1. Hydrogen

Hydrogen, which is the first element in the periodic table of elements, having the atomic number 1, is a non-metal, colorless, odorless, highly reactive, self-burning characteristic gas that has all the qualities to be a fuel for our automobiles and a future energy source. In contrast to conventional energy sources (coal, oil, natural gas), it is carbon free and hence environmentally friendly. The countdown for conventional sources of energy has already begun as they are depleting fast. Therefore, hydrogen is a perfect candidate to fulfill the energy needs of humans for the future. In addition to the aforementioned qualities, hydrogen is quite challenging as an energy source or fuel because of its availability. Although hydrogen is naturally present on the Earth in the combined state in both organic and inorganic compounds, for example, as water and hydrocarbons, it is scarcely present in the free and molecular state. Therefore, elemental hydrogen is artificially produced, and hence its safe and environmentally benign production is most important. When considering environmentally friendly hydrogen production, the obvious choice for the input energy is renewable energy, mainly solar energy.