DR. AKSHAYA JENA AND DR. KRISHNA GUPTAPOROUS MATERIALS, INC.,ITHACA, NEW YORK, USACharacterization of Pore Structure of Fuel Cell Components for Enhancing Performance

OutlineIntroductionThrough pore throat diameter, distribution, gas permeability & surface area by:Capillary Flow PorometryCapillary Condensation Flow PorometryHydrophobic through and blind pore volume & distribution by:VacuaporeThrough pore volume, diameter, distribution & liquid permeability by:Liquid Extrusion PorosimetrySummary and Conclusion

IntroductionPore structure governs kinetics of physicochemical processes & Flows of reactants and products in fuel cells.Quantitative measurement of pore structure is essential for Design, development and performance evaluation. Technologies for pore structure measurement are currently being developed to characterize the complex pore structure of fuel cell components. We will discuss several innovative techniques successfully developed and applied for evaluation of pore structure of fuel cell components.

Through Pore Throat Diameters, Distribution, Gas Permeability and Surface AreaImportance of Such Properties

Through Pores:Fluid flowPore Diameters:Capillary forces for liquid movementThroat diameters:Separation of undesirable particlesGas permeability:Overall rate of the processesThrough pore surface area:Physicochemical processesEffects of stress, chemical environments & temperature:Influence of operating conditions

Suitable Characterization Techniques

Advanced Capillary Flow PorometryCapillary Condensation Flow Porometry

Through Pore Throat Diameters, Distribution, Gas Permeability and Surface Area

Advanced Capillary Flow PorometryFor wetting liquid:Wetting Liquids fill pores spontaneouslyCannot come out spontaneouslyA pressurized inert gas can displace liquid from pores provided:Work done by Gas = Increase in Interfacial Free Energy

Basic Principle

Advanced Capillary Flow PorometryPressure needed to displace liquid from a pore:

p = 4 cos / D

p = differential gas pressure = surface tension of wetting liquid = contact angle of the liquidD = pore diameter

Pore diameter is defined for all pore cross-sections

Advanced Capillary Flow Porometry(Perimeter/Area)pore = (Perimeter/Area)cylindrical opening

Pore Diameter = Diameter of Cylindrical Opening

SKETCH

Advanced Capillary Flow PorometryMeasured differential pressure & gas flow through dry & wet sample yield pore structure

The TechniqueAccuratePressure transducersFlow transducersRegulatorsControllersSophisticated sample sealing mechanisms to direct flow in desired directionsInternal computersTo control sequential operationsTo execute automated testsAdvanced Flow Porometers

The TechniqueProper algorithmsTo detect stable pressure and flowTo acquire dataSoftwareTo convert acquired data to pore structure characteristicsTo present data in tabular, graphical and excel formatsAdvanced Flow Porometers

An Example:The PMI Advanced Capillary Flow Porometer

The PMI Advanced Capillary Flow PorometerFeatures:Sealing with uniform pressure by pneumatic piston-cylinder deviceAutomatic addition of measured amount of wetting liquid at appropriate time

The PMI Advanced Capillary Flow PorometerAppropriate design & strategic location of transducers to minimize pressure drop in the instrumentMinimal operator involvementUse of samples without cutting and damaging the bulk product

Analysis of Experimental DataDry Flow, Wet Flow & Differential PressureFlow rate and differential pressure measured in a solid oxide micro fuel cell component

Analysis of Experimental DataPore diameter computed from pressure to start flow = Through Pore Throat DiameterThrough Pore Throat Diameter

Analysis of Experimental DataComputed from pressure to initiate gas through wet sampleThe Largest Through Pore Throat Diameter(Bubble Point Pore Diameter)The largest pore size in a solid oxide micro fuel cell component

Analysis of Experimental Data50% of flow is through pores larger than the mean flow through pore throat diameterMFPD computed using pressure when wet flow is half of dry flowThe Mean Flow Through Pore Throat DiameterMean flow pore diameter of a solid oxide micro fuel cell component

Analysis of Experimental DataSmallest pore is computed using pressure at which wet and dry curves meetThe Smallest Through Pore Throat Diameter& The Pore Diameter RangePore diameter range measured in a solid oxide micro fuel cell component

Analysis of Experimental DataFlow DistributionFlow distribution in a membraneThe flow distribution is given by the distribution function, fFfF = -d [(Fw / Fd)p 100] / d DFw = wet flow, Fd = dry flow

Analysis of Experimental DataFlow DistributionArea under distribution function in any diameter range = % flow through pores in that range

