Novel sol-gel synthesis and characterization of oxide nanopowders for solid oxide fuel cells

In this research, three different sol-gel synthesis methods by using cost effective materials, such as sugar + pectin, sodium alginate beads or sodium alginate granules, have been investigated for nanopowders production in Solid Oxide Fuel Cells (SOFCs) application. Materials (cerium gadolinium oxide and nickel oxide) have been selected as model materials. Cerium gadolinium oxide (CGO) is one of the most important electrolyte materials in SOFC due to its high ionic conductivity at 500-800 "C, whilst nickel oxide (NiO) can be reduced into nickel in SOFC fuels (H2 or CH4) as catalyst in anode layer. First of all, a novel sol-gel method has been developed for the production of high purity nanopowders of Ce0.8Gd0.201.9 (CG02) solid solution using maltose or sucrose as an organic chelating agent and pectin for gelation. The results of this investigation indicate that the final particle size of approximately 10 nm can be obtained after calcination of the dried gel at , 500°C for 2 hours in ambient air. Powder X-ray diffraction (XRD) shows that all samples are single phase cubic CGO powders. The mean crystallite sizes calculated from XRD analysis using Rietveld refinement method agree with the morphological features observed by transmission electron microscopy (TEM). The nominal composition of CG02 has been found to be in excellent agreement with that determined by energy dispersive X-ray spectroscopy (EDS) and inductively coupled plasma - atomic emission spectrometry analysis (ICP-AES). The ionic conductivities of Ceo.8Gdo.201.9 samples are measured by AC-impedance which appears reasonably well with the reference data which will qualify the use of this material for SOFC as solid electrolyte and in the fabrication of composite electrodes. On the other hand, another novel and generic sol-gel method has been developed for the production of high purity metal oxide nanopowders using sodium alginate (Na-ALG). This has been demonstrated successfully employing NiO and CGO CG01 (Ce0.9Gd0.1O1.95) and CGO2 (Ce0.8Gd0.201.9) as model materials in this instance. For NiO, the results of this investigation indicate that the final particle size of -20 nm can be obtained after calcination of the predried beads at 500°C for 3 hours in ambient air. XRD shows that the obtained samples are single phase cubic NiO powders. Furthermore, freeze dried and X-ray micro-tomography (XMT) technologies are applied to observe the inside morphology of the Ni-ALG beads. XMT shows that nickel ions have been uniformly cross-linked in the alginate structure and remained stable after freeze drying evidenced by the bright green color of the freeze dried beads. Finally, NiO nanopowders can also be synthesized using Na-ALG granules. Moreover, this alginate method has also been demonstrated successfully employing CGO in two composites designated as CG01 (Ceo.9Gdo.101.95) and CG02 (Ceo.aGdo.201.9), respectively. The results indicate that the nanopowders having a final particle size of -7 nm can be obtained after calcination of ion-exchanged alginate precursor at 500 degC for 2 hours in ambient' air. The chemical structures of Na-ALG solution and CGO beads are analyzed by Fourier transform infrared spectroscopy (FTIR) which indicates that Ce3+/Gd3+ are ion-exchanged with Na+ after gelation. The nominal compositions of CG01 and CG02 have been found to be in excellent agreement with that determined by EDS and ICP-AES. The ionic conductivities of these two samples are measured by AC-impedance which appears reasonably well with the reference data which will also qualify the use of this material for SOFC as solid electrolyte. All of these new sol-gel methods are simple, environmentally friendly and non-toxic routes for a large scale production of high purity single phase nanopowders in a cost effective manner at significantly low temperatures.EThOS - Electronic Theses Online ServiceGBUnited Kingdo