Editing Solid oxide fuel cell (section)

==Research==
Research is going now in the direction of lower-temperature SOFCs (600&nbsp;°C). Low temperature systems can reduce costs by reducing insulation, materials, start-up and degradation-related costs. With higher operating temperatures, the temperature gradient increases the severity of thermal stresses, which affects materials cost and life of the system.<ref>{{cite book|last1=Ishihara|first1=Tatsumi|title=Perovskite Oxide for Solid Oxide Fuel Cells|url=https://archive.org/details/perovskiteoxidef00ishi|url-access=limited|date=2009|publisher=Springer|isbn=978-0-387-77708-5|page=[https://archive.org/details/perovskiteoxidef00ishi/page/n34 19]}}</ref> An intermediate temperature system (650-800&nbsp;°C) would enable the use of cheaper metallic materials with better mechanical properties and [[thermal conductivity]]. New developments in nano-scale electrolyte structures have been shown to bring down operating temperatures to around 350&nbsp;°C, which would enable the use of even cheaper steel and [[elastomer]]ic/[[polymer]]ic components.<ref name="ReferenceA">{{cite journal|last1=Wachsman|first1=Eric|last2=Lee|first2=Kang|title=Lowering the Temperature of Solid Oxide Fuel Cells|journal=Science|date=18 November 2011|volume=334|issue=6058|pages=935–9|doi=10.1126/science.1204090|pmid=22096189|bibcode=2011Sci...334..935W|s2cid=206533328}}</ref>

Lowering operating temperatures has the added benefit of increased efficiency. Theoretical fuel cell efficiency increases with decreasing temperature. For example, the efficiency of a SOFC using CO as fuel increases from 63% to 81% when decreasing the system temperature from 900&nbsp;°C to 350&nbsp;°C.<ref name="ReferenceA"/>

Research is also under way to improve the fuel flexibility of SOFCs. While stable operation has been achieved on a variety of hydrocarbon fuels, these cells typically rely on external fuel processing. In the case of [[natural gas]], the fuel is either externally or internally reformed and the [[sulfur]] compounds are removed. These processes add to the cost and complexity of SOFC systems. Work is under way at a number of institutions to improve the stability of anode materials for hydrocarbon oxidation and, therefore, relax the requirements for fuel processing and decrease SOFC balance of plant costs.

Research is also going on in reducing start-up time to be able to implement SOFCs in mobile applications.<ref>{{cite journal | last=Spivey | first=B. | title=Dynamic modeling, simulation, and MIMO predictive control of a tubular solid oxide fuel cell | journal=Journal of Process Control | year=2012 | doi=10.1016/j.jprocont.2012.01.015 | volume=22 | issue=8 | pages=1502–1520}}</ref> This can be partially achieved by lowering operating temperatures, which is the case for [[proton-exchange membrane fuel cell]] (PEMFCs).<ref>{{cite web|title=Fuel Cell Comparison|url=http://www.nedstack.com/technology/fuel-cell-comparison|website=Nedstack|access-date=6 November 2016}}</ref> Due to their fuel flexibility, they may run on partially reformed [[Diesel fuel|diesel]], and this makes SOFCs interesting as auxiliary power units (APU) in refrigerated trucks.

Specifically, [[Delphi Corporation|Delphi Automotive Systems]] are developing an SOFC that will power auxiliary units in automobiles and tractor-trailers, while [[BMW]] has recently stopped a similar project. A high-temperature SOFC will generate all of the needed electricity to allow the engine to be smaller and more efficient. The SOFC would run on the same [[gasoline]] or diesel as the engine and would keep the air conditioning unit and other necessary electrical systems running while the engine shuts off when not needed (e.g., at a stop light or truck stop).<ref>{{cite journal |last1=Lamp |first1=P. |last2=Tachtler |first2=J. |last3=Finkenwirth |first3=O. |last4=Mukerjee |first4=S. |last5=Shaffer |first5=S. |title=Development of an Auxiliary Power Unit with Solid Oxide Fuel Cells for Automotive Applications |journal=Fuel Cells |date=November 2003 |volume=3 |issue=3 |pages=146–152 |doi=10.1002/fuce.200332107}}</ref>

