Hydrogen capacity is the material property that has had the greatest focus of attention in the race to discover and improve the ultimate hydrogen storage material. While the other critical materials properties (kinetics, thermodynamics, stability, safety, cost and flavor) may have been pushed into the shadows by comparison, it is true that without a reasonably high capacity, a material can not be considered viable for on-board hydrogen storage. And yet, despite all of its appeal and the perceived simplicity of the concept, the hydrogen storage capacity of a material has not been, in fact, either well defined, or (in many cases) very easy to measure, particularly when the measurement of and accounting for gas-phase species other than hydrogen is brought into play. Unfortunately, focus on the glitter of what is not always gold has, on occasion, lead to the expenditure of great efforts, minds, and money on unproductive paths. 

The mechanisms of hydrogen uptake in a material can be separated into “Physisorption”, or “Covalent” and “Interstitial”, with a cross-over mechanism referred to as “Spillover”. 

A material’s hydrogen storage capacity is strongly dependent on the hydrogen-material binding mechanisms responsible for hydrogen uptake. Hydrogen storage mechanisms differ among molecularly bound hydrogen that is physisorbed, and hydrogen that is dissociated and covalently bound or occurs in the interstices of metal atoms in alloys. The strength of the hydrogen interactions among these various modes of hydrogen binding affects the overall manner in which a hydrogen storage material will be used in a real application. Because of the different energetics among the classes of hydrogen storage materials, different measurement approaches and definitions of storage capacity are needed to fully describe the hydrogen storage characteristics of these separate classes of materials. For this reason, we will review the hydrogen storage capacity definitions with respect to the different categories of storage materials differentiated by the type and strength of hydrogen binding mechanisms. To begin with, we separate hydrogen storage materials into three different classes.

1) Physisorbed hydrogen storage materials that store hydrogen by molecular physisorption or weak atomic chemisorption. These typically have high hydrogen storage capacities at cryogenic (77K) storage temperatures but some ambient temperature applications are being explored as well.

2) On-board reversible hydrogen storage materials that store dissociated hydrogen either covalently or as interstitially bound hydrogen. These are able to take up and release hydrogen at moderate temperatures and pressures allowing the materials to be directly recharged with hydrogen in-situ on a vehicle

3) Off-board regenerable hydrogen storage materials that store dissociated hydrogen as covalently bound hydrogen materials, but that require more complex off board chemical processes for regeneration.

These classes of materials tend to parallel the focus of the US DOE’s three different hydrogen storage materials development Centers of Excellence:

1) Sorption,

2) Metal Hydride, and

3) Chemical Hydrogen Storage.

In reality the boundaries are not distinct and there is much overlap between the materials and research centers.

1. Physisorbed Hydrogen Storage Materials

Hydrogen storage by physisorption or weak chemisorption of H2 on high-surface-area materials, such as activated carbon, nanostructured materials, and metal organic frameworks (MOFs) has the advantage of inherent reversibility, cyclability, generally rapid release and recharging kinetics and high capacities at moderate pressures (ca. 30 bar). Physisorption generally involves weak interactions of molecular hydrogen with the surface. However, surface adsorption of hydrogen can also include spillover of atomic H and Kubas type binding of molecular H2. Because these interactions are rather weak, materials that store H2 by physisorption have the drawback of low storage capacities at near ambient temperatures. Gravimetric storage densities only reach acceptable levels at cryogenic temperatures (77K). Up to now, most physisorption studies have focused on the storage capacity of the materials and have not included thermal performance of the storage system. The net storage capacity of an adsorption based system over a wide range of pressures and temperatures as well as system thermal requirements are important for real world applications. 

In physisorption, intermolecular forces between hydrogen molecules and surface atoms create a region near the surface of the material where the local hydrogen density is greater than in the bulk gas. This region is called the adsorbed phase, or more colloquially the boundary layer (BL), and may extend several hydrogen molecule diameters from the solid surface (shown in Figure 1). As in other transport fields, it is difficult to quantify the extent of the hydrogen BL because the density, used to define the boundary layer, asymptotically approaches that of the free gas phase. In order to avoid ambiguity, the physical chemist J.W. Gibbs proposed the concept of Gibbsian surface excess (GSE), a measure of the additional hydrogen stored in the boundary layer. GSE allows for the unambiguous and practical determination of the extent and advantage of the adsorption of molecular hydrogen to the surface of a material with respect to the hydrogen storage materials themselves and also with respect to the complete storage systems. The reader is referred to Sircar for a more detailed analysis of GSE theory and application.

