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

Highlights

• 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.

Abstract

As an environmentally friendly and high-density energy carrier, hydrogen has been recognized as one of the ideal alternatives for fossil fuels. One of the major challenges faced by “hydrogen economy” is the development of efficient, low-cost, safe and selective hydrogen generation from chemical storage materials. In this review, we summarize the recent advances in hydrogen production via hydrolysis and alcoholysis of light-metal-based materials, such as borohydrides, Mg-based and Al-based materials, and the highly efficient regeneration of borohydrides. Unfortunately, most of these hydrolysable materials are still plagued by sluggish kinetics and low hydrogen yield. While a number of strategies including catalysis, alloying, solution modification, and ball milling have been developed to overcome these drawbacks, the high costs required for the “one-pass” utilization of hydrolysis/alcoholysis systems have ultimately made these techniques almost impossible for practical large-scale applications. Therefore, it is imperative to develop low-cost material systems based on abundant resources and effective recycling technologies of spent fuels for efficient transport, production and storage of hydrogen in a fuel cell-based hydrogen economy.

1 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 (PEMFCs) 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% H₂ [12], just satisfying the present target of DOE. Ammonia (NH₃) is also highly valued as a potential hydrogen storage option except compressed H₂ gas, owing to its high hydrogen density (17.8 wt% and 0.120 kg H₂ L⁻¹ for gravimetric and volumetric H₂ density), low storage pressure and stability for long-term storage as well as high flexibility in its utilization [13]. In this regard, NH₃ can fulfill the demand to store the energy in time (stationary energy storage) and in space (energy export and import). However, NH₃ encounters high energy demand in both synthesis and decomposition for indirect utilization by the release of H₂. In case of liquid H₂, in spite of a much higher volumetric density (0.071 kg H₂ 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 H₂, 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 H₂ storage for light-duty fuel cell vehicles [11]

Table 2 A comparison between some typical lightweight materials and hydrogen mediums [16, 17]

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 AB₂ and AB₅ types (ZrFe₂, LaNi₅, 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 magnesium-based materials (MgH₂ 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 MgH₂ and 18.5 wt% for LiBH₄ even exceed the value for on-board applications set by DOE. Recently, Shui’s group [27] synthesized a multilayered Ti₂CT_x (T is a functional group) stack by incomplete hydrofluoric acid (HF) etching, and the as-prepared Ti₂CT_x showed an unprecedented hydrogen uptake of 8.8 wt% H₂ at room temperature and 60 bar H₂, 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 ZrCl₄ is an effective catalyst to considerably reduce the dehydrogenation temperature and activation energy for LiBH₄ [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 B₂H₆ [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 H₂ or gas-state hydrogen carriers that need further development and construction in infrastructures, such as the NH₃/H₂ pipelines, H₂/NH₃ 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, NaBH₄, 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 available storage and transportation facilities and their regulation can be well utilized to increase the readiness for the adoption of light metal-based materials.

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 metal-based 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 NaBH₄ regeneration process from hydrolysis by-product are discussed. NaBH₄ is considered as the most potential hydrolysable material.

2 Hydrogen Generation from Hydrolysis or Alcoholysis

The typical hydrolytic materials include metals/hydrides, ammonia borane (NH₃BH₃, denoted as AB) and borohydrides. Hydrogen supply from NaBH₄ 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 H₂ 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.

