| Description |
Solid oxide fuel cells (SOFCs) are electrochemical generators: they convert chemical energy into electrical energy using solid-state components. The electrolyte must simultaneously allow the passage of oxygen ions from cathode to anode, while preventing the passage of electrons from anode to cathode. Porous samples of Sm2O3-doped CeO2 (samaria-doped ceria, SDC) of composition Sm0.15Ce0.85O2-s were made. Electrical conductivity was measured using a 4- probe DC method over a wide range of temperatures and oxygen partial pressures. Conductivity rapidly stabilized at any given temperature consistent with the attainment of thermodynamic equilibrium corresponding to the imp0sed conditions. The ionic transference number of SDC at 400 oC in hydrogen was only ~ 0.4, which showed that the electrolytic domain of SDC at and above 400 oC was rather narrow. This also suggested that SDC is not a suitable electrolyte without a thin electron blocking layer. Sr2Fe1.5Mo0.5O6-8 (SFMO) powders were synthesized by combustion synthesis. Porous samples were formed and electrical conductivity was measured by a four-point DC technique over a temperature range from 200 oC to 800 oC in air and in hydrogen. It was observed that Sr2Fe1.5Mo0.5O6-8 is stable at 800 oC in water-containing atmospheres. However, it reacts with water at low temperatures. Reaction of Sr2FeuMo0.5O6-8 with water at low temperatures is a potential shortcoming of this material. The previous work demonstrated instability in water-containing atmospheres due to decomposition of SFMO into strontium hydroxides and di-hydroxides. In this work, we demonstrated that SFMO was also unstable in dry atmospheres, such as ultra-high-purity (UHP) hydrogen, and that different decay products result whether the imposed atmospheres are oxidizing or reducing. Powders with particles that are nanometers in diameter are desirable for creating materials with enhanced electrical, catalytic, and ionic properties. A method based on combustion synthesis for preparing nanocrystalline powder was investigated and samarium-doped ceria (SDC) was synthesized. The resulting products were characterized by XRD, TEM, and BET. The effect of fuel content in the starting mixture on the powder grain size was investigated, and the requirements for nanosized powders were demonstrated. Nanosized SDC powders showed improved performance in a fuel cell compared with conventional SDC powders. |
