Editing Fuel cell (section)

==Types of fuel cells; design==
Fuel cells come in many varieties; however, they all work in the same general manner. They are made up of three adjacent segments: the [[anode]], the [[electrolyte]], and the [[cathode]]. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load.

At the anode a [[catalyst]] ionizes the fuel, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot. The freed electrons travel through a wire creating an electric current. The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide.

[[File:Fuel Cell Block Diagram.svg|thumb|upright=1.35|A block diagram of a fuel cell]]
Design features in a fuel cell include:
* The electrolyte substance, which usually defines the ''type'' of fuel cell, and can be made from a number of substances like potassium hydroxide, salt carbonates, and phosphoric acid.<ref>{{Cite news| url=http://www.energygroove.net/technologies/fuel-cells| title=Fuel Cells - EnergyGroove.net|work=EnergyGroove.net|access-date=2018-02-06}}</ref>
* The most common fuel that is used is hydrogen.
* The anode catalyst, usually fine platinum powder, breaks down the fuel into electrons and ions.
* The cathode catalyst, often nickel, converts ions into waste chemicals, with water being the most common type of waste.<ref name=TTI>{{Cite news|url=https://textechindustries.com/high-performance-materials| title=Reliable High Performance Textile Materials| work=Tex Tech Industries|access-date=2018-02-06}}</ref>
* Gas diffusion layers that are designed to resist oxidization.<ref name=TTI/>

A typical fuel cell produces a voltage from 0.6 to 0.7&nbsp;V at a full-rated load. Voltage decreases as current increases, due to several factors:
* [[Overpotential|Activation loss]]
* Ohmic loss ([[voltage drop]] due to resistance of the cell components and interconnections)
* Mass transport loss (depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage).<ref name="Larminie2003">{{Cite book| last = Larminie | first = James | title = Fuel Cell Systems Explained, Second Edition | publisher = [[Society of Automotive Engineers|SAE International]] | date=1 May 2003| isbn = 978-0-7680-1259-0}}</ref>

To deliver the desired amount of energy, the fuel cells can be combined in [[series and parallel circuits|series]] to yield higher [[voltage]], and in parallel to allow a higher [[Electric current|current]] to be supplied. Such a design is called a ''fuel cell stack''. The cell surface area can also be increased, to allow higher current from each cell.

===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 />

=== Phosphoric acid fuel cell ===
{{Main|Phosphoric acid fuel cell}}
Phosphoric acid fuel cells (PAFCs) were first designed and introduced in 1961 by [[G. V. Elmore]] and [[H. A. Tanner]]. In these cells, phosphoric acid is used as a non-conductive electrolyte to pass protons from the anode to the cathode and to force electrons to travel from anode to cathode through an external electrical circuit. These cells commonly work in temperatures of 150 to 200&nbsp;°C. This high temperature will cause heat and energy loss if the heat is not removed and used properly. This heat can be used to produce steam for air conditioning systems or any other thermal energy-consuming system.<ref name="americanhistory.si.edu">{{Cite web|url=https://americanhistory.si.edu/fuelcells/phos/pafcmain.htm|title=Collecting the History of Phosphoric Acid Fuel Cells|website=americanhistory.si.edu}}</ref> Using this heat in [[cogeneration]] can enhance the efficiency of phosphoric acid fuel cells from 40 to 50% to about 80%.<ref name="americanhistory.si.edu"/> Since the proton production rate on the anode is small, platinum is used as a catalyst to increase this ionization rate. A key disadvantage of these cells is the use of an acidic electrolyte. This increases the corrosion or oxidation of components exposed to phosphoric acid.<ref>{{cite web|url=http://scopewe.com/phosphoric-acid-fuel-cells|title=Phosphoric Acid Fuel Cells|website=scopeWe - a Virtual Engineer|access-date=28 June 2013|archive-date=10 November 2013|archive-url=https://web.archive.org/web/20131110071945/http://scopewe.com/phosphoric-acid-fuel-cells/|url-status=usurped}}</ref>

