Advances in Hydrolysis of Magnesium and Alloys: A Conceptual Review on Parameters Optimization for Sustainable Hydrogen Production

Abstract

This review explores hydrogen production via magnesium hydrolysis, emphasizing its role in the energy transition. Articles were selected from the Scopus database based on novelty. Magnesium’s abundance, high reactivity, and potential for recycling industrial waste make it a strong candidate for sustainable hydrogen production. A key advantage is the use of non-potable water, enhancing environmental and economic benefits. A major challenge is the passivating Mg(OH)₂ layer, which limits hydrogen release. Recent advances mitigate this issue through additives (metals, oxides, salts), alloying (Ni, La, Ca), mechanical treatments (ball milling, cold rolling), and diverse reaction media (seawater, acids, saline solutions). These strategies significantly improve hydrogen yields and kinetics, enabling industrial scalability. Magnesium hydrolysis exhibits a wide activation energy range (3.5–102.6 kJ/mol), highlighting the need for optimization in additives, concentration, temperature, and medium composition. Critical factors include additive selection, particle size control, and alloying, while secondary additives have a minimal impact. This review underscores magnesium hydrolysis as a promising, circular, economy-compatible method for hydrogen generation. Despite challenges in balancing efficiency and environmental impact, recent advancements provide a solid foundation for scalable, sustainable hydrogen production.

1. Introduction

Hydrogen (H₂) plays a key role in the global energy transition, particularly in combating greenhouse gas (GHG) emissions. It represents a promising alternative to fossil fuels due to its high energy releases and its potential to reduce the carbon footprint of the most polluting sectors.

Hydrogen can be produced through various methods, including water electrolysis, photocatalytic water splitting, steam reforming of natural gas, and more recently, research focused on the hydrolysis of metals and their compounds. Among these, the hydrolysis of magnesium gains increased attention due to the availability and high reactivity of this metal. Magnesium hydride (MgH₂) is also particularly notable for its potential as a hydrogen production material (e.g., 1703 mL/g), compared to pure magnesium (i.e., 921 mL/g) [1,2,3].

The reactions involved in the hydrolysis of magnesium and its hydrides are:

$$\mathrm{Mg}+2{\mathrm{H}}_2\mathrm{O} \to \mathrm{Mg}(\mathrm{OH}{)}_2+{\mathrm{H}}_2\hspace{1em}\mathsf{\Delta }°H_1=-354.0 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

$${\mathrm{MgH}}_2+2{\mathrm{H}}_2\mathrm{O} \to \mathrm{Mg}(\mathrm{OH}{)}_2+2{\mathrm{H}}_2\hspace{1em}\mathsf{\Delta }°H_2=-277.9 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

Nevertheless, it is worth pointing out that the hydrolysis reaction is a redox reaction in which the solution involved plays the role of cathodic site (Reactions (1a/c)) and Mg (or Mg H₂) plays the role of anodic site. The complete reaction and so the mechanisms should be written as follows (in the case of Mg, the sum of Reaction (1a–c) is equal to Reaction (1) above):

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$

(1a)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to 2{\mathrm{HO}}^{-}+{\mathrm{H}}_2$$

(1b)

$$\mathrm{Mg} \to {\mathrm{Mg}}^{2+}+2{\mathrm{e}}^{-}$$

(1c)

Magnesium reacts with water to produce hydrogen gas (H₂); however, this process is impeded by the formation of a magnesium hydroxide (Mg(OH)₂) layer on the magnesium surface, which blocks water diffusion and stops the reaction, ultimately limiting hydrogen production [2,4,5,6,7,8,9,10]. The solubility of Mg(OH)₂ in water is 6.5 mg/L at 25 °C for a solubility constant Ks = 5.61 × 10⁻¹² mol³ L⁻³. This low solubility makes the layer stable in water, and it explains the blocking of the reaction.

As expected, the hydrolysis reaction is faster at higher temperatures. Nevertheless, most of the studies are conducted between 0 and 60 °C. Below 0 °C, the reaction is too slow (and salt have to be used to maintain water in the liquid state) and at temperature higher than 60 °C, the If the temperature exceeds 60 °C, the saturated vapor pressure of water becomes too high to be well controlled making the analysis of the experimental results a little unsure. It is also worth pointing out that the activation energy reported for the hydrolysis ranges from 3.5 kJ/mol to 102.6 kJ/mol depending on the experimental conditions, underscoring the importance of optimizing parameters.

One point is essential concerning the production of hydrogen by hydrolysis from Mg. Indeed, machining residues of magnesium alloy parts can be used. This then has a double advantage: (i) offering a second life to waste and producing hydrogen (energy vector) without initial energy input and (ii) producing Mg(OH)₂ which is an inert, non-hazardous product and which can therefore be stored without any special precautions (unlike Mg machining waste which must be stored away from air and humidity). Finally, it should be noted that magnesium hydroxide can be used (in the short term) to treat various problems, such as indigestion, aches, and heartburn. It also acts as a laxative and, therefore, helps relieve constipation. It is also a fire-fighting element. It, therefore, has many potential applications.

The production and use of hydrogen, especially through innovative and sustainable methods like the hydrolysis of magnesium compounds, experiences unprecedented scientific and industrial enthusiasm (Figure 1; the research was done with Scopus database by entering “Magnesium hydrolysis for hydrogen production” in both keyword and title). This renewed interest reflects the urgency and opportunities associated with integrating hydrogen-based solutions to decarbonize the global economy.

The aim of this review is to investigate the potential of hydrogen production through the hydrolysis of pure metallic magnesium, magnesium alloys, and also magnesium compounds. Numerous influencing parameters, such as the addition of salt, modification of the liquid solution (pure water, salted water, acid, etc.), the addition of solid oxides or other metals, the use of compounds, etc., will be detailed. However, the role of Mg(OH)₂, as well as the potential of MgH₂, will also be briefly described as they are involved in the hydrolysis of magnesium.

All those working on magnesium hydrolysis may find this work particularly useful. In particular, those new to the field will discover an overview that is as comprehensive as possible of current advancements. Similarly, researchers, industry professionals, and institutions interested in hydrogen production—and hydrogen in general—will find a wealth of information as well as answers to some of their questions.

To improve hydrolysis performance, several parameters can be adjusted, particularly magnesium and water. Figure 2 presents a scheme of the approaches used by researchers. Magnesium, metals, oxides, hydrides, etc., can be added to catalyze the reaction by creating a galvanic coupling (since this is an oxidation-reduction reaction). Although this method is relatively simple, it incurs additional costs and complicates recycling. Another commonly used approach is the addition of carbon or carbon-based additives due to carbon’s lubricating properties. The advantages and disadvantages of this method are similar to those of adding metals but to a lesser extent.

An alternative is to work with magnesium alloys, including industrial waste alloys. In this case, pre-treatment of the material is required (adding elements, mechanical treatment, etc.) because these alloys have better corrosion resistance, which slows down their hydrolysis. The main advantages of this method are: (i) it allows the reuse of end-of-life materials, (ii) it remains cost-effective, and (iii) it transforms a potentially hazardous product (magnesium alloy, which requires strict storage conditions) into an inert product (magnesium hydroxide). The major disadvantage is the need to adapt the initial treatment depending on the type of alloy, as this can vary significantly.

The second factor influencing hydrolysis performance is water. Adding salts, organic or inorganic acids, and using wastewater are feasible solutions. The advantages of these methods include (i) simplicity of implementation and (ii) low cost. However, they also present notable disadvantages: (i) the concentration and method of addition depend on the product to be hydrolyzed (the Mg/H2O couple), and (ii) the risk of corrosion of the equipment and potential danger, especially when acids are used.

Finally, for the Mg/liquid medium couple, it is possible to modify the temperature (since the reaction is thermally activated) and adjust the microstructure and/or composition of the mixture through mechanical treatments, with ball milling being the most commonly used method.

Recent advancements aim to enhance hydrolysis efficiency and hydrogen yield by addressing the passivation issue caused by the Mg(OH)₂ layer. Techniques such as continuous removal of the hydroxide layer, ball milling to reduce particle size and modify magnesium morphology, and adjusting aqueous solutions to create more corrosive environments using chloride salts, organic and inorganic acids, are explored. Additionally, alloying magnesium with other metals like rare earth metals (mainly La [10]) or transition metals (Co, Cu, Ni [5], Pd, Pt [9]) shows promising results. Moreover, light-activated hydrogen generation is an emerging approach with significant potential [7].

While the use of acids can significantly boost hydrogen production, it also brings challenges, such as corrosion and environmental pollution. Various strategies, including ball milling and alloying, are employed to enhance large-scale hydrolysis kinetics. The incorporation of nanostructures into magnesium particles and the application of laser-induced methods have further improved magnesium’s reactivity with water [6]. Moreover, ball milling of Mg₂NiH₄ powder with additives such as fused silica (FS) and NaCl has demonstrated their ability to enhance surface interactions and reaction rates. This technique modified the morphology of Mg₂NiH₄ powder, creating fractures that enhance reaction kinetics and increased hydrogen yield during hydrolysis [11]. The addition of salt or acid to the solution promotes the generation of micro-cracks and accelerates the hydrolysis of magnesium by breaking down its protective passivation layer. Salts, as well as acids, provide ions such as chloride that actively destabilize the surface layer of Mg(OH)₂. While the pH effect in acids tends to dominate over the ionic contribution, the presence of these ions increases the solubility of the Mg(OH)₂ layer. This destabilization facilitates greater interaction between water molecules and the magnesium surface, thereby enhancing reactivity and accelerating the overall rate of hydrogen production [12,13,14,15,16,17].

