Presentation on theme: "Prepared for defense practice talk"— Presentation transcript:
1 Prepared for defense practice talk Synthesis, characterization and modeling of porous electrodes for fuel cellsHao WenPrepared for defense practice talk3/29/2012
2 Fuel cells - overview Load Motor vehicles current AirCathodeAnodeElectrolytecurrentLoadPortable device power supplyFuel cells convert chemical energy into electricityApplications varies from high temperature high power output to room temperature portable power sources.Biofuel cellsBarton, S.C., AlCHE annual meeting
3 Multiscale porous electrode support Fuel transporte-ReactantsSupportToo much porosity lowers conductivityElectrolyteReactantse-ProductReactantsCatalyste-MesoporesInterfacial reactionCurrent collector
4 Synthesis of carbon porous electrodes Carbon nanotubeCarbonaceous foam monolithExfoliated graphiteTemplate introduced macro-poreSurface modification, compositing, and coating with catalystJ. Lu 2007, Chemistry of MaterialsO. Velev, 2000, Advanced MaterialsFlexer, 2010, Energy and Environmental Science
5 Porous Electrode Model Modeling schemeINPUTOUTUTGeometryRDEPRDEFilmPorous layerMeasurableImpedancePolarizationCyclic voltammetryHardly MeasurableConcentration profileActive regionPorous Electrode ModelKineticsPing pong bi biDifferential linear kineticsOptimizationElectrode thicknessPorosityFeeding rateTransportFuel / OxygenIn Channel, porous layer
6 Porous electrodes under study CNTCarbon fiberCarbon nanotube coated carbon fiber microelectrodePolystyrene derived macro-pore embedded CNT coated carbon fiber microelectrodeωdiameterPorous mediaSOFC composite cathodePorous rotating disk electrode
7 OutlineCarbon nanotube modified electrodes as support for glucose oxidation bioanodesPolystyrene bead pore formersAnalysis of transport within porous rotating disk electrodeSolid oxide fuel cell composite cathode model
8 Carbon Nanotube Modified Electrodes As Support For Glucose Oxidation Bioanodes
9 Substrate concentration gradient Carbon Paper / CNT ElectrodeCNT grown on carbon paperCNT growth time effectCurrent Collector100 µmSubstrate concentration gradientS. C. Barton et al, Electrochem. & Solid State Lett., 10, B96 (2007).
10 Transition from glass capillary tip to fiber Carbon Fiber MicroelectrodeGlass capillaryHeat pulled fine tipCu wireExposed fiberCarbon pasteEpoxyGlass endsTransition from glass capillary tip to fiber
12 Focused Ion Beam Cut Cross Section Carbon Fiber / CNT ElectrodeFocused Ion Beam Cut Cross Section5 μm1 μm+CNTfiberSEM Side ViewFiber electrode
13 Coating thickness and capacitance The initial increase is 7.9 µF/µgThicknessCNT coating layer density can be estimated: 1.0×10-6 µg µm-3Capacitance 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 ElectrolyteRedox hydrogelGlucose oxidaseGlucoseGlucono lactoneRedox polymer – the mediatore-Redox potential:PVI-[Os(bpy)2Cl]2+/3+0.23 V vs Ag/AgCle-e-Electronically conductivee-Carbon supportB. Gregg and A. Heller, J. Phys. Chem. 95, 5970 (1991).
15 CFME/CNT/Hydrogel Performance Redox polymer testPolarization curve1.76 x 104 Ω50 mV/sPotentiostatElectrochemical cellInternal resistanceInternal resistance1 mV/sPerformance summaryPerformance6.4 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.
17 Polystyrene introduced macro-pores Macroporosity was introduced to enhance transportMixingApplication to CFMEHeat TreatmentBiocatalystDriedPS removedPolystyrene beadsCarbon nanotubesN,N-DimethylformamideCNT matrix+fiber+fiber+fiberBiocatalystPS introduced poressonicationChai, G.S., Shin, I.S. & Yu, J.-S. Advanced Materials 16, (2004).
