Download - DR. AKSHAYA JENA AND DR. KRISHNA GUPTA POROUS MATERIALS, INC., ITHACA, NEW YORK, USA Characterization of Pore Structure of Fuel Cell Components for Enhancing.

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


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


(Perimeter/Area)pore = (Perimeter/Area)cylindrical opening

Pore Diameter = Diameter of Cylindrical Opening

SKETCH


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


Features: Sealing with uniform

pressure by pneumatic piston-cylinder device

Automatic addition of measured amount of wetting liquid at appropriate time


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


Pore diameter computed from pressure to start flow = Through Pore Throat Diameter

Through Pore Throat Diameter


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


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


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


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


Flow Distribution

Area under distribution function in any diameter range = % flow through pores in that range

dDf]100)F/F[(Fpdw


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


Pore Fraction Distribution

Flow fraction distribution of a membrane


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


Gas PermeabilityPermeability computed from dry flow

Flow rate through a dry sample


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


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


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


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


Condensation starts at the throat of a through pore and prevents gas flow

Dc = through pore throat diameter

Through Pore Throat Diameter


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


Change of Vapor Flow Rate

Variation of flow rate with pore diameter

Flow rate through a membrane


Pore DistributionExpressed in terms of distribution function, f

f = - d((F/AΔp)cumulative) / dD

Flow distribution in a membrane


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


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.


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.


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.


Measured volume is the cumulative through pore volume

Through Pore Volume

Pore volume of five thin layers of a fuel cell component


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


Through pore volume distribution function fv

Through Pore Volume Distribution

Pore volume distribution of Toray paperobtained by various techniques


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


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


Applications of these techniques have been illustrated with examples of measurements on fuel cell components

Thank You