Editing Solid oxide fuel cell (section)

==Operation==
[[File:SOFC-en.svg|thumb|upright=1.5|right|Cross section of three ceramic layers of a tubular SOFC. From inner to outer: porous cathode, dense electrolyte, porous anode]]
A solid oxide fuel cell is made up of four layers, three of which are [[ceramic]]s (hence the name). A single cell consisting of these four layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then connected in series to form what most people refer to as an "SOFC stack". The ceramics used in SOFCs do not become electrically and [[ion]]ically active until they reach very high temperature and as a consequence, the stacks have to run at temperatures ranging from 500 to 1,000&nbsp;°C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again.

===Balance of plant===
Most of the downtime of a SOFC is caused by the mechanical [[balance of plant]] (from components like the [[air preheater]], [[prereformer]], [[afterburner]], [[water heat exchanger]], and [[anode tail gas oxidizer]]) and the electrical balance of plant (including the [[power electronics]], [[hydrogen sulfide sensor]] and fans). By using internal reforming (converting methane into hydrogen internally), lower cooling requirements are needed, helping decrease the complexity and costs of the [[balance of plant]].<ref name="e-collection.ethbib.ethz.ch"/>

===Anode===
The ceramic [[anode]] layer must be very porous to allow the fuel to flow towards the electrolyte. Consequently, granular matter is often selected for anode fabrication procedures.<ref>{{cite journal| last1=Ott| first1=J | last2=Gan | first2=Y |last3=McMeeking |first3=R | last4=Kamlah | first4=M | title= A micromechanical model for effective conductivity in granular electrode structures | journal= Acta Mechanica Sinica | year= 2013| volume=29| issue=5| pages=682–698 |url=http://www.heterofoam.com/UserFiles/hetfoam/Documents/Ott%20et%20al%20Granular%20Electrodes%20Acta%20Mechanica%20Sinica%202013.pdf | doi=10.1007/s10409-013-0070-x| bibcode=2013AcMSn..29..682O | s2cid=51915676 }}</ref> Like the cathode, it must conduct electrons, with ionic conductivity a definite asset. The anode is commonly the thickest and strongest layer in each individual cell, because it has the smallest polarization losses, and is often the layer that provides the mechanical support. [[Electrochemistry|Electrochemically]] speaking, the anode's job is to use the oxygen ions that diffuse through the electrolyte to oxidize the hydrogen [[fuel]].
The [[redox|oxidation reaction]] between the oxygen ions and the hydrogen produces heat as well as water and electricity.
If the fuel is a light hydrocarbon, for example, methane, another function of the anode is to act as a catalyst for steam reforming the fuel into hydrogen. This provides another operational benefit to the fuel cell stack because the reforming reaction is endothermic, which cools the stack internally. The most common material used is a [[cermet]] made up of [[nickel]] mixed with the ceramic material that is used for the electrolyte in that particular cell, typically YSZ (yttria stabilized zirconia). One version of constructing the anode is to use [[tape casting]], after the slurry is prepared with resonant acoustic mixing. This low-impact mixing allows for the effective combination of NiO-YSZ slurries in about 30 minutes, more than 140 times faster than conventional [[ball mill]]ing (72 hours).<ref name="z599">{{cite journal | last1=Park | first1=Jeong Hwa | last2=Bae | first2=Kyung Taek | last3=Kim | first3=Kyeong Joon | last4=Joh | first4=Dong Woo | last5=Kim | first5=Doyeub | last6=Myung | first6=Jae-ha | last7=Lee | first7=Kang Taek | title=Ultra-fast fabrication of tape-cast anode supports for solid oxide fuel cells via resonant acoustic mixing technology | journal=Ceramics International | publisher=Elsevier BV | volume=45 | issue=9 | year=2019 | doi=10.1016/j.ceramint.2019.03.119 | pages=12154–12161}}</ref> The [[nanomaterial-based catalyst]]s help stop the grain growth of nickel. Larger grains of nickel would reduce the contact area that ions can be conducted through, which would lower the cell's efficiency. [[Perovskite (structure)|Perovskite materials]] (mixed ionic/electronic conducting ceramics) have been shown to produce a power density of 0.6 W/cm2 at 0.7 V at 800&nbsp;°C which is possible because they have the ability to overcome a larger [[activation energy]].<ref>{{Cite journal|doi = 10.1149/2.1321608jes|title = Hydrogen Oxidation Mechanisms on Perovskite Solid Oxide Fuel Cell Anodes|year = 2016|last1 = Zhu|first1 = Tenglong|last2 = Fowler|first2 = Daniel E.|author3-link=Kenneth Poeppelmeier |last3 = Poeppelmeier|first3 = Kenneth R.|last4 = Han|first4 = Minfang|last5 = Barnett|first5 = Scott A.|journal = Journal of the Electrochemical Society|volume = 163|issue = 8|pages = F952–F961}}</ref>

