Introduction to Metal Hydrides:
In regards to using hydrogen as fuel, the story tends to be quite enticing, one in which we have clean energy and zero emissions. We do seem to sidestep one issue and get caught up in the hype – safely storing hydrogen. Completing gaps in hydrogen fuel infrastructure is still the ‘elephant in the room’.
Hydrogen can be quite a pain to handle. For one, hydrogen is extremely flammable. Furthermore, because of its low density, hydrogen has the potential to pass through minute holes in a medium. Thus, because of those crevices, in combination with hydrogen’s tricky nature, storing hydrogen becomes challenging.
Classical methods of storing it in high-pressure tanks (up to 70 MPa) or cooling hydrogen to liquid at -253°C give us unacceptable solutions. Not merely are those processes complicated, from a safety standpoint, they’re downright frightening.
Hydrogen is also stored using metal hydrides, which provide an alternative in the form of solids. In contrast to conventional methods, these new storage technologies use solid-state hydrogen storage in which hydrogen is chemically incorporated into metal alloys. And the good news? This process does not use harsh conditions such as sharp pressure or cryogenic temperatures. This significantly eases the handling of hydrogen and its use in conventional energy supplies.
In this article, we explore details of how metal hydrides function, why they have an advantage over conventional storage, and what holds back widespread use.
What Are Metal Hydrides?
Metal hydrides consist of one or more metal cations (M+) and one or more hydride anions (H−).
Under pressure, metals strongly adhere to hydrogen, and this forms stable metal hydrides which can be conveniently carried in vehicles for storing hydrogen.
Examples:
Examples of metal hydrides are
• LaNi5H6
• MgH₂
• NaAlH₄.
The metals virtually “absorb” hydrogen like a sponge at specific pressure and temperature conditions, releasing it upon heating—a process of reversible absorption-desorption.
Hydrides of metals can be in powder or liquid form and are typically held in tanks at about 1 MPa. Metal hydrides for hydrogen storage are suitable for storage capacities up to 100 MWh.
What are complex metal hydrides for hydrogen storage?
Aluminum’s complex hydrides have also been seen to be appealing in terms of hydrogen storage compounds because of their high hydrogen content. However, due to the immense challenges in reversing hydrogen release reaction, such use of them has proven to be not practical.
How Do Metal Hydrides Work?
A breakdown of how metal hydrides work, particularly in the context of hydrogen storage is given below:
Absorption (Hydrogenation)
• Upon the addition of hydrogen gas (H₂) to an appropriate metal or alloy at moderate temperature and pressure, hydrogen atoms get absorbed by the metal.
• The atoms diffuse into the metal lattice and become incorporated into a metal hydride (MHₓ).
• It’s an exothermic process—it gives out heat.
LaNi₅ + 3H₂ → LaNi₅H₆
Storage
Upon absorption, the hydrogen is held in chemical bond within the metal matrix.
In this form, hydrogen is highly concentrated, frequently more so than in liquid hydrogen.
The system is compact, stable, and non-flammable, which is more secure than high-pressure or cryogenic storage.
Desorption
• Heat is used to release hydrogen.
• The metal-hydrogen bonds rupture at this temperature, releasing hydrogen gas.
• This is an endothermic process—it involves energy input.
Advantages of Metal Hydrides for Hydrogen Storage:
1. High storage density
• Metal hydrides have larger hydrogen storage capacity per unit volume than compressed hydrogen gas or liquid hydrogen.
• Some hydrides (e.g., those based on magnesium) have storage gravimetric densities rivaling other technologies.
2. Energy Efficiency
• In contrast to cryogenic storage and needs continuous refrigerating, metal hydrides simply need heat for liberating hydrogen.
• Fuel cell waste heat or process waste heat can be utilized to induce hydrogen desorption.
3. Long-Term Stability
• Hydrides of metals do not have hydrogen leakage over time, unlike pressurized tanks.
