Editing Solid oxide fuel cell (section)

===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" />