M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Brandon E. Bürgler Nonmetallic Inorganic Materials ETH Zürich Single

M a t e r i a l s

Swiss Federal

Institute of Technology

Zürich

Nonmetallic Materials

Brandon E. Bürgler

Nonmetallic Inorganic Materials

ETH Zürich

Single Chamber Solid Oxide Fuel Cells (SC-SOFC)

Thursday, March 19th, 2004

M a t e r i a l s

Swiss Federal


Zürich


Outline

• Single Chamber SOFCs

• Experiments

• Modelling Issues

• Outlook

SC-SOFC

M a t e r i a l s

Swiss Federal


Zürich


• Cathode and anode exposed to same gas mixture of fuel and oxidant

• Selectivity of electrodes for either oxidation or reduction reaction

Single Chamber SOFC

OCV

O2 + N2

H2

+ N2

H2O

O2

ocvCH4 + O2 + N2

CO + CO2 + N2 + H2

CH4 + N2 + O2

Conventional SOFC Single Chamber SOFC

SC-SOFC

M a t e r i a l s

Swiss Federal


Zürich


SC-SOFC ↔ conventional SOFC

Advantages

• Simplified cell sealing• Elimination of complex flow field structures• Fast start-up possible• Costs

Challenges

• Non-equilibrium gas mixture (explosive from 5 to 15% CH4 in air)

• Fuel utilisation?• Parasitic chemical reactions

SC-SOFC

M a t e r i a l s

Swiss Federal


Zürich


Basic Designs of SC-SOFCs

CH4 + air

a) classic

b) fully porous

c) planar

SC-SOFC

M a t e r i a l s

Swiss Federal


Zürich


Open Questions & Aims

• Which parameters influence the OCV and the

maximum power output?

• Fundamental model of SC-SOFC including non-

ideal electrodes and CH4 as the fuel

• High performance SC-SOFC

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Measurement Setup

CH4Air

CH4AirExhaust

Thermocouple

U

I

Electrical characterisation:

Galvanostatic 4-point

measurements

Temperature:• 400 - 700°C• dT/dt < 2.5°C/min

CH4-air mixture:

• Air: 100-400ml/min

• CH4: 100ml/min,

moistened (~3% H2O)

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Cell Design & Preparation

Current collector: Pt-mesh

Electrolyte: Ce0.9Gd0.1O1.95 (CGO)

Anode: 60wt% NiO,40wt% CGO

Cathode: Sm0.5Sr0.5CoO3-

10mm 0.2 – 0.53 mm

Current collector: Pt-mesh

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Fuel Cell Cross Section

Cathode (~140 m)

Anode (~160 m)

Electrolyte (~330m)

Pt-mesh, longitudinal section (~80 m)

Pt-mesh, cross section (~80 m )

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Open Circuit Voltage

MS14 (0.19mm electrolyte):

Reduction of NiO

strong heat generation

300 350 400 450 500 550 600 650 700 7500.0

0.2

0.4

0.6

0.8

1.0

OC

V [V

olt]

Temperature [°C]

100ml/min air (heating) 100ml/min air (cooling)

Experiments

M a t e r i a l s

Swiss Federal


Zürich


U-I Characteristics at different flow

0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

Current Density [mA/cm2]

Vol

tage

[V]

0

100

200

300

400

500P

ower D

ensity [m

W/cm

2]T = 600°C

Experiments

M a t e r i a l s

Swiss Federal


Zürich



0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

100ml/min air


Vol

tage

[V]

0

100

200

300

400

500P

ower D

ensity [m

W/cm

2]T = 600°C

Experiments

M a t e r i a l s

Swiss Federal


Zürich



0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

100ml/min air 200ml/min air


Vol

tage

[V]

0

100

200

300

400

500P

ower D

ensity [m

W/cm

2]T = 600°C

Experiments

M a t e r i a l s

Swiss Federal


Zürich



0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

100ml/min air 200ml/min air 300ml/min air


Vol

tage

[V]

0

100

200

300

400

500P

ower D

ensity [m

W/cm

2]T = 600°C

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Pmax = 440 mW/cm2 @ 100 ml/min CH4 and

