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DR. AKSHAYA JENA AND DR. KRISHNA GUPTA POROUS MATERIALS, INC., ITHACA, NEW YORK, USA...
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Transcript of DR. AKSHAYA JENA AND DR. KRISHNA GUPTA POROUS MATERIALS, INC., ITHACA, NEW YORK, USA...
DR. AKSHAYA JENA AND DR. KRISHNA GUPTAPOROUS MATERIALS, INC.,ITHACA, NEW YORK, USA
Characterization of Pore Structure of Fuel Cell
Components for Enhancing Performance
Outline
IntroductionThrough pore throat diameter, distribution, gas
permeability & surface area by: Capillary Flow Porometry Capillary Condensation Flow Porometry
Hydrophobic through and blind pore volume & distribution by: Vacuapore
Through pore volume, diameter, distribution & liquid permeability by: Liquid Extrusion Porosimetry
Summary and Conclusion
Introduction
Pore 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 Area
Importance of Such Properties
Through Pores: Fluid flow
Pore Diameters:Capillary forces for liquid movement
Throat diameters:Separation of undesirable particles
Gas permeability: Overall rate of the processes
Through pore surface area: Physicochemical processes
Effects 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 Porometry
For wetting liquid:Wetting Liquids fill pores spontaneously
Cannot come out spontaneously
A pressurized inert gas can displace liquid from pores provided:Work done by Gas = Increase in Interfacial
Free Energy
)l/g()L/s( Basic Principle
Advanced Capillary Flow Porometry
Pressure needed to displace liquid from a pore:
p = 4 γ cos θ / D
p = differential gas pressureγ = surface tension of wetting liquidθ = 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 Porometry
Measured differential pressure & gas flow through dry & wet sample yield pore structure
The Technique
Accurate Pressure transducers Flow transducers Regulators Controllers
Sophisticated sample sealing mechanisms to direct flow in desired directions
Internal computers To control sequential operations To execute automated tests
Advanced Flow Porometers
The Technique
Proper algorithms To detect stable pressure and flow To acquire data
Software To convert acquired data to pore structure
characteristics To present data in tabular, graphical and excel formats
Advanced Flow Porometers
An Example:
The PMI Advanced Capillary Flow Porometer
The PMI Advanced Capillary Flow Porometer
Features: Sealing with uniform
pressure by pneumatic piston-cylinder device
Automatic addition of measured amount of wetting liquid at appropriate time
The PMI Advanced Capillary Flow Porometer
Appropriate design & strategic location of transducers to minimize pressure drop in the instrument
Minimal operator involvement Use of samples without cutting
and damaging the bulk product
Analysis of Experimental Data
Dry Flow, Wet Flow & Differential Pressure
Flow rate and differential pressure measured in a solid oxide micro fuel cell component
Analysis of Experimental Data
Pore diameter computed from pressure to start flow = Through Pore Throat Diameter
Through Pore Throat Diameter
Analysis of Experimental Data
Computed from pressure to initiate gas through wet sample
The Largest Through Pore Throat Diameter
(Bubble Point Pore Diameter)
The largest pore size in a solid oxide micro fuel cell component
Analysis of Experimental Data
50% of flow is through pores larger than the mean flow through pore throat diameter
MFPD computed using pressure when wet flow is half of dry flow
The Mean Flow Through Pore Throat Diameter
Mean flow pore diameter of a solid oxide micro fuel cell component
Analysis of Experimental Data
Smallest pore is computed using pressure at which wet and dry curves meet
The Smallest Through Pore Throat Diameter
& The Pore Diameter Range
Pore diameter range measured in a solid oxide micro fuel cell component
Analysis of Experimental Data
Flow Distribution
Flow distribution in a membrane
The flow distribution is given by the distribution function, fF
fF = -d [(Fw / Fd)p × 100] / d DFw = wet flow, Fd = dry flow
0
10
20
30
40
50
60
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
diameter (microns)
Por
e si
ze d
istr
ibut
ion
Area = A
Analysis of Experimental Data
Flow Distribution
Area under distribution function in any diameter range = % flow through pores in that range
dDf]100)F/F[(Fpdw
Analysis of Experimental Data
Pore Fraction Distribution
Pore Fraction
jjj
jjj
F/f
]N/N[
Nj = the number of through pores of throat diameter Dj
Fj = [1/(4 γ cos θ / 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 Data
Pore Fraction Distribution
Flow fraction distribution of a membrane
Analysis of Experimental Data
Gas PermeabilityFrom Darcy’s Law:
F = k (A / 2μ l ps) (Ts / T) (pi + po) [pi – po]
F = gas flow rate in volume at STPps = standard pressureTs = standard temperaturek = permeabilityA = area μ = viscosityl = thicknessT = test temperature in Kelvinpi= inlet gas pressurepo = outlet gas pressure
Analysis of Experimental Data
Gas PermeabilityPermeability computed from dry flow
Flow rate through a dry sample
Analysis of Experimental Data
Through Pore Surface AreaKozeny-Carman equation relates through pore