2. Various Additives Used

2.1. Metals and Their Oxides and Hydrides

Research showed that the addition of certain transition metal oxides significantly improved the hydrogen production efficiency of magnesium powder. These oxides varied in catalytic effectiveness due to the differing electron-accepting properties of their constitutive metal elements. Furthermore, metals with higher valence states generally exhibited superior catalytic activity in magnesium hydrolysis reactions compared to those with lower valence states [18,19]. The reaction rate depended on the nature of the oxide. Hong et al. studied the effect of MgO addition on hydrolysis. The MgH₂ + 5%MgO produced the highest volume of hydrogen. The increased reactivity of MgH₂ particles with water leading to a higher H₂ generation rate and yield was attributed to the formation of numerous defects, the presence of clean surfaces, and the reduction in particle size of MgH₂ in the milled MgH₂ + 5% MgO powder [20]. In the study of Awad et al., various transition metals and oxides were studied (on the hydrolysis reaction of Mg-based mixtures). Transition metal oxides, specifically V₂O₅ and Nb₂O₅, enhanced hydrogen generation for both magnesium and its hydride, especially after mechanical treatment. For instance, Mg + 10 wt.% Nb₂O₅ milled for 1 h produced 95% of hydrogen in 8 min, while the mixture with V₂O₅ took 17 min. Nb₂O₅ outperformed V₂O₅ in hydrolysis kinetics due to higher defect density and a number of cracks. Shortening the ball milling time accelerated hydrolysis as 1-h milled time was better than a few hours. Longer milling time (i.e., 3 to 5 h) decreased yields due to magnesium hydride formation (as milling was performed under H₂). Among the transition metals, Mg + 10 wt.% Ni showed superior hydrolysis performance over Mg + 10 wt.% Fe, attributed to a higher corrosion rate and lower magnesium hydride content in the former [21]. Yang et al. studied the enhancement of the hydrolysis properties of Mg-MO_x (M = Al, Ti and Fe). Among the metal oxides tested, Mg-TiO₂ passivated was the most effective in improving hydrolysis performance. TiO₂ lowered the local pH around Mg during hydrolysis, enhancing the H₂ generation [22]. Huang et al. investigated the effects of various metal oxides on magnesium hydrolysis performance for hydrogen production. MoO₃ exhibited the strongest catalytic effect, followed by Fe₂O₃, Fe₃O₄, TiO₂, Nb₂O₅, and CaO. The addition of 5 wt.% of MoO₃ significantly improved hydrogen production, achieving a hydrogen volume of 888 mL/g. The study showed that transition metals with higher valence states enhanced the catalytic effects on magnesium hydrolysis (e.g., testing various Mo compounds (Mo, MoO₂, and MoO₃) revealed that higher Mo valence states led to increased hydrolysis rates). The mixture Mg + 5 wt.% MoO₃ showed the highest reactivity and a low activation energy of 12.1 kJ/mol [23,24]. The various valence state was associated with electrons mobility enhancing the redox process of hydrolysis. Huang et al. synthesized flower-shaped MoS₂ (600–800 nm) via hydrothermal methods and found that the petal-like morphology of MoS₂ enhanced both the reaction rate and maximum yield of H₂ formation. The Mg + 10 wt.% MoS₂ composite, prepared from this petal-shaped MoS₂, achieved a hydrogen production efficiency of 90.4% within 1 min. This improvement was attributed to the increased surface area of petal-shaped MoS₂, which aided the mechanical activation of Mg powder during ball milling, leading to smaller Mg particles and so larger specific surface and also reduced agglomeration [25].

As previously said, the hydrolysis of magnesium hydride (MgH₂) was interesting for the generation of H₂ as it could generate twice the hydrogen quantity than Mg (i.e., one H₂ from MgH₂ and the other from water). The limitations of these hydrides were (i) the slow hydrolysis reaction due, as for Mg, to the formation of Mg(OH)₂ layer onto the reactive material but also (ii) the lower exothermicity of the reaction (278 vs. 354 kJ/mol) compared with Mg [21]. The hydrolysis reaction of pure MgH₂ was fully suspended in 20 min, generating a total of 760 mL/g of hydrogen. Many attempts were made to accelerate this reaction [26]. Chen et al. added surfactants to the hydrolysis of MgH₂ that enhanced hydrogen generation by reducing surface tension. The addition of surfactant improved the liquid’s wetting effect on MgH₂ and increased initial hydrogen generation rates. However, a reduction in overall hydrogen generation was observed due to the foam formation that could trap hydrogen. Nevertheless, hydrogen generation efficiency might have been limited due to charge interactions: negatively charged surfactant groups could bind with H⁺, reducing its concentration and impacting hydrogen production, while positively charged groups might have inhibited Mg(OH)₂ growth by binding with OH⁻ [27,28,29,30].

A solution to increase the hydrogen released was to mix Mg with other metals or hydrides. Incorporating Mg-based alloys and their hydrides enhanced the hydrolysis performance of both Mg and MgH₂. The addition of alkali and alkali-earth metals boosted hydrogen generation due to their high reactivity with water [28,29,30]. Kong et al. suggested the use of Mg decomposable hydrides due to their high hydrolysis rate. They used MgH₂ and Mg(BH₄)₂ and obtained a yield of almost 100%. Mg-Ca hydrides were also studied. Tessier et al. found that two factors influenced the amount of hydrogen released. The mix with the longest milling time and the highest amount of calcium had a faster hydrolysis reaction and higher hydrogen yield. This was due to the CaH₂ layer formed during the ball milling. It was worth pointing out that this result was only partially in agreement with the one of Awad et al. [21], but the milling duration and equipment were different (i.e., the induced power of milling was different). MgH₂–20.3 mol% CaH₂ mixtures reached 80% yield after 30 min of hydrolysis. Calcium addition allowed a faster and more complete hydrolysis reaction of magnesium hydride. Even if calcium hydride was formed during milling, faster hydrolysis reaction and higher yield were achieved when calcium hydride was used directly as a starting component instead of calcium [30]. It has been demonstrated that Ca₄Mg₃H₁₄ alloy hydride, formed during milling, could be completely hydrolyzed at various temperatures, indicating that it reacted more easily with water than Mg. The kinetics of the hydrolysis reaction of Mg-Ca alloys showed that increasing the calcium (Ca) content lowered the activation energies. The activation energy for the hydrolysis reaction of a 30 wt.% Ca-Mg alloy hydride was calculated to be 8.3 kJ/mol, which was the lowest activation energy ever reported for the hydrolysis of Mg-based materials. As the Ca content decreased, the activation energies rose to 17.7 kJ/mol for a 20 wt.% Ca-Mg alloy hydride and further to 21.1 kJ/mol for a 10 wt.% Ca-Mg alloy [31]. Mg-alkali metals’ hydrides hydrolysis rate was studied by Jiang et al. It was demonstrated that MgLi alloy formed Li₃Mg₁₇ after ball-milling. During ball milling under hydrogen (3 MPa), Li₃Mg₁₇ was progressively transformed into Mg and LiH, accelerating hydrolysis and producing 1773 mL/g of hydrogen in 30 min [32]. Alloys like NaMgH₃ showed an extremely fast hydrolysis rate; a hydrogen generation of 1360 mL/g was possible within a few tenths of a second (≈50 s). This fast hydrolysis reaction of NaMgH₃ was related to its crystallographic structure: the NaMgH₃ crystallized in a perovskite structure type that enabled fast hydrogen motion [33]. Liu et al. discovered that LiBH₄ promoted the hydrolysis of Mg in pure water. The combined hydrolysis reactions of Mg and LiBH₄ in Mg-LiBH₄ composites created a synergistic effect, enhancing each other’s reactivity [34]. Li et al. proceeded to prepare Mg-Li alloy with a nanoporous structure by vapor deposition. Nanopores with diameters in the range between 100 nm and 600 nm were uniformly dispersed in the Mg-Li alloy matrix, and the maximum porosity reached was 42.4%. The amount of hydrogen produced by the nanoporous Mg-Li alloy reached more than 99% of the theoretical hydrogen production, and the hydrogen production rate could reach 367 mL/min/g at 50 °C. The porous structure in nanoporous Mg-Li reduced the Mg(OH)₂ passivation layer formed in its hydrolysis reaction [35].

Mg₃RE (RE = La, Ce, Pr, Nd) alloys were prepared via induction melting by Ouyang et al. [36]. The hydrogenated (by submitting the alloys to H₂ pressure) alloys contained phases like MgH₂, LaH₃, CeH₃, PrH₃, Nd₂H₅, and MmH₃. The presence of hydrides in the alloy allowed faster hydrolysis. The Mg₃Mm alloy produced 695 mL/g of hydrogen in 5 min and 828 mL/g in 15 min [36]. During the H-Mg₃RE hydrolysis, REH₃ could produce a reaction tunnel for MgH₂ hydrolysis, leading to the breaking of the Mg(OH)₂ passivation layer [37]. Zhong et al. assessed the energy efficiency of hydrolysis cycles for MgH₂, H-Mg₃La, and H-La₂Mg₁₇, finding efficiencies of 45.3%, 40.1%, and 41.1%, respectively, which confirmed the feasibility of hydrogen generation from these materials. A novel method for hydrogenating La₂Mg₁₇ at 25 °C was developed using ball milling under hydrogen (with Ni addition), yielding to a decomposition and the formation of a MgH₂—LaH₃—Ni composites that produced 892 mL/g of hydrogen in 10 min and 1208 mL/g in 40 min, with an activation energy of 52.9 kJ/mol [38]. Ma et al. focused on hydrogen generation through the hydrolysis of H-CaMg₂ and H-CaMg_1.9Ni_0.1 alloys. Both alloys exhibited rapid hydrolysis rates and high yields: H-CaMg₂ generated 800 mL/min, and H-CaMg_1.9Ni_0.1 achieved a 94.6% yield in 12 min, producing 1053 mL/g of hydrogen. The Ca₅Mg₉H₂₈ ternary hydride formed during the hydrogenation of CaMg_1.9Ni_0.1 significantly contributed to its excellent hydrolysis properties, with an apparent activation energy of 32.9 kJ/mol in deionized water [39].

In the work of Jiang et al., researches were focused on MgLi hydride. They firstly illustrated their comparison of the hydrolysis properties of various hydrides (shown in

Table 1. Comparison of the kinetic and hydrogen yield of various materials for hydrogen generation. Total H₂ production was calculated without water included. Adapted from Jiang et al. [32].

