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Review

Review on Magnesium Hydride and Sodium Borohydride Hydrolysis for Hydrogen Production

by
Nuraini Ruslan
1,
Muhammad Syarifuddin Yahya
1,*,
Md. Nurul Islam Siddique
2,
Ashish Prabhakar Yengantiwar
3,
Mohammad Ismail
1,
Md. Rabiul Awal
2,
Mohd Zaki Mohd Yusoff
4,
Muhammad Firdaus Asyraf Abdul Halim Yap
5 and
Nurul Shafikah Mustafa
1
1
Energy Storage Research Group, Faculty of Ocean Engineering Technology and Informatics, University Malaysia Terengganu, Kuala Terengganu 21030, Terengganu, Malaysia
2
Faculty of Ocean Engineering Technology and Informatics, University Malaysia Terengganu, Kuala Terengganu 21030, Terengganu, Malaysia
3
Department of Physics, Fergusson College (Autonomous), Savitribai Phule Pune University, Pune 411004, India
4
School of Physics and Material Studies, Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam 40450, Selangor, Malaysia
5
Faculty of Innovative Design and Technology, Universiti Sultan Zainal Abidin, Kuala Nerus 21300, Terengganu, Malaysia
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(10), 1376; https://doi.org/10.3390/cryst12101376
Submission received: 23 August 2022 / Revised: 24 September 2022 / Accepted: 25 September 2022 / Published: 28 September 2022
(This article belongs to the Special Issue State-of-the-Art Magnesium Alloys)
Browse Figures
Versions Notes

Abstract

:
Metal hydrides such as MgH2 and NaBH4 are among the materials for with the highest potential solid-state hydrogen storage. However, unlike gas and liquid storage, a dehydrogenation process has to be done prior to hydrogen utilization. In this context, the hydrolysis method is one of the possible methods to extract or generate hydrogen from the materials. However, problems like the MgH2 passivation layer, high cost and sluggish self-hydrolysis of NaBH4 are the known limiting factors for this process, but they can be overcome with the help of catalysts. In this works, selected studies have been reviewed on the performance of catalysts like chloride, oxide, fluoride, platinum, ruthenium, cobalt and nickel-based on the MgH2 and NaBH4 system. These studies show a significant enhancement in the amount of hydrogen released as compared to the hydrolysis of the pure MgH2 and NaBH4. Therefore, the addition of catalysts is proven as one of the options in improving hydrogen generation via the hydrolysis of MgH2 and NaBH4.

1. Introduction

A crucial component of a sustainable development community is the possible creation, preservation, and generation of new renewable, ecologically friendly energies, especially given the rapid use of energy sources [1]. Sources of energy including coal, oil, and gas, on the other hand, continue to rise in price, resulting in the urgent need for renewable energy sources [2]. Thereupon, hydrogen has been considered to be among the most suitable replacements to fossil fuels over the last ten years as a clean and energy carrier resulting from high energy requirement and intensified environmental degradation [3]. Moreover, hydrogen’s chemical energy per unit mass (142 MJ/kg) is almost three times more than any other fuel (for example, hydrocarbons’ chemical energy per unit mass itself is 47 MJ/kg) [4]. Hydrogen also produces water as its by-product from the combustion process which is environmentally-safe [5].
Hydrogen can be stored in gas, liquid and solid form. However, the high working pressure (70 MPa) for gaseous-state hydrogen storage and low temperature requirements (−253 °C) for liquid-state hydrogen storage had restrained their applications. Likewise, because hydrogen must be stored at 253 °C, it loses a huge proportion of energy (up to 40%), making it ineffective for long-term hydrogen storage. Solid-state hydrogen storage has some benefits over gaseous and liquid hydrogen storage. With substantial pressure and temperature requirements, the storage method offers a larger energy capacity. Besides that, unlike hydrogen storage in gas and liquid, hydrogen stored in solid need to undergo an extraction process so that the hydrogen can be utilized.
However, the global-scale utilization of hydrogen as a major energy source has been restricted by many issues starting from its production. Although hydrogen is inexhaustible and the richest element in the universe, it is normally bound to other elements to form compounds like hydrocarbon fuels, water and other organic forms. Owing to this characteristic, hydrogen needs to be extracted or to be produced prior to its utilization. However, up to this point, hydrogen production is coming from fossil fuels like natural gas, oil, naphtha and coal. Meanwhile, green hydrogen generation methods such as photocatalysis [6], biomass [7] and water electrolysis [7] have been studied. In recent years, more and more attention has been paid to hydrogen generation by hydrolysis of metal hydrides for their high theoretical hydrogen yield [8,9,10].
As stated by Brack et al. [5], hydrogen production via metal hydrides consists of two methods which are thermolysis and hydrolysis. The hydrogen is produced by heating for thermolysis and reaction with water for hydrolysis. Hydrolysis is more appealing for MgH2 and NaBH4 due to the spontaneous and exothermic process that can be improved with the help of a catalyst. Also, they can hydrolyze at a low temperature such as room temperature [11,12].
To date, the hydrolysis of metal hydrides is widely studied. This includes the hydrolysis of MgH2 and NaBH4. Both the overall amount of generated hydrogen and the kinetic parameters in the hydrolysis process are highly influenced by the type of hydride, active surface area, pH value and temperature of water [13,14,15,16,17]. Besides that, high cost of electrical energy usage is a significant constraint even when hydrogen can be readily produced by electrolysis from water [18]. Even so, with the process of hydrolysis, it is plausible to generate hydrogen by disintegrating metal hydrides at ambient temperature in an aqueous solution. Also, the hydrolysis process happens naturally at a temperature lower than the electrolysis process.
This article reviewed the hydrolysis of metal hydrides focusing on MgH2 and NaBH4 with the aid of catalysts such as chloride, oxide, fluoride, platinum, ruthenium, cobalt and nickel-based catalysts. The aim is to classify the types of catalysts and its effect that have been studied by other researchers on the hydrolysis of MgH2 and NaBH4.

