{{Short description|Type of electrolyzer}} {{Infobox electrolysis |electrolysistype = Alkaline Water Electrolysis |acatalyst=Ni/Co/Fe |ccatalyst=Ni/C-Pt |membranemat=NiO<ref>{{cite journal |last1=Divisek |first1=J. |last2=Schmitz |first2=H. |title=A bipolar cell for advanced alkaline water electrolysis |journal=International Journal of Hydrogen Energy |date=1 January 1982 |volume=7 |issue=9 |pages=703–710 |doi=10.1016/0360-3199(82)90018-0 |bibcode=1982IJHE....7..703D }}</ref>/Asbestos/polysulfone matrix and ZrO2 (Zirfon)/polyphenil sulfide<ref>{{cite journal |last1=Shiva Kumar |first1=S. |last2=Lim |first2=Hankwon |title=An overview of water electrolysis technologies for green hydrogen production |journal=Energy Reports |date=November 2022 |volume=8 |pages=13793–13813 |doi=10.1016/j.egyr.2022.10.127|s2cid=253141292 |doi-access=free |bibcode=2022EnRep...813793S }}</ref><ref>{{cite journal |last1=David |first1=Martín |last2=Ocampo-Martínez |first2=Carlos |last3=Sánchez-Peña |first3=Ricardo |title=Advances in alkaline water electrolyzers: A review |journal=Journal of Energy Storage |date=June 2019 |volume=23 |pages=392–403 |doi=10.1016/j.est.2019.03.001|bibcode=2019JEnSt..23..392D |hdl=2117/178519 |s2cid=140072936 |hdl-access=free }}</ref> |aptl=Ti/Ni/zirconium |cptl=Stainless steel mesh |bppmat=Stainless steel |celltemp=60-80&nbsp;°C<ref name="carmo2013a" /> |cellpress=<30 bar<ref name="carmo2013a" /> |curdens=0.2-0.4 A/cm<sup>2</sup><ref name="carmo2013a" /><ref name="Colli et al." /> |cellvolt=1.8-2.40 V<ref name="carmo2013a" /><ref name="Colli et al." /> |powdens=to 1.0 W/cm<sup>2</sup><ref name="carmo2013a" /> |cellvolteff=62–82% (HHV)<ref name="carmo2013a" /> |specengcomstack=4.2-5.9 kWh/Nm<sup>3</sup><ref name="carmo2013a" /> |specengcomsys=4.5-7.0 kWh/Nm<sup>3</sup><ref name="carmo2013a" /> |ploadrng=20-40%<ref name="carmo2013a" /> |cellare=< 4 m<sup>2</sup><ref name="carmo2013a" /> |h2prod=<760 Nm<sup>3</sup>/h<ref name="carmo2013a" /> |lifetimestack=<90,000 h<ref name="carmo2013a" /> |degrat=<3 μV/h<ref name="carmo2013a" /> |syslife=20-30 years<ref name="carmo2013a" /> }}

