N.A.Ali,M.Ismail

Energy Storage Research Group,Faculty of Ocean Engineering Technology and Informatics,Universiti Malaysia Terengganu,21030 Kuala Nerus,Terengganu,Malaysia

Abstract A solid-state storage system is the most practical option for hydrogen because it is more convenient and safer.Metal hydrides,especially MgH2,are the most promising materials that offer high gravimetric capacity and good reversibility.However,the practical application of MgH2 is restricted by slow sorption kinetics and high stability of thermodynamic properties.Hydrogen storage performance of MgH2 was enhanced by introducing the Mg-Na-Al system that destabilises MgH2 with NaAlH4.The Mg-Na-Al system has superior performance compared to that of unary MgH2 and NaAlH4.To boost the performance of the Mg-Na-Al system,the ball milling method and the addition of a catalyst were introduced.The Mg-Na-Al system resulted in a low onset decomposition temperature,superior cyclability and enhanced kinetics performances.The Al12Mg17 and NaMgH3 that formed in situ during the dehydrogenation process modify the reaction pathway of the Mg-Na-Al system and alter the thermodynamic properties.In this paper,the overview of the recent progress in hydrogen storage of the Mg-Na-Al system is detailed.The remaining challenges and future development of Mg-Na-Al system are also discussed.This paper is the firs review report on hydrogen storage properties of the Mg-Na-Al system.

Keywords: Mg-Na-Al system;MgH2;NaAlH4;Al12Mg17;hydrogen storage.

1.Introduction

1.1.Hydrogen energy and hydrogen storage

The massive development of the world’s population and economies combined with the increasing urbanisation culminated in a colossal rise in energy demand.The United States Energy Information and Administration estimated an increment of 28% of global energy demand between 2015 and 2040 [1].Global energy demand is anticipated to reach climax in 2035,whereas the global economy is predicted to experience a long depression after 2040 [2].Classic energy supply trends based on fossil fuel resources have triggered a massive rise in rates of CO2and other greenhouse gases(GHSs) in our atmosphere that contribute to global warming.Moreover,the world’s fossil fuel production has a dwindling stock of energy and is estimated to peak soon and then begin to decline [3-5].Therefore,the decarbonisation of the energy supply by utilising alternatives to clean and renewable energy is essential for sustainable development in the future.Technology based on renewable energy is favourable to the environment compared to that based on fossil fuels.At present,the rapid development in wind,solar,hydrogen,geothermal and nuclear energy has now offered a myriad of opportunities for exploiting renewable energy sources [6-11].Amongst the alternatives above,hydrogen energy has been recognised as a possible key factor in tackling the energy transition because it provides sizeable storage and fl xibility capabilities [12].

Fig.1.Present state of hydrogen storage technologies concerning DOE’s target [1].

