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Hydrogen bound in organic matter and in water makes up 70% of the earth's surface. Breaking up these bonds in water allows us produce hydrogen and then to use it as a fuel. There are numerous processes that can be used to break these bonds. Described below are a few methods for producing hydrogen that are currently used, or are under research and development. Most of the hydrogen now produced on an industrial scale by the process of steam reforming, or as a byproduct of petroleum refining and chemicals production. (reference)
Steam reforming uses thermal energy to separate hydrogen from the carbon components in methane and methanol, and involves the reaction of these fuels with steam on catalytic surfaces. The first step of the reaction decomposes the fuel into hydrogen and carbon monoxide. Then a "shift reaction" changes the carbon monoxide and water to carbon dioxide and hydrogen. These reactions occur at temperatures of 200oC or greater.
Another way to produce hydrogen is by electrolysis. Electrolysis separates the elements of water-H and oxygen (O)-by charging water with an electrical current. Adding an electrolyte such as salt improves the conductivity of the water and increases the efficiency of the process. The charge breaks the chemical bond between the hydrogen and oxygen and splits apart the atomic components, creating charged particles called ions. The ions form at two poles: the anode, which is positively charged, and the cathode, which is negatively charged. Hydrogen gathers at the cathode and the anode attracts oxygen.
Steam electrolysis is a variation of the conventional electrolysis process. Some of the energy needed to split the water is added as heat instead of electricity, making the process more efficient than conventional electrolysis. At 2,500oC water decomposes into hydrogen and oxygen. This heat could be provided by a solar energy concentrating device to supply the heat. The problem here is to prevent the hydrogen and oxygen from recombining at the high temperatures used in the process.
Thermochemical water splitting uses chemicals such as bromine or iodine, assisted by heat. This causes the water molecule to split. It takes several steps-usually three-to accomplish this entire process.
Photoelectrochemical processes use two types of electrochemical systems to produce hydrogen. One uses soluble metal complexes as a catalyst, while the other uses semiconductor surfaces. When the soluble metal complex dissolves, the complex absorbs solar energy and produces an electrical charge that drives the water splitting reaction. This process mimics photosynthesis.
The other method uses semiconducting electrodes in a photochemical cell to convert optical energy into chemical energy. The semiconductor surface serves two functions, to absorb solar energy and to act as an electrode. Light-induced corrosion limits the useful life of the semiconductor.
Biological and photobiological processes use algae and bacteria to produce hydrogen. Under specific conditions, the pigments in certain types of algae absorb solar energy. The enzyme in the cell acts as a catalyst to split the water molecules. Some bacteria are also capable of producing hydrogen, but unlike algae they require a substrate to grow on. The organisms not only produce hydrogen, but can clean up pollution as well.
