Editing Fuel cell (section)

===Proton-exchange membrane fuel cells===
{{main|Proton-exchange membrane fuel cell}}
[[File:PEM fuelcell.svg|thumb|upright=1.75|Construction of a high-temperature [[proton-exchange membrane fuel cell|PEMFC]]: Bipolar plate as [[electrode]] with in-milled gas channel structure, fabricated from conductive [[composite material|composites]] (enhanced with [[graphite]], [[carbon black]], [[carbon fiber]], and/or [[carbon nanotube]]s for more conductivity);<ref>{{cite journal | last1 = Kakati | first1 = B. K. | last2 = Deka | first2 = D. | year = 2007 | title = Effect of resin matrix precursor on the properties of graphite composite bipolar plate for PEM fuel cell | journal = Energy & Fuels | volume = 21 | issue = 3| pages = 1681–1687 | doi=10.1021/ef0603582}}</ref> [[Porous]] carbon papers; reactive layer, usually on the [[polymer]] membrane applied; polymer membrane.]] 
[[File:condensation.jpg|upright=1.55|thumb|Condensation of water produced by a PEMFC on the air channel wall. The gold wire around the cell ensures the collection of electric current.<ref>{{cite web|url=http://perso.ensem.inpl-nancy.fr/Olivier.Lottin/Ourfuelcells.html |title=LEMTA – Our fuel cells |publisher=Perso.ensem.inpl-nancy.fr |access-date=2009-09-21 |url-status=dead |archive-url=https://web.archive.org/web/20090621084543/http://perso.ensem.inpl-nancy.fr/Olivier.Lottin/Ourfuelcells.html |archive-date=21 June 2009 }}</ref>]]
[[File:SEM micrograph of an MEA cross section.jpg|thumb|SEM micrograph of a PEMFC MEA cross-section with a non-precious metal catalyst cathode and Pt/C anode.<ref>{{cite journal |last1=Yin |first1=Xi |last2=Lin |first2=Ling |last3=Chung |first3=Hoon T |last4=Komini Babu |first4=Siddharth |last5=Martinez |first5=Ulises |last6=Purdy |first6=Geraldine M |last7=Zelenay |first7=Piotr |title=Effects of MEA Fabrication and Ionomer Composition on Fuel Cell Performance of PGM-Free ORR Catalyst |journal=ECS Transactions |date=4 August 2017 |volume=77 |issue=11 |pages=1273–1281 |doi=10.1149/07711.1273ecst|bibcode=2017ECSTr..77k1273Y |osti=1463547 }}</ref> False colors applied for clarity.]]
In the archetypical hydrogen–oxide [[proton-exchange membrane fuel cell]] (PEMFC) design, a proton-conducting polymer membrane (typically [[nafion]]) contains the [[electrolyte]] solution that separates the [[anode]] and [[cathode]] sides.<ref>Anne-Claire Dupuis, Progress in Materials Science, Volume 56, Issue 3, March 2011, pp. 289–327</ref><ref>{{Cite web|url=http://personal.cityu.edu.hk/~kwanshui/Paper/IJHE2.pdf|archive-url=https://web.archive.org/web/20131105234936/http://personal.cityu.edu.hk/~kwanshui/Paper/IJHE2.pdf|url-status=dead|title=Measuring the relative efficiency of hydrogen energy technologies for implementing the hydrogen economy 2010|archive-date=5 November 2013}}</ref> This was called a ''solid polymer electrolyte fuel cell'' (''SPEFC'') in the early 1970s, before the proton-exchange mechanism was well understood. (Notice that the synonyms ''polymer electrolyte membrane'' and ''proton-exchange mechanism'' result in the same [[acronym]].)

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what are commonly referred to as multi-facilitated proton membranes. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen [[molecule]]s react with the electrons (which have traveled through the external circuit) and protons to form water.

In addition to this pure hydrogen type, there are [[hydrocarbon]] fuels for fuel cells, including [[diesel fuel|diesel]], [[methanol]] (''see:'' [[direct-methanol fuel cell]]s and [[indirect methanol fuel cell]]s) and chemical hydrides. The waste products with these types of fuel are [[carbon dioxide]] and water. When hydrogen is used, the CO{{sub|2}} is released when methane from natural gas is combined with steam, in a process called [[steam reforming|steam methane reforming]], to produce the hydrogen. This can take place in a different location to the fuel cell, potentially allowing the hydrogen fuel cell to be used indoors—for example, in forklifts.