200 ml/min Air at 600°C


0 500 1000 1500 20000.0

0.2

0.4

0.6

0.8

100ml/min air 200ml/min air 300ml/min air 400ml/min air


Vol

tage

[V]

0

100

200

300

400

500P

ower D

ensity [m

W/cm

2]T = 600°C

Experiments

M a t e r i a l s

Swiss Federal


Zürich


U-I characteristic at different Temperatures

0 500 1000 15000.0

0.2

0.4

0.6

0.8

500°C


Vo

ltag

e [V

olt]

0

100

200

300

400P

ow

er D

en

sity [mW

/cm2]

fAir = 200 ml/min

Experiments

M a t e r i a l s

Swiss Federal


Zürich



0 500 1000 15000.0

0.2

0.4

0.6

0.8

500°C 600°C


Vo

ltag

e [V

olt]

0

100

200

300

400P

ow

er D

en

sity [mW

/cm2]

fAir = 200 ml/min

Experiments

M a t e r i a l s

Swiss Federal


Zürich



0 500 1000 15000.0

0.2

0.4

0.6

0.8

500°C 600°C 700°C


Vo

ltag

e [V

olt]

0

100

200

300

400P

ow

er D

en

sity [mW

/cm2]

fAir = 200 ml/min

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Open Circuit Voltage

400 500 600 7000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

OC

V [

V]

Temperature [°C]


400 500 600 7000

50

100

150

200

250

300

max

. P

ower

Den

sity

[m

W/c

m2 ]

Temperature [°C]


Experiments

M a t e r i a l s

Swiss Federal


Zürich


Electrode Temperatures at OCV

100 200 300 400500

600

700

800

Tnorm

=600°C

Tnorm

=500°C

Tnorm

=700°C

Cathode Temperature Anode Tempearature

Air Flow [ml/min]

Te

mp

era

ture

[°C

]

Pronounced heat generation on the anode

Electrolyte thickness: 390m

Experiments

M a t e r i a l s

Swiss Federal


Zürich


Conclusions from Experiments

• Cells operate at T > 500°C

• Optimum conditions for maximum Power

output at T= 600°C and fair = 300 ml/min

• Pronounced heat generation at the anode

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Modelling of SC-SOFCs

1. Single Chamber SOFC versus Double Chamber: Driving force for ionic current?

2. Calculations of Equilibrium gas mixtures at anode

3. Mixed ionic electronic conducting electrolyte

Modelling

M a t e r i a l s

Swiss Federal


Zürich


What is the driving force for the ionic current?

Assumptions:

-Hydrogen as fuel, air as oxidant

-Reversible and perfectly selective electrodes for H2 or O2

-Electrolyte only O2--conductor

Riess, I., van der Put, P. J. (1995). "Solid oxide fuel cells operating on uniform mixtures of fuel and air." Solid State Ionics 82: 1-4.

Cathode

½ O2 (gas) + 2e- → O2- (C)

Anode

½ H2 (gas) → H+ (A) + 2e-

O2- (SE/A) + 2 H+ (A)→ H2O (gas)(C)

(A) (SE/A)

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Calculation of O2-

Combination of (7), (8) and (9) yields

½ O2 (gas) + 2e- ↔ O2- (C)

½ H2 (gas) ↔ H+ (A) + e-

O2- (SE/A) + 2 H+ (A)↔ H2O (gas)

GOCV =

2q

The Nernst Voltage is the same for

SC- and conventional SOFCs

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Comment

• Electrodes are not ideally selective nor reversible.

• Direct oxidation of the fuel (=parasitical) lowers OCV.

• Improving selectivity of the electrodes will improve efficiency and reduce fuel waste.

