Comparing the Different Interstitial Hydrides for Hydrogen Storage in Solid Form

Hydrogen storage in Solid Form – Intermetallic hydrides consist of metals or alloy transition metals (also called hydrogen storage/absorbing metals) in which hydrogen occupies interstitial tetrahedral and/or octahedral sites via a metallic bond.

The interstitial hydrates are alloys consisting of non-hydride forming metal B (ex. Cr, Mn, Fe) and hydride forming metal A (ex. Ti, V, Zr, Y). Examples of intermetallic compounds include AB2, A2B, A2B7, AB5, AB3, AB, and body-centered cubic (BCC) alloy structures.

This article compares these intermetallic hydrides for hydrogen storage.

AB Type Alloys 

In 1974, researchers discovered the first AB-type alloy, which was named TiFe. It desorbs and adsorbs hydrogen at room temperature, and the maximum hydrogen capacity is 1.9 wt%, similar to that of TiMn1.5.

Iron (Fe) and Titanium (Ti) are abundant in the earth’s crust and relatively cheap. As a result, there have been extensive studies of TiFe as a material for hydrogen storage.

However, the hydrogen absorption reaction is difficult to execute because, above 3 MPa and at 400 °C, limited hydrogen activation significantly affects hydrogen absorption.

Large hysteresis between the absorption and desorption plateaus, cycling degradation of hydrogen desorption/absorption and the two-step absorption (two-step plateau) are amongst the most serious issues limiting the practical utilization of the AB-type alloys.

Researchers have invested significant effort into substituting and doping with other elements to address those issues.

AB2 Type Alloys – Hydrogen storage in Solid Form

Laves phase alloys, also known as AB2 type alloys, consist of A elements such as Titanium (Ti) and Zirconium (Zr), and B elements such as Chromium (Cr), Manganese (Mn), and Vanadium (V).

The AB2 type alloys consist of hexagonal C36, cubic C15, and hexagonal C14. In AB2 alloys, Zr and Ti on one side and Cr, Mn, and V on the other side can be substituted for one another, resulting in a variety of typical alloys like ZrV2, ZrMn2, ZrCr2, TiMn1.5, and TiCr2. An AB2 alloy can absorb hydrogen in large amounts and has a maximum hydrogen capacity, H/M, of 1.5.

For hydrogen absorption and desorption, the temperature range of AB2 alloys is wider than AB5 alloys. For example, TiCr2-based alloys can desorb and absorb hydrogen at 200 K, indicating heat pump application potentials. In the case of Ti-Mn alloys, hydrogen is not absorbed by the stoichiometric TiMn2, whereas TiMn1.5 exhibits a high storage capacity for hydrogen at 1.9 wt%.

AB3-Type Alloys – Hydrogen storage in Solid Form

AB3- type alloys, RMg2Ni9 (R = Lanthan, Cerium, Praseodymium, Neodymium, Samarium, and Gadolinium) with (Lanthan (La), Magnesium (Mg))Ni3 phase was first reported in 1997. The crystal structure of the AB3- type alloy is the stacking of a [MgNi2 (AB2)] sub-unit and a [RNi5 (AB5)] sub-unit along the c axis. Several researchers have reported the hydrogen storage properties of La3MgNi14, La5Mg2Ni23, La2MgNi9, and ternary alloys.

Between 1 MPa and 10-3 MPa pressure range and at 333K, the maximum storage capacity of hydrogen (H/M) is 1.1% for La0.7Mg0.3Ni2.8Co0.5 and 0.8 for MmNi4.0Mn0.3Al0.3Co0.4. The hydrogen discharge capacity of La0.7Mg0.3Ni2.8Co0.5 could be about 410 mA·h·g-1, which is 1.3 times more than that of AB5-type alloys. This proves its high potential as the negative electrode of the Ni-MH battery. Several Ni-MH battery types already use AB3-type alloys.

AB5 Type Alloys 

In AB5 alloys, A refers to rare earth metals and B represents d-transition metals. An example of an AB5-type alloy is LaNi5. This alloy forms the LaNi5H6 hydride by absorbing 1.4 wt% of hydrogen, with hydrogen over metal ratio, H/M = 1. During catalytic spontaneous surface segregation, the process produces metallic Ni-precipitates. They absorb and desorb hydrogen at room temperature without requiring activation treatment.

Several researchers study MmNi5-based alloys where in LaNi5-based alloys, La is replaced by Mm (Mischmetal as a mixture of rare earth metals) and Ni is substituted by Mn, Co, and Al. The purpose is to enhance the storage characteristics of hydrogen for practical purposes. 

Additionally, MmNi5-based alloys have been successfully employed as a negative electrode material for Nickel-Metal Hydride (Ni-MH) batteries. Hybrid vehicle manufacturers, such as Toyota, have extensively used solid-state hydrides in their vehicles, including the Prius.

BCC Solid Solution Alloys 

Solid solution intermetallic compounds with body-centered cubic (BCC) crystals have attracted significant research interest as their high hydrogen capacity of H/M = 2 is greater than that of ABx intermetallic compounds.

The V-based BCC solid solution alloys like V-Ti-Cr, V-Ti-Mn, and V-Ti-Fe, which were first introduced in the 1980s, can absorb and re-absorb hydrogen through a two-step plateau. Because of the very low plateau pressure at room temperature, hydrogen absorbed in the first plateau is difficult to release. 

For BCC, at room temperature, the maximum capacity for reversible hydrogen storage is around 3 wt%. Researchers have devoted many attempts to developing effective methods for activating and speeding up the kinetics, producing a flat hydrogen absorption plateau, and reducing the degradation of the cycling capacity.

Reports suggest that one of the most effective methods for increasing the hydrogen storage capacity, improving the absorption of hydrogen, and flattening the hydrogen desorption plateau is by increasing the V content.

Future research – Hydrogen storage in Solid Form

At room temperature, interstitial hydrides can absorb and desorb hydrogen, but they have limited hydrogen capacity with an H/M ratio of ≤ 2. Researchers could carry out future research in this direction.

Further research may also concentrate on interstitial hydrides with high entropy alloys (HEAs). HEAs comprise of five or more elements with atomic ratios of 5% to 35% and have recently emerged as a new class material for hydrogen storage because of their flexible chemical composition.