Hydrogen Production via Hydrolysis and Alcoholysis of Light Metal-Based Materials: A Review

An overview of the recent advances in hydrogen production from light metal-based materials is presented, including hydrolysis of Mg-based alloys and hydrides, hydrolysis of Al-based alloys and hydrides and (catalyzed) hydrolysis/alcoholysis of borohydrides. Hydrogen production and storage in a close loop are achieved via hydrolysis and regeneration of borohydrides, demonstrating a promising step toward the large-scale application of chemical hydrogen storage materials in a fuel cell-based hydrogen economy.


Introduction
Hydrogen, the most abundant content in the universe, has a number of advantages over conventional fuels. It has a high energy density (142 MJ kg −1 ) and is environmentally friendly. As such, hydrogen energy economy was proposed by Hofman et al. [1] in the early 70s. Encouragingly, the emerging of proton exchange membrane fuel cells (PEM-FCs) in the mid-2000s made large-scale hydrogen applications achievable in vehicles or portable electronic devices [2][3][4]. Particularly, a commercially available car driven by 4 kg of hydrogen fuel can run 400 km with zero carbon oxide emissions [5]. The energy efficiency of this hydrogen 'burnt' process via electrochemically combining with oxygen in fuel cell may reach 70% with less Carnot efficiency loss compared to that in an internal combustion engine [6]. However, the major obstacles for the advent of the hydrogen economy are the absence of efficient strategies for both hydrogen storage and production. Therefore, it is urgent to develop effective solutions to solve these problems from the view of the futuristic aspect of the utilization of hydrogen in stationary, portable and automotive applications [7][8][9].
As it is known, hydrogen storage methods generally are classified into three types: solid-, liquid-and gas-state. Though ultrahigh-pressure hydrogen and cryogenic-liquid hydrogen technologies are relatively mature and have been applied in various prototype vehicles [10], the hydrogen density barely meets the targets determined by the US Department of Energy (DOE) [11]. For ultrahigh-pressure hydrogen gas, the hydrogen-storage targets of DOE upon onboard hydrogen applications in terms of gravimetric and volumetric density are 1.6 and 2.1 times higher (Table 1), respectively, than the values achieved to date using common 700-bar tanks. As far as we know, only the state-of-the-art 700-bar hydrogen tank designed by Toyota holds a hydrogen density of approximately 5.7 wt% H 2 [12], just satisfying the present target of DOE. Ammonia (NH 3 ) is also highly valued as a potential hydrogen storage option except compressed H 2 gas, owing to its high hydrogen density (17.8 wt% and 0.120 kg H 2 L −1 for gravimetric and volumetric H 2 density), low storage pressure and stability for long-term storage as well as high flexibility in its utilization [13]. In this regard, NH 3 can fulfill the demand to store the energy in time (stationary energy storage) and in space (energy export and import). However, NH 3 encounters high energy demand in both synthesis and decomposition for indirect utilization by the release of H 2 . In case of liquid H 2 , in spite of a much higher volumetric density (0.071 kg H 2 L −1 ) that even surpasses the ultimate targets of DOE at the temperature as low as -253 °C, the inevitable hydrogen loss resulted from heat transfer and a large amount of energy consumed to liquefy hydrogen severely impede its practical applications [8,14]. As same as liquid H 2 , besides the much unavoidable energy consumption required in the high-pressurized compression, the high cost and latent safety risks of hydrogen refueling stations are the obstacles for the large-scale utilization in civilian vehicles. Admittedly, solid hydrogen storage materials [15] are the most acceptable hydrogen carriers and have received a great deal of attentions due to their ideal hydrogen density, reliable safety and numerous modification methods that have been developed to tailor their practical dehydrogenation capacities in recent years. Here, a comparison of some typical hydrogen mediums in terms of cost, hydrogen storage capacity and safety is summarized, as shown in Table 2. Table 1 Current states vs targets for onboard H 2 storage for light-duty fuel cell vehicles [11] 1 Projected at 500,000 units/year 2 1 3 In the mid and late of 2000s, the heavy intermetallic binary compounds were initially emerged as hydrogen storage materials owing to their good cycling performance and rapid kinetics under moderate conditions. However, the AB 2 and AB 5 types (ZrFe 2 , LaNi 5 , etc.), representative members of heavy metal alloys family, merely enable ≤ 2 wt% of hydrogen sorption because of the heavyweight and hydrogen non-absorptive trait of B side elements 9,[18][19] . To meet the hydrogen storage targets given by DOE, scientists and researchers have been focusing toward novel lightweight hydrides [20][21][22]. Among these hydrogen materials, the most fascinating hydrides are magnesiumbased materials (MgH 2 as the host material) [23][24][25] and B-N compounds (borohydrides or ammonia borane) [26]. The gravimetric hydrogen densities of 7.6 wt% for MgH 2 and 18.5 wt% for LiBH 4 even exceed the value for onboard applications set by DOE. Recently, Shui's group [27] synthesized a multilayered Ti 2 CT x (T is a functional group) stack by incomplete hydrofluoric acid (HF) etching, and the as-prepared Ti 2 CT x showed an unprecedented hydrogen uptake of 8.8 wt% H 2 at room temperature and 60 bar H 2 , which is much higher than the ultimate targets of DOE. Unfortunately, most of light metal-based materials are considered to be irreversible under mild conditions, so a serious of tailoring strategies have been developed for hydrolysis and thermolysis. For example, it was found that ZrCl 4 is an effective catalyst to considerably reduce the dehydrogenation temperature and activation energy for LiBH 4 [28]. Furthermore, the hydrogen produced by the thermal decomposition is always accompanied with the emission of other explosive or toxic gas such as CO and/ or B 2 H 6 [29]. Generally, PEMFCs are very sensitive to the impurity of hydrogen, and even a little amount of impurity may cause the poisoning the catalysts [30]. Compared with the above approach, pure hydrogen supply from hydrolysis of light metal-based materials, including metal hydrides and borohydrides via reacting with water without external heat input, has a number of advantages, such as suitable operation temperature and well-controlled hydrogen release. Especially, hydrogen supply via hydrolysis is a self-humidification process, and such humid hydrogen can be conveyed directly into PEMFCs without dehumidification treatment and any performance loss [31]. Different from liquid H 2 or gas-state hydrogen carriers that need further development and construction in infrastructures, such as the NH 3 /H 2 pipelines, H 2 /NH 3 refueling stations and liquefaction devices, the storage and transportation of metal hydrides and borohydrides hold low potential risk and low capital investment because they are largely compatible with the current transport infrastructure [13]. For Mg-based and Al-based materials, they can be stored and transported in the form of bulks. Moreover, the formation of a coherent passive layer deposited on the surface of bulks may prevent further oxidation of hydrolysable materials. With respect to borohydrides, NaBH 4 , an example of the family of borohydrides, is a well-known hydrogen carrier due to its high hydrogen-storage capacity (10.8 wt%) [32,33]. It is easily dissolved in alkaline aqueous solution for safe, stable and long periods of storage, leading to a highly convenient transportation. Therefore, the currently Hydrolysis enables hydrogen extraction from liquid water. However, the performance of hydrolysis reaction is subject to the operation temperature. The hydrogen generation rate will be significantly reduced in a low-temperature climate and the hydrolysis process could even be directly frozen in subzero circumstances. Methanol has a very low freezing point (-97 ℃); thus, hydrogen supply from methanolysis is considered optimal for real-time hydrogen production in low-temperature climate or subzero areas. At mild conditions, the reversible hydrogen storage systems like the metal-based hydrides have the advantages of fast hydrogen injection and durability for repeated recycling, whereas the hydrogen storage properties are plagued by the sluggish de-/ hydrogenation kinetics, thermodynamic barriers (de-/rehydrogeneration temperature < 100 ℃, pressure < 10 atm) and cyclic performance [34]. In contrast, the device for hydrolysis hydrogen supply is very compact [35], and the hydrogen derived from water or light metal-based materials can be directly connected to the fuel cell to drive the motor. Significantly, water freight is safer and more convenient compared to high-pressure hydrogen storage and transportation. However, the controllability and utilization of enormous exothermicity of hydrolysis require further investigations.
In this review, we summarize the recent progress in the development of hydrolysis and alcoholysis of light metalbased materials, especially the Mg-/Al-based materials and borohydrides. To overcome the sluggish hydrolysis and low conversion, various methods have been developed, such as ball milling, catalysis, alloying, and solution modification. The different hydrolysis mechanisms of Al/Mg-based materials and sodium borohydride are discussed in detail. Furthermore, the recent advances in NaBH 4 regeneration process from hydrolysis by-product are discussed. NaBH 4 is considered as the most potential hydrolysable material.

Hydrogen Generation from Hydrolysis or Alcoholysis
The typical hydrolytic materials include metals/hydrides, ammonia borane (NH 3 BH 3 , denoted as AB) and borohydrides. Hydrogen supply from NaBH 4 hydrolysis was the most widely studied and has numerous advantages over the other hydrolytic materials, including half of hydrogen production from water, low operation temperature, environmentally benign by-product, well-controlled and high-purity hydrogen release [36][37][38], making it promising for on-board or onsite hydrogen supply. On the other hand, Mg-or Al-based materials are also widely discussed as hydrogen carriers, and they can supply high-purity H 2 according to real-time demands via contacting with water. Compared to costly borohydrides, hydrogen supply from the light-metal materials is affordable and sustainable because of the abundant content in the earth crust and the mature recycling process in the industry. The following sections mainly emphasize the hydrolysis/alcoholysis of borohydrides, Mg-/Al-based alloys and hydrides.

Highly Efficient Catalytic and Non-catalytic Alcoholysis/Hydrolysis of Borohydrides
Extensive efforts have been devoted to exploring highly efficient hydrolysis of borohydrides (NaBH 4 , Mg(BH 4 ) 2 , LiBH 4 , etc.) or AB due to their excellent hydrogen storage capacities [39][40][41] . For hydrogen application in fuel cells, if the water produced in the fuel cell part is redirected to LiBH 4 , then the H 2 generation capacity may increase to 37.0 wt% [42]. Compared with the expensive LiBH 4 , NaBH 4 with a 21.1 wt% H 2 generation capacity (the water produced in the fuel cell part is recycled to react with NaBH 4 and it is not taken into account in the case) is preferred as a more superior hydrolysable material, but its hydrolysis suffers from sluggish kinetics in neutral aqueous solutions. To lower the high kinetic barrier to an extent that would give a hydrogen generation rate closing to the requirement of practical applications, a variety of non-noble metal catalysts have been developed, such as Fe, Co, Ni or Pt, Ru, and Pd [43][44][45][46][47]. Especially, in the hydrolysis of borohydride aided by M 3 B (M = Cu, Ni, Fe), the catalytic activities are in the order of Cu < Ni < Co [48]. The Co-B-based types [49][50][51][52] are commonly admitted as reactive as noble metals and much more cost-effective, which exhibit saltant performance improvements. The enhanced performance results from the Co-B catalysts loaded on supports with a high surface distribution, where transition metals (Co, Ni, and Fe) act as active sites. The real hydrolysis by-product of NaBH 4 is NaBO 2 ·xH 2 O, and the real-time hydrolysis reaction is given as follows [53]:

3
That is, NaBH 4 could produce four equivalents of hydrogen through the hydrolysis process. Recently, Appiah-Ntiamoah et al.