Materials	H2 Production Kinetic	Total H2 Production (wt.%)
H-CaMg2	800 mL/g in 50 s	7.14 wt.%
H-CaMg1.9 Ni0.1	968 mL/g in 5 min	9.4 wt.%
H-30 wt.% Ca-Mg	755.7 mL/g in 1 h	6.7 wt.%
H-Mg3La	474 mL/g in 5 min	8.47 wt.%
H-Mg3LaNi0.1	446 mL/g in 5 min	9.05 wt.%
H-La2Mg17	653 mL/g in 10 min	11.2 wt.%
H-Mg3CeNi0.1	725 mL/g in 10 min	9.71 wt.%
H-Mg17Al12	1389 mL/g in 1 h	12.4 wt.%
H-MgLi	1263 mL/g in 5 min	15.8 wt.%

), which were hydrogenated either through reactive ball milling or hydrogenation under hydrogen pressure, excluding commercial hydrides from the analysis. The H-MgLi produced using this method demonstrated superior kinetics and a higher hydrogen yield. To do so, an inclusion of MgCl₂ significantly enhanced the hydrolysis kinetics of H-MgLi, as the Cl⁻ ions penetrated the Mg (OH)₂ passivation layer [32]. As previously mentioned, the formation of lithium hydride enhances hydrogen production. [32,40]

2.2. Borohydrides

Yun et al. worked on H₂ production of a mixture of AZ91D alloys (NB: metallurgic nomenclature: these alloys contained Al and Zn as predominant elements; as an example, AZ91 meant an alloy containing 9 wt.% Al and 1 wt.% Zn and Mg as balanced; these alloys exhibited excellent castability and had a low tendency for hot cracking, though this risk increased as the Zn content rose.) + borohydrides [41]. Borohydrides significantly improved the hydrolysis kinetics and hydrogen production capacity of AZ91D alloy waste at room temperature and pH = 7, with KBH₄ showing the most notable activation effect. The hydrogen production capacity increased from 402.7 mL/g for AZ91D alloy waste to 605.0 mL/g for AZ91D-KBH₄ within 10 min. Peak hydrogen production rates decreased drastically from 223.6 mL/g/min (after 0.83 min, i.e., 50 s) to 40.4 mL/g/min (after 11 min) with KBH₄. The hydrolysis yield after 5 min increased from 25% for AZ91D to 51% for AZ91D-KBH₄, and the activation energy of hydrolysis drastically decreased from 87.8 kJ·mol⁻¹ to 22.2 kJ/mol with the addition of KBH₄. Overall, this research as well as the one of Grojean et al. underscored the potential of using borohydrides as surface activators to enhance hydrogen production from Mg-based waste alloys, contributing to more sustainable and cost-effective hydrogen generation in neutral mediums (i.e., pure water or equivalent seawater) [41,42].

2.3. Halide (MCl_x and MF_x) Additives

The mechanism by which metal salts enhanced the hydrolysis of magnesium was rather complex, with various metal salts exerting different catalytic effects. The study of Mao et al. developed Mg-MFx (M = V, Ni, La, Ce) nanocomposites using an arc plasma method for efficient hydrogen generation via hydrolysis in MgCl₂ solution (NB: not NaCl as often) [43]. These composites showed significantly higher hydrogen yield and generation rates than commercial MgH₂, with Mg-VF₃ achieving the highest rate due to reduced particle agglomeration, maximizing water contact. Though Mg-NiF₂ and Mg-LaF₃ initially outpaced pure Mg in hydrogen generation, their rates declined after 15 min. The Mg-CeF₃ composite had the slowest rate overall, likely due to rapid solution alkalinization that led to the formation of CeO₂/Ce₂O₃, which hindered effective water contact. Overall, the arc plasma-prepared samples demonstrated enhanced hydrogen production and reaction rates despite slightly higher Ea values. For example, for pure Mg prepared by arc plasma, the activation energy was 102.6 kJ/mol H₂ as it was reported to be usually about 70 kJ/mol H₂ for Mg prepared by traditional method, highlighting the effectiveness of Mg-MFx composites for efficient hydrogen generation in MgCl₂ solution but also the poorest thermal effect (as Ea were higher) [43]. In the study of Wang et al., they focused their interests on Mg-metal chloride. While ball-milled pure Mg was unreactive with water, Mg-metal chloride composites demonstrated rapid initial hydrogen production with an incubation time of around 50 s for NiCl₂ and 20 s for FeCl₃ and CuCl₂. However, for most composites, the hydrogen generation quickly slowed down due to the passive Mg(OH)₂ layer formation, resulting in conversion yields below 80%. The Mg-CoCl₂ composite stood out, maintaining a high reaction rate and achieving a 96.6% yield. Ball milling with chlorides enhanced reactivity by creating surface defects and promoting Cl⁻ induced Mg corrosion, which helped Mg(OH)₂ detach, sustaining hydrolysis and boosting hydrogen generation (

Table 2. Hydrogen generation properties of ball-milled Mg-chloride with water. Adapted from Wang et al. [44].

Sample (0.5 g)	Hydrogen Generation Volume (L)	Conversion Yield (%)	Reaction Duration (s)
Mg	0.002	4.28	5
Mg-6% NiCl2	0.347	79.2	276
Mg-6% CoCl2	0.423	96.6	325
Mg-6% CuCl2	0.141	32.2	830
Mg-6% FeCl3	0.258	58.9	1684

Note: Duration refers to the time from the beginning of the reaction to the time when maximum hydrogen production is reached.

) [44].

Shetty et al. prepared active magnesium by adding NaCl and KCl salts via ball milling. KCl or NaCl on the surface of the magnesium particles was gradually dissolved during the reaction and exposed a new magnesium surface, leading to an increase of the hydrolysis kinetic and also to a complete reaction [45]. Ammonium chloride and AlCl₃ played a primary role in boosting the magnesium hydrolysis reaction. Al Bacha et al. examined the catalytic action of AlCl₃ in magnesium hydrolysis within a 0.5 M HCl solution. Their findings revealed that the Mg-AlCl₃ composite, produced by ball milling, displayed excellent hydrolysis characteristics. AlCl₃ generated Al³⁺ ions and Brønsted acid cations (H⁺), lowering the pH of the solution and enhancing the dissolution of the Mg(OH)₂ passivation layer. This process prevented the formation of a passivation and hindered the Mg(OH)₂ layer through interactions between cations and hydroxyl groups produced in hydrolysis [46]. Sun et al. investigated the hydrogen production performance of Mg-CoCl₂ composites, noting a significant hydrogen production rate of 524 mL·min⁻¹·g⁻¹ and an efficiency of 95% in pure water [47]. Zhong et al. synthesized Mg-CaH₂ composites, evaluating their hydrolysis reaction when adding NH₄Cl. The Mg-CaH₂–5 wt.% NH₄Cl composite demonstrated impressive performance, producing 720 mL of hydrogen per gram of composite within 1 min in pure water. The NH₄Cl addition during ball milling effectively minimized magnesium particle size, increasing the reactive surface area of the Mg-CaH₂ composite. The solubility of NH₄Cl enhanced the reaction by creating a fresh reactive surface upon dissolution, while NH₄⁺ ions, with a high affinity for OH⁻ ions, facilitated faster interaction between magnesium composites and water [48].

2.4. Carbon and Carbon-Based Additives

The study of Awad et al. [21] highlighted the impact of carbon additives on the hydrolysis of Mg-based mixtures for hydrogen generation. Carbon additives significantly enhanced the hydrolysis reaction rate and reduced activation energy compared to transition metals (TMs) or oxides, with mixtures containing both carbon and TMs showing the fastest kinetics. Carbon addition aided in preventing the formation of a passivation layer by facilitating MgCl₂ formation in chloride solutions, which maintained particle reactivity and promoted continuous hydrolysis. Mixtures with both carbon and metal displayed the lowest activation energies, with Mg + 5 wt.% G + 5 wt.% Ni achieving 14.34 kJ/mol, resulting in faster hydrogen release. Additionally, carbon’s lubricating effect enhanced the ball milling efficiency and thus helped obtain smaller particle sizes, improving reaction kinetics. The comparison between Mg + 5 wt.% G + 5 wt.% Ni and Mg + 10 wt.% G showed that adding 5 wt.% of both graphite and Ni significantly improved the hydrogen generation rate. As reported in Figure 3, the Mg + 5 wt.% G + 5 wt.%N composite achieved over 95% of the theoretical hydrogen yield within 2 min, whereas Mg + 10 wt.% G required more than 3 min to reach the same yield. A similar trend was observed in composites containing both graphite and Fe (Mg + 5 wt.% G + 5 wt.% Ni). Notably, just 1 h of mechanical milling was sufficient to optimize kinetics, with no further improvements from extended milling times [21]. Hydrogen generated from carbon-containing mixtures was then used to power a Proton Exchange Membrane Fuel Cell, demonstrating stable energy output.

In the study of Al Bacha et al., the impact of different ball milling techniques on the hydrolysis performance of Mg alloy waste was investigated. Milling Mg alloy alone under argon led to a 58% hydrogen yield within 5 min, mainly attributed to reduced particle size. Graphite’s lubricating effect preserved surface reactivity. The highest hydrolysis efficiency was achieved after 2 h ball milling (P6 ball miller) with graphite by reaching 98% hydrolysis in 5 min. Milling under argon further enhanced reactivity. Both milling strategies ((i) large scale miller: Uni Ball and (ii) Small scale miller: P6 Fritsch^®) demonstrated strong hydrolysis performance, supporting potential interest for industrial application [46]. These results are summarized in

Table 3. Hydrogen production by hydrolysis with Mg alloy milled with the UB mill after 5 min and 60 min of reaction in 0.6 M MgCl₂ solution, after 0.5 M HCl solution addition considered as being the total hydrogen amount produced and the calculated yield. Adapted from Al Bacha et al. [46].

Sample	Mill Gas	H2 Generation in 0.6 M MgCl2 After 5 min	Yield (%)	H2 Generation in 0.6 M MgCl2 After 60 min	Yield (%)	H2 (wt.%) Generation in 0.5 M HCl
Mg alloy		0.03 ± 0.03	0.5	1.1 ± 0.1	17.2	6.7 ± 0.2
Mg alloy 35 h Mg alloy 50 h Mg alloy 100 h	H2 H2 H2	3.5 ± 0.1 3.5 ± 0.1 3.2 ± 0.1	43.8 42.4 32.6	6.8 ± 0.2 6.3 ± 0.2 5.1 ± 0.1	83.8 75.3 52.2	8.1 ± 0.3 8.3 ± 0.3 9.7 ± 0.3
Mg alloy + G 20 h Mg alloy + G 50 h	Ar Ar	6.4 ± 0.2 6.0 ± 0.2	97.9 96.6	6.4 ± 0.2 6.1 ± 0.2	98.4 98.3	6.5 ± 0.3 6.2 ± 0.3
Mg alloy + G 20 h + AlCl3 20 h Mg alloy + G 50 h + AlCl3 20 h	Ar Ar	5.9 ± 0.2 6.0 ± 0.2	94.9 92.0	5.9 ± 0.2 6.1 ± 0.2	95.5 93.0	6.2 ± 0.3 6.6 ± 0.3

.

