Editing Fuel cell (section)

===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>