Dr. Akshaya Jena and Dr. Krishna Gupta

1
Characterization of Pore Structure of Fuel Cell
Components for Enhancing Performance

• Dr. Akshaya Jena and Dr. Krishna Gupta
• Porous Materials, Inc.,
• Ithaca, New York, USA

2
Outline

• Introduction
• Through 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

3
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.

4
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
5
Through Pore Throat Diameters, Distribution, Gas
Permeability and Surface Area

• Suitable Characterization Techniques
• Advanced Capillary Flow Porometry
• Capillary Condensation Flow Porometry

6
Advanced Capillary Flow Porometry
Basic Principle

• 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

7
Advanced Capillary Flow Porometry

• Pressure needed to displace liquid from a pore
• p 4 ? cos ? / D
• p differential gas pressure
• ? surface tension of wetting liquid
• ? contact angle of the liquid
• D pore diameter
• Pore diameter is defined for all pore
  cross-sections

8
Advanced Capillary Flow Porometry

• (Perimeter/Area)pore (Perimeter/Area)cylindrical
  opening
• Pore Diameter Diameter of Cylindrical Opening

SKETCH
9
Advanced Capillary Flow Porometry

• Measured differential pressure gas flow through
  dry wet sample yield pore structure

10
The Technique
Advanced Flow Porometers

• 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

11
The Technique
Advanced Flow Porometers

• 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

12
An Example
The PMI Advanced Capillary Flow Porometer
13
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

14
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

15
Analysis of Experimental Data
Dry Flow, Wet Flow Differential Pressure
Flow rate and differential pressure measured in a
solid oxide micro fuel cell component
16
Analysis of Experimental Data
Through Pore Throat Diameter

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

17
Analysis of Experimental Data
The Largest Through Pore Throat Diameter (Bubble
Point Pore Diameter)

• Computed from pressure to initiate gas through
  wet sample

The largest pore size in a solid oxide micro fuel
cell component
18
Analysis of Experimental Data
The Mean Flow Through Pore Throat Diameter

• 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

Mean flow pore diameter of a solid oxide micro
fuel cell component
19
Analysis of Experimental Data
The Smallest Through Pore Throat Diameter The
Pore Diameter Range

• Smallest pore is computed using pressure at which
  wet and dry curves meet

Pore diameter range measured in a solid oxide
micro fuel cell component
20
Analysis of Experimental Data
Flow Distribution

• The flow distribution is given by the
  distribution function, fF
• fF -d (Fw / Fd)p 100 / d D
• Fw wet flow, Fd dry flow

Flow distribution in a membrane
21
Analysis of Experimental Data
Flow Distribution

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

22
Analysis of Experimental Data
Pore Fraction Distribution

• Pore Fraction

• Nj the number of through pores of throat
  diameter Dj
• Fj 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

23
Analysis of Experimental Data
Pore Fraction Distribution
Flow fraction distribution of a membrane
24
Analysis of Experimental Data
Gas Permeability

• From Darcys Law
• F k (A / 2µ l ps) (Ts / T) (pi po) pi po
• F gas flow rate in volume at STP
• ps standard pressure
• Ts standard temperature
• k permeability
• A area
• µ viscosity
• l thickness
• T test temperature in Kelvin
• pi inlet gas pressure
• po outlet gas pressure

25
Analysis of Experimental Data
Gas Permeability

• Permeability computed from dry flow

Flow rate through a dry sample
26
Analysis of Experimental Data
Through Pore Surface Area

• Kozeny-Carman equation relates through pore
  surface area to flow
• F l / p A P3 / K(1 - P)2 S2 µ
• Z P2 p / 1 - P) S (2 p p ?) ½
• F flow rate in volume at average pressure
• p (p pi po / 2),
• and test temperature
• P porosity
• S surface area per unit volume of solid
• ? density of gas at average pressure
• K 5
• Z (48/13 p)

Flow rate through a dry sample
27
Analysis of Experimental Data
Through Pore Surface Area
Change of envelope surface area with flow rate
28
Enhanced Capability

• Advanced Porometers with special attachments can
  test samples under a variety of conditions

29
Enhanced Capability
Compression Cyclic Compression Porometry

• Sample under compressive stress or cyclic
  compressive stress

Effects of compressive stress on gas permeability
of GDL
30
Enhanced Capability
Controlled Thermal Chemical Environment
Porometry

• Sample under desired controlled humidity and
  temperature

The PMI Fuel Cell Porometer
31
Enhanced Capability
Microflow Porometry

• Samples exhibiting very low flow rates
• Fuel cell components
• Membranes
• Dense ceramics
• Tightly woven fabrics
• Tiny parts
• Silicon wafers
• Storage materials

Small flow rates through a fuel cell component
measured in the microflow porometer
32
Enhanced Capability
In-Plane Porometry (Directional Porometry)

• In-Plane pore structure of sample or pore
  structure of each layer of multilayer components
• Fuel cell components
• Battery separators
• Nonwoven filters
• Felts
• Paper

Pore structure of each layer of a ceramic
component
33
Capillary Condensation Flow Porometry
Basic Principle