Carbon materials differed from other catalysts as they lacked metal elements, preventing alloy formation with magnesium. Consequently, they did not facilitate magnesium hydrolysis through galvanic coupling. Graphite, specifically, offered superior lubrication, minimizing cold welding between magnesium particles during ball milling and enhancing the smaller size of the magnesium powder. During hydrolysis, graphite on the magnesium surface gradually detached, enabling larger water contact and reaction initiation. Moreover, the hydrophobicity of carbon materials effectively shielded magnesium from air moisture corrosion that could happen during handling [49,50,51]. Ma et al. prepared graphite-based magnesium composites by plasma-assisted ball milling and found that the milling promoted graphite peeling. The volume of hydrogen produced by the hydrolysis reaction of graphite-based magnesium composite was 614.3 mL/g (66% yield) [52].

The two-dimensional structure of graphite also allowed it to act as a support for Mg particles [53]. Chen et al. developed MgLi-Expandable Graphite (EG) composites via ball milling. Notably, the MgLi composite containing 10 wt.% EG exhibited the best hydrolysis performance, achieving a hydrogen production rate of 1147 mL/min/g. The EG helped maintain the reactivity of MgLi with water and protected it from degradation by air moisture. The main hydrolysis byproducts in the MgCl₂ solution were identified as Mg(OH)₂ and Mg₃Cl₂(OH)₄·4H₂O, with chloride ions contributing to the breakdown of the Mg(OH)₂ layer [54].

Hou et al. studied as-cast and high-energy ball milled (HEBM) Mg-Ni-La alloys for hydrogen (H₂) generation. Mg-based alloys exhibited various alloys that showed the best performance and the lowest activation energy (14.68 kJ·mol⁻¹). In high-energy ball milled (HEBM) alloys, (Mg10Ni)_1−xLa_x (x = 15), named 5 La (e.g., 5 stands for sample number 5) alloy demonstrated the highest capacity in the short term, outperforming both HEBM and smelted alloys. This superior performance was linked to a lower initial nucleation rate of Mg(OH)₂ and favourable microstructural characteristics. Overall, effective hydrogen generation involves the nucleation, growth, and contact of Mg(OH)₂ nuclei, making optimized Mg-based alloys promising candidates for solid-state hydrogen generators [55].

2.5. Mg-Al Mixtures and Alloys

Al content in Mg alloys resulted in better castability and an increase of ambient tensile, compressive, and fatigue strength, as well as improved corrosion resistance [56,57,58]. Aluminum and magnesium waste presented flammability and explosiveness risks, requiring treatment before disposal. By using a saline solution, it was possible to oxidize magnesium waste and harness the hydrogen production [12].

Zou et al. worked on ball milled Mg-Al alloys. The duration of ball milling time was modified (from 0 to 4 h), and the reactivity in the air was measured. Increasing milling time impacted the oxidation and hydrolysis of Mg-Al alloys by removing the oxide layer and creating surface defects, enhancing reactivity in seawater. High-energy ball milling improved the material’s performance by refining the particle structure [17]. In the study of Buryakovskaya et al., Mg-Al wastes were ball-milled with a Sn-Pb solder alloy additive to enhance hydrogen release during magnesium alloy corrosion. After testing different Sn-Pb contents and different durations in saline water, it was shown that one hour of milling with 5 wt.% Sn-Pb was optimal for hydrogen production with a yield of about 90% (Figure 4). This method could efficiently recycle (or offer a second life to) magnesium waste while generating hydrogen [12].

These alloys could also be used as reducing agents. It was the case in the study of Zhong et al. where Mg₁₇Al₁₂ was used to reduce NaB(OH)₄ to regenerate NaBH₄. A H⁻ anion in the regenerated NaBH₄ was directly transferred from the [OH]⁻ group to H⁺. During the reduction process, firstly, the Mg₁₇Al₁₂ alloy reacted with NaB(OH)₄ and generated NaBH₄, MgO, and Al-metal. Afterward, the Al-metal reacted with residual NaB(OH)₄ and produced NaBH₄ and Al₂O₃ [59]. In the study of Qin et al., NaH was added, and no extra hydrogen was added here. The H in the regenerated NaBH₄ was mainly derived from the H⁺ in the coordinated water. By adding a small amount of NaH, the highest yield reached 57% after optimizing the regeneration process. Also, it was found for the first time that not only Mg could participate in the reaction, but also Al. NaH could also promote the reaction [58].

As previously mentioned, AZ alloys were widely used in structural applications. Also, the alloys could develop microporosity. The most common alloy used was AZ91, thanks to its low cost and adequate mechanical properties and processing characteristics [56]. In the study of Yun et al. [41], borohydrides were incorporated into AZ91D alloy. Borohydrides, as surface activators, significantly enhanced the initial hydrolysis kinetics and hydrogen yield of AZ91D at room temperature in water. Also, Al Bacha et al. demonstrated that by enhancing the microstructure and corrosion characteristics of magnesium alloys, milling them with graphite and AlCl₃ increased hydrogen generation. While sequential milling with graphite and AlCl₃ achieved the same capacity in 5 min, milling with AlCl₃ lowered pH and avoided passive films, reaching 92% of theoretical hydrogen capacity in 60 min. Within 60 min, the brittle Mg₁₇Al₁₂ phase, which existed in the AZ91 alloy, enhanced surface defects creation and galvanic corrosion, allowing it to reach 80% hydrogen capacity in NaCl solution. Through the efficient milling of magnesium alloy waste, this study presented a sustainable method for producing hydrogen [59,60].

By utilizing atomic layer deposition (ALD) to deposit a ZrO₂ nanofilm on AZ31 alloy, it was possible to enhance the alloy’s corrosion resistance in NaCl solution, addressing the rapid degradation typically associated with magnesium alloys. The thin ZrO₂ layer effectively prevented electrolyte penetration and halted corrosion. However, after 72 h, galvanic interactions accelerated corrosion. While a PLGA (poly(lactic-co-glycolic) acid) coating temporarily improved corrosion resistance, acid-induced damage to the ZrO₂ layer ultimately promoted degradation. These highlighted both the strengths and limitations of PLGA/ZrO₂ hybrid coatings and ALD for corrosion prevention [61].

Other magnesium alloys, such as AM60 and AS41, also offered distinct properties. AM60, in which aluminum and manganese were added, was recognized for its excellent ductility and toughness, making it ideal for die-cast automotive components like wheels [62]. AS41, containing aluminum and silicon, was optimal for high-temperature performance and was used in applications such as engine crankcases in air-cooled systems [63,64].

Zheng et al. demonstrated that hydrogen production rates increased with temperature and the Mg/Al ratio under pH values from 1 to 9. Depending on alloy composition and hydrolysis duration, products ranged from pure layer double hydroxides (LDHs) to nanocomposites containing LDHs, Al(OH)₃, and alloy particles. A Mg/Al ratio of 70:30 in deionized water or diluted HCl produced a pure LDH phase with superior purity, crystallinity, and surface area compared to conventional methods. These findings suggested magnesium-aluminum alloys have potential interest for onboard hydrogen generation [65].

Yang et al. studied hydrogen production from Al-Mg-Li alloys at temperatures up to 1030 °C, starting from an aluminum-rich alloy. They showed that adding 15% lithium and magnesium to aluminum significantly enhanced hydrolysis, with over 50% and up to 89% of the aluminum reacting. Needle-like LiAlO₂ structures improved porosity, while magnesium disrupted the Al₂O₃ layer, increasing hydrogen output. The highest hydrogen production occurred in the later stages (higher temperature, close to 1030 °C) with 5% lithium samples [64]. The reaction followed three distinct stages, corresponding to the activity of Li, Mg, and Al. Li and Mg accelerated some Al reactions in the first two stages. Unlike typical aqueous Al hydrolysis, this process produced oxides (LiAlO₂, Li₂Al₄O₇, Al₂O₃, and MgO) instead of hydroxides. Magnesium combustion fragmented the passive Al₂O₃ layer, enhancing H₂ release by increasing surface reactivity. Needle-like LiAlO₂ structures formed, embedding into globular products, creating a porous structure that boosted hydrogen production. The study found that Li and Mg influenced the reaction differently, with optimal H₂ output from the 5% Li sample due to low energy requirements in the final reaction stages [64].

2.6. Mg-Metals Alloys

2.6.1. Mg-Ni Alloys

Nickel (Ni) played a crucial role in enhancing hydrogen production in magnesium (Mg)-based alloys [66,67]. By catalyzing hydrolysis, accelerating reaction rates, and improving yield, Ni significantly boosted hydrogen output. The formation of stable phases, such as Mg₂Ni, refined the alloy’s microstructure and reduced activation energy, enabling effective hydrogen generation even at relatively low temperatures. Additionally, Ni enhanced structural stability and corrosion resistance, making Mg-Ni alloys highly durable and efficient for hydrogen production applications. The activation energy was respectively 40.56 kJ/mol and 31.72 kJ/mol for Mg + 23.5 wt.% Ni and Mg + 23.5 wt.% Ni + 10 wt.% La eutectic alloys [68]. Grosjean et al. demonstrated that a Mg + 10 at.% Ni (i.e., Mg + 4.7 wt.% Ni) composite milled for 30 min achieved optimal performance, with a 100% conversion yield after 1 h of hydrolysis in a neutral (pH = 7) aqueous solution containing chloride ions. This reaction produced 6.4 wt.% hydrogen when accounting for water used in hydrolysis, increasing to 15.2 wt.% if water produced by a fuel cell was recycled. Therefore, the focus was on developing highly reactive materials that enabled complete hydrolysis in neutral aqueous environments [69]. Then, Oh et al. focused on designing Mg–Ni alloys to enhance hydrogen generation from seawater, specifically highlighting the Mg + 6.28 wt.% Ni (e.g., 2.7 at %; the alloy was named Mg–2.7 Ni alloy in the article). This achieved a hydrogen generation rate 1300 times faster than pure Mg. The study also demonstrated that a polymer electrolyte membrane fuel cell could stably produce 7.3 W for 20 min (1.215 kWh/kg of alloy) using hydrogen generated from the hydrolysis of 2 g of this alloy [66,67]. In the hydrolysis of magnesium alloys, magnesium acted as the anode, losing electrons, while other metals like Ni and Sn promoted the cathodic reaction, releasing hydrogen gas.