=== Solid acid fuel cell ===
{{Main|Solid acid fuel cell}}

Solid acid fuel cells (SAFCs) are characterized by the use of a solid acid material as the electrolyte. At low temperatures, [[solid acid]]s have an ordered molecular structure like most salts. At warmer temperatures (between 140 and 150{{nbsp}}°C for CsHSO<sub>4</sub>), some solid acids undergo a phase transition to become highly disordered "superprotonic" structures, which increases conductivity by several orders of magnitude. The first proof-of-concept SAFCs were developed in 2000 using cesium hydrogen sulfate (CsHSO<sub>4</sub>).<ref>{{Cite journal|last1=Haile|first1=Sossina M.|last2=Boysen|first2=Dane A.|last3=Chisholm|first3=Calum R. I.|last4=Merle|first4=Ryan B.|s2cid=4430178|date=2001-04-19|title=Solid acids as fuel cell electrolytes|journal=Nature| volume=410|issue=6831|pages=910–913|doi=10.1038/35073536|pmid=11309611|issn=0028-0836|bibcode=2001Natur.410..910H|url=https://authors.library.caltech.edu/14197/2/HAInature01supp.pdf |archive-url=https://web.archive.org/web/20170922051627/http://authors.library.caltech.edu/14197/2/HAInature01supp.pdf |archive-date=2017-09-22 |url-status=live}}</ref> Current SAFC systems use cesium dihydrogen phosphate (CsH<sub>2</sub>PO<sub>4</sub>) and have demonstrated lifetimes in the thousands of hours.<ref>{{Cite journal|last1=Haile|first1=Sossina M.|last2=Chisholm|first2=Calum R. I.|last3=Sasaki|first3=Kenji|last4=Boysen|first4=Dane A.|last5=Uda|first5=Tetsuya|date=2006-12-11|title=Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes|journal=Faraday Discussions| volume=134|doi=10.1039/B604311A|pmid=17326560|issn=1364-5498|pages=17–39|bibcode=2007FaDi..134...17H|url=https://authors.library.caltech.edu/7019/1/HAIfd07.pdf |archive-url=https://web.archive.org/web/20170815181049/http://authors.library.caltech.edu/7019/1/HAIfd07.pdf |archive-date=2017-08-15 |url-status=live}}</ref>

===Alkaline fuel cell===
{{Main|Alkaline fuel cell|Alkaline anion exchange membrane fuel cell}}
The alkaline fuel cell (AFC) or hydrogen-oxygen fuel cell was designed and first demonstrated publicly by Francis Thomas Bacon in 1959. It was used as a primary source of electrical energy in the Apollo space program.<ref>{{Cite journal|title = Francis Thomas Bacon. 21 December 1904 – 24 May 1992|last = Williams|first = K.R.|s2cid = 71613260|date = 1 February 1994|journal = Biographical Memoirs of Fellows of the Royal Society|doi = 10.1098/rsbm.1994.0001|volume = 39|pages = 2–9|doi-access = free}}</ref> The cell consists of two porous carbon electrodes impregnated with a suitable catalyst such as Pt, Ag, CoO, etc. The space between the two electrodes is filled with a concentrated solution of [[Potassium hydroxide|KOH]] or [[Sodium hydroxide|NaOH]] which serves as an electrolyte. H<sub>2</sub> gas and O<sub>2</sub> gas are bubbled into the electrolyte through the porous carbon electrodes. Thus the overall reaction involves the combination of hydrogen gas and oxygen gas to form water. The cell runs continuously until the reactant's supply is exhausted. This type of cell operates efficiently in the temperature range 343–413{{nbsp}}K (70 -140&nbsp;°C) and provides a potential of about 0.9{{nbsp}}V.<ref>Srivastava, H. C. ''Nootan ISC Chemistry'' (12th) Edition 18, pp. 458–459, Nageen Prakashan (2014) {{ISBN|9789382319399}}</ref> [[Alkaline anion exchange membrane fuel cell]] (AAEMFC) is a type of AFC which employs a solid polymer electrolyte instead of aqueous potassium hydroxide (KOH) and it is superior to aqueous AFC.

===High-temperature fuel cells===

====Solid oxide fuel cell====
{{Main|Solid oxide fuel cell}}
[[Solid oxide fuel cell]]s (SOFCs) use a solid material, most commonly a ceramic material called [[yttria-stabilized zirconia]] (YSZ), as the [[electrolyte]]. Because SOFCs are made entirely of solid materials, they are not limited to the flat plane configuration of other types of fuel cells and are often designed as rolled tubes. They require high [[operating temperature]]s (800–1000&nbsp;°C) and can be run on a variety of fuels including natural gas.<ref name=Types1>[http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html "Types of Fuel Cells"] {{webarchive|url=https://web.archive.org/web/20100609041046/http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html |date= 9 June 2010 }}. Department of Energy EERE website, accessed 4 August 2011</ref>

SOFCs are unique because negatively charged oxygen [[ion]]s travel from the [[cathode]] (positive side of the fuel cell) to the [[anode]] (negative side of the fuel cell) instead of [[proton]]s travelling vice versa (i.e., from the anode to the cathode), as is the case in all other types of fuel cells. Oxygen gas is fed through the cathode, where it absorbs electrons to create oxygen ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas at the anode. The reaction at the anode produces electricity and water as by-products. Carbon dioxide may also be a by-product depending on the fuel, but the carbon emissions from a SOFC system are less than those from a [[fossil fuel]] combustion plant.<ref>{{cite journal | last1 = Stambouli | first1 = A. Boudghene | year = 2002 | title = Solid oxide fuel cells (SOFCs): a review of an environmentally clean and efficient source of energy | journal = Renewable and Sustainable Energy Reviews | volume = 6 | issue = 5| pages = 433–455 | doi=10.1016/S1364-0321(02)00014-X| bibcode = 2002RSERv...6..433S }}</ref> The chemical reactions for the SOFC system can be expressed as follows:<ref>[http://www.fctec.com/fctec_types_sofc.asp "Solid Oxide Fuel Cell (SOFC)"]. FCTec website', accessed 4 August 2011 {{webarchive |url=https://web.archive.org/web/20120108053109/http://www.fctec.com/fctec_types_sofc.asp |date=8 January 2012 }}</ref>

