Universidad Complutense de Madrid, Departamento Física Aplicada III Universidad Complutense de Madrid, Departamento Física Aplicada III Rainer Schmidt Increased ionic conductivity in microwave hydrothermally synthesized rare–earth (RE) doped ceria Ce1-xRExO2-(x/2) Rainer Schmidt, Jesús Prado-Gonjal, Emilio Morán Universidad Complutense de Madrid Grupo de Física de Materiales Complejos Dpto. Fisica Aplicada III Solid State Chemistry Group Dpto. Química Inorgánica I O.8 25th June 2012

Universidad Complutense de Madrid, Departamento Física Aplicada III Universidad Complutense de Madrid, Departamento Física Aplicada III Rainer Schmidt Increased ionic conductivity in microwave hydrothermally synthesized rare–earth (RE) doped ceria Ce1-xRExO2-(x/2) - Introduction - Microwave assisted hydrothermal synthesis - Structural characterisation of RE doped ceria nano- powders - Impedance spectroscopy of sintered RE doped ceria ceramics

Solid Oxide Fuell Cells (SOFC) Introduction Rainer Schmidt Solid Oxide Fuell Cells (SOFC) Cathode: ½ O2 + 2e- O2- Anode: H2 + O2- H2O + 2e- Cell: H2 + ½ O2 H2O 3

Tubular Solid Oxide Fuell Cells Introduction Rainer Schmidt Tubular Solid Oxide Fuell Cells Operating temperature 600 - 1000 ºC Try to reduce! 4

Solid Oxide Fuell Cells for Clean Energy Applications Introduction Rainer Schmidt Solid Oxide Fuell Cells for Clean Energy Applications

Rare-Earth Doped Ceria as a SOFC Electrolyte Introduction Rainer Schmidt Rare-Earth Doped Ceria as a SOFC Electrolyte Ce4+ O2- CeO2 Ce4+ O2- RE3+ VO Ce1-xRExO2-(x/2) Fluorite Structure (CaF2) Alio-valent RE Doping High Ionic Oxygen Vacancy Conduction 6

Motivation of this work: Introduction Rainer Schmidt Motivation of this work: - In polycrystalline RE doped ceria Ce1-xRExO2-(x/2) the Grain Boundary (GB) areas constitute barriers for ionic oxygen charge transport - This limits the application of RE doped ceria as an electrolyte material for SOFCs - Increase the sintering activity of the ceria powders to obtain denser polycrystalline ceramics with higher GB ionic conductivities - Use of microwave assisted hydrothermal synthesis for producing nano-sized RE doped ceria powders with increased sintering activity - Fabricate dense pellets and determine the ionic GB and bulk ionic conductivities separately using impedance spectroscopy

“FAST CHEMISTRY” Aqueous Solutions Ce(NO3)3∙6H2O Sm(NO3)3∙6H2O Microwave Assisted Hydrothermal Synthesis Rainer Schmidt Aqueous Solutions Ce(NO3)3∙6H2O Sm(NO3)3∙6H2O -Gd(NO3)3∙6H2O -KOH flakes Milestone Ethos 1 Temperature Pressure Power 200º C 13 atm “FAST CHEMISTRY” 30 minutes synthesis!! Ce0.85RE0.15O1.925 (RE = Gd, Sm), Ce0.8(Gd0.1Sm0.1)O1.9 and CeO2-d J. Prado-Gonjal et al. Materials Research Bulletin, 44 (2009) 1734 8

Microwave Heating Mechanism Microwave Assisted Hydrothermal Synthesis Rainer Schmidt Microwave Heating Mechanism It is based upon the ability of a material to absorb electromagnetic energy directly and be heated. Heat can be generated from within the material itself as a result of the absorption of microwave energy by the material. Microwave Thermal energy is transferred to the material through conduction, convection and radiation of heat produced by external sources. Conventional Absorption Reflection The electromagnetic field polarizes dipolar substances which induces a dipole in the molecule. As a result increased rotation sets in which causes internal heating of the substance. Dielectric materials Metals Transmission Glass, teflon 9

