Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:
Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy, based on green hydrogen. 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
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.
Vion, , "Improved method of using atmospheric electricity", June 1860.
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.
Currently energy efficiency for electrolytic water splitting is 60%–70%.
Water splitting in photosynthesis
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 get 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.
Photoelectrochemical water splitting
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.
Photocatalytic water splitting
The conversion of solar energy to hydrogen by means of water splitting process is a way to achieve clean and renewable energy. This process can be more efficient if it is assisted by photocatalysts suspended directly in water rather than a photovoltaic or an electrolytic system, so that the reaction takes place in one step.
Radiolysis
Nuclear radiation routinely breaks water bonds. In the Mponeng gold mine, South Africa, researchers found in a naturally high radiation zone a community dominated by a new phylotype of Desulfotomaculum, feeding on primarily radiolytically produced H2. Spent nuclear fuel is also being investigated as a potential source of hydrogen.
Nanogalvanic aluminum alloy powder
An aluminum alloy powder invented by the U.S. Army Research Laboratory in 2017 was shown to be capable of producing hydrogen gas upon contact with water or any liquid containing water due to its unique nanoscale galvanic microstructure. It reportedly generates hydrogen at 100 percent of the theoretical yield without the need for any catalysts, chemicals, or externally supplied power.