:''Anode reaction'': 2H<sub>2</sub> + 2O<sup>2−</sup> → 2H<sub>2</sub>O + 4e<sup>−</sup>
:''Cathode reaction'': O<sub>2</sub> + 4e<sup>−</sup> → 2O<sup>2−</sup>
:''Overall cell reaction'': 2H<sub>2</sub> + O<sub>2</sub> → 2H<sub>2</sub>O

SOFC systems can run on fuels other than pure hydrogen gas. However, since hydrogen is necessary for the reactions listed above, the fuel selected must contain hydrogen atoms. For the fuel cell to operate, the fuel must be converted into pure hydrogen gas. SOFCs are capable of internally [[Fossil fuel reforming|reforming]] light hydrocarbons such as [[methane]] (natural gas),<ref name=uva20130213>{{cite web|title=Methane Fuel Cell Subgroup|url=http://artsandsciences.virginia.edu/cchf/research/fuelcells.html|publisher=University of Virginia|access-date=2014-02-13|date=2012|archive-date=22 February 2014|archive-url=https://web.archive.org/web/20140222181513/http://artsandsciences.virginia.edu/cchf/research/fuelcells.html|url-status=dead}}</ref> propane, and butane.<ref>{{cite journal|author1=A Kulkarni |author2=FT Ciacchi |author3=S Giddey |author4=C Munnings |author5=SPS Badwal |author6=JA Kimpton |author7=D Fini |title=Mixed ionic electronic conducting perovskite anode for direct carbon fuel cells|journal=International Journal of Hydrogen Energy|year=2012| volume=37|issue=24|pages=19092–19102| doi=10.1016/j.ijhydene.2012.09.141|bibcode=2012IJHE...3719092K }}</ref> These fuel cells are at an early stage of development.<ref>{{cite journal|author1=S. Giddey |author2=S.P.S. Badwal |author3=A. Kulkarni |author4=C. Munnings |title=A comprehensive review of direct carbon fuel cell technology|journal=Progress in Energy and Combustion Science| year=2012| volume=38|issue=3|pages=360–399|doi=10.1016/j.pecs.2012.01.003|bibcode=2012PECS...38..360G }}</ref>

Challenges exist in SOFC systems due to their high operating temperatures. One such challenge is the potential for carbon dust to build up on the anode, which slows down the internal reforming process. Research to address this "carbon coking" issue at the University of Pennsylvania has shown that the use of copper-based [[cermet]] (heat-resistant materials made of ceramic and metal) can reduce coking and the loss of performance.<ref>Hill, Michael. [http://www.ceramicindustry.com/Articles/Feature_Article/10637442bbac7010VgnVCM100000f932a8c0____ "Ceramic Energy: Material Trends in SOFC Systems"] {{Webarchive|url=https://web.archive.org/web/20110928023507/http://www.ceramicindustry.com/Articles/Feature_Article/10637442bbac7010VgnVCM100000f932a8c0____ |date=28 September 2011 }}. ''Ceramic Industry'', 1 September 2005.</ref> Another disadvantage of SOFC systems is the long start-up, making SOFCs less useful for mobile applications. Despite these disadvantages, a high operating temperature provides an advantage by removing the need for a precious metal catalyst like platinum, thereby reducing cost. Additionally, waste heat from SOFC systems may be captured and reused, increasing the theoretical overall efficiency to as high as 80–85%.<ref name=Types1/>

The high operating temperature is largely due to the physical properties of the YSZ electrolyte. As temperature decreases, so does the [[ionic conductivity (solid state)|ionic conductivity]] of YSZ. Therefore, to obtain the optimum performance of the fuel cell, a high operating temperature is required. According to their website, [[Ceres Power]], a UK SOFC fuel cell manufacturer, has developed a method of reducing the operating temperature of their SOFC system to 500–600 degrees Celsius. They replaced the commonly used YSZ electrolyte with a CGO (cerium gadolinium oxide) electrolyte. The lower operating temperature allows them to use stainless steel instead of ceramic as the cell substrate, which reduces cost and start-up time of the system.<ref>[http://www.cerespower.com/Technology/TheCeresCell/ "The Ceres Cell"] {{webarchive|url=https://web.archive.org/web/20131213064702/http://www.cerespower.com/Technology/TheCeresCell/ |date=13 December 2013 }}. ''Ceres Power website'', accessed 4 August 2011</ref>

====Molten-carbonate fuel cell====
{{Main|Molten carbonate fuel cell}}
[[Molten carbonate fuel cell]]s (MCFCs) require a high operating temperature, {{convert|650|°C|abbr=on|-1}}, similar to [[Solid oxide fuel cell|SOFCs]]. MCFCs use lithium potassium carbonate salt as an electrolyte, and this salt liquefies at high temperatures, allowing for the movement of charge within the cell – in this case, negative carbonate ions.<ref name=moltencarb>[http://www.fossil.energy.gov/programs/powersystems/fuelcells/fuelcells_moltencarb.html "Molten Carbonate Fuel Cell Technology"]. U.S. Department of Energy, accessed 9 August 2011</ref>