2. Influence of Catalysts on the Hydrogen Production of MgH2

This section focused on MgH2 hydrolysis and the utilization of several catalysts to enhance the hydrolysis process by previous researchers.

2.1. Why MgH2

Recently, several researchers have shifted their focus to hydrolysis of MgH2 as MgH2 has so far been considered to be among the most effective metal hydrides for hydrolysis. Although it has a lower capacity for hydrogen storage (7.6 wt.%), as compared to NaBH4 and NH3BH3, but it is relatively low cost [19]. When stoichiometric water is taken into account, it has been reported that the hydrogen yield of the hydrolysis reaction of MgH2 is 6.4 wt.% [20]. Furthermore, due to MgH2’s strong reducibility, when it interacts chemically with water at room temperature, as depicted in Equation (1), the hydrogen production can reach to 15.2% wt.% if the weight of water is not evaluated, making it particularly ideal for the fuel cell in terms of supplying hydrogen [13,21,22,23,24].
MgH2 + 2H2O → Mg(OH)2 + 2H2 ↑ ∆H = −277 kJ/mol
Equivalently, the by-product from MgH2 hydrolysis is insufficiently water-soluble, thus hydrolysis promptly stops when a passive layer of magnesium hydroxide forms on the surface of unreacted MgH2 [14]. The passive magnesium hydroxide layer blocks hydrolysis by reducing the kinetics of MgH2 due to its high pH value which is 10.44 [25]. After only 1 min of hydrolysis, the pH of the solution has increased to the point where Mg2+ solubility is minimal [20]. The maximum conversion of MgH2 in direct hydrolysis cannot surpassed 30% in 1 h, according to prior research, which is insufficient for practical implementation [16,25,26,27]. Therefore, several methods have been used to extract the passivation layer and boost MgH2 hydrolysis effects which include alloying, ball milling, modifying the aqueous solution and many others [11]. Because of the increased specific surface area, ball milling leads in a significant increase in the reactivity of MgH2, but the milling time should not surpass one hour because this will induce particle agglomeration and the hydrolysis properties of the resulting materials will deteriorate [28]. Similarly, the introduction of structural defects and the development of nanocrystalline structures are advantageous to MgH2 hydrolysis [29]. The example of set-up apparatus to study MgH2 hydrolysis by Chen et al. [30] was as in Figure 1.
However, optimum milling time needs to be studied as reported by Grosjean et al. [16] where they found that the 0.5 h of milling time was the best compared to 3 and 10 h of milling for the Mg and MgH2 hydrolysis. In addition, Bacha et al. [31,32] reported that Mg alloy that milled for more than 2 h with and without AlCl3 will have lower hydrolysis performance as compared to 5 and 10 h of milling time due to powder oxidation and homogeneity of the defect distribution.
Furthermore, hydrolysis at higher temperatures will increase the hydride hydrolysis. Previous researchers such as in [11,14,33,34] have shown and reported the significance of temperature on the hydrolysis performance of MgH2. Higher temperatures significantly boost the hydrogen yield for the hydrolysis process in terms of kinetics and capacity.

