Hydrogen is the most abundant of all elements on the planet Earth; it is well known in the form of vapor, liquid or ice. In nature however, hydrogen is not available in its pure form due to its high binding affinity: it either binds with oxygen to form water or with carbon to form hydrocarbon compounds. These compounds are usually relatively stable. In order to activate hydrogen as an energy carrier and energy source, it has to be extracted from the stable compounds. This requires an energy input. The simplest form to separate hydrogen and oxygen is electrolysis. As a result, the process creates pure hydrogen and pure oxygen. Whereas oxygen is usually released into the atmosphere, hydrogen can be reused by compression and liquefaction.