**Abstract**

The need for cleaner sources of energy has become a serious need now more than ever due to the rising effects of fossil fuels on the environment. Technological advancement in society today has necessitated the need for fast and robust materials that will match the speed at which society is moving forward. Hydrogen as an alternative source, has garnered a lot of attention due to its zero emission characteristic. In this chapter, a background on hydrogen storage and its impact on the 'envisaged green environment' is discussed. Graphene and borohydrides hydrogen storage materials are reviewed extensively and the kinetic models thereof. Furthermore, the reaction mechanism of graphene nanocomposites is also discussed.

**Keywords:** hydrogen storage, hydrides, kinetics, graphene, nanocomposites

#### **1. Introduction**

The generation of the 21st century has had to deal with the effects of centuries of environmental mismanagement due to industrialization and modernization. To put things into perspective, approximately 6587 billion metric tons of carbon dioxide, a main contributor to climate change, were emitted into the atmosphere in 2015 through the use of fossil fuels [1]. In 2017, these numbers increased and they are continually increasing every year (**Figure 1**).

As such, the implementation of green systems and processes has become a priority in order to try and mitigate the damages to the environment. The remedial actions implemented to reduce the effects on the environment include the institution of legislations that fine high carbon emitters, green technologies and green alternatives to support energy and electricity demands. Hydrogen is amongst the green alternative researched as a substitute for fossil fuels. However, it is acknowledged that the transition from fossil fuels or carbon based sources is not going to be an easy one because of the demand and the complexity of introducing a different system. The United States of America has already started introducing the hydrogen fuelled cars and currently, there is about 6558 hydrogen fuelled cars. This number is expected to surge in the coming years, in 2027 it is expected that there demand would increase to 70,000 units [2].

The implementation of the hydrogen economy is driven by the Department of Energy in the United States. The DoE set the standards and requirements for hydrogen storage materials for practical application in hybrid cars (**Figure 2**).

Various hydrogen storage materials have been synthesized and tested for hydrogen storage applications. However most of these materials have not met the

#### **Figure 2.**

*2020 hydrogen storage requirements for hybrid cars (Department of Energy, United States).*

requirements for applicable hydrogen storage. In this book chapter, the synthesis and first principles studies thereof of new classes of materials will be reviewed, their kinetics will also be reviewed in great details.

An immense effort has been put towards finding novel material for hydrogen storage that will have optimum conditions as indicated by the D.o.E. A great number of light-weight nanostructures consisting of nitrogen, carbon and boron have received favorable attention due to their large volume-to- surface ratio and lightweight characteristic [3]. These materials have been studied intensely and recent works, depicts an improvement and a ray of light in terms of practical applicable hydrogen storage in them.

The interest in these materials was instigated by their unique characteristics. Boron based nanostructures have similar porous structural characteristics with carbon based materials however, boron based materials are much more lighter [3] making them an interesting candidate for practical hydrogen storage. Boron based materials are synthesized using various methods such as hydrolysis and pyrolysis of chemical hydrides.

### **Hydrolysis:**

Hydrolysis is the liberation of hydrogen gas through the reaction of water with a hydride. The overall reaction is summed up as follows:

$$\text{MHx} + \text{xH}\_2\text{O} \rightarrow \text{M(OH)}\_\text{x} + \text{xH}\_2\tag{1}$$

$$\text{MXH}\_4 + 4\text{H}\_2\text{O} \rightarrow 4\text{H}\_2 + \text{MOH} + \text{H}\_3\text{XO}\_3 \tag{2}$$

Where x is the valence number of the metal, M is the metal and X is a trivalent group III element.

### **Pyrolysis:**

Pyrolysis is the breakdown of a substance due to heating. The overall pyrolysis reaction is summed up as follows:

$$\text{2M} + \text{xH}\_2 \rightarrow \text{2MHx} + \text{HEAT} \tag{3}$$

Where M is an alloy or a metal.

Boron hydrides are interesting chemical H2 storage materials that have high hydrogen capacity however the kinetics and thermodynamics of these materials have limited their practical use for hydrogen storage. Different approaches have been employed to decrease the kinetics and thermodynamics of these materials. Popular boron hydrides include LiBH4 (Lithium borohydride) and NH3BH3 (ammonia borane) [1]. The reaction mechanism with boron hydrides can be summed up into four main steps: 1. the decomposition or breakdown of the material through heating to generate hydrogen (Pyrolysis), 2. The interaction of the material with water to release hydrogen (pyrolysis), 3. Improvement of the kinetics through the addition of boron to the electrodes on the metal hydride battery, 4. The storage of hydrogen via the boron nitride nanotubes which can also release the hydrogen when heating [4]. A lot of remedial actions have been employed to improve the kinetics of these materials however the research still continue.
