**5. Energy storage: MOFs**

MOFs are known as coordination polymers developed from solid-state/zeolite chemistry and coordination chemistry [87, 88]. The use of MOFs in gas is extremely important because they are porous materials [89]. Ever since the establishment of MOFs by Hoskins and Robson, MOFs have grabbed great attention in porous materials because of their ability to store gases. Yaghi et al. went on to popularize research on MOFs after that, particularly after MOF-5 was reported [90]. It is theoretically possible to construct a MOF that is well-suited for a desired application (such as sensors, catalysis or separation) by carefully choosing nodes and linkers (including flaws, such as missing nodes and/or linkers). It's noteworthy that MOFs have an ad hoc naming scheme that often uses numbers (e.g. MOF-5) or names derived from the universities from which they originated (e.g. NU-1000) [91].

#### **5.1 Properties of MOFs**

Metal-organic frameworks (MOFs) are crystalline substances with a high surface area, high porosity, and the ability to efficiently adsorb hydrogen. At 77 K, MOFs exhibit good gravimetric hydrogen capabilities, and some of them have exceeded the US Department of Energy (DOE) objective. Numerous MOFs have been reported to display permanent porosity with pore windows between 5 and 25 Å. Through ligand extension, MOFs with large interior surface areas, extending 10,000 m<sup>2</sup> /g, and extremely high porosity (up to 90% free volume) have been created. The measurement of gas isotherms has proven crucial in determining if permanent porosity has been achieved after guests have left. MOFs can be created with the right tailoring to function as extremely selective molecular sieves, sensors or catalysts. Among other capabilities of MOFs, gas storage is one of the most promising uses for metal-organic frameworks. As shown in **Figure 1**, the isotherm forms, which are typically Type I with little to no hysteresis, show that durable microporous structures exist under reversible gas physisorption of tiny molecules (**Figure 7**).

#### **5.2 Metal hydrides for hydrogen storage**

Due to its high gravimetric energy density and environmental benefits, hydrogen has been proposed as a promising alternative to the widely used fossil fuels as an energy source [92]. Hydrogen can be generated and separated from a variety of sources including water, fossil fuels and biomass [93]. The use of hydrogen as an energy carrier can however be impeded by the lack of safe, energy-efficient and costeffective storage systems [94]. The most common storage modalities of hydrogen include (i) pressurized gas, (ii) cryogenic liquid and (iii) solid fuel via adsorption onto porous materials [95]. Since the storage of hydrogen in MOFs was innovatively proposed by Rosi et al. [96], a plethora of research has been conducted on the

**Figure 7.**

*Schematic representation of type I isotherms of MOFs (MOF 177, MOF-5, MOF-2) N2 sorption measured at 196,15°C showing the conformal microspores. Reprinted with permission from literature.*

modification and application of MOFs as hydrogen storage materials [97–99]. The tunability, topological structure and nanoconfined environments of MOFs provide ideal conditions for hydrogen capture, storage and release with considerable safety, convenience and efficiency [100]. Other properties of MOFs that make them attractive for hydrogen storage include their porous structures, high specific surface areas, exposed metal nodes, facile fabrication procedures, controllable chemical functionality and amenability to scale-up [101]. Although MOFs are favourable for hydrogen storage, high storage capacities (up to 4.5–7.5 wt%) are normally achieved at cryogenic temperatures (77 K) and high pressures [87, 94, 102]. The strong pressure and temperature dependence as well as storage capacity requirements of hydrogen physisorption on MOFs therefore limit their practical application [103]. Several approaches have thus been considered to improve the hydrogen storage capacities of MOFs, including the formation of composites by adding dopants and substituting the metal nodes within the MOFs [104]. Some researchers have even proposed nanoconfinement of other materials inside MOFs as an alternative approach to enhancing hydrogen storage [105]. Alternatively, several other materials have emerged as feasible candidates for efficient hydrogen storage such as in metal hydrides. Hydrogen forms metal hydrides with some metals and alloys leading to solid-state storage under moderate temperature and pressure [106]. The reaction of hydrogen with a metal to form a metal hydride results in the generation of heat, i.e. an exothermic reaction. When hydrogen is then required, the stored heat (ΔH) is utilized to release hydrogen from the hydride in an endothermic reaction [107]. Since hydrogen becomes part of the chemical structure of the metal, cryogenic temperature or high pressure are not required to break these chemical bonds; hence, most metal hydrides absorb and desorb hydrogen at ambient temperature and close to atmospheric pressure [108]. Metal hydrides have a higher hydrogen storage density than gaseous or liquid hydrogen; hence, they are volume-efficient storage materials [108]. The use of metal hydrides for hydrogen storage is also favourable over-pressurized gas and other hydrogen storage methods because of their gravimetric and volumetric storage capacities and safe operating pressures [21]. Nanostructured metal hydrides have particularly gained attention due to their improved reversibility, altered heats of hydrogen absorption/desorption and nano interfacial reaction pathways with fast rates [109].