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Operation Principles of a SC-SOFC

O2 + 4e- 2O2-

p(O2 )

p(O2 )

OCV

CH4 + air

CH4 + air

H2 + O2- 2H2O + 2e-

CO + O2- CO2 + 2e-

CH4 + 1/2O2 2H2 + CO

cathode2anode2

p(O )kTOCV = ln

4q p(O )

partial oxidation of methane

Modelling

M a t e r i a l s

Swiss Federal


Zürich



1. Single Chamber SOFC versus Double Chamber. Driving force for ionic current?



Modelling

M a t e r i a l s

Swiss Federal


Zürich


Calculation of equilibrium gas mixtures

Input:

T, Composition

X(C), X(O), X(H)

Minimisation of Gibbs

Free Energy

Output:

Concentrations of species: CH4, O2, H2, CO,

CO2

Thermocalc™

Basic idea:

Anode: Equilibrium reached very fast. pO2 ≈ 10-26 atm

Cathode: non-equilibrium gas mixtures remains unreacted

pO2 ≈ 0.05 – 0.17 atm

CH4 + Air

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Equilibria for Different CH4:O2 Ratios

0.0 0.2 0.4 0.6 0.8 1.0

0.0

0.2

0.4

0.6

0.8

1.0

frac

tion

in g

as p

hase

x(O)

CH4

CO

CO2

H2

H2O

O2

Suitable mixtures for SC-SOFCs

X (O)

T = 600°C

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Carbon deposition at low x(O)

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

mo

lar

fra

cti

on

x(O)

solid

fCH4 = 100 ml/min

fAir= 100 ml/min

Carbon deposition at low x(O)!!

gas

Modelling

M a t e r i a l s

Swiss Federal


Zürich






Modelling

M a t e r i a l s

Swiss Federal


Zürich


high pO2 conductance predominantly ionic.

low pO2 partial reduction n-type semiconduction Electronic conductivity ~ [Ce3+] ~ pO2

-1/4.

D. Schneider, M. Gödickemeier and L.J. GaucklerJ. of Electroceramics, 1, [2], (1997), 165-172

Conductivity of Ce0.8Sm0.2O1.9-x vs. pO2

Electrolyte is a mixed ionic electronic conductor

28 24 20 16 12 8 4 01

10

873 K

973 K

1073 K

tot [

S/m

]

-log (pO

2

/atm)

-1/4

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Transport Model for MIEC- SOFC electrolyte

Gödickemeier, M., Sasaki, K. and Gauckler, L. J. (1997)."Electrochemical Characteristics of Cathodes in Solid Oxide Fuel Cells based on Ceria Electrolytes."J. Electrochem. Soc. 144(5): 1635-1646.

j kT q N

j kT q n

i iNx i x

e enx i x

2

2

Nx

0

nn

qkT th

L V MC0

exp( ( ))

for n p O ( )2

14

V MC V V Vth Nernst th C th A( ) , , jV MC V

L

V

V MC Ve e

L th CellqkT Cell

qkT th Cell

( ) exp( )

exp( ( ( ) ))

1

1j V MC V

Li ith Cell ( )

MIEC

p(O2)low

p(O2)0

p(O2)L

p(O2)high

e

0 L

x

O 2 -

< < <

VCell

air side

fuel side

load

A Cext A ext C

1. transport equations

2. defect distribution

3. electrode overpotentials

jx

jx

i e 0 0

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Partial currents in MIEC – SOFC electrolytes

ionic & electronic current It

cell current It

Vcell

Vcell = VOC

-Ie = Ii

open circuit voltage

Vth

electronic current Ie

Vcell

electronic current Ie Ie(Vocv)

Vcell = 0Ie = 0

IeRe

Vcell

ionic current Ii

ionic current Ii

Ii(Vocv)

Vcell = Vth, app

Ii = 0

IiRi +Vth,C+ Vth,A

Modelling

M a t e r i a l s

Swiss Federal


Zürich


Conclusions




4. Thermal Reactor

----

M a t e r i a l s

Swiss Federal


Zürich


Acknowledgements

• Prof. Dr. L. J. Gauckler• A. Nicholas Grundy• Michel Prestat• SOFC group• The entire Nonmets Group

Diploma students• Marco Siegrist• Srdan Vasic

M a t e r i a l s

Swiss Federal


Zürich


Thank you for your kind attention

Documents

M a t e r i a l s Swiss Federal Institute of Technology Zürich Nonmetallic Materials Brandon E. Bürgler Nonmetallic Inorganic Materials ETH Zürich Single