2.6.2. Mg-Ca Alloys

Li et al. demonstrated that CaCl₂ enhanced hydrogen yield and conversion rates, improving the hydrolysis performance of Mg-Ca alloy hydrides (MCH). By dissolving passive layers on the magnesium surface, the addition of CaCl₂ to xMCH-CaCl₂ composites boosted hydrolysis efficiency, achieving conversion rates of up to 83.2% [70]. Similarly, Zhong et al. reported that Ca²⁺ ions in CaCl₂ solutions facilitated hydrogen production at low temperatures (253–283 K) through interactions with OH⁻ ions, although less effectively than Mg²⁺ or NH₄⁺ ions. Hydrogen yields significantly increased as the temperature rose, positioning CaCl₂ as a moderately effective catalyst for hydrogen generation [16]. Zhong et al. [48] demonstrated that ball-milling MCH-based materials with NH₄Cl improved both hydrolysis efficiency and air stability (which could appear as antithetical). Their study revealed that the MCH+5 wt.% NH₄Cl composite, milled for 0.5 h, achieved the highest hydrogen output. Pressing this composite into plates further enhanced air stability by reducing the hygroscopic nature of NH₄Cl and minimizing air exposure. During hydrolysis, chlorides released from the composite created new reactive surfaces, intensifying the reaction. Additionally, incorporating calcium into Mg-Ca alloys was shown to boost hydrogen production by increasing yields and reactivity while forming a stable oxide layer that reduced corrosion and prolonged the alloy’s lifespan [71].

2.6.3. Mg-Cu Alloys

Oh et al. studied Mg-Cu alloys, particularly Mg + 3 wt.% Cu (but wt.% of Cu ranged from 1 to 3). To ensure a rapid hydrogen generation, they aimed to precipitate the electrochemically noble phase Mg₂Cu at the grain boundaries. These noble precipitates significantly enhanced hydrolysis kinetics through synergistic galvanic and intergranular corrosion, resulting in a hydrogen generation rate that was 307 times faster than pure Mg. However, prolonged annealing reduced the noble precipitates, leading to a decline in hydrogen generation rates. The feasibility of using Mg-3 wt.% Cu for power generation was demonstrated with a single-cell PEMFC, where just 10 g of the alloy produced 7.25 W for 37 min. (e.g., in this study, the hydrolysis was performed in a 3.5 wt.% NaCl solution [72].

2.6.4. Mg-Li Alloys

Jiang et al., synthesized a MgLi alloy with 10 wt.% graphite to refine grain structure and enhance hydrolysis properties. By adding ethylene glycol (EG), agglomeration during ball milling was prevented, while adding MgCl₂ during the ball milling improved hydrolysis kinetics. This allowed the MgLi + 10 wt.% EG composite to obtain a yield of 97% in 1 min in a 0.5 M MgCl₂ solution. After 12 h of ball milling, the composite formed thin disks of 1–2 mm in diameter. Sieved and sawed MgLi produced maximum hydrogen of 12.1 wt.% and 13 wt.% of H₂ within 3 min, respectively. Furthermore, treated sawed MgLi RBMH in 1 M MgCl₂ generated 1354 mL/g after 5 min and 1419 mL/g after 30 min with an activation energy of 24.6 kJ/mol, demonstrating high hydrogen production efficiency [32]. Then, the study of Tao et al. investigated the effects of both temperature and NH₄Cl solution concentration on hydrogen production rates from a Mg + 8 wt.% Li alloy. Results showed that increasing NH₄Cl concentration enhanced the hydrolysis reaction due to the promoting effects of NH₄⁺, Cl⁻, and H⁺ ions. At temperatures of 25 °C and 45 °C, hydrogen production volume decreased with rising temperature, while higher temperatures (above 65 °C) led to increased hydrogen production. Higher concentrations of HCl and NH₄Cl increased both hydrogen production and reaction rates due to elevated H⁺ and NH₄⁺/Cl⁻ concentrations. Between 25 °C and 45 °C, hydrogen yield decreased with temperature because of the protective film formed (that hindered the reaction). At 65 °C (and beyond), both hydrogen volume and reaction rates increased as the protective film on the alloy surface that inhibited hydrolysis was dissolved, thereby promoting the reaction [73]. To finish, Jiang et al. found that ball-milled MgLi alloy formed Li₃Mg₁₇, which transformed into Mg and LiH, accelerating hydrolysis and producing 1773 mL/g of hydrogen in 30 min [32].

2.6.5. Mg-Mg-Low Melting Point Metallic Element Alloy

Xiao et al. investigated a series of activated Mg-based alloys with low melting point metals (Zn, Sn, Bi, and In) [74]. Their hydrogen-generation properties were systematically investigated in simulated seawater and methanol. The results demonstrated that the addition of low melting point metals could significantly accelerate the reaction of magnesium with both seawater and methanol. The alloy Mg-10 wt.% In exhibited the best hydrolysis properties among all the samples, with the maximum hydrogen generation rate of 7.4 mL/g/s and the hydrogen conversion yield of 93% at 30 °C in seawater. The excellent hydrolysis performance of Mg + 10 wt.% In could be attributed to its lower activation energy (12.4 kJ/mol, 11.5 kJ/mol, 13.4 kJ/mol and 13.1 kJ/mol for In, Zn, Sn and Bi respectively), lower corrosion potential (−1.346 V, −1.236 V, −1.003 V and −0.979 V for alloys with In, Zn, Sn and Bi respectively), and higher corrosion current density (5.13 × 10⁻⁷ A, 3.63 × 10⁻⁷ A, 2.88 × 10⁻⁷ A and 1.62 × 10⁻⁷ A for alloys with In, Zn, Sn and Bi respectively). Moreover, the active preservation method of activated Mg alloys by coating with dioctyl sebacate (DOS) was studied.

Meanwhile, Mg + 10 wt.% In also showed a very fast hydrogen generation rate in methanol, providing a simple and clean method to prepare magnesium methoxide. Moreover, the activity of the activated Mg alloy could be effectively preserved by coating DOS on its surface. If we wanted to compare it with other alloys shown in

Table 4. Hydrogen generation properties of the activated Mg alloys in seawater at 30 °C. Adapted from Xiao et al. [74].

Alloys	Hydrogen Generation Volume (mL/g) at RT	Conversion Yield (%)
Mg + 10 wt.% Zn	875	95.1
Mg + 10 wt.% In	856	93.0
Mg + 10 wt.% Sn	810	88.0
Mg + 10 wt.% Bi	775	84.2

, Mg + 10 wt.% Zn generated a higher volume of hydrogen than the other. Nevertheless, the maximum hydrogen generation rate (speed of the reaction, not reported in Table 4) is much lower (almost divided by 2) in the case of Zn addition [74].

2.6.6. Mg-RE Alloys

The La₂Mg₁₇ and Mg₃La phases were first hydrided, and both were studied for their hydrolysis properties. The hydrolysis reaction of hydrogenated Mg₃La was almost complete within 21 min, with faster kinetics and higher yield than that obtained on hydrogenated La₂Mg₁₇. Hydrogen production of 43.8 mL/min/g was obtained during the first 20 min of reaction with Mg₃La as the one with La₂Mg₁₇ was only 40.1 mL/g/min. These results were explained by the catalytic effect of LaH₃ formed during the hydriding process, which improved the corrosion of MgH₂ (due to galvanic coupling) [29] and which was more numerous in Mg₃La than in La₂Mg₁₇ (according to the chemical composition).

Incorporating Mg-based alloys and their hydrides enhanced the hydrolysis performance of Mg and MgH₂. The addition of alkaline earth metals significantly boosted hydrogen generation due to their high reactivity with water, but these metals oxidized easily when exposed to air, raising synthesis and storage costs. Transition metals (TMs) and rare earth elements (REs) improved hydrolysis by forming micro-galvanic cells and acting as catalysts, although they reduced the global hydrogen generation capacity due to their high molecular weight. While other Mg-based alloys like MgAl and MgSi had been explored, they showed lower hydrogen generation compared to alloys with TMs or REs. Mg-based hydrides were more thermodynamically stable, offering a long shelf life, whereas Mg oxidized readily, hindering its hydrolysis [28,75]. Also, other alloys were coupled with RE like in WE43 alloys. It was a corrosion-resistant magnesium-based alloy, comprising 4% Y and 3% REs. Al Bacha et al. examined the effects of milling time, additives (carbon and nickel), and their synergistic interactions on hydrolysis performance. A pre-milling duration of 3 h was essential to activate the alloy and reduce particle size. Milling with carbon enhanced hydrolysis by forming a protective layer, while nickel introduced microstructural defects and created micro-galvanic cells with magnesium in NaCl electrolyte. The addition of C and/or Ni further improved hydrolysis, with the sequence of their introduction significantly impacting performance, adding Ni before C leading to optimal results. This combination allowed a complete hydrolysis reaction within less than 10 min, showcasing the enhanced capabilities of this already corrosion-resistant alloy [76].

2.7. Mg₂X-Based Alloys (X = Ni, Cu, Sn, Si)

Alloys such as Mg₂Cu and Mg₂Ni were studied by Xie et al., who improved hydrolysis by promoting galvanic coupling (between the alloy and the Mg) and intergranular corrosion (at the interface). Doping Mg with small amounts of Ni, Cu, Sn, and Si tended to form Mg₂X alloys after hydrogenation or ball milling, enhancing hydrolysis efficiency because of galvanic coupling between Mg and the alloys and also because of intergranular corrosion. Xie et al. showed that Mg₂X catalysts in Mg-Mg₂X eutectic alloys formed galvanic cells that facilitate hydrolysis and broke down the Mg(OH)₂ layer, aiding hydrogen release. For example, Mg-Mg₂Cu generated 479 mL/g of hydrogen in 60 min with a reduced activation energy of 36.91 kJ/mol. The presence of Mg₂Cu, Mg₂Ni, and Mg₂Sn at grain boundaries further increased hydrogen production, though Mg₂Si presented safety issues due to the possible silane generation [77]. In the paper of Wang et al., they studied a straightforward, cost-effective process for creating Ni nanoparticles through the hydrolysis of Mg(OH)Ni alloy, which produces Mg(OH)₂, Ni, and hydrogen gas. After being arc-melted and grounded in argon, magnesium-Ni pellets are hydrolyzed in water, which quickly releases hydrogen at first and stabilizes at a pH value in the range of 10 to 11. As Ni atoms bonded together to form nanoparticles, they were shielded from oxidation by Mg’s strong affinity for oxygen until all of the Mg₂Ni was consumed. XRD analysis verified that Mg in Mg₂Ni reacted with OH⁻ to form Mg(OH)₂. Other metal nanoparticles, including Co, Cu, and Ag, could also be synthesized using this technique [78]. Then, Mg₂NiH₄ was also used for producing Ni nanoparticles (NiNPs) and hydrogen. Ball-milling Mg₂Ni under hydrogen formed Mg₂NiH₄ and MgH₂, both of which hydrolyzed in water or KOH, creating Mg(OH)₂, Ni, and H₂. Mg₂NiH₄ dissociated into Mg₂Ni before further hydrolysis [79]. Increasing the temperature from 25 °C to 40 °C further improved kinetics, with an activation energy of 29.45 kJ/mol. The reaction produced Mg(OH)₂ and MgNi₂ as by-products, indicating complete hydrogen release, necessitating further investigation of their reuse [80]. Mg-Mg₂X (Cu, Sn) eutectic alloys were studied for hydrogen production in seawater-like NaCl solution. The alloys’ nano-lamellar structure created galvanic cells that accelerated hydrolysis, with Mg-Mg₂Cu showing superior performance due to a higher potential difference towards Mg. Both alloys produced hydrogen continuously without external power, with activation energies of 36.91 kJ/mol (Mg-Mg₂Cu) and 38.19 kJ/mol (Mg-Mg₂Sn) [35,81]. The paper of Tan et al. explored Mg₂Si for hydrogen production via a simple one-step hydrolysis reaction, marking the first use of Mg₂Si for this purpose. Mg₂Si traditionally posed explosion risks due to silane (SiH4) generation upon hydrolysis, but here, adding fluorine ions (i.e., NH₄F solution) effectively converted silanes to hydrogen, reducing silane content significantly. Initial tests in NH₄Cl solution showed a mix of hydrogen and silanes, with the gas passed through NaOH to separate H₂ [82].

