Metal Hydrides | Encyclopedia MDPI

Metal Hydrides: Comparison

Please note this is a comparison between Version 2 by Camila Xu and Version 1 by Vamsi Krishna Kukkapalli.

metal hydride reactor
heat transfer
thermal management

1. Introduction

A metal hydride is a compound formed between a metal and hydrogen, in which the hydrogen atoms are bonded to the metal atoms through chemical bonds. Metal hydrides have a wide range of applications as energy storage materials, catalysts, and structural materials. There are various types of metal hydrides, each with their own unique properties and challenges. These include interstitial, substitutional, and complex types. Interstitial hydrides are those in which the hydrogen atoms are located between the metal atoms in the crystal lattice, while substitutional hydrides are those in which the hydrogen atoms replace metal atoms in the crystal lattice. In complex hydrides, the hydrogen atoms form covalent bonds with the metal atoms, resulting in a compound with a more complex chemical structure. Some of the most well-known metal hydrides include sodium aluminum hydride (NaAlH₄), magnesium hydride (MgH₂), and titanium hydride (TiH₂). These hydrides can store and release large amounts of hydrogen in a relatively safe and controlled manner, making them ideal for energy storage applications.

2. Metal Hydrides

One of their main characteristics is the ability to absorb and release hydrogen gas through processes known as hydriding and dehydriding, respectively ^[1]. This property makes them attractive for use as hydrogen storage materials, as they can store large amounts of hydrogen at relatively low pressures ^[2]. Metal hydrides can also be used as catalysts in chemical reactions and as structural materials due to their high strength and low density.

There are several advantages to using metal hydrides for hydrogen storage: High capacity: Metal hydrides have a high hydrogen storage capacity, meaning that they can store large amounts of hydrogen in a relatively small volume. This makes them a compact and efficient storage option. Safe: Metal hydrides are generally considered to be a safe storage option because they do not release hydrogen gas unless they are subjected to specific conditions, such as high temperatures or pressures. This reduces the risk of explosions or fires. Stable: Metal hydrides are stable and do not react with other materials, making them a safe and reliable storage option. Reusable: Metal hydrides can be used to store and release hydrogen multiple times, making them a reusable and environmentally friendly storage option. Lightweight: Metal hydrides are typically lightweight, making them a suitable storage option for applications where weight is a concern, such as in vehicles. Some of the applications of metal hydrides are provided in Table 1.

Table 1.

Some applications where metal hydrides are used.

