ZrFe2Hx via hydrogenation; it was noted to be a more useful catalyst as it was more homogenously distributed after milling.Desorption temperatures for as received MgH2, MgH2/ZrFe2 and MgH2 /ZrFe2Hx were 410, 368 and 290 o C respectively.At 280 o C and 2MPa H2 pressure, ball-milled MgH2 could absorb 4.5 wt.% H2 after 1 h while MgH2/ ZrFe2 and MgH2/ ZrFe2Hx nanocomposites could absorb 4.6 and 5.2 wt.% H2 respectively under the same condition.;it was further confirmed that MgH2/ ZrFe2Hx absorbed 4.7 wt.% H2 within the first 20 min.The catalyst hydride was also found to be responsible for a reduced activation energy (61.4 kJ/mol) because it facilitated easy dissociation of hydrogen molecules to atoms and transferred it to it to Mg/ZFe2Hx.Kumar et al. (2027) reported that when ZrCl4 was used as a catalyst in improving on hydrogen sorption kinetics of MgH2 via milling, metallic Zr or ZnCl3 was formed.Either phase according to the researchers modified the surface of MgH2 which was responsible for its hydrogenation and dehydrogenation kinetics.Activation energies of 40 and 92 kJ/mol were recorded for MgH2/ZnCl4 hydrogenation and dehydrogenation respectively while 70 kJ/mol (hydrogenation) and 150kJ/mol (dehydrogenation) were 0bserved for catalystfree MgH2.Refined grains possessed by catalyzed MgH2 (via the use of ZnCl4) was also noted to be responsible its improved hydrogenation and dehydrogenation kinetics.The refinement of grains occurred during ball milling and dehydrogenation.The product ZHx/MgO nanoparticles were confirmed to be formed after ball milling of MgH2 and ZrO2 for 20 h (Pukazhselvan et al. 2022).This phase was known to be responsible for the improvement of MgH2 sorption kinetics and stability.

Niobium (Nb)
A metastable NbH was formed by the hydrogenation and dehydrogenation of Mg from a 20 h ball milled MgH2/5 wt.% Nb composite (Huot et al. 2003).The formation of NbH decreased the energy barrier for MgH2 dehydrogenation.Effects of cyclic heating on milled MgH2/2mol% Nb2O5 was carried out by Friedrichs et al. (2006).After hydrogen had desorbed from MgH2, Nb2O5 was reduced to Nb; dissociated Mg reacted with oxygen from the additive (Nb2O5) to form MgO and MgNb2O3.67.These three components prevented MgH2 grain growth during heating and thus improved its hydrogen sorption kinetics.Jin et al. (2007b) ball milled MgH2 with 1 mol.% of niobium (V) fluoride (NbF5) for 15 min.The additive was reported to have melted during the milling period and a liquidsolid reaction between the two combining materials led to the formation of fin-like NbH layers along nanocrystalline MgH2 grain boundaries.The grain growth of nanocrystalline MgH2 was subdued by the NbH layers and this sustained the additive's catalytic effect up to 10 de/hydrogenation cycles.Role of F in MgF2 product (during dehydrogenation reaction) or Mg-H-F solid solution in the de/hydrogenation kinetics of MgH2, according to the researchers, needed further investigations.Niobium (V) fluoride was confirmed by Luo et al. (2008) to have a profound influence on the de/hydrogenation kinetics and storage capacity of MgH2.The composite which was processed by milling MgH2 together with 2 mol % NbF5 for 5 h could absorb 56 wt.% H2 in in 60 min and within the same period, it could desorb 5 wt.%H2.Results from XRD and XPS proved that Nb species with varying valence states between 0 and +5 was responsible for the improved kinetic performance of the composite.Further investigations on the active species and influence of F -in the de/hydrogenation reactions were recommended.Porcu et al. (2008) employed transmission electron microscopy (TEM) -based techniques to identify the structure and reaction between MgH2 and Nb2O5.The TEM analysis showed that during milling, Nb2O5 broke into fragments and were embedded within MgH2.The smallest fragments stuck to the MgH2 grains and got embedded within the grain boundaries.This happened after the grains were welded into larger particles.Heating the two compounds yielded reduction of Nb2O5 to Nb2O and MgO was formed.Inter diffusion of MgO and Nb2O5 yielded formation of the mixed oxide-MgNb2O3.67.Bi-metallic Nb-V film catalyst added to MgH2 was recorded to enhance its cycling stability, even beyond 500 de/hydrogenation cycles without causing sorption kinetics distortions (Tan et al. 2012).To achieve this, investigators claimed that the surface catalyst distribution and its stability determined the cyclic stability of MgH2.Ball milling was employed in doping nanosized amorphous NbHx nanoparticles into MgH2 (Zhang et al. (2015).The composite absorbed 4 wt.