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Overview

1.1 Introduction to Mg‐based Hydrogen and Electric Energy Storage Materials

The heavy reliance on fossil fuels has incurred serious environmental consequences because of the resultant carbon dioxide (CO2) emissions into the atmosphere, which are, however, driven by the accelerating energy demands due to global civilization and economic development [1]. The access to abundant, cheap, and clean energy has become our most essential foundation for economic prosperity and human civilization. Among all other new alternative clean energy sources, such as solar, biomass, and nuclear sources, hydrogen has been widely recognized as a clean, renewable, and high‐density energy carrier [2]. Although it is believed to be the long‐term solution for the world energy supply, the current hydrogen storage and transportation technologies remain the bottleneck challenge to be tackled [3]. Therefore, developing safe, effective, and economical technologies to store and transport hydrogen is an essential step to make it more competitive with respect to other fuels.

Nowadays, hydrogen is mainly stored in three forms: compressed gas storage, liquid‐state storage, and solid‐state storage [4]. Compressed hydrogen storage technology, as the most mature and widely implemented storage method, suffers from difficult‐to‐produce and expensive carbon‐fiber tanks, low volumetric energy density, and large energy consumption for hydrogen compression. Meanwhile, the liquefied hydrogen storage method requires an energetically unfavorable deep cooling to −253 °C, and up to 30% energy is required for liquefaction in real applications. Beside gas and liquid storage, hydrogen storage in a solid‐state form has been regarded as a viable alternative since it is possible to contain more hydrogen per unit volume than liquid or high‐pressure hydrogen gas while maintaining high safety of operation.

Among different energy storage materials, magnesium and magnesium‐based materials may play an important role in high‐density energy storage systems (Figure 1.1) [6]. On the one hand, they have been already intensively investigated in hydrogen storage and transportation technologies because of their natural abundance and availability, as well as their extraordinary high gravimetric (7.6 wt%) and volumetric (110 g l−1) storage densities [7]. Moreover, magnesium hydrides have been also used as a one‐time hydrogen carrier, where their water hydrolysis can give a doubled gravimetric capacity up to 15.2 wt% and a high volumetric capacity of 150 g l−1. On the other hand, rechargeable Mg‐ion batteries (RMBs) can also act as a promising alternative for high‐density energy storage systems beyond Li ion batteries (LIBs), because of their high volumetric capacity (3833 mA h cm−3) and dendrite‐free metal anodes [8].

Figure 1.1 The role of Mg‐based materials in hydrogen storage and batteries.

Source: Reproduced with permission from Sun et al. [5] Copyright 2018, Elsevier.

1.2 Overview of Mg‐based Hydrogen Storage Materials and Systems

Hydrogen has been considered a potential clean energy vector because of its high gravimetric energy density of 33.3 kWh kg−1, as compared to that of gasoline (12.4 kWh kg−1) and natural gas (13.9 kWh kg−1) [4]. Although highly appealing, the employment of hydrogen as an energy carrier is largely hindered by the lack of appropriate and economical storage and transportation solutions. In general, ideal hydrogen storage technologies should possess the following characteristics: (i) high volumetric and gravimetric hydrogen density; (ii) adequate recyclability; (iii) high safety; and (iv) best operated under ambient conditions [9]. Nowadays, hydrogen is mainly stored in three different forms: (i) compressed gas storage (e.g. 20, 35, and 70 MPa); (ii) liquid storage (−253 °C); and (iii) solid state in hydrides (e.g. metal hydrides and complex metal hydrides) [10]. It is worth noting that compressed hydrogen storage technology is currently the most mature and widely implemented storage method; however, it suffers from several major drawbacks: (i) difficult‐to‐produce and expensive carbon‐fiber tanks; (ii) poor volumetric energy density (e.g. 5.6 MJ l−1 at 70 MPa compared to gasoline of 32.0 MJ l−1); and (iii) a large energy consumption for the compression work (13–18% of hydrogen when compressed to 70 MPa) [11]. Meanwhile, the liquefied hydrogen storage method requires an energetically unfavorable deep cooling to −253 °C, and up to 30% energy is required for liquefaction in real applications [12]. Moreover, due to the boiling‐off phenomenon, a daily hydrogen loss of 1–2% has been considered. Therefore, the solid‐state storage method has been considered an alternative and promising method (e.g. metal hydrides) for hydrogen storage and transportation due to its high achievable volumetric hydrogen density and high safety. Such metal hydrides have been discovered since 1866, when Graham affirmed the high affinity of hydrogen for Pd [13]. However, metal hydrides have been considered for hydrogen storage purposes since the 1960s.