For the past decades,the use of hydrogen has been in the limelight owing to its abundance.Hydrogen can be stored in large quantities over a long period because it holds high energy content (142MJ/kg),three times higher than that in gasoline (46MJ/kg) [13,14].Hydrogen has high energy density and was proven to be most useful in the 1970s for NASA spacecraft and rocket propulsion [15].Moreover,it can be gained from different sources (renewable energy),such as wind,sun and sub-surface of the earth[16-18]through several processes,such as thermochemical cycle and water electrolysis [19,20].For example,108.7kg of hydrogen can be generated by electrolysis of 1 m3of water,and the energy of this amount is equal to 422L gasoline [21].Other than that,hydrogen can be used by combustion (to get mechanical energy and heat) or by electrochemical conversion (to get electricity and heat).Aside from this,hydrogen is a zero-emission or emission-free fuel that generates minimal or negligible greenhouse gases because it only releases water vapour as its byproduct [22].For these reasons,hydrogen energy is seen as the most potent way of securing renewable energy and contributing to the energy transition.Hence,it can be utilised in transportation (hydrogen fuel cell in vehicles),industries and electricity generation [23-27].Despite the promising feature of the hydrogen,several technical and scientifi challenges should be tackled to ensure safety,cost-effectiveness and an efficien hydrogen economy.The main issue that needs to be resolved is storage of hydrogen for the hydrogen energy application.The quality of the storage medium and its toxicity features must be carefully scrutinised to achieve the goals set.In the making of a hydrogen-powered vehicle,the hydrogen storage system needs to meet several requirements that accentuate compatibility,safety and cost-effectiveness.The U.S.Department of Energy (DOE) has highlighted series of guidelines for the on-board hydrogen storage;by the year 2025,a system of the gravimetric capacity of 5.5wt.%and volumetric capacity of 0.040kg H2/L should be developed [28].Suitable and sustainable material should be utilised for the hydrogen economy.Fig.1 displays the current state of hydrogen storage technologies and the forthcoming goals for on-board applications.In Fig.1,the gravimetric and volumetric capacity for the current hydrogen storage method that is mainly based on(i) high-pressure gaseous storage,(ii) cryogenic liquid storage and (iii) solid-state storage was between 1.5 and 6.0wt.%and between 20 and 40g/L,respectively.Even though high-pressure gas and liquid hydrogen storage is the most advanced technique and seem close to the DOE’s target,its safety and energy efficien y issues present obstacles for its application as a storage medium.Storing hydrogen in highpressure gaseous storage operates at very high pressure that is 300-700bar at room temperature.Other than that,there are energy losses of up to 12%-16% during the pressurisation [29].For the cryogenic liquid storage,it operates at very low temperature and pressure (-253°C,5-10bar),and the addition of refrigeration units is necessary to preserve the cryogenic state [30].To produce liquid hydrogen,at least 25% (30%-40% is more realistic) of the energy is lost [31].As safety concerns arise on these two storage methods,alternative methods are considered for the storage of hydrogen.Storage of hydrogen in the solid state has practical advantages from the safety viewpoint and storage capacity [32].Other than that,solid-state hydrogen storage operates at moderate temperature and pressure and offers high energy efficien y[33,34].Therefore,mainstream research has focused on developing a solid-state hydrogen storage system for on-board applications.

1.2.An overview of the Mg-Na-Al system

Promising candidates for solid-state hydrogen storage materials include metal hydrides (e.g.MgH2,TiH2and AlH3)and complex hydrides (e.g.NaAlH4,LiAlH4,NaBH4and LiBH4) [30,35-38].As MgH2offers favourable properties such as high hydrogen capacity (up to 7.6wt.%),MgH2has received special attention amongst the light metal hydrides and has thus been devoted as a promising material for onboard applications [39,40].MgH2holds the highest energy density (9MJ/kg) amongst all the reversible hydrides that are applicable to hydrogen storage [41].Besides,MgH2has the benefi of low cost with good reversibility [42,43].The decomposition of MgH2is shown as follows:

However,these favourable properties of MgH2for practical applications are restricted by its temperature for hydrogen decomposition that is still high (>400°C) and slow sorption kinetics [44-48].Slow sorption kinetics is due to the sluggish hydrogen diffusion rate on the bulk MgH2and poor decomposition of hydrogen on the surface of Mg [49].Meanwhile,an aluminium-based complex hydride,NaAlH4,has been viewed as a favourable host for storing hydrogen due to its mild operating temperature and pressure [50,51].It has been known as the only complex hydride that offers favourable thermodynamics and satisfactory gravimetric capacity for the use in the proton exchange membrane (PEM) fuel-cell system[52].NaAlH4belongs to the group of medium temperature hydrides because it started to decompose at a lower temperature.Moreover,NaAlH4is one of the light complex hydrides with high hydrogen capacity and low cost that is available in bulk [30,53].NaAlH4decomposes in three dehydrogenation stages with a total capacity of 7.5wt.%.Three stages of decomposition of NaAlH4are as follows [54-57]:

For the firs reaction,NaAlH4releases 3.7wt.% of hydrogen and takes place at a temperature of 185-230°C.The second reaction occurs at 260°C (release 1.9wt.% of hydrogen),and the third reaction occurs at a 435°C,releasing the remaining 1.9wt.% of hydrogen [58].Even though NaAlH4offers several benefit over conventional metal hydrides,it suffers from slow desorption kinetics and poor reversibility below 150°C [59-61].As an attempt to enhance the performances of MgH2and NaAlH4,numerous studies have been performed,including the reduction of the particle size through the ball milling process,addition with catalyst and destabilisation with other hydrides [62-77].Recently,numerous studies discovered that the outstanding performance of MgH2can be achieved with the aid of NaAlH4because of the mechano-chemical reaction that occurs between MgH2and NaAlH4(Mg-Na-Al system) and the formation of an intermediate phase that alter the thermal stability [78-81].Mg-Na-Al system has attracted considerable interest for solid-state hydrogen storage owing to the low onset of decomposition temperature as low as 60°C compared to the MgH2-catalyst system and good cyclability than the NaAlH4-catalyst system[82].The positive attribute of this Mg-Na-Al system that offers low operating temperature and good cyclability has made them compelling candidates for the near-term PEM fuel cell applications.In this review,we summarise the recent development with an effective tuning strategy of the Mg-Na-Al system that destabilises MgH2with NaAlH4.

2.Hydrogen storage performance of the Mg-Na-Al system

2.1.Reaction mechanism of the Mg-Na-Al system

The destabilisation concept has been approached as benefi cial in altering the thermodynamics and improving hydrogen sorption kinetics performances of metal/complex hydrides.Ismail et al.[78] discovered that the destabilised system of MgH2-NaAlH4improved the dehydrogenation performance compared with separate MgH2and NaAlH4.For the MgH2-NaAlH4system,the dehydrogenation occurs in four distinct steps as reported by Sartori et al.[79].The firs reaction occurs at a temperature of 170-220 °C and is attributed to the following reaction:

The second dehydrogenation stage occurs at a temperature of 280-330 °C that corresponds to the MgH2or NaMgH3relevant decomposition.This reaction step reflect to (i) the reaction of MgH2with Al and (ii) the decomposition of the excessive MgH2.The MgH2-relevant decomposition is as follows:

Fig.2.Possible decomposition reaction of MgH2 -NaAlH4 (1:1) [81].

The third reaction stage that proceeds around 330-360 °C correlated with the decomposition of NaMgH3as follows:

The last stage(decomposition of NaH)occurs between 360 and 375 °C,and the reaction mechanism is as in Eq.(4).

However,it has been reported that NaMgH3can also be formed during dehydrogenation of the MgH2-NaAl3H6composite as follows [83]:

The decomposition reaction of MgH2-NaAlH4and MgH2-Na3AlH6composite suggests the possibility of the intermediate hydrides Na3AlH6and NaMgH3coexisting as both can be formed from the same hydride.However,it has been suggested that the MgH2-NaAlH4composite dehydrogenates with only one intermediate hydride,which is NaMgH3or Na3AlH6,depending on the molar ratio of the composite[81].A study conducted by Bendyna et al.[81] proposed that the firs major decomposition process of the MgH2-NaAlH4composite may correspond to the formation of the intermediate hydrideβ-Na3AlH6,NaMgH3or both simultaneously since the two hydrides are formed at the similar temperature range.They proposed the decomposition pathway of MgH2-NaAlH4(1:1) as shown in Fig.2.

During the reaction process,the formation of the intermediate phase of Mg17Al12is believed to provide a dominant role in improving the dehydrogenation of the MgH2-NaAlH4system.The formation of the Mg17Al12phase as a reaction product is probably highly dispersed in the surface of other species and provides a favourable surface diffusion path for the hydrogen atom,thereby improving the thermodynamic system of MgH2-NaAlH4[84,85].The formation of the Mg17Al12also effectively destabilized the MgH2and NaAlH4by changing the reaction pathway thus leading to the lower dehydrogenation temperature [85,86].As demonstrated by Zhang et al.[84] for the MgH2-LiAlH4system,the Mg17Al12that formed as a reaction product of MgH2with Al during the dehydrogenation process may generate favourable pathway for hydrogen atom diffusion to the surface and recombination hence enhance the thermodynamic properties.

Fig.3.Dehydrogenation performance of MgH2-NaAlH4 composite milled at a different time conducted in a static vacuum (～0.1bar) at a heating rate of 3°C/min:(a) desorb hydrogen as a function of temperature and (b) desorb hydrogen and temperature as a function of time [81].