[54] synthesized a novel catalyst with a core-shell structure, where Co was loaded upon Fe 3 O 4 @C "active" support. The unique properties of the "active" Fe 3 O 4 @C promoted a synergistic catalytic reaction involving Co, Fe 3 O 4 , and C during NaBH 4 hydrolysis as shown in Fig. 1, delivering a hydrogen generation rate up to 1746 mL (g min) −1 . Holbrook [55] believed that the hydrolysis mechanism with transition catalyst could be classified into five steps as shown in Fig. 1a However, Fe exposed in the pores and Co could also from Fe 3 O 4 @C-Co to catalyze hydrogen release according to the mechanism proposed by Pena-Alonso via a synergistic effect as shown in Fig. 1b where hydrogen is firstly produced in the 3 rd step, and the entire reaction path is shortened. Moreover, the reusability and stability of Fe 3 O 4 @C-Co composite were investigated via successive catalytic runs, and there was negligible loss in the amount of H 2 generated after 5 runs. The Fe 3 O 4 @C-Co composite showed high recyclability performance in catalytic activity and structural integrity, signifying its real-life application prospects. Furthermore, Patel's team [56] doped with various transition metals in Co-B-based binary catalysts and explored the hydrolysis properties as shown in Fig. 2. The Co-B-based ternary or quaternary catalysts may display better catalytic activity than binary catalysts. Table 3 summarizes recent advances on Cobased catalysts and their catalytic performances for NaBH 4 hydrolysis. More information and applications about hydrogen production from NaBH 4 for fuel-cell systems could be referred from a recent review [57]. AB is considered as a leading contender in promising chemical hydrogen-storage materials for various applications due to its high hydrogen density (19.6 wt%) and high stability both in solid state and solution under ambient conditions, as well nontoxicity and high solubility [33,73]. It can release three equivalents of hydrogen vis thermolysis, but the third-step dehydrogenation requires more than 1200 ℃. Similarly, the developed catalysts for the hydrolysis of NaBH 4 , such as noble metal-based NPs and Co-based NPs deposited on supports, can also impel AB hydrolysis as well. Li et al. [74] synthesized CVD-Ni/ZIF-8 by chemical vapor deposition, which could promote ammonia borane to release 3 equivalents of hydrogen in 13 min. Later, Wang et al. [75] deposited Ni NPs in ZIP-8 by NaBH 4 reduction method, which promoted AB to complete reaction in 0.3 M NaOH solution within 5 min with a TOF value of 85.7 mol H2 mol cat −1 min −1 . Interestingly, it was found that H + in the acid could slow the reaction, and a certain concentration of OH − remarkably improved hydrogen evolution. Therefore, a switch was designed to control hydrogen supply by adjusting the pH value of the solution. In addition, the reusability of the nanocatalyst NiNPs/ZiF-8 was examined by the continuous addition of a new proportion of AB aqueous solution when the previous run was completed. It was found that the activity of NiNPs/ZiF-8 was essentially retained until the fifth run and there was almost no loss in the amount of H 2 generated during the cycling test. He et al. [76] also got the same result that OH − in aqueous solution is crucial in determining the hydrolysis kinetics of AB through the kinetic isotope effect (KIE). Wang et al. [77] further explored the hydrolysis mechanism of Ni 2 Pt@ZIF-8 and found that OH − acted as a catalyst promoter, making the NP more electron-rich, which could favor the oxidative addition of water, as shown in Fig. 3. The presence of OH − boosts H 2 evolution that becomes 87 times faster than in its absence with Ni 2 Pt@ZiF-8. The kinetic isotope effects using D 2 O showed that cleavage by oxidative addition of an O-H bond of water onto the catalyst surface is the rate-determining step of this reaction, enabling significant progress in catalyst design toward convenient H 2 generation from hydrogen-rich substrates in the near future. Although the introduction of the catalyst can enhance the reaction to some extent, the difficulty and cost in recovering the catalyst, however, is an issue. Therefore, it is required to develop catalyst-free hydrogen supply systems from lightmetal-based materials. Recently, Ouyang and co-workers investigated the non-catalytic hydrolysis of some borohydrides [36,78,79]. For instance, they found that the hydrogen generation rate for NaBH 4 hydrolysis could be accelerated by doping with ZnCl 2 without involving catalysts. It was  found that NaBH 4 -35 wt% ZnCl 2 achieved the optimal hydrogen yield of 1964 mL g −1 H 2 with a considerable hydrogen production rate of 1124 mL g −1 within only 5 min [79].
Interestingly, they observed the existence of NaZn(BH 4 ) 3 ( Fig. 4) after ball milling the mixture of NaBH 4 -ZnCl 2 and further investigated the hydrolysis performance of pure , with the larger the ligand and the higher the denticity, and the smaller amount of B 2 H 6 being produced [81].
As is well known, the hydrogen generation performance would deteriorate markedly followed by temperature decrease. To solve this issue, alcoholysis and alcoholysis/hydrolysis composite hydrogen generation systems for NaBH 4 have been developed [37,[82][83][84][85]. For example, hydrogen release from NaBH 4 in ethylene glycol/water solutions in the presence of CoCl 2 catalyst could be quickly launched even at -10 ~ 20 °C, fulfilling 100% of fuel conversion within only a few minutes. What's more, the hydrogen density of the alcoholysis/ hydrolysis composite system with optimized composition may reach 4 wt%. This demonstrated that a superior-performance hydrogen generation system with a wide range of operational temperature may be developed for practical hydrogen source for mobile/portable applications [37].