%H2 at 100 o C while at a higher temperature (300 o C), it absorbed 3.3 wt.% H2.It was reported that the nanosized amorphous NbHx acted as charge transfer between Mg 2+ and H − , which played a major role in the improved hydrogen storage performance of MgH2.Hydrogen storage of MgH2/Nb2O5 composite was investigated by Pukazhselvan et al. (2016).Two different milling media-zirconia and steel, were used and the system was processed by mechanochemically reacting the combining compounds for 30 h.The product was characterized with Nb -dissolved MgO, which provided a catalytic effect on the hydrogen storage properties of MgH2.Influence of high energy ball milling (HEBM) with isothermal catalytic synergic behaviour of 10 wt.% Nb2O5 and TiF3 on dehydrogenation of MgH2 was investigated by Zhang et al. (2016).Dehydrogenation temperature of MgH2/TiF3 was 341 o C, which was 76 o C lower than as-received MgH2; addition of Nb2O5 to MgH2 resulted in hydrogen being released at a reduced temperature of 336 o C. A combination of these two catalysts to MgH2 culminated in 310 o C desorption temperature.The non-isothermal synergetic catalytic effect of TiF3 and Nb2O5 was attributed to electronic exchange reactions with hydrogen molecules, which improved the recombination of hydrogen atoms during dehydrogenation.A two-dimensional Nb4C3Tx (Mxene) synthesized via chemical exfoliation has been introduced to MgH2 (Liu et al. 2019).Adding 5 wt.% of the additive to the matrix demonstrated good adsorption kinetics.Multilayer structure with OH-terminations and F terminations were generated on the surface of the additive.NbHx was formed after ball milling and de/hydrogenation cycles.The NbHx was found to be evenly dispersed in the matrix.Onset temperature for ball milled MgH2 was approximately 297 o C while that of the composite was recorded as 151 0 C. Within 800 0 C, the composite releases H2 completely within 800 s.Yahya et al. (2018) ball milled 1-20 wt.% K2NbF7 with MgH2 powders.The composite comprising MgH2/5 wt.% K2NbF7 was the most effective as it lowered the dehydrogenation temperature of MgH2 to 225 o C.This composite absorbed 5.1 wt.% H2 after 43 s at 320 o C while asmilled could absorb this quantity within 76 s at the same temperature.At lower temperature, the composite absorbed 4.7 wt.% H2 after 30 min while as-milled MgH2 could absorb 0.7 wt.% at the same time.The active species that led to the improvement of MgH2 hydrogen storage properties were KH and Nb that were formed during the process.Hollow spherical o-Nb2O5 made of 50 nm wall thickness and mossy surfaces was synthesized and ball milled with MgH2 for 24 h to improve its hydrogen storage properties (Zhang et al. 2020).The composite (MgH2/7 wt.% o-Nb2O5) desorbed 6.4 wt.% H2 at 195 o C; at room temperature, the dehydrogenated composite began to reabsorb hydrogen, and 5.7 wt.% H2 was achieved at 150 o C. It was reported that high valence Nb +5 state of Nb2O5 was gradually lowered to Nb +4 and Nb +2 and finally Nb 0 during | 1153 ISSN: 2252-4940/© 2023.The Author(s).Published by CBIORE milling followed by first dehydrogenation cycle.It was concluded that the in situ formed low-valence Nb acted as de facto catalytic species which lowered the kinetic barriers of MgH2 hydrogen sorption.This culminated in its decreased dehydrogenation/hydrogenation temperatures Nanoparticles of V4Nb18O55 have been synthesized to aid the catalytic effect of Nb2O5 on MgH2 hydrogen storage properties (Meng et al. 2022).Desorption temperature of MgH2 was reduced to 225 o C; synergistic effect between both V 5+ and Nb 5+ improved hydrogen desorption properties of MgH2.When MgH2 was doped with 5 wt.