Study On Solid Electrolyte Of Intermediate Temperature Used In Fuel Cells

The solid oxide electrolytes of intermediate temperature used in fuel ceils, such as CeO₂ and Bi₂O₃-based electrolyte materials, were investigated systematically. The effects of doping kinds of oxides on the electrical, mechanical properties and microstnicture were evaluated. Some optimum electrolyte compositions were selected based on the experimental results and theoretical analysis.In the previous work, CeO₂-based electrolytes needed a high sintering temperature. It was shown according to this researching work that the sintering temperature decreased from 1550℃ to 1450℃ when the electrolytes were doped with Sm₂O₃ and Gd₂O₃. In the meantime, the total doping concentration decreased from 10mol% which single dopant needed to 8mol%, and the producing process was improved significantly. The amount of intergranular glass phase increased and the electrical conductivity decreased with the heightening of the sintering temperature.One of the main disadvantages of CeO₂-based electrolyte materials was lower strength. Doping Al₂O₃ in CeO₂-based electrolyte materials could increase the mechanical strength, and the doping concentration of Al₂O₃ corresponding to the maximum strength was 3mol%. The reason was that doping Al₂O₃ could bring on compressive stress and prevent the micro-crack from propagation. On the other hand, Al₂O₃ distributed in the grain boundary and caused the crack deflection. Doping Al₂O₃ improved sintering ability also: the sintering temperature decreased to below 1550℃. Doping Al₂O₃ decreased the electrical conductivity.Doping ZrO₂ in CeO₂-based electrolyte materials could increase strength of CeO₂-based electrolyte as well. The reason was ZrO₂ toughed the grain boundary through solution and segregation. The fracture after doping was transgranular instead of intergranular. Doping ZrO₂ also improved sintering ability, decreased the sintering temperatures and increased the growing rate of grains. In the meantime, the electrical conductivity decreased and the activation energy increased. The reason was that solubility of ZrO₂ decreased the lattice constant of cubic ceria-based electrolyte and heightened the transition potential barrier of oxygen ions.Doping CaO in CeO₂-based electrolyte materials improved the relative density and sintering ability significantly, decreased the sintering temperature as well. The electrical conductivity decreased and the activation energy increased after doped CaO, but the decreasing extent of electrical conductivity was less than that of samples doped with Al₂O₃ or ZrO₂. This showed that the effect of CaO on the decreasing ofelectrical conductivity was mild. CaO appeared after the doping concentration exceeding 20mol%. CaO of low concentration could transform impurity into glass-phase. The grass phase distributed in the grain boundary and increased the sintering density.The experimental results showed that doping B2O3 in Bi2O3-based electrolyte could lighten the reducing of Bi2O3 in hydrogen atmosphere. Doping ZrO2 prevented Bi2O3-Y2O3 system from cubic to rhombohedral phase transition, and the doped samples remained cubic phase after long term ageing at 650 °C for 300h. The undoped samples presented obvious phase transition after 180h ageing, and after 300h ageing, the cubic phase transformed fully to rhombohedral phase. ZrO2 suppressed phase transition through decreasing the anion concentration. Complexly doped with B2O3 and ZrO2, Bi2O3-based electrolyte could gain good synthetic property.The calculating results showed that the minimum potential trace of oxygen vacancies were approaching the FC cubic center, then transiting to the vacant lattice position through an arc trace, not passing through the center. The main transition forms were between second adjacent or second adjacent and first adjacent. The transition potential energy was equal to the electrical activation energy. The calculating results of the association energies for doping anion and oxygen vacancies indicated that Al3+ combined easily with oxygen vacancy, but Gd3+ and other rare-earth metal anions were not easy to associate with oxygen vacancy. Therefore, doping a modicum of A12O3 would decrease the electrical conductivity significantly. This work presented the theoretical fundamental for anticipating the properties of CeO2-based electrolyte.The work in this dissertation proofed that the solid oxide electrolytes doped with multiple components were superior to that doped with single component. The former materials had better synthetic properties, for the properties could be adjusted to some extent. Therefore, the complex doping will be an important field of solid electrolytes researching in the future.