Schematic illustration of (a) physisorbed hydrogen and (b) the distinction between excess and bulk gas phase hydrogen

2. On-Board Reversible Hydrogen Storage Materials

On-board reversible hydrogen storage materials consists of a wide range of materials, with the sole requirement that the thermodynamics allow the discharged materials to be recharged on-board a vehicle under reasonable pressures and temperatures Many if not all of the members of this class of storage material release hydrogen endothermically. If the release of hydrogen is very endothermic, heat might have to be supplied to the material to achieve reasonable rates. However, recharging endothermic release materials with hydrogen of course releases heat, and managing the rate of this heat release is often a challenge for on-board regeneration. Within this class of materials, the thermodynamics of release on the order of 30 kJ/mol H2 provides the ideal stability for the reversible uptake and release of hydrogen at room temperature and 1 atmosphere. Solid on-board hydrogen storage materials include among other things, interstitial metal hydrides, covalently bound metal hydrides including multi-component hydride systems and metal amides, borohydrides, etc. With respect to these “reversible” materials the interstitial metal hydrides and covalent complex metal hydrides have received the most research focus over the last few decades. A brief look at these two classes of materials is presented here in view of capacity measurements.

There are three general types of metal hydrides: interstitial metal hydrides, covalent metal hydrides, and covalent complex metal hydrides. Interstitial metal hydrides are materials such as LaNi5Hx, where hydrogen atoms are found in interstitial sites within the metal atom substructure. The hydrogen bonding is often complex but relatively weak, involving multicenter bonding between hydrogen and the metal. Examples of complex saline hydrides include lithium aluminum hydride, LiAlH4, and sodium borohydride, NaBH4. Covalent metal hydrides are discrete compounds such as MgH2, AlH3, among others, where the bonding between hydrogen and metal is very covalent and localized and strong. Complex covalent hydrides are compounds such as metal borohydrides, metal amides, and mixtures thereof, and the bonding between hydrogen and either B or N, etc is highly covalent and strong.

Most metal hydride formation reactions are exothermic and relatively stable below their dissociation temperatures. This means that under suitable activation conditions, the formation reaction will be spontaneous, and the hydrogen will remain stored within the material until a certain desorption temperature is reached.

3. Off-board Regenerable Hydrogen Storage Materials

There are numerous approaches to hydrogen storage that result in “spent fuel” that is not feasible to regenerate on-board, and these storage materials are considered here. The key feature of these materials is that they typically release hydrogen in an overall exothermic set of reaction sequences, and the dehydrogenated products are too thermodynamically stable to rehydrogenate in a practical sense. These materials may require chemical regeneration in an off-board process. Perhaps the most explored example of an off-board regenerable storage material is ammonia borane (AB), NH3-BH3, a molecular solid that releases hydrogen stepwise to liberate hydrogen, heat, and “BNHx”, where x is determined by the severity of the dehydrogenation conditions. Losing one mole or 6.5 wt. % of H2/mole of AB results in the known compound cyclotriborazane. Losing two moles or 13 wt. % of hydrogen generates the molecular compound borazylene. Loss of additional hydrogen (up to 16 wt. % may be readily delivered) presumably results in the formation of cross-linked borazylene units, forming so-called polyborazylene, and other BNHx oligomers or polymers.

Regeneration of polyborazylene with H2 pressure is too endothermic and not feasible in a practical sense, and so spent AB must be regenerated using a chemical process. Another subclass of potentially off-board regenerable covalent hydrogen storage materials are certain organic hydrocarbon compounds, where hydrogen is stored as C-H bonds, and released endothermically. Researchers at Air Products have led the way in exploring a variety of endothermic hydrogen release compounds.

Another example of an off-board regenerable storage material that received some attention is the simple molecular solid sodium borohydride. Controlled hydrolysis of concentrated aqueous solutions of sodium borohydride release hydrogen and generate sodium borate. Rehydrogenation of sodium borate is too endothermic, and so regeneration of the borohydride from the borate in an energetically efficient pathway is a significant technical challenge. In all of these cases, the bonds that are being broken are quite strong and covalent B-H or N-H bonds, with bond strengths on the order of 300-400 kJ/mol.