2.2. Chloride-Based Catalysts

Chloride salts are considered as a potential catalytic material in hydrolysis. One of the reasons is because Cl− can destabilize or disintegrate the passivation layer of magnesium hydroxide by a pit corrosion process [11]. Figure 2 highlights the function of chloride ions in the hydrolysis process. According to Tegel et al. [13], it is evident that the pH of the reaction mixture and the reaction’s overall yield are only positively influenced by additives that contain anions from powerful Brønsted acids, such as Cl−. The fact that hydride acts as a base and raises the reaction’s pH can be used to explain this observation. As a result, magnesium hydride rapidly develops strong passivation layers that stop any further reaction.
Gan et al. [11] studied hydrolysis of MgH2 catalyzed with 0.1 M and 0.5 M AlCl3 from 5 °C to 30 °C. From the result, for both 0.1 M and 0.5 M of AlCl3, the hydrogen yield was the highest at 30 °C. For comparison, at 10, 20 and 30 min, the amount of hydrogen yield for 0.1 M of AlCl3 was 805 mL/g, 1004 mL/g and 1061 mL/g. In contrast, for the 0.5 M of AlCl3, at a similar time frame, the amount of hydrogen yield was 1487 mL/g, 1683 mL/g and 1700 mL/g, for the 10, 20 and 30 min, respectively. Meanwhile, for the activation energy, the reported values were 35.68 kJ/mol (0.1 M AlCl3) and 21.64 kJ/mol (0.5 M AlCl3). Korablov et al. [36] suggested the MgH2 + 5 wt.% AlCl3 sample as an effective hydrogen generating material due to the efficient hydrogen production of 557 mL/g and a conversion rate of 30.3% within 10 min of hydrolysis.
Into the bargain, Huang et al. [14] reported the effect of 0.5 wt.% and 4.5 wt.% of NH4Cl from 20 °C to 90 °C. From their work, they found that at 90 °C, the amount of hydrogen yield was 1000 mL/g (10 min), 1100 mL/g (20 min) and 1154 mL/g (30 min) for 0.5 wt.% of NH4Cl. As for the 4.5 wt.% of NH4Cl, under a similar time and a much lower temperature (60 °C), the amount of hydrogen yield was 1604 mL/g (10 min) and constant (1660 mL/g) from 20 min to 30 min. The reported activation energy was 50.858 kJ/mol for 0.5 wt.% of NH4Cl and 30.373 kJ/mol for 4.5 wt.% of NH4Cl. A vast improvement of activation energy was observed when the catalyst was added which the value decreased from 58.058 kJ/mol.
Moreover, Li et al. [37] investigated the effect of 3%, 5% and 10% of NaCl, MgCl2 and NH4Cl at 25 °C. Based on their result, NH4Cl showed the best performance with the highest hydrogen yield while NaCl recorded the lowest value of hydrogen yield. The amount of hydrogen yield for 3% of NH4Cl was 581 mL/g (10 min), 761 mL/g (20 min) and 872 mL/g (30 min). Next, for 3% of NaCl, the recorded value for hydrogen yield was 413 mL/g (10 min), 540 mL/g (20 min) and 620 mL/g (30 min). Based on the best result obtained in this research, it can be seen that 10% of NH4Cl greatly improves the hydrolysis performance of MgH2 with the hydrogen yield of 916 mL/g (10 min), 1105 mL/g (20 min) and 1208 mL/g (30 min). On top of that, NH4Cl also recorded the highest conversion rate in 60 min (85.69%) as compared to both NaCl (64.64%) and MgCl2 (68.95%).
Besides that, Zhou et al. [25] disclosed the outcome of MgH2-0.5 M MgCl2 and MgH2-H2O systems. According to their result, at 30 °C, the amount of hydrogen yield for H2O was measured as 257 mL/g (10 min), 291 mL/g (20 min) and 318 mL/g (30 min). Under similar temperature, with the use of 0.5 M MgCl2 as a catalyst, the amount of hydrogen yield for 10, 20 and 30 min was 1431 mL/g, 1580 mL/g and 1621 mL/g respectively. This demonstrated that the initial result of 0.5 M MgCl2 was nearly five times better than H2O. In addition, the value of conversion rate for 0.5 M MgCl2 was marked at 96% in just 30 min which is higher than H2O (20%). When MgCl2 was added to the mixture, the pH value decreased in conformance with Equation (2), promoting the decomposition of Mg(OH)2su and Mg(OH)2aq (Mg(OH)2su and Mg(OH)2aq referred to Mg(OH)2 developed on the surface of MgH2 and that formed in the solution, respectively). When Mg(OH)2su dissolves, a fresh surface of MgH2 was exposed to the solution and reacts with water and HCl in accordance with Equation (3). As a result, every mole of MgH2 that reacted produced two moles of hydrogen. According to Equation (4), once the Mg(OH)2su passive film had developed and had slowed the release of hydrogen, HCl would then dissolve the passive layer. To maintain the solution’s low pH, Mg2+ from Mg(OH)2su and MgH2 simultaneously replaces Mg2+ used in the formation of Mg(OH)2aq.
MgCl2 + 2H2O − Mg(OH)2aq↓ + 2HCl
MgH2 + 2HCl − MgCl2 + 2H2↑
Mg(OH)2su↓ + 2HCl − MgCl2 + 2H2O
In line with Berezovets et al. [38], hydrolysis performance improved as the relative quantity of MgCl2 increased, with the best results coming from the stoichiometric ratio MgH2 + 0.7MgCl2 (MgCl2/MgH2 weight ratio of 12.75/100). This resulted in a 1025 mL (H2)/g MgH2 hydrogen production. Maximum hydrogen production reached 89% of theoretical hydrogen generation capacity within 150 min of starting the hydrolysis, with 35% of hydrogen released in the first 10 min and a hydrogen generation rate of 800 mL/min/g MgH2.
The latest research done by Zhou et al. [39] centralized on the hydrolysis of MgH2 with the addition of 10 wt.% of CoCl2. Based on their research, they found that the interaction of MgH2 with deionized water exhibited unfavourable kinetics, yielding only 377 mL/g of hydrogen after 60 min and 30 °C, whereas adding acid chloride improved the hydrolysis performance greatly. Under the same conditions, a 3 h milled MgH2 doped with 10 wt.% of CoCl2 produced 1481.7 mL/g of hydrogen and initiated the value of activation energy to be 17.59 kJ/mol. The activation energy reported by Coşkuner Filiz [10] was 20 kJ/mol alongside the ideal concentration of catalyst was found to be 6.25 wt.% CoCl2, with the quickest hydrogen generation rate of 18.55 mL/min/g and full conversion of MgH2 to Mg(OH)2.