References	Technology Area	Engineering System	Pros	Cons	Suggested Hydrides
^[3]—Krane et al., 2022	Energy Storage	Two-reactor metal hydride system	High energy density Non-toxic and non-flammable suitable for long-term storage. Can be cycled without degradation in capacity Can operate at a wide range of temperatures and pressures and low environmental impact	Slow charging Limited number of experimental validations to support the dynamic model	MgH₂, TiFeH₂, LaNi₅H₆
^[4]—Zhang et al., 2023	Catalysis	Ammonia synthesis	High catalytic activity and selectivity for ammonia synthesis Lanthanum hydride species improves the catalytic performance	Limited understanding of the mechanism behind the improved catalytic performance Potential deactivation of the catalyst over time	Lanthanum hydride
^[5]—Lv, Y. et al., 2023	Energy storage	Magnesium hydride conversion electrode	High energy density Abundant raw materials and detailed understanding of ion diffusion and hysteresis in magnesium hydride conversion electrodes	Slow reaction kinetics Low cycling stability Challenges in improving ion diffusion and charge transfer kinetics Limited practical application due to the need for high operating temperatures	Magnesium hydride
^[6]—Zhou et al., 2023	Fuel cells	Hydrogen feeding system	Comprehensive comparison of RE-based and Ti-based multicomponent metal hydrides	Limited on long-term stability	RE-based and Ti-based multicomponent metal hydrides
^[7]—Tiwari and Sharma, 2023	Energy storage	Metal hydride reactor and thermocline-based heat storage system	Proposed a hybrid energy storage system using metal hydrides and thermocline heat storage Efficient thermal energy storage and release	Limited discussion on real-world implementation and system optimization Limited experimental validation	MgH₂
^[8]—Alok Kumar, P. Muthukumar, 2022	Hydrogen storage and purification		Methane poisoning characteristics studied experimentally	Limited discussion on the impact of methane poisoning on system performance	La_0.9Ce_0.1Ni₅
^[9]—Krishnamoorthy et al., 2023	Battery modeling	Lithium–ion and nickel–metal hydride batteries	Provides efficient battery models for performance studies		Nickel-metal hydride battery
^[10]—Zhang et al., 2022	Chemicals	CO₂ capture	Insight into CO₂ capture by nickel hydride complexes	Limited discussion of practical applications Further optimization of the nickel hydride complexes is needed to enhance performance	Nickel hydride complexes
^[11]—Brestovič, T. et al. 2022	Metal Hydride Compressors	Heat pump-based compression system	Improved heat transfer efficiency due to the integration of a heat pump with a metal hydride compressor Utilization of low-temperature heat sources Increased COP values up to 2.2 in comparison to conventional heat pumps	The use of metal hydride beds for compression may cause a decrease in the COP Additional heating of the gas after compression may be required Low heat transfer rates in the metal-.hydride bed may result in higher operating temperatures, which may negatively impact the overall performance of the system Limited scalability due to material properties	LaNi₅
^[12]—Massaro et al., 2023	On-board hydrogen storage technologies	Fuel cell systems for aircraft electrification	High energy density of hydrogen, which provides high specific energy and range for aircraft On-board hydrogen storage technologies, such as metal hydrides, offer high storage capacity and low weight Fuel cell systems provide high efficiency and low noise emissions	Low volumetric energy density of hydrogen, which requires large storage volumes Safety concerns due to the high flammability and explosiveness of hydrogen Limited availability of hydrogen infrastructure
^[13]—Nguyen and Shabani, 2022	Metal hydride hydrogen storage	Standalone solar hydrogen systems	High volumetric and gravimetric hydrogen storage capacity of metal hydrides The ability to use renewable energy sources, such as solar energy, to power the metal hydride system Improved thermal management using phase change materials can enhance system performance	High cost of metal hydride materials Slow kinetics for hydrogen uptake and release Limited cycle life due to material degradation
^[14]—Eadi et al., 2023	Hydrogen gas sensing	Pd-Ni alloy thin films	Pd-Ni alloy thin films exhibit high sensitivity and selectivity towards hydrogen gas detection The sensing performance of Pd-Ni alloy thin films can be improved by optimizing the deposition parameters Pd-Ni alloy thin films are a promising candidate for practical hydrogen sensing applications due to their low cost and ease of fabrication	Pd-Ni alloy thin films can be sensitive to other gases, such as CO and H₂S, which can interfere with hydrogen gas detection The sensing performance of Pd-Ni alloy thin films can be affected by environmental factors, such as temperature and humidity Pd-Ni alloy thin films may require periodic calibration to maintain accurate hydrogen gas detection
^[15]—Kumar et al., 2022	Metal hydride-based hydrogen storage	Standalone microgrids	Metal hydride-based hydrogen storage systems can provide a reliable and efficient means of energy storage for standalone microgrids Metal hydride systems have the potential for high energy density storage compared to other energy storage technologies Metal hydride systems are environmentally friendly and have no harmful emissions	Metal hydride systems can be expensive to manufacture and maintain The hydrogen uptake and release kinetics of metal hydride systems can be slow, leading to reduced system performance Metal hydride systems can be affected by temperature and humidity changes, which can reduce system efficiency	MgH₂, TiFeH₂, LaNi_4.8Al_0.2H_11.3