For LiBH 4 hydrolysis, the catalyst-free hydrolysis reaction never surpasses 50% of its theoretical yield due to the low solubility of the LiBO 2 -based by-product in water that deposits on LiBH 4 and limits the full utilization of the hydride [86]. Kojima et al. [87] reported that the hydrogen densities increased with the increase in the dropped water (H 2 O/LiBH 4 ) and followed by a reduction. These densities may show maximum values at H 2 O/LiBH 4 = 1.3. To enhance the sluggish kinetics and low conversion efficiency for LiBH 4 hydrolysis, a series of strategies have been adopted toward H 2 release at approximately a stoichiometric equivalent, including the hydrolysis system of LiBH 4 doped with multiwalled carbon nanotubes (MWCNTs) [88] or diethyl ether addition [89], the non-catalytic hydrolysis of LiBH 4 /NH 3 BH 3 composite system [90], and the catalytic hydrolysis reaction system of LiBH 4 solution over nano-sized platinum dispersed on LiCoO 2 (Pt-LiCoO 2 ) [91], etc. Considering the affordability and sustainability, it is imperative to develop low-cost and nonnoble metal catalysts that hold similar activity and stability with noble metals in the conversion and utilization of LiBH 4 hydrolysis system. Recently, Zhu's group [92] firstly adopted the transition-metal chlorides (CoCl 2 , NiCl 2 , FeCl 3 ) to promote the hydrolysis behaviors of LiBH 4 . Among the above catalysts, CoCl 2 showed faster hydrogen kinetics, delivering a hydrogen generation rate ranging from 421 to 41,701 mL H 2 min −1 g −1 with a maximum conversion of 95.3%. These values are much higher than the value of 225 mL H 2 min −1 g −1 with Pt-LiCoO 2 . Moreover, NH 3 was introduced to tailor the uncontrollable kinetics of LiBH 4 by forming its ammoniates (LiBH 4 ·xNH 3 , x = 1, 2, 3). In the presence of CoCl 2 , LiBH 4 ·xNH 3 could stably release over 4300 mL H 2 g LiBH4 −1 Fig. 4 XRD patterns of a NaBH 4 -ZnCl 2 composites ball-milled for different durations. Reprinted with permission from Ref. [79], Copyright 2017 Elsevier, and b purified NaZn(BH 4 ) 3 and its standard PDF card. Reprinted with permission from Ref. [36], Copyright 2017 Royal Society of Chemistry with a hydrogen capacity of ~ 7.1 wt% and a H 2 yield of 97.0%, while it reacts with a stoichiometric amount of H 2 O. However, the difficulty in regenerating the utilized LiBH 4 and the associated high cost hamper their large-scale applications. In the near future, developing convenient and economical methods for LiBH 4 regeneration is a linchpin, as it acts as hydrogen carrier in off-/on-board applications.

Hydrogen Production via Hydrolysis of Mg-based Alloys or Its Hydrides
Compared to borohydrides, the hydrolysis from light metals and metal hydrides for down-to-earth hydrogen supply has a number of advantages, including low-cost, abundant element contents, environmentally benign products of oxidation, etc. [38,[93][94][95]. Generally, it is widely accepted that the hydrolysis reaction of Mg or MgH 2 is rapidly interrupted by a passive Mg(OH) 2 layer deposited on the surface of Mg-based materials, leading to poor hydrolysis performance. To date, numerous methods, such as ball milling, alloying, aqueous solution modification or catalysis [96][97][98][99], have been applied to enhance the sluggish kinetics. Recently, Ouyang' group [100] synthesized flower-like MoS 2 spheres via a one-step hydrothermal method. The as-prepared MoS 2 composes of many uniform spherical nanoparticles (Fig. 5), resulting in larger surface areas than its bulk counterpart. The Mg-10 wt% MoS 2 composite could release over 90% of theoretical hydrogen capacity in 1 min. Also, they investigated the catalytic effects of the transition metal Mo and its compounds (MoS 2 , MoO 2 , and MoO 3 ) upon hydrolysis of Mg in seawater [99]. The results showed that the distribution of MoS 2 catalyst in the Mg matrix became increasingly homogeneous with the increase in milling time (Fig. 6). The unique structure and uniformly dispersed MoS 2 could significantly accelerate the hydrolysis process of Mg. Moreover, the reusability and stability of MoS 2 were investigated via successive catalytic runs. As shown in Fig. 7, there was a slight drop in the amount of H 2 generated after 5 runs, and the catalytic activity of retrieved MoS 2 was completely retained without decrease in H 2 evolution rate. They believed that the markedly enhanced activity could be attributed to the synergistic effect of grinding and the galvanic corrosion between Mg-and Mo-based additives.
