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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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Presentation on theme: "1 Synthesis, characterization and modeling of porous electrodes for fuel cells -Hao Wen -Prepared for defense practice talk -3/29/2012."— Presentation transcript:

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

2 2 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 FuelAir Cathode Anode Electrolyte current Load Biofuel cells Barton, S.C., AlCHE annual meeting

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

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

5 5 Modeling scheme Porous Electrode Model INPUT OUTUT 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 Hardly Measurable Concentration profile Active region Optimization Electrode thickness Porosity Feeding rate

6 6 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

7 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 Outline

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

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

10 10 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

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

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

13 13 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/cm 2 Capacitance The initial increase is 7.9 µF/µg Thickness CNT coating layer density can be estimated: 1.0×10 -6 µg µm -3

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

15 15 CFME/CNT/Hydrogel Performance Internal resistance Performance summary Performance 6.4 fold increase of current density at 0.5 V to 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 10 4 Ω Potentiostat Electrochemical cell Internal resistance

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

17 17 Polystyrene introduced macro-pores PS removed Dried sonication Polystyrene beads Carbon nanotubes N,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, (2004). CNT matrix Macroporosity was introduced to enhance transport PS introduced pores

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

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

20 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 21 Analysis Of Transport Within Porous Rotating Disk Electrode (PRDE)

22 22 Porous rotating disk electrode (PRDE) electrode ω RDE 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

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

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

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

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

27 27 Geometric parameters Permeability effect Electrode 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

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

29 29 Experimental setup – Symmetric cell IC electrolyte O2O2 VoVo VoVo VoVo VoVo VoVo VoVo VoVo VoVo VoVo 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

30 30 Phenomena to be considered IC electroly te IC MC Gas Charge transfer vacancy electrons gas Gas diffusion Reaction Vacancy migration and diffusion Electron conductio n SOFC composite cathode

31 31 High infiltration fitting Analytical expression: where Effective diffusivity takes account of migration. Vacancy mostly transport through migration. 1e-7 cm 2 /s cm 2 /s Large MIEC conductivity

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

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

34 34 Conclusions

35 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 Conclusions

36 36 Thanks!

37 37 Backup Slides

38 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 Hydrogel density: 1.6 g/cm 3 + biocatalyst Estimated: 20% porosity fiber CNT

39 39 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/cm 2 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 + biocatalyst fiber No CNT Coating thickness

40 40 Glucose Concentration Study Electrode K m,app mM I max mA cm -2 Turnover s -1 Bare µg cm -1 CNT µg cm -1 CNT Michaelis-Menten kinetics fitted 0.5 V

41 41 PRDE fitting parameters

42 42 High infilatraion SOFC fitting

43 43 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

44 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 Conclusions – CNT/CFME

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

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

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

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

49 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 Conclusions

50 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 PRDE - Conclusions

51 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 Conclusions - SOFC

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


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