Solid-state hydrogen storage systems have received increased attention in recent years due to their high stability and ease of transportation. Researched materials for hydrogen storage include carbon-based materials, hollow glass microspheres, metal hydrides, nitrides, imides, amides, zeolites, and metal-organic frameworks (MOFs).

Hydrogen storage in solid-state can be accomplished either by physical bonding (physisorption) or chemical bonding (chemisorption). Physisorption has faster desorption/adsorption cycles and higher energy efficiency, for example as in MOF and carbon-based materials. Chemisorption allows large amounts of gas to be absorbed, is irreversible, and requires higher temperatures for gas release, for example as in metal hydrides, nitrides, and amides.

Powerpaste

The main component of PowerPaste is magnesium hydride. PowerPaste is produced by mixing magnesium hydride with mineral oil. When reacted with water at specific temperatures and pressures, PowerPaste releases hydrogen, which is used to power fuel cells. PowerPaste is reported to have a hydrogen storage capacity of 10 wt%.

PowerPaste has been developed by the researchers at the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM). The institute reported that PowerPaste can be used in any fuel cell for vehicle propulsion. Also, the reaction of PowerPaste and water in a fuel cell can be controlled so that hydrogen is only produced when needed.

One of the most important advantages of magnesium hydride that have led to its increased application for hydrogen storage is its high gravimetric storage capacity of up to 10 wt% in slurry form. It also allows for more hydrogenation cycles. The practical applications of magnesium-based materials are limited due to their slow rates of desorption and adsorption. The materials require large temperatures to release hydrogen. Ball milling and suitable catalysts can enhance the kinetics of adsorption and desorption. Recent studies show that mechanical ball milling of magnesium hydride can pulverize it into micro-and nano sized particles. This largely enhances the energy of hydrogen sorption. Adding palladium to the surface of magnesium can help the dissociation of hydrogen. Palladium enhances the rates of adsorption even at room temperatures.

Carbon-based Materials

As one of the most common elements on earth, carbon is well-known for its high gas adsorption ability. Carbon has been utilized as a detoxifier and purifier for many decades due to its ability to be produced in extremely fine powdered form. In powdered form, carbon has a highly porous structure and the propensity to interact with gaseous molecules.

Carbon-based materials for hydrogen storage include carbon nanotubes (CNT) and graphite nanofibers. These materials exhibit high capacities for hydrogen storage. Carbon-based materials having higher microporosity have very high adsorbing characteristics toward gaseous molecules.

Studies show that micropores do not affect the capacity of adsorption practically. Micropores are only essential for kinetic reaction rate and gas compression. Generally, the hydrogen storage capacity of carbon-based materials is directly proportional to the BET surface area.

The adsorption of hydrogen on carbon porous materials at moderate temperatures is due to molecular physisorption. Only a very small amount of hydrogen can be adsorbed by these materials because the molecular interaction in physisorption is very weak. This is true even at 90 bar pressure. Also, temperature plays a very important role in hydrogen storage in pure carbon-based materials.

The properties of carbon-based materials that make them attractive for hydrogen storage include good chemical stability, extensive pore structure, a broad variety of structural forms, and low density. Also, the structures of the materials can easily be modified using a broad range of activation, carbonization, and preparation conditions.

Metal-organic Frameworks (MOFs)

Due to their tunable functionality and pore size, porous materials have a promise for onboard hydrogen storage. Metal-organic frameworks (MOFs) are synthetic nanoporous materials that have high capacities for hydrogen storage.

The key properties of MOFs include large porous volume, high enthalpy, high surface area, and low density. Studies have focused on designing and synthesizing porous MOFs with nanometer-scale pores. The potential applications of MOFs include adsorption, separation sciences, drug synthesis and delivery, and catalysis.

The use of MOFs to store hydrogen was initially reported in 1999. The MOF material was MOF-5, which showed a high capacity of 4.5 wt% for hydrogen storage at 0.8 bar and 77 K. However, at room temperature and 20 bar pressure, the capacity was nearly 1 wt%.

More recent studies have shown that the hydrogen adsorption capacity of MOF-5 varies with surface area, temperature, and pressure. At 77 K temperature and 1 bar temperature, MOF-5 storage capacity for hydrogen can be maximum 1.32 wt%. At 10 bar pressure and above, the hydrogen storage capacity can be 1.6 wt% and at room temperature and 67 bar pressure, the capacity can be 0.2 wt%.

