Synthesis, characterization and
modeling of porous electrodes for
fuel cells
- Hao Wen
- Prepared for defense practice talk
- 3/29/2012
1
Fuel cells - overview
Motor vehicles
Load
current
Cathode
Electrolyte
Fuel
Anode
Portable device power supply
Air
Fuel cells convert chemical energy into electricity
Applications varies from high temperature
high power output to room temperature
portable power sources.
Biofuel cells
http://www.fllibertarian.org/
Barton, S.C., AlCHE annual meeting
2
Multiscale porous electrode support
Fuel transport
eReactants
Support
Too much porosity lowers conductivity
Electrolyte
Reactants
eProduct
Reactants
Catalyst
e-
Mesopores
Interfacial reaction
Current collector
3
Synthesis of carbon porous electrodes
Carbon nanotube
Carbonaceous foam monolith
Exfoliated graphite
Template introduced macro-pore
Surface modification,
compositing, and
coating with catalyst
www.nanocyl.com
J. Lu 2007, Chemistry of Materials
O. Velev, 2000, Advanced Materials
Flexer, 2010, Energy and Environmental Science
4
Modeling scheme
OUTUT
INPUT
Geometry
RDE
PRDE
Film
Porous layer
Kinetics
Ping pong bi bi
Differential linear kinetics
Transport
Fuel / Oxygen
In Channel, porous layer
Measurable
Impedance
Polarization
Cyclic voltammetry
Porous
Electrode
Model
Hardly Measurable
Concentration profile
Active region
Optimization
Electrode thickness
Porosity
Feeding rate
5
Porous electrodes under study
CNT
Carbon fiber
Carbon nanotube coated carbon
fiber microelectrode
Polystyrene derived macro-pore embedded
CNT coated carbon fiber microelectrode
ω
Porous media
SOFC composite cathode
diameter
Porous rotating disk electrode
6
Outline
• Carbon nanotube modified electrodes as
support for glucose oxidation bioanodes
• Polystyrene bead pore formers
• Analysis of transport within porous
rotating disk electrode
• Solid oxide fuel cell composite cathode
model
7
Carbon Nanotube Modified Electrodes As
Support For Glucose Oxidation Bioanodes
8
Carbon Paper / CNT Electrode
CNT grown on carbon paper
CNT growth time effect
Current Collector
Substrate
concentration
gradient
S. C. Barton et al, Electrochem. & Solid State Lett., 10, B96 (2007).
100 µm
9
Carbon Fiber Microelectrode
Glass capillary
Heat pulled fine tip
Cu wire
Epoxy
Exposed fiber
Carbon paste
Glass ends
Transition from glass capillary tip to fiber
10
Fabrication Procedure
Carbon nanotubes
N,N-Dimethylformamide
CNT Coating
Biocatalyst coating
sonication
CNT Dispersion
11
Carbon Fiber / CNT Electrode
fiber
CNT
5 μm
1 μm
Focused Ion Beam Cut Cross Section
SEM Side View
Fiber electrode
12
Coating thickness and capacitance
50
2
40
15
30
CNT/CFME
CFME
Current / µA
2
20
10
10
1
0
-1
5
Coating thickness / µm
Active surface area / cm
20
Capacitance
Thickness
-2
0.40 0.42 0.44 0.46 0.48 0.50
Potential / V vs Ag|AgCl
0
0
0
2
4
6
8
10
Loading mass / µg cm
12
14
-1
• Capacitance measured in 20 mM PBS
solution with 0.1 M NaCl.
• The coating thickness was measured
digitally by optical micrograph.
• Surface area conversion factor: 1.5
μF/cm2
• Capacitance
• The initial increase is 7.9 µF/µg
• Thickness
• CNT coating layer density can be
estimated: 1.0×10-6 µg µm-3
13
Biocatalyst test system
Electrolyte
Redox hydrogel
Glucose
Glucose
oxidase
Glucono lactone
Redox polymer – the mediator
e-
eeCarbon support
Electronically conductive
Redox potential:
PVI-[Os(bpy)2Cl]2+/3+
0.23 V vs Ag/AgCl
B. Gregg and A. Heller, J. Phys. Chem. 95, 5970 (1991).
14
CFME/CNT/Hydrogel Performance
1.76 x 104 Ω
Redox polymer test
50 mV/s
Electrochemical cell
Internal resistance
Polarization curve
1 mV/s
Current Density at 0.5 V vs Ag|AgCl / mA cm
-2
18
16
Exp
Fitted Line
Potentiostat
14
12
Internal resistance
10
8
6
4
2
0
10
20
30
Surface area / cm
40
50
2
Performance summary
50 mM glucose, 20 mM phophate buffer solution,
0.1 M NaCl as supporting electrolyte, 37.5 ⁰C, 150
rpm stirring bar, nitrogen saturated.
• Performance
• 6.4 fold increase of current density at 0.5 V to 16.63
mA cm-2.
