## 3 Materials
### Anode  
The primary objective of anode is the oxidation of fuel and to provide a path to electrons produced in oxidation reaction so they can reach at current collector. The SOFC anode should have properties like good electronic conductivity, sufficient permeability, good electro-catalytic activity, phase relevance with current collector and electrolyte, good microstructural strength to operate at SOFC working temperature and a relating comparable thermal expansion coefficient with electrolyte [12; 35; 68; 43]. \[\mathcal{O}^{2-}\left(\text{electrolyte}\right)\rightarrow\frac{1}{2}\mathcal{O}^{2 }\left(\text{gas}\right)+2e^{-}\left(\text{Anode}\right) \tag{2}\]The most frequently used anode with YSZ electrolyte consists of Ni and YSZ whereas Ni doped ceria is used in case of ceria based electrolytes. Ni is a good catalyst to dissociate hydrogen at working temperature of SOFC and also bears excellent electrical conductivity of about 2 \(\times\) 10\({}^{4}\) S\(\cdot\) cm\({}^{-1}\)[68; 69; 70; 71; 72]. NiO also has a melting temperature (1955 \({}^{\circ}\)C) higher than the NiO/YSZ anode fabrication temperature. Since both phases of Ni-YSZ anode contribute greatly toward its performance so there should be equilibrium in their amount for adequate outcome. Usually, Ni to YSZ phase volume ratio lies between 35:65 and 55:45 [72]. Coarsening of Ni particles can be avoided by the addition of YSZ phase in the anode and it also provides extra triple phase boundaries by extension of effective reaction region [72; 73]. Along with anode composition, the properties like particle shape and size of precursor oxide powders also affect anode performance to a great extent. Different deposition techniques, processing steps and heat treatment are the factor on which optimum particle size and shape is dependent. It is reported that the ideal size of particles for excellent screen printing is 0.1 \(\upmu\)m in case of YSZ and 0.2 \(\upmu\)m for NiO [72; 73; 74]. With these initiating materials, an anodic over potential of just 90 mV was observed with a current density of 350 mA/cm\({}^{2}\) at temperature 800 \({}^{\circ}\)C, and deterioration of only 1.7% after 4300 h of continuous working at 900 \({}^{\circ}\)C [74]. The content of impurity in the starting oxide materials NiO-YSZ is also another factor that affects the performance of anode. It has been found that when content of common impurities like Na\({}_{2}\)O and SiO\({}_{2}\) increase from 100 ppm, it reduces the efficiency of the anode effectively within only hundred hours by intermixing the phases of glassy sodium silicate at interface of anode/electrolyte particularly [75]. The anode supported SOFCs are also under extensive research since they provide better performance as compared to conventional electrolyte supported SOFCs [10; 12; 68; 69; 70; 71; 72; 73; 74]. In anode supported SOFCs, methods like tape casting or die pressing is used to prepare anode while a number of thin film deposition techniques are used to deposit a thin layer of electrolyte over the anode. In anode supported SOFCs, careful attention is given to microstructure, porosity and thickness of the anode. The anode should be dense enough so that the electrolyte layer can be deposited on them but at the same time it should be porous enough to allow diffusion of gas through it so that it can reach at anode electrolyte interface. In Ni-YSZ anode supported SOFCs, high power densities greater than 1 W/cm\({}^{2}\) have been reported to be needed for some commercial systems [72; 74; 76]. Some problems that Ni-YSZ anode encounter during its operation in SOFCs involve deterioration of Ni particles with the passage of time, low stability toward the impurities of fuel, and allows coking when fuels like natural gas and hydrocarbons are used [77]. It has been reported that the coking problem can be reduced by addition of metals like Ru, Au or Rh (1-4 mol %) which can also further reduced by using catalyst and ceria [68; 72; 77].