{{Short description|Technology for purifying hydrogen}} '''Hydrogen purification''' is any technology used to purify hydrogen. The impurities in hydrogen gas depend on the source of the H2, e.g., petroleum, coal, electrolysis, etc. The required purity is determined by the application of the hydrogen gas. For example, ultra-high purified hydrogen is needed for applications like proton exchange membrane fuel cells.<ref name=Ull/>

==Purification technologies==

===Low temperature methods=== The default large-scale purification of H<sub>2</sub> produced in oil refineries exploits its very low boiling point of −253 °C. Most impurities have boiling points well above this temperature. Low temperature methods can be complemented by scrubbing to remove particular impurities.<ref name=Ull>{{cite book |doi=10.1002/14356007.o13_o04 |chapter=Hydrogen, 3. Purification |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |last1=Häussinger |first1=Peter |last2=Lohmüller |first2=Reiner |last3=Watson |first3=Allan M. |isbn=978-3-527-30385-4 }}</ref>

===Palladium membrane hydrogen purifiers=== Hydrogen can be purified by passing through a membrane composed of palladium and silver. Permeability of the former to hydrogen was discovered back in the 1860s.<ref>{{Cite book |last1=Sammells |first1=Anthony F. |url=https://books.google.com/books?id=gCQjw-cyulAC&pg=PA78 |title=Nonporous Inorganic Membranes: For Chemical Processing |last2=Mundschau |first2=Michael V. |date=2006-12-13 |publisher=John Wiley & Sons |isbn=978-3-527-60858-4 |language=en}}</ref> An alloy with a ca. 3:1 ratio for Pd:Ag is more structural robust than pure Pd, which is the active component that allows the selective diffusion of H<sub>2</sub> through it. Diffusion is faster near 300 °C. This method has been used for purification of hydrogen on a laboratory scale, but not in industry. Silver-palladium membranes are unstable toward alkenes and sulfur-containing compounds.<ref name=Ull/>

Dense thin-metal membrane purifiers are compact, relatively inexpensive and simple to use.<ref>[http://www.hydrogen.bham.ac.uk/h2purify.htm Hydrogen purification membranes]</ref><ref>[http://www.igb.fraunhofer.de/en/competences/interfacial-materials/membranes/metal-membranes.html Dense metal membranes for hydrogen purifying]</ref>

===Pressure swing adsorption=== Pressure swing adsorption is used for the removal of carbon dioxide (CO<sub>2</sub>) as the final step in the large-scale commercial synthesis of hydrogen. It can also remove methane, carbon monoxide, nitrogen, moisture and in some cases, argon, from hydrogen.

==Applications== === Metalorganic vapour phase epitaxy=== Hydrogen purifiers are used in metalorganic vapour phase epitaxy reactors for LED production.<ref>{{cite journal |title=Hydrogen purifiers proving vital to LED production |journal=III-Vs Review |date=June 2006 |volume=19 |issue=5 |pages=19 |doi=10.1016/S0961-1290(06)71698-2|doi-access=free }}</ref>