18 FIB-SEM cross-sectional view CNT only on CFMEPS + CNT + CFMEPS removed by heat treatmentHydrogel coated CFME
19 PS removed by heat treatment SEM side viewCNT only on CFMEPS + CNT + CFMEPS removed by heat treatmentHydrogel coated CFME
20 Electrochemical testBoth 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) ωFlow field within porous mediaFlat surface;Well-solved fluid flow field.Kinematic viscositypermeabilityAssuming fast kineticsThe analytical flow field assume infinite PRDE radiusNam, B. & Bonnecaze, R.T. , Journal of The Electrochemical Society 154, F191(2007).
23 Experimental system to be modeled carbonaceous foam electrode74% porosityHierarchical multi-scale porosityωExperimental data to be modeledRDE2190 µg cm-22190 µg cm-2340 µg cm-2100 mM glucose0.5 V vs. Ag/AgClMediator (redox polymer)Electrochemical reactionsThe redox potential: 350 mV vs Ag/AgCl.PAA-PVI-[Os(4,4’-dichloro-2,2’-bipyridine)2Cl+/2+]
24 Electrolye solved flow field Model setupPRDEElectrolyteZero fluxElectrolye solved flow fieldEnzyme reaction rateInterface continuity
25 Fitting results by considering diffusion Phenomena considered:Diffusion at all rotations;Boundary layer in electrolyte;Natural convection;
26 Concentration profile Convection dominantDiffusion dominant regionDiffusion is dominant in low rotation, and high rotation, but closer to current collector surface
27 Electrode thickness effect Geometric parametersElectrode thickness effectPermeability effectLarge 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 MIECIC electrolyteO2VoGold C.C.LCM porous C.C.MIEC/IC electrodePtAVMixed ionic and electronic conductorConducting both electrons and oxygen ions;Active for oxygen exchange reaction;Nano-particles on IC surfacesICIonic conductorTransport oxygen ions;Insulating to electrons;Compressed into electrolytes;GoalPolarization resistance and its origin
30 Phenomena to be considered SOFC composite cathodeCharge transferVacancy migration and diffusionICelectrolyteICvacancyElectron conductionMCelectronsGasgasReactionGas diffusion
31 High infiltration fitting Large MIEC conductivityAnalytical expression:where1e-7 cm2/scm2/sEffective 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 cluseterRandom packing of two components
35 ConclusionsPorous 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.
38 Hydrogel Coating on CFME/CNT CNT:13 µg/cmhydrogel:0 (left) to 76.8 µg /cm (right).For 13 µg/cm CNT on 1 cm CFME, 40 µg hydrogel isThus, 1 µg CNT can contain up to 3.1 µg hydrogelfiberCNTbiocatalyst+Hydrogel density: 1.6 g/cm3Estimated: 20% porosityMake a improved version of the bottom plot to better present the ideaMake the units consistent on the plot with “/cm”
39 CNT Free Control Experiments fiberbiocatalystNo CNTCoating thickness+Coating morphology and maximum glucose oxidation current in 50 mM glucoseOnly 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 VMichaelis-Menten kinetics fitted parametersElectrodeKm,app mMImax mA cm-2Turnover s-1Bare10.33.10.54 µg cm-1 CNT8.812.72.310 µg cm-1 CNT7.517.2
43 Validation of heat treatment temperature TGA analysisValidation of heat treatment temperatureOur treatment T: 450 °CTemperature 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.
48 Thickness change summary CNT loading mass was fixed at 2 µg cm-1
49 ConclusionsIntroducing 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 - ConclusionsA 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 - SOFCComposite 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 processesDifferential Volume ElementNo analytical solution possible.ICVoMC/IC chargetransferINPUT - OUTPUTOUTPUTINPUTMCVovacancyINPUT - OUTPUTRXNelectroneGasoxygen