2.2.The role of milling time on the dehydrogenation performance of the Mg-Na-Al system

The technique of ball milling is one of the effective ways of improving the surface performance and enhancing the desorption rate of the hydride.Different milling times have different effects on dehydrogenation properties of the hydrides.Bendyna et al.[81] conducted a study on the influence of different milling times on the dehydrogenation performance of the Mg-Na-Al system.They discovered that prolonged milling time presents outstanding dehydrogenation performance of MgH2-NaAlH4.Fig.3 depicts the dehydrogenation performance of MgH2-NaAlH4milled for different milling times.

Fig.4.TG profil of MgH2 and NaAlH4 and MgH2-NaAlH4 composite with different molar ratio [91].

In Fig.3,the composite sample that is milled for a longer time begins to release hydrogen at a lower temperature (～150°C)compared to the sample milled for 1 min.Other than that,the temperature required to complete the firs dehydrogenation step was also reduced with the longer milling time.The firs dehydrogenation step for the sample milled for 1min,30min,120min and 360min was completed at 290 °C,230 °C,210°C and 220 °C,respectively.This condition may be attributed to the effect of high-energy ball milling on the sample.As reported in the previous study,prolonged milling time of hydrides reduces the crystallite and particle size of the sample and forms various surface defects;this leads to faster hydrogen diffusion and enhances the hydrogen storage performance[87-90].

2.3.Effect of the molar ratio of the Mg-Na-Al system

Rafi-ud-di et al.[91]conducted a study on the Mg-Na-Al system with different molar ratios (1:2,1:1 and 2:1),as illustrated in Fig.4.They discovered that increasing the amount of MgH2reduces the decomposition temperature of NaAlH4.For instance,the onset decomposition temperature for the 1MgH2-2NaAlH4composite occurs at 175 °C and terminates at 255 °C for the firs dehydrogenation step,and decomposition for the second dehydrogenation step proceeds at 278°C and terminates at 326 °C.For the sample with a different molar ratio (1MgH2-1NaAlH4),the firs dehydrogenation step started at 170 °C and concluded at 248 °C,whereas for the second dehydrogenation step,the 1MgH2-1NaAlH4sample started to decompose at 275 °C and concluded at 335 °C.Surprisingly,for the 2MgH2-1NaAlH4system,the firs dehydrogenation step proceeds at 152-225 °C,and the second dehydrogenation step occurs at 240-300 °C,which indicating that the decomposition process occurs at a lower temperature compared to the those of 1MgH2-2NaAlH4and 1MgH2-1NaAlH4systems.Besides,the 2MgH2-1NaAlH4composite sample has exhibited four decomposition stages.The results suggested that the enhancement of the 2MgH2-1NaAlH4system may be due to more refinemen of powder in the composite sample.

Fig.5.XRD patterns of the:(i) 1MgH2-1NaAlH4 composite (a) before dehydrogenation,(b) after dehydrogenation at 225°C,(c) after dehydrogenation at 250°C,(c) after dehydrogenation at 350°C and (d) after dehydrogenation at 400°C;and (ii) 2MgH2-1NaAlH4 composite a) before dehydrogenation,(b) after dehydrogenation at 230°C (c) after dehydrogenation at 310°C (c) after dehydrogenation at 360°C and (d) after dehydrogenation at 390°C [91].

In terms of the reaction pathway of the MgH2-NaAlH4system,the phase composition before and after dehydrogenation is shown in Fig.5.For the 1MgH2-1NaAlH4composite sample,after dehydrogenation at 350 °C,the peak containing Al12Mg17and Mg is visible in the XRD pattern that concludes there is a reaction occurs between MgH2and Al and the decomposition of excessive MgH2as in the Eqs.(6) and(7) while the Al phase cannot be detected.After continuous heating to 400 °C,the Na phase appeared,and the NaH phase disappeared,indicating that decomposition of NaH occurs as in Eq.(4).These three dehydrogenation steps for the 1MgH2-1NaAlH4composite sample correlated with the complete dehydrogenation of NaAlH4,Na3AlH6and NaH.However,for the 2MgH2-1NaAlH4composite sample,the phase of NaMgH3together with Al12Mg17was observed when the sample was heated to 310 °C.The phase of NaMgH3that formed during the decomposition reaction of the 2MgH2-1NaAlH4sample exhibits that the reaction takes place through formation of the intermediate phase.As the formation of NaMgH3cannot be detected for the 1MgH2-1NaAlH4sample,this indicates that the formation of NaMgH3can be seen for a higher ratio of MgH2/NaAlH4.