=== PEM fuel cells === Fuel cell electric vehicles commonly use polymer electrolyte membrane fuel cells (PEMFC) that are susceptible to a range of impurities. Impurities impact PEMFC using a range of mechanisms, these may include poisoning the anode hydrogen oxidation reaction catalysts, reducing the ionic conductivity of the ionomer and membrane, altering wetting behaviour of components or blocking porosity in diffusion media. The impact of some impurities like carbon monoxide, formic acid, or formaldehyde is reversible with PEMFC performance recovering once the supply of impurity is removed. Other impurities, for example sulphurous compounds, may cause irreversible degradation.<ref>{{Cite journal|last=X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang and J. Shen|year=2007|title=A review of PEM hydrogen fuel cell contamination:Impacts, mechanisms, and mitigation|journal=Journal of Power Sources|volume=165|issue=2|pages=739–756|doi=10.1016/j.jpowsour.2006.12.012|bibcode=2007JPS...165..739C|s2cid=95246225 }}</ref> The permissible limits of hydrogen impurities are shown below. {| class="wikitable" |+Fuel Quality Specification For Gasseous Hydrogen Supplied to PEMFC Road Vehicles <ref name=":02">{{Cite web|last=|title=ISO 14687:2019|url=https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html|access-date=2021-10-18|website=|language=en}}</ref> ! !Maximum Permissible Concentration / μmol mol<sup>−1</sup> |- |Total non-hydrogen gasses |300 |- |Water |5 |- |Total Hydrocarbons Except Methane [Carbon atom basis] |2 |- |Methane |100 |- |Oxygen |5 |- |Helium |300 |- |Nitrogen |300 |- |Argon |300 |- |Carbon Dioxide |2 |- |Carbon Monoxide |0.2 |- |Total Sulphur Compounds [Sulphur atom basis] |0.004 |- |Formaldehyde |0.2 |- |Formic Acid |0.2 |- |Ammonia |0.1 |- |Halogenated Compounds [Halogen ion basis] |0.05 |- |Maximum Particulate Concentration |1&nbsp;mg kg<sup>−1</sup> |} Efforts to assess the compliance of hydrogen supplied by hydrogen refuelling stations against the ISO-14687 standard have been performed.<ref name=":1">{{Cite journal|last1=Aarhaug|first1=Thor Anders|last2=Kjos|first2=Ole|last3=Bacquart|first3=Thomas|last4=Valter|first4=Vladimir|last5=Optenhostert|first5=Thomas|date=2021-08-18|title=Assessment of hydrogen quality dispensed for hydrogen refuelling stations in Europe|journal=International Journal of Hydrogen Energy|series=HYDROGEN ENERGY SYSTEMS|language=en|volume=46|issue=57|pages=29501–29511|doi=10.1016/j.ijhydene.2020.11.163|s2cid=230535934|issn=0360-3199|doi-access=free|bibcode=2021IJHE...4629501A |hdl=11250/3025287|hdl-access=free}}</ref><ref>{{Cite journal|last1=Aarhaug|first1=Thor A.|last2=Kjos|first2=Ole S.|last3=Ferber|first3=Alain|last4=Hsu|first4=Jong Pyong|last5=Bacquart|first5=Thomas|date=2020|title=Mapping of Hydrogen Fuel Quality in Europe|journal=Frontiers in Energy Research|volume=8|pages=307|article-number=585334 |doi=10.3389/fenrg.2020.585334|bibcode=2020FrER....885334A |issn=2296-598X|doi-access=free|hdl=11250/2770289|hdl-access=free}}</ref><ref>{{Cite web|title=HYDRAITE public report D3.1 {{!}} HYDRAITE|url=https://hydraite.eu/public-reports/|access-date=2021-10-18|language=en-US}}</ref> While the hydrogen was generally found to be 'good'<ref name=":1" /> violations of the standard have been reported, most frequently for nitrogen, water and oxygen.

=== Combustion engines and appliances === Combustion applications are generally more tolerant of hydrogen impurities than PEFMC, as such the ISO-14687 standard for permissible impurities is less strict.<ref name=":03">{{Cite web|last=|title=ISO 14687:2019|url=https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html|access-date=2021-10-18|website=|language=en}}</ref> This standard has itself been criticised with revisions proposed to make it more lenient and therefore suitable for hydrogen distributed through a repurposed gas network.<ref name=":222">{{Cite web|title=WP2 Report Hydrogen Purity|url=https://www.hy4heat.info/reports|access-date=2021-10-18|website=Hy4Heat|language=en-US}}</ref> {| class="wikitable" |+Fuel Quality Specification For Gaseous Hydrogen Supplied to Combustion Engines and Appliances <ref name=":04">{{Cite web|last=|title=ISO 14687:2019|url=https://www.iso.org/cms/render/live/en/sites/isoorg/contents/data/standard/06/95/69539.html|access-date=2021-10-18|website=|language=en}}</ref> !Impurity !Maximum Permissible Concentration / μmol mol<sup>−1</sup> |- |Total non-hydrogen gasses |20 000 |- |Water |Non-condensing |- |Total Hydrocarbons [Carbon atom basis] |100 |- |Carbon Monoxide |1 |- |Sulphur [Sulphur atom basis] |2 |- |Combined water, oxygen, nitrogen, argon |19 000 |- |Permanent Particulates |Shall not contain an amount sufficient to cause damage. |}

== Sources of impurities == The presence of impurities in hydrogen depends on the feedstock and the production process. Hydrogen produced by electrolysis of water may routinely include trace oxygen and water. Hydrogen produced by reforming of hydrocarbons contains carbon dioxide and carbon monoxide as well as sulphur compounds.<ref name=":222" /> Some impurities may be added deliberately, for example odorants to aid detection of gas leaks.<ref>{{Cite web|title=Hydrogen Odorant and Leak Detection Project Closure Report|url=https://sgn.co.uk/sites/default/files/media-entities/documents/2020-09/Hydrogen_Odorant_and_Leak_Detection_Project_Closure_Report_SGN.pdf}}</ref>