Al₃Mg₂ (which is, by the way, richer in Al than in Mg) was studied by Cuzacq et al. This study investigated the formation of the intermetallic phase Al₃Mg₂ during solid-state sintering and its impact on hydrolysis properties. This intermetallic phase could be formed by reacting a mixture of 80 wt.% Al + 20 wt.% Mg heated at 400 °C for 10 min dwell time and 15 MPa pressure. At higher pressures, the system approached equilibrium, yielding Al, an FCC solid solution of Mg in Al, and Al₃Mg₂ without extending time or temperature. Hydrogen (H₂) production in simulated seawater was observed for all materials. Denser materials exhibited slower hydrolysis kinetics due to reduced surface reactions and increased bulk contributions. Importantly, the presence of Al₃Mg₂ enhanced H₂ production, attributed to galvanic coupling between Al₃Mg₂ and Al. This demonstrated the positive role of Al₃Mg₂ in improving H₂ generation efficiency (but then lower corrosion resistance), particularly at consistent porosity levels [83].

In another study, Uchiyama et al., worked on different Mg-Ca alloys (Mg + 10 wt.% Ca, Mg + 15 wt.% Ca, Mg + 16.2 wt.% Ca, and Mg + 20 wt.% Ca). The reaction rates significantly decreased at high temperatures due to the aggregation of hydroxides in the lamellar structure, which prevented contact with the NaCl solution. The hydrolysis reaction of the Mg-Mg₂Ca eutectic alloy occurred even in bulk form, showing a hydrogenation rate comparable to that of pure Mg powder pre-treated by ball milling [84].

2.8. Mg₃M-Based Alloys (M = La, Ce, Pr, Nd, Mm, In) and Ternary (and More) Alloys

As previously mentioned, Ouyang et al. developed Mg₃RE alloys (where RE = La, Ce, Pr, Nd, Mm) via induction melting, leading to hydrogenated alloys with phases like MgH₂ and hydrides of La, Ce, Pr, Nd, and Mm (36.0 at. % La, 45.0 at.% Ce, 15.0 at.% Nd, 5.0 at.% Pr). Mg₃Mm alloys showed the most rapid hydrolysis, releasing 695 mL/g of hydrogen in 5 min and 828 mL/g in 15 min, with a total hydrogen production of 9.79 wt.%. The MmH₃ phase promoted hydrogen ion diffusion by forming a conductive microelectronic system (usually named micro galvanic coupling). Additionally, reducing the particle size of Mg₃La and incorporating Ni or Al improved hydrogen generation through galvanic corrosion. Moreover, modifying the reaction media also leads to various kinetic behavior [85]. The Mg₃In alloy also enhanced hydrolysis due to its lower activation energy and higher corrosion current density [36].

Xu et al. investigated the hydrolysis performance of ternary alloys CaMg₂ (CM), CaMg₂Ni_0.1 (CMN), CaMg₂Zn_0.1 (CMZ), and CaMg₂Ti_0.1 (CMT) for hydrogen production. CMT achieved the highest yield of 1815 mL/g at 353 K (80 °C) and a decrease of the activation energy (Ea) from 37.77 kJ/mol to 15.66 kJ/mol due to the catalytic effects of TiO₂ and TiH₂ (present in CMT). In CMZ, the formation of Zn(OH)₂ from MgZn₂, which competed for OH⁻ with Mg(OH)₂, allowed the reduction of the formation of the dense Mg(OH)₂ layer and then promoted ongoing hydrolysis. The CMN alloy showed safe, gradual hydrolysis, aided by the uniform distribution of Ni particles from the Mg₂Ni and CM phases. Overall, adding elements to the CM alloy enhanced hydrolysis properties and kinetics [86]. The study of Buryakovskaya et al. explored hydrogen generation from twelve samples made of mixed Mg–Al–Zn and Mg–Nd–Zr–Zn scrap alloys, incorporating 0, 2.5, 5, and 10 wt.% low-melting-point Rose alloy (i.e., Bi–Sn–Pb Alloy) through high-energy ball milling for 0.5, 1, and 2 h. Testing in a 3.5 wt.% NaCl solution revealed that ball milling created lattice imperfections, enhancing susceptibility to corrosion. Most samples achieved optimal hydrogen yields with 1 h of activation, while the 10 wt.% Rose alloy performed best at 0.5 h due to its larger surface area. The addition of 2.5 wt.% Rose negatively affected hydrogen release, likely due to Pb and Sn inhibiting corrosion. Prolonged milling resulted in a dense oxide film that reduced performance. Therefore, the results suggested that the method could effectively utilize mixed Mg-based waste for hydrogen production even if the yield was not optimal [13].

Mg-Ni-Sn system was investigated by Oh et al. highlighting that Mg₂Sn at the grain boundary acted as the initiation site for pitting corrosion. The dissolved Sn in the alloy caused pitting corrosion by locally breaking the surface oxide film in the Mg matrix in seawater. The Mg alloy showed an excellent hydrogen generation that was 1700 times faster than that of pure Mg due to the combined action of galvanic and intergranular corrosion, as well as pitting corrosion in seawater. As the solution temperature was increased from 30 to 70 °C, the hydrogen generation rate from the hydrolysis of the Mg-2.7 wt.% Ni-x wt.% Sn alloy was drastically increased. The activation energy for the hydrolysis was 43.13 kJ/moL. Based on the increase of the hydrogen generation rate, the contributions to the increase from Ni and Sn were calculated to be 80% and 20%, respectively [87].

Alasmar et al. worked on a composite system made of Mg and NdNiMg₁₅. It was more effective in Nd-Ni-Mg (1:1:15) ball-milled mixtures than in NdNiMg₁₅ compounds, achieving 100% hydrolysis in 7 min due to structural defects induced by ball milling. Another severe plastic deformation process was used: cold rolling. It was shown that adding 15 wt.% NaCl during cold rolling enhanced the hydrolysis of NdNiMg₁₅, also reaching 100% yield after 10 min. Chloride salts like NaCl and NdCl₃ improved milling efficiency and conductivity and lowered activation energy, boosting hydrolysis performance. However, extended milling duration (5 h) caused oxidation of Mg, hindering the reaction, making optimization of the milling time essential (e.g., determined to be 3 h) [88]. Hou et al. worked with Ce and Ni to form Mg-Ni-Ce. Increasing cerium (Ce) content in Mg–Ni–Ce alloys reduced micro-mass transfer channels due to the decreased amount of the eutectic structure and the increased amount of Mg dendrites, enhancing Ce solubility and Mg–Ni–Ce reactivity. At 15 wt.% Ce, the formation of Mg₁₂Ce intermediate promoted galvanic cell formation with Mg phases, accelerating hydrogen generation via electrochemical corrosion. In 3.5 wt.% NaCl, the 5 wt.% Ce alloy achieved a high hydrogen generation of 840 mL/g compared to 641 mL/g for the 0 wt.% Ce alloy, with conversion yields of 90, 89, and 82% for 5 wt.% Ce, 10 wt.% Ce and 15 wt.% Ce alloys, respectively [89].

Magnesium alloys could also be coupled with Long Period Stacking Ordered (LPSO) structures, particularly in systems like Mg-Y-Zn, Mg-Gd-Zn, and Mg-Y-Ni. It was the case in the work of Legrée et al. It offered lower corrosion resistance and so efficient hydrogen (H₂) production via hydrolysis. This study examined four different LPSO additions (Mg₉₁Cu₄Y₅, Mg₉₁Ni₄Y₅, Mg₉₁Ni₄Gd₅, and Mg₉₁Ni₄Sm₅) to magnesium, measuring their hydrolysis behaviour on powders and their electrochemical properties on bulk samples. Results showed that LPSO 14 H structures had high corrosion current densities, enabling rapid corrosion in seawater, which favoured H₂ generation, as shown in Figure 5. Additionally, variations in rare earth (Y, Gd, Sm) and transition metals (Ni, Cu) altered the alloys’ microstructure and corrosion rates; alloys with complex LPSO phases and higher Ni content corroded faster, enhancing H₂ production efficiency [90].

Mg and Ni were used with La doping in the work of Hou et al. The as-cast Mg_23.5Ni alloy had a lamellar Mg-Mg₂Ni eutectic structure, and adding 10 wt.% La introduced the active La₂Mg₁₇ phase to form Mg_23.5Ni₁₀La. Both alloys (with and without La) produced H₂ in NaCl solution, with higher temperatures increasing the production rate and capacity. Mg_23.5Ni₁₀La showed better reaction kinetics and hydrogen yield than Mg_23.5Ni. The hydrolysis process differed: Mg_23.5Ni involved a three-dimensional reaction, while Mg_23.5Ni₁₀La followed one-dimensional diffusion. The La₂Mg₁₇ phase enhanced H₂ production by improving hydrolysis and forming colloidal Mg(OH)₂. It was pointed out that Mg_23.5Ni₁₀La produced 679 mL/g with a 98% yield [55].