2.1 Highly Efficient Catalytic and Non-catalytic Alcoholysis/Hydrolysis of Borohydrides

Extensive efforts have been devoted to exploring highly efficient hydrolysis of borohydrides (NaBH₄, Mg(BH₄)₂, LiBH₄, etc.) or AB due to their excellent hydrogen storage capacities ^39–41. For hydrogen application in fuel cells, if the water produced in the fuel cell part is redirected to LiBH₄, then the H₂ generation capacity may increase to 37.0 wt% [42]. Compared with the expensive LiBH₄, NaBH₄ with a 21.1 wt% H₂ generation capacity (the water produced in the fuel cell part is recycled to react with NaBH₄ 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 M₃B (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 NaBH₄ is NaBO₂·xH₂O, and the real-time hydrolysis reaction is given as follows [53]:

$${\text{NaBH}}_{4} + \left( {2 + x} \right){\text{H}}_{2} {\text{O}} \to {\text{NaBO}}_{2} \cdot x{\text{H}}_{2} {\text{O}} + 4{\text{H}}_{2}$$

(1)

That is, NaBH₄ 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 Fe₃O₄@C “active” support. The unique properties of the “active” Fe₃O₄@C promoted a synergistic catalytic reaction involving Co, Fe₃O₄, and C during NaBH₄ 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. Firstly, the chemisorption of BH₄⁻ on the metal atom site produces M-BH₃ and M-H (step 1–3). Then, an electron from M-BH₃ is transferred to the M site and BH₃ is discarded, so the electronegative M site attracts H⁺ in water to form a new M-H. And a consumption of the two M-H can release one H₂ molecule, then the BH₃ legacy and OH– will form BH₃(OH)⁻ (step 4–5). Subsequently, the stable intermediate BH₃(OH)⁻ successively provides three active hydrogens, which will attack three H₂O to form BOH₄⁻ finally and release 3 mol of H₂ (step 5–6). However, Fe exposed in the pores and Co could also from Fe₃O₄@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 Fe₃O₄@C–Co composite were investigated via successive catalytic runs, and there was negligible loss in the amount of H₂ generated after 5 runs. The Fe₃O₄@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 Co-based catalysts and their catalytic performances for NaBH₄ hydrolysis. More information and applications about hydrogen production from NaBH₄ for fuel-cell systems could be referred from a recent review [57].

Fig. 1

Copyright 2019 Elsevier

Schematic illustration for NaBH₄ hydrolysis on a Fe₃O₄@C–Co and, b Fe₃O₄@C–X–Co (X = temperature). Reprinted with permission from Ref. [54].

Fig. 2

Copyright 2010 Elsevier

Hydrogen generation yield as a function of reaction time obtained by hydrolysis of alkaline NaBH₄ (0.025 M) with a Co–Ni–B, b Co–Fe–B, c Co–Cu–B, d Co–Cr–B, e Co–Mo–B, and f Co–W–B with different χ_M values (where M = Ni, Fe, Cu, Cr, Mo, and W). Insets show the maximum H₂ generation rate (R_max) as a function of χ_M. Reprinted with permission from Ref. [56].

Table 3 Comparison of some Co-based catalysts and their catalytic performance for NaBH₄ hydrolysis

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 NaBH₄, 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 NaBH₄ 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 H₂ 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 Ni₂Pt@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 H₂ evolution that becomes 87 times faster than in its absence with Ni₂Pt@ZiF-8. The kinetic isotope effects using D₂O 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 H₂ generation from hydrogen-rich substrates in the near future.

Fig. 3

Copyright 2018 American Chemical Society

Proposed mechanism for the hydrolysis of AB catalyzed by NiPt@ZIF-8. Reprinted with permission from Ref. [77].

