Concerning the production of hydrogen from renewable sources, that is a more clean and interesting way, the processes can be roughly divided into:

• production from biomass

• production from water.

Concerning the production of hydrogen from biomass, none of the proposed processes have reached industrial maturity. The various alternatives (gasification, pyrolysis and subsequent reforming of the produced liquid fraction, production and reforming of ethanol, production through biological processes based on phenomena of photosynthesis or fermentation) all still require research, development and demonstration. Anyway the premises are good, taking into account the different materials used. Hydrogen can also be produced by splitting into its components (hydrogen and oxygen) through various processes, among which the most consolidated is electrolysis. Schematically this is represented by the following reaction:

H₂O + electricity -> H₂ + 1/2 O₂ (1.9)

You can see immediately that the electrolysis shows exactly the inverse reaction occurring in fuel cells. Therefore, the entire process of production and consumption is environmentally sustainable if, as mentioned above, a corresponding amount of clean electricity is available to power the electrolysis process. One could immediately think of the sun as the source of this energy, exploitable through the use of photovoltaic conversion facilities. The current technology can be considered reliable and adequate, though not yet competitive. Indeed, through the use of photovoltaic, solar energy can produce electrolytic hydrogen and oxygen. As end product of the recombination of hydrogen and oxygen it generates a quantity of pure water almost equal to that of departure, thus closing the cycle with no emissions of pollutant. In principle, therefore, it would be possible to extract from water all the needed hydrogen in order to meet clean energy needs of humanity. The problem now is the cost. By electrolysis of water, it is true that one can virtually obtain pure hydrogen, but only at a price that can be economically acceptable in a still distant era, when the technology will hopefully enable low costs electricity production from renewable sources (or nuclear). The dissociation of water may also be done using thermochemical processes that need high temperature (800-1000 ° C) obtained from different sources (first of all thermal solar energy). Research and development aimed to demonstrate the industrial feasibility of these processes seem to be very interesting. There are other processes already mentioned, still in the laboratory scale, such as photoconversion, which breaks down the water body using biological or synthetic materials, and photoelectrochemical processes, that use as an electric current generated by semiconductors for the same purpose.