1. Introduction

## 1.1 Hydrogen as the future renewable energy vector

In the last two centuries, the energy supply has been based on fossil fuels (natural gas, liquid fuels, and coal). This strong dependence of our modern society on fossil fuels has economic- and geo-political consequences and causes detrimental environmental footprints. Figure 1A shows the world fossil fuel demand-supply trend from the 1990s until 2017. In 2017, the world fossil fuel demand (97.8 barrel per day) is over the supply (97.4 barrel per day) [1]. For these reasons, in the last decades, many efforts have been put for the development and research of energy renewable sources. The market of the renewable energies achieved a new global record in 2015 with \$285 billion investment on this technology, exceeding the previous record of \$278 billion on 2011 [2]. The use of renewable energy has many

energy density is extremely low (0.003 kWh/dm<sup>3</sup>

DOI: http://dx.doi.org/10.5772/intechopen.82433

storage systems.

kWh/dm<sup>3</sup>

Figure 2.

127

densities of 1.2 kWh/dm3

metric energy density (0.8 kWh/dm<sup>3</sup>

volumetric energy density of H2 is notably lower than the traditional fossil fuels (0.008 kWh/dm<sup>3</sup> for natural gas, 10.5 kWh/dm<sup>3</sup> for liquid hydrocarbons, and 35 kWh/dm<sup>3</sup> for processed coal). Figure 2B shows the volumetric storage density for different traditional and hydrogen storage systems. As seen, there is a huge gap between the volumetric energy density of the light and cheap tank system for liquid hydrocarbons such as gasoline and the volumetric energy density of hydrogen

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage

Liquefaction and compression are the traditional physical methods to store hydrogen. Nowadays, available liquid hydrogen storage systems can achieve energy

253°C (liquefied hydrogen temperature) and this requires 20–30% of its energy content. Moreover, hydrogen losses because of evaporation are between 0.3 and 3% per day. On the contrary, gas storage systems can confine hydrogen for long periods without losses. The gas hydrogen vessels need similar technology and infrastructure to that of the established compressed natural gas tanks. Reinforced gas hydrogen containers for 350 and 700 bar of pressure are commercially available. These gas storage systems can achieve volumetric energy densities up to 0.9

Solid storage systems in hydride compounds are so far tested on small and middle scale in laboratories. As an example, a tested solid storage system has lower volu-

[14]. However, solid storage systems work at milder temperature and pressure conditions than the physical storage systems, i.e., in the range of temperatures between 25 and 400°C and under pressure in the range of 10–100 bar of H2, depending on the hydride material. This means that extreme cryogenic temperature of 253°C or extreme high pressure of 700 bar is not necessary. It brings two main advantages from the safety and energy point of view: (1) mechanical properties of the vessel's material are not demanding as for gas storage containers, (2) energy losses because of hydrogen liquefaction, compression, or boil-off are avoided. Nonetheless, the volumetric energy density of the solid storage hydrogen system is still low, particularly comparing with the gasoline storage system. Moreover, there are critical topics like materials' properties (hydride material), charging and discharging conditions of the systems (pressure, time, and heat transfer), charging

(A) Volumetric energy density for pure: hydrogen, liquid hydrocarbons, natural gas, and coal [3].

(B) Volumetric energy density for storage systems [14].

, but about 15% of the fuel energy content is required for compression.

. However, cryogenic hydrogen storage systems work at

) than the traditional physical storage systems

). Figure 2A shows that the

Figure 1.

(A) Evolution of the world demand and supply of fossil fuels in millions of barrels per day (includes all kinds of fossil fuels) [1] and (B) Gravimetric energy density for pure: hydrogen, liquid hydrocarbons, natural gas and coal [3].

potential benefits, including a reduction in CO2 emissions, the diversification of energy vectors, and a reduced dependency on fossil fuel markets. In this regard, hydrogen (H2) is considered as a potential energy vector because it is abundant and its combustion generates a large amount of energy and water: H2 +½O2 ! H2O + Energy. Figure 1B shows that H2 exhibits far higher gravimetric energy density than other traditional fossil fuels: 33.3 kWh/kg for H2, 13.9 kWh/kg for natural gas, and 12.4 kWh/kg for liquid hydrocarbons [3–6]. Furthermore, H2 can be directly produced from clean sources such as, for instance, from electrolysis of water or from biomass with light from the sun and biological micro-organism. These renewable methods for hydrogen production can avoid the coal reforming, though this technology is under development and it is not available on industrial scale yet [7].

Nowadays, H2 is used as feedstock in the petrochemical and fertilizers industries. There are restricted technological applications of H2 as for example in portable devices like mobile phones, computers, and even some hydrogen driven cars. In spite of all the advantages of H2 to be considered as the new energy carrier and the evolution of the hydrogen technology showing some technological applications, there are some major issues to be solved before introducing the "hydrogen technology" into the massive market [8]. The hydrogen technology is still under early research and development phase, the current costs of this new technology and the lack of established infrastructure are the main constraints [9–11]. Among the technological problems for the implementation of the "hydrogen economy," one of the main bottleneck is the lack of safe, compact, energetic-, and cost-efficient hydrogen storage system, especially for mobile applications such as means of transports [12, 13].

On one hand, H2 has the highest gravimetric energy density in comparison with the traditional fossil fuels, Figure 1B. On the other hand, H2 has extremely low density at room temperature (0.089 g/dm3 under 1 bar), hence its volumetric
