Hydrogen storage in solid hydrides

The hydrogen economy is an alternative to the current energy situation based, fundamentally, on the consumption of fossil fuels with the consequent and recognised economic, geostrategic and ecological problems [1]. It is based on producing, transporting, storing and using the energy vector hydrogen according to the cycle shown in Fig. 1. Ideally, hydrogen would be generated locally using non-fossil fuel sources (solar, wind, tidal, etc.), for subsequent transport, storage and use in a fuel cell, generating water as the only waste, which would then be used again in the cycle.

However, one of the main problems of the hydrogen economy lies in the difficulty of storing the hydrogen produced in an economical and safe manner. Conventional techniques based on storing hydrogen in high-pressure cylinders have improved substantially, with pressures of 800 bar now being used, but there are still drawbacks related to the need to use high volumes to store a sufficient amount of hydrogen and the unresolved problem of finding cheap ways of compressing hydrogen at such high pressures. Some of these issues (size of the hydrogen container) can be partially solved by using liquid hydrogen, but both the high energy and economic cost of maintaining liquid hydrogen (T = 21 K) make this practically unfeasible.

In this context, accumulating hydrogen in solid compounds through chemical reactions (Eq.1) appears as a very attractive alternative given their enormous versatility, as many of them are capable of storing more hydrogen per unit volume than liquid hydrogen itself (Table.1), presenting greater safety. The suitability of solid compounds to absorb and desorb hydrogen depends on parameters such as the pressure and temperature of charge/discharge, the speed of these processes as well as their cyclability. These properties are closely linked to the thermodynamic and kinetic properties of hydride formation and decomposition [2]. In order to know them, it is necessary to characterise the Gibbs energy of the reaction as well as the activation energies of the different stages occurring during the absorption/desorption of hydrogen in the material, respectively.

In principle, a compound is considered suitable if it is capable of absorbing/desorbing hydrogen at moderate pressures and temperatures, i.e. between 1-10b and at temperatures between 0 and 100ºC. From a thermodynamic point of view, this means that the enthalpy of reaction (Eq. 1) should be around 30-40 kJ/mol H2. The main compounds that fulfil this condition are the so-called intermetallic compounds, i.e. compounds with a chemical formula AB5, AB2 and AB (A and B being elements with high and low affinity to hydrogen, respectively) and which have a weak metallic bond to hydrogen. These compounds have been extensively investigated and are capable of absorbing/desorbing hydrogen in a few minutes with good reversibility and are suitable for stationary applications (e.g., to store hydrogen from renewable sources such as solar and wind) [2]. Figure 2 shows an accumulator cartridge based on these compounds, designed and built in our group for mainly didactic purposes. On the other hand, because their storage capacity per unit mass is not very high, they are not usually used in mobile applications.

In this case, it is necessary to find hydrides with higher gravimetric capacity. For this, the hydrides must be based on light elements such as alkaline or alkaline-earth elements i.e. ionic hydrides (LiH, NaH, CaH2…). Among them, the most interesting is magnesium hydride (MgH2) due to the abundance of magnesium, its low reactivity and its high storage capacity (7.6 wt.%). On the other hand, its ionic character means that the hydrogen absorption/desorption reaction has an enthalpy of 76 kJ/mol, preventing its use at reasonable temperatures and making it difficult to implement. Currently, an enormous effort is being made to lower this enthalpy and accelerate the kinetics of the reaction by methods such as nanostructuring, formation of Mg-based compounds (Mg2Ni, Mg2Si…) and synthesis of nanoparticles.

Finally, there are other families of hydrides based on light elements, namely complex hydrides, including borohydrides, with the general chemical formula Mn+[BH4]n- and alanates (Mn+[AlH(n+3)]n-), where M is an n-valent metal. However, despite their high gravimetric hydrogen accumulation capacities, which reach values around 17-18 wt%, they present reversibility problems and require the use of high temperatures and pressures for cycling so that they could currently only be used in single-use applications. This type of behaviour is similar to that shown by compounds consisting of nitrogen and boron i.e. aminoboranes (NH3BH3).

In short, the problem of accumulating large quantities of hydrogen in a safe and economically viable way has not yet been solved and constitutes one of the bottlenecks for the implementation of the hydrogen economy. In this framework, solid state hydrogen storage is a field that presents great prospects due to the wide range of hydride families that could be used ad hoc in different applications (stationary, mobile, single-use, high temperature…) offering also advantages related to their safety, efficiency and capacity compared to conventional methods. As a result, there are currently compounds that are capable of accumulating up to 150 kg H2/m3 and almost 20% by weight. In many of these hydrides (mainly the metal hydride family) the technology is mature and commercial accumulators have been developed for years, but in other families (ionic and complex hydrides) both fundamental research and technical development are still needed to offer viable solutions to the accumulation problem and, consequently, to achieve greater implementation of the hydrogen economy.

[1] J. Rifkin, ‘The hydrogen economy’ 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, January-March, 2010, pp. 63-68.