[[Rolls-Royce plc|Rolls-Royce]] is developing solid-oxide fuel cells produced by [[screen printing]] onto inexpensive ceramic materials.<ref>{{cite journal |last1=Gardner |first1=F.J |last2=Day |first2=M.J |last3=Brandon |first3=N.P |last4=Pashley |first4=M.N |last5=Cassidy |first5=M |title=SOFC technology development at Rolls-Royce |journal=Journal of Power Sources |date=March 2000 |volume=86 |issue=1–2 |pages=122–129 |doi=10.1016/S0378-7753(99)00428-0|bibcode=2000JPS....86..122G }}</ref> Rolls-Royce Fuel Cell Systems Ltd is developing an SOFC gas turbine hybrid system fueled by natural gas for power generation applications in the order of a megawatt (e.g. [[Futuregen]]).{{citation needed|date=August 2016}}

3D printing is being explored as a possible manufacturing technique that could be used to make SOFC manufacturing easier by the Shah Lab at Northwestern University. This manufacturing technique would allow SOFC cell structure to be more flexible, which could lead to more efficient designs. This process could work in the production of any part of the cell. The 3D printing process works by combining about 80% ceramic particles with 20% binders and solvents, and then converting that slurry into an ink that can be fed into a 3D printer. Some of the solvent is very volatile, so the ceramic ink solidifies almost immediately. Not all of the solvent evaporates, so the ink maintains some flexibility before it is fired at high temperature to densify it. This flexibility allows the cells to be fired in a circular shape that would increase the surface area over which electrochemical reactions can occur, which increases the efficiency of the cell. Also, the 3D printing technique allows the cell layers to be printed on top of each other instead of having to go through separate manufacturing and stacking steps. The thickness is easy to control, and layers can be made in the exact size and shape that is needed, so waste is minimized.<ref>{{cite journal|title=Northwestern group invent inks to make SOFCs by 3D printing|journal=Fuel Cells Bulletin|doi=10.1016/S1464-2859(15)70024-6|volume=2015|page=11|year=2015|issue=1 |bibcode=2015FCBu.2015Q..11. }}</ref>

[[Ceres Power]] Ltd. has developed a low cost and low temperature (500–600 degrees) SOFC stack using cerium gadolinium oxide (CGO) in place of current industry standard ceramic, [[yttria]] stabilized [[zirconia]] ([[YSZ]]), which allows the use of [[stainless steel]] to support the ceramic.<ref name=ceres>
{{cite web
 |url         = http://www.cerespower.com/Technology/TheCeresCell/
 |title       = The Ceres Cell
 |work        = Company Website
 |publisher   = Ceres Power
 |access-date  = 30 November 2009
|url-status     = dead
 |archive-url  = https://web.archive.org/web/20131213064702/http://www.cerespower.com/Technology/TheCeresCell/
 |archive-date = 13 December 2013
|df          = dmy-all
}}
</ref>

Solid Cell Inc. has developed a unique, low-cost cell architecture that combines properties of planar and tubular designs, along with a Cr-free [[cermet]] interconnect.{{citation needed|date=December 2023}}

The high temperature electrochemistry center (HITEC) at the University of Florida, Gainesville is focused on studying ionic transport, electrocatalytic phenomena and microstructural characterization of ion conducting materials.<ref>{{cite web |url=http://hitec.mse.ufl.edu/ |title=HITEC |publisher=Hitec.mse.ufl.edu |access-date=8 December 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131212104558/http://hitec.mse.ufl.edu/ |archive-date=12 December 2013  }}</ref>

SiEnergy Systems, a Harvard spin-off company, has demonstrated the first macro-scale thin-film solid-oxide fuel cell that can operate at 500 degrees.<ref>[https://www.technologyreview.com/2011/04/20/259492/cooling-down-solid-oxide-fuel-cells/ Cooling Down Solid-Oxide Fuel Cells]. Technologyreview.com. 20 April 2011. Retrieved 27 November 2011.</ref>