'''Chemical  Reaction:'''

H<sub>2</sub> + O<sup>2-</sup> —> H<sub>2</sub>O + 2e<sup>-</sup>

However, there are a few disadvantages associated with YSZ as anode material. Ni coarsening, carbon deposition, reduction-oxidation instability, and sulfur poisoning are the main obstacles limiting the long-term stability of Ni-YSZ. Ni coarsening refers to the evolution of Ni particles in doped in YSZ grows larger in grain size, which decreases the surface area for the catalytic reaction. Carbon deposition occurs when carbon atoms, formed by hydrocarbon pyrolysis or CO disproportionation, deposit on the Ni catalytic surface.<ref>{{Citation|last1=Bao|first1=Zhenghong|title=Chapter Two - Catalytic Conversion of Biogas to Syngas via Dry Reforming Process|date=1 January 2018|url=http://www.sciencedirect.com/science/article/pii/S2468012518300026|work=Advances in Bioenergy|volume=3|pages=43–76|editor-last=Li|editor-first=Yebo|publisher=Elsevier|language=en|access-date=14 November 2020|last2=Yu|first2=Fei|doi=10.1016/bs.aibe.2018.02.002 |editor2-last=Ge|editor2-first=Xumeng|url-access=subscription}}</ref> Carbon deposition becomes important especially when hydrocarbon fuels are used, i.e. methane, syngas. The high operating temperature of SOFC and the oxidizing environment facilitate the oxidation of Ni catalyst through reaction Ni + {{frac|1|2}} O<sub>2</sub> = NiO. The oxidation reaction of Ni reduces the electrocatalytic activity and conductivity. Moreover, the density difference between Ni and NiO causes volume change on the anode surface, which could potentially lead to mechanical failure. Sulfur poisoning arises when fuel such as natural gas, gasoline, or diesel is used. Again, due to the high affinity between sulfur compounds (H<sub>2</sub>S, (CH<sub>3</sub>)<sub>2</sub>S) and the metal catalyst, even the smallest impurities of sulfur compounds in the feed stream could deactivate the Ni catalyst on the YSZ surface.<ref>{{Cite book|last=Rostrup-Nielsen|first=J. R.|title=Progress in Catalyst Deactivation |chapter=Sulfur Poisoning |year=1982|editor-last=Figueiredo|editor-first=José Luís|chapter-url=https://link.springer.com/chapter/10.1007/978-94-009-7597-2_11|series=NATO Advanced Study Institutes Series|language=en|location=Dordrecht|publisher=Springer Netherlands|pages=209–227|doi=10.1007/978-94-009-7597-2_11|isbn=978-94-009-7597-2}}</ref>

