Rajalekshmi Chockalingam, Vasantha R.W. Amarakoon, and Herbert Giesche New York State College of Ceramics at Alfred University, Alfred, NY, USA Alumina

Rajalekshmi Chockalingam, Vasantha R.W. Amarakoon,

and Herbert Giesche

New York State College of Ceramics at Alfred University, Alfred, NY, USA

Alumina / Cerium Oxide

Nano-Composite Electrolyte

for

Solid Oxide Fuel Cell Applications

NYSCC Alfred University

Center for Advanced Ceramic Technology



But first: “Where on earth is Alfred ?”



eOHOHAnodeat 2: 22

2

22 2

2

1: OeOCathodeat

OHOHOverall 222 2

1:

Anode: Ni + YSZ

Cathode: La1−xSrxMnO3-δ

Electrolyte: 8 mol% Y2O3 stabilized ZrO2

Operated at close to 10001000°C°C.

http://en.wikipedia.org/wiki/Image:Fcell_diagram_sofc.gif

Alternative Electrolyte Materials



For Example:

Gadolinium doped Ceria

Ce0.8Gd0.2O1.9

Leading to lower operation temp.

However !!!

under reducing conditions:

Ce+IV → Ce+III electronic conduction.

Idea !!!

Electron Trapping Interfaces.

S.M. Haile, “Fuel cell materials and components,” Acta materialia, 51, 5981-6000 (2003)

Conduction Band

Acceptor states

Fermi Level

Valence Band

ΦL

2

1

02

dqNW

W= width of the depletion layer

εo= permittivity of free space

ε’= dielectric constant

Φ= potential barrier

q= electronic charge

Nd= charge carrier density

ZnO varistor microstructure Al2O3/CeO2 composite microstructure

Semiconductor Phase

Insulating Phase

Grain Boundary

CB

VB

Fermi Level

Acceptor States

ΦL

N N P P

N

P

P N

“Electron Trapping”



So, how do we make such a microstructure?



Coated Nano-particles.

Densify/sinter and retain microstructure.(microwave sintering; fast & uniform)


Use two suspensions of particles with opposite charge.

Zeta-potential (pH)

Surfactant adsorption

Porous coating; weak adhesion forces;requires large difference in particle size

Heterocoagulation

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Alfred University

Precipitate coating material onto seed particles.

Essentially “any” precipitation reaction can be used.

As long as it is a “controlled” (slow) precipitation

Dense and uniform coating

Heteronucleation


Silica-Yttria: Schematic Examples of Microstructures


Example


Silica spheres coated with yttria.

Heteronucleation Example



Excess silica cores remain after phase transformation and sintering.

Visualized by

etching with HF.

Heteronucleation Example cont.



Alfred University

Alumina core (seed)

Cobalt and Manganese surface layer; acceptor states at the interface

Gadolinium doped Ceria (50 to 100 nm) oxygen-ion-conductor; ‘continuous phase’

Microwave sintering to retainthe proposed microstructure

Schematic of the “new” nano-composite electrolyte.



T h erm om eter

S tirrer

C on sta n t tem p era tu re b a th

A l2 O 3 -M n -C o -S O L

Al(OC4H9)3 H2O (75°C)

Hydrolysis under vigorous stirring for 30 min

Peptization with HNO3 & Aging at 95°C for 5 days

Al2O3 SOLMn (NO3)2 6H2O + H2O

Co (NO3)2 6H2O

+ H2O

NH4OH

+ H2O

Mn, Co coated Al2O3 Sol

Stirr at 90°C for 4 hrs & Age 24 hrs

Synthesis of Mn, Co doped Al2O3 Sol



Coating of Gd doped CeO2 on Al2O3 Sol

T h erm om eter

S tirrer

C on sta n t tem p era tu re b a th

A l2 O 3 -M n -C o -2 0 % G d d o p e d C e O 2

Gd(NO3)3 6H 2O

+H2O

Aging at RT for 24 hours

Dry and heat Treatment

Mn, Co coated

Al2O3 Sol

Forming and sintering

Gd0.2Ce0.8O1.9coated on Mn, Co coated Al2O3 Sol

NH4OH + H2O

Vigorous stirring at 93°C for 6 hours

Ce(NO3)3.6H2O

+H2O

Microwave Sintering Set up

Figure (A) 2.45 GHz MW Furnace and Figure (B) Sample set up with alumina insulation box and Thermocouple.

A B



Sintering: Temperature Profile



0

200

400

600

800

1000

1200

1400

1600

0 250 500 750 1000 1250 1500 1750Time in minutes

Tem

per

atu

re°C

Conventional Microw ave

XRD Results for Gd0.2Ce0.8O1.9 -0.34%Mn-0.34%Co-Al2O3



0

500

1000

1500

2000

2500

20 30 40 50 60 70

2 theta

Inte

nsi

ty

900°C - 6h

900°C - 4h

900°C - 2h

700°C - 2h

500°C - 2h

300°C - 2h

room temp.

♥ matches Gd0.2Ce0.8O1.9


SEM Micrographs: Al2O3-0.34%Mn,Co-Gd0.2Ce0.8O1.9

MW1250C-40min

500nm500nm

CONV-1350C-5hr




3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

1200 1250 1300 1350 1400 1450 1500

Temperature (°C)

De

nsi

ty (

g/c

m3 )

GdC(MW)Coat-B(MW)Coat-C(MW)GdC (CON)Coat-B (CON)Coat-C (CON

Microwave sintering Conventional sintering



-1

1

3

5

7

9

11

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4

1000/T (K)

ln(s

T)

(S

cm

-1 K

)

GdC CON

Coat-B CON

Coat-C CON

Coat-B MW

Coat-C MW

Impedance Spectroscopy in Air (‘Ionic Conductivity’)



-5

-4

-3

-2

-1

0

1

2

3

4

-30 -25 -20 -15 -10 -5 0

Log Po2

Lo

g (s ) (

S/c

m)

GdC CON

Coat-B CON

Coat-C CON

600°C

Electrical Conductivity as a Function of Oxygen Partial Pressure



-5

-4

-3

-2

-1

0

1

2

3

4

-30 -25 -20 -15 -10 -5 0

Log Po2

Lo

g (

s) (

S/c

m)

GdC CON

Coat-B CON

Coat-C CON

800°C

Electrical Conductivity as a Function of Oxygen Partial Pressure


• Coated powders lead to unique microstructure.

• Microwave sintering is substantially faster.

• Submicron grain size can be retained

• Increased hardness.

• Electron trapping states at the alumina-ceria interface reduce electronic conductivity.

• Al2O3 inclusions have no major effect on ionic-conductivity.

Conclusions


• Better control of coating - thickness and uniformity.

• Test of other material combinations.

• Measure oxygen conductivity ‘directly’ (transference number).

• Test in a ‘real’ device !!!

What’s next?

Documents

Rajalekshmi Chockalingam, Vasantha R.W. Amarakoon, and Herbert Giesche New York State College of Ceramics at Alfred University, Alfred, NY, USA Alumina