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

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 http://www.fllibertarian.org/

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

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

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

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

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

Carbon Nanotube Modified Electrodes As Support For Glucose Oxidation Bioanodes

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).

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

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

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

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

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).

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 16.63 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.

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

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, 2057-2061(2004).

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

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

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

Analysis Of Transport Within Porous Rotating Disk Electrode (PRDE)

Porous rotating disk electrode (PRDE) ω http://www.pineinst.com/ 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).

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+]

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

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

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

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

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

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

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

High infiltration fitting Large MIEC conductivity Analytical expression: where 1e-7 cm2/s 0.0012 cm2/s Effective diffusivity takes account of migration. Vacancy mostly transport through migration.

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

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

Conclusions

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.

Thanks!

Backup Slides

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”

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.

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

PRDE fitting parameters

High infilatraion SOFC fitting

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

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.

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

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

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

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

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

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;

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.

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