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 light-metal-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 NaBH₄ hydrolysis could be accelerated by doping with ZnCl₂ without involving catalysts. It was found that NaBH₄-35 wt% ZnCl₂ achieved the optimal hydrogen yield of 1964 mL g⁻¹ H₂ with a considerable hydrogen production rate of 1124 mL g⁻¹ within only 5 min [79]. Interestingly, they observed the existence of NaZn(BH₄)₃ (Fig. 4) after ball milling the mixture of NaBH₄-ZnCl₂ and further investigated the hydrolysis performance of pure NaZn(BH₄)₃ [36]. The results showed that NaZn(BH₄)₃ enabled the hydrogen release of 1740 mL g⁻¹ in 5 min with a total hydrogen yield up to 97%. Because the ligands neighboring the metal cations in the borohydride involve the hydrogen elimination barrier and the stability of BH₄⁻ [80], they introduced NH₃ to achieve a rate-controlled hydrogen supply of NaZn(BH₄)₃ by forming its ammoniate. Similarly, they also studied the effect of ammonia complex number on hydrogen production kinetics by Mg(BH₄)₂ hydrolysis [78]. Obviously, the hydrogen evolution behaviors could be well-controlled via altering ammonia complex number upon Mg(BH₄)₂, whereas it sacrificed hydrogen yield. The hydrogen yields of Mg(BH₄)₂·0.5NH₃, Mg(BH₄)₂·NH₃, Mg(BH₄)₂·2NH₃, Mg(BH₄)₂·3NH₃, and Mg(BH₄)₂·6NH₃ are 2376, 2029, 1780, 1665, and 1180 mL (H₂) g⁻¹, respectively. Similarly, Mg(BH₄)₂ can possess different hydrolytic behaviors when coordinated with various organic ligands (including Mg(BH₄)₂ × xE₂O, Mg(BH₄)₂ × diglyme and MgBH₄ × 3THF), with the larger the ligand and the higher the denticity, and the smaller amount of B₂H₆ being produced [81].

Fig. 4

Copyright 2017 Elsevier, and b purified NaZn(BH₄)₃ and its standard PDF card. Reprinted with permission from Ref. [36], Copyright 2017 Royal Society of Chemistry

XRD patterns of a NaBH₄-ZnCl₂ composites ball-milled for different durations. Reprinted with permission from Ref. [79],

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 NaBH₄ have been developed [37, 82,83,84,85]. For example, hydrogen release from NaBH₄ in ethylene glycol/water solutions in the presence of CoCl₂ 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 LiBH₄ hydrolysis, the catalyst-free hydrolysis reaction never surpasses 50% of its theoretical yield due to the low solubility of the LiBO₂-based by-product in water that deposits on LiBH₄ 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 (H₂O/LiBH₄) and followed by a reduction. These densities may show maximum values at H₂O/LiBH₄ = 1.3. To enhance the sluggish kinetics and low conversion efficiency for LiBH₄ hydrolysis, a series of strategies have been adopted toward H₂ release at approximately a stoichiometric equivalent, including the hydrolysis system of LiBH₄ doped with multiwalled carbon nanotubes (MWCNTs) [88] or diethyl ether addition [89], the non-catalytic hydrolysis of LiBH₄/NH₃BH₃ composite system [90], and the catalytic hydrolysis reaction system of LiBH₄ solution over nano-sized platinum dispersed on LiCoO₂ (Pt–LiCoO₂) [91], etc. Considering the affordability and sustainability, it is imperative to develop low-cost and non-noble metal catalysts that hold similar activity and stability with noble metals in the conversion and utilization of LiBH₄ hydrolysis system. Recently, Zhu’s group [92] firstly adopted the transition-metal chlorides (CoCl₂, NiCl₂, FeCl₃) to promote the hydrolysis behaviors of LiBH₄. Among the above catalysts, CoCl₂ showed faster hydrogen kinetics, delivering a hydrogen generation rate ranging from 421 to 41,701 mL H₂ min⁻¹ g⁻¹ with a maximum conversion of 95.3%. These values are much higher than the value of 225 mL H₂ min⁻¹ g⁻¹ with Pt-LiCoO₂. Moreover, NH₃ was introduced to tailor the uncontrollable kinetics of LiBH₄ by forming its ammoniates (LiBH₄·xNH₃, x = 1, 2, 3). In the presence of CoCl₂, LiBH₄·xNH₃ could stably release over 4300 mL H₂ g_LiBH4⁻¹ with a hydrogen capacity of ~ 7.1 wt% and a H₂ yield of 97.0%, while it reacts with a stoichiometric amount of H₂O. However, the difficulty in regenerating the utilized LiBH₄ and the associated high cost hamper their large-scale applications. In the near future, developing convenient and economical methods for LiBH₄ regeneration is a linchpin, as it acts as hydrogen carrier in off-/on-board applications.

