Water splitting

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Diagram of the chemical equation of the electrolysis of water, a form of water splitting.

Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:[1]

2 H2O → 2 H2 + O2

Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy. 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. Water splitting using solar radiation has not been commercialized.


Atmospheric electricity utilization for the chemical reaction in which water is separated into oxygen and hydrogen. (Image via: Vion, US patent 28793. June 1860.)
Electrolyser front with electrical panel in foreground

Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen (H2):[2]

Water electrolysis ship Hydrogen Challenger

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]

High-temperature electrolysis (also HTE or steam electrolysis) is a method for the production of hydrogen from water with oxygen as a by-product. brary.unt.edu/ark:/67531/metadc270782/}}</ref>

Water splitting in photosynthesis[edit]

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 photosystem I initiates electron transfer to a series of electron acceptors, eventually reducing NADP+ to NADPH. The oxidized photosystem I captures electrons from photosystem II through a series of steps involving plastoquinone, cytochromes, and plastocyanin. Oxidized photosystem II oxidizes the oxygen-evolving complex (OEC), which converts water into O2 and protons.[4][5] Since the active site of the OEC contains manganese, much research has aimed at synthetic Mn compounds as catalysts for water oxidation.[6]

An algae bioreactor for hydrogen production.

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.[7]

Photoelectrochemical water splitting[edit]

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.[8][9][10][11]

Catalysis and proton-relay membranes are often the focus on development.[12]

Photocatalytic water splitting[edit]

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]


Energetic nuclear radiation can break the chemical bonds of a water molecule. 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.[15]

Thermal decomposition of water[edit]

In thermolysis, water molecules split into hydrogen and oxygen. For example, at 2,200 °C (2,470 K; 3,990 °F) about three percent of all H2O are dissociated into various combinations of hydrogen and oxygen atoms, mostly H, H2, O, O2, and OH. Other reaction products like H2O2 or HO2 remain minor. At the very high temperature of 3,000 °C (3,270 K; 5,430 °F) more than half of the water molecules are decomposed. At ambient temperatures only one molecule in 100 trillion dissociates by the effect of heat.[16] The high temperatures and material constraints have limited the applications of this approach.

Other research includes thermolysis on defective carbon substrates, thus making hydrogen production possible at temperatures just under 1,000 °C (1,270 K; 1,830 °F).[17]

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 Copper-chlorine cycle is a cogeneration system using the waste heat from nuclear reactors, specifically the CANDU supercritical water reactor.[19]


Concentrating solar power can achieve the high temperatures necessary to split water. Hydrosol-2 is a 100-kilowatt pilot plant at the Plataforma Solar de Almería in Spain which uses sunlight to obtain the required 800 to 1,200 °C (1,070 to 1,470 K; 1,470 to 2,190 °F) to split water. Hydrosol II has been in operation since 2008. The design of this 100-kilowatt pilot plant is based on a modular concept. As a result, it may be possible that this technology could be readily scaled up to megawatt range by multiplying the available reactor units and by connecting the plant to heliostat fields (fields of sun-tracking mirrors) of a suitable size.[20]

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 thermochemical cycles have been described for water splitting by thermolysis.[22] These cycles promise to produce hydrogen and oxygen from water and heat without using electricity.[23] Since all the input energy for such processes is heat, they can be more efficient than high-temperature electrolysis. This is because the efficiency of electricity production is inherently limited. Thermochemical production of hydrogen using chemical energy from coal or natural gas is generally not considered, because the direct chemical path is more efficient.

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)


