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Concept
Water splitting
Summary
Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:
2 H2O → 2 H2 + O2
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 of water
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%.
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.
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High-temperature electrolysis
High-temperature electrolysis (also HTE or steam electrolysis or HTSE) is a technology for producing hydrogen from water at high temperatures. High temperature electrolysis is more efficient economically than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and also because the electrolysis reaction is more efficient at higher temperatures. In fact, at 2500 °C, electrical input is unnecessary because water breaks down to hydrogen and oxygen through thermolysis.
Hydrogen production
Hydrogen production is the family of industrial methods for generating hydrogen gas. As of 2020, the majority of hydrogen (~95%) is produced from fossil fuels by steam reforming of natural gas and other light hydrocarbons, partial oxidation of heavier hydrocarbons, and coal gasification. Other methods of hydrogen production include biomass gasification, methane pyrolysis, and electrolysis of water. Methane pyrolysis and water electrolysis can use any source of electricity including solar power.
Sulfur–iodine cycle
The sulfur–iodine cycle (S–I cycle) is a three-step thermochemical cycle used to produce hydrogen. The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled. The S–I process requires an efficient source of heat. The three reactions that produce hydrogen are as follows: I2 + SO2 + 2 H2O 2 HI + H2SO4 (); Bunsen reaction The HI is then separated by distillation or liquid/liquid gravitic separation.