Hydrogen storage in metal hydrides

It will provide a safe and simple storage solution, solving problems with hydrogen

Hydrogen fuel is an alternative to the present energy situation, which is predominantly based on fossil fuels and consequently their economic, geostrategic and ecological problems [1]. The Hydrogen economy is based on the production, transportation, storage and use of the hydrogen energy vector, as shown in Fig.1. Ideally, hydrogen would be generated locally using non-fossil resources (e.g. renewable energy generation), it would then be transported, stored and used in a fuel cell. The only waste material would be water, which could be reused in the cycle.

Fig.1. Showing the Hydrogen cycle

However, one of the main problems in the hydrogen economy is the difficulty in storing hydrogen in a safe and inexpensive way. Conventional techniques based on hydrogen storage in high-pressure tanks have substantially improved and now it is being stored at pressures of 800 bars. But there are still unsolved problems. In order to store a sufficient quantity of fuel, very large volumes are required. In addition to this it is necessary to investigate more cost-effective methods of compressing hydrogen at such high pressures. One solution to the large volume of containers can be addressed by using liquid hydrogen instead of gas, however the high energy and economic costs to keep hydrogen in liquid state (T=21K) make this solution unfeasible.

In this context, new methods for hydrogen storage have been sought. One alternative is the storage of hydrogen molecules in solid compounds using chemical reactions (Eq.1). This is an attractive alternative because of its versatility and because solid compounds can store more hydrogen per unit of volume than liquid hydrogen itself (Table.1), as well increasing the safety. The suitability of a solid compound to absorb and deabsorb hydrogen depends on several parameters such as required pressure and temperature of the charge/discharge, speed of the process and cyclability. These properties are closely linked to the thermodynamic and kinetic properties to form and decompose hydrides [2]. To know them, it is necessary to characterise the Gibbs energy of the reaction and the activation energies of the different stages that takes place in the absorption/deabsorption of the hydrogen in the material respectively.

Ecuation 1

Table.1. Gravimetric and volumetric densities of different hydrides, compared to the density of liquid hydrogen and gaseous hydrogen at high pressure. *Mass of the container is not considered

A compound is suitable if it can absorb/deabsorb hydrogen at moderate pressure and temperature ranging 1-10b and 0-100ºC. From a thermodynamic point of view, it means that the enthalpy of the reaction (Ec.1) must range between 30-40kJ/mol of H₂. The most suitable compounds are the known as intermetallic compounds whose chemical formula is AB₅, AB₂ and AB (A and B being elements with high and low affinity with hydrogen, respectively) and that present a weak metallic bond with hydrogen. Such compounds have been well researched, and they can absorb/desorb hydrogen in just few minutes. They present a good reversibility and they are ideal for stationary applications such as the storage of hydrogen generated from renewable energy sources like solar and wind [2]. Figure 2 shows a storage cartridge based on these compounds that has been design by our research group for educational applications. In contrast, as the storage capacity by mass unit of intermetallic compounds is not very high, they are not usually used in portable applications.

Fig.2. Storage cartridge developed by MIRE Research Group. Volume=50cm³. Capacity similar to a 100cm³ tank at 250 bars (2.2g of H₂)

Thus, it is necessary to find hydrides with a higher gravimetric capacity. So, hydrides must be based on light elements such as alkaline or alkaline earth metals (e.g. ionic hydrides such as LiH, NaH, CaH₂). An appealing option is Magnesium Hydride (MgH₂) as Magnesium is very abundant, its reactivity is low, and its storage capacity is high (7.6% weight). In contrast, its ionic character implies that the enthalpy of the hydrogen absorption/desorption reaction is 76kJ/mol, which prevents its use at reasonable temperatures and makes implementation difficult. Now, huge efforts are being made to reduce this enthalpy and accelerate the reaction kinetics. Some of the methods used are nanostructuration, formation of Mg compounds (e.g. Mg₂Ni, Mg₂Si) and synthesis of nanoparticles.

Finally, there are other hydrides families that are based on light elements. These include complex hydrides, such as borohydrides, whose general chemical formula is Mⁿ⁺[BH₄]^n- and the alanates (Mⁿ⁺[AlH_(n+3)]^n-), where M stands for a metal with a valency of n. However, besides their high gravimetric capacities to store hydrogen, ranging 17-18% in weight, they have reversibility problems and cycling requires high temperature and pressure, so they could only be used in single-use applications. This behaviour is similar to the one of nitrogen and boron compounds, such as amine-boranes (NH₃BH₃).

Summing up, the problem to store a large quantity of hydrogen in a safe and economic way has not been solved yet and it is a bottleneck to implement the hydrogen economy as a fuel alternative. In this framework, the storage of hydrogen in a solid state presents good opportunities as there is a wide range of hydrides families that could be used ad hoc in different applications (stationary, mobile, single-use, high temperature…). They also have added advantages of increasing the safety, efficiency and capacity in comparison with conventional methods. There are now compounds that can store up to 150kg H₂/m³ at a 20% in weight. The technology in most of these hydrides is mature and commercial storage units have been developed for years. However, in other families (ionic and complex hydrides) more research and technical development is necessary to find out feasible solutions to the storage problem and, consequently, increase the implementation of the hydrogen economy.

[1] J. Rifkin, “La economía del hidrógeno” Ed. Paidós, 2002.

[2] A. Fernández, C. Sánchez, O. Friedrichs, J.R. Ares, F. Leardini, J. Bodega, J.F. Fernández “Hidruros sólidos como acumuladores de hidrógeno” Revista Española de Física, RSEF, enero-marzo, 2010, pags. 63-68.