## 3 Materials
### Interconnect  
In SOFC stack electrical connection between the adjacent cells is provided by interconnect. The materials of interconnect must have properties like high thermal conductivity, high electrical conductivity, and matching thermal expansion coefficients with other components of the cell. It must also be impermeable to gas, have high mechanical strength and phase stability in the extreme operating environment of fuel cell [35; 87; 88]. The interconnect materials used in SOFC operating in temperature range of 500-1000 \({}^{\circ}\)C are usually composed of metals, ceramics or their composites. The commonly used ceramic interconnect material is basically Perovskite lanthanum chromite (LaCrO\({}_{3}\)) in which doping of elements like Sr and Ca is done in order to obtain desired thermal and electrical properties along with the phase stability in extreme operating environments of SOFC [35; 89]. Nickel and calcium doped yttrium chromites are other materials that have shown their compatibility with YSZ in terms of resistance to zirconate formation and sinterability in air. However, low thermal and electrical conductivities have been observed in case of ceramic interconnects. The high fabrication cost of ceramic interconnects limits it to tubular geometry that requires less material [90]. Thus, metallic interconnects are considered to be better option as compared to ceramic interconnects. The drawback of metallic interconnect is that they undergo oxidation (corrosion), contamination of cathode takes place due to evaporation of chromium and mismatching of thermal expansion coefficient with other ceramic materials of SOFC components. The most used materials which have matching thermal expansion coefficients with ceramic components are usually alloys of chromite and ferrite stainless steel [91]. Plansee Ducralloy (94 CrSFe/Y\({}_{2}\)O\({}_{3}\)) was the foremost chromium alloy used as ceramic material replacement [91; 92]. This alloy has shown better resistance to oxidation, bears good mechanical strength and exhibits matching thermal expansion coefficient with other components of SOFC. However, cost of fabrication for this material is very high. On the other hand, although ferrite stainless steel offers low fabrication cost along with matching thermal expansion coefficient, oxidation characteristics are poor that in cathode environment, resulting in oxide scale growth. This growth leads to high area specific resistance [91]. The vapor phase of Cr\({}^{6+}\) species reacts with LSM in the presence of oxygen/water to develop new phases. Furthermore, it diffuses to electrolyte/cathode interface and deposits chromia and other species resulting in devastating cell deterioration [87]. In order to avoid all this protective coating layer of materials like reactive element oxides, spinels, nitrides and conductive perovskites, _etc._, have been suggested by researchers. The best of all coatings on ferrite stainless steel is the chromium free spinel coating (Mn,Co)\({}_{3}\)O\({}_{4}\)[35; 91; 92; 93; 94; 95; 96; 97; 98; 99]. \begin{table}
\begin{tabular}{c c c c} \hline
**Anode** & **\(\boldsymbol{\sigma_{e}}\) (S- cm\({}^{-1}\))** & **Rp (\(\Omega\)-cm\({}^{2}\))** & **Temperature (\({}^{\circ}\)C)** \\ \hline Y\({}_{0.3}\)Ce\({}_{0.7}\)O\({}_{2-\delta}\) & 10 & 0.1 & 900 \\ La\({}_{0.4}\)Sr\({}_{0.6}\)TiO\({}_{3}\) & \(\sim\)360 & 0.7 & 800 \\ La\({}_{0.35}\)Sr\({}_{0.65}\)TiO\({}_{3}\)-Ce\({}_{0.7}\)La\({}_{0.3}\)O\({}_{2}\) (7:3 mol ratio) & 40 & 0.2–0.4 & 800 \\ La\({}_{0.25}\)Sr\({}_{0.75}\)Cr\({}_{0.5}\)Mn\({}_{0.5}\)O\({}_{3}\) & 3 & 0.1–0.3 & 800 \\ Sr\({}_{0.88}\)Sr\({}_{0.08}\)TiO\({}_{3}\) & 64 & 1–10 & 800 \\ Pr\({}_{0.4}\)Sr\({}_{0.2}\)Co\({}_{0.2}\)Fe\({}_{0.7}\)Nb\({}_{0.1}\)O\({}_{3-\delta}\) & - & 0.16 & 900 \\ Y\({}_{0.2}\)Zr\({}_{0.2}\)Zr\({}_{0.62}\)Ti\({}_{0.1}\)O\({}_{3-\delta}\) & 0.15 & 1–10 & 800 \\ La\({}_{0.6}\)Sr\({}_{0.4}\)Fe\({}_{0.9}\)Mn\({}_{0.3}\) + 2 wt \% Pd & - & 0.8 in CH\({}_{4}\) & 800 \\ YSZ-40 wt \% CeO\({}_{2}\) + 1 wt \% Pd & - & 0.26 & 900 \\ \hline \end{tabular}
\end{table}