Editing Hydrogen (section)

==== Thermochemical ====
[[Water splitting]] is the process by which water is decomposed into its components.  Relevant to the biological scenario is this simple equation:
:{{chem2|2 H2O  ->  4 H+ + O2 + 4e-}}
The reaction occurs in the [[Light-dependent reactions|light reactions]] in all [[photosynthetic]] organisms. A few organisms, including the alga ''[[Chlamydomonas reinhardtii]]'' and [[cyanobacteria]], have evolved a second step in the [[dark reaction]]s in which protons and electrons are reduced to form {{chem2|H2}} gas by specialized hydrogenases in the [[chloroplast]].<ref>{{cite journal|last1=Kruse|first1=O.|last2=Rupprecht|first2=J.|last3=Bader|first3=K.|last4=Thomas-Hall|first4=S.|last5=Schenk|first5=P. M.|last6=Finazzi|first6=G.|last7=Hankamer|first7=B.|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells|journal=The Journal of Biological Chemistry|date=2005|volume=280|issue=40|pages=34170–7|doi=10.1074/jbc.M503840200|pmid=16100118|s2cid=5373909|url=http://espace.library.uq.edu.au/view/UQ:75490/UQ75490_OA.pdf|access-date=24 August 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015735/https://espace.library.uq.edu.au/data/UQ_75490/UQ75490_OA.pdf?Expires=1611885542&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=Qmpjq4YH0rwOJNqiSZ5M7-E5cYH~Dm2B-4kasb1eH66pVWPlvPNRj7TfcTKR1lDhF0--bkJdtE~yrSWwcZAA8FzxAA3MXY99mHTIOxyD3s73Dai1bwrLNuOkibXTVo6WbY5RKv7JAhXJ2sUV~TDIphC4Qikr0AWk5z-dwdY997n0NzcdTlqr0sn5n9WsOari3pJ0wRuL0w6Ged~HhrQ6ClrheilhtRo43U6HuaATFKEAuUM682rv4gvRCEVR1ljVOW0jwruB0SAJszTOZAbqNtb3V0SJh0x7wI8~ZZrp-XYqqzLDsWOB9w3ttyGSpLjcE2LvI7ty5vUljlfBGbnnLg__|url-status=live|doi-access=free}}</ref>

Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to more efficiently generate {{chem2|H2}} gas even in the presence of oxygen.<ref>{{cite web
|first1= Hamilton O.
|last1= Smith
|last2= Xu
|first2= Qing
|date= 2005
|url= http://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|title= IV.E.6 Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacteria System
|work= FY2005 Progress Report
|publisher= United States Department of Energy
|access-date= 6 August 2016
|archive-url= https://web.archive.org/web/20161229231756/https://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|archive-date= 29 December 2016
|url-status= live
}}</ref> Efforts have also been undertaken with genetically modified [[Biological hydrogen production (Algae)|alga in a bioreactor]].<ref>{{cite news| last=Williams| first=C.| title=Pond life: the future of energy| work=Science| publisher=The Register| date=24 February 2006| url=https://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| access-date=24 March 2008| archive-url=https://web.archive.org/web/20110509143055/http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| archive-date=9 May 2011| url-status=live}}</ref>

Relevant to the thermal water-splitting scenario is this simple equation:
:{{chem2|2 H2O  ->  2 H2 + O2}}
More than 200 thermochemical cycles can be used for [[water splitting]]. Many of these cycles such as the [[iron oxide cycle]], [[cerium(IV) oxide–cerium(III) oxide cycle]], [[zinc zinc-oxide cycle]], [[sulfur-iodine cycle]], [[copper-chlorine cycle]] and [[hybrid sulfur cycle]] have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|title=Development of solar-powered thermochemical production of hydrogen from water|first1=Al|last1=Weimer|date=25 May 2005|publisher=Solar Thermochemical Hydrogen Generation Project|access-date=21 December 2008|archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|archive-date=17 April 2007|url-status=live}}</ref> A number of labs (including in [[France]], [[Germany]], [[Greece]], [[Japan]], and the [[United States]]) are developing thermochemical methods to produce hydrogen from solar energy and water.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|title=Development of Solar-Powered Thermochemical Production of Hydrogen from Water, DOE Hydrogen Program, 2007|author=Perret, R.|access-date=17 May 2008|archive-url=https://web.archive.org/web/20100527212241/http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|archive-date=27 May 2010}}</ref>