Moreover,Plerdsranoy et al.[92] studied the different molar ratio of the MgH2-NaAlH4composite that is infiltrate into carbon aerogel scaffold (CAS).They revealed that increasing the amount of MgH2content on the hydride composite results in high surface area and pore volume.Table 1 below presents the texture parameter of the 2MgH2-1NaAlH4and 1MgH2-1NaAlH4nanoconfine sample.Besides,a high amount of hydrogen content in nano 2MgH2-1NaAlH4provided a higher amount of Al12Mg17after dehydrogenation,resulting in superior reversibility.

Table 1 Texture parameters of nanoconfine MgH2-NaAlH4 samples [92].

2.4.Effect of additives on the Mg-Na-Al system

Fig.6.XRD patterns of the Nb2O5-doped MgH2-NaAlH4 system (1:1) (a)before dehydrogenation,(b) after dehydrogenation at 200°C,(c) after dehydrogenation at 300°C,(d) after dehydrogenation at 350°C and (e) Nb2O5-doped MgH2-NaAlH4 system (2:1) upon heating to 300°C [91].

The addition of a catalyst is one of the key factors that promote significan improvement on the de/rehydrogenation performance of the Mg-Na-Al system.The sorption kinetics performances were greatly enhanced with the aids of a catalyst that helps the dissociation process of the hydrogen molecules in the hydrides.Rafi-ud-di et al.[91] revealed that remarkable sorption kinetics performances of MgH2and NaAlH4can be achieved by combining with one another.They also revealed that doping with Nb2O5significantl improves the dehydrogenation performances of the MgH2-NaAlH4system.The addition of Nb2O5to the MgH2-NaAlH4system presents a superior catalytic effect on the de/rehydriding kinetics and results in a lower dehydrogenation temperature.Undoped MgH2started to decompose at 345°C,whereas undoped NaAlH4decompose at 178°C.Samples of the MgH2-NaAlH4composite have exhibited four dehydrogenation stages and started to decompose at around 150-175°C.The improvement in the composite system is believed owing to the reaction between MgH2and NaAlH4that alters the thermodynamics of the reaction by either reducing the enthalpy of the reaction or leading to a balance of the reaction towards one direction.After the addition of Nb2O5,a striking effect on the dehydrogenation performances of a binary composite and a unary component can be detected.The onset decomposition temperature for the Nb2O5-doped MgH2-NaAlH4occurs at around 70-90°C,which is significantl lower than that of the undoped sample.The existence of Nb2O5nanopowder may contribute to the creation of the surface defect by initiating significan refinemen of powder particles during the milling process,which then ameliorates the dehydrogenation and the sorption kinetics performances [93].To structurally elucidate the catalytic mechanism of the Nb2O5catalyst,the XRD measurement for the Nb2O5-doped MgH2-NaAlH4system before and after dehydrogenation was conducted at different temperature are shown in Fig.6.As shown in Fig.6(a),after milling,it can be seen that the peak of Nb2O5can be detected suggesting that the Nb2O5nanoparticle remains stable with the composite matrix during the high impact milling process.After heated to 200°C,the Nb2O5monocrystalline disappeared.However,the parallel growth of the newly reduced niobium species with different oxidation states,NbO2and NbO was detected.During further heating,the Mg liberated from the hydrogen induces a fast reduction of the niobium oxide due to its very low redox potential,leading to a new peak corresponding to MgO.Moreover,the reaction between Mg/MgH2and the reduced niobium oxide species has yield the formation of the MgNb2O3.67phase.The formation of the MgNb2O3.67phase is possible due to the MgO and NbO that exhibit similar crystal structures and both have similar lattice parameters with comparable lengths between metal and oxygen atoms.Hence,the evolution of the additive to the real active species has taken place in the form of a mixed MgNb2O3.67phase.These ternary oxide Mg-Nb oxide facilitate the hydrogen diffusion by acting as a pathway by the formation of metastable niobium hydride [94].When the desorption process is completed,a steady state is reached consisting mainly of phases with the oxidation states rich in +1(NbH) and +2(NbO).Moreover,as shown in Fig.6(e),it can be seen that the peak of NaMgH3was not detected by XRD for the Nb2O5-doped MgH2-NaAlH4system (2:1) suggesting that the presences of Nb2O5modify the desorption pathway by inhibiting the formation of NaMgH3.This result indicates that the four decomposition stages of 2MgH2-NaAlH4have transformed into three decomposition stage owing to the addition of Nb2O5.