Examples of commonly used metal hydrides as solid-state hydrogen storage materials are binary metal hydrides which include MgH2, TiH2 and AlH3 [110, 111]. The

*Recent Progress on Metal Hydride and High Entropy Materials as Emerging Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.113105*

low cost and good reversibility of MgH2 have made it particularly popular, as well as the fact that it holds the highest energy density (9 MJ/kg) among all the reversible hydrides that apply to hydrogen storage [112]. The sorption kinetics involved in hydrogen generation with MgH2 are however quite sluggish and it has a high thermodynamic stability requiring temperatures that exceed 300°C for the desorption of hydrogen [113]. Upon alloying, metal hydrides exist either as intermetallic or complex hydrides (alanates, borohydrides and nitrides) which have also been more extensively studied due to their high hydrogen storage capacities [109].

Alternative metals have also been proposed as additives to aid the shortcomings of these metal hydrides. Palladium, for instance, acts not only as a catalyst to facilitate the uptake and dissociation of hydrogen in other metal hydrides but it can also protect the surface from corrosion [28]. Although palladium can absorb large volumetric quantities of hydrogen at room temperature and atmospheric pressure to form palladium hydride, it (as well as other PGMS) is not often considered as a sole hydrogen storage material since it is somewhat expensive and has a low gravimetric hydrogen density [113]. Extensive research has been conducted on the use and optimization of various metal hydrides to optimize the conditions and increase the hydrogen storage capacity of these materials as shown in **Table 4**.

#### **5.3 Application of MOFs and metal hydrides in batteries**

The power output of these renewable energy resources such as solar, hydro and wind power is highly fluctuating and intermittent which invites the parallel implementation of electrochemical energy conversion and storage technologies, such as rechargeable batteries [122]. Such storage technologies make sustainable energy utilization easy and efficient [123]. Rechargeable lithium-ion batteries (LIBs) with zero emissions, now particularly dominate the energy storage and conversion devices market, which reduces our reliance on conventional energy resources [124]. The development of high-capacity electrode materials for LIBs, however, is still necessary to meet the sustained growing demand for energy. Thus, research centred on the optimization of efficient conducting materials in energy storage devices such as batteries has soared.


#### **Table 4.**

*Summary of hydrogen storage parameters and efficiencies of different metal hydrides.*

The high porosity, versatile functionalities, diverse structures and controllable chemical compositions of MOFs offer various possibilities for generating adequate electrode materials for rechargeable batteries [125]. The porous structure of MOFs enables a facile electrolyte penetration and ion transportation, while the designable components promise the incorporation of electroactive sites, offering infinite possibilities for the search for candidate electrode materials for different battery systems [126]. Despite these attractive features, MOFs (and their derivatives) as electrode materials face various challenging issues, which impede their practical applications. They suffer from poor electrical conductivity, low tap density, and irreversible structural degradation upon the charge/discharge processes [125].

