An overview of hydrogen production from Al-based materials

: A profound overview of the recent development for on-time, on-demand hydrogen production from light metal-based hydrolysis is presented. Hydrogen energy is one of the clean and renewable energy sources which has been recognized as an alternative to fossil fuels. In addition, aluminum is the most suitable light activity metal for hydrolysis materials attributed to its safety, environmental friendliness, high-energy density, inexpensive, and low density with high strength ratio. In general, dense oxide ﬁ lms formed act as a barrier on aluminum surfaces. Accordingly, e ﬀ ective removal of the oxide ﬁ lm is a key measure in solving the Al – water reaction. In this review, recent advances in addressing the main drawbacks including high-purity aluminum with acid – alkali solutions, nano-pow-ders of aluminum or composite with acid – base solutions, ball-milled nano-powders, alloying blocks, and gas atomization powders are summarized. The characteristics of these three technologies and the current research progress are summarized in depth. Moreover, it is essential to promote low-cost aluminum-based materials based on e ﬀ ective hydrogen production e ﬃ ciency and explore ways for practical large-scale applications.


Introduction
Energy is considered a "strategic commodity," while the volume of global energy trade has developed substantially over the past 30 years.For global countries, energy security is the cornerstone of national security and a primary consideration in energy trade strategies and policies [1,2].Currently, more than 80% of human energy consumption is accounted for by fossil fuels (coal, oil, and natural gas).Nevertheless, renewable energy stands in contrast to traditional biomass, which is being used far more quickly than they are being replenished.Derse et al. [3] noted that "Hydrogen network management is one of the focal issues for a cleaner and sustainable world.The developed model is compatible with the real world issues by incorporating safety conditions.The sensitivity analysis indicates that distances are effective on the total cost."An urgent demand for developing clean and renewable green energy sources has been evoked [4].On the one hand, the main cause of global warming was fossil fuels with an increase in the atmospheric greenhouse gas levels [5,6].On the other hand, uniformly distributed fossil fuels which are controlled by certain countries globally raise conflicts regarding the security of supply and distribution in developing countries.Ultimately, finite sources of energy will be eventually depleted, and global competition and price increase of these resources cannot be ruled out in the future [7].In response to increasing energy demand and low carbon needs, green and renewable energy has become a political and economic focal point finally [8].For instance, Europe has committed to reducing greenhouse gas emissions by 80-95% below the 1990 levels by 2050 [9].Of course, widening the utilization of solar, wind, hydro, and tidal natural energy can significantly reduce greenhouse gas emissions; however, they cannot be regulated because of intermittent sources [10,11].The grid requires a certain percentage of stationary energy as a base; thus, storing clean energy is critical during this transformation.In order to phase out fossil fuels and limit climate change, hydrogen energy is considered the most promising option [9,12].Hydrogen energy is a clean and renewable energy source and several advantages are as follows: 1) Abundant reserves.Mainly in the form of hydrocarbons and water, and hydrogen is one of the most readily available elements on earth [13].2) Excellent energy density (122 kJ/g).Approximately 2. 75 times more than that of hydrocarbon fuels [14].3) Non-toxic and non-polluting.Water is the only combustion product [15,16].4) Wide range of applications.High heat capacity can be benefited from hydrogen combustion to produce mechanical work in a heat engine, or as an energy material for fuel cells to generate electricity directly [17][18][19].
