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Synthesis, characterization and modeling of porous electrodes for
fuel cells- Hao Wen
- Prepared for defense practice talk- 3/29/2012
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Fuel cells - overview
Fuel cells convert chemical energy into electricity
Applications varies from high temperature high power output to room temperature portable power sources.
Motor vehicles
Portable device power supply
Fuel Air
CathodeAnod
e
Electrolyte
current
Load
http://www.fllibertarian.org/
Biofuel cells
Barton, S.C., AlCHE annual meeting
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Multiscale porous electrode support
Catalyst
Mesoporese-
e-
Reactants
Fuel transport
Product
Too much porosity lowers conductivity
Support
Electrolyte
Reactants
Reactantse-
Interfacial reaction
Current collector
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Synthesis of carbon porous electrodes
Carbon nanotube
www.nanocyl.comJ. Lu 2007, Chemistry of Materials
Exfoliated graphite
Carbonaceous foam monolith
Template introduced macro-pore
O. Velev, 2000, Advanced MaterialsFlexer, 2010, Energy and Environmental Science
Surface modification, compositing, and
coating with catalyst
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Modeling scheme
Porous Electrode
Model
INPUT OUTUT
GeometryRDEPRDEFilmPorous layer
KineticsPing pong bi biDifferential linear kinetics
TransportFuel / OxygenIn Channel, porous layer
MeasurableImpedancePolarizationCyclic voltammetry
Hardly MeasurableConcentration profileActive region
OptimizationElectrode thicknessPorosityFeeding rate
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Porous electrodes under study
Carbon fiber
CNT
Carbon nanotube coated carbon fiber microelectrode Polystyrene derived macro-pore embedded
CNT coated carbon fiber microelectrode
SOFC composite cathode
Porous media
ω
diameter
Porous rotating disk electrode
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• 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
Outline
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Carbon Nanotube Modified Electrodes As Support For Glucose Oxidation Bioanodes
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S. C. Barton et al, Electrochem. & Solid State Lett., 10, B96 (2007).
Curre
nt C
olle
ctor
100 µm
CNT grown on carbon paper
CNT growth time effect
Substrate concentration
gradient
Carbon Paper / CNT Electrode
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Carbon Fiber Microelectrode
Transition from glass capillary tip to fiber
Cu wire
Epoxy
Glass capillary
Carbon paste
Heat pulled fine tip
Exposed fiber
Glass ends
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sonication
Carbon nanotubesN,N-Dimethylformamide
CNT DispersionCarbon Fiber
Pipett
e
CNT suspension
CNT Coating Biocatalyst coating
CNT Coated Fiber
Biocatalyst Coating
Pipett
e
Fabrication Procedure
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SEM Side View
+CNTfiber
Fiber electrode
Focused Ion Beam Cut Cross Section
5 μm 1 μm
Carbon Fiber / CNT Electrode
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Coating thickness and capacitance
•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
50
40
30
20
10
0
Act
ive
surfa
ce a
rea
/ cm
2
14121086420Loading mass / µg cm-1
20
15
10
5
0
Coating thickness / µm
2
1
0
-1
-2
Cur
rent
/ µA
0.500.480.460.440.420.40Potential / V vs Ag|AgCl
CNT/CFME CFME
Capacitance Thickness
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Biocatalyst test system
B. Gregg and A. Heller, J. Phys. Chem. 95, 5970 (1991).
Carbon support
e-
Redox hydrogel
Glucose oxidase
Redox polymer – the mediator
e-
e-
Redox potential:PVI-[Os(bpy)2Cl]2+/3+
0.23 V vs Ag/AgCl
Glucose
Glucono lactone
e-
Electronically conductive
Electrolyte
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CFME/CNT/Hydrogel Performance
Internal resistance
Performance summary• Performance
•6.4 fold increase of current density at 0.5 V to 16.63 mA cm-2.
Redox polymer test
Polarization curve
50 mM glucose, 20 mM phophate buffer solution, 0.1 M NaCl as supporting electrolyte, 37.5 ⁰C, 150 rpm stirring bar, nitrogen saturated.
1 mV/s
50 mV/s
1.76 x 104 Ω
Potentiostat
Electrochemical cell
Internal resistance
18
16
14
12
10
8
6
4
2
Cur
rent
Den
sity
at 0
.5 V
vs
Ag|
AgC
l / m
A c
m-2
50403020100
Surface area / cm2
Exp Fitted Line
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Polystyrene Bead Template Introduced Macro-pores In Carbon Nanotube Porous Matrix
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Polystyrene introduced macro-pores
PS removedDried
sonication
Polystyrene beadsCarbon nanotubesN,N-Dimethylformamide
Mixing
+fiber
Application to CFME Heat Treatment
+
fiber
Biocatalyst
+
fiber
Biocatalyst
Chai, G.S., Shin, I.S. & Yu, J.-S. Advanced Materials 16, 2057-2061(2004).
