Hello, everyone. In this lecture, we are going to talk about the detailed mechanism, which is a later with the oxide ion and proton conduction. As shown here, we already discussed about the required properties for SOFC component, including electrolyte, interconnector, anode and cathode. So based on this required properties that as shown here, so there are so many promising materials have been developed. For example, calcium, magnesium, scandium, yttrium doped zirconia, and samarium gadolinium, yttrium doped ceria, and calcium, strontium, barium, gadolinium, yttrium doped bismuth oxide and strontium magnesium conduct, lanthanum gallate can be used as an electrolyte material. These materials are oxide ion conductors and also silicon and doped barium cerate can also be used to pull low temperature of the solid oxide fuel set the because this is on proton conductor. And Strontium-doped lanthanum chromite oxide and calcium-doped lanthanum chromite oxide and alloys are the nickel-chrome and feritic steel and ducrolloy are used as an interconnecting material. And for anode, the various assignment of nickel YSZ, ruthen YSZ, nickel SDC, SDC is the samaria-doped ceria. And nickel GDC, GDC is the gadolinia-doped ceria, and nickel, ruthenium, platinum can also be used as an anode material. And lanthanum manganite oxide-based material and lanthanum cobalt oxide-based material can be used as a cathode material. More on electrolyte material, doped zirconia is widely used the current SOFC. It show issue high oxide ion conductivity and also show low electronic conductivity. As you know, the required required property in electrolyte material is high ionic conductivity and low electronic conductivity. And by doping of the some important atoms with 3+ charges at zirconium 4+ charges site, we can generate oxygen vacancy. And oxygen zirconia also show good stability under high and low oxygen partial pressure. Then let's think about the effect of doping elements and content. High doping content the results in high density of oxygen vacancies, but too high doping contents result in oxygen or oxygen vacancy ordering, oxygen vacancy clustering, and decrease in effective mobility. So the oxide ion conductivity should be decreased due to these phenomena. And as shown here, cerium ion, Nd ion, samarium ion, gadolinium ion, ytterium, ytterbium, scandium ion can be used as a doping element. And same doping contents means same oxygen vacancy density. But these material shows the difference in mobility due to lattice strain and association. This can be understood by the difference in ionic radius. So the zirconium, the host ion, the ionic radius is about 0.78 angstrom with coordination number of 7. And cerium iron has much larger size compared to two host zirconium ion. The ionic radius of cerium 3+ is about 1.143 angstrom and you can find very similar ionic radius in scandium. The ionic radius of scandium 3+ is about 0.87 angstrom. So due to this similarity in ionic radius, the scandium is the very promising doping element for zirconia. But in many cases, due to the cost of problem, yttrium is widely used to for doping element. And in order to increase the oxide ion conductivity, we should consider also free volume and lattice symmetry. And then, let's think about the chemical reaction of doped zirconia. So, you know the operating temperature of SOFC is ranged between 600 to 1000 degrees C. In this temperature range, there is no reaction between lanthanum manganite oxide, this is a cathode material, and nickel YSZ, this is an anode material, and lanthanum chromite oxide, this is an interconnector material. But at 1100 degrees C, the reaction between YSZ and lanthanum manganite oxide, it is cathode, that can be occurred. So they formed the insulating lanthanum zirconium oxide film. So operating temperature of SOFC should be kept under 1100 degrees C. And thermal expansion coefficient matching is another important parameter. So as shown here, the thermal expansion coefficient of YSZ electrolyte material is about ten times 10x10 to the minus 6 per degree C. So difference in thermal expansion coefficient between four important component material of SOFC makes interface strain and then result in crack. So sometimes, in order to increase the mechanical property of electrolyte material, we can use the alumina as an additive material. The doped ceria is another promising candidate for an electrolyte material. Actually, the oxide ion conductivity of gadolinium-doped ceria and samarium-doped ceria is much higher than that of doped zirconia. But as shown here, this material shows end-type conduction behavior at lower oxygen partial pressure. So you remember that require the physical property. Electrical property of electrolyte is high oxide ion conductivity and low electronic conductivity. But this like doped ceria has high electronic conductivity. And then, another important material for on electrolyte is doped lanthanum gallium oxide. We can make oxygen vacancy by doping of barium and strontium with 2+ charge at lanthanum site with 3+ charging and also make oxygen vacancies by doping of magnesium with 2+ charges at gallium site with 3+ charge. And oxygen transfer is later waste transfer from oxygen site to oxygen vacancy site. So it is very important to consider the set of point, that is the critical radius between two A site atoms and one B site atom as shown here. Let's think about the lattice structure of a perovskite ABO3. It composed of larger site A-site cation and small side B-site cation, as shown here. You can find the coordination number of B-site cation is 6, but coordination number of A-site cation is 12. So this like perovskite structure has larger flexibility for incorporating cation of different size in order to make some defect structure. And it has high tolerance of vacancy formation. The crystal structure of a perovskite has structured oxide is later with just like Goldschmidt tolerance factor, t. T, as given by this equation, are the ionic radius of A-site cation plus ionic radius of oxygen over root 2 times ionic radius of B-site cation plus ionic radius of oxygen. If this parameter is near 1, that means there's a high latest symmetry. So sometimes, we can find the very important displacement phenomena in this structure, in this perovskite material such as B-site cation displacement and A-site cation displacement. This is related with the ferroelectric property of barium titanate. The detailed information will be given in later. And actually, let's return to the lanthanum gallium oxide, the calculated tolerance factor of lanthanum gallium oxide is about 0.949. So lattice symmetry with pure lanthanum gallate is not high. So, you know, tolerance factor is related with the lattice symmetry. So in order to increase the lattice symmetry, we should engineer the lattice structure of lanthanum gallate, and simultaneously, we should make the oxygen vacancy. As shown here, the ionic radius of barium and ionic radius of strontium is larger than that of host lanthanum ion. So by doping of barium and strontium, we can increase the tolerance factor, then increase the lattice symmetry of lanthanum gallate. And then, let's think about the concentration of protonic defect. The first stage of the formation of protonic defect is formation of oxygen vacancy and then incorporation of water vapor, then we can make protonic defect. And then, as shown here, just like protonic defect is related with the water vapor pressure and also related with the water solubility limits. So and then proton mobility is ruled by hop and turn mechanism, with the phase changing, the state changing between ground state and various state of hydrogen. So in order to obtain the high proton mobility, we should make high symmetric structure because high symmetric structure, they result in three-dimensional conduction. And low symmetry makes 1-dimensional conduction, so mobility of a proton should be decreased. And anode, anode is a fuel electrode, it should be porous layer. And due to the small polarization loss, this like anode can be used as the thickest layer, it is support. An anode should have high electronic conductivity and also high ionic conductivity. So Cermet, cermet is the composite material of ceramic and metal. So cermet of nickel and YSZ is usually used as an anode material. And for cathode, cathode is an air electrode. So thin porous layer on the electrolyte is a cathode. At cathode, oxygen reduction takes place. And cathode also should have high electronic conductivity and also high ionic conductivity. So strontium doped thalanthanum manganite oxide has good compatibility with doped zirconia electrolyte and also has similar coefficient of thermal expansion and has low level of chemical reactivity. But it's ionic conductivity islow. So we should increase the triple phase boundary which is composed of electrolyte, air, oxygen, the electrode. And sometimes, we can use some composition attuned material such as strontium ion conductor and lanthanum cobalt 2 oxide. Thank you.