Hydrogen is a key chain between hydrogen-consuming industries and several essential sectors such as the electricity grid, gas grid, transportation, residential, agriculture, and energy storage [20].Therefore, hydrogen energy is the most promising clean energy source at this stage [21,22].Currently, fossil fuel hydrogen production methods have occupied the largest proportion of industrial hydrogen production (to date, the total volume of hydrogen consumption was 48% from natural gas, 30% from heavy oils and naphtha, and 18% from coal) [23][24][25][26].Clearly, it is not in line with the strategic requirements of sustainable development in the world [27].
Thus, alternative methods of hydrogen production are urgently needed [24,[28][29][30][31][32].As shown in Figure 1, the production, application, and storage of hydrogen play a key role, yet it is always a challenge.As the density of hydrogen is only 7.14% of air density [33] and the ignition energy is only 0.02 mJ [14,34,35], special attention should be paid to safety issues during transportation.Metal hydrolysis behaves as a feature of convenient storage and transportation in hydrogen technology [36], which can provide on-time and on-demand hydrogen production.On the one hand, the replacement of large volumes, large weights, high-pressure resistant, or adiabatic hydrogen storage bottles can effectively avoid the safety problems during high-pressure gaseous hydrogen storage applications and low-temperature liquid hydrogen storage.On the other hand, infrastructure construction is suitable for gas applications in various special scenarios such as disaster area rescue, emergency power supply, field detection, single-armed combat, etc.
The theoretical hydrogen release content of aluminum (Al) is 11.1%, which is higher than that of Na (4.34%), Mg (8.3%), and Fe (3.57%) [37].The production of hydrogen by hydrolysis reaction of hydrides (Mg and MgH 2 , NaBH 4 , LiAlH 4 , Al, etc.) recently attracted increased attention because of its safety and its amenability to mild reaction conditions.For example, in recent years, more and more attention has been paid to hydrogen generation by hydrolysis of metals, metal hydrides, or formic acid for their high theoretical hydrogen yield [38][39][40][41].Generally, the energy efficiencies of the MgH 2 , H-Mg 3 La, and H-La 2 Mg 17 hydrolysis cycles were 45.3, 40.1, and 41.1%, respectively, for half of the energy released by the exothermic reaction from this cycle was collected for recycling.However, the maximum energy efficiency (49.91%) indicates that the reaction of MgH 2 with NaBO 2 to form NaBH 4 during ball milling is feasible [42,43].Additionally, aluminum is the most abundant metal in the earth's crust with a low density of 2,700 kg/m 3 .Likewise, it is of low cost and has high gravimetric and volumetric energy density and high energy efficiency (31.1 MJ/kg and 83.8 MJ/L) [44].The Al-water reactions are as follows: (1) (2) According to reactions (1) and ( 2), 1 mol Al can produce 1.5 mol H 2 .Under theoretically standard conditions, 1,245 mL of hydrogen can be generated from 1 g of aluminum.The end-products of Al-water reaction (Al(OH) 3 , AlO(OH)) are non-toxic, non-polluting, and easy to collect and store.Other advantages include wide applications in flame retardants [45,46], adsorbents [47], etc.Therefore, the Al-water reaction can provide safe, environmentally friendly, and portable hydrogen energy.Furthermore, regeneration is a potential method for reducing the cost, and a one-step approach for hydrogen production, storage, and transportation is a new method via Li(Na)BH 4 regeneration using Mg-based alloys and as the new topics for hydrogenenergy process chain and hydrogen economics [48][49][50].
This overview briefly introduced various hydrogen production technologies for aluminum-based materials, compared the development trends of each technology, and provided an outlook on the future advanced methods for aluminum-based materials.Theoretically, since the Gibbs free energy ΔG < 0 for the Al-water reaction, it means the reaction direction can conduct spontaneously with water.However, a dense alumina film is formed on the Al-based material surface for its activity, which prevents the Al-water reaction from proceeding [51,52].Hence, eliminating the oxide layer is a key point to the problem of Al-water reaction.