2.2 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 MgH₂ 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 MoS₂ spheres via a one-step hydrothermal method. The as-prepared MoS₂ composes of many uniform spherical nanoparticles (Fig. 5), resulting in larger surface areas than its bulk counterpart. The Mg-10 wt% MoS₂ 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 (MoS₂, MoO₂, and MoO₃) upon hydrolysis of Mg in seawater [99]. The results showed that the distribution of MoS₂ catalyst in the Mg matrix became increasingly homogeneous with the increase in milling time (Fig. 6). The unique structure and uniformly dispersed MoS₂ could significantly accelerate the hydrolysis process of Mg. Moreover, the reusability and stability of MoS₂ were investigated via successive catalytic runs. As shown in Fig. 7, there was a slight drop in the amount of H₂ generated after 5 runs, and the catalytic activity of retrieved MoS₂ was completely retained without decrease in H₂ 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.

Fig. 5

Copyright 2017 Elsevier

SEM images of a bulk and b as-prepared MoS₂, c high-magnification SEM image showing a small zone of the as-prepared MoS₂. Reprinted with permission from Ref. [100].

Fig. 6

Copyright 2017 Royal Society of Chemistry

SEM images of the Mg-10 wt% MoS₂ composite milled for various durations: a 0.1 h, b 0.5 h, c 1 h, d 3 h, and e 5 h, f and g high-magnification SEM images showing a small zone of the Mg-10 wt% MoS₂ composite milled for 1 h. Reprinted with permission from Ref. [99].

Fig. 7

Copyright 2017 Royal Society of Chemistry

Cyclic curve of hydrogen evolution via hydrolysis of Mg-10 wt% retrieved MoS₂ milled for 1 h in seawater. Reprinted with permission from Ref. [99].

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 rare-earth 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_1.9Ni_0.1 under room temperature, as opposed to 450 °C for pure CaMg₂. Thus, the H-CaMg_1.9Ni_0.1 could achieve a hydrogen yield of 1053 mL g⁻¹ in only 12 min, approximately twice as much as that of CaMg_1.9Ni_0.1. In this regard, they doped a small amount of Ni toward CaMg₂ via ball milling [108]. The hydrogen yield of the hydrogenated CaMg₂-0.1Ni sample could increase from 853 to 1147 mL H₂ 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 H₂ 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 NaBH₄, 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 CaMg₂ alloy and its hydrides for hydrogen generation. The as-prepared CaMg₂ could generate 858 mL H₂ 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 H₂ g⁻¹ within 180 min, respectively). Even at − 20 °C, there was still over 600 mL H₂ g⁻¹ released at a conversion rate of 70.7% within 100 min for methanolysis, demonstrating its prominent advantage for hydrogen production, especially in winter or subzero areas.

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 MgH₂ nanoparticles together with the promotion effect of MgCl₂ solution. A near-theoretical amount of H₂ (1820 mL g⁻¹) was released within 20 min in 1 M MgCl₂ solution without any pretreatment of the MgH₂ nanoparticles (800 nm). By separating Mg(OH)₂ through filtration and recycling the MgCl₂ solution, the hydrogen capacity of this system may approach the theoretical value of 6.45 wt% with continuous MgH₂ and water feeding. Recently, Tan et al. [113] reported that the hydrolysis performance of Mg₂Si could be notably improved by using NH₄F solution. The fluorine ion was introduced to restrain the release of silanes during the hydrolysis reaction of Mg₂Si. Due to its high chemical affinity to silicon ion, it is possible for F⁻ to break the Si–H bond and form H₂ and SiF₆²⁻ in aqueous solution. As the concentration of the NH₄F solution increased to 13.0%, the hydrogen yield of Mg₂Si reached the maximum, producing 616 mL H₂ 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 NH₄Cl 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%.