%Nb nanocatalyst (prepared via surfactant assisted ball milling technique), stable NbH was formed; this acted as active catalytic unit diminished the energy obstacle and boost MgH2 kinetics (Nyahuma et al. 2022).The composite began to release hydrogen at 186.7 o C, while additive-free s MgH2 stated hydrogen release at 347 o C. Niobium (V) oxide nanoparticles grafted on MOF (Nb2O5@MOF) has been synthesized and doped into MgH2 (Zhang et al. (2023), A slight loss in composite's hydrogen storage capacity was attributed to the formations of MgO and NbO emanating from the reactions between MgH2 and Nb2O5.However, there was a synergetic effect between Nb2O5 and MOF which enhanced hydrogen drift between Mg/MgH2 boundaries; it also prevented Mg/MgH2 from agglomeration and growth.Ultrafine and steadily dispersed NbC (niobium carbide) nanoparticles enclosed by carbon substrate (NbC/C) was produced via carbon thermal shock method and ball milled with MgH2 (Jia et al. 2023).Particles of MgH2 were refined by NbC while the substrate (carbon) destabilized Mg-H bond.In addition, multi valence electron transfer existed between positively charged Nb ions and NbHx (formed in situ).In addition, the electron transfer also occurred between Mg and H atoms which influenced Mg/MgH2 reversible transformation.These were the outcomes of the catalytic influence of NbC/C.

Summary and Conclusion
Researches involving the use of Ti, Ni, V, Mo, Fe, Cr, Co, Zr and Nb in their monoatomic forms, alloys (with transition or other metals), intermetallics and composites to better MgH2 hydrogen storage features have been reviewed.These transition metalbased additives are often doped in MgH2 via ball milling in the presence or absence of hydrogen.Depending on the milling time and other conditions considered, reaction often occurs during milling which culminates in the formation of new phase(s) (see Figure 6); in situations where combining materials remain unchanged after milling, transformation occurs during de/hydrogenation which engenders the existence of new phase(s).Asides causing uniform distribution of additives throughout MgH2 matrix, mechanical milling has also been proven to create suitable surface area that acts as catalyst sites.In addition to the prevalence of some monoatomic transition metal after hydrogenation-dehydrogenation cycles, phases such as TiH2, Mg2Ni, Mg2NiH4, V2O, VH2, MoSe, Mg2FeH6, NbH and Nb2O5 have imparted catalytic effects through creation of diffusion channels for hydrogen by weakening Mg -H bond strength.This reduces hydrogen de/sorption temperatures, remove energy barrier for de/hydrogenation (which results to activation energy reduction) and in turn, hastens MgH2 hydrogen absorption and desorption kinetics.Grain growth of MgH2 can also be prevented by the catalysts during heating to improve its hydrogen storage capacity.Transition metals such as Ti, Fe and Nb can exist in multiple valence states and they often aid charge transfer between Mg + and H -to positively influence hydrogen sorption kinetics.It is also observed that the hydrogen storage operation of MgH2/transition metal -based materials will depend on the kind of additive used (including formulations), MgH2/additive mixing ratio, ball milling time, ball-to-combining materials ratio and de/hydrogenation cycle (including temperature and holding time involved).There is need for more investigations to be carried out on nanostructured binary and ternary transition metal -based materials (alloys, intermetallics and or their combinations) as additives to amplify the hydrogen storage properties of MgH2.In addition, the already established compounds (TiH2, Mg2Ni, Mg2NiH4, V2O, VH2, MoSe, Mg2FeH6, NbH, and Nb2O5) formed after ball milling or de/hydrogenation can be processed and directly doped into MgH2.