Like SOFCs, MCFCs are capable of converting fossil fuel to a hydrogen-rich gas in the anode, eliminating the need to produce hydrogen externally. The reforming process creates {{CO2}} emissions. MCFC-compatible fuels include natural gas, [[biogas]] and gas produced from coal. The hydrogen in the gas reacts with carbonate ions from the electrolyte to produce water, carbon dioxide, electrons and small amounts of other chemicals. The electrons travel through an external circuit, creating electricity, and return to the cathode. There, oxygen from the air and carbon dioxide recycled from the anode react with the electrons to form carbonate ions that replenish the electrolyte, completing the circuit.<ref name=moltencarb/> The chemical reactions for an MCFC system can be expressed as follows:<ref>[http://www.fctec.com/fctec_types_mcfc.asp "Molten Carbonate Fuel Cells (MCFC)"]. FCTec.com, accessed 9 August 2011 {{webarchive |url=https://web.archive.org/web/20120303125426/http://www.fctec.com/fctec_types_mcfc.asp |date=3 March 2012 }}</ref>

:''Anode reaction'': CO<sub>3</sub><sup>2−</sup> + H<sub>2</sub> → H<sub>2</sub>O + CO<sub>2</sub> + 2e<sup>−</sup>
:''Cathode reaction'': CO<sub>2</sub> + ½O<sub>2</sub> + 2e<sup>−</sup> → CO<sub>3</sub><sup>2−</sup>
:''Overall cell reaction'': H<sub>2</sub> + ½O<sub>2</sub> → H<sub>2</sub>O

As with SOFCs, MCFC disadvantages include slow start-up times because of their high operating temperature. This makes MCFC systems not suitable for mobile applications, and this technology will most likely be used for stationary fuel cell purposes. The main challenge of MCFC technology is the cells' short life span. The high-temperature and carbonate electrolyte lead to corrosion of the anode and cathode. These factors accelerate the degradation of MCFC components, decreasing the durability and cell life. Researchers are addressing this problem by exploring corrosion-resistant materials for components as well as fuel cell designs that may increase cell life without decreasing performance.<ref name=Types1/>

MCFCs hold several advantages over other fuel cell technologies, including their resistance to impurities. They are not prone to "carbon coking", which refers to carbon build-up on the anode that results in reduced performance by slowing down the internal fuel [[Fossil fuel reforming|reforming]] process. Therefore, carbon-rich fuels like gases made from coal are compatible with the system. The United States Department of Energy claims that coal, itself, might even be a fuel option in the future, assuming the system can be made resistant to impurities such as sulfur and particulates that result from converting coal into hydrogen.<ref name=Types1/> MCFCs also have relatively high efficiencies. They can reach a fuel-to-electricity efficiency of 50%, considerably higher than the 37–42% efficiency of a phosphoric acid fuel cell plant. Efficiencies can be as high as 65% when the fuel cell is paired with a turbine, and 85% if heat is captured and used in a [[cogeneration|combined heat and power]] (CHP) system.<ref name=moltencarb/>

FuelCell Energy, a Connecticut-based fuel cell manufacturer, develops and sells MCFC fuel cells. The company says that their MCFC products range from 300&nbsp;kW to 2.8&nbsp;MW systems that achieve 47% electrical efficiency and can utilize CHP technology to obtain higher overall efficiencies. One product, the DFC-ERG, is combined with a gas turbine and, according to the company, it achieves an electrical efficiency of 65%.<ref>[http://www.fuelcellenergy.com/products.php "Products"]. FuelCell Energy, accessed 9 August 2011 {{webarchive |url=https://archive.today/20130111041426/http://www.fuelcellenergy.com/products.php |date=11 January 2013 }}</ref>

===Electric storage fuel cell===
The electric storage fuel cell is a conventional battery chargeable by electric power input, using the conventional electro-chemical effect. However, the battery further includes hydrogen (and oxygen) inputs for alternatively charging the battery chemically.<ref>''{{US patent|8354,195}}''</ref>

