In this work, we explored RF(Radio-Frequency) thermal plasma synthesis routes for Ni-CeO2 nano-catalyst and GDC(Gd doped CeO2) nano-powders, which are aimed at fuel reforming and electrolyte applications in IT-SOFC(intermediate temperature solid oxide fuel cell), respectively. For this purpose, solid precursors were prepared by mixing Ni metal and CeO2 of several ㎛ in size at the mole ratio of 5:5 and 3:7 for Ni-CeO2 nano-catalyst while CeO2 and Gd2O3 powders were also mixed at the cation ratio of 8:2 and 9:1 for GDC(Gd doped CeO2) nano-powders. Then, the prepared mixture precursors were injected into RF thermal plasma for the in-flight treatment along the centerline of the torch.
The in-flight RF thermal plasma treatment of the mixture precursor, such as Ni-CeO2 solid mixture can be expected to cause a preferential evaporation of Ni metal, which may lead to the formation of the highly dispersed active metal sites on the CeO2 surfaces in spite of the high Ni content of 50 mol%. Meanwhile, the precursor powders consisting of Gd2O3 and CeO2, if they are vaporized simultaneously in the same RF torch, can be predicted to co-condense into the GDC nano powders. As presented from numerical analysis results on the plasma-particle (such as Ni, CeO2 and Gd2O3) interaction in this work, for example, Ni metal can experience a preferential evaporation at relatively low power level of ~10 kW while full evaporation of CeO2 and Gd2O3 powders needs RF thermal plasma with high power level of ~ 50 kW. According to these basic numerical results based on the physical properties of precursor materials, we tested each synthesis route using RF thermal plasma systems with the power levels of 60 kVA and 200 kVA, respectively.
As for the synthesis routes of Ni-CeO2 nano-catalyst, firstly, it was confirmed that Ni-CeO2 nano catalysts with high nickel content up to 50 mol% can be successfully prepared in the form of highly dispersed active Ni sites on the CeO2 surfaces using RF thermal plasma. In addition, XRD and H2-TPR results of the as-prepared catalysts revealed that many of these active Ni sites formed a NiO solid solution layer on a CeO2 surface. This can help the resistance of the as-prepared catalyst to carbon deposition at intermediate temperature of ≥ 550 ℃ during the partial oxidation of methane, which was verified from the performance test results of the as-prepared Ni-CeO2 catalysts for partial oxidation of methane, showing a methane conversion rate of ≥ 70 % as well as the selectivities of CO and H2 greater than 90 % at 550 ℃ - 650 ℃. In particular, no carbon deposition is found for the as-prepared catalyst used in the 24 h test at 550 ℃ in spite of the high Ni content of 50 mol%. From these advantages of the as-prepared catalyst, the as-prepared catalysts is expected to be applied as a fuel reforming catalyst for IT-SOFC.
For the GDC nano powders prepared by RF thermal plasma at a plate power level of ~140 kVA, FE-SEM images showed that micron-sized mixture precursor was reformed into nano powders of ≤ 50 nm. Considering very short residence time of several ㎲ in RF torch, this reformation indicates that the evaporation of solid precursor took a place simultaneously due to the high plate power of ~140 kVA. In addition, TEM-EELS and X-ray diffraction patterns of the as-synthesized powders showed that Gd atoms were incorporated into the CeO2 lattices, which means that Gd doping process can be realized during the quenching of the vapor species from Gd2O3 and CeO2 as expected previously. In order to estimate the applicability as an electrolyte material for IT-SOFC, the as-prepared GDC nano powders were sintered and the sintered body was used as a target of RF sputtering device to produce GDC thin films. Then, the electrical conductivities of the GDC thin films were measured according to Van der Pauw’s method. For example, we obtained a sintered body with the density of 89% and the grain sizes of 250 ~ 400 nm at sintering temperature of 1,250 ℃. When served as a target of RF sputtering device at the substrate temperature of 600 ℃ and the gas composition condition of 80% Ar - 20% O2, it was found that this sintered body can produce a GDC thin film with high crystallinity, which normally leads to high electrical conductivity. For the as-deposited GDC thin films with thickness of ~1 ㎛, the electrical conductivity of 10-2 S/cm was measured and this is allowable value for a thin film electrolyte of IT-SOFC. From these theoretical and experimental result, it was concluded that RF thermal plasmas can provide a fast and facile route to the synthesis of ceria based nano-composite, such as Ni-CeO2 nano-catalyst and GDC electrolyte nano powders for IT-SOFC applications.