2.3. Oxide-Based Catalysts

As proclaimed by Awad et al. [35], only MgO has been investigated in the reaction of hydrolysis even though many other metal oxides were already tested as catalysts in the reaction of hydrogen sorption for Mg or MgH2. In spite of that, the high reactivity of MgH2 with water leads to an increment in the rate of hydrogen yield due to the presence of numerous defects and clean surfaces in conjunction with the decreasing size of particles throughout the milled process of the MgH2-MgO system [40].
Kojima et al. [26] researched the reaction of hydrolysis for MgH2 catalyzed by 50 mg of Pt-LiCoCO2 and 2 wt.% of acetic acid at 20 °C. By referring to their result, for MgH2 + 2 wt.% of acetic acid, the reported percentage of hydrogen yield was 60% (10 min), 68% (20 min) and 72% (30 min). Comparatively, MgH2 + 50 mg of Pt-LiCoCO2 + 2 wt.% of acetic acid showed the best kinetic performance with the percentage of hydrogen yield 84% (10 min), 90% (20 min) and 96% (30 min). This validated that the use of Pt-LiCoCO2 as a catalyst helped in increasing the hydrogen yield percentage up to more than 20%. In addition, in 60 min, almost 100% of hydrogen was generated when the system was catalyzed by Pt-LiCoCO2.
In addition, research done by Awad et al. [35] fixated on the effectiveness of carbons, transition metals and oxides such as 10 wt.% of Nb2O5 and V2O5 as catalysts. The effect of 10 wt.% of Nb2O5 and V2O5, ball-milled from 1 h to 5 h at 25 °C was studied. As stated from their result, 10 wt.% of Nb2O5, ball milled for 1 h, exhibited the percentage of hydrogen yield 96% in 10 min and 100% in both 20 and 30 min. In comparison, the exhibited percentage of hydrogen yield for 10 wt.% of V2O5 was 80% (10 min), 96% (20 min) and 100% (30 min) under the same condition. This confirmed that Nb2O5 was more effective as a catalyst and it also owned an activation energy of 31.46 kJ/mol.
The impacts of graphite (C) and Fe2O3 on the kinetics of MgH2 hydrolysis were explored by Yang et al. [41]. When compared to pure MgH2, they found that both MgH2 + C and MgH2 + C + Fe2O3 samples performed better in terms of hydrolysis reaction rate and hydrogen conversion rate. The MgH2 + C + Fe2O3 sample had the highest hydrolytic yield rate and conversion rate of all of them. The 30 min conversion rate of MgH2 + C + Fe2O3 sample at 80 °C is 9.8% greater than that of the MgH2 + C sample, and 42.6% greater than that of pure MgH2. The hydrolysis activation energy of pure MgH2 was also discovered to be 55.57 kJ/mol. The introduction of C and Fe2O3 to MgH2 + C and MgH2 + C + Fe2O3 samples reduced the activation energy values to 43.40 kJ/mol and 36.92 kJ/mol, correspondingly.
Recently, Xie et al. [42] investigated the systems of MgH2 doped with 10% of Nb2O5 and 10% of CeO2. On the hydrolysis performance of MgH2, Nb2O5 is discovered to have superior facilitating effects than CeO2. Within 60 min and in the temperature of 20 °C, MgH2 doped with 10% of Nb2O5 and 10% of CeO2 generated 705 mL/g and 474 mL/g of hydrogen in distilled water, respectively. In a 5% of MgCl2 solution, the sample of MgH2 doped with 10% of Nb2O5 generated 1222 mL/g of hydrogen, equating to a conversion rate of 74.6%. This proved that the addition of MgCl2 to water can also improve the kinetics and conversion rate of the hydrolysis reaction. Nevertheless, in MgCl2 solution, an air-exposed MgH2 doped with 10% of Nb2O5 sample barely generated any hydrogen. Meanwhile, after 24 h of air exposure, the sample of MgH2 doped with 10% of CeO2 could generate 250 mL/g of hydrogen. This is because CeO2 can alter oxidation resistance.
In the same year, Naseem et al. [43] studied the Mo and B2O3 catalysts for the co-hydrolysis of Al and MgH2. In the 1 M AlCl3 solution at ambient temperature, the Al12Mg17-1.5 wt.% Mo and Al12Mg17-3.5 wt.% B2O3 composites yielded 1341.6 mL/g and 1304 mL/g of hydrogen. Moreover, the greatest hydrogen yield was reached when both Mo and B2O3 were included as additions, with 1420.2 mL/g of hydrogen released in the 1 M AlCl3 solution at ambient temperature. After 2 h of milling in 0.1, 0.5, and 1 M AlCl3 solutions, the hydrolysis of Al12Mg17-1.5 wt.% Mo-3.5 wt.% B2O3 yielded evident activation energies of 16.4, 13.3, and 9.4 kJ/mol, correspondingly. Additionally, the H3BO3 that was produced from the interaction with water caused a concentration of H+ ions to increase, further shattering the layers of MgAl2O4 and Mg(OH)2. This procedure increased the number of new surfaces available for the subsequent hydrolysis reaction, which also improved the hydrolysis kinetics.