In addition to doping catalysts, alloying and ball milling have been proved to be effective means to enhance the hydrolysis performance of Mg. Ouyang et al. [97,[102][103][104][105][106] systematically studied the hydrolysis behaviors of Mg-RE alloy and its hydrides. They found that rareearth elements could facilitate the hydrogen absorption of Mg-based alloys, resulting in higher hydrogen yields for the hydrolysis of hydrogenated Mg-RE. Ma et al. [107] revealed that Ni could promote the hydrogenation of CaMg 1.9 Ni 0.1 under room temperature, as opposed to 450 °C for pure CaMg 2 . Thus, the H-CaMg 1.9 Ni 0.1 could achieve a hydrogen yield of 1053 mL g −1 in only 12 min, Reprinted with permission from Ref. [100]. Copyright 2017 Elsevier approximately twice as much as that of CaMg 1.9 Ni 0.1 . In this regard, they doped a small amount of Ni toward CaMg 2 via ball milling [108]. The hydrogen yield of the hydrogenated CaMg 2 -0.1Ni sample could increase from 853 to 1147 mL H 2 g −1 in 5 min with hydrogenation durations ranging from 0.5 to 1.5 h. On the other hand, Ouyang et al. [109] found that the hydrolysis properties of Mg can be greatly enhanced with the addition of expanded graphite by plasma-assisted milling. The obtained Mg-graphite composite could release 614.3 mL H 2 g −1 in 25 min with a hydrolysis conversion rate of 83.5%. They also synthesized refined hydrogenated MgLi (H-MgLi) by reactive ball milling [110], producing ~ 15.8 wt% hydrogen in 5 min. As same as NaBH 4 , the hydrogen generation behaviors of Mg would deteriorate markedly followed by decreased temperature. To remove the troublesome freezing issue of the water solution system in low-temperature conditions, Ouyang et al. [111] adopted pure methanol, methanol/water and methanol/ethanol solutions to react with CaMg 2 alloy and its hydrides for hydrogen generation. The as-prepared CaMg 2 could generate 858 mL H 2 g −1 within only 3 min at room temperature, while it reacted vigorously with methanol, as opposed to a low hydrogen yield with ethanol and water (395 and 224 mL H 2 g −1 within 180 min, respectively). Even at − 20 °C, there was still over 600 mL H 2 g −1 released at a conversion rate of 70.7% within 100 min for methanolysis, demonstrating its  Aqueous solution modification is also an effective strategy to tailor the hydrogen behaviors of Mg-based materials. In real application, large excess of water is required to ensure complete hydrolysis of Mg, resulting in significant capacity loss. The formation of insoluble Mg(OH) 2 enables simple separation and repeated using of water, which minimizes the hydrogen capacity loss caused by the excessive water. In this regard, Li et al. [112] solved the issue by using MgH 2 nanoparticles together with the promotion effect of MgCl 2 solution. A near-theoretical amount of H 2 (1820 mL g −1 ) was released within 20 min in 1 M MgCl 2 solution without any pretreatment of the MgH 2 nanoparticles (800 nm). By separating Mg(OH) 2 through filtration and recycling the MgCl 2 solution, the hydrogen capacity of this system may approach the theoretical value of 6.45 wt% with continuous MgH 2 and water feeding. Recently, Tan et al. [113] reported that the hydrolysis performance of Mg 2 Si could be notably improved by using NH 4 F solution. The fluorine ion was introduced to restrain the release of silanes during the hydrolysis reaction of Mg 2 Si. Due to its high chemical affinity to silicon ion, it is possible for F − to break the Si-H bond and form H 2 and SiF 6 2− in aqueous solution. As the concentration of the NH 4 F solution increased to 13.0%, the hydrogen yield of Mg 2 Si reached the maximum, producing 616 mL H 2 g −1 in 30 min at 25 °C. The L.G. Sevastyanova et al. [101] systematically explored the effect of salt solutions and the transition metals on magnesium hydrolysis (Fig. 8) and found (1) the NH 4 Cl solution exhibited the fastest initial reaction rate, but the conversion yield reached the maximum in NaCl solution, (2) aqueous solutions of alkaline or alkali earth metal chlorides at a salt content over 3 wt% would effectively improve the hydrolysis performance (the optimal amount being 4-15 wt%), (3) the transition metals can also cause reduction of the hydrogen yield if it is over 10 wt%. Correspondingly, Table 4 lists the varieties of some Mg-based materials and their hydrolysis properties. Nearly all hydrolysis materials enable the solution concentration being at least 3 wt% and the amount of oxidation addition not exceeding 10 wt%.