Reaction with acid or alkali solution
Al 2 O 3 is the main component of the dense film on the surface of aluminum which can react with both acid and alkali solutions at room temperature.The reaction mechanism among HCl is displayed as follows: (3) The acid solution reacts with Al 2 O 3 first to destroy the outer oxide layer and then contacts the inner pure aluminum matrix, as evident from reactions (3) and (4).This reaction can evoke hydrogen generation directly.However, the H + concentration decreases and the pH increases during the reaction.The hydrogen generation rate is bound to decrease whenever the acid supplementation is stopped.However, aluminum reacts differently with alkali solutions, and the surface aluminum passivation layer dissolved more easily than in acidic solutions.The reaction equations are given as follows [53][54][55]: (5) First, the Al 2 O 3 layer begins to dissolve in NaOH solution at the outer oxide layer.In other words, aluminum starts to lose its protective film and NaOH in the solution is consumed by the oxide layer and NaAl(OH) 4 is produced, which in turn decomposes into NaOH again.Finally, aluminum directly reacts with water.During this whole process, NaOH does not participate in the Al-water reaction and only plays a role as a catalyst.Hiraki et al. [56] described the reaction of aluminum with sodium hydroxide solution which is shown in Figure 2. The schematic diagram of the experimental apparatus is shown in Figure 2(a), while the hydrogen generation and rate were detected with a flow meter.As shown in Figure 2(b), the content of H 2 increases with time and the hydrogen generation rate starts to decrease after a certain time.When H 2 is produced, the solution temperature increases rapidly.In addition, the pH of the solution decreases in the first period and then increases.The consumption of NaOH is mainly due to the formation of Al(OH) 4 − first, and then Al(OH) 4 − decomposes to Al(OH) 3 and OH − .This reaction continues and moves in circles.Additionally, NaOH promotes the Al-water reaction more efficiently than KOH and Ca(OH) 2 [57], while the reaction rate can be further increased by the temperature and the surface area of aluminum increased.In addition, the reaction rate is also strongly influenced by the concentration of the base solution.
The consistency of CO 2 in the air decreases the concentration gradient of free hydroxides in aqueous solutions and the reaction rate [58,59].Although this kind of reaction provides a compact source of hydrogen, it is not suitable for practical use because highly concentrated acid and alkali solutions can corrode the equipment, and strict control of the pH of the solution is required to achieve good hydrolysis results.

Hydrolysis reactions of ballmilled aluminum matrix composites
A powerful means of activating aluminum is mechanical ball milling [60] which crushes the powder into particles by strongly impacting, grinding, and stirring the aluminum powder through mechanical alloying and milling [61,62].In milling, large particles of metal are ground into small particles, and ball milling builds on this by converting small particles into nano-powder and forming new compounds between different metals; therefore, ball milling is a combination of mechanical alloying and milling.Generally, powders are susceptible to agglomeration during ball milling, which can deeply affect the hydrolysis reaction.Therefore, mixing aluminum with grinding aids (such as salts, metal oxides, or metal hydrides) for ball milling can effectively improve the hydrolytic activity of aluminum.During the ball milling process, the physical interaction occurs between the grinding aid and pores, cracks, and even fractures in the microstructure of the aluminum-based material, which not only deeply expands the specific surface area but also the distortion of the lattice structure and leads to elevated energy in the imperfections or defects of the metal matrix.
In this process, many active reaction sites are formed to promote the Al-water reaction [63,64].The details of different grinding aids are discussed below.

Salt additives
Generally, NaCl is a common grinding aid in aluminum ball milling due to its low cost, safety, non-toxicity, good solubility, and environmentally friendly.NaCl addition in ball milling directly continuously chops or destroys the aluminum oxide layer of the aluminum powder and prevents the agglomeration of the powder.After activation of aluminum by mechanical treatment, NaCl particles embedded on the aluminum surface and a large number of pits on the aluminum surface emerged after dissolving in water, which increases the contact area of the Al-water reaction and promotes the reaction rate.Yolcular and Karaoglu [65] analyzed the relationship between the NaCl content and ball milling time for the aluminum powder during hydrogen production.It can be seen that without NaCl, as the ball milling time increases, the aluminum powder grows into spherical particles of 1-2 mm diameter due to agglomeration.If the NaCl content is increased from 5 to 20%, the diameter of the composite powder further decreased and the hydrogen generation rate reached 1,300 mL/(g min), as shown in Figure 3.The mixture with the addition of NaCl has a laminar structure and can exhibit better hydrolytic properties.
Irankhah et al. [66] explored the promotion of Al-water reaction by NaCl, KCl, and BaCl 2 .In comparison with NaCl, KCl and BaCl 2 could cover the surface of Al effectively.As a result, BaCl 2 could significantly reduce the induction time of the reaction, while the alloy powders of hydrogen yield with BaCl 2 adjunction was considerably lower than that of NaCl and KCl.The hydrogen generation curve with different salt additives is shown in  on the surface, resulting in a decline in the specific surface area and consequently a decrease in the hydrogen yield.It is worth mentioning that the by-products of Co or Ni can be easily separated by their magnetic properties and reused in the subsequent hydrogen production cycle.