He et al. worked with High Entropy Alloys (HEAs), and more precisely, rapidly solidified Al–Mg-Ga-In-Sn alloys. In their study, they demonstrated much faster reaction times and higher hydrogen generation rates with water compared to as-cast alloys, reacting within 5 min at 70 °C even with 10 wt.% Mg. The hydrogen generation rate was highly temperature-dependent, with rates reaching 952 mL/min/gat 70 °C for a 2 wt.% Mg alloy, while the as-cast equivalent achieved only 14.8 mL/min/g at 70 °C. Rapid solidification refined Al grain size to 2–4 μm and reduced precipitation phases, enhancing hydrogen yield and reaction rates. In contrast, in as-cast alloys, Mg promoted In and Sn segregation and intermetallic formation, decreasing the reaction rate and hydrogen yield as Mg content increased [91,92,93].

3. Environment Enhancing Hydrogen Generation

3.1. Chloride Salt

3.1.1. Sodium Chloride (NaCl)

Seawater (or, more precisely, equivalent seawater) was commonly used in corrosion studies of Mg-based alloys because of its potential abundance and relatively low environmental impact. A 3.5 wt.% NaCl solution was considered equivalent to seawater [13,15,17,94,95]. Consequently, the presence of acids enhanced the corrosion of the alloy. However, adding NaCl solution to the Mg-Al alloy did not improve its hydrolysis efficiency, as NaCl dissolution was an endothermic process. In their study, only 15% of Mg₁₇Al₁₂ reacted in a 3.5 wt.% NaCl solution, suggesting that complete hydrolysis of Mg-Al alloys was unlikely [15]. Zhong et al. worked in a 22.6 wt.% NaCl solution at low temperature. The Mg-Ca-based hydride exhibited the lowest hydrogen generation rate, producing 237 mL/g within 5 min. In the NaCl solution, the Mg-Ca hydride exhibited a high activation energy (Ea) of 30.02 kJ/mol, indicating lower hydrolysis reaction kinetics. Since H⁺ should have moved to the anode to be reduced to H₂, high Ea may have represented that the H⁺ mobility was relatively low despite a high H⁺ concentration [16,93]. When the temperature was increased from −20 °C (253 K) to 10 °C (283 K), hydrogen generation efficiency significantly improved, with a 146% increase in yield in the NaCl solution (Figure 6) [16]. As previously mentioned, NaCl as a salt additive during ball milling prevented particle agglomeration and so increased the surface reaction [12,13,15,17,93,94]. Lowering the pH of the solution further accelerated the reaction kinetics, primarily due to the increased concentration of ionized species and the breakdown of the Mg(OH)₂ passivation layer. Chloride ions adsorbed onto the Mg surface, penetrating the passivation layer and converting Mg(OH)₂ into soluble MgCl₂, which enhanced hydrolysis kinetics [14,16,17,79,93,95]. This process intensified pitting corrosion on Mg by creating numerous defects. These defects could have acted as sites that initiated hydrolysis reactions [14,15,17].

Liu et al. investigated the effects of NaCl concentration and temperature on the hydrogen generation rate of nanoporous Mg. It was indicated, but not explained, that an NaCl concentration of 5 wt.% was optimal, yielding a maximum hydrogen generation of 933 mL/g [94]. Additionally, they demonstrated that increasing temperature enhanced the hydrogen generation process, as described by the Arrhenius equation (Equation (1)):

$$\ln k=\ln k_0-{\displaystyle \frac{Ea}{RT}}$$

(1)

where k represents the reaction rate, k₀ is the reaction constant, Ea is the Activation energy (kJ/mol), R is the gas constant (8.314 J/mol/K), and T is the temperature in Kelvin.

3.1.2. Magnesium Chloride (MgCl₂)

Studies demonstrated that MgCl₂ significantly enhanced Mg Hydrolysis, with Mg conversion reaching up to 90% at 53 °C [14,96]. The MgCl₂ solution exhibited the smallest pH increase and so the lowest post-reaction pH, allowing for maximum H₂ generation at 273 K (0 °C), reaching 1132 mL/g in 60 min. This solution demonstrated superior hydrolysis reaction kinetics due to a low activation energy of 3.56 kJ/mol. The hydrolysis performance, in terms of cations used in the solution, could be classified as follows: Na⁺ < Ca²⁺ < Mg²⁺ < NH₄⁺ [16]. In a related study, Buryakovskaya et al. focused on hydrogen production at low temperatures (−40 °C to 0 °C) in a 21 wt.% MgCl₂ solution, achieving a hydrogen production rate of 37 mL/min/g at 0 °C, which declined to zero at −30 °C [96].

Additionally, Al Bacha et al. investigated the effect of directly adding 5 wt.% MgCl₂ as an additive during the ball milling of Mg-Al alloy (and not in the solution). The additional heat released from the exothermic dissolution of the salt further enhanced both the yield and kinetics of the hydrolysis reaction, achieving a maximum yield of 12% in 60 h, which was almost double the one without MgCl₂ addition [15].

3.1.3. Calcium Chloride (CaCl₂)

Zhong et al. studied the effect of CaCl₂ salts on the hydrogen generation performance of Mg-Ca alloy hydride (MCH) during ball milling, finding that CaCl₂ enhanced hydrogen yield more effectively than MgCl₂, with 15MCH-CaCl₂ achieving a yield of 1141 mL/g at 25 °C. Such results were in perfect agreement with [16]. This improvement was attributed to the significantly higher solubility of Ca(OH)₂ (1.73 g/L at 20 °C) in water compared to Mg(OH)₂ (0.000064 g/L at 25 °C), which facilitated the hydrolysis process of MCH. However, similar to Mg²⁺, the common ion effect of Ca²⁺ limited the dissolution of Ca(OH)₂, thereby limiting reaction progression during hydrolysis [97]. MCH exhibited the lowest hydrolysis efficiency in the NaCl solution, whereas in the CaCl₂ solution, hydrolysis performance increased notably with temperature. Specifically, as the temperature increased from −20 °C (253 K) to 10 °C (283 K), hydrogen yield rose by 173% in CaCl₂. This improvement was explained by reduced passivation in the CaCl₂ solution, as fewer surface layers formed on MCH particles due to the lower solubility of the resulting byproduct, which minimized surface blockage and enhanced reaction kinetics [16].

3.1.4. Potassium Chloride (KCl)/Lithium Chloride (LiCl)

Grosjean et al. showed that the enhanced conversion of magnesium in the presence of KCl is attributed to chloride ions destabilizing the Mg(OH)₂ passive layer, leading to pitting corrosion. Chloride ions replaced hydroxide ions, forming more soluble MgCl₂, which facilitated the localized breakdown of the passivation layer. As previously mentioned, at 20 °C, the solubility of MgCl₂ was 542 g/L, and the solubility of Mg(OH)₂ was 0.000064 g/L. Milled Mg powders, particularly those milled for 30 min, were more prone to pitting corrosion due to milling-induced defects that promoted chloride accumulation. However, prolonged milling duration reduced reactivity due to oxidation and a decrease in surface area (known as the cold welding phenomenon during the milling process). In contrast, KCl had a minimal impact on the hydrolysis of magnesium hydride (MgH₂), with only an increase in conversion yield from 26% in pure water to 37% in KCl for the 30-min milled sample, likely due to the insulating properties of MgH₂ that limited corrosion. Therefore, it could be concluded that the anion (e.g., Cl⁻) was not the only one responsible for the improvement of the hydrolysis performance [42].

Grosjean et al. also demonstrated, in another study, that the addition of KCl in aqueous media was notably more effective in enhancing the conversion yield of Mg powder than its incorporation in solid form. KCl achieved a 45% conversion yield after 1 h of hydrolysis, which was lower compared to the performance of LiCl under similar conditions. The use of LiCl resulted in a conversion yield of 57%. The salts were ranked in terms of their effectiveness as follows: MgCl₂ > LiCl > NaCl > KCl [14].

It is worth pointing out that when the salt was added directly to the Mg or Mg alloys, the results on hydrolysis properties often differed. Even if it was not explained, it was probably due to the dispersion of the additives in the solid form (which was more complex than in the liquid solution).

In their study, Buryakovskaya et al. investigated the effect of adding 20 wt.% KCl to a Mg-Al alloy to improve hydrogen production. At 35 °C, this mixture demonstrated the highest hydrogen evolution rate, achieving a 49% yield (e.g., 460 mL/g) within the first hour, though hydrogen production decreased at lower temperatures. To further enhance hydrogen yield, a modified mixture containing 10 wt.% KCl and 10 wt.% Wood’s alloy (containing 9.7 wt.% Sn, 40.4 wt.% Pb, 9.67 wt.% Cd and 40 wt.% Bi) was added. The powder with 20 wt.% KCl displayed the lowest activation energy of 7 kJ/mol, indicating highly favourable reaction kinetics. In contrast, the sample containing both 10 wt.% KCl and 10 wt.% Wood’s alloy showed a higher activation energy of 30.6 kJ/mol. Initially, the 20 wt.% KCl powder exhibited the highest hydrogen evolution rate, likely due to its fine particles and substantial specific surface area, which promoted rapid reaction onset. However, over a 4-h period, this sample produced the lowest overall hydrogen yield, likely due to the formation of a dense, compact product layer on particle surfaces that restricted further reaction progress [93].

3.1.5. Transition Metal Chlorides (NiCl₂, CoCl₂, CuCl₂, FeCl₂, MnCl₂)

Kantürk Figen et al. investigated the effect of 1 M solutions of various salts on hydrogen generation rates. Although their result showed that the NiCl₂ solution achieved the highest initial conversion rate, FeCl₃ exhibited the highest hydrogen generation rate overall. Among the solutions tested, CuCl₂ was the least reactive, achieving only a 60% conversion (Figure 7) [97].

Further studies indicated that MnCl₂ and NiCl₂ effectively enhanced hydrolysis, achieving up to 90% Mg conversion at 53 °C [97]. To further enhance hydrogen production, Kantürk Figen et al. introduced a Mg + 2.5 M NiCl₂ solution into Marmara and Aegean seawater (and not equivalent seawater as often), observing improvement in Mg conversion and hydrogen generation performance. The study identified H⁺ mobility and concentration as critical determinants of the volume of hydrogen gas produced. They found that while the hydrolysis reaction duration was influenced by changes in temperature and molarity, the initial hydrogen generation rate was predominantly affected by temperature. The Ea for hydrogen generation was calculated to be 21.12 kJ/mol in 2.5 M NiCl₂ solutions, with increasing NiCl₂ concentrations further reducing the Ea for H₂ evolution. The increase in total ion concentration, due to the addition of NiCl₂, effectively lowered the Ea required for hydrogen generation [98].

