Solid State Approach: La9.33Si6O26 Electrolyte as a Replacement for YSZ in Solid Oxide Fuel Cells

By: Scott Wilhour, Penn State, MatSEMentor: Martha Mecartney, UCI, ChEMS

Outline

• What are SOFCs?• Purpose of this study• Apatite-type lanthanum silicate• Key points to remember• Experimental process• Results• Conclusions• Future work

How SOFCs Work• What is it?

– Electrochemical energy generation device

– Operates like a battery, no recharging

• As long as fuel is supplied to it, the fuel cell will produce energy in the form of electricity and heat

Purpose of this Study

• Use the apatite structure for an electrolyte as an alternative to currently used yttria stabilized zirconia (YSZ)

• High-conductivity electrolyte to promote the utility of the solid oxide fuel cell– Lower the internal SOFC temperature

• solve the leaking-seal problem due to high operating temperatures in YSZ SOFCs

New Electrolyte La28Si18O78

(La9.33Si6O26)

• High oxide ion conductivity at low T

• Conduction occurs via an interstitial mechanism

• Has lower operating temperature than YSZ

KEY POINTS

• Nanocrystalline grain size as small as possible (~20 nm) for high conductivity

• Hypothesis is that when smaller grains increase the grain boundary area, should get improved conductivity since materials have interstitial transport of the oxygen ions and grain boundaries should have more space for interstitials– As a result, nanocrystalline apatite would

have a higher oxide-ion conductivity to use in future fuel cells than currently used YSZ

Experimental Process• Procedure for making lanthanum silicate

powders:– La2O3 and SiO2 raw powders are mixed in 4:5 ratio– Mill powders for 8 hours (Attrition and Cryo)– Pack powder into a mold– Cold Isostatic Press (CIP) to mold the pellet– Heat treatment in furnace at 1450°C for 2 hours– Reaction produces lanthanum-silicate based apatite

(La9.33Si6O26)

• Analysis of materials synthesized– XRD ~ used to determine phase composition and

grain size– SEM ~ used to determine grain size– IS ~ used to determine material’s impedance and

consequently its ionic conductivity (future work)

Results – XRD

• It was concluded that all of the diffraction peaks of the sample can be assigned to La28Si18O78. This means that there was no contamination from impurities

• Grain sizes were then determined using Scherrer’s method of peak broadening– Attrition-milled lanthanum silicate yielded

~47nm grains– Cryo-milled lanthanum silicate yielded ~29nm

grains, within the range needed for high conductivity

XRD for La2O3

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XRD for Attritor milled La28Si18O78

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XRD for Cryo-milled La28Si18O78

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Results – SEM

Attrition-Milled Cryo-Milled

Results – SEM

Attrition-Milled Cryo-Milled

Conclusions

• Apatite-type lanthanum silicate (La9.33Si6O26) exhibits smaller grain sizes on the order of 20-30 nm from XRD data when cryo-milled

• This shows that achieving a fine grain size in apatite ceramics is possible with cryo-milling.

Future Work

• Impedance Spectroscopy was not conducted due to time constraints, and remains as the final step in this study to measure ionic conductivity as a function of grain size in lanthanum silicate

• Oxygen ion conductivity in lanthanum silicate electrolyte will need to be compared to yttria stabilized zirconia to see if it is superior

Acknowledgements

• Principal Investigator – Professor Martha Mecartney

• Graduate Mentor – Mai Ng

• Aminah Rumjahn

• Peter Dillon

• IMSURE Program

• National Science Foundation