Oxide additives
The addition of oxide in the ball milling also directly makes close contact between aluminum powder and oxide powder and reduces the generation of the oxide film, so that the modified aluminum powder can react with water to generate hydrogen continuously.
Deng et al. [67] investigated the effect of different oxides (γ-Al 2 O 3 , α-Al 2 O 3 , TiO 2 ) on the hydrogen production rate of the Al-water reaction.After the oxide was added, the surface of the aluminum particles was covered with fine oxide particles, which were not as smooth as the original aluminum particles, as shown by the SEM and TEM micrographs in Figure 5(a-h); the hydrogen production curve is shown in Figure 5   the highest hydrogen yield.In addition, an interesting phenomenon was found during the reaction of Bi 2 O 3 .The reaction would stop when the experimental temperature was lowered to room temperature, and the generation rate returned to the original when the temperature was 80°C.

Addition of low melting point metals
The advantages of adding the low melting point metals to ball-milled aluminum are as follows: (1) discontinuous oxide film on the aluminum surface; (2) higher energy potential of the melting point metal than that of aluminum, and galvanic cell corrosions occur in water; and (3) prohibition of the production of Al(OH) 3 on the aluminum surface.

Binary Al composites
From Table 1, it is evident that Bi and Sn worked as active metals for binary Al composites.Nevertheless, the hydrogen yield from binary Al composites is low (85%) Hydrogen production from Al-based materials  7 and the amount of active metal added is comparably high with large consumption.

Ternary Al composites
By the hydrolysis performance of Bi and Sn in binary composites, researchers started to investigate different ratios of Al-Bi-Sn alloys.Table 2 shows some of the recent articles on ternary composites.The hydrogen yield of Al-Bi-Sn alloys with a 5 wt% (Bi + Sn) content is close to the theoretical values at room temperature.Even with the same composition of the material, different ball milling procedures (ball milling time, ball-to-powder ratio) displayed different hydrogen production performances.Microstructures show that Bi and Sn promote structural disruption of Al during the mechanochemical treatment, which easily reduces the size of Al particles.Bi and Sn are relatively uniformly distributed throughout the Al particles promoting the galvanic cell reaction between the Al and Bi/Sn during the Al-water reaction.
In addition, researchers also investigated the effect of ball milling on Al-Bi-In and Al-In-Sn powders.As shown in Table 1, the Al-In mixture is essentially non-hydrolytic with water.However, Al-In-Sn powders show good hydrolysis after the gradual replacement of a portion of In with Sn.The hydrogen yield of Al-7.5Bi-2.5Inpowders is close to the theoretical values.Hence, Bi and In were completely dispersed on the aluminum surface, destroying the protective layer on the aluminum surface which is different from Bi-Sn.
Unlike the previous two composites, Al-In-Sn formed new phases during ball milling -In 3 Sn and InSn 4 .Nearly 100% hydrogen yield was achieved with Al-5In-5Sn.
Taking into account the costs of the raw material, ternary Al-Bi-Sn powder is a particularly suitable choice, and with a suitable ball milling process, excellent hydrogen production can be achieved for practical applications in future.

Quaternary and multiple Al composites
The progress of hydrogen production performance of quaternary Al composites is summarized in Table 3.It can be seen that the hydrogen production performance of the quaternary Al composite is excellent, whereas it has no obvious advantage over the ternary Al composites, and it is more difficult to recover the fourth-series Al composites due to their complexity.