Besides,Ismail et al.[82] proposed that the addition of TiF3to the MgH2-NaAlH4system had better dehydrogenation performance.The composite 4MgH2-NaAlH4-TiF3system exhibited faster desorption kinetics performance and lower activation energy.The desorption kinetic result indicates that the 4MgH2-NaAlH4-TiF3system desorbed 2.4wt.% of hydrogen in a duration of 10 min,whereas the undoped 4MgH2-NaAlH4system was only able to desorb<1.0wt.% of hydrogen at the same time period.In terms of activation energy,the apparent activation energy of the first second,and third dehydrogenation step for the pristine 4MgH2-NaAlH4system was 148kJ/mol,142kJ/mol and 138kJ/mol,respectively.However,with the addition of TiF3,the activation energy was reduced to 71kJ/mol,104kJ/mol and 124kJ/mol,respectively.These improvements may be attributed to the in situ formation of the Ti-Al phase,which accelerates the reaction of the 4MgH2-NaAlH4-TiF3system.Other studies demonstrated that the enhanced dehydrogenation properties of MgH2-NaAlH4were achieved by doping with TiO2nanopowder [95].They found out that the onset decomposition temperature for the MgH2-NaAlH4-TiO2sample started at 90 °C,which is lower than the unary and undoped MgH2-NaAlH4sample.Other than that,the composite sample of MgH2-NaAlH4-TiO2exhibits faster dehydrogenation kinetics.Within 1 hour,the doped sample was able to release 6.4wt.% of hydrogen below 350°C,whereas the undoped sample only released 5.5wt.% of hydrogen under the same condition.In terms of cyclability,the composite sample of MgH2-NaAlH4-TiO2performed superior reversibility with the ability to rehydrogenate 4.2wt.%of hydrogen in the firs cycle,whereas the undoped sample rehydrogenated 3.3wt.% of hydrogen.Furthermore,the MgH2-NaAlH4-TiO2composite maintained kinetics performance well with only a small capacity loss.The improvement of the MgH2-NaAlH4performance with the addition of TiO2may be due to the reduced Ti species that enable the heterogeneous nucleation of the intermediate compound (Mg-Al)alloys at the MgH2/NaAlH4interfaces and thus enhance the cycling kinetics.Other than that,other factors that result in the superior hydrogen performance of the composite sample may be due to surface modification As illustrated in Fig.7,there is an existence of the substantial refinemen of the TiO2-doped sample.Nano TiO2impregnation has resulted in massive surface modificatio resulting in many distorted and disordered surface areas.The presence of TiO2increases the brittleness of the hydride particles that will ultimately shift the balance between fracturing agglomerations to the smaller particle size because TiO2is harder than MgH2and NaAlH4.

Fig.7.The FESEM image of (a) undoped MgH2-NaAlH4,(b) undoped 2MgH2-1NaAlH4,(c) milled MgH2-NaAlH4-TiO2 and (d) MgH2-NaAlH4-TiO2 after rehydrogenation [95].