Hydrogen Production via Hydrolysis of Al-based Alloys or Its Hydrides
The distribution of aluminum is more abundant than magnesium, being third only to oxygen and silicon. Aluminum is a safe and cheap metal as well as electrochemically active element; thus, it may be a more appropriate candidate for the process of hydrogen production [31,128]. The catholic use of aluminum is for the applications in batteries [129], like the aluminum-air battery that has an aluminum-based anode. While this aluminum-based battery has potential prospect in electric In addition, OH − can dissolve the passive layer and form AlO 2 − to generate hydrogen even at room temperature. Taking the most commonly used NaOH solution as an example, the hydrogen generation is proposed as follows [130]: (2) 2Al + 6H 2 O + 2NaOH → 2NaAl(OH) 4 + 3H 2 (3) NaAl(OH) 4 → NaOH + Al(OH) 3 Initially, the hydrogen generation reaction consumes sodium hydroxide, but when the NaAl(OH) 4 concentration exceeds the saturation limit, it leads to the NaOH regeneration process accompanying aluminum hydroxide formation. Therefore, only water is consumed during the whole hydrogen supply as shown by the reactions (4 and  [127] 5), and the hydrolysis by-products are the non-polluting bayerite (Al(OH) 3 ) and boehmite (AlOOH) [2,131,132]. Though the addition of OH − is considered as the simplest and the most effective approach for promoting the Al/ H 2 O reaction [133], the use of an aqueous NaOH solution causes corrosion of system apparatus. Therefore, novel technologies that enable a combination of a minimized quantity of NaOH and rapid H 2 generation kinetics are highly desirable. Wang et al. [134,135] found that a combined usage of sodium hydroxide (NaOH) and sodium stannate (Na 2 SnO 3 ) can simultaneously address the Al/ H 2 O reaction kinetics and alkali corrosion problems. The addition of a small amount of Na 2 SnO 3 causes a remarkable decrease of NaOH concentration without compromising the hydrogen generation performance of the system. In comparison with the traditional Al/H 2 O system using aqueous NaOH solution, the new system exhibits a series of advantages in hydrogen generation performance, manipulability and adaptability; all are relevant to the development of practical aluminum-based hydrogen generation systems for mobile or portable applications. Notably, aluminum can be regenerated from the by-products by mature industrial technologies, the Bayer process [136] from bauxite ore (AlOOH) and the Hall-H′eroult process [137] from alumina. Since Belitskus [130] first proposed the Al-water reaction to provide hydrogen in the 1970s, crucial efforts have been put into action to overcome the hydrolysis obstacle caused by the formation of the Al 2 O 3 layer. Ball milling, as a frequently used method for increasing the hydrolysis performance of Mg-based materials, has proved to be effective for Al-based materials [138][139][140][141][142]. Yan et al. [140] milled an Al-10 mol% LiH-10 mol% KCl mixture for 10 h and obtained a hydrogen yield of 97.1% in 10 min at 60 ℃. The effects of metal chlorides to aluminum were similar to magnesium in hydrolysis. Firstly, chlorides can decrease the grain size during ball milling, and secondly, chlorides can also raise galvanic corrosion of magnesium or aluminum. Thirdly, Cl − could damage the Mg(OH) 2 or Al(OH) 3 layer. Except mechanical activation by ball milling, torsional pressure and ultrasonic assistance, chemical activation of aluminum, such as by alloying, is also applicable. Originally, mercury was utilized for chemical activation of aluminum [143]. While mercury is a toxic substance and is not recommended for use in large scale, the new method of alloying to activate aluminum for aluminum-water reaction is sought after [144][145][146][147].
It has been confirmed that the hydrolysis properties have been enormously boosted up by alloying low melting point metals (LMPM) such as Ga, In, Sn and Zn with Al. Bulychev et al. [144] investigated the hydrolysis properties of aluminum alloy containing different accounts of LMPM. They found that the hydrogen supply virtually did not proceed without the presence of gallium, and the absence of indium in the alloy also led to a sharp decrease in the hydrolytic ability. But this alloy showed a terrible stability even stored under an inert atmosphere or in vacuum. They believed that this might be related to the presence of dispersed solid phases and a liquid phase (eutectic) distributed over the grain boundary space (Fig. 9). Parmuzinaa [145] held a point of view that the liquid eutectics based on gallium brought about eutectic penetration into aluminum grain boundaries, which destructed the inter-crystal contacts and resulted in the formation of aluminum monocrystal powders covered by eutectic thin film. Dong et al. [148] demonstrated that the presence of a liquid phase in the Al-Ga and Al-Ga-In-Sn alloys was decisive for the alloys to react with water and produce H 2 with an average yield of 83.8% in all 80 trials. The reaction temperature correlated well with the reported Al-Ga binary eutectic melting point of 26.6 ℃ and Ga-In-Sn ternary eutectic melting point of 10.7 ℃. When they changed the reaction temperature to make the alloys completely solid without liquid phase distribution, no hydrogen was produced. Interestingly, in many experiments, it was found that at 20-30 ℃, hydrogen generation from Al-Ga alloys stopped after only a certain extent [147,[149][150][151][152][153], but the reaction would resume if the system temperature was raised to resuscitate the liquid eutectic phase.
However, compared to the binary and ternary systems, the activity of the quaternary Al-Ga-In-Sn alloy was greatly improved and it could be fully reactive even at room temperature, indicating that the presence of a liquid eutectic phase in the Al-based alloy was essential. Liquid In 3 Sn and InSn 4 were indeed observed in the Al-Ga-In-Sn quaternary system [154]. Qian Gao et al. [150] compared the hydrolysis properties of Al-Ga-InSn 4 and Al-Ga-In 3 Sn alloys (Fig. 10). They concluded that the eutectic reaction of Al with InSn 4 was crucial, and Al could transfer from Al grains to intermetallic compounds to react with water continuously. Recently, Lu et al. [155] investigated the hydrolysis performance and activation mechanism of Al  [150]. Copyright 2015 Elsevier 85wt%-Ga 68.5 In 21.5 Sn 10 alloy (Fig. 11). Combined with EDX analysis, the marked regions in the SEM images shown in Fig. 11c, d could be identified as In 3 Sn phase (A), Al-Ga solid solution (matrix B), and C GaInSn liquid alloy (GIS) (C) and Al-Ga solid solution (matrix D). Especially, they emphasized the promotion of Al-water reaction with respect to the presence of low-melting eutectic liquid alloy GIS [156] and the In 3 Sn phase. The Al-water reaction can be summarized in two steps. Firstly, a certain amount of Al atoms, which are solvated in the GIS and In 3 Sn phases, are active and could react with the water freely. Secondly, the local temperature of the reaction site evidently increases due to a highly exothermic reaction, which can further promote the transportation of Al atoms to the interface and then react with water continuously.
