Electrical Properties Of CeO₂ Based Electrolyte And Their Application In Intermediate-Temperature Solid Oxide Fuel Cells

SOFC (Solid Oxide Fuel Cell) is the fourth generation fuel cell. Firstly, with all solid components, SOFC eliminates problems liquid electrolyte fuel cell faces, such as corrosion and leakage of liquid electrolytes. Second, it is unnecessary to use noble metals as electrodes because SOFC works at high temperature, thus the cost of cells can be minimized. At the same time, the high quality heat it emits can be fully used. The overall energy conversion efficiency of the thermal-electric system can be added up to 80%. It can use a large of different fuels, the hydrogen, carbon monoxide to the natural gas, even other combustive gases.The low-temperature operation of solid oxide fuel cells (SOFCs) is advantageous from the viewpoints of operational stability and the use of cost-effective gas sealing and interconnecting materials. Decreasing the operating temperature is the main purpose of the research on SOFC. Ceria-based solid solutions have been acknowledged to be the most promising electrolytes for intermediate temperature SOFC (IT-SOFC) science their ionic conductivity is higher than yttria stabilized zirconia (YSZ) in the intermediate temperature range. We studied the oxygen ionic conductivities of ceria-based solid solutions in this paper.Acceptor-doped CeO₂ is present in the form of polycrystals, consequently, the grain boundaries often have a significant influence on overall properties. In the low-temperature regime, the specific grain boundary conductivity is known to be 2-3 orders lower than the bulk conductivity. Two main origins for high grain-boundary resistivity are the space-charge layers near the grain-boundary and the siliceous intergranular phase. A novel method in which the space charge layers effect and the impurity factor were taken into consideration to analysis the grain boundary transport in acceptor-doped CeO₂ of normal purity is presented, and the origin of the grain size effects for acceptor-doped CeO₂ is investigated. Ce0.85Sm0.15O1.925 samples have been prepared using the glycine-nitrate process. The power densities and current densities of single cells based on the Ce0.85Sm0.15O1.925 electrolytes with different grain size were evaluated. The Ni0.9Cu0.1Ox and Ce0.85Sm0.15O1.925 oxide powders were used as anode powders, the BaCo0.7Fe0.2Nb0.1O3-δoxide powders were used as cathode powders. The effects of microstructure on their electrical properties were investigated by X-ray diffraction (XRD), density measurements, scanning electron microscopy (SEM) and AC impedance spectroscopy. The phase of Ce0.85Sm0.15O1.925 sintered in air at 1150oC, 1250oC, 1350oC,and 1450oC for 10h was confirmed by X-ray diffraction to have the fluorite structure, the lattice parameter independent of the amount of the grain size. All samples exhibit a homogeneous structure, the average grain sizes increase with increasing the sintering temperatures. The bulk conductivity is nearly independent of the grain size, lower sintered densities produce a small decrease in the bulk conductivity when the relative density is below 95%. The grain boundary conductivity and total conductivity increased with decreasing grain size. The space charge potential was nearly independent of grain size. The increase of the apparent specific grain boundary conductivity was resulted from the increase of the conduction path width determined by the siliceous phase at grain boundaries. The power densities and current densities of single cells based on Ce0.85Sm0.15O1.925 electrolytes with different grain size are evaluated. The power densities and current densities of single cells increase with decreasing the grain size of Ce0.85Sm0.15O1.925 electrolytes. The maximum power density of the cell is 648mWcm-2 at 800oC for Ce0.85Sm0.15O1.925 sintered at 1250oC for 10h.Series of solid solutions of (Ce0.85Sm0.15)1-xTbxO₂-δwith x=0-0.05 were synthesized by glycine-nitrate process. All prepared (Ce0.85Sm0.15)1-xTbxO₂-δ(x=0-0.05) powders were sintered at 1250oC, 1350oC and 1450oC,respectively. X-ray diffraction patterns indicate that all samples crystallize to a single phase of cubic fluoride structure. The lattice parameters decrease until x=0.02 and then increase with increasing the content of Tb for samples sintered at 1250oC, the lattice parameters are nearly independent of the content of Tb for samples sintered at 1450oC. The change of electrical conductivities of solid solutions for (Ce0.85Sm0.15)1-xTbxO₂-δ(x=0-0.05) was not obvious, and the maximum conductivity was found at x=0.01 when sintered at 1250oC. The reason may be that co-doping 0.01 terbium in doped ceria will increase the oxygen vacancy concentration slightly, but with dopant concentration increasing continue, the oxygen vacancy V··o and the dopants Tb′Ce and Sm′Ce tend to associate and form [V··o Tb′Ce ] and [V··o Sm′Ce ] pairs, resulting in a decrease in [V··o ].To improve the grain boundary conductivity of acceptor-doped CeO₂, many attempts have been made to reduce the GB effect through adding various dopants such as Fe2O3, Co3O4, MgO, Al2O3 and CaO. Though the addition of these oxides gives rise to an important effect on grain boundary transport properties, the exact role of the additive is still debatable. The addition of Fe2O3, Co3O4, MgO, MnO₂ and CaO would improve the grain growth of SDC. NiO is an important component for the fabrication of anode in SOFCs, which are in direct contact with the electrolytes. During fabrication and operation, diffusion of Ni cations from the anode to the electrolyte has been widely reported in the literature. Therefore, it would be of great interest to study the effect of small amount of NiO on the properties of acceptor-doped CeO₂ electrolyte for the design and fabrication of high-performance SOFC stacks. The effects of NiO addition on the microstructures and electrical conductivities of Ce0.85Sm0.15O1.925 sintered at 1250oC and 1450oC were investigated. The performances of the cells based on Ce0.85Sm0.15O1.925+xNiO electrolytes were also evaluated, the Ni0.9Cu0.1Ox and Ce0.85Sm0.15O1.925 oxide powders were used as anode powders, the BaCo0.7Fe0.2Nb0.1O3-δoxide powders were used as cathode powders. X-ray diffraction patterns indicate that all the compositions used are single phase, and Ni2+ is not dissolved into the Ce0.85Sm0.15O1.925 lattice. NiO promoted grain growth of SDC during the sintering process. NiO have an scavenging effect on SiO₂ impurity after sintered at 1450oC for 10 h, and the space charge potential was nearly independent on the the content of NiO. The V-I characteristics of single cell show that the power density of the cell using SDC-NiO as composite electrolyte was higher than the cell using SDC as electrolyte. The maximum output power density was 648mW/cm2 at 800 oC for the cell with SDC+0.5NiO sintered at 1450oC as electrolyte. The maximum output power density was 860mW/cm2 at 800 oC for the cell with SDC+0.5NiO sintered at 1250oC as electrolyte. The improvement of maximum output power density was resulted from the constrain effect of NiO on SiO₂.