For simplicity, hydrogen storage properties measurements can be broken down into three basic categories: storage system level performance measurements, materials development measurements, and fundamental science measurements. It is important to understand the purpose of a particular experimental investigation before making measurements because the experimental setup and procedures can vary greatly depending on the purpose of the measurements and results and conclusions can be misleading if they are not presented in the proper context. Of course, these categories overlap and support each other. Testing the design and performance of a storage system, comparing storage properties across new or improved materials, and gaining insight into the underlying phenomena of hydrogen storage require experimental apparatuses and procedures that may be significantly different. Care should be taken to match the experiment well to the purpose of the study to avoid misleading results.

Purpose 1: System Performance

System level measurements are concerned with the performance of materials at an operational scale. The primary difference between experimentation at the system level versus other levels is the importance of application-specific considerations. Depending on the utilization of the hydrogen storage system, any combination of properties may be most important for performance optimization. Properties such as energy density, reversibility and safety at standard operating conditions are some of the primary ‘real world’ considerations. For example, one consideration central to portable hydrogen storage systems like those envisioned for transportation is energy density because the system must be transported. Stationary applications, in contrast, may be more concerned with total energy than energy density, and cost.

To gain useful system performance information, enough material should be tested to provide results representative of the material’s behavior in a full-sized operating storage system. In general, this means between 10 grams and 1 kilogram of storage material. The authors believe 100 grams is a good tradeoff between the practical issues of synthesis and handling of the materials and a characteristic, uninterrupted volume of material that would be found in a large-scale system. It is recommended to take samples from different portions of larger batch to get a good average material representation.

An example of an experimental setup that may be appropriate at a system performance level but not for other purposes, is the use of flow meters to study rates of hydrogen uptake or release. This requires that the material performance has already been well characterized at a smaller scale. This is because conventional thermal mass flow meters generally have fairly narrow dynamic measurement ranges (often less than one decade). Thus, some prior knowledge of the material’s performance is needed to select flow meters with the appropriate flow range. This can be a severe limitation when a material’s hydrogen sorption/desorption rates may vary over several orders of magnitude depending on state of charge, temperature and pressure. An alternative is Coriolis flow meters, which have a robust dynamic measurement range, maintaining accuracy over several decades.

Purpose 2: Materials Development

The purpose of experimentation at the analytical level (typically tens of milligrams to a few grams) is to guide the development of advanced materials with desirable hydrogen storage properties. It is important to realize that hydrogen storage properties of a material can be strongly influenced by any number of individual sample characteristics including total chemical composition and distribution, microscopic and macroscopic structure, etc. In the realm of materials development, parametric and comparative studies are generally the best approach to producing reliable and productive studies. There is an extremely wide range of material characteristics (including such things as doping and nano-structuring) that can be modified and manipulated to produce profound changes in hydrogen storage properties. For example, the effect of elemental substitution in the LaNi5 family of Intermetallic compounds (LaNi5-xAx) is demonstrated in Figure 1. In this case Ni has been substituted with tin or aluminum to form materials with decidedly different hydrogen storage properties, including changing the thermodynamic stability of the hydride that is formed.

Van’t Hoff diagram representing the hydriding thermodynamic properties of several different LaNi5-based materials

In order to conduct instructive parametric studies, it is especially important that measurements are conducted under identical conditions (pressure, temperature, sample size, etc.) using identical measuring equipment. This minimizes the number of free variables associated with the experiment and allows for the accurate determination of the effect of a specific material property. The effect of some material properties on hydrogen storage can be quite small and easily masked by variation in testing conditions and measuring equipment if the proper precautions are not taken.

Purpose 3: Fundamental Science

As the title suggests, fundamental science research is concerned with identifying and understanding the physics and chemistry that governs a material’s hydrogen storage properties. This fundamental knowledge can be used in research at the materials development and system performance levels. It may help to validate models that describe hydrogen-material interactions or aid in making enlightened decisions on new directions in improving materials. Without some level of fundamental understanding, system and materials development measurements would be conducted solely through intuition or painstaking systematic studies. By using knowledge gained from fundamental science, researchers can eliminate much of the trial and error and more efficiently conduct measurements for material development. With regard to fundamental science, samples may be small (gram quantities) and indeed very small (milligram powders, liquids and thin films) as synthesis is achieved on lab-scales. As a consequence, very sensitive instruments will be needed to perform measurement on sample weighing as low as tens of milligrams. Also, fundamental science implies the modeling of the measurements and hence, intrinsic behavior should be clearly separated from instrumental effects to match a particular model.