Water splitting
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Water splitting is the general term for a chemical reaction in which water is converted into oxygen and hydrogen. Water splitting is actively researched because demand for cheap hydrogen is expected to rise with the new hydrogen economy. Various techniques for water splitting have been issued in water splitting patents [1] in the United States.
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[edit] Methods
[edit] Artificial
In chemistry and manufacturing, electrolysis is a method of separating chemically bonded elements and compounds by passing an electric current through them. One important use of electrolysis of water or artificial photosynthesis (photoelectrolysis in a photoelectrochemical cell) is to produce hydrogen . This has been suggested as a way of shifting society toward using hydrogen as an energy carrier for powering electric motors and internal combustion engines. (See hydrogen economy.)
Thermal decomposition, also called thermolysis, is defined as a chemical reaction whereby a chemical substance breaks up into at least two chemical substances when heated. Water, when heated to well over 2000 degrees Celsius, breaks up into its components - hydrogen and oxygen.
[edit] Natural
Water splitting donates electrons to power the electron transport chain in photophosphorylation of photosynthesis.
[edit] Research
Research is being conducted over photocatalysis,[2] the acceleration of a photoreaction in the presence of a catalyst. Its comprehension has been made possible ever since the discovery of water electrolysis by means of the titanium dioxide. Artificial photosynthesis is a research field that attempts to replicate the natural process of photosynthesis, converting sunlight, water and carbon dioxide into carbohydrates and oxygen. Recently, this has been successful in splitting water into hydrogen and oxygen using an artificial compound called Nafion.[3]
High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product. Other research includes thermolysis on defective carbon substrates, thus making hydrogen production possible at temperatures just under 1000°C[4]
The iron oxide cycle is a series of thermochemical processes used to produce hydrogen. The iron oxide cycle consists of two chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled. The iron oxide process requires an efficient source of heat.
The sulfur-iodine cycle (S-I cycle) is a series of thermochemical processes 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.
More than 200 different thermochemical cycles are known to produce hydrogen, research is concentrated on the following cycles:
| Thermochemical cycle | LHV Efficiency | Temperature (°C/F) |
|---|---|---|
| Cerium(IV) oxide-cerium(III) oxide cycle (CeO2/Ce2O3) | ? % | 2,000 °C (3,632 °F) |
| Hybrid sulfur cycle (HyS) | 43 % | 900 °C (1,652 °F) |
| Sulfur iodine cycle (S-I cycle) | 38 % | 900 °C (1,652 °F) |
| Cadmium sulfate cycle | 46 % | 1,000 °C (1,832 °F) |
| Barium sulfate cycle | 39 % | 1,000 °C (1,832 °F) |
| Manganese sulfate cycle | 35 % | 1,100 °C (2,012 °F) |
| Zinc zinc-oxide cycle (Zn/ZnO) | 44 % | 1,900 °C (3,452 °F) |
| Hybrid cadmium cycle | 42 % | 1,600 °C (2,912 °F) |
| Cadmium carbonate cycle | 43 % | 1,600 °C (2,912 °F) |
| Iron oxide cycle (Fe3O4/FeO) | 42 % | 2,200 °C (3,992 °F) |
| Sodium manganese cycle | 49 % | 1,560 °C (2,840 °F) |
| Nickel manganese ferrite cycle | 43 % | 1,800 °C (3,272 °F) |
| Zinc manganese ferrite cycle | 43 % | 1,800 °C (3,272 °F) |
| Copper-chlorine cycle (Cu-Cl) | 41 % | 550 °C (1,022 °F) |
Source: Development of solar-powered thermochemical production of hydrogen from water [5]
[edit] Patents
- Vion, U.S. Patent 28,793, "Improved method of using atmospheric electricity", June 1860.
[edit] References
- ^ Patent Database Search Results: ttl/"water splitting" in US Patent Collection
- ^ Strategies for the Development of Visible-light-driven Photocatalysts for Water Splitting Akihiko Kudo, Hideki Kato1 and Issei Tsuji Chemistry Letters Vol. 33 (2004) , No. 12 p.1534
- ^ [1]
- ^ Kostov, M. K.; Santiso, E. E.; George, A. M.; Gubbins, K. E.; and Nardelli, M. Buongiorno (2005) (PDF). Dissociation of Water on Defective Carbon Substrates. Physical Review Letters. http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000095000013136105000001&idtype=cvips&prog=normal. Retrieved 2007-11-05.
- ^ Development of solar-powered thermochemical production of hydrogen from water
