Water splitting

Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: Efficient and economical photochemical water splitting would be a technological breakthrough that could underpin a hydrogen economy. No industrially practical version of water splitting with pure water has been demonstrated, but the two component reactions (H2 production and O2 production) are well known. The water splitting of seawater and other salt water is used industrially to make chlorine, however, and the waste hydrogen collected comprises about five percent of the world's supply. A version of water splitting occurs in photosynthesis, but hydrogen is not produced. The reverse of water splitting is the basis of the hydrogen fuel cell. Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen (H2) due to an electric current being passed through the water. In power to gas production schemes, the excess power or off peak power created by wind generators or solar arrays is used for load balancing of the energy grid by storing and later injecting the hydrogen into the natural gas grid. Production of hydrogen from water is energy intensive. Potential electrical energy supplies include hydropower, wind turbines, or photovoltaic cells. Usually, the electricity consumed is more valuable than the hydrogen produced so this method has not been widely used. In contrast with low-temperature electrolysis, high-temperature electrolysis (HTE) of water converts more of the initial heat energy into chemical energy (hydrogen), potentially doubling efficiency to about 50%. Because some of the energy in HTE is supplied in the form of heat, less of the energy must be converted twice (from heat to electricity, and then to chemical form), and so the process is more efficient. A version of water splitting occurs in photosynthesis, but the electrons are shunted, not to protons, but to the electron transport chain in photosystem II. The electrons are used to convert carbon dioxide into sugars. When photosystem I gets photo-excited, electron transfer reactions gets initiated, which results in reduction of a series of electron acceptors, eventually reducing NADP+ to NADPH and PS I is oxidized. The oxidized photosystem I captures electrons from photosystem II through a series of steps involving agents like plastoquinone, cytochromes and plastocyanine. The photosystem II then brings about water oxidation resulting in evolution of oxygen, the reaction being catalyzed by CaMn4O5 clusters embedded in complex protein environment; the complex is known as oxygen evolving complex (OEC). In biological hydrogen production, the electrons produced by the photosystem are shunted not to a chemical synthesis apparatus but to hydrogenases, resulting in formation of H2. This biohydrogen is produced in a bioreactor. Using electricity produced by photovoltaic systems potentially offers the cleanest way to produce hydrogen, other than nuclear, wind, geothermal, and hydroelectric. Again, water is broken down into hydrogen and oxygen by electrolysis, but the electrical energy is obtained by a photoelectrochemical cell (PEC) process. The system is also named artificial photosynthesis.