Materials preparation and handling can have a dramatic effect on performance results but is often an overlooked step in understanding and performing hydrogen sorption measurements. Thus, the determination of a sample’s purity or at least its chemical composition is clearly important. For reversible materials, it is critical to measure hydrogen uptake and release over several cycles as the formation of unintended by-products (NaCl, H2O, NH3, LiH, other stable hydrides…..) versus true hydrogen storage will typically present a decrease in the apparent hydrogen storage capacity of the material. Mass-spectrometry of the evolved gases is highly recommended. In addition, because the manner in which impurities can produce misinterpreted results is generally differently in volumetric versus gravimetric measurements, it is highly recommended that unexpected or too-good-to-be-true results from one technique be carefully evaluated using at a minimum the other measurement technique.

The most obvious consideration in the synthesis or preparation of a sample for use in quantifying the hydrogen storage capacity of a material are impurities or wholly inactive side products in the sample. Impurities, in addition to potentially detracting from overall capacity, may have an effect on the actual hydrogen uptake or release capacity and/or kinetics.

The presence of unaccounted for-secondary hydrogen absorption phases may skew the interpretation of the capacity of a material if all of the hydrogen uptake or release is attributed to only one component of a multi-component sample. For example, the presence of a catalytic metal that itself forms a reversible hydride. Or the presence of unstable oxides that may react with hydrogen to form water. These issues are often most consequential for high-surface-area materials because the level of impurities generally scale with the amount of active surface area. Impurity issues are also most difficult to identify in high-surface-area materials because while impurities may be present in significant quantities, they may be dispersed in only a few monolayers, present as nano-particles, or amorphous in nature and, therefore, not readily observable using common analytical methods such as standard X-ray diffraction.

The secondary component may not be a hydride former at all, but may cause hydrogen release through a side reaction. For example, a sample of unreacted NaAlH₄ + xTiCl₃ may produce more hydrogen on its first desorption than expected from the decomposition of NaAlH₄ → NaH + Al + 3/2H₂. This is because in the process of decomposition NaH may react with xTiCl3 to form xNaCl releasing x/2H₂. In this case, the impurity or additive, could contribute to an overestimation of hydrogen storage capacity of the actual active storage material based on measurement of the initial quantity of released hydrogen. Such side reactions are clearly an important consideration in the determination of the hydrogen storage capacity of off-board rechargeable chemical hydrides.

The presence of unaccounted for “inactive” impurities can lead to an underestimation of the capacity of the active component of a storage material. This may be the most common and often the most difficult problem to address. If secondary components are present difficult to detect impurities (e.g. nanocrystalline, poorly crystallized or glassy phases), they will be difficult to quantify or even identify in using standard techniques such as X-ray diffraction If the impurities are of the same composition as the storage phase, but only structurally different than the hydrogen sorption component, it becomes extremely difficult to assess the true hydrogen capacity of a material based on a sample of an unknown mixture of phases. For example, in the case of nanostructured carbon materials (specifically single wall carbon nanotubes, SWCNTs) it has been very challenging to determine how much of a sample actually consists of SWCNTs versus amorphous carbon, graphite, multiwall carbon nanotubes, how catalysts used in the synthesis and other components introduced in processing and purification are associated with the various phases and their roles, etc. Different techniques such as TEM and pulsed neutron diffraction have been used to try to quantitatively determine the actual SWCNT content of samples. With impurity concentrations on the order of 30-40% even in highly purified samples, a thorough analysis of a sample’s elemental and structural composition is needed before measured quantities of ab/adsorbed hydrogen can be ascribed to any single material as its true hydrogen storage capacity.

The properties of most hydrogen storage materials are very sensitive to impurities, both as solid contaminants and gas phase impurities. It is well known, for example, that the addition of small amounts of additives (alloying or impurities metals) to hydride-forming alloys used in nickel-metal hydride batteries will cause large changes in electrical capacity, kinetics and cyclability. A great amount of work has been carried out in perfecting the composition (now ~ 10 elements) of AB5 battery alloys. One such example is shown in Figure 1 where the Ni portion of the AB5 alloy LaNi5 has been substituted with small quantities of other elements to improve the long-term cycling capacity. With the commercialization of gaseous hydrogen storage materials and systems, it is very likely that similar compositional “fine tuning” will be employed to improve storage material performance.

Cycle life behavior of LaNi5-XGeX. alloys with comparison to a good commercial mischmetal-based, multi-component alloy also evaluated at JPL.

An important consideration is that solids contaminates can have similar beneficial or negative effects on a material’s performance. For example, it is very common to use ball-milling preparation to introduce catalysts, create defects, or produce nano-structured materials. The milling pot and balls themselves may introduce metals into the samples (Fe, Cr, W, C….). Enhancement of the kinetics may not necessarily be coming solely from intentionally added catalysts, but rather the components of the ball mill may be participating as well. This has been observed by comparing the diameter milling balls when they are new with those that have been used for many hours of milling. The amount of material loss due to mechanical attrition was significant.

An additional consideration with respect to the mechanical milling preparation of materials is the possibility of the introduction of gas-phase impurities that may be deleterious to the materials synthesis. One generally performs milling of the sample in an inert gas environment. There are two ways to do this. One is to introduce the entire mechanical mill into an inert gas glove box, and even then, ppm level of oxygen and water may affect the outcome of the anticipated products. There is also the added complications of needing to setup the milling machine in the glovebox, potential perform maintenance or repairs in the glovebox as well as all of the vibration problems the mill imposes on the glovebox and delicate instruments and items in the glovebox.

The other method is to simply seal the milling vial in an inert gas glove box and subsequently perform the milling outside of the glove box. This second method may leave open question of how well the milling vial is sealed against introduction of oxygen and moisture from the surrounding air that could leak into the vessel if the vessel is poorly sealed. The seals on milling vials are typically made of an elastomer and are not particularly suited to seal against a pressure differential. So, for example, as the vial heats up on milling, so too will the inert gas in the vial heat and expand building up pressure in the vial. The excess pressure may be released across the elastomer seal to equalize the pressure. After milling if the milling vial will cool and the pressure will drop inside of the vial. If it is still in an air environment there is a chance that the lower pressure in the vial will draw air into the vial contaminating or reacting with the sample materials. Introduction of ambient air into the vessel during milling could obviously alter the desired composition and in turn affect the hydrogen storage properties of the materials or even cause unanticipated chemical reactions within the vial that could lead to potential safety concerns.

A well-controlled series of sorption tests of the materials after exposure to known quantities (ppm levels) of contaminants should provide insight into the sensitivity of the materials to impurities during preparation and/or testing. However, it is difficult at best to perform hydrogen sorption testing on materials in an ultra-high purity environment, and it is unlikely that in the real world application for which the materials are being developed that any hydrogen storage system will ever operate under UHP conditions.

Reference List:

• Read, C., Thomas, G., Ordaz, G., and Satyapal, S., “U.S. Department of Energy’s System Targets for On-Board Vehicular Hydrogen Storage”, Material Matters (2007), vol 2, issue 2, p. 3-4.

• “DOE Targets for On-Board Hydrogen Storage Systems for Light-Duty Vehicles Current R&D Focus is on 2015 Targets with Potential to Meet Ultimate Targets”, DOE, EERE publication, (revision Feb, 2009)

• Satyapal, S., “Current State of the Art”, Figures by DOE: Thomas, G., (2007), Sandrock, G., (2008) Presentation “Hydrogen Storage”, 2008 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting June 9, 2008.