While accomplishing the global market usage of hydrogen requires addressing numerous concerns, such as hydrogen production, transportation, and application, the primary constraint on the growth of a hydrogen-based energy ecosystem lies in the nonexistence of a perfect hydrogen storage material. Magnesium hydride is very promising among several hydrogen-storage materials because of its high hydrogen capacity, low cost, and good reversibility. However, this material’s poor hydrogen intake and release rate restrict its commercial application. This challenge may be overcome by using functional additives to make magnesium hydride a prospectus hydrogen storage material.

Magnesium hydride is chemically stable, even at temperatures such as 280 0C (approximately 540 0F). This means it is difficult to make some changes, such as adding more hydrogen or taking out hydrogen from the system. Hence, to solve this problem, the chemical stability of Magnesium hydride must be altered. Studies show that breaking the magnesium hydride particles to finer size may expose many new sites of magnesium hydride to exchange hydrogen. Pure magnesium hydride with finer particles shows a little improvement in its functioning. Restructuring magnesium hydride molecules by adding suitable additives to the hydride molecules is a cheap chemical process.

Which additives are suitable for magnesium hydride?

In general, researchers use two types of additives with magnesium hydrides to improve the rate of loading and unloading of hydrogen: catalysts and other metal hydrides. Catalysts such as metals, metal oxides, carbon, metal halides, and covalent compounds are used. Metals such as cobalt, copper, iron, germanium, manganese, niobium, palladium, vanadium, etc., are the most common ones. For better performance, researchers apply metal oxides such as alumina, ceria, titania, silica, chromium oxide, copper oxide, iron oxides, and others. Fluorides of cerium, titanium, and copper also show enhanced performance. A few studies on incorporating carbon materials such as graphite, CNTs, and graphenes show a positive effect in enhancing hydrogen adsorption and desorption rate.

Mixing other metal hydrides with magnesium hydride helps to overcome the limitations of slow uptake and discharge of hydrogen. Hydrides of lighter metals, such as lithium aluminum hydride (LiAlH4), show superior performance when mixed with magnesium hydride. Other metal hydrides such as aluminum hydride (AlH3), sodium aluminum hydride (NaAlH4), and lithium boron hydride (LiBH4) have shown promising results. Composites of the metal hydrides are also mixed in varying proportions, such as LiNH4-LiBH4 and LiBH4-Al, and utilized to improve the activity of magnesium hydrides.

How do additives function with magnesium hydride?

Researchers incorporate additives into magnesium hydride to address various issues, such as increasing hydrogen storage capacity, improving the kinetics of hydrogen uptake and release, enhancing reversibility in cyclic operation, and reducing operational temperature. One of the effective methods for achieving such properties is to expose more surface area by downsizing the material using mechanical grinding, such as ball milling. When milling additives with magnesium hydride, they penetrate deeper into the pores of magnesium hydride.

Catalytic additives facilitate easier separation of hydrogen from magnesium hydride. Due to the presence of the catalyst, the energy barrier for the hydrogenation of magnesium-to-magnesium hydride reduces; therefore, the operational temperature also reduces. Since metals exhibit multiple ionic states, they can alter the hydrogen release mechanism from magnesium hydride and improve the rates. Adding carbon materials, especially nanotubes, creates channels for hydrogen flow in the magnesium hydride matrix and boosts the hydrogen exchange rate. 

The incorporation of hydrides from lighter metals into the magnesium hydride matrix addresses the challenges related to thermodynamic stability and kinetics of hydrogen release, thereby enhancing the performance of hydrogen storage. Adding lighter metals creates more void space and alters the chemical stability of the hydride. The mixed metal hydrides interact and form highly stable metal alloys that help release the hydrogen from the mixed hydride system. Due to the presence of such mixed metals in the magnesium hydride systems, the hydrogen release is possible at 90 0C, which otherwise requires around 350 0C with pure magnesium hydride.

Performance Enhancement of Hydrogen Storage with Additives

A few studies reported that magnesium hydride, when ball-milled with metals such as Cobalt and nickel in the presence of hydrogen gas, increases the gas loading rate by at least 2.5 times.  A separate study shows ball milling metals such as titanium, vanadium, manganese, iron, and nickel 5 % with magnesium hydride can increase the hydrogen storage capacity by up to 5 times, hydrogen absorption rate by 8 times, and desorption rate by 2 to 10 times in ambient conditions.

Mechanical grinding of chromium oxides, alumina, and ceria with magnesium hydroxide can enhance the hydrogen absorption and desorption rate by 2 -6 times. Adding titania can also increase the rates by more than one order of magnitude. When mixed with magnesium hydride, fluorides of niobium and titanium reportedly show a 4-fold enhancement in the rates at moderate temperatures like 80 0C. Similarly, the carbon materials enhance the rates when used with MgH4.

When 10 % aluminum hydride is mechanically milled with MgH4, the hydrogen release rate increases by more than 7.5 % when heated from ambient temperature to 500 0C. A study shows that adding 20 % lithium aluminum hydride in the MgH4 matrix can reduce the onset temperature for hydrogen release. Additional 5 % iron oxide in that mixed matrix can further reduce the onset temperature by more than 40 0C. Therefore, incorporating additives into the magnesium hydride improves operational temperature, hydrogen intake and release rates, and lifetime, which are crucial for commercial application.

The perspective of magnesium hydride as future energy storage material

Magnesium hydride is a material suitable for application in hydrogen storage. Researchers have successfully resolved a few drawbacks of magnesium hydride by adding suitable additives, such as lowering the operating temperature, increasing the hydrogen uptake and release rate, and improving the lifetime. To realize magnesium hydride at a commercial level, it is crucial to address some serious issues. Understanding the mechanism of hydrogen uptake and release at the atomic level may lead to the discovery of suitable additives for better rates. Identifying multifunctional additives that can resolve more than one issue can be a great step towards that. A limited number of cyclic operations with the material is still one of the challenging issues, and it requires crucial attention. Anticipations suggest that further research on the magnesium hydride system will lead to important developments, making it a widely usable product.