Fig. 8

Copyright 2014 Elsevier

Yields of hydrogen release due to magnesium powder oxidation in the presence of alkali, alkaline earth and ammonia halides (1 g of Mg in 30 mL of salt solution). Halide concentration was maintained approximately the same: 0.85 M (curves 1–7); 0.93 M for NH₄Cl (curve 9) and 0.31 M NH₄Cl + 0.85 M NaCl (curve 8). Reprinted with permission from Ref. [101].

Table 4 Comparisons of some ball-milling Mg-based materials and their hydrolysis performances

2.3 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 vehicles, it is inhibited by the undesirable parasitic corrosion reaction or the formation of a dense oxide layer. But the reaction actually produces hydrogen.

In addition, OH⁻ can dissolve the passive layer and form AlO₂⁻ 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]:

$$2{\text{Al}} + \, 6{\text{H}}_{2} {\text{O}} + \, 2{\text{NaOH}} \to \, 2{\text{NaAl}}\left( {{\text{OH}}} \right)_{4} + 3{\text{H}}_{2}$$

(2)

$${\text{NaAl}}\left( {{\text{OH}}} \right)_{4} \to {\text{NaOH}} + {\text{Al}}\left( {{\text{OH}}} \right)_{3}$$

(3)

$$2{\text{Al}} + \, 6{\text{H}}_{2} {\text{O}} \to \, 2{\text{Al}}\left( {{\text{OH}}} \right)_{3} + 2{\text{H}}_{2}$$

(4)

$$2{\text{Al}} + \, 4{\text{H}}_{2} {\text{O}} \to 2{\text{AlOOH}} + 3{\text{H}}_{2}$$

(5)

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 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/H₂O 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 H₂ generation kinetics are highly desirable. Wang et al. [134, 135] found that a combined usage of sodium hydroxide (NaOH) and sodium stannate (Na₂SnO₃) can simultaneously address the Al/H₂O reaction kinetics and alkali corrosion problems. The addition of a small amount of Na₂SnO₃ causes a remarkable decrease of NaOH concentration without compromising the hydrogen generation performance of the system. In comparison with the traditional Al/H₂O 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 Al₂O₃ 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 H₂ 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.

Fig. 9

Copyright 2005 Elsevier

SEM images of multicomponent aluminum alloy (Ga:In:Sn:Zn:Al = 5.3:2.0:5.4:7.3:80.0) sections (× 800). A After preparation, B after annealing at 450 °C for 20 h. C and D after storing as-cast and annealed alloys for 1 month. Reprinted with permission from Ref. [144].

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 In₃Sn and InSn₄ were indeed observed in the Al–Ga–In–Sn quaternary system [154]. Qian Gao et al. [150] compared the hydrolysis properties of Al–Ga–InSn₄ and Al–Ga–In₃Sn alloys (Fig. 10). They concluded that the eutectic reaction of Al with InSn₄ 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 85wt%–Ga_68.5In_21.5Sn₁₀ alloy (Fig. 11). Combined with EDX analysis, the marked regions in the SEM images shown in Fig. 11c, d could be identified as In₃Sn 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 In₃Sn 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 In₃Sn 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.

Fig. 10

Copyright 2015 Elsevier

Water temperature effect on hydrogen generation of Al–Ga–InSn₄ alloy and Al–Ga–In₃Sn alloy (0.5 g alloy ingot in 100 mL water), a Al–Ga–InSn₄ alloy and b Al–Ga–In₃Sn alloy. Reprinted with permission from Ref. [150].

Fig. 11

Copyright 2019 Elsevier

SEM images of fracture surfaces of Al 85 wt%–Ga_68.5In_21.5Sn₁₀ alloy ingots, a, b images of the quaternary alloy, c enlarged image of intermetallic compounds, and d image of the LMPA at the grain boundary. Reprinted with permission from Ref. [155].