'''Alkaline water electrolysis''' is a type of [[electrolysis]] that is characterized by having two [[electrodes]] operating in a liquid alkaline electrolyte. Commonly, a solution of [[potassium hydroxide]] (KOH) or [[sodium hydroxide]] (NaOH) at 25-40 wt% is used.<ref name="Marian2022">{{cite journal |last1=Chatenet |first1=Marian |last2=Pollet |first2=Bruno G. |last3=Dekel |first3=Dario R. |last4=Dionigi |first4=Fabio |last5=Deseure |first5=Jonathan |last6=Millet |first6=Pierre |last7=Braatz |first7=Richard D. |last8=Bazant |first8=Martin Z. |last9=Eikerling |first9=Michael |last10=Staffell |first10=Iain |last11=Balcombe |first11=Paul |last12=Shao-Horn |first12=Yang |last13=Schäfer |first13=Helmut |title=Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments |journal=Chemical Society Reviews |date=2022 |volume=51 |issue=11 |pages=4583–4762 |doi=10.1039/d0cs01079k|pmid=35575644 |pmc=9332215 }}</ref> These electrodes are separated by a diaphragm, separating the product gases and transporting the hydroxide ions (OH<sup>−</sup>) from one electrode to the other.<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|page=4901|year=2013|doi=10.1016/j.ijhydene.2013.01.151|bibcode=2013IJHE...38.4901C}}</ref><ref>{{cite web|title=Alkaline Water Electrolysis|url=http://www.eolss.net/sample-chapters/c08/e3-13-03-02.pdf|publisher=Energy Carriers and Conversion Systems|access-date=19 October 2014}}</ref> A recent comparison showed that state-of-the-art nickel based water electrolysers with alkaline electrolytes lead to competitive or even better efficiencies than acidic [[polymer electrolyte membrane electrolysis|polymer electrolyte membrane water electrolysis]] with [[platinum group]] metal based electrocatalysts.<ref name="schalenbach2016">{{cite journal|last=Schalenbach|first=M|author2=Tjarks G |author3=Carmo M | author4=Lueke W |author5=Mueller M |author6=Stolten D|title=Acidic or Alkaline? Towards a New Perspective on the Efficiency of Water Electrolysis|journal=Journal of the Electrochemical Society|volume=163|issue=11|pages=F3197|year=2016|doi=10.1149/2.0271611jes|s2cid=35846371|url=https://publications.rwth-aachen.de/record/681185|doi-access=free}}</ref>

The technology has a long history in the chemical industry. The first large-scale demand for hydrogen emerged in late 19th century for [[Lifting gas#Hydrogen|lighter-than-air aircraft]], and before the advent of [[steam reforming]] in the 1930s, the technique was competitive.{{cn|date=April 2024}}

Hydrogen-based technologies have evolved significantly since the initial discovery of hydrogen and its early application as a buoyant gas approximately 250 years ago. In 1804, the Swiss inventor Francois Isaac de Rivaz secured a patent for the inaugural hydrogen-powered vehicle. This prototype, equipped with a four-wheel design, utilised an internal combustion engine (ICE) fuelled by a mixture of hydrogen and oxygen gases. The hydrogen fuel was stored in a balloon, and ignition was achieved through an electrical starter known as a Volta starter. The combustion process propelled the piston within the cylinder, which, upon descending, activated a wheel through a ratchet mechanism. This invention could be viewed as an early embodiment of a system comprising [[hydrogen storage]], conduits, valves, and a conversion device.<ref name=":0">{{Citation |last=Jordan |first=Thomas |title=Hydrogen technologies |date=2022 |work=Hydrogen Safety for Energy Applications |pages=25–115 |publisher=Elsevier |doi=10.1016/b978-0-12-820492-4.00005-1|isbn=978-0-12-820492-4 }}</ref>

Approximately four decades after the military scientist Ritter developed the first electrolyser, the chemists Schoenbein and Sir Grove independently identified and showcased the fuel cell concept. This technology operates in reverse to electrolysis around the year 1839. This discovery marked a significant milestone in the field of hydrogen technology, demonstrating the potential for hydrogen as a source of clean energy.<ref name=":0" />

==Structure and materials== [[File:Alkaline water electrolyser.png|thumb|Scheme of alkaline water electrolyzers. The catalysts are added to the anode and cathode to reduce the overpotential.<ref name="zeng2010">{{cite journal |last1=Zeng |first1=Kai |last2=Zhang |first2=Dongke |title=Recent progress in alkaline water electrolysis for hydrogen production and applications |journal=Progress in Energy and Combustion Science |date=June 2010 |volume=36 |issue=3 |pages=307–326 |doi=10.1016/j.pecs.2009.11.002|bibcode=2010PECS...36..307Z }}</ref>]]

