# Reversible Hydrogen Storage Using Nanocomposites
## 1 Introduction  
The increased deployment of fossil fuels produces an exorbitant amount of greenhouse gases such as CO2[1; 2; 3], which has already caused an increase in the Earth's temperature and will continue to do so unless CO2 emissions are significantly curtailed. In addition to the occurrence of global warming due to combustion of fossil-based fuels, there is wide agreement that these fossil fuels are running out [4], therefore increasing the urgency of finding new energy alternatives. One such alternative, especially for use in mobile applications, is hydrogen [5; 6]. Hydrogen is an ideal fuel, particularly for transportation because it has the highest energy density per mass (149 MJ\(\cdot\)g\({}^{-1}\)) among the known fuel sources, and it is a safe, environmentally friendly fuel source that can be easily transformed to useful work without releasing harmful emissions. If burned in internal combustion engines (ICE), hydrogen can be combusted at high compression ratios, which leads to improved engine efficiencies, and substantially reduced pollutants compared to fossil fuels [7; 8; 9]. A better alternative to utilize hydrogen in mobile platforms is to burn it catalytically with oxygen using fuel cells toobtain desired work output. This catalytic conversion of hydrogen and oxygen to work produces primarily heat and water vapor as byproducts [10]. Hydrogen storage is a vital component between hydrogen production and utilization, for a full-fledged hydrogen economy as shown in Figure 1[11; 12; 13; 14]. Hydrogen economy will be realized only when the hydrogen production, transportation/distribution, storage, and utilization (see Figure 1) are streamlined [15; 16; 17]. Hydrogen storage system is an essential part of any fuel cell or hydrogen-ICE automobile [19; 20]. The roadblocks for the widespread commercialization of the hydrogen powered automobile are the development of safe and reliable hydrogen storage and transportation systems. The required properties of hydrogen storage systems for automotive applications are low cost, high volumetric and gravimetric storage capacity, favorable cycling kinetics, and near ambient operation temperature. Improving these properties requires better understanding of the fundamental physicochemical processes of solid-gas reactions at the nanoscale. There are four major approaches to storing hydrogen: (i) compressed-gaseous, (ii) liquefied, (iii) solid-state (on-board regenerable), and (iv) chemical (off-board regenerable) [21]. Compressed-gaseous and liquefied hydrogen storage systems are considered as physical hydrogen storage methods. Both of these methods are mature and compressed-gaseous hydrogen storage is the only method employed in commercially available fuel-cell vehicles such as Honda Clarity, Toyota Mirai, and Hyundai Nexo. The main disadvantages of compressed-gaseous hydrogen storage are a high energy penalty for compression, which consumes 15% of total internal energy of hydrogen for pressures up to 350 bar, and high cost carbon-fiber wound cylinders to withstand pressures up to 700 bar [22]. Storing hydrogen at such a large pressure is acceptable to users as long as the safety is demonstrated, which is a considerable challenge. Liquefied hydrogen is the other major form of physical hydrogen storage. It provides a compact medium for on-board storage [23], but again, the energy penalty for liquefaction is quite high (liquefaction process consumes around 30% of total internal energy of hydrogen) and it is impossible to prevent the loss of hydrogen through boil-off during long dormancy periods [24]. Liquefied hydrogen tanks operate at atmospheric pressures, and they are perfectly insulated to minimize heat transfer to hydrogen at 20 K from surroundings. There is a promising hybrid physical-chemical storage method called cryo-compressed hydrogen storage [25].