===SOEC===
A [[solid oxide electrolyser cell]] (SOEC) is a solid oxide fuel cell set in [[Regenerative fuel cell|regenerative mode]] for the [[electrolysis of water]] with a solid oxide, or [[ceramic]], [[electrolyte]] to produce oxygen and [[hydrogen gas]].<ref>{{cite journal | author1=Anne Hauch | author2=Søren Højgaard Jensen | author3=Sune Dalgaard Ebbesen | author4=Mogens Mogensen | title=Durability of solid oxide electrolysis cells for hydrogen production | journal=Risoe Reports | year=2009 | volume=1608 | pages=327–338 | url=http://www.risoe.dk/rispubl/reports/ris-r-1608_327-338.pdf | url-status=dead | archive-url=https://web.archive.org/web/20090711161522/http://www.risoe.dk/rispubl/reports/ris-r-1608_327-338.pdf | archive-date=11 July 2009 | df=dmy-all }}</ref>

SOECs can also be used to do electrolysis of CO<sub>2</sub> to produce CO and oxygen<ref>{{cite journal |author1=Rainer Küngas
|author2=Peter Blennow |author3=Thomas Heiredal-Clausen |author4=Tobias Holt |author5=Jeppe Rass-Hansen |author6=Søren Primdahl |author7=John Bøgild Hansen | title=eCOs - A Commercial CO2 Electrolysis System Developed by Haldor Topsoe | journal=ECS Trans. | year=2017 | volume=78 |issue=1 | pages=2879–2884 | doi=10.1149/07801.2879ecst|bibcode=2017ECSTr..78a2879K }}</ref> or even co-electrolysis of water and CO<sub>2</sub> to produce syngas and oxygen.

===ITSOFC===
SOFCs that operate in an intermediate temperature (IT) range, meaning between 600 and 800&nbsp;°C, are named ITSOFCs. Because of the high degradation rates and materials costs incurred at temperatures in excess of 900&nbsp;°C, it is economically more favorable to operate SOFCs at lower temperatures. The push for high-performance ITSOFCs is currently the topic of much research and development. One area of focus is the cathode material. It is thought that the oxygen reduction reaction is responsible for much of the loss in performance so the catalytic activity of the cathode is being studied and enhanced through various techniques, including catalyst impregnation. The research on NdCrO<sub>3</sub> proves it to be a potential cathode material for the cathode of ITSOFC since it is thermochemically stable within the temperature range.<ref>Nithya, M., and M. Rajasekhar. "Preparation and Characterization of NdCrO3 Cathode for Intermediate Temperature Fuel Cell Application." ''International Journal of Applied Chemistry'' 13, no. 4 (2017): 879-886.</ref>

Another area of focus is electrolyte materials. To make SOFCs competitive in the market, ITSOFCs are pushing towards lower operational temperature by use of alternative new materials. However, efficiency and stability of the materials limit their feasibility. One choice for the electrolyte new materials is the ceria-salt ceramic composites (CSCs). The two-phase CSC electrolytes GDC (gadolinium-doped ceria) and SDC (samaria-doped ceria)-MCO<sub>3</sub> (M=Li, Na, K, single or mixture of carbonates) can reach the power density of 300-800&nbsp;mW*cm<sup>−2</sup>.<ref>{{Cite journal|last=Zhu|first=Bin|title=Functional ceria–salt-composite materials for advanced ITSOFC applications|journal=Journal of Power Sources|volume=114|issue=1|pages=1–9|doi=10.1016/s0378-7753(02)00592-x|year=2003|bibcode=2003JPS...114....1Z}}</ref>

===LT-SOFC===
Low-temperature solid oxide fuel cells (LT-SOFCs), operating lower than 650&nbsp;°C, are of great interest for future research because the high operating temperature is currently what restricts the development and deployment of SOFCs. A low-temperature SOFC is more reliable due to smaller thermal mismatch and easier sealing. Additionally, a lower temperature requires less insulation and therefore has a lower cost. Cost is further lowered due to wider material choices for interconnects and compressive nonglass/ceramic seals. Perhaps most importantly, at a lower temperature, SOFCs can be started more rapidly and with less energy, which lends itself to uses in portable and transportable applications.{{citation needed|date=December 2023}}