The electrodes are typically separated by a thin porous foil, commonly referred to as diaphragm or separator. The diaphragm is non-conductive to electrons, thus avoiding electrical shorts between the electrodes while allowing small distances between the electrodes. The ionic conductivity is supplied by the aqueous alkaline solution, which penetrates in the pores of the diaphragm. [[Asbestos]] diaphragms have been used for a long time due to their effective gas separation, low cost, and high [[chemical stability]]; however, their use is restricted by the [[Rotterdam Convention]].<ref name="Smolinka2021">{{cite book |last1=Smolinka |first1=Tom |title=Electrochemical Power Sources: Fundamentals, Systems, and Applications: Hydrogen Production by Water Electrolysis |date=2021 |publisher=Elsevier |isbn=978-0-12-819424-9}}</ref> The state-of-the-art diaphragm is Zirfon, a composite material of [[zirconia]] and [[Polysulfone]].<ref>{{cite web|title=AGFA Zirfon Perl Product Specification| url=http://www.agfa.com/specialty-products/solutions/membranes/zirfon-perl-utp-500/|access-date=29 January 2019 |archive-url=https://web.archive.org/web/20180423012500/http://www.agfa.com/specialty-products/solutions/membranes/zirfon-perl-utp-500/ |archive-date=2018-04-23 }}</ref> The diaphragm further avoids the mixing of the produced hydrogen and oxygen at the cathode and anode,<ref name="schalenbach2016zirfon">{{cite journal|last=Schalenbach|first=M|author2=Lueke W |author3=Stolten D| title=Hydrogen Diffusivity and Electrolyte Permeability of the Zirfon PERL Separator for Alkaline Water Electrolysis |journal= Journal of the Electrochemical Society |volume=163|issue=14|pages=F1480–F1488|year=2016|doi= 10.1149/2.1251613jes|s2cid=55017229|url=http://jes.ecsdl.org/content/163/14/F1480.full.pdf}}</ref><ref name="Haug2017">{{cite journal|last=Haug|first=P|author2=Koj M |author3=Turek T| title=Influence of process conditions on gas purity in alkaline water electrolysis|journal=International Journal of Hydrogen Energy|volume=42|issue=15|pages=9406–9418|year=2017|doi=10.1016/j.ijhydene.2016.12.111|bibcode=2017IJHE...42.9406H}}</ref> respectively. The thickness of asbestos diaphragms ranges from 2 to 5 mm, while Zirfon diaphragms range from 0.2 to 0.5 mm.<ref name="Smolinka2021" />

Typically, nickel based metals are used as the electrodes for alkaline water electrolysis.<ref>{{Cite journal |last1=Zhou |first1=Daojin |last2=Li |first2=Pengsong |display-authors=et al. |date=2020 |title=Recent Advances in Non-Precious Metal-Based Electrodes for Alkaline Water Electrolysis |url=https://onlinelibrary.wiley.com/doi/10.1002/cnma.202000010 |journal=ChemNanoMat |language=en |volume=6 |issue=3 |pages=336–355 |doi=10.1002/cnma.202000010 |s2cid=213442277 |issn=2199-692X|url-access=subscription }}</ref> Considering pure metals, Ni is the least active non-noble metal.<ref name="Quanio2014">{{cite journal|last=Quaino|first=P|author2=Juarez F |author3=Santos E| author4=Schmickler W| title=Volcano plots in hydrogen electrocatalysis–uses and abuses |journal=Beilstein Journal of Nanotechnology |volume=42|pages=846–854|year=2014|doi= 10.3762/bjnano.5.96 |pmid=24991521| pmc=4077405 }}</ref> The high price of good noble metal electrocatalysts such as platinum group metals and their dissolution during the oxygen evolution<ref name="Schalenbach-dissolution">{{cite journal|last=Schalenbach |first=M |display-authors=et al | title=The electrochemical dissolution of noble metals in alkaline media |journal= Electrocatalysis|volume=9|issue=2 |pages=153–161|year=2018|doi=10.1007/s12678-017-0438-y |s2cid=104106046 }}</ref> is a drawback. Ni is considered as more stable during the oxygen evolution,<ref name="Cherevko2016 ">{{cite journal|last=Cherevko |first=S |display-authors=et al | title=Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability |journal=Catalysis Today |volume=262|pages=170–180|year=2016|doi= 10.1016/j.cattod.2015.08.014}}</ref> but stainless steel has shown good stability and better catalytic activity than Ni at high temperatures during the [[Heterogeneous_water_oxidation |Oxygen Evolution Reaction (OER)]].<ref name="Colli et al.">{{cite journal|last=Colli |first=A.N. |display-authors=et al | title=Non-Precious Electrodes for Practical Alkaline Water Electrolysis |journal= Materials|volume=12|issue=8 |page=1336|year=2019|doi=10.3390/ma12081336 |pmid=31022944 |doi-access=free |pmc=6515460 |bibcode=2019Mate...12.1336C }}</ref>