Other additives
The effects of additions of different mixtures on the hydrolysis effect of the composite have been analyzed.Wu et al. [79] prepared Al-10 mol% CaH 2 -10 mol% NiCl 2 and it gave a hydrogen yield of 92.1% at 75°C with a maximum hydrogen generation rate of 1566.3 mL/(g min) after 3 h ball milling.The hydrolysis process leads to the formation of a new compound Ca 2 Al(OH) 6 Cl( H 2 O) 2 , which stimulates to remove the dense by-product layer.Liu et al. [80] developed a mixture of Al and Li 3 AlH 6 which gave 92.8% of hydrogen yield at room temperature.Chen et al. [81] added 5 wt% NaCl during ball milling of Al-10 wt% Bi(OH) 3 , and the hydrogen generation rate elevated to 1,000 mL/(g min) at 30°C.Liao et al.
On the contrary, when sintered in a plasma discharge sintering furnace, the product of blocks gave 100% theoretical hydrogen yield, as shown in Figure 6(c).Also, the sintered material reflects excellent oxidation resistance and still displays a 75.5% hydrogen yield after being exposed to air for 30 days.The main reason is that the number of carbon defects and cracks increased by the sintering process on the surface of the block.The effect of different processing methods on the surface microstructure of the Al-Bi@C material is shown in Figure 6(d and e).As shown in Figure 6(f), the intensity ratio of the D band to the G band (ID/IG) is 0.948 for Al-Bi@C milled and 0.952 for Al-Bi@C SPS.This indicates that ball milling increases the carbon defects and the SPS process further enhances carbon defects, which helps to improve the reactivity of the block.In particular, the sintering process not only increases the hydrogen generation rate of Al composites but also improves its oxidation resistance.Therefore, this method is expected to have practical uses in the field of hydrogen supply.
Generally, aluminum powder composites prepared by ball milling had the advantages of high hydrogen yield, high hydrogen generation rate, and short reaction induction time.The higher the activity, the higher the requirements for preparation, transportation, and storage.However, its short storage time is an obvious disadvantage.Furthermore, the high cost and time-consuming ball milling procedure is another obstacle not suitable for large-scale processing.Nonetheless, it cannot be overlooked that the high activity provided by the ball-milled aluminum powder can provide a large amount of hydrogen swiftly in an emergency and has a wide range of prospects for commercial application.

Low melting point metalactivated aluminum alloys 4.1 Block alloys
When hydrargyrum is dropped on the aluminum plate where the oxide film has just been removed, tall blocks will gradually grow on the aluminum plate.As shown in Figure 7(a-c), it was found that hydrargyrum can help the

