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Title: MEA Manufacturing How We Make MEAs and Why


1
MEA Manufacturing How We Make MEAs and Why?!?!
  • Edison Materials Technology Center
  • MEA Manufacturing Symposium
  • Dayton, Ohio
  • Jack Brouwer
  • National Fuel Cell Research Center
  • August 21, 2007

2
OUTLINE
  • Introduction to Important Concepts
  • Fuel Cell Operation
  • General Manufacturing Considerations and
    Techniques
  • How are MEAs Manufactured?
  • Overall Manufacturing Processes
  • Backing Layers
  • Catalyst Layers
  • Full Membrane Electrode Assemblies (MEA)
  • Why are MEAs Manufactured This Way?
  • What are Some Issues and Challenges?

3
Introduction
Basic Process Fuel Cell
Cathode
Anode
4
Introduction
  • TRIPLE PHASE BOUNDARY
  • Interface between Gas, Electrode and Electrolyte
    phases
  • Place where electrochemistry actually takes place
    (molecular level)
  • Increasing the available TPB is how one
    increases performance (e.g., power density)

5
Fuel Cell Stack
  • Fuel Cell Stack
  • Increase voltage (and power) to useful levels
  • Bundle or stack together many electrode-electrolyt
    e assemblies
  • Stack
  • Implies output thatscales with fuelcell surface
    area(vs. volume fortraditional engines)

From Cammera, MC Power, 2000
6
Introduction
  • Fuel Cell Stack

Larminie and Dicks, 2003
7
Introduction
  • Fuel Cell Stack

Larminie and Dicks, 2003
8
Introduction
  • Fuel Cell Stack

Larminie and Dicks, 2003
9
Introduction
  • Fuel Cell Stack

Larminie and Dicks, 2003
10
Introduction
  • Fuel Cell Membrane Electrode Assembly (MEA)
  • Basic building block

Larminie and Dicks, 2003
11
Introduction
  • Membrane Electrode Assembly (MEA) features
  • Membrane
  • Ion conductor
  • Electron insulator
  • Impermeable to gases
  • Air and Fuel Electrodes
  • Promote oxygen reduction electrochemistry (air
    cathode)
  • Promote hydrogen oxidation electrochemistry (fuel
    anode)
  • Produce as much active TPB area as possible
  • Non-reactive and degradation resistant in acidic,
    highly reactive, electric field, , conditions
  • Other
  • Mechanical strength
  • Corrosion/degradation resistance
  • Amenable to sealing, manifolding, compression,

12
OUTLINE
  • Introduction to Important Concepts
  • Fuel Cell Operation
  • General Manufacturing Considerations and
    Techniques
  • How are MEAs Manufactured?
  • Overall Manufacturing Processes
  • Backing Layers
  • Catalyst Layers
  • Full Membrane Electrode Assemblies (MEA)
  • Why are MEAs Manufactured This Way?
  • What are Some Issues and Challenges?

13
How Are MEAs Manufactured?
  • PEMFC Unit Cell Components

From Hirschenhofer et al., 2002
14
How Are MEAs Manufactured?
  • PEMFC Unit Cell Components 5 or 7 layers
  • Membrane
  • Nafion, perflourinated sulfonic acid (PFSA)
    polymer
  • Hydrocarbon membrane
  • Composites with poly-vinylidene difluoride (PVDF)
    or other structural components
  • Polybenzimidizol (PBI) for higher temperature
    PEMFC
  • 2 Catalyst Layers (anode and cathode)
  • 2 Backing / Gas Diffusion Layers (anode and
    cathode)
  • 2 Integrated Gaskets or Seals (7 layer only)

15
How Are MEAs Manufactured?
  • MEA requirements
  • Intimate contacting of catalyst with electrolyte
  • Intimate contacting of catalyst with support
  • Sufficient availability of catalyst to reactants
  • Continuous path for ions from TPB to electrolyte
  • Continuous path for electrons from TPB to backing
    layer / current collector
  • Sufficient porosity in electrode to allow
    reactant flow in from and product flow out to gas
    phase
  • Sufficient hydrophobicity in electrode to repel
    water, inhibit pooling
  • Sufficient density, flexibility, and robustness
    in electrolyte layer
  • Absorb water
  • Conduct ions
  • Resist electronic conduction
  • Fully separate reactant gases
  • (ONLY a partial list)

16
How Are MEAs Manufactured?
  • MEA Requirements (contd)
  • Primarily depend upon random percolation of
    phases to obtain all properties three
    constituents

