Brandon E. B

1
Single Chamber Solid Oxide Fuel Cells (SC-SOFC)
Brandon E. Bürgler Nonmetallic Inorganic
Materials ETH Zürich
Thursday, March 19th, 2004
2
Outline

• Single Chamber SOFCs
• Experiments
• Modelling Issues
• Outlook

3
Single Chamber SOFC
O2 N2
N2
O2
OCV
H2
H2O
Conventional SOFC
Single Chamber SOFC

• Cathode and anode exposed to same gas mixture of
  fuel and oxidant
• Selectivity of electrodes for either oxidation or
  reduction reaction

4
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

5
Basic Designs of SC-SOFCs
6
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

7
Measurement Setup

• Temperature
• 400 - 700C
• dT/dt lt 2.5C/min
• CH4-air mixture
• Air 100-400ml/min
• CH4 100ml/min, moistened (3 H2O)

Electrical characterisation Galvanostatic
4-point measurements
8
Cell Design Preparation
Current collector Pt-mesh
Anode 60wt NiO, 40wt CGO
Electrolyte Ce0.9Gd0.1O1.95 (CGO)
0.2 0.53 mm
10mm
Cathode Sm0.5Sr0.5CoO3-?
Current collector Pt-mesh
9
Fuel Cell Cross Section
Anode (160 ?m)
Pt-mesh, cross section (80 ?m )
Electrolyte (330?m)
Cathode (140 ?m)
Pt-mesh, longitudinal section (80 ?m)
10
Open Circuit Voltage
MS14 (0.19mm electrolyte)
11
U-I Characteristics at different flow
T 600C
12
U-I Characteristics at different flow
T 600C
13
U-I Characteristics at different flow
T 600C
14
U-I Characteristics at different flow
T 600C
15
U-I Characteristics at different flow
T 600C

• Pmax 440 mW/cm2 _at_ 100 ml/min CH4 and 200
  ml/min Air at 600C

16
U-I characteristic at different Temperatures
fAir 200 ml/min
17
U-I characteristic at different Temperatures
fAir 200 ml/min
18
U-I characteristic at different Temperatures
fAir 200 ml/min
19
Open Circuit Voltage
20
Electrode Temperatures at OCV
Electrolyte thickness 390?m
? Pronounced heat generation on the anode
21
Conclusions from Experiments

• Cells operate at T gt 500C
• Optimum conditions for maximum Power output at T
  600C and fair 300 ml/min
• Pronounced heat generation at the anode

22
Modelling of SC-SOFCs

• Single Chamber SOFC versus Double Chamber
  Driving force for ionic current?
• Calculations of Equilibrium gas mixtures at anode
• Mixed ionic electronic conducting electrolyte

23
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
Anode ½ H2 (gas) ? H (A) 2e- O2- (SE/A) 2 H
(A)? H2O (gas)
Cathode ½ O2 (gas) 2e- ? O2- (C)
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.
24
Calculation of DmO2-
½ O2 (gas) 2e- ? O2- (C)
½ H2 (gas) ? H (A) e-
O2- (SE/A) 2 H (A)? H2O (gas)
Combination of (7), (8) and (9) yields
The Nernst Voltage is the same for SC- and
conventional SOFCs
25
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.

26
Operation Principles of a SC-SOFC
CH4 air
CH4 1/2O2 ? 2H2 CO
partial oxidation of methane
CO O2- ? CO2 2e-
p(O2 )
H2 O2- ? 2H2O 2e-
OCV
p(O2 )
O2 4e- ? 2O2-
CH4 air
27
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

28
Calculation of equilibrium gas mixtures
CH4 Air
Basic idea Anode Equilibrium reached very fast.
pO2 10-26 atm Cathode non-equilibrium gas
mixtures remains unreacted pO2 0.05 0.17 atm
Thermocalc
Output Concentrations of species CH4, O2, H2,
CO, CO2
Minimisation of Gibbs Free Energy
29
Equilibria for Different CH4O2 Ratios
Suitable mixtures for SC-SOFCs
T 600C
X (O)
30
Carbon deposition at low x(O)
fCH4 100 ml/min fAir 100 ml/min
gas
solid
?Carbon deposition at low x(O)!!
31
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

32
Conductivity of Ce0.8Sm0.2O1.9-x vs. pO2
high pO2 ? conductance predominantly ionic. low
pO2 ? partial reduction ? n-type semiconduction
Electronic conductivity Ce3 pO2-1/4.
? Electrolyte is a mixed ionic electronic
conductor
D. Schneider, M. Gödickemeier and L.J.
GaucklerJ. of Electroceramics, 1, 2, (1997),
165-172
33
Transport Model for MIEC- SOFC electrolyte
anode
cathode
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.
34
Partial currents in MIEC SOFC electrolytes
35
Conclusions

• Single Chamber SOFC versus Double Chamber.
  Driving force for ionic current?
• Calculations of Equilibrium gas mixtures at anode
• Mixed ionic electronic conducting electrolyte
• Thermal Reactor

----
36
Acknowledgements

• Prof. Dr. L. J. Gauckler
• A. Nicholas Grundy
• Michel Prestat
• SOFC group
• The entire Nonmets Group
• Diploma students
• Marco Siegrist
• Srdan Vasic

37
Thank you for your kind attention