Transcript Document

Simulazione ed ottimizzazione di
elettrodi per celle a combustibile ad
ossidi solidi
Marco Cannarozzo, Paola Costamagna
Genova, 12 Dicembre 2007
Incontri del Dottorato di Ingegneria Chimica e dei Materiali
DICHEP - UNIGE
What is a fuel cell?
Combustion:
H 2  O2 
 H 2O  heat
Fuel Cell (SOFC):
H2  O
2

 H 2O  2e

1

2
O2  2e 
 O
2
Fuel cells combine hydrogen and
oxygen electrochemically to produce
electricity with high efficency.
Composite electrodes model
Composite electrodes benefits:
H2O
H2
e-
Electronic
current
•Reduction of aging effects caused by agglomeration of Ni in
the anode.
•Stress reduction caused by thermal expansion;
SOFC
Anode
•Enlargement of the active area for the electrochemical
reaction in the whole electrode thickness, in solid electrolyte
fuel cell;
Ionic
current
O2-
Continuous conducting networks for both conductors between
current collector and electrolyte
Electrochemical kinetic
Percolation
theory
H 2  O 2 
 H 2O  2e 
Mass and
charge
transport
model
Charge transport and electrochemical kinetic
C.C.
eq
el
Electrolyte
iel
 dVio
eff



