Electrochemical removal of NOx by scandium doped zirconia membrane reactor with ceria buffer layer

초록

NOx is harmful exhausts from automobiles and stationary sources such as electric power plant, which cause serious environmental problems, for example, acid rain, respiratory diseases and so on. In past a couple of years, our research group has proposed novel type of membrane reactors for NO decomposition. The membrane reactor consists of yttria-stabilized zirconia (YSZ) electrolyte, composite catalytic layer (NiO-YSZ), and Pt electrodes. The obtained membrane reactor showed higher NOx decomposition efficiency than commercial ones that use porous noble metal thick layer like Pt or Pd as cathode. Such a high efficiency was considered to be due to microstructure of the NiO-YSZ composite cathode. On the other hands, the membrane reactor requires relatively high voltage and the voltage should be further increased if the membrane reactor operates at low temperature, since YSZ is used as a electrolyte which shows low ionic conductivity at lower temperature. In this study, we proposed new type of the electrochemical cell for NO decomposition, which consists of a scandium stabilized zirconia (ScSZ) electrolyte, a thin gadolinium doped ceria (GDC) buffer layer, composite catalytic electrode with a perovskite crystal structure (LSCF). ScSZ is an attractive candidate as a solid electrolyte, due to its high ionic conductivity at the intermediate temperature range, 500° to 700°C, and good mechanical properties, for example, three-point bending strength; its ionic conductivity is approximately two or three times higher than that of 8YSZ. However, perovskite catalytic materials like LSCF tend to react with ZrO2, forming reaction products such as SrZrO3. This would be the reason why we integrate CGO buffer layer with the ScSZ electrolyte and LSCF electrode. Pulsed laser deposition (PLD) was selected as a thin film processing the GDC buffer layer. It is expected that dense and homogeneous thin GDC can be deposited on the ScSZ electrolyte.