Editing Electrolysis (section)

===Electrolysis of acidified water===
{{main|Electrolysis of water}}
Electrolysis of water produces [[hydrogen]] and oxygen in a ratio of 2 to 1 respectively.
:2 H<sub>2</sub>O{{abbr|(l)|liquid}} → 2 H<sub>2</sub>{{abbr|(g)|gaseous}} + O<sub>2</sub>{{abbr|(g)|gaseous}} {{pad|2em}} ''E''° = +1.229&nbsp;V

The [[Energy conversion efficiency|energy efficiency]] of water electrolysis varies widely. The efficiency of an electrolyser is a measure of the enthalpy contained in the hydrogen (to undergo combustion with oxygen or some other later reaction), compared with the input electrical energy. Heat/enthalpy values for hydrogen are well published in science and engineering texts, as 144&nbsp;MJ/kg (40 kWh/kg).  Note that fuel cells (not electrolysers) cannot use this full amount of heat/enthalpy, which has led to some confusion when calculating efficiency values for both types of technology.  In the reaction, some energy is lost as heat. Some reports quote efficiencies between 50% and 70% for alkaline electrolysers (50 kWh/kg);<ref name="polly2023">{{cite web |last1=Martin |first1=Polly |title=Green hydrogen {{!}} Which type of electrolyser should you use? Alkaline, PEM, solid oxide or the latest tech? |url=https://www.hydrogeninsight.com/electrolysers/green-hydrogen-which-type-of-electrolyser-should-you-use-alkaline-pem-solid-oxide-or-the-latest-tech-/2-1-1480577 |website=rechargenews.com |language=en |date=5 July 2023}}</ref> however, higher practical efficiencies are available with the use of [[polymer electrolyte membrane electrolysis]] and catalytic technology, such as 95% efficiency.<ref name="carmo2013a">{{cite journal|last=Carmo|first=M |author2=Fritz D |author3=Mergel J |author4=Stolten D |title=A comprehensive review on PEM water electrolysis|journal=Journal of Hydrogen Energy|volume=38|issue=12|pages=4901|year=2013 |doi=10.1016/j.ijhydene.2013.01.151|bibcode=2013IJHE...38.4901C }}</ref><ref>{{cite web
 |title=Chapter 3: Production of Hydrogen. Part 4: Production from electricity by means of electrolysis
 |work=HyWeb: Knowledge – Hydrogen in the Energy Sector
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 |author1=Zittel, Werner
 |author2=Wurster, Reinhold
 |publisher=Ludwig-Bölkow-Systemtechnik GmbH
 |date=8 July 1996
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The [[National Renewable Energy Laboratory]] estimated in 2006 that 1&nbsp;kg of hydrogen (roughly equivalent to 3&nbsp;kg, or 4 liters, of petroleum in energy terms) could be produced by wind powered electrolysis for between US$5.55 in the near term and US$2.27 in the longer term.<ref name="NRELElectrolysis">{{cite web|url=http://www.nrel.gov/docs/fy06osti/39534.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nrel.gov/docs/fy06osti/39534.pdf |archive-date=2022-10-09 |url-status=live |title=Wind Energy and Production of Hydrogen and Electricity – Opportunities for Renewable Hydrogen – Preprint |access-date=20 October 2008 |author1=Levene, J.  |author2=Kroposki, B. |author3=Sverdrup, G. |date=March 2006 |work=National Renewable Energy Laboratory}}</ref>

About 4% of hydrogen gas produced worldwide is generated by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the [[Haber process]], and converting heavy petroleum sources to lighter fractions via [[hydrocracking]]. Onsite electrolysis has been utilized to capture hydrogen for hydrogen fuel-cells in [[hydrogen vehicles]].