2.4. Fluoride-Based Catalyst

In consonance with Mao et al. [33], fluorides are also anticipated as being efficient catalysts to stimulate the reaction of hydrolysis by considering the fact that the properties of fluorides are close to those of chlorides and bromides. On top of that, the hydrogen absorption and resistance of oxidation will significantly improve due to the development of the fluoride layer on top of the magnesium particles. Nonetheless, a limited amount of research projects have been published on the fluorides’ catalytic performances for the hydrolysis of MgH2.
Correspondingly, the hydrolysis of MgH2 catalyzed by 10 wt.% of VF3, NiF2, LaF3 and CeF3 from 20 °C to 50 °C was studied by Mao et al. [33]. Based on their study, for 10 wt.% of VF3, NiF2, LaF3 and CeF3, the highest hydrogen yield was detected at 50 °C. Meanwhile, 10 wt.% of VF3 showed the fastest hydrolysis kinetic with the amount of hydrogen yield 1491 mL/g (5 min), 1534 mL/g (10 min) and 1550 mL/g (15 min). This contrasts with 10 wt.% of NiF2 which showed the slowest hydrolysis kinetic with the amount of hydrogen yield 1136 mL/g (5 min), 1173 mL/g (10 min) and 1179 mL/g (15 min). In the meantime, the values of activation energy for 10 wt.% of VF3 and NiF2 were recorded as 95.56 kJ/mol and 88.61 kJ/mol respectively.

2.5. Other Catalysts

The milled MgH2 with the addition of catalyst brings benefits including the synergies among nanostructuring and catalyst supports, allowing the synthesis of numerous products with greater potential for hydrogen yield [44]. There are many other types of catalyst which were found to be effective for the hydrolysis of MgH2. For instance, since Ge was viewed as an efficient catalyst for ammonia borane, so Ge was presumed to be effective in MgH2 [44].
Research done by Adeniran et al. [44] focused on the use of Ge (5–20 wt.%) as a catalyst for hydrolysis of MgH2 in 30% concentration of citric acid and acetic acid from 30 °C to 50 °C. According to their work, at 30 °C, for 5 wt.% of Ge in 30% citric acid, the amount of hydrogen yield was 700 mL/g (10 s), 1100 mL/g (20 s) and 1350 mL/g (30 s). Also at 30 °C, for 5 wt.% of Ge in 30% acetic acid, the amount of hydrogen yield was 1000 mL/g (10 s), 1300 mL/g (20 s) and 1360 mL/g (30 s). This proved that the hydrolysis kinetic for 5 wt.% of Ge in 30% acetic acid was faster than in 30% acetic acid.
Other than that, Ma et al. [45] have carried out an investigation on the hydrolysis of MgH2-LiNH2 (Lithium Amide) system with different ratios of MgH2 to LiNH2 (1:1, 2:1, 4:1, 8:1, 16:1 and 32:1) from 0 °C to 40 °C. Based on their work, at 25 °C, the 4MgH2-LiNH2 system exhibited the highest kinetic performance with the amount of hydrogen yield 950 mL/g (10 min), 970 mL/g (20 min) and 1000 mL/g (30 min). Under the same temperature, the 32MgH2-LiNH2 system appeared as the lowest kinetic performance with the amount of hydrogen yield 620 mL/g (10 min), 690 mL/g (20 min) and 700 mL/g (30 min). Simultaneously, the listed activation energy for 4MgH2-LiNH2 and 32MgH2-LiNH2 systems was 16.0 kJ/mol and 52.5 kJ/mol respectively.
A study on the hydrolysis of MgH2 catalyzed by different concentrations of citric acid (0.005 mol dm−3, 0.01 mol dm−3, 0.02 mol dm−3, 0.05 mol dm−3 and 0.1 mol dm−3) at 25 °C was done by Hiraki et al. [46]. From their result, 0.1 mol dm−3 of citric acid owned an outstanding kinetic performance with the amount of hydrogen yield 1700 mL/g from 10 min to 30 min. In comparison, 0.005 mol dm−3 of citric acid showed poor kinetic performance with the amount of hydrogen yield 400 mL/g from 10 min to 30 min. This showed that the amount of hydrogen yield for 0.1 mol dm−3 of citric acid has reached up to 3 times better than 0.005 mol dm−3 of citric acid. Concurrently, the reported rates of conversion were 99% (0.1 mol dm−3 of citric acid) and 20% (0.005 mol dm−3 of citric acid).
Next, the effectiveness of 10 at% of Ni at 25 °C in pure water and 1 M KCl was observed by Grosjean et al. [16]. Firstly, for 10 at% of Ni in pure water, the percentage of hydrogen conversion yield was 20% (10 min), 23% (20 min) and 25% (30 min). In contrast, the percentage of hydrogen conversion yield for 10 at% of Ni in 1 M KCl was 26% (10 min), 30% (20 min) and 34% (30 min). This verified that the percentage of hydrogen conversion yield for 10 at% of Ni in 1 M KCl showed an increase of more than 5% when compared to 10 at% of Ni in pure water.
Liu et al. [34] investigated the effect of 10 wt.%, 20 wt.% and 30 wt.% of Ca in the MgH2 system from 25 °C to 70 °C. Based on their work, it was found that the best hydrolysis kinetic was recorded at 70 °C. Along with that, at 70 °C, the best hydrolysis kinetic was obtained from 30 wt.% of Ca with the amount of hydrogen yield 1150 mL/g (10 min), 1250 mL/g (20 min) and 1350 mL/g (30 min). Meanwhile, poor hydrolysis kinetic was obtained from 10 wt.% of Ca with the amount of hydrogen yield 800 mL/g (10 min), 1050 mL/g (20 min) and 1200 mL/g (30 min) at the same temperature. As for the activation energy, 30 wt.% of Ca recorded the lowest value (8.3 kJ/mol) while 10 wt.% of Ca recorded the highest value (21.1 kJ/mol).