== Methods for analysis == As the permissible concentrations for many impurities are very low this sets stringent demands on the sensitivity of the analytical methods. Moreover, the high reactivity of some impurities requires use of a properly passivated sampling and analytical systems.<ref>{{Cite journal|last1=Bacquart|first1=Thomas|last2=Moore|first2=Niamh|last3=Hart|first3=Nick|last4=Morris|first4=Abigail|last5=Aarhaug|first5=Thor A.|last6=Kjos|first6=Ole|last7=Aupretre|first7=Fabien|last8=Colas|first8=Thibault|last9=Haloua|first9=Frederique|last10=Gozlan|first10=Bruno|last11=Murugan|first11=Arul|date=2020-02-14|title=Hydrogen quality sampling at the hydrogen refuelling station – lessons learnt on sampling at the production and at the nozzle|url=https://www.sciencedirect.com/science/article/pii/S0360319919340510|journal=International Journal of Hydrogen Energy|series=22nd World Hydrogen Energy Conference|language=en|volume=45|issue=8|pages=5565–5576|doi=10.1016/j.ijhydene.2019.10.178|bibcode=2020IJHE...45.5565B |hdl=11250/2689927 |s2cid=213820032|issn=0360-3199|hdl-access=free|url-access=subscription}}</ref> Sampling of hydrogen of is challenging and care must be taken to ensure that impurities are not introduced to the sample and that impurities do not absorb on or react within the sampling equipment, there are currently different methods for sampling but rely on filling a gas cylinder from the refuelling nozzle of a refuelling station.<ref>{{Cite journal|last1=Arrhenius|first1=Karine|last2=Aarhaug|first2=Thor|last3=Bacquart|first3=Thomas|last4=Morris|first4=Abigail|last5=Bartlett|first5=Sam|last6=Wagner|first6=Lisa|last7=Blondeel|first7=Claire|last8=Gozlan|first8=Bruno|last9=Lescornez|first9=Yann|last10=Chramosta|first10=Nathalie|last11=Spitta|first11=Christian|date=2021-10-11|title=Strategies for the sampling of hydrogen at refuelling stations for purity assessment|journal=International Journal of Hydrogen Energy|language=en|volume=46|issue=70|pages=34839–34853|doi=10.1016/j.ijhydene.2021.08.043|s2cid=239636011|issn=0360-3199|doi-access=free|bibcode=2021IJHE...4634839A |hdl=11250/3010363|hdl-access=free}}</ref> Efforts are underway to standardise and compare sampling strategies.<ref>{{Citation|title=Practice for Sampling of High Pressure Hydrogen and Related Fuel Cell Feed Gases|url=http://dx.doi.org/10.1520/d7606-17|publisher=ASTM International|doi=10.1520/d7606-17|access-date=2021-11-01|url-access=subscription}}</ref><ref>{{Citation|title=DIN ISO/TS 22002-3:2017-09|url=https://www.iso.org/obp/ui/#iso:std:iso:19880:-1:ed-1:v1:en|publisher=|access-date=2021-11-01}}</ref> A combination of different instruments is needed to assess hydrogen samples for all of the components listed in ISO 14687-2.<ref>{{Cite journal|last1=Murugan|first1=Arul|last2=Brown|first2=Andrew S.|date=2015-03-22|title=Review of purity analysis methods for performing quality assurance of fuel cell hydrogen|url=https://www.sciencedirect.com/science/article/pii/S0360319915000804|journal=International Journal of Hydrogen Energy|language=en|volume=40|issue=11|pages=4219–4233|doi=10.1016/j.ijhydene.2015.01.041|bibcode=2015IJHE...40.4219M |issn=0360-3199|url-access=subscription}}</ref> Techniques suitable for individual impurities are indicated in the table below. {| class="wikitable" |+Example Analytical Methods for Assessing The Concentration of Impurities in Hydrogen<ref>{{Cite web|title=Hydrogen purity|url=https://www.npl.co.uk/products-services/gas/hydrogen-purity|access-date=2021-10-18|website=NPLWebsite|language=en}}</ref><ref>{{Cite journal|last1=Bacquart|first1=Thomas|last2=Arrhenius|first2=Karine|last3=Persijn|first3=Stefan|last4=Rojo|first4=Andrés|last5=Auprêtre|first5=Fabien|last6=Gozlan|first6=Bruno|last7=Moore|first7=Niamh|last8=Morris|first8=Abigail|last9=Fischer|first9=Andreas|last10=Murugan|first10=Arul|last11=Bartlett|first11=Sam|date=2019-12-31|title=Hydrogen fuel quality from two main production processes: Steam methane reforming and proton exchange membrane water electrolysis|journal=Journal of Power Sources|language=en|volume=444|article-number=227170|doi=10.1016/j.jpowsour.2019.227170|bibcode=2019JPS...44427170B|s2cid=208754564|issn=0378-7753|doi-access=free}}</ref> !Impurity !Possible Analytical Methods !Detection Limits |- |Total non-hydrogen gasses | | |- |Water |Quartz crystal microbalance or CRDS |1.3 or 0.030 |- |Total Hydrocarbons Except Methane [Carbon atom basis] |GC-Methaniser-FID |0.1 |- |Methane |GC-Methaniser-FID, GC-EPD |0.1 |- |Oxygen |GC-PDHID, GC-EPD |0.3 |- |Helium |GC-TCD |10 |- |Nitrogen |GC-PDHID, GC-EPD |1 |- |Argon |GC-PDHID, GC-EPD |0.3 |- |Carbon Dioxide |GC-Methaniser-FID, GC-EPD |0.02 |- |Carbon Monoxide |GC-Methaniser-FID, GC-EPD |0.02 |- |Total Sulphur Compounds [Sulphur atom basis] |GC-SCD, GC-EPD |0.001 |- |Formaldehyde |GC-Methaniser-FID |0.1 |- |Formic Acid |FTIR |0.2 |- |Ammonia |GC-MS or UV-visible spectroscopy or FTIR |1 or 0.03 or 0.1 |- |Halogenated Compounds (Halogen Ion Equivalent) |TD-GC-MS |0.016 |} Techniques such as electrochemical sensors <ref>{{Cite journal|last=Mukundan|first=Rangachary|title=Development of an Electrochemical Hydrogen Contaminant Detector|journal=Journal of the Electrochemical Society|year=2020|language=en|volume=167|issue=14|pages=147507|doi=10.1149/1945-7111/abc43a|bibcode=2020JElS..167n7507M|s2cid=226341724|doi-access=free}}</ref><ref>{{Cite journal|last1=Noda|first1=Z.|last2=Hirata|first2=K.|last3=Hayashi|first3=A.|last4=Takahashi|first4=T.|last5=Nakazato|first5=N.|last6=Saigusa|first6=K.|last7=Seo|first7=A.|last8=Suzuki|first8=K.|last9=Ariura|first9=S.|last10=Shinkai|first10=H.|last11=Sasaki|first11=K.|date=2017-02-02|title=Hydrogen pump-type impurity sensors for hydrogen fuels|url=https://www.sciencedirect.com/science/article/pii/S0360319916336138|journal=International Journal of Hydrogen Energy|language=en|volume=42|issue=5|pages=3281–3293|doi=10.1016/j.ijhydene.2016.12.066|bibcode=2017IJHE...42.3281N |issn=0360-3199|url-access=subscription}}</ref> and mass spectrometry.<ref>{{Cite web|title=HydrogenSense|url=https://www.vandf.com/en/products/analyzers/hydrogensense/,%20https://www.vandf.com/en/products/analyzers/hydrogensense/|access-date=2021-10-27|website=www.vandf.com|language=en}}{{Dead link|date=August 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