Binary and ternary Al alloys
Woodall et al. [85] found that Al-72 wt% Ga and Al-50 wt% Ga show hydrolysis at 26-27°C, which is clearly due to the melting point of the Al (Ga) solid solution (about 26.6°C).
Along with the hydrolysis reaction process and temperature, Ga dissolves in the Al matrix which does not participate in the reaction and only acts as an active component similar to a catalyst.Nevertheless, the hydrogen yield of Al-Ga alloys is deficient, which cannot satisfy the practical requirements.The emergence of binary alloys provides us with the possibility of hydrogen generation from the Al-water reaction but the hydrogen yield is low and the amount of a single precious metal is too large to be suitable for general applications.Otherwise, Al-Ga alloys not only display lower hydrogen production efficiency but also exhibit mechanical brittleness.In particular, Zhou et al. [86,87] verified this conclusion by first-principles calculations and found that the solid solution of Ga significantly reduces the strong toughness of the aluminum matrix.Al-Ga alloys are observed to produce H 2 at a reaction temperature of about 26°C which is close to the melting point of Al-Ga eutectic.Therefore, lowering the melting point of the low melting point phase in the alloy to reduce the initial reaction temperature is an important idea.He et al. [88] investigated the reactivity of Al-Ga-In and Al-Ga-Sn alloys with water.The heat absorption peaks of Al-Ga-In and Al-Ga-Sn appear at 15°C and 19°C, respectively, as shown by DSC, which basically coincide with the melting points of Al-Ga-In and Al-Ga-Sn eutectics of 15.3 and 20.5°C, respectively.It was found that the Al-Ga-In alloys showed reactivity at about 10°C and the Al-Ga-Sn alloys started to show reactivity at 20°C.The reason for this is that the aluminum-water reaction is exothermic, so the reaction temperature is slightly below the melting point.The 15 wt% (Ga + In) Al-Ga-In alloys have a hydrogen yield of more than 90% in water at 20°C.In comparison, the hydrogen yield of the 15 wt% (Ga + Sn) Al-Ga-Sn is only 12%.Although the hydrogen yield of the Al-Ga-In alloys is much higher than that of Al-Ga-Sn alloys, the hydrogen generation rate is lower than Al-Ga-Sn alloys.In general, the low melting point phase determines the starting temperature of the hydrolysis reaction, and the initial temperature at which the reaction point begins to occur follows the melting point Al-Ga-In < Al-Ga-Sn < Al-Ga, and the different compositions seriously affect the H 2 generation rate and H 2 yield.In addition, there is microgalvanic cell corrosion in the aluminum-water reaction of the alloys, which means that the difference between Ga, In, and Sn and Al also affects the hydrogen generation rate.
Consequently, further research is needed to develop alloys that combine hydrogen generation rates, hydrogen yields, and lower reaction temperatures.

Quaternary and multi-alloys
Wang et al. [89] observed the Al-Ga-In-Sn alloys showed a heat absorption peak at 11.2°C by the DSC test, slightly above the melting point of 10.7°C for Ga-In-Sn eutectic.
When the mass ratio of In/Sn is equal to 1.5:0.7, the In 3 Sn phase appears, and when the mass ratio is equal to 1:1, not only the In 3 Sn phase but also the InSn 4 phase appears in the alloys.du Preez and Bessarabov [77] further observed that the formation of In and Sn intermetallic phases is determined by the molar ratio of In/Sn and In 3 Sn was the dominant In-Sn phase in composites with an In/Sn ratio of <0.97.