===Biofuel cell===
{{Main article|Microbial fuel cell|Enzymatic biofuel cell}}
A biofuel cell converts chemical energy from biological substances into electrical energy using biological catalysts, such as enzymes or microorganisms. The process involves the oxidation of a fuel, like glucose, at the anode, releasing electrons and protons. The electrons travel through an external circuit to generate electrical current, while at the cathode, oxygen is typically reduced to water or hydrogen peroxide, completing the circuit.<ref>{{cite journal |last1=Huang |first1=Wengang |last2=Zulkifli |first2=Muhammad Yazid Bin |last3=Chai |first3=Milton |last4=Lin |first4=Rijia |last5=Wang |first5=Jingjing |last6=Chen |first6=Yuelei |last7=Chen |first7=Vicki |last8=Hou |first8=Jingwei |title=Recent advances in enzymatic biofuel cells enabled by innovative materials and techniques |journal=Exploration |date=August 2023 |volume=3 |issue=4 |doi=10.1002/EXP.20220145|pmid=37933234 |pmc=10624391 }}</ref> Applications include wastewater treatment and renewable energy production.<ref>{{cite journal |last1=Babanova |first1=Sofia |title=Biofuel Cells – Alternative Power Sources |journal=International Scientific Conference (FMNS2013), Blagoevgrad (Bulgaria) |date=2009 |url=https://inis.iaea.org/records/4884q-dkm61 |language=en}}</ref> Conductive polymers may be used to improve electron transfer between enzymes and electrodes.
<ref>{{cite journal |last1=Kižys |first1=Kasparas |last2=Zinovičius |first2=Antanas |last3=Jakštys |first3=Baltramiejus |last4=Bružaitė |first4=Ingrida |last5=Balčiūnas |first5=Evaldas |last6=Petrulevičienė |first6=Milda |last7=Ramanavičius |first7=Arūnas |last8=Morkvėnaitė-Vilkončienė |first8=Inga |title=Microbial Biofuel Cells: Fundamental Principles, Development and Recent Obstacles |journal=Biosensors |date=3 February 2023 |volume=13 |issue=2 |pages=221 |doi=10.3390/bios13020221|doi-access=free |pmid=36831987 |pmc=9954062 }}</ref>

The integration of nanomaterials, such as carbon nanotubes and metal nanoparticles, are used to enhance the performance of BFCs. These materials increase the surface area of electrodes and facilitate better electron transfer, resulting in higher power densities. Three-dimensional porous structures and graphene-based materials, have been used to improve conductivity and stability, and hybrid biofuel cells that combine BFCs with supercapacitors or secondary batteries are being developed to provide stable and continuous energy output.<ref>{{cite journal |last1=Kwon |first1=Cheong Hoon |last2=Ko |first2=Yongmin |last3=Shin |first3=Dongyeeb |last4=Kwon |first4=Minseong |last5=Park |first5=Jinho |last6=Bae |first6=Wan Ki |last7=Lee |first7=Seung Woo |last8=Cho |first8=Jinhan |title=High-power hybrid biofuel cells using layer-by-layer assembled glucose oxidase-coated metallic cotton fibers |journal=Nature Communications |date=26 October 2018 |volume=9 |issue=1 |page=4479 |doi=10.1038/s41467-018-06994-5|pmid=30367069 |pmc=6203850 |bibcode=2018NatCo...9.4479K }}</ref> BFCs are being explored as power sources for implantable devices like pacemakers and biosensors.to potentially eliminate the need for traditional batteries, and fiber-type EBFCs show potential in implantable applications.<ref>{{cite journal |last1=Cai |first1=Jingsheng |last2=Shen |first2=Fei |last3=Zhao |first3=Jianqing |last4=Xiao |first4=Xinxin |title=Enzymatic biofuel cell: A potential power source for self-sustained smart textiles |journal=iScience |date=February 2024 |volume=27 |issue=2 |pages=108998 |doi=10.1016/j.isci.2024.108998|pmid=38333690 |pmc=10850773 |bibcode=2024iSci...27j8998C }}</ref> The power density of BFCs, however, is generally lower than that of conventional energy sources, the stability of enzymes and microorganisms over extended periods is another concern, and scalability and commercial viability also pose hurdles.<ref>{{cite journal |last1=Wang |first1=Linlin |last2=Wu |first2=Xiaoge |last3=Su |first3=B. S. Qi-wen |last4=Song |first4=Rongbin |last5=Zhang |first5=Jian-Rong |last6=Zhu |first6=Jun-Jie |title=Enzymatic Biofuel Cell: Opportunities and Intrinsic Challenges in Futuristic Applications |journal=Advanced Energy and Sustainability Research |date=August 2021 |volume=2 |issue=8 |doi=10.1002/aesr.202100031|doi-access=free |bibcode=2021AdESR...200031W }}</ref>