Current research is focused on reducing or replacing Ni content in the anode to improve long-term performance. The modified Ni-YSZ containing other materials including CeO<sub>2</sub>, Y<sub>2</sub>O<sub>3</sub>, La<sub>2</sub>O<sub>3</sub>, MgO, TiO<sub>2</sub>, Ru, Co, etc. are invented to resist sulfur poisoning, but the improvement is limited due to the rapid initial degradation.<ref>{{Cite journal|last1=Sasaki|first1=K.|last2=Susuki|first2=K.|year=2006|title=H2S Poisoning of Solid Oxide Fuel Cells|url=https://iopscience.iop.org/article/10.1149/1.2336075/meta|journal=Journal of the Electrochemical Society|volume=153|issue=11|pages=11|doi=10.1149/1.2336075|bibcode=2006JElS..153A2023S|url-access=subscription}}</ref> Copper-based cerement anode is considered as a solution to carbon deposition because it is inert to carbon and stable under typical SOFC oxygen partial pressures (pO<sub>2</sub>). Cu-Co bimetallic anodes in particular show a great resistivity of carbon deposition after the exposure to pure CH<sub>4</sub> at 800C.<ref name=":0">{{Cite journal|last1=Ge|first1=Xiao-Ming|last2=Chan|first2=Siew-Hwa|last3=Liu|first3=Qing-Lin|last4=Sun|first4=Qiang|year=2012|title=Solid Oxide Fuel Cell Anode Materials for Direct Hydrocarbon Utilization|url=https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.201200342|journal=Advanced Energy Materials|language=en|volume=2|issue=10|pages=1156–1181|doi=10.1002/aenm.201200342|bibcode=2012AdEnM...2.1156G |s2cid=95175720 |issn=1614-6840|url-access=subscription}}</ref> And Cu-CeO<sub>2</sub>-YSZ exhibits a higher electrochemical oxidation rate over Ni-YSZ when running on CO and syngas, and can achieve even higher performance using CO than H<sub>2</sub>, after adding a cobalt co-catalyst.<ref>{{Cite journal|last1=Costa-Nunes|first1=Olga|last2=Gorte|first2=Raymond J.|last3=Vohs|first3=John M.|date=1 March 2005|title=Comparison of the performance of Cu–CeO2–YSZ and Ni–YSZ composite SOFC anodes with H2, CO, and syngas|url=http://www.sciencedirect.com/science/article/pii/S037877530401064X|journal=Journal of Power Sources|language=en|volume=141|issue=2|pages=241–249|doi=10.1016/j.jpowsour.2004.09.022|bibcode=2005JPS...141..241C|issn=0378-7753}}</ref> Oxide anodes including zirconia-based fluorite and perovskites are also used to replace Ni-ceramic anodes for carbon resistance. Chromite i.e. La<sub>0.8</sub>Sr<sub>0.2</sub>Cr<sub>0.5</sub>Mn<sub>0.5</sub>O<sub>3</sub> (LSCM) is used as anodes and exhibited comparable performance against Ni–YSZ cermet anodes. LSCM is further improved by impregnating Cu and sputtering Pt as the current collector.<ref name=":0" />

===Electrolyte===
The electrolyte is a dense layer of ceramic that conducts oxygen ions. Its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. The high operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. However, as the operating temperature approaches the lower limit for SOFCs at around {{nowrap|600 °C,}} the electrolyte begins to have large ionic transport resistances and affect the performance. Popular electrolyte materials include [[yttria-stabilized zirconia]] (YSZ) (often the 8% form 8YSZ), scandia stabilized zirconia ([[ScSZ]]) (usually 9&nbsp;mol%&nbsp;Sc<sub>2</sub>O<sub>3</sub> – 9ScSZ) and [[gadolinium doped ceria]] (GDC).<ref>{{cite journal |author1=Nigel Sammes |author2=Alevtina Smirnova |author3=Oleksandr Vasylyev | title=Fuel Cell Technologies: State and Perspectives | journal=NATO Science Series, Mathematics, Physics and Chemistry | year=2005 | volume=202 | pages=19–34 | doi=10.1007/1-4020-3498-9_3|bibcode=2005fcts.conf.....S }}</ref> The electrolyte material has crucial influence on the cell performances.<ref name="elmater">{{cite journal | doi=10.1038/35104620 | author=Steele, B.C.H., Heinzel, A. | title=Materials for fuel-cell technologies | journal=Nature | year=2001 | volume=414 | issue=15 November | pages=345–352 | pmid=11713541| bibcode=2001Natur.414..345S | s2cid=4405856 }}</ref> Detrimental reactions between YSZ electrolytes and modern cathodes such as [[lanthanum strontium cobalt ferrite]] (LSCF) have been found, and can be prevented by thin (<100&nbsp;nm) [[ceria]] diffusion barriers.<ref>{{cite journal |author1=Mohan Menon |author2=Kent Kammer | title= Processing of Ce<sub>1-x</sub>Gd<sub>x</sub>O<sub>2-δ</sub> (GDC) Thin Films from Precursors for Application in Solid Oxide Fuel Cells| journal= Advanced Materials Research| year=2007 | volume=15–17 | pages=293–298 | doi=10.4028/www.scientific.net/AMR.15-17.293|s2cid=98044813 |display-authors=etal}}</ref>