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 GaIn₁₀ in 4 min, and the liquid metal surface remained shiny, meaning that GaIn₁₀ 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.

Fig. 12

Copyright 2016 Elsevier

Surface morphology comparisons among different liquid metals in aluminum–water reaction. a Ga. b GaSn₁₀. c GaZn_3.6. d GaIn₁₀. Reprinted with permission from Ref. [153].

Table 5 Comparisons of some Al-based materials and their hydrolysis performances

2.4 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 437 kJ heat and 1.5 mol hydrogen. An amount of 363 kJ energy can be produced unambiguously from this 1.5 mol hydrogen if it can be thoroughly utilized. Similarly, the hydrolysis of 1 mol magnesium generates 354 kJ heat and 1 mol hydrogen. While the exothermicity is huge during the metal–water hydrolysis, there were only few efforts that tried to transform the thermal energy into other forms of useful energy. In particular, Zhong et al. [180] calculated the energy efficiencies in the hydrolysis cycles of MgH₂, H–Mg₃La and H–La₂Mg₁₇. The maximum energy efficiencies of MgH₂, H–Mg₃La, and H–La₂Mg₁₇ were estimated to be 45.3%, 40.1%, and 41.1%, respectively, meaning roughly half of the energy released by the exothermic reaction was collected. Xiao et al. [181] firstly conceived and designed the Al-based hydrolysis battery, 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-H₂ 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 YH₂ thin film were used as the anode and the cathode, respectively. As the hydrolysis battery was activated, the YH₂-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.

Fig. 13

Copyright 2018 Wiley Online Library

Schematic illustration of the Al hydrolysis battery a and the conventional Ni-MH battery b and their operation principle. Synchronized optical transmittance at 500 nm (the upper panel) and potential profiles (the lower panel) of the YH_x-Pd electrode during galvanostatic process in the hydrolysis battery c and Ni-MH battery d. The current density is 0.2 and 0.05 mA cm⁻² for the Al hydrolysis battery and the Ni-MH battery, respectively. The potential profile of the Al electrode during the operation of the hydrolysis battery is also shown. Inset: photographs of the YH_x-Pd electrode at different stages as indicated by the corresponding number in the transmittance curve. Photographs of lighting the LED by the Al hydrolysis battery e and the MH-Ni battery (f). The electrolyte of hydrolysis battery and Ni-MH battery in e, f is 1 M KOH. Reprinted with permission from Ref. [181].

3 Recent Advances in Regeneration Process of Borohydrides from Hydrolysis By-products

It has been demonstrated that hydrogen supply from NaBH₄ 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 NaBH₄ regeneration by recycling its real-time hydrolysis products (NaBO₂·2H₂O and NaBO₂·4H₂O) 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 LiBH₄ or LiAlH₄ production.

Recently, more attentions were shifted to the preparation and regeneration of NaBH₄ for achieving its large-scale practical applications. In the industry of chemical production, NaBH₄ is usually synthesized by the Brown–Schlesinger process [184] and the Bayer process [185]. The synthesis reactions of Schlesinger and Bayer methods are given as follows:

$$4{\text{NaH}} + {\text{B}}\left( {{\text{OCH}}_{3} } \right)_{3} \to {\text{NaBH}}_{4} + 3{\text{NaOCH}}_{3} \left( {225\sim275 \, ^\circ {\text{C}}} \right)$$

(6)

$${\text{Na}}_{2} {\text{B}}_{4} {\text{O}}_{7} + 16{\text{Na}} + 8{\text{H}}_{2} + 7{\text{SiO}}_{2} \to {\text{NaBH}}_{4} + 7{\text{Na}}_{2} {\text{SiO}}_{3} \left( {450\sim500 \, ^\circ {\text{C}}} \right)$$