===Comparison of fuel cell types===
{| class="wikitable sortable plainrowheaders" style="text-align:center;"
 ! rowspan=2 | Fuel cell name
 ! rowspan=2 | Electrolyte
 ! rowspan=2 | Qualified [[electric power|power]] (W)
 ! rowspan=2 | Working temperature (°C)
 ! colspan=2 | [[Electrical efficiency|Efficiency]]
 ! rowspan=2 | Status
 ! rowspan=2 | Cost (USD/W)
|-
 ! Cell
 ! System
|-
 ! scope="row" |Electro-galvanic fuel cell
 |Aqueous alkaline solution<!--e.g., potassium hydroxide-->
 |
 |{{Good|{{Sort|39|< 40}}}}
 | ||
 |{{Partial|Commercial / Research}}
 |3-7
|-
 ! scope="row" |[[Formic acid fuel cell|Direct formic acid fuel cell]] (DFAFC)
 |Polymer membrane (ionomer)
 |{{Okay|{{Sort|49|< 50 W}}}}
 |{{Good|{{Sort|39|< 40}}}}
 | ||
 |{{Okay|Commercial / Research}}
 |10-20
|-
 ! scope="row" |[[Alkaline fuel cell]]
 |Aqueous alkaline solution <!--e.g., potassium hydroxide-->
 |{{Okay|{{Sort|10000|10–200 kW}}}}
 |{{Good|{{Sort|79|< 80}}}}
 |{{Good|{{Sort|65%|60–70%}}}} || {{Good|62%}}
 |{{Partial|Commercial / Research}}
 |50-100
|-
 ! scope="row" |[[Proton-exchange membrane fuel cell]]
 |Polymer membrane (ionomer) <!--e.g. [[Nafion]] or [[Polybenzimidazole fiber]]-->
 |{{Good|{{Sort|100|1 W – 500 kW}}}}
 |{{Okay|{{Sort|125|50–100 (Nafion)<ref>{{cite web |url=http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/pdfs/fc_comparison_chart.pdf |title=Fuel Cell Comparison Chart |access-date=2013-02-10 |url-status=dead |archive-url=https://web.archive.org/web/20130301120203/http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/pdfs/fc_comparison_chart.pdf |archive-date=1 March 2013}}</ref><br />120–200 (PBI)}}}}<ref>{{Cite journal| title = Thermal management strategies for a 1 kWe stack of a high temperature proton exchange membrane fuel cell | journal=Applied Thermal Engineering | date = 15 December 2012 | doi = 10.1016/j.applthermaleng.2012.04.041 | author = E. Harikishan Reddy | volume = 48| pages = 465–475 | last2 = Jayanti | first2 = S| bibcode=2012AppTE..48..465H }}</ref>
 |{{Okay|{{Sort|60%|50–70%}}}} || {{Okay|{{Sort|40%|30–50%<ref name="BadwalGiddey"/>}}}}
 |{{Partial|Commercial / Research}}
 | 50–100
|-
 !scope=row|[[Metal hydride fuel cell]]
 |[[Aqueous]] [[alkali]]ne solution
 |
 |{{Okay|{{Sort|0|> −20<br />(50% P<sub>peak</sub> @ 0&nbsp;°C)}}}}
 |  || 
 |{{Partial|[[Research and development|Commercial / Research]]}}
 |30-200
|-
 !scope=row|[[Zinc–air battery]]
 |Aqueous alkaline solution<!--e.g., potassium hydroxide-->
 |
 |{{Good|{{Sort|39|< 40}}}} ||
 |
 |{{yes2}}[[Mass production]]
 |150-300
|-
 ! scope="row" |[[Direct carbon fuel cell]]
 |Several different
 |
 |{{Bad|{{Sort|775|700–850}}}}
 |{{Good|80%}} || {{Good|70%}}
 |{{Partial|Commercial / Research}}
 |18
|-
 ! scope="row" |[[Direct borohydride fuel cell]]
 |Aqueous alkaline solution<!--e.g., potassium hydroxide-->
 |
 |{{Good|70}}
 | ||
 |{{yes2|Commercial}}
 |400-450
|-
 !scope=row|[[Microbial fuel cell]]
 |Polymer membrane or [[humic acid]]
 |
 |{{Good|{{Sort|39|< 40}}}}
 |  || 
 |{{Beta|Research}}
 |10-50
|-
 !scope=row|Upflow microbial fuel cell (UMFC)
 |
 |
 |{{Good|{{Sort|39|< 40}}}}
 |  || 
 |{{Beta|Research}}
 |1-5
|-
 !scope=row|[[Regenerative fuel cell]]
 |Polymer membrane ([[ionomer]])
 |
 |{{Good|{{Sort|49|< 50}}}}
 |  || 
 |{{Partial|Commercial / Research}}
 |200-300
|-
 !scope=row|[[Direct methanol fuel cell]]
 |Polymer membrane (ionomer)
 |{{Okay|{{Sort|0.1|100 mW – 1 kW}}}}
 |{{Okay|{{Sort|105|90–120}}}}