As temperature decreases, the maximum theoretical fuel cell efficiency increases, in contrast to the Carnot cycle. For example, the maximum theoretical efficiency of an SOFC using CO as a fuel increases from 63% at 900&nbsp;°C to 81% at 350&nbsp;°C.<ref>{{Cite journal |last1=Choi |first1=Sihyuk |last2=Yoo |first2=Seonyoung |last3=Kim |first3=Jiyoun |last4=Park |first4=Seonhye |last5=Jun |first5=Areum |last6=Sengodan |first6=Sivaprakash |last7=Kim |first7=Junyoung |last8=Shin |first8=Jeeyoung |last9=Jeong |first9=Hu Young |last10=Choi |first10=YongMan |last11=Kim |first11=Guntae |last12=Liu |first12=Meilin |date=2013-08-15 |title=Highly efficient and robust cathode materials for low-temperature solid oxide fuel cells: PrBa0.5Sr0.5Co2−xFexO5+δ |journal=Scientific Reports |language=en |volume=3 |issue=1 |page=2426 |doi=10.1038/srep02426 |issn=2045-2322 |pmc=3744084 |pmid=23945630}}</ref>

This is a materials issue, particularly for the electrolyte in the SOFC. YSZ is the most commonly used electrolyte because of its superior stability, despite not having the highest conductivity. Currently, the thickness of YSZ electrolytes is a minimum of ~10 μm due to deposition methods, and this requires a temperature above 700&nbsp;°C. Therefore, low-temperature SOFCs are only possible with higher conductivity electrolytes. Various alternatives that could be successful at low temperature include gadolinium-doped ceria (GDC) and erbia-cation-stabilized bismuth (ERB). They have superior ionic conductivity at lower temperatures, but this comes at the expense of lower thermodynamic stability. CeO2 electrolytes become electronically conductive and Bi2O3 electrolytes decompose to metallic Bi under the reducing fuel environment.<ref>{{Cite journal |last1=Hibino |first1=Takashi |last2=Hashimoto |first2=Atsuko |last3=Inoue |first3=Takao |last4=Tokuno |first4=Jun-ichi |last5=Yoshida |first5=Shin-ichiro |last6=Sano |first6=Mitsuru |date=2000-06-16 |title=A Low-Operating-Temperature Solid Oxide Fuel Cell in Hydrocarbon-Air Mixtures |url=https://www.science.org/doi/10.1126/science.288.5473.2031 |journal=Science |language=en |volume=288 |issue=5473 |pages=2031–2033 |doi=10.1126/science.288.5473.2031 |pmid=10856213 |issn=0036-8075|url-access=subscription }}</ref>

To combat this, researchers created a functionally graded ceria/bismuth-oxide bilayered electrolyte where the GDC layer on the anode side protects the ESB layer from decomposing while the ESB on the cathode side blocks the leakage current through the GDC layer. This leads to near-theoretical open-circuit potential (OPC) with two highly conductive electrolytes, that by themselves would not have been sufficiently stable for the application. This bilayer proved to be stable for 1400 hours of testing at 500&nbsp;°C and showed no indication of interfacial phase formation or thermal mismatch. While this makes strides towards lowering the operating temperature of SOFCs, it also opens doors for future research to try and understand this mechanism.<ref>{{cite journal | last1 = Wachsman | first1 = E. | last2 = Lee | first2 = Kang T. | year = 2011 | title = Lowering the Temperature of Solid Oxide Fuel Cells | journal = Science | volume = 334 | issue = 6058| pages = 935–939 | doi=10.1126/science.1204090| pmid = 22096189 | bibcode = 2011Sci...334..935W | s2cid = 206533328 }}</ref>

[[File:Ion Conductivity.png|thumb|upright=1.5|right|Comparison of ionic conductivity of various solid oxide electrolytes]]

Researchers at the Georgia Institute of Technology dealt with the instability of BaCeO<sub>3</sub> differently. They replaced a desired fraction of Ce in BaCeO<sub>3</sub> with Zr to form a solid solution that exhibits proton conductivity, but also chemical and thermal stability over the range of conditions relevant to fuel cell operation. A new specific composition, Ba(Zr0.1Ce0.7Y0.2)O3-δ (BZCY7) that displays the highest ionic conductivity of all known electrolyte materials for SOFC applications. This electrolyte was fabricated by dry-pressing powders, which allowed for the production of crack free films thinner than 15 μm. The implementation of this simple and cost-effective fabrication method may enable significant cost reductions in SOFC fabrication.<ref>{{Cite journal |last1=Zuo |first1=C. |last2=Zha |first2=S. |last3=Liu |first3=M. |last4=Hatano |first4=M. |last5=Uchiyama |first5=M. |date=2006-12-18 |title=Ba(Zr 0.1 Ce 0.7 Y 0.2 )O 3–δ as an Electrolyte for Low-Temperature Solid-Oxide Fuel Cells |url=https://onlinelibrary.wiley.com/doi/10.1002/adma.200601366 |journal=Advanced Materials |language=en |volume=18 |issue=24 |pages=3318–3320 |doi=10.1002/adma.200601366 |issn=0935-9648}}</ref> However, this electrolyte operates at higher temperatures than the bilayered electrolyte model, closer to 600&nbsp;°C rather than 500&nbsp;°C.

