Electrolysis is the technical name for using electricity to split water into its constituent elements, hydrogen and oxygen. The splitting of water is accomplished by passing an electric current through water. The electricity enters the water at the cathode, a negatively charged terminal, passes through the water and exists via the anode, the positively charged terminal. The hydrogen is collected at the cathode and the oxygen is collected at the anode. Electrolysis produces very pure hydrogen for use in the electronics, pharmaceutical and food industries.
Relative to steam reforming, electrolysis is very expensive. The electrical inputs required to split the water into hydrogen and oxygen account for about 80% of the cost of hydrogen generation. Potentially, electrolysis, when coupled with a renewable energy source, can provide a completely clean and renewable source of energy. In other circumstances, electrolysis can couple with hydroelectric or off-peak electricity to reduce the cost of electrolysis.
Photoelectrolysis, known as the hydrogen holy grail in some circles, is the direct conversion of sunlight into electricity. Photovoltaics, semiconductors and an electrolyzer are combined to create a device that generates hydrogen. The photoelectrolyzer is placed in water and when exposed to sunlight begins to generate hydrogen. The photovoltaics and the semiconductor combine to generate enough electricity from the sunlight to power the electrolyzer. The hydrogen is then collected and stored. Much of the research in this field takes place in Golden, Colorado at the National Renewable Energy Laboratory.
Photobiological production of hydrogen involves using sunlight, a biological component, catalysts and an engineered system. Specific organisms, algae and bacteria, produce hydrogen as a byproduct of their metabolic processes. These organisms generally live in water and therefore are biologically splitting the water into its component elements.
Currently, this technology is still in the research and development stage and the theoretical sunlight conversion efficiencies have been estimated up to 24%. Over 400 strains of primitive plants capable of producing hydrogen have been identified, with 25 impressively achieving carbon monoxide to hydrogen conversion efficiencies of 100%.
In one example, researchers have discovered that the alga, Chlamydomonas reinhardtii, possesses an enzyme called hydrogenase that is capable of splitting water into its component parts of hydrogen and oxygen. The researchers have determined the mechanism for starting and stopping this process, which could lead to an almost limitless method for producing clean, renewable hydrogen.
The algae need sulfur to grow and photosynthesize. Scientists found that when they starved the algae of sulfur, in an oxygen-free environment, the algae reverted to a hydrogenase-utilizing mode. This mechanism was developed over millions of years of evolution for survival in oxygen-rich and oxygen-free environments. Once in this cycle, the algae released hydrogen, not oxygen. Further research is necessary to improve the efficiencies of the engineered plant systems, collection methods and the costs of hydrogen generation.