why is fuel-cell technology not a pollution free alternative to the internal combustion gasoline engine?

At present, hydrogen is mostly used as fuel for rocket engines. In the future, hydrogen might replace more conventional fuels in traditional internal combustion engines. If hydrogen fuel cell technology becomes widespread, then the use of internal combustion engines may be phased out.

Although there are multiple ways of producing free hydrogen, those methods require converting combustible molecules into hydrogen or consuming electric energy. Unless that electricity is produced from a renewable source—and is not required for other purposes—it seems hydrogen does not solve any energy crisis. The disadvantage of hydrogen in many situations is its storage. Liquid hydrogen has extremely low density (14 times lower than water) and requires extensive insulation—whilst gaseous hydrogen requires heavy tankage. Even when liquefied, hydrogen has a higher specific energy but the volumetric energetic storage is still roughly five times lower than petrol. The ‘Hydrogen on Demand’ process (see direct borohydride fuel cell) creates hydrogen as it is needed, but has other issues such as the high price of the sodium borohydride which is the raw material.