• They can be charged and discharged repeatedly without serious degradation.
Challenges and Research Directions Associated With Metals Hydrides
Although metal hydrides have various advantages, a few obstacles still remain:
1. Weight and Price
Some of those high-capacity hydride materials, for instance, magnesium hydride, result in heavy weights, which restricts its appliciability. Rare-earth or complex hydrides are costly to manufacture.
2. Slow Kinetics
Some metal hydrides need catalysts or high temperatures for hydrogen to release easily. Scientists are investigating alloyed and nanostructured hydrides for enhancing reaction rates.
3. Limited Reversibility
Not all hydrides can be recharged easily; some of them deteriorate after several cycles. The new compounds, for example, titanium-coated sodium alanate, exhibit better cycling stability.
Applications of Metal Hydrides for Safe Hydrogen Storage
The applications of metal hydrides in different industries, because of their storage capacity for hydrogen at low pressures and high densities in comparison to compressed gas, are as follows:
1. Portable Power
Suitable for compact, robust hydrogen supply in emergency backup, telecommunications, and military applications.
2. Stationary Energy Storage
Facilitates renewable energy storage by storing excess hydrogen and releasing it when necessary.
3. Aerospace & Submarines
Offers safe, low-noise hydrogen storage in constrained or harsh environments.
4. Compression & Purification of Hydrogen
Some hydrides purify and compress hydrogen without mechanical components.
The following applications underscore the prospect of metal hydrides for enabling a safer and diverse hydrogen economy.
Future of Metal Hydrides: Innovations on the Horizon
Scientists are working actively to address these issues. Here’s where we’re headed:
1. Nano-engineered Hydride
By decreasing particle size to the nano level, scientists can accelerate hydrogen absorption/desorption rates and reduce operating temperatures. Such materials demonstrate good performance in laboratory settings.
2. Hybrid Storage Systems
Future technologies can integrate metal hydrides and compressed gas to balance storage density, release rate, and safety—delivering a best-of-both-worlds.
3. New Alloy Discoveries
Advanced materials modeling and AI-based materials exploration accelerate the process of seeking low-cost, light-weight, thermally stable alloys with improved hydrogen storage capabilities.
4. Thermal Management Solutions
The incorporation of waste heat recovery or integrated heating systems in hydride tanks can rectify thermal problems and ensure efficiency in the system.
Conclusion: Metal Hydrides – A Path with Potential for Expansion
Metal hydrides provide an improved, compact method of storing hydrogen without cryogenic temperatures or extreme pressures of conventional technology. With their state as solids, they become suitable for use in vehicles, mobile power, and even aerospace.
Despite this, critical challenges still persist—slow kinetics, material expense, weight, and limited reversibility. Gaps in research still remain in terms of achieving low-cost, lightweight hydrides with superior kinetics and cycling stability. System integration and scalable solutions for deployment in practical applications also need to be developed.
With nanoengineering advances, AI-aided materials discovery, and hybrid storage design, metal hydrides are gradually moving from benchside novelty to real-world solution. They might be the foundation for safe, compact energy storage as the hydrogen economy expands.
Looking Ahead:
The use of metal hydrides offers one potential solution to safely storing hydrogen in solid form and overcoming some of the problems related to hydrogen storage in gaseous form. Yet, in order for hydrogen to fully live up to its potential as an energy carrier, infrastructure for its delivery must be taken into consideration. That includes hydrogen pipelines, which also have issue areas in terms of engineering, safety, and regulation. In our next blog, Hydrogen Pipelines: Infrastructure Challenges, we examine those problem areas in detail, emphasizing the need for an extensive pipeline system to facilitate growing hydrogen use.
Read More on Liquid Hydrogen Storage Technologies….
Resources:
Advances in hydrogen storage with metal hydrides: Mechanisms, materials, and challenges.
https://sciencedirect.com/science/article/pii/S036031992400778X
