Relating Microstructure and Ionic Conductivity in Calcium Doped Ceria for Solid Oxide Fuel Cell (SOFC) Electrolytes

Cruz Hernandez, Materials Science and Engineering Mentor: Will Bowman and Dr. Peter Crozier

School for the Engineering of Matter, Transport, and Energy

• A SOFC is an electrochemical conversion device that offers an efficiency around 85% (with combined heat and power).

• Cerium based SOFC offer high ionic conductivity at intermediate temperatures (400-700oC). Ionic conductivity can be improved by doping with appropriate elements.

• The purpose of this research is to study how the microstructure and ionic conductivity are affected by varying dopant concentrations of calcium in ceria.

O2 + 4e- → 2O2-

Cathode

CH4 + 4O2→ CO2 + 2H2O

Anode Electrolyte

• Transmission electron micrograph of spray dried ceria following calcination.

• A solution of nitrate precursors is sprayed into reactor at 400oC to form powder.

• Powder calcined at 800oC for 4 hours for full decomposition.

• Calcined powder pressed at 5 kip for 5 min. • Pellets sintered at 14000C for 24 hours.

• The XRD spectra indicate that the cubic fluorite structure of ceria was maintained during calcium doping.

[Ca] Davg % ρ

2% 5.9μm 95

5% .97μm 94

10% 1.4μm 81

• Ceria with 2% calcium had the lowest conductivity and 5% had the highest conductivity.

Conclusion • Spray drying is a useful technique for

doping ceria electrolytes. • XRD shows cubic fluorite structure for

all compositions. • SEM analysis shows grain size

dependence on Ca concentration. • Electrical characterization shows that

conductivity varies with Ca concentration.

• 5% Ca doping gives highest grain and grain boundary conductivity

25 30 35 40 45 50 55

Arb

itra

ry U

nit

s

2ϑ degrees

X-ray Diffraction of Ceria with 2, 5 and 10% Calcium CaDC2

CaDC5

CaDC10

111

200 220

(c)

5μm

(b)

5μm

(a)

10μm

Filter

Hot air gun

Powder Collector Solution Sprayer

Exhaust

• Scanning electron micrographs of 2%, 5% and 10% calcium doped ceria (a-c). • Average grain size (Davg) and % theoretical density.

-12

-10

-8

-6

-4

-2

0

2

0 0.5 1 1.5 2 2.5 3

Log

(σ*T

)

1000/T (1/K)

CaxCe(1-x)O(2-δ) ionic conductivity

CaDC_2

CaDC_5

CaDC_10

fcmGDC10 grain

gb2

gb5

gb10

fcmGDC10 gb

Grain conductivity

Grain boundary conductivity

2% Ca

5% Ca

10% Ca

GDC10 (ref)

2% Ca

5% Ca

10% Ca

GDC10 (ref)

Background and Introduction

Materials Preparation by Spray Drying

Results and Conclusions

Acknowledgements We gratefully acknowledge the LeRoy Eyring Center for Solid State Science and the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University. This material is based upon work supported by the National Science Foundation under NSF DMR 1308085.