3. Influence of Catalysts on the Hydrogen Production of NaBH4

Section 3 introduces the hydrolysis of NaBH4 and the implementation of different types of catalysts to improve the NaBH4 hydrolysis done by previous researchers.

3.1. Why NaBH4

In 2007, the US Department of Energy recommended a no-go on NaBH4 hydrolysis [47]. However, the no-go recommendation is only for the on-board systems due to the high “net system” cost in order to regenerate sodium borate (NaBO2) back to NaBH4 fuel. In addition, the water-driven NaBH4 was seen as impossible to meet the capacity performance targets for 2010 [48]. However, due to NaBH4’s high hydrogen storage capacity, non-combustible and exemplary constancy of its alkaline solution, it is regarded as a possible choice for the generation of hydrogen through hydrolysis [49] and stands as a promising hydrogen carrier for the on-demand power supply [47]. In addition, studies on the economic potential NaBH4 regeneration process were extensively done by researchers [8,9,47,50,51]. Ideally, the hydrolysis reaction of NaBH4 is given as [8]:
NaBH4 + 2H2O → NaBO2 + 4H2
However, since the hydrolysis of NaBH4 only happen with the introduction of water, the actual the hydrolysis reaction of NaBH4 in water is given by [8]:
NaBH4 + (2 + x)H2O → NaBO2.xH2O + 4H2
where x, which might have a value of 2 or 4 depending on the reaction circumstances, is the excess hydration factor [52].
Producing hydrogen by hydrolyzing sodium borohydride is not without issues. The amount of water needed is a significant problem. The stoichiometric chemical reaction is shown as in equation 1, however in reality, at least four molar equivalents of water are needed for every mole of NaBH4. This is happened due to two factors; (i) sodium metaborate (NaBO2) hydrates very quick and (ii) low solubility of NaBH4 (55 g for every 100 g of H2O at 25 °C) [53].
Based on NaBH4’s low effective gravimetric hydrogen storage capacity, high cost, and ineffectiveness for recycling its hydrolysis by-product (NaBO2), the United States DOE recommended against using it [54]. In the interim, the process of NaBH4 hydrolysis progresses smoothly at room temperature and is an exothermic reaction and in order to stimulate this reaction, it is essential to include acid or a relevant metal catalyst [12]. This is because NaBH4’s self-hydrolysis without any catalyst is very sluggish [3,55,56].

3.2. Platinum-Based Catalysts

In line with Zhang et al. [57], the outlandish cost and minimal availability cause Pt difficult to be considered for a wide range of applications even though many existing works consistently show the notable reaction of Pt in catalyzing NaBH4 hydrolysis. Thus, to attain bimetallic catalysts through the inclusion of non-costly metals is an enticing approach that will substantially lower the cost, retaining only minor costly metal products. It will also generate efficacy among the distinct metals that can help in increasing the catalytic performance. Table 1 shows several previous latest studies for the hydrolysis of NaBH4 which used Pt as a catalyst.

3.3. Ruthenium-Based Catalysts

Ru has been perceived as a compelling catalyst not only for its impeccable catalytic performance but also due to its low cost contrasted to several other noble metals [65]. Conversely, Ru’s deficiency and expensive price are the obstructions on the path to industrial purposes [66]. Accordingly, further improvement in the catalytic efficiency of Ru for hydrolysis of NaBH4 is indispensable which leads to Ru being extensively studied. Thus, Table 2 displays prior studies for NaBH4 hydrolysis with the use of Ru as a catalyst.

