1. Prepared for defense practice talk
Synthesis, characterization and modeling of porous electrodes for fuel cells
Hao Wen
Prepared for defense practice talk
3/29/2012

2. Fuel cells - overview Load Motor vehicles current
Air
Cathode
Anode
Electrolyte
current
Load
Portable device power supply
Fuel cells convert chemical energy into electricity
Applications varies from high temperature high power output to room temperature portable power sources.
Biofuel cells
Barton, S.C., AlCHE annual meeting

3. Multiscale porous electrode support
Fuel transport e-
Reactants
Support
Too much porosity lowers conductivity
Electrolyte
Reactants e-
Product
Reactants
Catalyst e-
Mesopores
Interfacial reaction
Current collector

4. Synthesis of carbon porous electrodes
Carbon nanotube
Carbonaceous foam monolith
Exfoliated graphite
Template introduced macro-pore
Surface modification, compositing, and coating with catalyst
J. Lu 2007, Chemistry of Materials
O. Velev, 2000, Advanced Materials
Flexer, 2010, Energy and Environmental Science

5. Porous Electrode Model
Modeling scheme
INPUT
OUTUT
Geometry
RDE
PRDE
Film
Porous layer
Measurable
Impedance
Polarization
Cyclic voltammetry
Hardly Measurable
Concentration profile
Active region
Porous Electrode Model
Kinetics
Ping pong bi bi
Differential linear kinetics
Optimization
Electrode thickness
Porosity
Feeding rate
Transport
Fuel / Oxygen
In Channel, porous layer

6. Porous electrodes under study
CNT
Carbon fiber
Carbon nanotube coated carbon fiber microelectrode
Polystyrene derived macro-pore embedded CNT coated carbon fiber microelectrode ω
diameter
Porous media
SOFC composite cathode
Porous rotating disk electrode

7. 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

8. Carbon Nanotube Modified Electrodes As Support For Glucose Oxidation Bioanodes

9. Substrate concentration gradient
Carbon Paper / CNT Electrode
CNT grown on carbon paper
CNT growth time effect
Current Collector
100 µm
Substrate concentration gradient
S. C. Barton et al, Electrochem. & Solid State Lett., 10, B96 (2007).

10. Transition from glass capillary tip to fiber
Carbon Fiber Microelectrode
Glass capillary
Heat pulled fine tip
Cu wire
Exposed fiber
Carbon paste
Epoxy
Glass ends
Transition from glass capillary tip to fiber

11. N,N-Dimethylformamide
Fabrication Procedure
sonication
Carbon nanotubes
N,N-Dimethylformamide
CNT Dispersion
Carbon Fiber
Pipette
CNT suspension
CNT Coating
Biocatalyst coating
CNT Coated Fiber
Biocatalyst Coating
Pipette

12. Focused Ion Beam Cut Cross Section
Carbon Fiber / CNT Electrode
Focused Ion Beam Cut Cross Section
5 μm
1 μm +
CNT
fiber
SEM Side View
Fiber electrode

13. Coating thickness and capacitance
The initial increase is 7.9 µF/µg
Thickness
CNT coating layer density can be estimated: 1.0×10-6 µg µm-3
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

14. Biocatalyst test system
Electrolyte
Redox hydrogel
Glucose oxidase
Glucose
Glucono lactone
Redox polymer – the mediator e-
Redox potential:
PVI-[Os(bpy)2Cl]2+/3+
0.23 V vs Ag/AgCl e- e-
Electronically conductive e-
Carbon support
B. Gregg and A. Heller, J. Phys. Chem. 95, 5970 (1991).

15. CFME/CNT/Hydrogel Performance
Redox polymer test
Polarization curve
1.76 x 104 Ω
50 mV/s
Potentiostat
Electrochemical cell
Internal resistance
Internal resistance
1 mV/s
Performance summary
Performance
6.4 fold increase of current density at 0.5 V to mA cm-2.
50 mM glucose, 20 mM phophate buffer solution, 0.1 M NaCl as supporting electrolyte, 37.5 ⁰C, 150 rpm stirring bar, nitrogen saturated.

16. Polystyrene Bead Template Introduced Macro-pores In Carbon Nanotube Porous Matrix

17. Polystyrene introduced macro-pores
Macroporosity was introduced to enhance transport
Mixing
Application to CFME
Heat Treatment
Biocatalyst
Dried
PS removed
Polystyrene beads
Carbon nanotubes
N,N-Dimethylformamide
CNT matrix +
fiber +
fiber +
fiber
Biocatalyst
PS introduced pores
sonication
Chai, G.S., Shin, I.S. & Yu, J.-S. Advanced Materials 16, (2004).