^[16]—Tian et al., 2022	Hybrid rocket propulsion	Solid-fuel additives	Addition of metal and metalloid solid-fuel additives can improve the combustion performance of hydroxy-terminated polybutadiene-based hybrid rocket motors Metal and metalloid additives can increase the density and specific impulse of the rocket motor Additives can also reduce the nozzle ablation rate, which can extend the lifetime of the motor	The effect of the additives on the mechanical properties of the motor is not well understood The production and processing of the solid-fuel additives can be expensive and difficult The use of metal and metalloid additives may require additional safety measures due to the potential for increased reactivity and flammability
^[17]—Lee et al., 2022	Hydrogen storage	Magnesium hydrogen tank	The two-in-one flexible high-temperature micro-sensor developed in this study can provide real-time monitoring of surface temperature and strain of magnesium hydrogen tanks The sensor is low-cost, lightweight, and can be easily integrated into existing tank designs Real-time monitoring of tank conditions can improve safety and performance in hydrogen storage systems	The study focuses solely on the development and testing of the micro-sensor and does not address other aspects of hydrogen storage technology The use of magnesium as a hydrogen storage material has some drawbacks, including low hydrogen storage capacity and high reactivity with air and moisture	MgH₂
^[18]—Sezgin et al., 2022	Hydrogen energy systems	Underwater applications	Hydrogen fuel cells are a promising power source for underwater applications due to their high efficiency, low noise, and zero emissions Hydrogen can be stored in metal hydride tanks, which have high energy density and can operate at low pressures Hydrogen systems can provide longer endurance and greater range than traditional battery-powered systems	The high cost and complexity of hydrogen systems may limit their adoption in some applications The need for refueling infrastructure and the limited availability of hydrogen fuel may also be barriers to adoption	TiFe, LaNi₅, AB₂ (MmNi_3.6Co_0.7Mn_0.4Al_0.3), MgH₂, LiBH₄
^[19]—Kailiang Ren, Jiajia Miao, et al., 2022	Electrochemical energy storage	Batteries	LaFeO₃ coated with C/Ni exhibited superior electrochemical performance at high temperatures compared to bare LaFeO₃ The coating with C/Ni helps to increase the conductivity of the material and improves its electrochemical properties	The preparation of the C/Ni-coated LaFeO₃ requires additional processing steps The long-term stability of the coating is unknown	NiMH
^[20]—Dinesh Dashbabu, E. Anil Kumar, I.P. Jain, 2022	Hydrogen compression	Metal hydride hydrogen compressor	Metal hydride hydrogen compressor with Al-substituted LaNi₅ hydride can effectively compress hydrogen Study provides a comprehensive thermodynamic analysis of the compressor	The performance of the compressor decreases at higher operating temperatures The thermal conductivity of the metal hydride decreases with increasing Al content	LaNi₅H₆-xAlx
^[21]—Sayantan Jana et al., 2022	Heating and cooling	Embedded cooling tube type metal hydride reactor	High thermal conductivity Energy-efficiency Rapid heating/cooling Absence of moving parts Long life cycle	Limited by heat transfer efficiency Relatively low hydrogen storage capacity Higher cost compared to conventional systems	Mg₂NiH₄, Top of Form
^[21]—Sayantan Jana et al., 2022	Heating and cooling	Embedded cooling tube type metal hydride reactor			bottom of form
^[22]—Singh et al., 2022	Renewable energy	Reversible SOFC, hydrogen storage, rankine cycle, absorption refrigeration	Novel energy storage system design based on multiple technologies	Need for further testing and optimization	LaNi_4.8Al_0.2, MgH₂, Ni-MH alloy, CaH₂
^[23]—H. Chang, Y.B. Tao, H. Ye, 2023	Hydrogen and thermal storage	Sandwich reaction bed filled with metal hydride and thermochemical material	Proposed a new sandwich reaction bed for hydrogen and thermal storage Conducted numerical simulations to investigate the performance of the system under different operating conditions	No experimental validation of the numerical results was conducted

Overall, metal hydrides offer several advantages over traditional hydrogen storage methods and may be a more efficient and safe option in certain applications. In Figure 1, hydrogenation enthalpies and entropies of various hydride materials and their suitable applications are provided.

Figure 1. Hydrogenation enthalpies and entropies of various hydride materials and their suitable applications: (A) heat pumps, (B) heat storage, (C) hydrogen storage, (D) hydrogen compression ^[24].

Overall, metal hydrides have the potential to play a significant role in a variety of applications, and their properties and behavior are an active area of research in the field of materials science. Type AB₅ metal hydrides (for which LaNi₅ serves as the paradigm) and type AB₂ metal hydrides are the most well known for hydrogen absorption (such as Mn₂Zn). The AB₅ group has excellent hydrogenation ability at ambient temperature. However, its hydrogen capacity is typically in the range of 1 to 1.5 wt%. Metal hydrides based on magnesium (Mg and Mg₂Ni) display unacceptably slow rates of hydrogenation and dehydrogenation even after significant activation at 673 K (400 °C) ^[25].

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