Water splitting
Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:[1]
Efficient and economical water splitting would be a technological breakthrough that could underpin a
Electrolysis
Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen (H2):[2]
Production of hydrogen from water is energy intensive. 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%.[citation needed] 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.[citation needed]
energy efficiency for electrolytic water splitting was 60%–70% in 2020.[3]
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 reduce carbon dioxide, which eventually becomes incorporated into sugars.
Photo-excitation o
In
Photoelectrochemical water splitting
Using electricity produced by
Catalysis and proton-relay membranes are often the focus on development.[12]
Photocatalytic water splitting
The conversion of solar energy into hydrogen by means of water splitting process might be more efficient if it is assisted by photocatalysts suspended in water rather than a photovoltaic or an electrolytic system, so that the reaction takes place in one step.[13][14]
Radiolysis
Energetic nuclear radiation can break the chemical bonds of a water molecule. In the
Thermal decomposition of water
In
Other research includes
One side benefit of a nuclear reactor that produces both electricity and hydrogen is that it can shift production between the two. For instance, a nuclear plant might produce electricity during the day and hydrogen at night, matching its electrical generation profile to the daily variation in demand. If the hydrogen can be produced economically, this scheme would compete favorably with existing grid energy storage schemes. As of 2005, there was sufficient hydrogen demand in the United States that all daily peak generation could be handled by such plants.[18]
The hybrid thermoelectric
Solar-thermal
Material constraints due to the required high temperatures are reduced by the design of a membrane reactor with simultaneous extraction of hydrogen and oxygen that exploits a defined thermal gradient and the fast diffusion of hydrogen. With concentrated sunlight as heat source and only water in the reaction chamber, the produced gases are very clean with the only possible contaminant being water. A "Solar Water Cracker" with a concentrator of about 100 m2 can produce almost one kilogram of hydrogen per sunshine hour.[21]
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 352
For all the thermochemical processes, the summary reaction is that of the decomposition of water:[23]
Thermochemical cycle | LHV Efficiency |
Temperature (°C/F) |
---|---|---|
Cerium(IV) oxide–cerium(III) oxide cycle (CeO2/Ce2O3) | ? % | 2,000 °C (3,630 °F) |
Hybrid sulfur cycle (HyS) | 43% | 900 °C (1,650 °F) |
Sulfur–iodine cycle (S–I cycle) | 38% | 900 °C (1,650 °F) |
Cadmium sulfate cycle | 46% | 1,000 °C (1,830 °F) |
Barium sulfate cycle | 39% | 1,000 °C (1,830 °F) |
Manganese sulfate cycle | 35% | 1,100 °C (2,010 °F) |
Zinc–zinc oxide cycle (Zn/ZnO) | 44% | 1,900 °C (3,450 °F) |
Hybrid cadmium cycle | 42% | 1,600 °C (2,910 °F) |
Cadmium carbonate cycle | 43% | 1,600 °C (2,910 °F) |
Iron oxide cycle (Fe3O4/FeO) | 42% | 2,200 °C (3,990 °F) |
Sodium manganese cycle | 49% | 1,560 °C (2,840 °F) |
Nickel manganese ferrite cycle | 43% | 1,800 °C (3,270 °F) |
Zinc manganese ferrite cycle | 43% | 1,800 °C (3,270 °F) |
Copper–chlorine cycle (Cu–Cl) | 41% | 550 °C (1,022 °F) |
References
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- ^ Monash University (17 August 2008). "Monash team learns from nature to split water". EurekAlert.
- ^ Melis T (2008). "II.F.2 Maximizing Light Utilization Efficiency and Hydrogen Production in Microalgal Cultures" (PDF). DOE Hydrogen Program - Annual Progress Report. U.S. Department of Energy. pp. 187–190.
- ^ Kleiner K (31 Jul 2008). "Electrode lights the way to artificial photosynthesis". New Scientist.
- ^ Bullis K (31 Jul 2008). "Solar-Power Breakthrough. Researchers have found a cheap and easy way to store the energy made by solar power". MIT Technology Review.
- ^ http://swegene.com/pechouse-a-proposed-cell-solar-hydrogen.html[dead link]
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- ^ Yildiz B, Petri MC, Conzelmann G, Forsberg C (2005). "Configuration and Technology Implications of Potential Nuclear Hydrogen System Applications" (PDF). Argonne National Laboratory. University of Chicago. Archived from the original (PDF) on 27 Sep 2007. Retrieved 3 Mar 2010.
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- ^ Bürkle D, Roeb M (2008). "DLR scientists achieve solar hydrogen production in a 100-kilowatt pilotplant" (PDF). DLR - German Aerospace Center. Archived from the original on 4 Jun 2011.
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- ^ Weimer A (2006). "Development of Solar-powered Thermochemical Production of Hydrogen from Water" (PDF). DOE Hydrogen Program.
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