surface area to flow
[F l / p A] = {P3 / [K(1 - P)2 S2 μ]}
+ [Z P2 π] / [1 - P) S (2 π p ρ) ½]
F = flow rate in volume at average pressure
p (p = [pi + po / 2]), and test temperature
P = porosityS = surface area per unit volume of solidρ = density of gas at average pressureK = 5Z = (48/13 π)
Flow rate through a dry sample
Analysis of Experimental Data
Through Pore Surface Area
Change of envelope surface area with flow rate
Enhanced Capability
Advanced Porometers with special attachments can test samples under a variety of conditions
Enhanced Capability
Sample under compressive stress or cyclic compressive stress
Compression & Cyclic Compression Porometry
0.00
0.50
1.00
1.50
2.00
2.50
0 50 100 150 200 250 300 350
Com pression pressure (ps i)
Pe
rme
ab
ilit
y (
10
-12 m
2)
GDL A
GDL B
Effects of compressive stress on gas permeability of GDL
Enhanced Capability
Sample under desired controlled humidity and temperature
Controlled Thermal & Chemical Environment
Porometry
The PMI Fuel Cell Porometer
Enhanced Capability
Samples exhibiting very low flow rates Fuel cell components Membranes Dense ceramics Tightly woven fabrics Tiny parts Silicon wafers Storage materials
Microflow Porometry
Small flow rates through a fuel cell component measured in the microflow
porometer
Enhanced Capability
In-Plane pore structure of sample or pore structure of each layer of multilayer components Fuel cell components Battery separators Nonwoven filters Felts Paper
In-Plane Porometry (Directional Porometry)
Pore structure of each layer of a ceramic component
Capillary Condensation Flow Porometry
Capillary Condensation Flow Porometry is a recently patented novel technique
Condensation of Vapor of a Wetting Liquid in Pores
Vapor at p<po cannot condense
Vapor at p<po can condense in pores
p = pressure of vapor, po = eq. vapor pressure
Basic Principle
Capillary Condensation Flow Porometry
Free Energy Balance shows → condensation occures in pores smaller than Dc
Basic Principle
Dc = - [4 V γl/v cos θ / RT] / [ ln (p/po)]
V = molar volume of condensed liquid R = gas constant γl/v = surface tension T= test temperature
θ = contact angle Dc = pore diameter
Capillary Condensation Flow Porometry
Flow of Vapor through Empty Pores
A small imposed vapor pressure gradient causes flow through empty pores greater than Dc
Basic Principle
The Technique
Measured vapor pressure in equilibrium with the sample yields Dc
Measured rate of pressure change in the downstream side yields flow rate
An Example:
The PMI Capillary Condensation Flow Porometer
Analysis of Experimental Data
Condensation starts at the throat of a through pore and prevents gas flow
Dc = through pore throat diameter
Through Pore Throat Diameter
Analysis of Experimental Data
Measured Flow Rate = Flow through all pores > Dc
Molecular flow is applicable to flow through such small pores
(F/AΔp)cumulative = (Ts/T) (π/12τpsl)(8RT/πM)½
[ΣD Dmax Ni(Di)3]
A = area of sample p = pressure drop across the samplel = sample thickness T= test temperature in KM = molecular weight, Ni= number of pores of diameter Di
F= flow rate in volume at STP, ps and Ts
= average tortuosity of pores and is equal to ( L/l) where L is the length of capillary,
D = pore diameter computed by adding to Dc a small correction term for thickness of adsorbed layer
Change of Vapor Flow Rate
Analysis of Experimental Data
Change of Vapor Flow Rate
Variation of flow rate with pore diameter
Flow rate through a membrane
Analysis of Experimental Data
Pore DistributionExpressed in terms of distribution function, f
f = - d((F/AΔp)cumulative) / dD
Flow distribution in a membrane
Analysis of Experimental Data
Number of Pores of Diameter, Di
Number of pores computed using the following relation
f = (Ts/T) (π/12τpsl)(8RT/ πM) ½ [3Ni(Di)2]
Strengths of the Technique
The diameters of pores down to a few nanometers and flow through these small pores are measured
Test pressure on the sample is almost zeroExtreme test conditions are avoidedThere is no stress on the sample and
structural distortion or damage to the sample is negligible
Strengths of the Technique
Only through nanopores are measured and blind pores are ignored unlike the gas adsorption technique
Throat diameters are measuredA wide variety of vapors can be usedMeasuring technique is simple
Hydrophobic Through and BlindPore Volume and Distribution
Hydrophobic and hydrophilic pores are relevant for: Water management Transport of reactants Reaction rates Flow rates of reaction products
Vacuapore
Basic PrincipleHydrophilic pores are spontaneously wetted by
water Hydrophobic pores repel water because
γ (water/solid) > γ (gas/solid)Pressure on water results in water intrusion Intrusion volume is pore volumePore diameter computed from intrusion pressure
Work done by water = Increase in surface free energy
D = - 4 γ cos θ / p
The Technique
Recently patented techniqueFeatures:
Removal of air from the pores, the sample chamber and water
Application of desired compressive stress on the sample
Optional in-plane intrusion of water
The Technique
Vacuapore
Analysis of Experimental Data
Only hydrophobic through and blind pore diameters are measured.