It has been proven that alloying Al with low melting point metals is an effective approach to inhibit the formation of a coherent passivation layer and promote the hydrolysis kinetics. Liu et al. [153] tested Al on four different liquid alloys to produce hydrogen. It was found that aluminum completely dissolved in liquid GaIn 10 in 4 min, and the liquid metal surface remained shiny, meaning that GaIn 10 was stable during entire reaction process (Fig. 12). They designed pure Ga as a reactor and successively inlaid Al into it, and the process still achieved a great conversion yield after 5 times cycle without any dead-weight issues involved in system. Table 5 summarizes the varieties of some Al-based materials and their hydrolysis properties.

Hydrogen Production via Hydrolysis of Al-based Alloys or Its Hydrides
Hydrolysis of metals or metal hydrides is a highly exothermic reaction; full hydrolysis of 1 mol aluminum generates where the hydrolysis of Al was decoupled into a battery by pairing an Al foil with a hydrogen-storage electrode. In the hydrolysis battery, 8-15% of the hydrolysis heat was converted into usable electrical energy, leading to much higher energy efficiency compared to that of direct hydrolysis-H 2 fuel cell approach. The schematic illustration of the hydrolysis battery is shown in Fig. 13, where the hydrolysis reaction of Al is a redox reaction. Thus, Al foil and a Pd-capped YH 2 thin film were used as the anode and the cathode, respectively. As the hydrolysis battery was activated, the YH 2 -Pd electrode would convert into YH 2+x phase (x ≈ 1, the hydrogenated state), attaining the electrons flowed from Al. Desirably, the higher utilization of hydrolyzed thermal energy and more efficient kinetics controllability require further investigation. Reprinted with permission from Ref. [153]. Copyright 2016 Elsevier

Recent Advances in Regeneration Process of Borohydrides from Hydrolysis By-products
It has been demonstrated that hydrogen supply from NaBH 4 hydrolysis is a potential system for hydrogen generation. However, the hydrolysis reactions are plagued by irreversibility, and the resulting high-cost strikingly restrains the large-scale practical applications of these hydrolytic materials. Recently, Ouyang et al. developed a facile and economical method for NaBH 4 regeneration by recycling its real-time hydrolysis products (NaBO 2 ·2H 2 O and NaBO 2 ·4H 2 O) for the first time without hydrides input [182,183]. This may provide important insights for retrieving other hydrogen supply irreversible systems with high efficiency, such as LiBH 4 or LiAlH 4 production. Recently, more attentions were shifted to the preparation and regeneration of NaBH 4 for achieving its large-scale practical applications. In the industry of chemical production, NaBH 4 is usually synthesized by the Brown-Schlesinger process [184] and the Bayer process [185]. The synthesis Though the above technologies are mature, they are unsuitable for NaBH 4 hydrolysis applications because of the fancy raw materials (Na or NaH) and high-energy consumption processes. Thus, suitable methods for NaBH 4 synthesis have been developed with low-cost raw materials instead of sodium or its hydride. MgH 2 was used to react with anhydrous borax (Na 2 B 4 O 7 ) for NaBH 4 synthesis by ball milling method at room temperature (RT). Here, the NaBH 4 yield may reach 78% with the addition of Na 2 CO 3 [186]. This method introduces not only a novel reducing agent (MgH 2 ), but also an energy-efficient strategy for NaBH 4 synthesis. Enlightened by this, RT ball milling became attractive in NaBH 4 synthesis studies, by which Na and MgH 2 could react with B 2 O 3 with the NaBH 4 yield of ~ 25% [187]. As Na was replaced by safe and cheap NaCl, NaBH 4 could also be produced [188]. Subsequently, high-pressure milling was also developed to synthesize NaBH 4 . For instance, the synthesis of NaBH 4 could be achieved by ball milling the hybrid of NaH and MgB 2 under 120 bar H 2 pressure with the yield of ca. 18% [189].
Importantly, considering the sustainability and environmental friendliness, NaBH 4 regeneration from NaBO 2 ·xH 2 O, the hydrolysis by-product, is appealing as the regeneration and hydrolysis form a recycling system. Since Kojima et al. [190] firstly achieved the regeneration of NaBH 4 via reacting MgH 2 with NaBO 2 under 70 bar H 2 pressure at 550 °C with a ~ 97% yield of NaBH 4 , NaBO 2 has become the main research object for NaBH 4 regeneration. Later, the thermochemistry process was substituted by RT ball milling because of high energy consumption under extreme conditions (high reaction temperature and high hydrogen pressure). Hsueh et al. [191][192][193] adopted MgH 2 to react with anhydrous NaBO 2 by ball milling under inert atmosphere. The conversion yields of NaBH 4 were > 70%, which indicated that ball milling is advisable for the reaction between MgH 2 and NaBO 2 . Recently, Ouyang et al. [182,183,194] successfully achieved the regeneration of NaBH 4 (Fig. 14) by applying the real hydrolysis by-product (NaBO 2 ·2H 2 O and NaBO 2 ·4H 2 O) as raw material with Mg-based reducing agents (Mg, Mg 2 Si and Mg 17 Al 12 ) at ambient conditions, where the troublesome heat-wasting process to obtain NaBO 2 using a drying procedure at over 350 °C from NaBO 2 ·xH 2 O was omitted. The regeneration yield of NaBH 4 may reach 78%. Significantly, the charged H − stored in NaBH 4 was completely converted from protonic H + in water bound to NaBO 2 . Particularly, it was found that the regeneration yield of NaBH 4 was up ~ 90%, while MgH 2 acted as reducing agent [195]. Recently, Ouyang et al. [196] found that high-energy ball milling of magnesium (Mg) with the mixture of Na 2 B 4 O 7 ·xH 2 O (x = 5, 10) and Na 2 CO 3 (obtained by exposing an aqueous solution of NaBO 2 to CO 2 ) resulted in the formation of NaBH 4 with a high yield of 80% under ambient conditions. In their approach, after ball milling for just 10 min, only B 4 O 5 (OH) 4 2− was detected ( Fig. 15 (1) Fig. 15(2, 4)). In the following step, the cleavage of (B)-O-H (O bonded with sp 2 boron) formed the H 2 BOH intermediate ( Fig. 15 (5) [197]. By replacing the majority of MgH 2 with low-cost Mg, an attractive yield of 78.6% was obtained. These reactions occurred without extra hydrogen gas inputs, meaning the low-cost and sustainable regeneration. More detailed information toward NaBH 4 regeneration can be found in a recent review [198].