3.4. Cobalt-Based Catalysts

Noble metals are costly and hard to come by. Therefore, it is extremely desirable to adopt catalyst systems based on the less expensive transition metals. One of the most popular active metals and a transition metal catalyst, Co, exhibits superior catalytic activity to other transition metal catalysts when used to hydrolyze NaBH4 [75,76,77]. In comparison to Co catalysts made with Co(NO3)2, Co(CH3COO), and CoSO4, those made with CoCl2 as a precursor have the highest catalytic activity [78,79,80]. Co-B was given significant recognition for its superior catalytic behaviour, reasonable price and convenient method of preparation [81]. Next, by dissolving the particles into an acceptable support material, the catalytic performance of Co-B can be optimized [82]. Table 3 presents previous works done when Co was used as a catalyst for NaBH4 hydrolysis.

3.5. Nickel-Based Catalysts

It is essential to look at the lower-priced alternatives like Ni since the price of cobalt, which is in high demand in the creation of lithium-ion batteries, is rising [93]. Although Ni does not directly involved in the hydrogen generation reaction, it greatly improves the efficiency of the process [94]. Furthermore, because of the shift in electronic states of the active metals, it has been discovered that combining Ni with a metalloid atom (such as B or P) will boost the catalytic activity [95]. Table 4 exhibits previous works resulting from using Ni as a catalyst for NaBH4 hydrolysis.

4. Conclusions

In conclusion, metal hydrides like MgH2 and NaBH4 can be used to store hydrogen via the solid-state form. Consequently, the hydrolysis method is one of the methods accustomed to release the stored hydrogen. However, problems like the MgH2 passivation layer, high cost and sluggish self-hydrolysis of NaBH4 are the known limiting factors that can be overcome with the help of catalysts. Based on selected studies reviewed in this article, catalysts like chloride, oxide, fluoride, platinum, ruthenium, cobalt and nickel-based are proven to be significant to enhance the amount of hydrogen released as compared to non-catalyzed MgH2 and NaBH4 system.

Author Contributions

Writing—original draft, N.R., M.S.Y., M.N.I.S. and A.P.Y.; writing—review and editing, N.R., M.S.Y., M.R.A., M.Z.M.Y., M.F.A.A.H.Y. and N.S.M.; supervision, M.S.Y. and M.I.; funding acquisition, M.S.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Higher Education Malaysia, grant number FRGS/1/2020/STG05/UMT/02/3 (Malaysia Fundamental Research Grant Scheme).

