Editing Fuel cell (section)

====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>