High surface area Ni catalysts can be achieved by dealloying of nickel-zinc<ref name="Colli et al." /> or nickel-aluminium alloys in alkaline solution, commonly referred to as [[Raney nickel]]. In cell tests the best performing electrodes thus far reported consisted of plasma vacuum sprayed Ni alloys on Ni meshes<ref name="Schiller1995">{{cite journal|last=Schiller |first=G |author2=Henne R|author3=Borock V| title=Vacuum Plasma Spraying of High-Performance Electrodes for Alkaline Water Electrolysis |journal=Journal of Thermal Spray Technology |volume=4|issue=2 |page=185|year=1995|bibcode=1995JTST....4..185S|doi=10.1007/BF02646111|s2cid=137144045 }}</ref> <ref name="Schiller1998">{{cite journal|last=Schiller |first=G |author2=Henne R|author3=Mohr P| author4=Peinecke V| title=High Performance Electrodes for an Advanced Intermittently Operated 10-kW Alkaline Water Electrolyzer |journal=International Journal of Hydrogen Energy|volume=23|issue=9 |pages=761–765|year=1998|doi= 10.1016/S0360-3199(97)00122-5|bibcode=1998IJHE...23..761S }}</ref> and hot dip galvanized Ni meshes.<ref name="Schalenbach2018-3">{{cite journal|last=Schalenbach |first=M |display-authors=et al | title= An alkaline water electrolyzer with nickel electrodes enables efficient high current density operation|journal= International Journal of Hydrogen Energy |volume=43 |issue=27 |pages=11932–11938 | year=2018|doi= 10.1016/j.ijhydene.2018.04.219 |bibcode=2018IJHE...4311932S |s2cid=103477803 }}</ref> The latter approach might be interesting for large scale industrial manufacturing as it is cheap and easily scalable, but unfortunately, all the strategies show some degradation.<ref name="NaeemehEsfandiari">{{cite journal|last=Esfandiari |first=N |display-authors=et al | title= Metal-based cathodes for hydrogen production by alkaline water electrolysis: Review of materials, degradation mechanism, and durability tests|journal= Progress in Materials Science |volume=143 |article-number=101254 | year=2024| doi= 10.1016/j.pmatsci.2024.101254}}</ref>

==Electrochemistry== === Anode reaction === In alkaline media oxygen evolution reactions, multiple adsorbent species (O, OH, OOH, and OO<sup>–</sup>) and multiple steps are involved. Steps 4 and 5 often occur in a single step, but there is evidence that suggests steps 4 and 5 occur separately at pH 11 and higher.<ref name="Scott2019">{{cite book |last1=Scott |first1=Keith |title=Electrochemical methods for hydrogen production |date=2020 |publisher=Royal Society of Chemistry |isbn=978-1-78801-378-9 |location=Cambridge}}</ref><ref>{{cite journal |last1=Diaz-Morales |first1=Oscar |last2=Ferrus-Suspedra |first2=David |last3=Koper |first3=Marc T. M. |date=2016 |title=The importance of nickel oxyhydroxide deprotonation on its activity towards electrochemical water oxidation |journal=Chemical Science |volume=7 |issue=4 |pages=2639–2645 |doi=10.1039/C5SC04486C |pmc=5477031 |pmid=28660036 |doi-access=free}}</ref>