(7)

Though the above technologies are mature, they are unsuitable for NaBH₄ hydrolysis applications because of the fancy raw materials (Na or NaH) and high-energy consumption processes. Thus, suitable methods for NaBH₄ synthesis have been developed with low-cost raw materials instead of sodium or its hydride. MgH₂ was used to react with anhydrous borax (Na₂B₄O₇) for NaBH₄ synthesis by ball milling method at room temperature (RT). Here, the NaBH₄ yield may reach 78% with the addition of Na₂CO₃ [186]. This method introduces not only a novel reducing agent (MgH₂), but also an energy-efficient strategy for NaBH₄ synthesis. Enlightened by this, RT ball milling became attractive in NaBH₄ synthesis studies, by which Na and MgH₂ could react with B₂O₃ with the NaBH₄ yield of ~ 25% [187]. As Na was replaced by safe and cheap NaCl, NaBH₄ could also be produced [188]. Subsequently, high-pressure milling was also developed to synthesize NaBH₄. For instance, the synthesis of NaBH₄ could be achieved by ball milling the hybrid of NaH and MgB₂ under 120 bar H₂ pressure with the yield of ca. 18% [189].

Importantly, considering the sustainability and environmental friendliness, NaBH₄ regeneration from NaBO₂·xH₂O, 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 NaBH₄ via reacting MgH₂ with NaBO₂ under 70 bar H₂ pressure at 550 °C with a ~ 97% yield of NaBH₄, NaBO₂ has become the main research object for NaBH₄ 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 MgH₂ to react with anhydrous NaBO₂ by ball milling under inert atmosphere. The conversion yields of NaBH₄ were > 70%, which indicated that ball milling is advisable for the reaction between MgH₂ and NaBO₂. Recently, Ouyang et al. [182, 183, 194] successfully achieved the regeneration of NaBH₄ (Fig. 14) by applying the real hydrolysis by-product (NaBO₂·2H₂O and NaBO₂·4H₂O) as raw material with Mg-based reducing agents (Mg, Mg₂Si and Mg₁₇Al₁₂) at ambient conditions, where the troublesome heat-wasting process to obtain NaBO₂ using a drying procedure at over 350 °C from NaBO₂·xH₂O was omitted. The regeneration yield of NaBH₄ may reach 78%. Significantly, the charged H⁻ stored in NaBH₄ was completely converted from protonic H⁺ in water bound to NaBO₂. Particularly, it was found that the regeneration yield of NaBH₄ was up ~ 90%, while MgH₂ acted as reducing agent [195]. Recently, Ouyang et al. [196] found that high-energy ball milling of magnesium (Mg) with the mixture of Na₂B₄O₇·xH₂O (x = 5, 10) and Na₂CO₃ (obtained by exposing an aqueous solution of NaBO₂ to CO₂) resulted in the formation of NaBH₄ with a high yield of 80% under ambient conditions. In their approach, after ball milling for just 10 min, only B₄O₅(OH)₄²⁻ was detected (Fig. 15(1)), suggesting that the reaction started with this compound containing two BO₄ tetrahedra and two BO₃ triangles. The B–O bond with a bond length of 1.4418 Å in the BO₄ tetrahedra is weaker than that (1.3683 Å) in the BO₃ triangle. Thus, the B–O bond in the BO₄ tetrahedra preferentially broke via a B–O–Mg–H intermediate, forming B–H and Mg–O (Fig. 15(2, 4)). In the following step, the cleavage of (B)–O–H (O bonded with sp² boron) formed the H₂BOH intermediate (Fig. 15(5)), in which B acted as the Lewis acidic site that accepted H⁻ from MgH₂ leading to the formation of the final products, BH₄⁻ and MgO. On the other hand, OH⁻ bonded with sp³ boron (Fig. 15(3, 4)) was also substituted by H⁻ from MgH₂, forming BH₄⁻. Furthermore, they achieved a higher yield of 93.1% for a short duration (3.5 h) by ball milling hydrated borax (Na₂B₄O₇·10H₂O and/or Na₂B₄O₇·5H₂O) with different reducing agents such as MgH₂, Mg, and NaH under ambient conditions [197]. By replacing the majority of MgH₂ 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 NaBH₄ regeneration can be found in a recent review [198].