The principle of hydrogen production in Al-Ga-In-Sn alloys is mainly based on the fact that the addition of low melting point metals generates eutectic phases with very low melting points (In 3 Sn, InSn 4 ), and even though most Ga dissolves in Al, trace amounts of Ga dissolve in the InSn eutectic phase, forming a Ga-In-Sn eutectic phase (named GIS phase).The GIS phase also dissolves a certain amount of Al atoms, completely surrounding these Al atoms.Al atoms react preferentially with water, even if the Al atoms in the eutectic phase have completed reacting.They will be supplemented by Al atoms with increased activity due to the solid solution of Ga and continue to undergo hydrolysis reactions.The low melting point phase not only protects the solid-solution Al atoms from oxidation but also provides a channel for the Al atoms to diffuse to the reaction sites [91]; these lower melting point "channels" allow the alloys to react with water at lower temperatures.The reaction mechanisms of the alloys are shown in Figure 8.In order to produce larger amounts of H 2 , it is necessary to maintain a suitable number of "channels" and the eutectic phase must be small and uniformly distributed [92,93].If the channels are too small, the activation energy will be reduced, thereby slowing down the hydrolysis reaction and reducing the hydrogen generation rate.If the channels are too large, at the same time, the content of rare and precious metals will increase, which will increase the cost.Only when the low melting point eutectic phase is uniformly and finely distributed on the surface of the aluminum substrate can all the aluminum be activated to the maximum.
Al-Ga-In-Sn alloys have excellent hydrogen production properties, and researchers added other metals (Bi, Zn, Cu, Mg, Li) or compounds (Al 2 O 3 , Al-5Ti-B) to achieve one or more goals of cost reduction, reaction rate modulation.and strength increase.Ga and In are expensive, so the proportion of their use needs to be kept as low as possible.Huang et al. [94] added Bi to 80 wt% Al-Ga-In-Sn alloys to replace In, compared with the Al-Ga-In-Sn alloys, the hydrogen generation rate was smooth and controllable, and the hydrogen yield could reach the theoretical value.An et al. [95] increased the Al content to 90 wt% on this basis, and the hydrogen yield at 40°C was already close to 100%, with a maximum hydrogen generation rate of 780 mL/(g min).Wei et al. [96] explored the effect of partial substitution of InSn 4 with Cu on the hydrogen production effect and found that the hydrogen yield and hydrogen generation rate of Al-Ga-InSn 4 -Cu alloy ingots with 3 wt% Cu content were better than Al-Ga-InSn 4 alloys.
Along with an increase in the hydrogen yield, the hydrogen generation rate in the alloys needs to be regulated.Wei et al. [97] replaced Al in the Al-Ga-In 3 Sn alloys   Hydrogen production from Al-based materials  13 generation rate of the alloys without Zn.While the presence of Zn in the alloys is similar to that of Ga, mainly in the form of Al-Zn solid solution, it can retard the peak hydrogen generation rate mainly due to the formation of an Al-Zn micro-galvanic cell.In addition, Cu and Mg can effectively slow down the hydrogen generation rate but have less effect on the hydrogen yield [101][102][103][104]. Soluble materials are critical in horizontal well fracturing technology for unconventional oil and gas field development.The water-soluble nature of the alloys allows them to be used as a downhole soluble fracturing material, subject to compressive performance requirements.With the addition of Cu and Mg, the Al grain size becomes significantly finer and the strength of the Al-Ga-In-Sn alloys increases [101][102][103].The functions of the different materials are summarized in Table 4.