Currently, given the state of the field for LT-SOFCs, progress in the electrolyte would reap the most benefits, but research into potential anode and cathode materials would also lead to useful results, and has started to be discussed more frequently in literature.

===SOFC-GT===
An [[SOFC-GT]] system is one which comprises a solid oxide fuel cell combined with a gas turbine. Such systems have been evaluated by [[Siemens Westinghouse]] and [[Rolls-Royce plc|Rolls-Royce]] as a means to achieve higher operating efficiencies by running the SOFC under pressure. [[SOFC-GT]] systems typically include anodic and/or cathodic atmosphere recirculation, thus increasing [[efficiency]].{{citation needed|date=December 2023}}

Theoretically, the combination of the SOFC and gas turbine can give result in high overall (electrical and thermal) efficiency.<ref>{{cite journal |author1=S.H. Chan |author2=H.K. Ho |author3=Y. Tian | title=Multi-level modeling of SOFC-gas turbine hybrid system | journal=International Journal of Hydrogen Energy | year=2003 | volume=28 | issue=8 | pages=889–900 | doi=10.1016/S0360-3199(02)00160-X|bibcode=2003IJHE...28..889C }}</ref> Further combination of the SOFC-GT in a combined cooling, heat and power (or [[trigeneration]]) configuration (via [[HVAC]]) also has the potential to yield even higher thermal efficiencies in some cases.<ref>{{cite journal |author1=L. K. C. Tse |author2=S. Wilkins |author3=N. McGlashan |author4=B. Urban |author5=R. Martinez-Botas | title=Solid oxide fuel cell/gas turbine trigeneration system for marine applications | journal=Journal of Power Sources | year=2011 | volume=196 | issue=6 | pages=3149–3162 | doi=10.1016/j.jpowsour.2010.11.099|bibcode=2011JPS...196.3149T }}</ref>

Another feature of the introduced hybrid system is on the gain of 100% CO<sub>2</sub> capturing at comparable high [[Energy efficiency (physics)|energy efficiency]]. These features like zero CO<sub>2</sub> emission and high energy efficiency make the power plant performance noteworthy.<ref>{{Cite journal|last1=Isfahani|first1=SNR|last2=Sedaghat|first2=Ahmad|date=15 June 2016|title=A hybrid micro gas turbine and solid state fuel cell power plant with hydrogen production and CO2 capture|journal=International Journal of Hydrogen Energy|volume=41|issue=22|pages=9490–9499|doi=10.1016/j.ijhydene.2016.04.065|s2cid=100859434 }}</ref>