18. FIB-SEM cross-sectional view
CNT only on CFME
PS + CNT + CFME
PS removed by heat treatment
Hydrogel coated CFME

19. PS removed by heat treatment
SEM side view
CNT only on CFME
PS + CNT + CFME
PS removed by heat treatment
Hydrogel coated CFME

20. 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

21. Analysis Of Transport Within Porous Rotating Disk Electrode (PRDE)

22. Porous rotating disk electrode (PRDE)ω
Flow field within porous media
Flat surface;
Well-solved fluid flow field.
Kinematic viscosity
permeability
Assuming fast kinetics
The analytical flow field assume infinite PRDE radius
Nam, B. & Bonnecaze, R.T. , Journal of The Electrochemical Society 154, F191(2007).

23. Experimental system to be modeled
carbonaceous foam electrode
74% porosity
Hierarchical multi-scale porosity ω
Experimental data to be modeled
RDE
2190 µg cm-2
2190 µg cm-2
340 µg cm-2
100 mM glucose
0.5 V vs. Ag/AgCl
Mediator (redox polymer)
Electrochemical reactions
The redox potential: 350 mV vs Ag/AgCl.
PAA-PVI-[Os(4,4’-dichloro-2,2’-bipyridine)2Cl+/2+]

24. Electrolye solved flow field
Model setup
PRDE
Electrolyte
Zero flux
Electrolye solved flow field
Enzyme reaction rate
Interface continuity

25. Fitting results by considering diffusion
Phenomena considered:
Diffusion at all rotations;
Boundary layer in electrolyte;
Natural convection;

26. Concentration profile
Convection dominant
Diffusion dominant region
Diffusion is dominant in low rotation, and high rotation, but closer to current collector surface

27. Electrode thickness effect
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

28. Solid Oxide Fuel Cell Composite Cathode Impedance Model With Low Electronic Conductivity

29. Experimental setup – Symmetric cell
MIEC
IC electrolyte O2 Vo
Gold C.C.
LCM porous C.C.
MIEC/IC electrode Pt A V
Mixed ionic and electronic conductor
Conducting both electrons and oxygen ions;
Active for oxygen exchange reaction;
Nano-particles on IC surfaces IC
Ionic conductor
Transport oxygen ions;
Insulating to electrons;
Compressed into electrolytes;
Goal
Polarization resistance and its origin

30. Phenomena to be considered
SOFC composite cathode
Charge transfer
Vacancy migration and diffusion IC
electrolyte IC
vacancy
Electron conduction MC
electrons
Gas
gas
Reaction
Gas diffusion

31. High infiltration fitting
Large MIEC conductivity
Analytical expression:
where
1e-7 cm2/s
cm2/s
Effective diffusivity takes account of migration.
Vacancy mostly transport through migration.

32. MIEC lwo to high loadings
Fitting parameter:
MIEC conductivity;
Surface exchange reaction rate;
MIEC conductivity explained with percolation theory

33. Percolation prediction of conductivity
Percolation theory assumption:
Bethe lattice approximation for finite cluseter
Random packing of two components

34. Conclusions

35. 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.

36. Thanks!

37. Backup Slides

38. 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
fiber
CNT
biocatalyst +
Hydrogel density: 1.6 g/cm3
Estimated: 20% porosity
Make a improved version of the bottom plot to better present the idea
Make the units consistent on the plot with “/cm”

39. 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.

40. 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

41. PRDE fitting parameters

42. High infilatraion SOFC fitting

43. Validation of heat treatment temperature
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

44. 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.

45. MIEC infiltration volume fraction
9.2%
22.8%
23.3%
42.7%
Jason Nicholas, 217th ECS meeting

46. PS packing scheme within CNT matrix
PS sparsely embedded
CNT only
Close packing
PS close-packing;
CNT incomplete filling
PS only

47. Heat treatment effect on thickness
CNT only
28 wt% PS
58 wt% PS
73 wt% PS

48. Thickness change summary
CNT loading mass was fixed at 2 µg cm-1

49. 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

50. 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;

51. 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.

52. Comprehensive Model setup - SOFC
Comprehensive Case including all processes
Differential Volume Element
No analytical solution possible. IC Vo
MC/IC charge
transfer
INPUT - OUTPUT
OUTPUT
INPUT MC Vo
vacancy
INPUT - OUTPUT
RXN
electron e
Gas
oxygen