The different components of a PEMFC are
# bipolar plates,
# [[electrode]]s,
# [[catalyst]],
# membrane, and
# the necessary hardware such as current collectors and gaskets.<ref>{{cite journal | last1 = Kakati | first1 = B. K. | last2 = Mohan | first2 = V. | year = 2008 | title = Development of low cost advanced composite bipolar plate for P.E.M. fuel cell | journal = Fuel Cells | volume = 08 | issue = 1| pages = 45–51 | doi=10.1002/fuce.200700008| s2cid = 94469845 }}</ref>

The materials used for different parts of the fuel cells differ by type. The bipolar plates may be made of different types of materials, such as, metal, coated metal, [[graphite]], flexible graphite, C–C [[composite material|composite]], [[carbon]]–[[polymer]] composites etc.<ref>{{cite journal | last1 = Kakati | first1 = B. K. | last2 = Deka | first2 = D. | year = 2007 | title = Differences in physico-mechanical behaviors of resol and novolac type phenolic resin based composite bipolar plate for proton exchange membrane (PEM) fuel cell | journal = Electrochimica Acta | volume = 52 | issue = 25| pages = 7330–7336 | doi=10.1016/j.electacta.2007.06.021}}</ref> The [[membrane electrode assembly]] (MEA) is referred to as the heart of the PEMFC and is usually made of a proton-exchange membrane sandwiched between two [[catalyst]]-coated [[carbon paper]]s. Platinum and/or similar types of [[noble metal]]s are usually used as the catalyst for PEMFC, and these can be contaminated by [[carbon monoxide]], necessitating a relatively pure hydrogen fuel.<ref name=WGS>Coletta, Vitor, ''et al.'' [https://hal.archives-ouvertes.fr/hal-03132190/document/#page=3 "Cu-Modified SrTiO 3 Perovskites Toward Enhanced Water-Gas Shift Catalysis: A Combined Experimental and Computational Study" ], ''ACS Applied Energy Materials'' (2021), vol. 4, issue 1, pp. 452–461</ref> The electrolyte could be a polymer [[artificial membrane|membrane]].

