Sol Gel Approach: Lanthanum Silicates as a Replacement for Yttria Stabilized Zirconia (YSZ) in Solid Oxide Fuel Cell (SOFC) Electrolytes Aminah Rumjahn Chemical Engineering and Material Science University of California, Davis University of California, Irvine Chemical Engineering and Material Science PI: Martha Mecartney Graduate student: Mai Ng

Outline Intro to Solid Oxide Fuel Cells (SOFCs) Intro to Solid Oxide Fuel Cells (SOFCs) Motivation for Work and Goals Motivation for Work and Goals Background of Apatite Background of Apatite Experimental procedure Experimental procedure Data Data Results Results Conclusion/Future Work Conclusion/Future Work

SOFCs

Motivation Yttria Stabilized Zirconia (YSZ) is traditional material used for electrolyte Yttria Stabilized Zirconia (YSZ) is traditional material used for electrolyte  Main disadvantage: High operating temp  Main disadvantage: High operating temp Diffusion equation: D = D 0 exp(-Q/RT) Diffusion equation: D = D 0 exp(-Q/RT) New materials must be considered New materials must be considered Good electrolyte: stable, lower operating temp, high oxygen ion conductivity Good electrolyte: stable, lower operating temp, high oxygen ion conductivity Apatite = lower operating temp and high oxygen ion conductivity Apatite = lower operating temp and high oxygen ion conductivity

Grain Size Must Be SMALL! Studies have shown some nanocrystalline ceramics have high ionic conductivityStudies have shown some nanocrystalline ceramics have high ionic conductivity Ionic conductivity governed by grain boundariesIonic conductivity governed by grain boundaries Ions in oxy-apatite travel faster in interstitial regionsIons in oxy-apatite travel faster in interstitial regions Hypothesis is more grain boundaries = higher conductivityHypothesis is more grain boundaries = higher conductivity Small grains  More grain boundaries = More efficient electrolyte material

Apatite Silicate-based apatite with SiO4 tetrahedra (yellow). The calculated pathway for oxygen diffusion is shown. *M S Islam, University of Bath A type of mineral Structure  Hexagonal monoclinic Rare earth oxy-apatites: La, Ce, Gd, Sm Specifically, apatite-type lanthanum silicates exhibit highest ionic conductivity

About Sol Gel Solution chemistry basedSolution chemistry based Inorganic metal salts/metal organic compounds (metal alkoxides)Inorganic metal salts/metal organic compounds (metal alkoxides) Hydrolysis and polymerization forms liquid SOLHydrolysis and polymerization forms liquid SOL Condensation forms solid GELCondensation forms solid GEL Heat treatment  crystalline ceramicsHeat treatment  crystalline ceramics AdvantagesAdvantages composition highly controllablecomposition highly controllable low temperatureslow temperatures homogenous mixinghomogenous mixing more freedom for applications -- coat = thin filmsmore freedom for applications -- coat = thin films SOLGEL

Procedure Dissolve lanthanum nitrate hexahydrate in ethanol and acetic acidDissolve lanthanum nitrate hexahydrate in ethanol and acetic acid Add tetraethylorthosilicate (TEOS)Add tetraethylorthosilicate (TEOS)  Sol  Sol Dry overnightDry overnight  Gel  Gel Heat treatments: Decompose at 600°C for 4hrs and Calcine at 1000°C for 2hrsHeat treatments: Decompose at 600°C for 4hrs and Calcine at 1000°C for 2hrs  Solid oxy-apatite (La 9.33 Si 6 O 26 )  Solid oxy-apatite (La 9.33 Si 6 O 26 )

Cryomilling No previous studies on cryomilling of ceramics No previous studies on cryomilling of ceramics Success with cryomilling of metals Success with cryomilling of metals  Reduced grain size of Al to ~26nm*  Reduced grain size of Al to ~26nm* Cryogenic = very low temps  liqN 2 (-200°C) Cryogenic = very low temps  liqN 2 (-200°C) *F.Zhou, D.Witkin, S.R. Nutt, E.J. Lavernia, Mater. Sci. Eng. A (2004)

Results: XRD STANDARDCRYOMILLED Water Contamination!! Secondary phase: La 2 SiO 5

Scherrer Equation t = 0.9λ t = 0.9λ Bcos (θ B ) Bcos (θ B ) t = crystallite size λ = wavelength of Cu filament (1.54Å) B = width of peak at ½I max θ B = angle of peak NOT APPLICABLE FOR SIZES > 200nm Crystallite size of standard sample: ~21nm Crystallite size of cryomilled sample: ~14nm

Results: SEM  Standard Sol Gel Cryomilled Sol Gel 

Results: SEM STANDARD

Results: SEM CRYOMILLED

Conclusions Fabricated apatite-type La 9.33 Si 6 O 26 through sol gel route Fabricated apatite-type La 9.33 Si 6 O 26 through sol gel route Scherrer formula gives similar crystallite sizes Scherrer formula gives similar crystallite sizes SEM shows cryomilled powders are less agglomerated SEM shows cryomilled powders are less agglomerated Uncertain of the effects of cryomilling!!

Future Work Fabrication of sintered pellets to conduct impedance spectroscopy (IS) to determine ionic conductivity of La 9.33 Si 6 O 26 Fabrication of sintered pellets to conduct impedance spectroscopy (IS) to determine ionic conductivity of La 9.33 Si 6 O 26 More Cryomilling!!! More Cryomilling!!!  Devise a better collection method to  Devise a better collection method to avoid water contamination avoid water contamination  Vary the milling time  Vary the milling time  Characterization tests  Characterization tests  Density measurements  Density measurements

Professor Martha Mecartney and Graduate student Mai Ng for their enthusiasm, guidance and support Mecartney and Mumm groups UC Irvine and the UROP team for the IMSURE program NSF for financial support Zeiss Center of Excellence for microscopy support Acknowledgements