Besides,Mustafa and Ismail [96] reported that the superior kinetics performance of the 4MgH2-NaAlH4system was obtained by doping with a potassium hexafluorotitanat(K2TiF6).The composite system of 4MgH2-NaAlH4-K2TiF6has favourable de/rehydrogenation kinetics performance compared to the undoped 4MgH2-NaAlH4system,as displayed in Fig.8.

As illustrated in the Fig.8,at 320 °C,the 4MgH2-NaAlH4-K2TiF6composite absorbed 4.5wt.% of hydrogen within 30 min,whereas the undoped sample absorbed 3.5wt.% of hydrogen.Furthermore,the composite sample of 4MgH2-NaAlH4-K2TiF6desorbed 3.5wt.% of hydrogen,whereas the undoped sample desorbed only 2.3wt.%of hydrogen within the same time duration.The 4MgH2-NaAlH4-K2TiF6system also maintained good cyclability as shown in Fig.9.However,after a prolonged time,there is some degradation.For instance,after completing the 10th cycle,the hydrogen absorption capacity reduces from 4.4wt.% to 3.3wt.%.This degradation that occurs may be due to the intermediate phase between the hydrides and catalysts and the formation of new species during the heating process.Other than that,the activation energy of the 4MgH2-NaAlH4-K2TiF6composite was lower than that of undoped 4MgH2-NaAlH4.The calculated activation energy for the composite system was 96.4kJ/mol,and that of 4MgH2-NaAlH4was 124.9kJ/mol.The addition of the catalyst on the Mg-Na-Al system significantl reduced the onset decomposition temperature and activation energy compared to that of undoped MgH2,doped MgH2,undoped NaAlH4,doped NaAlH4and undoped MgH2-NaAlH4,as demonstrated in Table 2.Based on Table 2,it is evident that the onset decomposition temperature and calculated activation energy for the MgH2-NaAlH4system doped with a catalyst was lower than that of unary MgH2and NaAlH4.

Table 2 The decomposition temperature and activation energy comparison between MgH2,NaAlH4 and MgH2-NaAlH4 composite.

Moreover,Bhatnagar et al.[113] reported that the notable dehydrogenation performance of the MgH2-NaAlH4composite was achieved with the addition of carbon nanostructures(CS),namely single-wall carbon nanotubes (SWCNTs) and graphene nanosheets(GNS).The MgH2-NaAlH4sample with the addition of 1.5wt.% GNS+0.5wt.% SWCNT presented superior dehydrogenation performances.The onset decomposition for this composite starts at 156 °C for the firs stage,263 °C for the second stage,355 °C for the third stage and 433 °C for the fourth stage;this shows a reduction by 16°C,27 °C,32 °C and 37 °C compared to that of undoped MgH2-NaAlH4.The improvement of the MgH2-NaAlH4system may be due to the function of SWCNT that hindered the restacking of the GNS layer,thus making it more efficien as a micro confinemen agent.The synergetic effect between GNS+SWCNT offers superior thermal conductivity and greater dispersion compared to separate GNS and SWCNT.Fig.10 is a schematic representation of the catalytic mechanism of GNS+SWCNT in the MgH2-NaAlH4system.

Fig.8.The performances of (a) rehydrogenation and (b) dehydrogenation kinetics of doped and undoped 4MgH2-NaAlH4 composite at 320 °C [96].

Fig.9.The cyclability study of the K2TiF6-doped MgH2-NaAlH4 composite [96].

Another study found that the dehydrogenation performance of the MgH2-NaAlH4system was enhanced when it was nanoconfine into a carbon aerogel scaffold (CAS) [92].It discovered that the multi-dehydrogenation step of MgH2-NaAlH4and 2MgH2-NaAlH4(three to four steps) was reduced to two dehydrogenation steps when confine in CAS as illustrated in Fig.11.Other than that,the dehydrogenation temperature for the MgH2-NaAlH4confine sample was also reduced to 216 °C and 358 °C,which are 40 °C and 27°C lower than that of the milled MgH2-NaAlH4sample that releases hydrogen at 256 °C and 384 °C.For the 2MgH2-NaAlH4confine sample,the dehydrogenation temperature decreased to 206 °C and 345 °C,which are 46 °C and 19°C lower than that of milled 2MgH2-NaAlH4.Moreover,the content of released hydrogen of the nanoconfine sample was enhanced to 80% and 68%,respectively,of the theoretical hydrogen capacity,compared to that of milled MgH2-NaAlH4and 2MgH2-NaAlH4samples (71% and 38%).The higher hydrogen content of the nanoconfine sample was attributed to the formation of Al after the dehydrogenation that reacted with MgH2and formed Al12Mg17that resulted in superior reversibility.The formation of Al12Mg17plays a significan role in improving the thermodynamics of the Mg-Na-Al system.