==See also== * Gas separation * Hydrogen pinch * Membrane gas separation * Membrane reactor * [https://www.scribd.com/doc/7201923/Solar-Hydrogen 45 Home Power #67 • October / November 1998] (home electrolyzers) * Hydrogen station * Hydrogen fuel * Proton-exchange membrane fuel cell * [https://www.sintef.no/projectweb/metrohyve-2/ MetroHyVe2 Project] * [https://hydraite.eu/ Hydraite Project] * [https://www.npl.co.uk/products-services/gas/hydrogen-purity National Physical Laboratory Hydrogen Purity]

==References== {{reflist}}

==External links== *[http://www.uop.com/objects/SelRevampHydroPurif.pdf 3 industrial hydrogen purifier systems] {{Webarchive|url=https://web.archive.org/web/20070715212652/http://www.uop.com/objects/SelRevampHydroPurif.pdf |date=2007-07-15 }} *[http://www.powerandenergy.com Power and Energy, Inc.] *[http://www.powerandenergy.com/pdfs/Setting_A_New_Benchmark_for_Hydrogen_Delivery.pdf Setting a new benchmark for hydrogen delivery] {{DEFAULTSORT:Hydrogen Purifier}} Category:Hydrogen technologies Category:Chemical equipment Category:Industrial gases Category:Gas technologies Category:Hydrogen