• Capillary Condensation Flow Porometry is a
  recently patented novel technique
• Condensation of Vapor of a Wetting Liquid in
  Pores
• Vapor at pltpo cannot condense
• Vapor at pltpo can condense in pores
• p pressure of vapor, po eq. vapor pressure

34
Capillary Condensation Flow Porometry
Basic Principle

• Free Energy Balance shows ? condensation occures
  in pores smaller than Dc

Dc - 4 V ?l/v cos ? / RT / ln (p/po) V
molar volume of condensed liquid R gas
constant ?l/v surface tension T test
temperature ? contact angle Dc pore
diameter
35
Capillary Condensation Flow Porometry
Basic Principle

• Flow of Vapor through Empty Pores
• A small imposed vapor pressure gradient causes
  flow through empty pores greater than Dc

36
The Technique

• Measured vapor pressure in equilibrium with the
  sample yields Dc
• Measured rate of pressure change in the
  downstream side yields flow rate

37
An Example
The PMI Capillary Condensation Flow Porometer
38
Analysis of Experimental Data
Through Pore Throat Diameter

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

39
Analysis of Experimental Data
Change of Vapor Flow Rate

• Measured Flow Rate Flow through all pores gt Dc
• Molecular flow is applicable to flow through such
  small pores
• (F/A?p)cumulative (Ts/T) (p/12tpsl)(8RT/pM)½
• SD Dmax Ni(Di)3
• A area of sample ?p pressure drop across
  the sample
• l sample thickness T test temperature in K
• M 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

40
Analysis of Experimental Data
Change of Vapor Flow Rate
Variation of flow rate with pore diameter
Flow rate through a membrane
41
Analysis of Experimental Data
Pore Distribution

• Expressed in terms of distribution function, f
• f - d((F/A?p)cumulative) / dD

Flow distribution in a membrane
42
Analysis of Experimental Data
Number of Pores of Diameter, Di

• Number of pores computed using the following
  relation
• f (Ts/T) (p/12tpsl)(8RT/ pM) ½ 3Ni(Di)2

43
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 zero
• Extreme test conditions are avoided
• There is no stress on the sample and structural
  distortion or damage to the sample is negligible

44
Strengths of the Technique

• Only through nanopores are measured and blind
  pores are ignored unlike the gas adsorption
  technique
• Throat diameters are measured
• A wide variety of vapors can be used
• Measuring technique is simple

45
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

46
Vacuapore
Basic Principle

• Hydrophilic pores are spontaneously wetted by
  water
• Hydrophobic pores repel water because
• ? (water/solid) gt ? (gas/solid)
• Pressure on water results in water intrusion
• Intrusion volume is pore volume
• Pore diameter computed from intrusion pressure
• Work done by water Increase in surface free
  energy
• D - 4 ? cos ? / p

47
The Technique

• Recently patented technique
• Features
• 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

48
The Technique

• Vacuapore

49
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.

50
Analysis of Experimental Data

• Volume distribution is given as function, fv
• fv - dV / d log D
• Hydrophobic and hydrophilic pore distributions
  obtained from results of Vacuapore and Mercury
  Intrusion Porosimeter.

51
Analysis of Experimental Data
Pore size distribution in GDL of a PEMFC

• Hydrophobic pores 50.3, MPD 17.1 ?m
• Hydrophilic pores 49.7, MPD lt16.3 ?m

52
Unique Feature

• Capable of measuring
• Hydrophobic large and small pore diameters
• In-plane pore structure
• Influence of compressive stress on pore structure

53
Through Pore Volume, Diameter and Distribution
and Liquid Permeability

• Important characteristics of flow permitting
  pores

54
Liquid Extrusion Porosimetry
Basic Principle

• Sample supported by membrane
• Largest Membrane Pore lt 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

55
Liquid Extrusion Porosimetry
Basic Principle

• Measured volume of liquid flowing out of membrane
  yields pore volume
• Pressure yields pore diameter
• p 4 ? cos ? / D

56
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

57
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.

58
Analysis of Experimental Data
Through Pore Volume

• Measured volume is the cumulative through pore
  volume

Pore volume of five thin layers of a fuel cell
component
59
Analysis of Experimental Data
Through Pore Diameter

• All diameters between the mouth and the throat
  are measured
• Diameters between the throat and the exit are not
  measured

Pore diameters measurable by several techniques
60
Analysis of Experimental Data
Through Pore Volume Distribution

• Through pore volume distribution function fv

Pore volume distribution of Toray paper obtained
by various techniques
61
Analysis of Experimental Data
Liquid Permeability

• Permeability is defined by Darcys law
• F k (A / ? l) (pi - po)
• F volume flow rate
• k permeability
• A area
• ? Viscosity
• (pi - po) differential pressure
• Instrument measures liquid flow rate
• Permeability is computed using the equation

62
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.

63
Unique Features

• Pore Structure Characteristics of pores in Toray
  paper using a number of techniques

Characteristics Through Blind Hydrophobic Hydrophilic
Pore Volume 75 25 29 71
Diameter, ?m 60 40 35 50
Kind of Pore Hydrophilic Hydrophobic Blind Through
64
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

65
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

66
Summary and Conclusions

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

67
Thank You