Fig 1.(a) Percentage constituents of greenhouse gas (US EPA, 2023) (b) Major activities that result to CO2 emission.

Fig 3 .
Fig 3. Schematic illustration of absorption and desorption of MgH2

Fig 4 .
Fig 4. Hydrogen absorption capacities of Mg/Ti-based composites as a function of ball milling time(Berezovets et al. 2022)

Fig 6 .
Fig 6.Simple possible reactions involving MgH2 with transition metals/transition metal-based compounds, M= Transition metal; X = one of H, O, C, S or elements in group VII on the periodic table, N.B.Equation could be complex if M is a transition metal -based alloy or if additives are more than one.
Magnesium chloride and Mg2Ni acted as catalyst on MgH2.It was also noted that the additive removed nucleation barrier which enabled hydrogen desorption occur at a low driving force.Within 60 s, 5, 17 wt.%H2 was absorbed at 300 0 C for MgH2/NiCl2 composite while it took 400 s to absorb 3.51 wt.% H2 at the same temperature for undopped ball milled MgH2.
1) | 1147 ISSN: 2252-4940/© 2023.The Author(s).Published by CBIORE Li et al. (2021) 2021)i2 composite.Majority of the Mg were transformed to MgH2 after hydrogenation while some reacted with MgNi2 to form Mg2NiH4 that later transformed to Mg2Ni during dehydrogenation.These two phases enhanced hydrogen adsorption and desorption of MgH2 as 2.5 wt.% H2 was absorbed at 200 o C while at 300 o C, 2.6 wt.% H2 was released.Magnesium hydride was investigated to be capable of absorbing 5.3 wt.% H2 at 300 0 C within 5 min when doped with 2mol% nano Ni powders of approximately 90 nm via high pressure ball milling under 10 MPa H2 pressure(Rahwanto et al. 2021).The nano Ni powders provided adequate reaction surface for MgH2 during milling.Li et al. (2021)doped Ni/ Ni/tubular g-C3N4 (TCN) into Mg by milling for 5 h and under 4 MPa H2 pressure, the milled sample was kept for 40 h to form MgH2/Ni/TCN composite.During milling, the additive coated the Mg surface and Ni particles reacted with Mg to form a phase that comprised Mg, Ni and H.The C atom, being a good conductor of heat and electron, was considered good for the behavior of the composite when hydrogenated and dehydrogenated at 400 o C; it also prevented the growth of particles.Reversible conversion from Mg to MgH2 and reaction ISSN: 2252-4940/© 2023.The Author(s).Published by CBIORE Polanski et al. (2011)cles at the same temperature; the coarsening of the composites microstructure was responsible for the slow desorption rate.Coarsening of the microstructure was maintained up to 350 o C. Cycling effects on MgH2 /10 wt.% Cr2O3 nanopowder composites was studied byPolanski et al. (2011).At 325 o C, the ball milled nanocomposite was put through 150 de/absorption cycles.Progressive reduction of nanocomposite's hydrogen storage capacity was witnessed to determine the thermal stability of MgH2/0.2 mol % Cr2O3 naocomposite.At 350 0 C, the composite possessed the best absorption kinetics after 17 cycles and it witnessed the least kinetics after 1000 cycles at 300 o C. Absorption rate of 47 kW/kg was maintained at 300 o C after 1000 cycles while desorption rate reduced to 4.5 kW/kg.During the cycling process, the crystallite size which was initially 21 nm | 1150 ISSN: 2252-4940/© 2023.The Author(s).Published by CBIORE grew