Fig. 14

Copyright 2018 MDPI

a XRD patterns of the NaBO₂–Mg₃La hydride hybrids and the product after ball milling the NaBO₂-Mg₃La hydride mixture. b XRD pattern of products via ball milling the mixture of NaBO₂·2H₂O-MgH₂ in 1:5.5 mol ratio for 15 h. c XRD curve of products via ball milling the mixture of NaBO₂·2H₂O-5 Mg for 15 h. d XRD spectra of the products after ball milling Mg₂Si and NaBO₂·2H₂O mixtures (in 2:1 mol ratio). Reprinted with permission from Ref. [198].

Fig. 15

Copyright 2020 Wiley Online Library

Proposed reaction mechanism between Mg, Na₂CO₃, and Na₂B₄O₇·10H₂O to form NaBH₄. Reprinted with permission from Ref. [196].

In the past few years, numerous reports have been published dealing with the regeneration of NaBH₄-based spent fuels (NaBO₂·xH₂O or Na₂B₄O₇·xH₂O), whereas the studies upon the regeneration of LiBH₄-based spent products were quite limited. Bilen et al. [199] firstly utilized MgH₂ and LiBO₂ to synthesize LiBH₄ by means of mechano-chemical reaction. Instead of its elements, the hydrolytic product of LiBH₄ (LiBO₂) was adopted as raw material, which may greatly reduce the application cost of LiBH₄ by recycling spent products. However, the tricky heating-wasting process for obtaining anhydrous LiBO₂ at elevated temperature (~ 470 ℃) is inevitable [200]. Stimulated by the successful regeneration of NaBH₄, Ouyang et al. [201] reported a facile method to regenerate LiBH₄ by ball milling its real hydrolysis by-product (LiBO₂·2H₂O) with Mg under ambient conditions with a yield of ~ 40%. This method bypasses the energy-intensive dehydration procedure to remove water from LiBO₂·2H₂O and does not require high-pressure H₂ gas, therefore leading to much reduced costs. Interestingly, it is expected to effectively close the loop of LiBH₄ regeneration and hydrolysis, enabling a wide deployment of LiBH₄ for hydrogen storage and application. As same as NaBH₄ or LiBH₄, KBH₄ could also be synthesized by mechano-chemical reaction. Bilen et al. [202] successfully synthesized KBH₄ by ball milling KCl, MgH₂, and B₂O₃ in a milling reactor. By tailoring the reactant ratio (MgH₂/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 NaBH₄, 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 NaBO₂ recycling, it was found that MgH₂ 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 NaBH₄-based spent fuels (NaBO₂·xH₂O or Na₂B₄O₇·xH₂O), they can be reduced to NaBH₄ with different reductants (MgH₂, Mg, or Mg₂Si) via ball milling, and the highest yield of NaBH₄ may reach 93.1%. Moreover, this process, that uses hydrated metaborate or borax, bypasses the energy-intensive dehydration procedure to obtain anhydrous NaBO₂ or Na₂B₄O₇ without the requirement of high-pressure H₂ gas; therefore, it could lead to much reduced costs. The boron compounds bound with water may act as hydrogen sources stored in NaBH₄ instead of MgH₂. As expected, low-cost waste Al or Al-based alloys may be attractive for achieving the regeneration of NaBH₄ via ball milling, enabling a wide deployment of NaBH₄ for hydrogen applications. This strategy may provide a new conceptual basis for the development of LiBH₄ production or other borohydrides.

4 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 aluminum-based 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–B-based 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 H₂. 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.