Before concentration and capacity can be discussed in depth, it is important to differentiate between capacity, a material property, and the concentration variable in hydrogen storage measurements.

Concentration and capacity provide information about the hydrogen content of a sample. Concentration is the temporal measure of hydrogen content in a sample and is the most fundamental variable in hydrogen storage; it is also the one important variable that cannot easily be measured directly in the material. In all cases, calculated relationships are used to determine hydrogen concentration from measurable (independent) variables. Concentration is the fundamental quantity needed for determining capacity and many other storage properties, including kinetics, thermodynamics and cycle life. Factors that affect concentration measurements will invariably affect the determination of other vital properties of a hydrogen storage material.

A simple example illustrates the difference between concentration and capacity. Figure 1 shows an idealized PCT isotherm of LaNi5, a well-known hydrogen absorbing material, at two different stages of hydriding. At a specific temperature with a corresponding plateau pressure of 1 bar, the theoretical material capacity of LaNi5 is 1.38 wt.% H2 corresponding to LaNi5H6. Neglecting hydrogen in solution in the alloy or the hydride, this is the maximum steady-state hydrogen concentration achievable at a given temperature. Thus, the capacity of a storage material is equal to the maximum concentration when fully hydrided (or 1.38 wt.% for LaNi5H6 as shown in Figure 1). The sample’s hydrogen concentration is a measure of hydrogen content which depends on pressure, temperature, time (kinetics and thermodynamics), and the quantity of hydrogen to which the sample is exposed. For example, a sample that has only been exposed to enough hydrogen in one dose to convert half of the sample to the LaNi5H6 hydride will have a hydrogen that increases in time from 0 to 0.69 wt.% H2. When equilibrium is achieved, the sample’s final concentration will be 0.69 wt.% H2. Regardless, the sample’s capacity under these conditions is still 1.38 wt.% H2.

Figure 1: Illustration of the relationship between concentration and capacity for a flat plateau hydride.

In this regard, concentration is treated as a variable and not a property of a material. Thus, concentration is used to describe the state of the material in the same way temperature is used. A material’s capacity, on the other hand, is a specific material property that is dependent temperature and pressure.

From the broadest definition, “capacity” represents the maximum steady-state hydrogen content in a material and capacity measurements are central to developing materials and systems that meet the criteria established by the US Department of Energy (DOE) for on-board hydrogen storage. However, hydrogen capacity can be defined in a number of different ways which depend not only on how hydrogen is stored in a material (binding mechanisms) but also on the issues being addressed with respect to capacity (fundamental vs. application questions). Hydrogen binding mechanisms and important definitions of “capacity” are presented in the next two sections.

Below are testing reports by H-Sorb 2600 high pressure hydrogen adsorption analyzer for LaNi 5 materials.