Hydrogen
While hydrogen is the most abundant element in the universe, and the nuclear fusion of hydrogen powers the stars, on earth virtually all of the hydrogen available is bound into water molecules or into hydrocarbons and therefore requires a substantial input of energy to free it for use as a fuel to combine with free oxygen in fuel cells. For this reason, hydrogen is today not really an energy source but rather is an energy transfer medium.
There are a few ways to produce free hydrogen; one is by electrolysis of water and the other two are by high temperature pyrolysis of natural gas or by the steam reforming or hydrocarbons or coal. When fossil fuels are burned to free this hydrogen, they contribute to greenhouse gas emissions so a true hydrogen economy must await the arrival of more widespread use of nuclear fission or fusion processes or solar powered electrolysis in order to become a realistic energy source.
In the United States today it costs about six dollars to produce one kilogram of hydrogen and this one kilogram contains about the same energy content as one gallon of gasoline. Once freed, the combining of hydrogen with free oxygen in fuel cells will make for an extraordinarily green form of energy since the waste byproduct is pure water vapor.
For this to work efficiently on a large scale, however, will require excavating larger reserves of Platinum-group metals in order build the requisite number of fuel cells. The United States Geological Survey today estimates that total global reserves of platinum that can be mined are roughly 100 million kilograms. An estimated 3 billion automobiles might be able to employ fuel cell technology by the year 2050 if the platinum loading can be reduced to a goal of 5.7 grams from the current 22.6 grams per vehicle.
If this cannot be achieved, it will become necessary to move mining operations off planet in order to mine the richer platinum reserves found in near earth asteroids, or perhaps onto the lunar highlands where there is evidence of higher densities of platinum group metals such as iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum than are available on the surface of the earth.