Gas-atomized alloys
The gas atomization process is a widely used powder manufacturing process that offers good uniformity and the possibility of mass production.It is a process in which a high-speed, high-pressure media flow is generated by an atomizing nozzle to crush the molten metal into fine droplets, which are subsequently spherified, cooled, and solidified into a metal powder.In addition, gas atomization is more suitable for mass production than ball milling [105].
As previously mentioned, ball-milled Al-Bi-Sn composites reflected high H 2 yield and H 2 generation rates, yet this composite cannot be prepared by melting because it is a difficult miscible alloy that can easily cause solid-liquid separation.Wang et al. [106] prepared Al-10wt%Bi-10wt% Sn powder by gas atomization and the powder showed outstanding hydrogen generation performance, with 80.4% H 2 yield at 0°C for 88 min and 91.3% at 30°C for 16 min with water.Liu et al. [108] prepared Al-20wt%Bi powder with a core-shell structure, as shown in Figure 10(a-g).The powder reacted at room temperature and reached 100% theoretical H 2 yield, as shown in Figure 10(h).
The composite powder prepared by gas atomization has a significant H 2 generation rate at low temperatures and the process is simpler than ball milling.However, the disadvantages are similar to ball milling, which can lead to unsafe storage and short storage times.

Conclusions
Hydrogen, with its high calorific value, low density, and only water as a combustion product is a promising renewable energy source.Nevertheless, there are safety risks in storage and transport, so there is a need to develop a safe, environmentally friendly, and portable hydrogen energy application.Hydrolysis of aluminum-based materials can improve this role.Surface passivation of aluminum in air remains a major obstacle to the reaction of aluminum with water and this study provides a detailed review of the main methods for removing Al 2 O 3 or destroying it from aluminum surfaces: 1) The reaction of aluminum with acid-alkali solutions is the simplest principle of hydrogen production and has high requirements for experimental equipment.2) Ball milling of aluminum powder can produce hydrogen quickly and is suitable for rapid hydrogen production under emergency conditions, but extra attention should be paid to the safety hazards and short storage time of aluminum powder as well as the time and cost of ball milling.
3) The melting and casting have the advantages of a simple process, low cost, high efficiency, and easy factory mechanization that are difficult to replace.Although the hydrogen yield and the hydrogen generation rate are lower than ball milling, it has the additional advantage of long storage times under dry conditions.4) The gas materialization adds a process to turn the solute into a powder when casting.The properties of the powder are close to powder prepared by ball milling, and advantages are similar to those of the ball milling, with poorer safety and shorter storage times.
In brief, Al-based materials have been successfully used in mobile hydrogen production equipment, hydrogen fuel cells, and other fields, and it has wide application prospects in many fields.For Al-based materials to have a better commercial future, there are still some issues that need continued attention: 1) As the alloys are water-soluble and can be strengthened by adding reinforcing metals, other scenarios have been developed for use and for developing applications for a wider range of use environments.2) In order to improve energy efficiency and reduce raw material costs, the use of high-purity aluminum is gradually being replaced by scrap aluminum or industrial pure aluminum.3) For cost and environmental considerations, the focus should be on scientific research and analysis in terms of by-product analysis, collection, and reuse.

Figure 1 :
Figure 1: Importance of hydrogen in integrating different energy sectors.

Figure 4 .
Chai et al. [61] investigated the effect of CoCl 2 and NiCl 2 on the Al-water reaction and found that the highest hydrogen yield was reached when the CoCl 2 concentration was increased from 0.5 to 1 M. Nevertheless, the hydrogen yield diminished significantly when the CoCl 2 concentration was further raised to 2 M. The effect of CoCl 2 and NiCl 2 concentrations on the hydrogen yield also showed similar trends.The reason was the formation of Co/ Al and Ni/Al galvanic cells, which cause Co or Ni to aggregate

Figure 2 :
Figure 2: (a) Schematic diagram of the experimental apparatus (b) Time evolution of the temperature and pH of the NaOH aqueous solution, the accumulated volume and the generation rate of hydrogen [56].
(i).Due to the low nucleation and conversion potential, the density of the oxide film of the ball-milled Al is lower than before.Consequently, the new particles can readily react with water for hydrogen production.Dupiano et al. [68] explored the effect of the addition of five different metal powders (MoO 3 , Bi 2 O 3 , CuO, MgO, Al 2 O 3 ) on the hydrogen production of the Al-water reaction.The surface structure of oxide powder additions detected by SEM is shown in Figure 5(j-n) and the hydrolysis reactions at 80°C are shown in Figure 5(o).It is obvious that Al-Bi 2 O 3 has the fastest reaction rate and

Table 4 :
Effect of adding different materials to Al-Ga-In-Sn alloys found that 1 wt% Al 2 O 3 contributed to the raised hydrogen generation rate of the alloys but had no effect on the hydrogen yield.Grain refinement is an important method to enhance the hydrogen generation rate.Du et al.[98] added 0.12 wt% Al-5Ti-B to Al-Ga-In-Sn alloys; the grain size was reduced from 129 to 57 μm, as shown in Figure9, and the alloys achieved 87% H 2 yield at 30°C.Wei et al.[97] found that replacing In and Sn with Al 2 O 3 can slow down the hydrogen generation rate of the alloys but has little effect on the hydrogen yield.He et al.[99] prepared Al-Ga-In-Sn-Li alloys with different Li contents.On increasing the Li content, the hydrogen generation rate of the alloys decreased significantly, while the hydrogen yield decreased slightly.Liu et al.[100] investigated the effect of Zn by replacing some of In 3 Sn on the hydrogen production properties of the Al-Ga-In 3 Sn alloys and realized that at 2 wt% Zn content, the hydrogen generation rate of the alloys decreased significantly compared to the hydrogen

Funding information:
This work was supported by the Key Research and Development Program of Shaanxi (Program

Table 1 :
A summary of ball-milled binary Al composites

Table 2 :
A summary of ball-milled ternary Al composites

Table 3 :
A summary of quaternary and quinary ball milled Al composites