In the past few years, numerous reports have been published dealing with the regeneration of NaBH 4 -based spent fuels (NaBO 2 ·xH 2 O or Na 2 B 4 O 7 ·xH 2 O), whereas the studies upon the regeneration of LiBH 4 -based spent products were quite limited. Bilen et al. [199] firstly utilized MgH 2 and LiBO 2 to synthesize LiBH 4 by means of mechano-chemical reaction. Instead of its elements, the hydrolytic product of LiBH 4 (LiBO 2 ) was adopted as raw material, which may greatly reduce the application cost of LiBH 4 by recycling spent products. However, the tricky heating-wasting process for obtaining anhydrous LiBO 2 at elevated temperature (~ 470 ℃) is inevitable [200]. Stimulated by the successful regeneration of NaBH 4 , Ouyang et al. [201] reported a facile method to regenerate LiBH 4 by ball milling its real hydrolysis by-product (LiBO 2 ·2H 2 O) with Mg under ambient conditions with a yield of ~ 40%. This method bypasses Reprinted with permission from Ref. [198]. Copyright 2018 MDPI Fig. 15 Proposed reaction mechanism between Mg, Na 2 CO 3 , and Na 2 B 4 O 7 ·10H 2 O to form NaBH 4 . Reprinted with permission from Ref. [196]. Copyright 2020 Wiley Online Library the energy-intensive dehydration procedure to remove water from LiBO 2 ·2H 2 O and does not require high-pressure H 2 gas, therefore leading to much reduced costs. Interestingly, it is expected to effectively close the loop of LiBH 4 regeneration and hydrolysis, enabling a wide deployment of LiBH 4 for hydrogen storage and application. As same as NaBH 4 or LiBH 4 , KBH 4 could also be synthesized by mechanochemical reaction. Bilen et al. [202] successfully synthesized KBH 4 by ball milling KCl, MgH 2 , and B 2 O 3 in a milling reactor. By tailoring the reactant ratio (MgH 2 /KCl) and the milling time, the yield of the reaction reached maximum values, whereas the definite value was not given.
Application of borohydride hydrolysis is limited by limit of their effective regeneration. Though the great achievements have been attained in the regeneration of NaBH 4 , simplifying synthetic routes and increasing regeneration yield that enable the efficient energy storage and conversion of the "one-pass" hydrogen fuel are two critical targets for large-scale applications. For the anhydrous NaBO 2 recycling, it was found that MgH 2 has the best reducing effect. However, its high cost, resulting from the high hydrogenation temperature of Mg, limits the application of such methods. For the direct NaBH 4 -based spent fuels (NaBO 2 ·xH 2 O or Na 2 B 4 O 7 ·xH 2 O), they can be reduced to NaBH 4 with different reductants (MgH 2 , Mg, or Mg 2 Si) via ball milling, and the highest yield of NaBH 4 may reach 93.1%. Moreover, this process, that uses hydrated metaborate or borax, bypasses the energy-intensive dehydration procedure to obtain anhydrous NaBO 2 or Na 2 B 4 O 7 without the requirement of high-pressure H 2 gas; therefore, it could lead to much reduced costs. The boron compounds bound with water may act as hydrogen sources stored in NaBH 4 instead of MgH 2 . As expected, lowcost waste Al or Al-based alloys may be attractive for achieving the regeneration of NaBH 4 via ball milling, enabling a wide deployment of NaBH 4 for hydrogen applications. This strategy may provide a new conceptual basis for the development of LiBH 4 production or other borohydrides.

Conclusions
The present review narrates the recent research progress of hydrogen generation via hydrolysis or alcoholysis by light metal-based materials for potential off-or on-board hydrogen applications, predominantly including borohydrides and Mg-/Al-based materials. The mechanisms of catalytic borohydride hydrolysis and activation of aluminumbased materials via alloying are depicted. Various common methods such as ball milling, catalysis, alloying, and solution modification for improving hydrolysis kinetics are described in detail. In summary, ball milling can refine the particles size to increase reaction activity, but it is unsuitable for practical use in the transportation and storage of the powder. For the hydrolysis of borohydrides, the Co-Bbased materials are commonly considered as reactive as noble metals and much more cost-effective. Other metals and Co may form a synergistic effect in Co-B-based ternary or quaternary catalysts. The (catalyzed) hydrolysis of Mg-/Al-based materials has been summarized. The alcoholysis operated at low temperatures can supply hydrogen for special subzero circumstances. The cost is substantially decreased in regeneration of sodium borohydride, making hydrolysis/alcoholysis more practical for on-site hydrogen applications or fuel cells with the advantages of mild operating temperature, environmentally benign by-products, precise controllable of hydrogen release and high-purity H 2 . However, the major exothermicity of hydrolysis reactions has not received enough attention, which is even more than the hydrogen energy. The improvement of controllability of hydrolysis helps to design novel on-board hydrogen supply systems.
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