Acknowledgments

The authors would like to express gratitude to Ministry of Higher Education Malaysia, Universiti Malaysia Terengganu and all the collaborators of this project.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 01376 g001 550
Figure 1. The set-up apparatus to study MgH2 hydrolysis ((1) water bath, (2) 250 mL three-necked flask, (3) thermometer, (4) condenser, (5) gas washing bottle (6) beaker (7) analytical balance) (source: [30]).
Figure 1. The set-up apparatus to study MgH2 hydrolysis ((1) water bath, (2) 250 mL three-necked flask, (3) thermometer, (4) condenser, (5) gas washing bottle (6) beaker (7) analytical balance) (source: [30]).
Crystals 12 01376 g001
Crystals 12 01376 g002 550
Figure 2. The role of chloride ions in the hydrolysis reaction mechanism. Reprinted with permission from Ref. [35]. 1976, Elsevier Science & Technology Journals.
Figure 2. The role of chloride ions in the hydrolysis reaction mechanism. Reprinted with permission from Ref. [35]. 1976, Elsevier Science & Technology Journals.
Crystals 12 01376 g002
Table
Table 1. Previous latest studies on the hydrogen yield rate due to the effect of platinum on NaBH4 hydrolysis.
Table 1. Previous latest studies on the hydrogen yield rate due to the effect of platinum on NaBH4 hydrolysis.
CatalystsH2 Yield RateTemperature (°C)Activation Energy (kJ/mol)YearReference
PtCofiber/Cu3700 mL/min.gcat30–7062.602018[58]
Pt-Co3O423,916 mL/min.gcat25–5543.522019[59]
Pt/MWCNT159 mmol/min.gcat27–6727.002019[60]
Pt/Al2O3/cordierite53 mL/minautothermal-2019[61]
PdPt372.52 mL/min.gcat20–3513.932021[62]
Ag-Pt294.78 mL/min.gcat20–3525.612022[63]
N@Pt-Ag367.01 mL/min.gcat20–3516.022022[64]
Table
Table 2. Previous latest studies on the hydrogen yield rate due to the effect of ruthenium on NaBH4 hydrolysis.
Table 2. Previous latest studies on the hydrogen yield rate due to the effect of ruthenium on NaBH4 hydrolysis.
CatalystsH2 Yield RateTemperature (°C)Activation Energy (kJ/mol)YearReference
Ru/ZIF22,400 mL/min.gcat30–6039.002018[65]
Ru-Co3O420,628 mL/min.gcat25–5528.262019[59]
RuMo/CNT82,758.43 mL/min.gcat20–5035.112020[67]
Ru/CoOx@NPC8019.5 mL/min.gcat25–4559.302021[68]
Ru9.8/r-CoP9783.3 mL/min.gcat25–4545.302021[69]
Ru/NH2-magadiite36,515 ± 500 mL/min.gRu5–2054.502021[70]
Ru/Co19.67 mL/min30–6030.182021[71]
RuW/MWCNT198,397.2 mL/min.gcat20–5016.322022[72]
Ru-TiO2-Ti3C2434,500 mL/min.gRu30–6050.962022[73]
Ru complex299,220 mL/min.gcat20–5025.802022[74]
Annotation: Ru complex = fabricated via 5-Amino-2,4-dichlorophenol-3,5-ditertbutylsalisylaldimine ligand and RuCl3-H2O salt.
Table
Table 3. Previous latest studies on the hydrogen yield rate due to the effect of cobalt on NaBH4 hydrolysis.
Table 3. Previous latest studies on the hydrogen yield rate due to the effect of cobalt on NaBH4 hydrolysis.
CatalystsH2 Yield RateTemperature (°C)Activation Energy (kJ/mol)YearReference
Co/Ti4N3Tx526 mL/min.gcat30–6044.232021[83]
Co@150PC11,086.4 mL/min.gCo15–5531.252021[84]
Co-Cr-B/NG2231.7 mL/min.gcat15–4538.412022[85]
Co/CuO–NiO–Al2O36460 mL/min.gcat25–5531.592022[86]
Co0 hollow microshells1560 mL/min.gcat25–4055.5 ± 1.92022[87]
Co–W–B/PPy13,470 mL/min.gcat20–3549.182022[88]
Co/CoxOy Nanofilm4067 mL/min.gcat30–4543.192022[89]
Co@C-462-1455392 mL/min.gCo25–4032.702022[90]
Co0.45W0.551770 mL/min.gcat0–3555.562022[91]
Co/HTNT1750 mL/min.gCo20–5029.682022[92]
Annotation: Co@C-462-145 = 462 μL of ammonia, 145 °C of hydrothermal temperature, Co/HTNT = immobilized Co on the surface of titanate nanotubes and Co@150PC = Co nanoparticles supported on bagasse-derived porous carbon.
Table
Table 4. Previous latest studies on the hydrogen yield rate due to the effect of nickel on NaBH4 hydrolysis.
Table 4. Previous latest studies on the hydrogen yield rate due to the effect of nickel on NaBH4 hydrolysis.
CatalystsH2 Yield RateTemperature (°C)Activation Energy (kJ/mol)YearReference
Ni22,500 mL/min.g10–6046.602019[96]
Ni–Zn430 mL/min.g25-2019[97]
Ni5P4/Fe175 mL/min.g3053.412019[98]
Pine-needle-like NiCo2O41904.76 mL/min.g25–5547.692020[99]
Network-like NiCo2O42251.14 mL/min.g25–5552.212020[99]
Ball-cactus-like NiCo2O46219 mL/min.g25–5555.792020[99]
Dandelion-like NiCo2O43125 mL/min.g25–5563.442020[99]
Ni/TiO2110.87 mL/min.gcat20–6025.112021[100]
NiB258970 mL/min.gcat6548.052021[101]
Nickel (II) complex/CeO243.392 mL/min.gcat20–5020.582021[102]
Ni/Dolomite88.16 mL/min.gcat30–6038.332022[103]
Annotation: NiB25 = with the addition of 25 μL Triton.
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Ruslan, N.; Yahya, M.S.; Siddique, M.N.I.; Yengantiwar, A.P.; Ismail, M.; Awal, M.R.; Mohd Yusoff, M.Z.; Abdul Halim Yap, M.F.A.; Mustafa, N.S. Review on Magnesium Hydride and Sodium Borohydride Hydrolysis for Hydrogen Production. Crystals 2022, 12, 1376. https://doi.org/10.3390/cryst12101376

AMA Style

Ruslan N, Yahya MS, Siddique MNI, Yengantiwar AP, Ismail M, Awal MR, Mohd Yusoff MZ, Abdul Halim Yap MFA, Mustafa NS. Review on Magnesium Hydride and Sodium Borohydride Hydrolysis for Hydrogen Production. Crystals. 2022; 12(10):1376. https://doi.org/10.3390/cryst12101376

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Ruslan, Nuraini, Muhammad Syarifuddin Yahya, Md. Nurul Islam Siddique, Ashish Prabhakar Yengantiwar, Mohammad Ismail, Md. Rabiul Awal, Mohd Zaki Mohd Yusoff, Muhammad Firdaus Asyraf Abdul Halim Yap, and Nurul Shafikah Mustafa. 2022. "Review on Magnesium Hydride and Sodium Borohydride Hydrolysis for Hydrogen Production" Crystals 12, no. 10: 1376. https://doi.org/10.3390/cryst12101376

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