15
Polystyrene Bead Template Introduced Macropores In Carbon Nanotube Porous Matrix
16
Polystyrene introduced macro-pores
Macroporosity was introduced to enhance transport
Mixing
Application to CFME
Dried
Polystyrene beads
Heat Treatment
Biocatalyst
PS removed
CNT matrix
Carbon nanotubes
N,N-Dimethylformamide
PS introduced pores
fiber
fiber
fiber
Biocatalyst
sonication
Chai, G.S., Shin, I.S. & Yu, J.-S. Advanced Materials 16, 2057-2061(2004).
17
FIB-SEM cross-sectional view
CNT only on CFME
PS removed by heat treatment
PS + CNT + CFME
Hydrogel coated CFME
18
SEM side view
CNT only on CFME
PS removed by heat treatment
PS + CNT + CFME
Hydrogel coated CFME
19
Electrochemical test
• Both active medaitor and
glucose oxidation current
doubled;
• Larger loading of PS over close
packing with total filled CNT
led to decrease in performance
20
Analysis Of Transport Within Porous Rotating Disk
Electrode (PRDE)
21
Porous rotating disk electrode (PRDE)
RDE
PRDE
ω
electrode
http://www.pineinst.com/
Flat surface;
Well-solved fluid flow field.
2
1
i  0.62 nF A D  v
3
2

Flow field within porous media
permeability
1
6
C
Assuming fast kinetics
Kinematic viscosity
The analytical flow field assume infinite PRDE radius
Nam, B. & Bonnecaze, R.T. , Journal of The Electrochemical Society 154,
F191(2007).
22
Experimental system to be modeled
Experimental data to be modeled
carbonaceous foam electrode
RDE
2190 µg cm-2
ω
2190 µg cm-2
340 µg cm-2
• 74% porosity
• Hierarchical multi-scale porosity
100 mM glucose
0.5 V vs. Ag/AgCl
Mediator (redox polymer)
Electrochemical reactions
G lucose      glucono lactone + 2e
glucose oxidase
-
M ediatoroxided + e-  M ediatorreduced
M ediatorreduced  M ediatoroxided + e
The redox potential: 350 mV vs Ag/AgCl.
electrode
PAA-PVI-[Os(4,4’-dichloro-2,2’bipyridine)2Cl+/2+]
23
Model setup
PRDE
Zero flux
dc
=0
dz
Electrolyte
Electrolye solved flow field
z=0
d 2c
¶c
D0 2 - qze
=0
dz
¶z
d 2c
¶c
-e R ( c ) + Deff 2 - qze
=0
dz
¶z
Enzyme reaction rate
e iqz
Interface continuity
PRDE
= q ze
electrolyte
¶c
¶c
Deff
+ qze c = qzec + D0
¶z
¶z
R ( c) =
Vmax
1+ K m / M o + K s / c
e(U -E0 )nF/RT
Mo =
iM total
(U -E0 )nF/RT
1+ e
24
Fitting results by considering diffusion
• Phenomena considered:
Diffusion at all rotations;
Boundary layer in electrolyte;
Natural convection;
25
Concentration profile
Convection dominant
Diffusion dominant region
Diffusion is dominant in low rotation, and high
rotation, but closer to current collector surface
26
Geometric parameters
Electrode thickness effect
Permeability effect
• Large thickness doesn’t lead to higher current at low
rotations due to limited active region;
• Higher permeability generate higher current at
lower rotations
27
Solid Oxide Fuel Cell Composite Cathode
Impedance Model With Low Electronic
Conductivity
28
Experimental setup – Symmetric cell
MIEC
Mixed ionic and electronic conductor
O2
Conducting both electrons and oxygen ions;
Active for oxygen exchange reaction;
Nano-particles on IC surfaces
Pt
IC
Gold C.C.
Ionic conductor
LCM porous C.C.
MIEC/IC electrode
IC electrolyte
Vo
Vo
Vo
Vo
Vo
Vo
Vo
Vo
Vo
Transport oxygen ions;
Insulating to electrons;
Compressed into electrolytes;
A
V
Goal
Polarization resistance and its origin
29
Phenomena to be considered
SOFC composite cathode
Charge transfer
Vacancy migration and diffusion
IC
vacancy
electroly
te
Reaction
IC
MC
electrons
Gas
gas
Electron
conductio
n
Gas diffusion
30
High infiltration fitting
Large MIEC conductivity
Analytical expression:
Z = Rel +
-zz
2 cr + jw
where
RTL
amc L2 r0
zz =
,cr =
zv ze F 2 Deff C¥
Deff e mcC¥
Deff = Dv +
1e-7 cm2/s
e ics ic RT
e mc ( zv F ) C¥
2
0.0012 cm2/s
• Effective diffusivity takes account of migration.
• Vacancy mostly transport through migration.