====Proton-exchange membrane fuel cell design issues====
; Cost: In 2013, the Department of Energy estimated that 80&nbsp;kW automotive fuel cell system costs of {{USD|67}} per kilowatt could be achieved, assuming volume production of 100,000 automotive units per year and {{USD|55}} per kilowatt could be achieved, assuming volume production of 500,000 units per year.<ref>
Spendelow, Jacob and Jason Marcinkoski. [http://www.hydrogen.energy.gov/pdfs/13012_fuel_cell_system_cost_2013.pdf "Fuel Cell System Cost – 2013"] {{webarchive|url=https://web.archive.org/web/20131202225059/http://www.hydrogen.energy.gov/pdfs/13012_fuel_cell_system_cost_2013.pdf |date= 2 December 2013 }}, DOE Fuel Cell Technologies Office, 16 October 2013 ([https://web.archive.org/web/20131202225059/http://www.hydrogen.energy.gov/pdfs/13012_fuel_cell_system_cost_2013.pdf archived version])
</ref> Many companies are working on techniques to reduce cost in a variety of ways including reducing the amount of platinum needed in each individual cell. [[Ballard Power Systems]] has experimented with a catalyst enhanced with carbon silk, which allows a 30% reduction (1.0–0.7&nbsp;mg/cm<sup>2</sup>) in platinum usage without reduction in performance.<ref>
{{Cite news
 | title = Ballard Power Systems: Commercially Viable Fuel Cell Stack Technology Ready by 2010
 | date = 29 March 2005
 | url = http://www.fuelcellsworks.com/Supppage2336.html
 | access-date = 2007-05-27
 | archive-url = https://web.archive.org/web/20070927050617/http://www.fuelcellsworks.com/Supppage2336.html
 | archive-date = 27 September 2007
 | url-status=dead
}}</ref> [[Monash University]], [[Melbourne]] uses [[poly(3,4-ethylenedioxythiophene)|PEDOT]] as a [[cathode]].<ref name="Online">
{{cite web
 |last=Online |first=Science
 |url=http://www.abc.net.au/news/stories/2008/08/02/2322139.htm
 |archive-url=https://web.archive.org/web/20080806174903/http://www.abc.net.au/news/stories/2008/08/02/2322139.htm
 |url-status=dead
 |archive-date=6 August 2008
 |title=2008 – Cathodes in fuel cells
 |publisher=Abc.net.au
 |date=2 August 2008
 |access-date=2009-09-21
}}</ref> A 2011-published study<ref>
{{cite journal
 | doi=10.1021/ja1112904
 | pmid=21413707
 | volume=133 | issue=14
 | title=Polyelectrolyte Functionalized Carbon Nanotubes as Efficient Metal-free Electrocatalysts for Oxygen Reduction
 | journal=Journal of the American Chemical Society
 | pages=5182–5185
 | last1 = Wang | first1 = Shuangyin
 | s2cid=207063759
 | year=2011
 | bibcode=2011JAChS.133.5182W
 }}</ref> documented the first metal-free electrocatalyst using relatively inexpensive doped [[carbon nanotube]]s, which are less than 1% the cost of platinum and are of equal or superior performance. A recently published article demonstrated how the environmental burdens change when using carbon nanotubes as carbon substrate for platinum.<ref>
{{cite journal
 |last1=Notter|first1=Dominic A.
 |last2=Kouravelou|first2=Katerina
 |last3=Karachalios|first3=Theodoros
 |last4=Daletou|first4=Maria K.
 |last5=Haberland|first5=Nara Tudela
 |title=Life cycle assessment of PEM FC applications: electric mobility and μ-CHP
 |journal=Energy Environ. Sci.
 |date=2015
 |volume=8|issue=7|pages=1969–1985
 |doi=10.1039/C5EE01082A
|bibcode=2015EnEnS...8.1969N
 }}</ref>
; Water and air management<ref>{{cite web|url=http://www.ika.rwth-aachen.de/r2h/index.php/Water_and_Air_Management_for_Fuel_Cells |title=Water_and_Air_Management |publisher=Ika.rwth-aachen.de |access-date=2009-09-21 |url-status=dead |archive-url=https://web.archive.org/web/20090114182615/http://www.ika.rwth-aachen.de/r2h/index.php/Water_and_Air_Management_for_Fuel_Cells |archive-date=14 January 2009}}</ref><ref>{{Cite journal|last1=Andersson|first1=M.|last2=Beale|first2=S. B.|last3=Espinoza|first3=M.|last4=Wu|first4=Z.|last5=Lehnert|first5=W.|date=2016-10-15|title=A review of cell-scale multiphase flow modeling, including water management, in polymer electrolyte fuel cells|journal=Applied Energy|volume=180|pages=757–778|doi=10.1016/j.apenergy.2016.08.010|bibcode=2016ApEn..180..757A }}</ref> (in PEMFCs): In this type of fuel cell, the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, the resistance across it increases, and eventually, it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to manage water in cells are being developed like [[electroosmotic pump]]s focusing on flow control. Just as in a combustion engine, a steady ratio between the reactant and oxygen is necessary to keep the fuel cell operating efficiently.
; Temperature management: The same temperature must be maintained throughout the cell in order to prevent destruction of the cell through [[thermal loading]]. This is particularly challenging as the 2H<sub>2</sub> + O<sub>2</sub> → 2H<sub>2</sub>O reaction is highly exothermic, so a large quantity of heat is generated within the fuel cell.
; Durability, [[service life]], and special requirements for some type of cells: [[Stationary fuel cell applications]] typically require more than 40,000 hours of reliable operation at a temperature of {{convert|-35|to|40|C|F}}, while automotive fuel cells require a 5,000-hour lifespan (the equivalent of {{convert|150000|miles|km|abbr=on|sigfig=2|order=flip|disp=or}}) under extreme temperatures. Current [[service life]] is 2,500 hours (about {{convert|75,000|mi|km|abbr=on|sigfig=2|order=flip|disp=or}}).<ref>{{cite web |url=http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/accomplishments.pdf |title=Progress and Accomplishments in Hydrogen and Fuel Cells |access-date=2015-05-16 |url-status=dead |archive-url=https://web.archive.org/web/20151123185414/http://www1.eere.energy.gov/hydrogenandfuelcells/pdfs/accomplishments.pdf |archive-date=23 November 2015}}</ref> Automotive engines must also be able to start reliably at {{convert|-30|°C|°F|abbr=on}} and have a high power-to-volume ratio (typically 2.5&nbsp;kW/L).
; Limited [[carbon monoxide]] tolerance of some (non-PEDOT) cathodes.<ref name=WGS />