1

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