{| style="border:1px solid #ccc; width:60%;" |- | <math>\mathrm{OH}^- \rightarrow \mathrm{OH}^* + \mathrm{e}^- </math> | style="text-align:right; vertical-align:top;" | <math>\left ( 1 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | <math>\mathrm{OH}^* + \mathrm{OH}^- \rightarrow \mathrm{O}^* + \mathrm{H}_2 \mathrm{O} + \mathrm{e}^-</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 2 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | <math>\mathrm{O}^* + \mathrm{OH}^- \rightarrow \mathrm{OOH}^* + \mathrm{e}^-</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 3 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | <math>\mathrm{OOH}^* + \mathrm{OH}^- \rightarrow \mathrm{OO}^{-*} + \mathrm{H}_2 \mathrm{O}</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 4 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | <math>\mathrm{OO}^{-*} \rightarrow \mathrm{O}_{2(g)} + \mathrm{e}^-</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 5 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | Overall anode reaction: <math>2\mathrm{OH}^- \rightarrow \mathrm{H}_2 \mathrm{O} + \frac{1}{2}\mathrm{O}_2 + 2 \mathrm{e}^- \quad (E^0 = + 0.40 \, \mathrm{V \; vs. \; SHE})</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 6 \right )</math> |}

Where the * indicate species adsorbed to the surface of the catalyst.

=== Cathode reaction === The hydrogen evolution reaction in alkaline conditions starts with water adsorption and dissociation in the Volmer step and either hydrogen desorption in the Tafel step or Heyrovsky step. {| style="border:1px solid #ccc; width:60%;" |- | Volmer step: <math>2\mathrm{H}_2 \mathrm{O} + 2\mathrm{e}^- \rightarrow 2\mathrm{H}^* + 2\mathrm{OH}^{-}</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 7 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | Tafel step: <math>2\mathrm{H}^* \rightarrow \mathrm{H}_2</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 8 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | Heyrovsky step: <math>\mathrm{H}_2 \mathrm{O} + \mathrm{H}^* + \mathrm{e}^- \rightarrow \mathrm{H}_2 + \mathrm{OH}^-</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 9 \right )</math> |}

{| style="border:1px solid #ccc; width:60%;" |- | Overall cathode reaction: <math>2\mathrm{H}_2 \mathrm{O} + 2\mathrm{e}^- \rightarrow \mathrm{H}_2 + 2\mathrm{OH}^- \quad (E^0 = - 0.83 \, \mathrm{V \; vs. \; SHE})</math> | style="text-align:right; vertical-align:top;" | <math>\left ( 10 \right )</math> |}

==Advantages compared to PEM water electrolysis== In comparison to [[Proton exchange membrane electrolysis]], the advantages of alkaline water electrolysis are mainly:<ref>{{Cite journal |last1=Shiva Kumar |first1=S. |last2=Himabindu |first2=V. |date=2019-12-01 |title=Hydrogen production by PEM water electrolysis – A review |journal=Materials Science for Energy Technologies |volume=2 |issue=3 |pages=442–454 |doi=10.1016/j.mset.2019.03.002 |bibcode=2019MSET....2..442S |issn=2589-2991|doi-access=free }}</ref>

# Has a longer track record of industrial use, proven reliability, and lower initial costs, making it a more mature option for large-scale hydrogen production. # Higher durability due to an exchangeable electrolyte and lower dissolution of anodic catalyst. # Unlike PEM electrolysis, alkaline electrolysis does not require expensive or scarce precious metals like platinum or iridium for the electrodes. This reduces the overall cost and material dependencies.

==Disadvantage== One disadvantage of alkaline water electrolysers is the low-performance profiles caused by the commonly used thick diaphragms that increase ohmic resistance, the lower intrinsic conductivity of OH− compared to H+, and the higher gas crossover observed for highly porous diaphragms.<ref name=":1">{{Cite journal |last1=Martínez-Rodríguez |first1=Angel |last2=Abánades |first2=Alberto |date=November 2020 |title=Comparative Analysis of Energy and Exergy Performance of Hydrogen Production Methods |journal=Entropy |language=en |volume=22 |issue=11 |page=1286 |bibcode=2020Entrp..22.1286M |doi=10.3390/e22111286 |issn=1099-4300 |pmc=7712718 |pmid=33287054 |doi-access=free}}</ref>

==See also== *[[Proton exchange membrane electrolysis]] *[[Solid oxide electrolyzer cell]] *[[Anion exchange membrane electrolysis]]

==References== {{Reflist}}

[[Category:Chemical processes]] [[Category:Electrochemistry]] [[Category:Electrolysis]] [[Category:Industrial gases]] [[Category:Hydrogen production]]