3.Future development

The Mg-Na-Al system is a candidate for the future solidstate hydrogen storage material.The Mg-Na-Al system leads to enhanced dehydrogenation performance,low onset decomposition temperature and faster dehydrogenation rate compared to unary MgH2and NaAlH4.Even though the addition of the catalyst ameliorates the dehydrogenation performance of the Mg-Na-Al system,there is still a challenge of boosting the re/dehydrogenation performance of the Mg-Na-Al system.More focus work should be done,and the following suggestions are made:

Fig.10.Illustration of the GNS+SWCNT mechanism in the MgH2-NaAlH4 system [113].

Fig.11.Simultaneous TG-DSC-MS/DSC-MS of (i) milled and (ii) nanoconfine (A) MgH2-NaAlH4 and (B) 2MgH2-NaAlH4 [92].

i) The addition of catalysts has shown great potential for the improved performance of the Mg-Na-Al system.Up to this date,the addition of TiF3catalyst to the Mg-Na-Al system significantl lowered the onset decomposition temperature as well as activation energy.The Mg-Na-Aldoped TiF3sample started to desorb hydrogen at 60°C which is reduced by 100°C than undoped Mg-Na-Al system.It is interesting to study the effect of another catalyst to boost the performance of the Mg-Na-Al system and to understand the exact role of the catalyst.

ii) The milling time has been found to affect the dehydrogenation performance of the Mg-Na-Al system.However,it is meaningful to study the surface modificatio and exploration of the size effect on the thermodynamic properties of the Mg-Na-Al system.

iii) Mg-Na-Al has been viewed as a lightweight material that has great potential to be used in a mobile application.However,it is necessary for one to focus on the thermodynamic study of the Mg-Na-Al system.

iv) The study on reversible release and hydrogen uptake (cycling) of the Mg-Na-Al system under moderate conditions should be the main focus,and a significan breakthrough is required to solve the degradation of hydrogen capacity during the re/dehydrogenation process.

v) To prevent the loss of Na that easily evaporated,modifica tions have to be made to NaAlH4.For instance,the introduction of surfactant on NaAlH4would act as a capping ligand stabilizing the NaAlH4nanoparticle [114].Other than that,doping the Mg-Na-Al sample with a material that consists of core-shell structure could be interesting as the core-shell structure will provide a cage for the sample to remain confine during the release of hydrogen and the Na evaporation could be inhibited.

Many challenges are discovered in this field and much research to be conducted to develop a proper system for Mg-Na-Al and make it convenient for a hydrogen energy application.

4.Conclusion

In summary,NaAlH4is an excellent candidate to be combined with MgH2to destabilise the MgH2effectively.In addition,the sorption kinetic performance of NaAlH4has also been improved by MgH2.The MgH2-NaAlH4system exhibited outstanding dehydrogenation performance compared to unary MgH2and NaAlH4.The performance of the Mg-Na-Al system was enhanced by the introduction of the ball milling method and the addition of a catalyst.The addition of the catalyst significantl reduced the onset of the decomposition temperature and resulted in superior kinetics performance.The formation of the intermediate phases of Al12Mg17and NaMgH3during the dehydrogenation process provided a significan role that alters the reaction pathway by creating a favourable pathway and surface recombination for hydrogen diffusion and thus improved the thermodynamic system of MgH2-NaAlH4.

Acknowledgment

This work was supported by the Ministry of Higher Education Malaysia through the Fundamental Research Grant Scheme (FRGS/1/2019/STG07/UMT/02/5).The authors also thank the Universiti Malaysia Terengganu for providing the facilities to carry out this project.