===DCFC===
For the direct use of solid coal fuel without additional gasification and reforming processes, a [[direct carbon fuel cell]] ([http://www.materialsviews.com/direct-carbon-fuel-cells-ultra-low-emission-technology-power-generation/ DCFC]) has been developed as a promising novel concept of a high-temperature energy conversion system. The underlying progress in the development of a coal-based DCFC has been categorized mainly according to the electrolyte materials used, such as solid oxide, molten carbonate, and molten hydroxide, as well as hybrid systems consisting of solid oxide and molten carbonate binary electrolyte or of liquid anode (Fe, Ag, In, Sn, Sb, Pb, Bi, and its alloying and its metal/metal oxide) solid oxide electrolyte.<ref>{{cite journal | last1 = Giddey | first1 = S | last2 = Badwal | first2 = SPS | last3 = Kulkarni | first3 = A | last4 = Munnings | first4 = C | year = 2012 | title = A comprehensive review of direct carbon fuel cell technology | journal = Progress in Energy and Combustion Science | volume = 38 | issue = 3| pages = 360–399 | doi = 10.1016/j.pecs.2012.01.003 | bibcode = 2012PECS...38..360G }}</ref> People's research on DCFC with GDC-Li/Na<sub>2</sub>CO<sub>3</sub> as the electrolyte, Sm<sub>0.5</sub>Sr<sub>0.5</sub>CoO<sub>3</sub> as cathode shows good performance. The highest power density of 48&nbsp;mW*cm<sup>−2</sup> can be reached at 500&nbsp;°C with O<sub>2</sub> and CO<sub>2</sub> as oxidant and the whole system is stable within the temperature range of 500&nbsp;°C to 600&nbsp;°C.<ref>{{Cite journal|last1=Wu|first1=Wei|last2=Ding|first2=Dong|last3=Fan|first3=Maohong|last4=He|first4=Ting|date=30 May 2017|title=A High Performance Low Temperature Direct Carbon Fuel Cell|journal=ECS Transactions|language=en|volume=78|issue=1|pages=2519–2526|doi=10.1149/07801.2519ecst|bibcode=2017ECSTr..78a2519W|osti=1414432|issn=1938-6737|url=https://www.osti.gov/biblio/1414432}}</ref>

'''SOFC operated on [[landfill gas]]'''

Every household produces waste/garbage on a daily basis. In 2009, Americans produced about 243 million tons of municipal solid waste, which is 4.3 pounds of waste per person per day. All that waste is sent to landfill sites. Landfill gas which is produced from the decomposition of waste that gets accumulated at the landfills has the potential to be a valuable source of energy since methane is a major constituent. Currently, the majority of the landfills either burn away their gas in flares or combust it in mechanical engines to produce electricity. The issue with mechanical engines is that incomplete combustion of gasses can lead to pollution of the atmosphere and is also highly inefficient.{{citation needed|date=December 2023}}

The issue with using landfill gas to fuel an SOFC system is that landfill gas contains hydrogen sulfide. Any landfill accepting biological waste will contain about 50-60 ppm of hydrogen sulfide and around 1-2 ppm mercaptans. However, construction materials containing reducible sulfur species, principally sulfates found in gypsum-based wallboard, can cause considerably higher levels of sulfides in the hundreds of ppm. At operating temperatures of 750&nbsp;°C hydrogen sulfide concentrations of around 0.05 ppm begin to affect the performance of the SOFCs.{{citation needed|date=December 2023}}

Ni + H<sub>2</sub>S → NiS + H<sub>2</sub>

The above reaction controls the effect of sulfur on the anode.

This can be prevented by having background hydrogen which is calculated below.

At 453 K the equilibrium constant is 7.39 x 10<sup>−5</sup>

ΔG calculated at 453 K was 35.833 kJ/mol

Using the standard heat of formation and entropy ΔG at room temperature (298 K) came out to be 45.904 kJ/mol

On extrapolation to 1023 K, ΔG is -1.229 kJ/mol

On substitution, K<sub>eq</sub> at 1023 K is 1.44 x 10<sup>−4</sup>. Hence theoretically we need 3.4% hydrogen to prevent the formation of NiS at 5 ppm H<sub>2</sub>S.<ref>{{Cite thesis|last=Khan|first=Feroze|date=1 January 2012|title=Effect of Hydrogen Sulfide in Landfill Gas on Anode Poisoning of Solid Oxide Fuel Cells|url=http://rave.ohiolink.edu/etdc/view?acc_num=ysu1338838003|publisher=Youngstown State University}}</ref>