In the past three decades, magnesium and magnesium‐based materials have been intensively investigated as potential hydrogen storage carriers due to their natural abundance and availability, as well as their extraordinary high gravimetric and volumetric storage densities [5]. Among several high potential hydride systems, magnesium hydrides exert a high volumetric and gravimetric hydrogen density (110 kg m−3 and 7.6 wt%), making it one of the most widely studied hydrogen storage materials (Figure 1.2). It is worth noting that these values are much higher than those of compressed hydrogen, i.e. 23 kg m−3 at 35 MPa and 38 kg m−3 at 70 MPa, and 71 kg m−3 of liquid hydrogen (−253 °C). In 1951, Wiberg first synthesized MgH2 by heating Mg at 570 °C and 20 MPa H2 using MgI2 catalysts directly [6]. Once MgH2 is formed, the reversible reaction between magnesium and hydrogen can be described by the following equation: MgH2(s) → Mg(s) + H2(g). For this reaction, the measured changes of enthalpy (ΔH) and entropy (ΔS) are 74.1 ± 0.4 kJ mol−1 and 133.4 ± 0.7 J K−1 mol−1, which entails an equilibrium pressure of 1 bar at 283 °C. Therefore, when selecting hydrogen storage materials, the ΔH and the ΔS of hydrogenation and dehydrogenation are among the most important parameters. Such values can be easily derived from the PCT isotherms using the van't Hoff plot. Beside thermodynamic considerations, the kinetic properties of MgH2 are also pivotal when selecting a suitable hydrogen storage system. In fact, achieving the fastest hydrogenation/dehydrogenation kinetics is an indispensable goal for MgH2. Alloying, catalyzing, nano‐structuring, and combining with complex hydrides, are among the most effective strategies to improve the hydrogen storage kinetics and thermodynamics of MgH2 [8].

Figure 1.2 An overview of essential metal hydrides for hydrogen storage applications.

Source: Reproduced with permission from Sun et al. [8]. Copyright 2018 Elsevier.

In practice, Mg‐based materials must be processed and placed in a hydrogen storage tank (HST) for efficient storage and transportation of hydrogen. However, when processed into pellets and packed in HSTs, Mg‐based alloys suffer from sluggish hydrogen desorption kinetics, which largely impedes their practical applications, due to the large enthalpy change of hydrogen desorption reaction and the low powder thermal conductivity of Mg‐based alloys [14]. And this phenomenon unfortunately becomes more significant in large‐scale Mg‐based HSTs. To enhance the hydrogen desorption kinetics of large‐scale Mg‐based HSTs, appropriate types of heating are indispensable to provide a high heating efficiency [15, 16]. Currently, direct electrical heating is one conventional way to heat Mg‐based HSTs; however, it becomes strenuous to apply due to the limited heat‐exchange surfaces and accumulated heat during the hydrogen absorption process with the increase of the HST scale. Heat transfer fluid (HTF) tube is another more efficient way to heat Mg‐based HSTs with more uniform control of the local temperature due to the increased heat exchange areas [17–19]. Before carrying out real experiments, numerical simulations are always applied to assist the design of high‐efficiency HSTs.

Magnesium hydride can also be used to produce hydrogen through hydrolysis with water, offering a doubled gravimetric capacity of 15.2 wt% and a high volumetric capacity of 150 g l−1 [20]. The hydrolysis of magnesium materials refers...