X-Ray Diffraction and Rietveld Refinement Structural Characterization of Nano-Powders Rainer Schmidt X-Ray Diffraction and Rietveld Refinement Fullprof software

Refined Unit Cell Parameters Structural Characterization of Nano-Powders Rainer Schmidt Refined Unit Cell Parameters Space group: Fm-3m (#225) a (Å) Agreement factors CeO2-δ a = 5.4211 (1) Å Rp = 3.65 ; Rwp = 4.87 ; Rexp = 2.75 ; χ2 = 3.14 Ce0.85Gd0.15O1.925 a = 5.4297 (1) Å Rp = 3.24 ; Rwp = 4.51 ; Rexp = 2.48 ; χ2 = 3.31 Ce0.85Sm0.15O1.925 a = 5.4345 (1) Å Rp = 3.69 ; Rwp = 5.30 ; Rexp = 2.58 ; χ2 = 4.24 Ce0.8Gd0.1Sm0.1O1.9 a = 5.4259 (2) Å Rp = 3.28 ; Rwp = 4.31 ; Rexp = 2.72 ; χ2 = 2.51 H.J. Whitfield, D. Roman, A.R. Palmer, J. Inorg. Nucl. Chem. 28 (1966) 2817. G. Brauer, H. Gradinger, Z. Anorg. Allgem. Chem. 276 (1954) 209. 11

Transmission Electron Microscopy (TEM) of Nano-Powders Structural Characterization of Nano-Powders Rainer Schmidt Transmission Electron Microscopy (TEM) of Nano-Powders 12

Scanning Electron Microscopy (SEM) Impedance Spectroscopy Measurements Rainer Schmidt Scanning Electron Microscopy (SEM) Massive grain growth as compared to the powders High sintering activity of the powders!

Impedance Spectroscopy Impedance Spectroscopy Measurements Rainer Schmidt Impedance Spectroscopy (440 K – 560 K @ 10 mHz – 3 MHz) Constant-Phase Element (CPE): For n = 1 : Ideal Capacitor 0.8 ≤ n ≤ 1: Non-ideal capacitor Interface pike visible Blocking electrode interface Ionic conductivity 14

Imaginary part of impedance vs frequency Impedance Spectroscopy Measurements Rainer Schmidt Imaginary part of impedance vs frequency Two regular thermally activated relaxation peaks (bulk + GB) One irregular interface contribution (blocking effect) GB resistance larger than bulk resistance large GB barrier effect 15

Dielectric permittivity e' vs frequency Impedance Spectroscopy Measurements Rainer Schmidt Dielectric permittivity e' vs frequency Two regular permittivity plateaus (bulk + GB) One irregular interface contribution (blocking effect) 16

GB resistance vs temperature trends for RE doped ceria ceramics Impedance Spectroscopy Measurements Rainer Schmidt GB resistance vs temperature trends for RE doped ceria ceramics Lowest GB resistance for Sm doped ceria preferential for application 17

Electronic contribution Impedance Spectroscopy Measurements Rainer Schmidt Bulk resistance vs temperature trends for RE doped ceria ceramics Activation energy and resistivity of CeO2-d too low for ionic conductivity Electronic contribution 18

Conclusions Rainer Schmidt Microwave assisted hydrothermal synthesis is an effective method to synthesise RE doped ceria powders with high sintering activity Line width broadening in XRD pattern, TEM images and BET Iso-therms confirmed small powder particle size and high surface area Impedance spectroscopy is a powerful tool to separately analyze the GB and bulk ionic conductivities in ceria ceramics Sm doped ceria shows the lowest GB resistance and may be preferential for application Undoped CeO2-d shows the signs of electronic charge transport most likely due to oxygen vacancies and mixed Ce4+/Ce3+ valence

Clear signs of an electronic contribution Impedance Spectroscopy Measurements Rainer Schmidt Signs for electronic conductivity in CeO2-d Additional interface permittivity plateau + additional interface Z ' – Z '' semicircle Clear signs of an electronic contribution 20