 |{{Bad|{{Sort|25%|20–30%}}}} || {{Bad|{{Sort|15%|10–25%<ref name="BadwalGiddey">{{cite journal|last1=Badwal|first1=Sukhvinder P. S.|last2=Giddey|first2=Sarbjit S.|last3=Munnings|first3=Christopher|last4=Bhatt|first4=Anand I.|last5=Hollenkamp|first5=Anthony F.|title=Emerging electrochemical energy conversion and storage technologies|journal=Frontiers in Chemistry|date=24 September 2014|volume=2|pages=79|doi=10.3389/fchem.2014.00079|pmid=25309898|pmc=4174133|bibcode=2014FrCh....2...79B|doi-access=free}}</ref>}}}}
 |{{Partial|Commercial / Research}}
 |125
|-
 !scope=row|[[Reformed methanol fuel cell]]
 |Polymer membrane (ionomer)
 |{{Okay|{{Sort|5|5 W – 100 kW}}}}
 |{{Okay|{{Sort|200|250–300 (reformer)<br />125–200 (PBI)}}}}
 |{{Bad|{{Sort|55%|50–60%}}}} || {{Bad|{{Sort|33%|25–40%}}}}
 |{{Partial|Commercial / Research}}
 |8.50
|-
 !scope=row|[[Direct-ethanol fuel cell]]
 |Polymer membrane (ionomer)
 |{{Okay|{{Sort|0|< 140 mW/cm²}}}}
 |{{Okay|{{Sort|26|> 25<br />? 90–120}}}}
 |  || 
 |{{Beta|Research}}
 |12
|-
 !scope=row|[[Flow Battery#Classes of flow batteries|Redox fuel cell]]{{Broken anchor|date=2024-06-19|bot=User:Cewbot/log/20201008/configuration|target_link=Flow Battery#Classes of flow batteries|reason= The anchor (Classes of flow batteries) [[Special:Diff/655911549|has been deleted]].}} (RFC)
 |Liquid electrolytes with [[redox]] shuttle and polymer membrane (ionomer)
 |{{Good|{{Sort|1000|1 kW – 10 MW}}}}
 |  || 
 |
 |{{Beta|Research}}
 |12.50
|-
 !scope=row|[[Phosphoric acid fuel cell]]
 |Molten [[phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>)
 |{{Good|{{Sort|999999|< 10 MW}}}}
 |{{Bad|{{Sort|175|150–200}}}}
 |{{Okay|55%}} || {{Okay|{{Sort|40%|40%<ref name="BadwalGiddey"/><br />Co-gen: 90%}}}}
 |{{Partial|Commercial / Research}}
 | 4.00–4.50
|-
 !scope=row|[[Solid acid fuel cell]]
 |H<sup>+</sup>-conducting oxyanion salt (solid acid)
 |{{Okay|{{Sort|505|10 W – 1 kW}}}}
 |{{Bad|{{Sort|250|200–300}}}}
 |{{Okay|{{Sort|58|55–60%}}}}
 |{{Okay|{{Sort|42|40–45%}}}}
 |{{Partial|Commercial / Research}}
 |15
|-
 !scope=row|[[Molten carbonate fuel cell]]
 |Molten alkaline [[carbonate]] <!--e.g., [[sodium bicarbonate]] NaHCO<sub>3</sub>-->
 |{{Good|{{Sort|100000000|100 MW}}}}
 |{{Bad|{{Sort|625|600–650}}}}
 |{{Okay|55%}} || {{Okay|45–55%<ref name="BadwalGiddey"/>}}
 |{{Partial|Commercial / Research}}
 |1000
|-
 !scope=row|[[Tubular solid oxide fuel cell]] (TSOFC)
 |O<sup>2−</sup>-conducting ceramic [[oxide]] <!--e.g., [[zirconium dioxide]], ZrO<sub>2</sub>-->
 |{{Good|{{Sort|99999999|< 100 MW}}}}
 |{{Bad|{{Sort|975|850–1100}}}}
 |{{Good|{{Sort|63%|60–65%}}}} || {{Good|{{Sort|57%|55–60%}}}}
 |{{Partial|Commercial / Research}}
 |3.50
|-
 !scope=row|[[Protonic ceramic fuel cell]]
 |H<sup>+</sup>-conducting ceramic oxide
 |
 |{{Bad|700}}
 |  || 
 |{{Beta|Research}}
 |80
|-
 !scope=row|Planar [[solid oxide fuel cell]]
 |O<sup>2−</sup>-conducting ceramic [[oxide]] <!--e.g., [[zirconium dioxide]], ZrO<sub>2</sub> Lanthanum Nickel Oxide La<sub>2</sub>XO<sub>4</sub>, X = Ni, Co, Cu-->
 |{{Good|{{Sort|99999999|< 100 MW}}}}
 |{{Bad|{{Sort|975|500–1100}}}}
 |{{Good|{{Sort|63%|60–65%}}}} || {{Good|{{Sort|57%|55–60%<ref name="BadwalGiddey"/>}}}}
 |{{Partial|Commercial / Research}}
 |800
|-
 !scope=row|[[Enzymatic Biofuel Cells|Enzymatic biofuel cells]]
 |Any that will not denature the enzyme <!--usually aqueous [[buffer]]-->
 |
 |{{Good|{{Sort|39|< 40}}}}
 |  || 
 |{{Beta|Research}}
 |10
|-
 !scope=row|[[Magnesium-air fuel cell]]
 |Salt water
 |
 |{{Good|{{Sort|−20|−20 to 55}}}}
 |{{Good|{{Sort|90%|90%}}}}||
 |{{Partial|Commercial / Research}}
 |15
|}