=== RSOC ===
The need for innovation in the energy storage market has brought upon research in [[Reversible solid oxide cell|reversible solid-oxide cells (RSOCs)]]. These cells are able to make a power-power conversion<ref name=":1">{{Cite journal |last=Venkataraman |first=Vikrant |last2=Pérez-Fortes |first2=Mar |last3=Wang |first3=Ligang |last4=Hajimolana |first4=Yashar S. |last5=Boigues-Muñoz |first5=Carlos |last6=Agostini |first6=Alessandro |last7=McPhail |first7=Stephen J. |last8=Maréchal |first8=François |last9=Van Herle |first9=Jan |last10=Aravind |first10=P. V. |date=2019-08-01 |title=Reversible solid oxide systems for energy and chemical applications – Review & perspectives |url=https://www.sciencedirect.com/science/article/abs/pii/S2352152X19301185?fr=RR-2&ref=pdf_download&rr=915219129d1a2f75 |journal=Journal of Energy Storage |volume=24 |pages=100782 |doi=10.1016/j.est.2019.100782 |issn=2352-152X|url-access=subscription }}</ref> by operating alternatively as solid oxide fuel cells (SOFCs) and [[Solid oxide electrolyzer cell|solid oxide electrolysis cells (SOECs)]], which can aide in the next generation of green energy technologies that are facing energy transportation and storage difficulties. In 2023, 34% of the energy provided to the grid was “rejected” or lost in the process, meaning that it did not reach the final output that it was meant for (i.e. heat emitted from a lightbulb instead of only photon emissions).<ref>{{Cite web |title=Flowcharts |url=https://flowcharts.llnl.gov/ |access-date=2025-02-20 |website=flowcharts.llnl.gov}}</ref> A surefire method to convert [[green hydrogen]] to energy, or energy to hydrogen as storage, would help considerably reduce energy losses and improve the total efficiency of emerging green energy technologies.
[[File:RSOC working principle.svg|thumb|403x403px|RSOC depiction in both SOEC and SOFC configurations]]
RSOCs operate similarly to normal SOFCs but can either consume hydrogen fuel to produce electricity (energy production) and oxygen or work the other way around and consume electricity and oxygen to produce hydrogen (energy storage). Because of this, the use of the terms “anode” and “cathode” are obsolete as both electrodes can operate as either an anode or cathode depending on the direction of fuel consumption. Better terminology would be the “fuel electrode” and “oxygen electrode”,<ref name=":1" /> where the fuel cathode either consumes and oxidizes the fuel (SOFC configuration) or reduces the products to provide fuel (SOEC configuration). The oxygen electrode would either reduce oxygen (SOFC configuration) or oxidize oxygen ions to produce oxygen gas (SOEC configuration).<ref>{{Cite web |title=Reversible Solid Oxide Fuel Cell (R-SOFC) |url=https://netl.doe.gov/carbon-management/fuel-cells |access-date=2025-02-20 |website=netl.doe.gov |language=en}}</ref> Similarly to SOFCs, the fuel electrode is typically made of a mixture of nickel and yttrium-stabilized zirconia. The oxygen electrode is typically made of [[perovskite]] materials, like [[Lanthanum strontium cobalt ferrite|lanthanum strontium cobalt ferrite (LCSF)]] and lanthanum strontium chromite (LSC). 

Research interest for RSOCs greatly involves the oxygen cathode material. As mentioned above, the traditional material for an RSOC oxygen cathode is a perovskite. Perovskites (formula ABO<sub>3</sub>) are used because of their cubic structure that allows for oxygen conductivity.<ref>{{Cite journal |last=Shen |first=Minghai |last2=Ai |first2=Fujin |last3=Ma |first3=Hailing |last4=Xu |first4=Hui |last5=Zhang |first5=Yunyu |date=2021-12-17 |title=Progress and prospects of reversible solid oxide fuel cell materials |url=https://www.sciencedirect.com/science/article/pii/S2589004221014358?ref=pdf_download&fr=RR-2&rr=91520cfb2f9ff0e4#sec4 |journal=iScience |volume=24 |issue=12 |pages=103464 |doi=10.1016/j.isci.2021.103464 |issn=2589-0042|pmc=8661483 }}</ref> Among perovskites, the ones used for an RSOC have a lanthanide or an alkaline earth metal ion in the A site and a small radius transition metal in the B site. Common problems arise when considering the various operating modes, high temperatures, and need for longevity.

Various combinations of perovskites have been and are currently being researched for the resulting performance in an RSOC. [[Doping (semiconductor)|Sr-doped]] LaMnO<sub>3</sub> is the most traditional perovskite, but its low ionic conductivity results in poor performance in SOEC configuration. Perovskites with cobalt instead of manganese in the B site are of great research because of their high electronic conductivity. Strontium (Sr) and Barium (Ba) doping in the A site is common because it enhances the [[Pseudocapacitance|pseudo capacitance]] of the perovskite. Many more combinations of different metals in perovskite structures are undergoing research for their use in RSOC and [[Perovskite solar cell|solar cell]] applications.