Sevastyanova et al. demonstrated that substituting NaCl with transition metal chlorides significantly enhanced the oxidation rate of both Mg and its alloys. Their findings revealed a significant enhancement in the oxidation process, with an optimal reaction rate of 1610 mL/g/min and a total hydrogen production of 890 mL/g achieved within 10 min. These results were obtained during the oxidation of an Mg + 10% Cu alloy in a 0.064 M CoCl₂ solution [99].

3.1.6. Aluminum Chloride (AlCl₃)

Al Bacha et al. examined the impact of an AlCl₃ solution on hydrogen generation, finding that it reduced particle size and crystallinity during ball milling, thereby increasing surface area and promoting crack formation. The hygroscopic nature of AlCl₃ resulted in HCl formation, which effectively dissolved the Mg(OH)₂ passivation layer, reducing the pH of the reaction medium while generating protons and releasing heat [60,96]. This pH decrease enhanced Mg(OH)₂ solubility, promoting continuous hydrolysis and improving reaction kinetics. Additionally, the co-milling of Mg with graphite and AlCl₃ further enhanced hydrolysis efficiency, resulting in a 16% increase in hydrogen generation in less than 5 min [15]. AlCl₃ demonstrated optimal performance in enhancing hydrogen generation and was further shown to improve the electrolysis performance of Mg₁₇Al₁₂ [60]. Similarly, Liu et al. explored the effect of AlCl₃ on the Mg-LiBH₄ system, in which adding 1 wt.% AlCl₃ promoted magnesium powder hydrolysis. However, higher AlCl₃ concentrations led to grain growth and a decrease in specific surface area, which subsequently lowered conversion yield [34]. Buryakovskaya et al. conducted a study focusing on hydrogen generation at very low temperatures, approximately −40 °C, using a range of AlCl₃ concentrations. They also observed that higher concentrations of AlCl₃ decreased the hydrogen release rate under these conditions. This reduction was attributed to a decrease in mobility and reactivity of chloride ions (Figure 8) [96]. Additionally, it was shown that the addition of 3 mol% AlCl₃ (i.e., 18.64 wt.%) significantly enhanced the activation effect, achieving a 93.86% yield at 25 °C [100].

3.1.7. Ammonium Chloride (NH₄Cl)

The study by Zhong et al. demonstrated that in an NH₄Cl solution, MCH exhibited the highest initial hydrolysis performance, producing 1198 mL/g of hydrogen within the first 5 min and achieving the maximum hydrogen production amount. The final conversion rate reached 89% at 0 °C (273 K), and the activation energy in NH₄Cl solution was 15.37 kJ/mol. Despite the low temperature, the hydrogen yield of MCH remained the highest in NH₄Cl solutions. In an NH₄Cl solution, OH⁻ ions tended to migrate toward NH₄⁺ ions surrounding the MCH particles rather than toward Mg²⁺ and Ca²⁺ ions on the MCH surface due to the stronger affinity of OH⁻ for NH₄⁺. The interaction between NH₄⁺ and OH⁻ produced NH₃ and H₂O. Furthermore, the pH of the NH₄Cl solution was the lowest (pH = 5.16) among the chloride solutions studied, which explained the superior initial hydrolysis performance and the highest hydrogen yield observed. However, the generated NH₃ was highly soluble in water, leading to a rapid increase in the solution’s pH, which subsequently impeded the continuation of the reaction [16].

3.2. Organic Acids (Citric, Acetic and Oxalic Acids)

Yu et al. examined the impact of citric acid addition to seawater on hydrogen production efficiency as citric acid was a weak organic acid, cheap and nontoxic. Their findings indicated that introducing 5 wt.% citric acid allowed to produce 19 L of H₂, while increasing the citric acid concentration to 30 wt.% generated 66 L of H₂ over a 3-h period (for 70 g of sample). It is worth pointing out that only 56 L of H₂ was obtained when 8 wt.% NaCl was used, demonstrating the interest of citric acid addition. In highly concentrated citric acid solutions with NaCl, a notably low activation energy (Ea = 8.40 kJ/mol when 5 wt.% citric acid was used) was observed, suggesting that both the mobility and concentration of H+ ions affected the total hydrogen yield. Consequently, while 30 wt.% citric acid allowed reaching a high overall H₂ volume, further increased beyond this concentration led to diminished H₂ production. The reason was as follows: the high citric acid concentration increased H+ concentration, contributing to an elevated cumulative H₂ volume. However, the increased solution viscosity in highly concentrated citric acid reduced H+ mobility, which limited the initial reaction rate [92]. Similarly, Uan et al. reported that 20 wt% citric acid seawater significantly outperformed 5 wt%, with 70 g of magnesium scrap producing 40 L of H₂ and 140 g generating 55 L of H₂ within 3 h [101]. Additionally, the non-volatile nature of citric acid minimized the contamination of the hydrogen gas produced, ensuring high-purity output. Across a wide concentration range, citric acid maintained a pH between 3 and 5, effectively preventing the formation of Mg(OH)₂ passivation layers on MgH₂ particle surfaces [102]. Acetic acid, also known as ethanoic acid, was tested at a concentration of 15 wt.% in seawater to evaluate its effectiveness in enhancing hydrogen generation. Results indicated that increasing the NaCl concentration within the acetic acid solution did not impact the H₂ yield much, which remained almost stable across different NaCl levels (from 0 to 8 wt.%). In a 15 wt.% acetic acid solution without NaCl, the Low-grade Magnesium Scrap (LGMS) sample produced 40 L of H₂ (using 70 g of sample) over a 3 h period [92]. The highest hydrogen generation rate was observed with a 30 wt.% acetic acid solution at 50 °C [4]. However, due to potential hazards associated with high acetic acid concentrations, experiments using more concentrated acetic acid solutions were not conducted. Acetic acid’s efficiency in enhancing hydrogen production could be attributed to its reactivity with Mg(OH)₂, forming magnesium acetate, an easily soluble compound, which prevented the buildup of a passivating Mg(OH)₂ layer on the magnesium surface.

Oxalic acid maintained an almost stable pH = 2 during the hydrolysis process. This pH stability was primarily due to the low solubility of magnesium oxalate (0.0038 g/L at 20 °C) in water, which necessitated a highly acidic environment to achieve complete hydrolysis. Consequently, a more acidic medium was required to drive the reaction to completion, ensuring effective hydrogen generation [102].

The introduction of organic acids markedly enhanced the H₂ yield, increasing from 40% to 100% at a concentration of 1 and 3 wt.% of acetic acid (AA), respectively. The effect of different organic acids on maximum H₂ yield was comparable, with acetic acid showing a slightly higher yield than the others (Figure 9).

3.3. Inorganic Acids

Hydrochloric Acid (HCl), Sulfuric Acid (H₂SO₄), Nitric Acid (HNO₃)

Tayeh et al. examined the influence of pH on the hydrolysis reaction of MgH₂ by employing HCl, HNO₃, and H₂SO₄. Their results demonstrated that H₂SO₄ notably enhanced the hydrolysis kinetics, resulting in a greater volume of hydrogen release. This accelerated reaction rate with H₂SO₄ was likely due to its high enthalpy of formation, which exceeded that of Mg(OH)₂, as well as its nature as a di-acid, providing additional protons to drive the reaction. Additionally, the study revealed that lowering the pH to 2 by increasing the concentration of HCl in deionized water significantly promoted the hydrolysis process, thereby increasing the hydrogen release rate (Figure 10) [103].

3.4. Nitrogen-Based Inorganic Salts

Ammonium Nitrate (NH₄NO₃)

Cui et al. examined the influence of ammonium nitrate (NH₄NO₃) at varying concentrations on the corrosion behavior of magnesium in a 0.1 M (i.e., 0.58 wt.%) NaCl solution. In the NaCl-only environment, corrosion rates initially declined over the first 48 h, followed by fluctuations as immersion time increased. The addition of NH₄NO₃ introduced multiple ions into the solution, including Na⁺, Cl⁻, NH₄⁺, NO₃⁻, NH₄OH, H⁺, and OH⁻, which created an acidic medium due to the hydrolysis of NH₄⁺. The presence of NH₄NO₃ significantly accelerated the corrosion rate, largely through the destabilization of the protective Mg(OH)₂ layer, primarily due to the activity of NH₄⁺ ions. At a concentration of 0.001 M NH₄NO₃ (0.0058 wt.%), localized intact regions on the magnesium surface were observed, whereas a higher concentration of 0.1 M NH₄NO₃ added resulted in a uniformly corroded specimen surface, indicating more pervasive corrosion effects [104].

3.5. Brewery Wastewater

Depending on the medium, seawater and brewery wastewater exhibited distinct effects on magnesium-based hydrolysis for hydrogen generation. In a study by Akbarzadeh et al. using brewery wastewater with 30 wt.% acetic acid, a hydrogen yield of 66.4% was achieved within 15 min, along with a 62.4% reduction in chemical oxygen demand (COD), demonstrating its dual-purpose utility [85]. Unlike seawater, which relied on added salts or elevated temperatures to enhance hydrolysis, brewery wastewater inherently provided acidity that prevented the formation of Mg(OH)₂ passivation layers, maintaining favourable reaction conditions without requiring additional additives. This process would allow recycling both wastes: Mg alloys and brewery water.

4. Conclusions

This review examines hydrogen production through magnesium hydrolysis, highlighting its significance in the energy transition. Articles were selected from the Scopus database based on their novelty. Magnesium’s abundance, high reactivity, and potential for industrial waste recycling make it a strong candidate for sustainable hydrogen production. A key advantage is its ability to utilize non-potable water, offering both environmental and economic benefits.

Magnesium hydrolysis presents a promising route for clean hydrogen generation. However, the formation of a passivating Mg(OH)₂ layer hinders the reaction. To overcome this, various strategies—such as adding salts, metal oxides, and alloys and applying mechanical treatments—have been developed to enhance reaction kinetics and hydrogen yield.

A growing research interest focuses on innovative solutions, including magnesium alloys with transition and rare earth metals, as well as repurposing industrial magnesium waste for cost-effective and sustainable hydrogen production. Advances in nanostructuring and synthesis techniques further improve reaction efficiency.

To enable large-scale applications, optimizing experimental conditions and refining reaction mechanisms remain crucial. Future research should prioritize enhancing material reactivity, reducing production costs, and integrating hydrolysis into broader hydrogen energy systems.

In conclusion, magnesium hydrolysis offers a viable solution to energy and environmental challenges while leveraging an abundant, recyclable resource. However, technological and industrial advancements are necessary to ensure its efficient, scalable, and competitive implementation within the hydrogen economy.