Glossary of terms in table:
{{further|glossary of fuel cell terms}}
; [[Anode]]: The electrode at which oxidation (a loss of electrons) takes place. For fuel cells and other galvanic cells, the anode is the negative terminal; for electrolytic cells (where electrolysis occurs), the anode is the positive terminal.<ref name="fctpglossary"/>
; [[Aqueous solution]]<ref>[http://www.merriam-webster.com/dictionary/aqueous "Aqueous Solution"]. Merriam-Webster Free Online Dictionary</ref>{{defn|Of, relating to, or resembling water}}{{defn|Made from, with, or by water.}}
; [[Catalyst]]: A chemical substance that increases the rate of a reaction without being consumed; after the reaction, it can potentially be recovered from the reaction mixture and is chemically unchanged. The catalyst lowers the activation energy required, allowing the reaction to proceed more quickly or at a lower temperature. In a fuel cell, the catalyst facilitates the reaction of oxygen and hydrogen. It is usually made of platinum powder very thinly coated onto carbon paper or cloth. The catalyst is rough and porous so the maximum surface area of the platinum can be exposed to the hydrogen or oxygen. The platinum-coated side of the catalyst faces the membrane in the fuel cell.<ref name="fctpglossary"/>
; [[Cathode]]: The electrode at which reduction (a gain of electrons) occurs. For fuel cells and other galvanic cells, the cathode is the positive terminal; for electrolytic cells (where electrolysis occurs), the cathode is the negative terminal.<ref name="fctpglossary"/>
; [[Electrolyte]]: A substance that conducts charged ions from one electrode to the other in a fuel cell, battery, or electrolyzer.<ref name="fctpglossary">[http://www1.eere.energy.gov/hydrogenandfuelcells/glossary.html#c "Fuel Cell Technologies Program: Glossary"] {{webarchive|url=https://web.archive.org/web/20140223003718/http://www1.eere.energy.gov/hydrogenandfuelcells/glossary.html |date=23 February 2014 }}. Department of Energy Energy Efficiency and Renewable Energy Fuel Cell Technologies Program. 7 July 2011. Accessed 3 August 2011.</ref>
; Fuel cell stack: Individual fuel cells connected in a series. Fuel cells are stacked to increase voltage.<ref name="fctpglossary"/>
; Matrix: something within or from which something else originates, develops, or takes form.<ref>[http://www.merriam-webster.com/dictionary/matrix "Matrix"]. Merriam-Webster Free Online Dictionary</ref>
; [[Membrane (selective barrier)|Membrane]]: The separating layer in a fuel cell that acts as electrolyte (an ion-exchanger) as well as a barrier film separating the gases in the anode and cathode compartments of the fuel cell.<ref name="fctpglossary"/>
; [[Molten carbonate fuel cell]] (MCFC): A type of fuel cell that contains a molten carbonate electrolyte. Carbonate ions (CO<sub>3</sub><sup>2−</sup>) are transported from the cathode to the anode. Operating temperatures are typically near 650&nbsp;°C.<ref name="fctpglossary"/>
; [[Phosphoric acid fuel cell]] (PAFC): A type of fuel cell in which the electrolyte consists of concentrated phosphoric acid (H<sub>3</sub>PO<sub>4</sub>). Protons (H+) are transported from the anode to the cathode. The operating temperature range is generally 160–220&nbsp;°C.<ref name="fctpglossary"/>
; [[Proton-exchange membrane fuel cell]] (PEM): A fuel cell incorporating a solid polymer membrane used as its electrolyte. Protons (H+) are transported from the anode to the cathode. The operating temperature range is generally 60–100&nbsp;°C for Low Temperature Proton-exchange membrane fuel cell (LT-PEMFC).<ref name="fctpglossary"/> PEM fuel cell with operating temperature of 120-200&nbsp;°C is called [[High Temperature Proton Exchange Membrane fuel cell|High Temperature Proton-exchange membrane fuel cell]] (HT-PEMFC).<ref>{{Cite book|last=Araya|first=Samuel Simon|url=https://www.worldcat.org/oclc/857436369|title=High temperature PEM fuel cells - degradation & durability : dissertation submitted to the Faculty of Engineering and Science at Aalborg University in partial fulfillment of the requirements for the degree of Doctor of Philosophy|date=2012|publisher=Aalborg University, Department of Energy Technology|isbn=978-87-92846-14-3|location=Aalborg|oclc=857436369}}</ref> 
; [[Solid oxide fuel cell]] (SOFC): A type of fuel cell in which the electrolyte is a solid, nonporous metal oxide, typically zirconium oxide (ZrO<sub>2</sub>) treated with Y<sub>2</sub>O<sub>3</sub>, and O<sup>2−</sup> is transported from the cathode to the anode. Any CO in the reformate gas is oxidized to CO<sub>2</sub> at the anode. Temperatures of operation are typically 800–1,000&nbsp;°C.<ref name="fctpglossary"/>
; [[Solution (chemistry)|Solution]]<ref>[http://www.merriam-webster.com/dictionary/solution "Solution"]. Merriam-Webster Free Online Dictionary</ref>{{defn|An act or the process by which a solid, liquid, or gaseous substance is homogeneously mixed with a liquid or sometimes a gas or solid.}}{{defn|A homogeneous mixture formed by this process; especially : a single-phase liquid system.}}{{defn|The condition of being dissolved.}}