Analysis of Experimental DataPore Fraction DistributionPore FractionNj = the number of through pores of throat diameter DjFj = [1/(4 cos / pj)4] [(Fw,j / F d,j) (Fw,j-1 / Fd,j-1)]pj = differential pressure to remove wetting liquid from pore of diameter Dj

Analysis of Experimental DataPore Fraction DistributionFlow fraction distribution of a membrane

Chart2

0.000038

0.000042

0.000046

0.000048

0.000049

0.000051

0.000055

0.00006

0.000065

0.000067

0.000066

0.000063

0.000061

0.000063

0.000067

0.000071

0.000075

0.00018

0.000681

0.001314

0.002376

0.004061

0.034131

0.157645

0.272848

0.277387

0.204321

0.028494

0.003771

0.002707

0.002476

0.001919

0.001723

0.001228

0.000887

0.000739

Diameter, microns

Ni / Sum(Ni)

Chart1

-1.48-2.59-0.92-10.13-6.94

-1.530.4-0.811.80.57

-0.812.181.878.326.37

3.8434.734.734.734.7

SS#1

SS#2

SS#3

Hovosorb

Papere

Surface tension, dynes/cm

(D-Dav), micrometers

Sheet1

Surface tensionss#1ss#2ss#3Hovopaper

wetting liquid25.0425.428.7150.4943.77

1623.56-1.4822.81-2.5927.79-0.9240.36-10.1336.83-6.94

20.123.51-1.5325.80.427.9-0.8152.291.844.340.57

22.324.23-0.8127.582.1830.581.8758.818.3250.146.37

34.728.883.84

Sheet2

0.9660.000038

0.92810.000042

0.8930.000046

0.86050.000048

0.83030.000049

0.80210.000051

0.77580.000055

0.75110.00006

0.7280.000065

0.70620.000067

0.68580.000066

0.66610.000063

0.64760.000061

0.63010.000063

0.61350.000067

0.59770.000071

0.58280.000075

0.56850.00018

0.5550.000681

0.54460.001314

0.53460.002376

0.52490.004061

0.51580.034131

0.50740.157645

0.50050.272848

0.49380.277387

0.48420.204321

0.47810.028494

0.4710.003771

0.46510.002707

0.4590.002476

0.45390.001919

0.44850.001723

0.4430.001228

0.4380.000887

0.43290.000739

Sheet3

Analysis of Experimental DataGas PermeabilityFrom Darcys Law:F = k (A / 2 l ps) (Ts / T) (pi + po) [pi po]F = gas flow rate in volume at STPps = standard pressureTs = standard temperaturek = permeabilityA = area = viscosityl = thicknessT = test temperature in Kelvinpi= inlet gas pressurepo = outlet gas pressure

Analysis of Experimental DataGas PermeabilityPermeability computed from dry flow

Flow rate through a dry sample

Analysis of Experimental DataThrough Pore Surface AreaKozeny-Carman equation relates through pore surface area to flow[F l / p A] = {P3 / [K(1 - P)2 S2 ]} + [Z P2 ] / [1 - P) S (2 p ) ]F = flow rate in volume at average pressure p (p = [pi + po / 2]), and test temperatureP = porosityS = surface area per unit volume of solid = density of gas at average pressureK = 5Z = (48/13 )Flow rate through a dry sample

Analysis of Experimental DataThrough Pore Surface AreaChange of envelope surface area with flow rate

Enhanced CapabilityAdvanced Porometers with special attachments can test samples under a variety of conditions

Enhanced CapabilitySample under compressive stress or cyclic compressive stressCompression & Cyclic Compression PorometryEffects of compressive stress on gas permeability of GDL

Enhanced CapabilitySample under desired controlled humidity and temperatureControlled Thermal & Chemical Environment PorometryThe PMI Fuel Cell Porometer

Enhanced CapabilitySamples exhibiting very low flow ratesFuel cell componentsMembranesDense ceramicsTightly woven fabricsTiny partsSilicon wafersStorage materialsMicroflow PorometrySmall flow rates through a fuel cell component measured in the microflow porometer

Enhanced CapabilityIn-Plane pore structure of sample or pore structure of each layer of multilayer componentsFuel cell componentsBattery separatorsNonwoven filtersFeltsPaperIn-Plane Porometry (Directional Porometry)Pore structure of each layer of a ceramic component

Capillary Condensation Flow PorometryCapillary Condensation Flow Porometry is a recently patented novel technique

Condensation of Vapor of a Wetting Liquid in PoresVapor at p

Capillary Condensation Flow PorometryFree Energy Balance shows condensation occures in pores smaller than DcBasic PrincipleDc = - [4 V l/v cos / RT] / [ ln (p/po)]

V = molar volume of condensed liquidR = gas constant l/v = surface tensionT= test temperature = contact a