If the conductivity for oxygen ions in SOFC can remain high even at lower temperatures (current target in research ~500&nbsp;°C), material choices for SOFC will broaden and many existing problems can potentially be solved. Certain processing techniques such as thin film deposition<ref name="Charpentier2000">{{cite journal|last1=Charpentier|first1=P|title=Preparation of thin film SOFCs working at reduced temperature|journal=Solid State Ionics|volume=135|issue=1–4|year=2000|pages=373–380|issn=0167-2738|doi=10.1016/S0167-2738(00)00472-0|s2cid=95598314}}</ref> can help solve this problem with existing materials by:

* reducing the traveling distance of oxygen ions and electrolyte resistance as resistance is proportional to conductor length;
* producing grain structures that are less resistive such as columnar grain structure;
* controlling the microstructural nano-crystalline fine grains to achieve "fine-tuning" of electrical properties;
* building composite possessing large interfacial areas as interfaces have been shown to have extraordinary electrical properties.

===Cathode===
The [[cathode]], or air [[electrode]], is a thin porous layer on the electrolyte where oxygen reduction takes place. The overall reaction is written in [[Kröger-Vink Notation]] as follows:

:<math> \frac{1}{2}\mathrm{O_2(g)} + 2\mathrm{e'} + {V}^{\bullet\bullet}_o \longrightarrow {O}^{\times}_o </math>
<!-- was: 1/2O<sub>2</sub>(g) + 2e' + V<sub>o</sub><sup>**</sup> -> O<sub>o</sub><sup>x</sup> -->

Cathode materials must be, at a minimum, electrically conductive. Currently, [[lanthanum strontium manganite]] (LSM) is the cathode material of choice for commercial use because of its compatibility with doped zirconia electrolytes. Mechanically, it has a similar coefficient of thermal expansion to YSZ and thus limits stress buildup because of CTE mismatch. Also, LSM has low levels of chemical reactivity with YSZ which extends the lifetime of the materials. Unfortunately, LSM is a poor ionic conductor, and so the electrochemically active reaction is limited to the [[triple phase boundary]] (TPB) where the electrolyte, air and electrode meet. LSM works well as a cathode at high temperatures, but its performance quickly falls as the operating temperature is lowered below 800&nbsp;°C. In order to increase the reaction zone beyond the TPB, a potential cathode material must be able to conduct both electrons and oxygen ions. Composite cathodes consisting of LSM YSZ have been used to increase this triple phase boundary length. Mixed ionic/electronic conducting (MIEC) ceramics, such as perovskite [[LSCF]], are also being researched for use in intermediate temperature SOFCs as they are more active and can make up for the increase in the activation energy of the reaction.<ref>{{Cite journal |last1=Shen |first1=F. |last2=Lu |first2=K. |date=August 2018 |title=Comparison of Different Perovskite Cathodes in Solid Oxide Fuel Cells |url=https://onlinelibrary.wiley.com/doi/10.1002/fuce.201800044 |journal=Fuel Cells |language=en |volume=18 |issue=4 |pages=457–465 |doi=10.1002/fuce.201800044 |s2cid=104669264 |issn=1615-6846|url-access=subscription }}</ref>

===Interconnect===
The interconnect can be either a metallic or ceramic layer that sits between each individual cell. Its purpose is to connect each cell in series, so that the electricity each cell generates can be combined. Because the interconnect is exposed to both the oxidizing and reducing side of the cell at high temperatures, it must be extremely stable. For this reason, ceramics have been more successful in the long term than metals as interconnect materials. However, these ceramic interconnect materials are very expensive when compared to metals. Nickel- and steel-based alloys are becoming more promising as lower temperature (600–800&nbsp;°C) SOFCs are developed. The material of choice for an interconnect in contact with Y8SZ is a metallic 95Cr-5Fe alloy. Ceramic-metal composites called "cermet" are also under consideration, as they have demonstrated thermal stability at high temperatures and excellent electrical conductivity.