iio
io
 dx

 dVel    eff i

el el
 dx
 diio
di
  el

dx
 dx
  Veleq  Vioeq  Vel  Vio 
V
anode
I
I
iio


 Vioeq




p
x
1  0
RT pH 2O x 
H
O
0
0
2


 H 2O   H 2  RT ln
 Vanodo 
ln

2 F  
pH 2 x  
2F
pH 2  x 
0
G



anodo

0

Electrochemical kinetic (Butler Volmer):
p
diel
F  
 F  pH 2O



  A  i0  H0 2 exp 

exp

1




 
0
dx
RT  
 RT  pH 2O

 pH 2
x
Mass transport and balance
Transport
equations
Dk is function of the single
component
DK H 2O  DK H 2
Transport equations
Anodic semi-reaction: H 2  O2  H 2O
Equimolarity condition for gases: N H 2  N H 2O  0
Since the Knudsen diffusivity of hydrogen and water are different, due to the previous
condition, a total convective flow must exist; it can be evaluated imposing the equimolarity
and the expression obtained is:
D* is now a function of the gas composition
and then of the x coordinate
Percolation theory
The percolation theory has been used to estimate the contact area between particles and
the effective conductivities for ionic and electronic conductors.
1e5
8000
P=1 rel=0.05
P=1 rel=0.1
P=1 rel=0.2
P=0.5 rel=0.05
P=2 rel=0.1
area (cm2/cm3)
6000
effective conductivity (S/cm)
7000
elec eff cond
1173 K
1073 K
973 K
1e4
5000
4000
3000
2000
1e3
1e2
1e1
1
1000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
elec. cond. volumetric fraction
0.8
0.9
0
0.2
0.4
0.6
elec. cond. volum fraction
0.8
1
Results
Comparison between the two versions of the model
0.18
Total overpotential (V)
Total overpotential (V)
0.2
0.15
0.1
new model
0.05
0
0
previous model
1
2
rel (cm)
3
4
-5
x 10
previous model
new model
0.16
0.14
0.12
0.1
0.08
0.06
0
0.005
0.01
0.015
thickness (cm)
0.02
The two models are different where the concentration polarizations are important, then
for thick electrodes or with a fine microstructure.
The new version of the model can be useful to simulate
electrodes of electrode supported fuel cell
Results along the electrode thickness
Varying parameter: radius of electronic conducting particles
•Overpotential
•Current
0.5
0.18
0.16
0.05 micron
0.1 micron
0.2 micron
0.45
0.4
0.35
0.12
current (A/cm2)
overpotential (V)
0.14
0.1
0.08
0.06
0.05 micron (elect curr)
0.3
0.1 micron (ionic curr)
0.25
0.1 micron (elect curr)
0.2
0.2 micron (ionic curr)
0.2 micron (elect curr)
0.15
0.04
0.1
0.02
0
0
0.05 micron (ionic curr)
0.05
0.2
0.4
0.6
x/thickness
0.8
1
0
0
0.2
0.4
0.6
x/thickness
0.8
(φ=0.5,P=1,T=1173K,p=1atm,ph20=0.6atm,I=5000A/m2, thickness 150 micron)
Decreasing the dimension of the particles, the active area per unit volume increases, then the
reaction takes place nearer the electrolyte
1
Optimization (1)
Total overpotential as a function of the electrode volumetric composition
New version of the model
Previous version of the model (only activation
and ohmic losses)
0.2
0.2
Total overpotential (V)
Total overpotential (V)
30 micron
50 micron
80 micron
150 micron
0.15
0.1
0.15
0.1
30 micron
50 micron
80 micron
150 micron
0.05
0
0.2
0.4
0.6
elec. cond vol. fraction
0.8
1
0.05
0
0.2
0.4
0.6
elec. cond vol. fraction
0.8
In the previous version of the model the total electrode overpotential is asymptotic with
thickness, while including concentration polarizations there is a optimum
1
Optimization (2)
Increasing particle dimension
0.18
Total overpotential (V)
0.16
0.14
Bigger pore
dimension
0.12
200
0.1
Less “active area”
useful for the
electrochemical
reaction
150
100
0.08
50
0.06
0
0.5
1
1.5
elec cond. particles dimension (cm)
2
x 10
-5
Lower concentration
polarizations (due to
a better diffusion)
Positive effect
Higher activation
polarizations
Negative effect
Summarizing:
thickness
Particles dimension
Concentration
losses
Activation losses
Application to RRFCS IPSOFC
equivalent conductivity (S/cm)
Integration of comp. el. model into 1-D model
0.01
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0
44% Ni
55% Ni
0
5
10
15
20
An increase of the
anode thickness
increases the
conductivity
because the TPB is
spread in a deeper
zone.
25
anode thickness (microns)
The increase of the Ni content causes a decrease of the equivalent conductivity
because the ionic conductive network is weakened, and for these thickness it is
important to have a strong ionic conduction because the TPB is extended to the
whole thickness.
Summarizing: it is important to have an anode composition near to the
electronic percolation threshold.
Conclusion and future applications
•Development of the model for composite electrodes;
•Optimization of thickness, microstructure and composition;
•Study of the degradation effects on the electrode performance and
optimization of the electrode for the whole operating life;
Further studies and applications:
•O2- transport and effect on concentration polarizations;
•Shifting and reforming reaction;
•Heat balance (in order to integrate the model into the cell model);
•Cathode simulation taking into account of the LSM mixed conductivity;
•Optimization of multilayered electrodes;
•Percolation theory applied also to the porosity.
Pubblications
•Cannarozzo M., Grosso S., Agnew G., Del Borghi A., Costamagna P., Effects of mass transport on the
performance of composite sofc electrodes, Journal of Fuel Cell Science and Technology, February 2007,
vol. 4, issue 1, pp. 99-106
•Cannarozzo M., Grosso S., Agnew G., Costamagna P., Effects of mass transport on the performance of
SOFC composite electrodes, Proceedings of the 1st European Fuel Cell Technology and Applications
Conference, ISBN No. 0-7918-4209-6, Roma, p.112, 14-16 dicembre 2005, copyright © 2005.
•Cannarozzo M., Costamagna P., Modelling of aging of full size SOFCs, 3rd Real-SOFC Workshop on
Modelling and understanding degradation in SOFC, p. 22, Lucerna, CH, 2-3 luglio 2006.
•Cannarozzo M., Bozzolo M., Costamagna P., Simulation of the performance of cermet electrodes for
SOFC, proceedings of the 10th european inter-regional conference on ceramics, ISBN 978-0-9546104-18, pp.279-289, 6-10 Settembre 2006.
•Cannarozzo M., Costamagna P., Simulation of the effects of mass transport on the electrochemical
performance of SOFC composite electrodes, Abstract of the Gei-Era 2007, Cagliari, Italia, 15-20 luglio
2007.
•Cannarozzo M., Costamagna P., Simulation of mass transport in SOFC composite electrodes, submitted
to Journal of Applied Electrochemistry.
•Cannarozzo M., Costamagna P., Modelling degradation in SOFCs, submitted to Journal of Power
Sources.