Ionic Conductivity Performance Of Tri-doped Solid Oxide Electrolytes Based On Ceria

In this paper, thermodynamic and kinetic theory of solid oxide fuel cell, conductivity and doping theory of electrolyte and plenty of experimental studies were summarized. It was attractive that the performance of CeO2-based electrolytes can be significantly improved by single-and double-doping with earth, alkaline earth and transition metal oxides. Based on above, we would like to explore tri-doped CeO2-based electrolyte materials, and to study the effect of dopants on microstructure, grain, grain boundary and total ionic conductivity and conductivity activation energy of electrolytes. Additional, it will be also taken into consideration that the power generation performance, ohmic polarization, activation polarization of single cells supported by tri-doped CeO2-based electrolytes.Ce0.90-2xGd0.10YxCaxO2-δelectrolyte materials tri-doped with rare and alkaline earth metal elements were prepared using method of nitrate-citrate self-combustion. The materials were cold pressed into electrolyte substrates. Ag slurry was screen printed onto both sides of the electrolyte substrates as electrodes to get Ag electrodes symmetric cells for conductivity tests. The anode and cathode were coated onto the electrolyte substrates to get single cells for electrical performance tests. The phase of electrolyte materials was characterized by XRD. The morphology and microstructure of the surface and cross section of single cells were characterized by SEM. The electrochemical characterizations were conducted on Ag electrode symmetric cells to get AC impedance spectroscopy, and the ionic conductivities of the electrolytes were calculated from data of electrochemical impedance spectroscopy. The electrical properties of single cells were tested.The results show that Ce0.90-2xGd0.10YxCaxO2-δelectrolyte materials tri-doped with Gd2O3, Y2O3 and CaO have a cubic fluorite structure and no visible lattice distortion. The grain boundary conductivities of tri-doped Ce0.90-2xGd0.10YxCaxO2-δelectrolytes have a strong dependence on the concentrations of Y3+ and Ca2+ the grain boundary resistance has maximum as x=0.025, the concentration of Ca2+, and then it decreases with x increasing. The grain boundary resistance of tri-doped electrolytes is much smaller than that of single-doped GDC, indicating that Ca2+ can significantly reduce the grain boundary resistance of CeO2-based electrolyte. The total conductivities of tri-doped Ce0.90-2xGd0.10YxCaxO2-δelectrolyte have obvious advantages compare to the single-doping. The total electrical conductivities firstly increases with x and then decreases, the maximum of 2.45×10-2 S·cm-1 occurs to the electrolyte with x=0.05 at 725℃. The total conductance activation energy decreases with x increasing. In addition, the positive effect of doping on the conductivity of ceria-based electrolytes in low-temperature region is more pronounced than that in high-temperature region.In the same operating conditions, the electrical properties of Ni-GDC/Ce0.90-2xGd0.10YxCaxO2-δ/LSM single cells firstly increase and then decrease with x increasing. The max current density and electric power density occur to the single cell supported by the electrolyte with x=0.05 of at 900℃, their values are 484 mA·cm-2 and 128.3 mW·cm-2, respectively. Decreasements of open circuit voltages of the cells in the operating temperature range of 750～950℃are own to the electronic conductivity through the electrolytes. Activation polarization losses of the single cells decrease with the operating temperature. The greater activation polarization loss results in the poor electrical properties of single cells, showing that activation polarization seriously weakened the cells electrical performance.