  1. ^ Kudo, Akihiko; Miseki, Yugo (2009). "Heterogeneous photocatalyst materials for water splitting". Chem. Soc. Rev. 38 (1): 253–278. doi:10.1039/b800489g. PMID 19088977.
  2. ^ Kumar, Mohit; Meena, Bhagatram; Subramanyam, Palyam; Suryakala, Duvvuri; Subrahmanyam, Challapalli (2022-11-11). "Recent trends in photoelectrochemical water splitting: the role of cocatalysts". NPG Asia Materials. 14 (1): 1–21. doi:10.1038/s41427-022-00436-x. ISSN 1884-4057.
  3. ^ Yan, Zhifei; Hitt, Jeremy L.; Turner, John A.; Mallouk, Thomas E. (9 June 2020). "Renewable electricity storage using electrolysis". Proceedings of the National Academy of Sciences. 117 (23): 12558–12563. Bibcode:2020PNAS..11712558Y. doi:10.1073/pnas.1821686116. PMC 7293654. PMID 31843917.
  4. ^ Yano J, Kern J, Sauer K, Latimer MJ, Pushkar Y, Biesiadka J, et al. (November 2006). "Where water is oxidized to dioxygen: structure of the photosynthetic Mn4Ca cluster". Science. 314 (5800): 821–5. Bibcode:2006Sci...314..821Y. doi:10.1126/science.1128186. PMC 3963817. PMID 17082458.
  5. ^ Barber J (March 2008). "Crystal Structure of the Oxygen-Evolving Complex of Photosystem II". Inorganic Chemistry. 47 (6): 1700–10. doi:10.1021/ic701835r. PMID 18330964.
  6. ^ Monash University (17 August 2008). "Monash team learns from nature to split water". EurekAlert.
  7. ^ 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.
  8. ^ Kleiner K (31 Jul 2008). "Electrode lights the way to artificial photosynthesis". New Scientist.
  9. ^ 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.
  10. ^ http://swegene.com/pechouse-a-proposed-cell-solar-hydrogen.html[dead link]
  11. ^ del Valle F, Ishikawa A, Domen K, Villoria De La Mano JA, Sánchez-Sánchez MC, González ID, et al. (2009). "Influence of Zn concentration in the activity of Cd1–xZnxS solid solutions for water splitting under visible light". Catalysis Today. 143 (1–2): 51–59. doi:10.1016/j.cattod.2008.09.024.
  12. ^ Chu S, Li W, Hamann T, Shih I, Wang D, Mi Z (2017). "Roadmap on solar water splitting: current status and future prospects". Nano Futures. 1 (2): 022001. Bibcode:2017NanoF...1b2001C. doi:10.1088/2399-1984/aa88a1. S2CID 3903962.
  13. ^ Navarro Yerga RM, Alvarez Galván MC, del Valle F, Villoria de la Mano JA, Fierro JL (2009). "Water Splitting on Semiconductor Catalysts under Visible-Light Irradiation". ChemSusChem. 2 (6): 471–485. doi:10.1002/cssc.200900018. PMID 19536754.
  14. ^ Navarro RM, del Valle F, Villoria De La Mano JA, Álvarez-Galván MC, Fierro JL (2009). de Lasa HI, Rosales BS (eds.). Photocatalytic water splitting under visible Light: concept and materials requirements. Advances in Chemical Engineering. Vol. 36. pp. 111–143. doi:10.1016/S0065-2377(09)00404-9. ISBN 9780123747631.
  15. ^ Lin LH, Wang PL, Rumble D, Lippmann-Pipke J, Boice E, Pratt LM, et al. (2006). "Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome". Science. 314 (5798): 479–82. Bibcode:2006Sci...314..479L. doi:10.1126/science.1127376. PMID 17053150. S2CID 22420345.
  16. ^ Funk JE (2001). "Thermochemical hydrogen production: past and present". International Journal of Hydrogen Energy. 26 (3): 185–190. doi:10.1016/S0360-3199(00)00062-8.
  17. ^ {{cite journal|vauthors=Kostov MK, Santiso EE, George AM, Gubbins KE, Nardelli MB|year=2005|title=Dissociation of Water on Defective Carbon Substrates|journal=Physical Review Letters|volume=95|issue=13|pages=136105|bibcode=2005PhRvL..95m6105K|doi=10.1103/PhysRevLett.95.136105|pmid=16197155
  18. ^ 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.
  19. ^ Naterer GF, Suppiah S, Lewis M, Gabriel K, Dincer I, Rosen MA, et al. (2009). "Recent Canadian Advances in Nuclear-Based Hydrogen Production and the Thermochemical Cu-Cl Cycle". International Journal of Hydrogen Energy. 34 (7): 2901–2917. doi:10.1016/j.ijhydene.2009.01.090.
  20. ^ 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.
  21. ^ "H2 Power Systems". Archived from the original on 4 Mar 2012.
  22. ^ Weimer A (2006). "Development of Solar-powered Thermochemical Production of Hydrogen from Water" (PDF). DOE Hydrogen Program.
  23. ^ a b Weimer A (2005). "Development of Solar-powered Thermochemical Production of Hydrogen from Water" (PDF). DOE Hydrogen Program.

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