Metal hydrides on the other hand have been widely investigated not only in LIBs but also in nickel-metal hydrides (Ni-MHy) batteries [127, 128]. Due to their attractive properties, Ni-MHy batteries have often been used for both electric and hybrid vehicles because they provide several advantages compared to lead-acid batteries [129]. In the conventional Ni-MH battery configuration, the charge-discharge processes occur as depicted in **Figure 8**.

Metal hydrides have received increasing interest as materials in these kinds of batteries, both as electrodes and ion conductors [130]. Metal hydride-based materials have the potential to be negative materials for LIBs, owing to their high theoretical Li storage capacity, relatively low volume expansion, and suitable working potential with very small polarization [131]. They also owe their efficiency to their large specific capacity and low voltage hysteresis compared to other conversion materials used for LIBs [124]. Among various metal hydrides, MgH2 has particularly gained popularity as an anode material for LIBs [132]. Various modification techniques including the addition of TiH2, as well as other materials (catalyst/carbon or suitable binders) and nanocrystallization, have been implemented to remedy the drawbacks associated with the electrochemical performances of metal hydrides [133]. These include their kinetics

**Figure 8.**

*Ni-MHy batteries. Reprinted with permission from [129].*

*Recent Progress on Metal Hydride and High Entropy Materials as Emerging Electrocatalysts… DOI: http://dx.doi.org/10.5772/intechopen.113105*

limitations, structural reorganization, capacity fading and volumetric change during the discharge/charge process [124].

For instance, Yang et al. fabricated MgH2-based composites with expanded graphite (EG) and TiO2 using a plasma-assisted milling process to improve the electrochemical performance of MgH2. The resulting MgH2–TiO2–EG composites showed an increase in the initial discharge capacity and cycling capacity compared with a pure MgH2 electrode. A stable discharge capacity of 305.5 mAh<sup>g</sup><sup>1</sup> could be achieved after 100 cycles for the 20 h-milled MgH2–TiO2–EG-20 h composite electrode and the reversibility of the conversion reaction of MgH2 could be greatly enhanced [134].

Additionally, Mo et al. designed a three-dimensional hierarchical metal hydride/ graphene composite (LiNa2AlH6/3DG) that showed outstanding cycling stability with LiBH4 as a solid electrolyte. An ultra-high capacity of 861 mAh<sup>g</sup><sup>1</sup> at the current density of 5 A g<sup>1</sup> and a long cycle life of 500 cycles with capacity retention of 97% was achieved [135]. Moreover, Weeks et al. analysed and compared the physical and electrochemical properties of an all-solid-state cell utilizing LiBH4 as the electrolyte and aluminium as the active anode material. An initial capacity of 895 mAh<sup>g</sup><sup>1</sup> was observed and is close to the theoretical capacity of aluminium due to the formation of a LiAl (1:1) alloy. This demonstrated the possibility of utilizing other high-capacity anode materials with a LiBH4-based solid electrolyte in all-solid-state batteries [136].

#### **5.4 Challenges and advantages of MOFs and metal hydride**

Many beneficial features have evolved into a wide variety of MOFs that is high degree of porosity, high surface area, flexible architecture, multifunctional chemical properties and tuneable structure. MOFs are widely used in many applications, including catalysts, supercapacitor, adsorbents, sensors, environmental protection and drug delivery, due to their simplicity of design and homogeneous and finetuneable pore architectures. However, a number of disadvantages have also limited the use of these innovative materials in practical applications, including high production costs, poor selectivity, low capacity and challenges with recycling and regeneration. Furthermore, poor electrical conductivity and stability of conventional MOFs inhibit development and application. With many short coming associated with MOFs, synthetic challenges in the discipline emerge from understanding and regulating both structural and compositional complexity because of the enormous array of conceivable topologies and compositions. In addition, despite the enormous variety of known structural types, there have been no definitive findings of shape-selective catalysis in MOFs although zeolite community has a highly established understanding of shapeselective catalysis. Zhang et al. reported the attempts to improve MOFs shapes and widen their application through the introduction of graphene as a template to grow the MOFs which avoids agglomeration, and diminish poor electrical conductivity and stability [137].