Measured pressure yields pore diameter of hydrophobic through and blind pores.
Measured intrusion volume of water = Cumulative pore volume of hydrophobic through and blind pores.
Analysis of Experimental Data
Volume distribution is given as function, fvfv = - dV / d log D
Hydrophobic and hydrophilic pore distributions obtained from results of Vacuapore and Mercury Intrusion Porosimeter.
Analysis of Experimental Data
Hydrophobic pores: 50.3%, MPD = 17.1 m Hydrophilic pores: 49.7%, MPD = <16.3 m
0
0.5
1
1.5
2
2.5
3
0.001 0.01 0.1 1 10 100 1000
pore diam eter ( )
V
/lo
g(d
iam
ete
r)
H2O
Hg
Pore size distribution in GDL of a PEMFC
Unique Feature
Capable of measuring:Hydrophobic large and small pore diametersIn-plane pore structureInfluence of compressive stress on pore
structure
Through Pore Volume, Diameter and Distribution and Liquid Permeability
Important characteristics of flow permitting pores
Liquid Extrusion Porosimetry
Basic PrincipleSample supported by membrane
Largest Membrane Pore < Smallest Sample Pore
Pores of sample & membrane filled with wetting liquid
Gas pressure displaced liquid from sample pores flows out through liquid filled pores of membrane
Gas pressure sufficient to remove liquid from sample pores does not remove liquid from membrane pores
Liquid Extrusion Porosimetry
Basic PrincipleMeasured volume of liquid flowing out of
membrane yields pore volumePressure yields pore diameter
p = 4 γ cos θ / D
The Technique
Cylindrical sample chamber holds a support screen and membrane
Chamber below the support screen connected to a container placed on a weighing balance
The Technique
O-ring seals against the wall of the sample chamber and the membrane
The pressure of the inert gas on the wet sample is increased to displace liquid from pores.
Analysis of Experimental Data
Measured volume is the cumulative through pore volume
Through Pore Volume
Pore volume of five thin layers of a fuel cell component
Analysis of Experimental Data
All diameters between the mouth and the throat are measured
Diameters between the throat and the exit are not measured
Through Pore Diameter
Pore diameters measurable by several techniques
Analysis of Experimental Data
Through pore volume distribution function fv
Through Pore Volume Distribution
Pore volume distribution of Toray paperobtained by various techniques
Analysis of Experimental Data
Permeability is defined by Darcy’s law:F = k (A / l) (pi - po)
F = volume flow ratek = permeabilityA = area = Viscosity(pi - po) = differential pressure
Instrument measures liquid flow ratePermeability is computed using the equation
Liquid Permeability
Unique Features
Highly versatile. Tests can be performed:
With sample under compressive stress At elevated temperatures Under chemical environments In variable humid atmospheres Using a wide variety of liquids With a wide variety of samples
Complete pore structure can be evaluated by combining various techniques.
Unique Features
Pore Structure Characteristics of pores in Toray paper using a number of techniques
Characteristics
Through Blind HydrophobicHydrophili
c
Pore Volume 75% 25% 29% 71%
Diameter, m 60 40 35 50
Kind of PoreHydrophili
cHydrophobic
Blind Through
Summary and Conclusions
Recently developed pore structure characterization techniques appropriate for fuel cells have been discussed Capillary Flow Porometry Capillary Condensation Flow Porometry Vacuapore Liquid Extrusion Porosimetry
Summary and Conclusions
These techniques are capable of determining pore structure characteristics of through pores relevant for fuel cell components. Pore throat diameter Largest pore diameter Mean flow pore diameter Flow distribution Pore fraction distribution Gas permeability Pore diameters of nanopores Nanopore distribution Envelope surface area Pore volume Pore volume distribution Liquid permeability
Summary and Conclusions
Applications of these techniques have been illustrated with examples of measurements on fuel cell components
Thank You