31
MIEC lwo to high loadings
Fitting parameter:
MIEC conductivity;
Surface exchange reaction rate;
MIEC conductivity explained with
percolation theory
32
Percolation prediction of conductivity
• Percolation theory assumption:
Bethe lattice approximation for finite cluseter
Random packing of two components
33
Conclusions
34
Conclusions
• Porous electrodes, including carbon based porous fiber
electrode, macro-pore embedded porous electrode,
porous rotating disk electrode, and porous composite
cathode for SOFC, were studied;
• Carbon nanotube and the modification with bead
template lead to better electrode performance;
• Porous rotating disk electrode with diffusion and
convection considered at all rotations yields a model
that fits well to experiments;
• Limited MIEC conductivity can explain the observed
large resistance in SOFC cathode with insufficient MIEC
loadings.
35
Thanks!
36
Backup Slides
37
Coating layer thickness / µm
Hydrogel Coating on CFME/CNT
40
with CNT
without CNT
•
CNT:13 µg/cm
•
hydrogel:0 (left) to 76.8 µg /cm
(right).
• For 13 µg/cm CNT on 1 cm
CFME, 40 µg hydrogel is
30
20
fiber
CNT
biocatalyst
• Thus, 1 µg CNT can contain up
to 3.1 µg hydrogel
10
0
0
20
40
60
80
-1
Hydrogel mass / µg cm
100
Hydrogel density: 1.6 g/cm3
Estimated: 20% porosity
38
CNT Free Control Experiments
fiber
biocatalyst
No CNT
Coating thickness
Coating morphology and maximum glucose
oxidation current in 50 mM glucose
•
•
•
Only 1 µm thickness of hydrogel film is required for the 90% of optimum performance.
Optimum performance is at 9 µm.
The current density is 2.5 mA/cm2 for 15 µm coating thickness, which was the control for later CNT
coated CFMEs.
39
Glucose Concentration Study
@ 0.5 V
Michaelis-Menten kinetics fitted parameters
Electrode
Km,app
mM
Imax
mA cm-2
Turnover
s-1
Bare
10.3
3.1
0.5
4 µg cm-1 CNT
8.8
12.7
2.3
10 µg cm-1 CNT
7.5
17.2
3.1
40
PRDE fitting parameters
41
High infilatraion SOFC fitting
42
TGA analysis
Validation of heat treatment temperature
Our treatment T: 450 °C
Temperature ramp: 10 °C/min to 105 °C,
hold 15 minutes to get rid of water, 10
°C/min to 900 °C until fully burned away
43
Conclusions – CNT/CFME
• Modified CFME bioelectrode allows observation and
quantification of methodologies for increasing surface
area and current density.
• CNT modification lead to 4000-fold increase in
capacitive surface area and over 6-fold increase in
glucose oxidation current density.
44
MIEC infiltration volume fraction
9.2%
22.8%
23.3%
42.7%
Jason Nicholas, 217th ECS meeting 45
PS packing scheme within CNT matrix
CNT only
PS close-packing;
CNT incomplete filling
PS sparsely embedded
Close packing
PS only
46
Heat treatment effect on thickness
CNT only
58 wt% PS
28 wt% PS
73 wt% PS
47
Thickness change summary
CNT loading mass was fixed at 2 µg
cm-1
48
Conclusions
• Introducing macropores via PS particle templating was
shown to increase accessible surface area and
performance;
• Peak redox polymer and enzymatic activity properties
that also doubled;
• The hydrophilicity of the carboxylated CNT layer enabled
total infiltration of biocatalytic hydrogel, as revealed by
FIB-SEM
49
PRDE - Conclusions
• A model based on convective and diffusive transport of
substrate in porous rotating disk electrode was
proposed;
• It explains the non-zero current at low rotation speeds,
and still show the signature sigmoidal trend of current
versus rotation rate;
• Almost perfect fitting to published PRDE experimental
data;
50
Conclusions - SOFC
• Composite cathode impedance performances were
modeled at varying loadings and temperatures;
• The diffusion, migration of oxygen vacancies and MIEC
electronic conduction were considered;
• Low MIEC loading leads to lower conductivity, which
can be explained with percolation theory.
51
Comprehensive Model setup - SOFC
Comprehensive Case including all processes
Differential Volume Element
No analytical solution possible.
Vo
0=
e ics ic ¶2 mv,ic
( zv F )
2
¶y 2
INPUT - OUTPUT
- aic N
MC/IC charge
transfer
¶Cv e mc DvC¥ ¶2 mv,mc
=
- nv amc r + aic N
Vo e mc
¶t
RT
¶y 2
INPUT - OUTPUT
e
oxygen
0=
se
( ze F )2
IC
RXN
¶ me
- neamc r
¶y 2
INPUT
OUTPUT
vacancy
MC
electron
2
Gas
¶x
¶2 x
e gasC0 = e gas DgC0 2 - ng amc r
¶t
¶y
52