GasDiffusion Layer
Catalyst Layer
Membrane
From S.S. Kocha., Handbook of Fuel Cells, 2003
17
How Are MEAs Manufactured?
  • Membrane
  • Most common is perfluorinated sulfonic acid
    (PFSA) polymer Nafion is one of these
  • H ion conduction in PFSA membranes is by
    migration in water filled pores
  • Critical Properties
  • high ion conductivity
  • low gas permeability
  • durability at temperature
  • adequate strength
  • cost and ease of processing
  • Functions
  • separate H2 from air
  • conduct protons H
  • prevent electron conduction

18
How are MEAs Manufactured?
  • PEMFC Membrane
  • Nafion electrolyte structure Teflon backbone
    with vinyl side chains terminated with sulfonic
    acid (SO3H)

From US Fuel Cell Council, 2005
19
How are MEAs Manufactured?
  • PEMFC Membrane
  • Nafion electrolyte structure (contd)

From US Fuel Cell Council, 2005
20
How Are MEAs Manufactured?
  • PEMFC Electrolyte
  • Maximize loading of acidic functional groups
    without permitting excessive swelling in the
    presence of water
  • Retain membrane strength flexibility as acid
    level rises
  • Control domain size and clusters to produce
    continuous pathways of proton conduction from one
    side to the other
  • Retain sufficient water for high conductivity
  • Develop fabrication techniques to give
    reproducible membrane behavior
  • Optimize the polymer properties so that the
    membrane will adhere to the catalyst and carbon
    particles at the electrodes
  • Produce membranes MEAs with longer useful life
  • Produce lower cost membranes MEAs

21
How Are MEAs Manufactured?
  • Catalyst Layer
  • Some call this the electrode
  • Recall need for catalyst itself (e.g., Pt, Pt
    alloys, PtRu) and catalyst supports (carbon) and
    ionomer (Nafion)
  • Critical Properties
  • high catalyst surface area
  • stability of supports
  • catalyst/ionomer contact
  • porosity for gas/water transport
  • low precious metal content
  • controllable processing
  • uniform dispersions, coatability
  • Functions
  • oxidize hydrogen on the anode, ½ H2 ?H e-
  • reduce oxygen on the cathode, ½ O2 2H 2 e- ?
    H2O
  • conduct electrons to and from GDL
  • conduct protons to and from PEM
  • diffuse gases and water vapor between catalyst
    surface and GDL

22
How Are MEAs Manufactured?
  • Catalyst Material - Carbon-supported catalyst
  • Pt black with 10-20mg/cm2 ? 4 mg/cm2 have
    historically been used for alkaline and acid
    (PEM) fuel cells alike
  • Lower Pt loadings have been accomplished by
    supporting/ dispersing Pt on carbon to obtain
    much higher Pt surface area

Idealized structure (not drawn to scale)
Larminie and Dicks, 2003
23
How Are MEAs Manufactured?
  • Catalyst Manufacturing
  • Activated Carbon is a popular catalyst support
  • High surface area, high porosity carbon
  • Obtained by carbonization of suitable precursors
    and subsequent thermal or chemical activation
  • Common starting material include wood/sawdust,
    coconut shells, charcoal, lignite, bituminous
    coal etc.
  • Macro-, meso- and micro-pore structure can be
    modified as needed
  • Ability to be surface modified
  • Stability in both acidic and basic media
  • Good chemical resistance and electrical
    conductivity
  • Carbon nano-fibers, nano-tubes, , are being
    investigated

24
How Are MEAs Manufactured?
  • Catalyst Manufacturing
  • Surface oxidation of original carbon support
    before application of the catalyst
  • Done to remove metallic particles
    (dimineralization), small carbonaceous
    impurities, ash and sulfur from the support
  • Can be done with gas phase or liquid phase
    oxidation
  • Common oxidizing agents
  • Nitric acid, HNO3
  • Sufuric acid, H2SO4
  • Phosphoric acid, H3PO4
  • Hydrogen peroxide, H2O2
  • plasma
  • ozone

25
How Are MEAs Manufactured?
  • Catalyst Manufacturing
  • Colloidal method
  • Finely dispersed catalyst phase in continuous
    phase
  • Aqueous media method
  • Liquid phase reduction of chloroplatinic
    acid-H2PtCl6.6H2O to colloidal Pt using reducing
    agents such as sodium dithionite, sodium citrate,
    sodium borohydride, sodium bisulfite
  • Colloidal Pt stabilized by use of a surfactant
  • Addition to a slurry of carbon material to form
    the supported catalyst
  • Ethylene glycol method
  • Titrate chloroplatinic acid solution into
    ethylene glycol (EG) suspension of oxidized
    carbon
  • pH of solution adjusted to 13 by solution of
    NaOH in EG
  • Mixture refluxed under inert conditions for 4
    hours at 140ºC to obtain colloidal Pt deposited
    on carbon
  • Filtration, washing, drying steps to obtain
    supported electrocatalyst

