Editing Solid oxide fuel cell (section)

===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.