CNT matrix
Macroporosity was introduced to enhance transport
PS introduced pores
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FIB-SEM cross-sectional view
CNT only on CFME PS + CNT + CFME
PS removed by heat treatment Hydrogel coated CFME
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SEM side view
CNT only on CFME PS + CNT + CFME
PS removed by heat treatment Hydrogel coated CFME
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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
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Analysis Of Transport Within Porous Rotating Disk Electrode (PRDE)
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Porous rotating disk electrode (PRDE)
2 113 620.62i nFAD v C
electrode
ω
RDE
http://www.pineinst.com/
Flat surface;Well-solved fluid flow field.
Flow field within porous media
The analytical flow field assume infinite PRDE radius
Nam, B. & Bonnecaze, R.T. , Journal of The Electrochemical Society 154, F191(2007).
Assuming fast kinetics
Kinematic viscosity
permeability
PRDE
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Experimental system to be modeled
Experimental data to be modeled
PAA-PVI-[Os(4,4’-dichloro-2,2’-bipyridine)2Cl+/2+]
carbonaceous foam electrode
• 74% porosity • Hierarchical multi-scale porosity
ω
glucose oxidase -
oxided reduced
-reduced oxided electrode
Glucose glucono lactone + 2eMediator + e- Mediator
Mediator Mediator +e
Electrochemical reactions
Mediator (redox polymer)
The redox potential: 350 mV vs Ag/AgCl.
100 mM glucose0.5 V vs. Ag/AgCl
2190 µg cm-2
340 µg cm-2
RDE2190 µg cm-2
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Model setup
PRDE Electrolyte
Zero flux
Interface continuity
Enzyme reaction rate
Electrolye solved flow field
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Fitting results by considering diffusion
• Phenomena considered:Diffusion at all rotations;Boundary layer in electrolyte;Natural convection;
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Concentration profile
Diffusion is dominant in low rotation, and high rotation, but closer to current collector surface
Diffusion dominant region
Convection dominant
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Geometric parameters
Permeability effectElectrode thickness 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
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Solid Oxide Fuel Cell Composite Cathode Impedance Model With Low Electronic Conductivity
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Experimental setup – Symmetric cell
IC electrolyte
O2
Vo Vo Vo Vo Vo Vo Vo Vo Vo
Gold C.C.
LCM porous C.C.
MIEC/IC electrode
Pt
A
V
Ionic conductor
Transport oxygen ions;Insulating to electrons;Compressed into electrolytes;
Mixed ionic and electronic conductor
Conducting both electrons and oxygen ions;Active for oxygen exchange reaction;Nano-particles on IC surfaces
MIEC
IC
Goal
Polarization resistance and its origin
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Phenomena to be considered
ICelectrol
yte
IC
MCGas
Charge transfer
vacancy
electrons
gas
Gas diffusionReaction
Vacancy migration and diffusion
Electron conduction
SOFC composite cathode
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High infiltration fitting
Analytical expression:
where
• Effective diffusivity takes account of migration.• Vacancy mostly transport through migration.
1e-7 cm2/s 0.0012 cm2/s
Large MIEC conductivity
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MIEC lwo to high loadings
Fitting parameter:MIEC conductivity;Surface exchange reaction rate;
MIEC conductivity explained with percolation theory
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Percolation prediction of conductivity
• Percolation theory assumption:Bethe lattice approximation for finite cluseterRandom packing of two components
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Conclusions
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• 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.
Conclusions
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Thanks!
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Backup Slides
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Hydrogel Coating on CFME/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
• Thus, 1 µg CNT can contain up to 3.1 µg hydrogel
Hydrogel density: 1.6 g/cm3
+
biocatalyst
Estimated: 20% porosity
fiberCNT
40
30
20
10
0
Coa
ting
laye
r thi
ckne
ss /
µm
100806040200Hydrogel mass / µg cm-1
with CNT without CNT
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CNT Free Control Experiments
• 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.
Coating morphology and maximum glucose oxidation current in 50 mM glucose
+
biocatalystfiber
No CNT
Coating thickness
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Glucose Concentration Study
Electrode Km,app mM
Imax
mA cm-2Turnover
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
Michaelis-Menten kinetics fitted parameters@ 0.5 V
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PRDE fitting parameters
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High infilatraion SOFC fitting
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TGA analysis
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
Our treatment T: 450 °C
Validation of heat treatment temperature
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• 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.
Conclusions – CNT/CFME
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MIEC infiltration volume fraction
9.2% 22.8% 23.3% 42.7%
Jason Nicholas, 217th ECS meeting
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PS packing scheme within CNT matrix
CNT onlyPS sparsely embedded
Close packing
PS onlyPS close-packing;
CNT incomplete filling
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Heat treatment effect on thickness
CNT only 28 wt% PS
58 wt% PS 73 wt% PS
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Thickness change summary
CNT loading mass was fixed at 2 µg cm-1
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• 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
Conclusions
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• 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;
PRDE - Conclusions
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• 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.
Conclusions - SOFC
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Comprehensive Model setup - SOFCComprehensive Case including all
processesNo analytical solution possible.
MCINPUT OUTPUT
IC
Gas
INPUT - OUTPUT
Vo
electron
vacancy
Differential Volume Element
RXN
MC/IC chargetransfer
e
Vo
oxygen
INPUT - OUTPUT