The University of Michigan developed a MOF material, University of Michigan Crystalline Material-2 (UMCM-2) MOF, using a multi-ligand strategy to increase the surface area. The material has a surface area of 5200 m2/g and consists of Zn4O metal clusters. The metal clusters are linked together by four trigonal planar ligands and two linear dicarboxylates in an octahedral geometry. The material has a hydrogen storage capacity of 6.9 wt% at 4.6 MPa and 77 K.

Zeolites

In 1954, the Union Carbide Company in the U.S. introduced zeolites as industrial adsorbents. Zeolites are hydrated microporous crystalline aluminosilicates, which have rigid, open, and infinite 3D structures with high-internal-surface areas up to 1000 m2/g. Zeolites are considered attractive candidates for solid-state hydrogen storage due to their adsorption properties.

The hydrogen storage capacity of zeolites varies significantly with the type of zeolites. A recent study reported that a high silica zeolite of the MFI group (ZSM-5) with a 430 m2/g surface area can adsorb about 0.7 wt% of hydrogen at 1 bar pressure and 77 K temperature.

Another study reported that zeolite A (LTA) can store hydrogen in a capacity of 2 wt% or more if all cage sites are filled. So far, many zeolites with different pore structures have been studied for hydrogen storage. The maximum hydrogen capacity of zeolites at 1.5 MPa and 77 K is 1.8 wt% and at 0.96 MPa and 35K is 4.5 wt%.

Hollow Glass Microspheres (HGM)

Studies have described hollow glass microspheres as a possible solid medium for controlled hydrogen storage and release. A commercial procedure for making HGMs is the spraying of a glass frit inside a flame. During the process, chemical agents like urea or sulfur must be added to the glass frit to make the frit blow outwards, forming the hollow spheres.

Hollow glass microspheres with approximate wall thicknesses of 1.5 lm and diameters of 1-200 lm are regarded as practical for hydrogen transport and storage. Nonetheless, the strength of HGMs is one of the major concerns for using the materials for hydrogen storage.

However, using engineered glass to make HGMs can increase the amount of hydrogen that the HGM can store. Engineered glasses are nearly 50 times stronger than normal glasses.

A study by researchers at the Savannah River National Laboratory (SRNL) in the U.S. reported that Porous Walled-Hollow Glass Microspheres (PW-HGMs) consist of tiny micron-sized glass balloons and each batch contains spheres of diameters 2-100 lm. The differentiating feature of the PW-HGM is the interconnected porosity of their outer walls. The micro balloons can be filled with absorbents and other materials via the open channels to provide a contained environment to form a novel type of glass-absorbent composite. Hydrogen can enter through the microspheres and can be stored on the absorbents. This results in a solid-state and contained storage.

Metal Imides, Amides, and Nitrides

Metal Imides, Amides, and Nitrides are formed via chemisorption. These compounds show a high capacity for solid-state hydrogen storage at low operating temperatures compared to other storage systems that undergo chemisorption.

Lithium nitride can reversibly uptake hydrogen in very large amounts. Research at the National University of Singapore showed that the hydrogen storage capacity of lithium nitride (11.4 wt%) is nearly 50% more than magnesium hydride (6.5 wt%). However, lithium nitride may not be ready for practical application soon because of high-temperature requirement for hydrogen release.

Metal imides can store hydrogen up to 7 wt%. Just like metal nitrides, metal imides require high operating temperatures in practice. This disadvantage limits their real-life application. Studies indicate that adding alkali earth metals like calcium or magnesium can dramatically reduce the hydrogen storage temperatures. Also, it increases the pressures of desorption and allows high hydrogen storage capacities to be accomplished.

Metal amides, imides, and nitrides are very expensive, and in a few cases, the hydrogen desorption/absorption is irreversible. In other cases, the release of hydrogen requires very high temperatures. For CNT, HGM, and MOF, the main disadvantage is their low hydrogen storage capacity under mild operating conditions.

Future research directions

The different methods of hydrogen storage depend on the application, whether stationary or mobile. Although there are several ways to store hydrogen efficiently, opportunities for new potential materials and methods for hydrogen storage always exists. Research must be carried out on these new materials and methods to advance the hydrogen storage sector further. Research should also be carried out for understanding the electronic behavior of hydrogen interaction with metals and other elements for developing new compounds from lightweight metals and hydrogen.