26
How Are MEAs Manufactured?
  • PEMFC Electrode Catalyst Layer
  • TEM micrograph of catalyst layer
  • Small dark particles are Pt catalyst (2-5nm
    diameter)
  • Larger gray particles are carbon support
    (50-100nm diameter)

From Larminie and Dicks, 2003
27
How Are MEAs Manufactured?
  • Gas Diffusion Layer (GDL)
  • Some distinguish between electrode backing and
    GDL
  • Critical Properties
  • electrical conductivity
  • wetting characteristics, and stability
  • gas permeability, K
  • stability of porosity
  • compressibility
  • thickness controlled
  • surface smoothness
  • process-ability and cost

EB Electrode Backing carbon papers
carbon cloths carbon non-wovens
Gas Diffusion Layer carbon black dispersion
GDL

EB
  • Functions
  • conduct electrons between catalyst and bi-polar
    plate
  • diffuse/convect gases to catalyst layers
  • facilitate transport of water to and from flow
    field

28
How Are MEAs Manufactured?
  • Gas Diffusion Backing Layer
  • Carbon fiber based
  • Most common is a co-polymer manufactured from
    90 polyacrylonitrile (PAN)
  • Solvent spinning process followed by
    carbonization at1200-1350oC in nitrogen
  • Filament diameter of 12-14mm
  • Use chopped fibers for papermanufacturing
  • Use longer fibers woven intocloth

From Mathias et. al., Handbook of Fuel Cells,
2003
29
How Are MEAs Manufactured?
  • Gas Diffusion Backing Layer
  • Non-woven carbon fiber paper manufacturing
  • Paper-making (wet-laid process using
    conventional paper-making equipment)
  • Resin impregnation (carbonizable thermoset resin
    introduced)
  • Molding (compression molded to desired shape and
    thickness, then cured to 175oC)
  • Heat treatment (carbonization/graphitization in
    which 30-40 of the mass is lost, mostly below
    1000oC, graphitization occurs above 2000oC)

30
How Are MEAs Manufactured?
  • Gas Diffusion Backing Layer
  • Wet-laid filled papers
  • add carbon or graphite powders to the paper
    (manufactured as above) and bind them with
    Teflon (PTFE)
  • Carbon fiber cloth
  • Produce carbon fiber yarn
  • Carbonize (or graphitize) yarn in a continuous,
    batch or combination of continuous and batch
    processes
  • Weave yarn into a fabric for mechanical integrity
    (vs. resin)
  • Dry-laid materials
  • Dry laying of PAN fibers onto a fiber fleece
  • Hydro-entangle the fibers by jet impingement
  • Oxidatively stablize the mat of fibers
  • Carbonization to 1000-1500oC
  • Option fill with carbon or graphite powders and
    resin

31
How Are MEAs Manufactured?
  • Various methods for manufacturing GDL or backing
    layers

From Mathias et. al., Handbook of Fuel Cells,
2003
32
How Are MEAs Manufactured?
Wet-laid Carbon Filled Paper
Dry-laid Carbon Filled Paper
  • SEMmicrographsof variousbacking layers

Dry-laid Non-Filled Paper
From Mathias et. al., Handbook of Fuel Cells,
2003
33
How Are MEAs Manufactured?
  • Overall PEMFC Electrode-Electrolyte Interface
    Construction
  • Catalyst layer and backing layer
  • Intimate contacting of electronic, ionic, and
    catalyst phases
  • Idealized structure (not drawn to scale)

Backinglayer
Catalyst layer
From Larminie and Dicks, 2003
34
How Are MEAs Manufactured?
  • Typical MEA Fabrication Steps
  • Carbon particles and catalyst (usually Pt)
    particles manufactured to form a carbon/catalyst
    agglomerate (supported catalyst)
  • Supported catalyst is combined with an ionomer
    solution and a liquid carrier (e.g., alcohol) and
    applied to the membrane or GDL
  • The catalyst layer is dried to remove the liquid
    and leave a uniform layer of carbon supported
    catalyst
  • The membrane, catalyst layer and GDL are affixed
    together with heat and pressure

35
How Are MEAs Manufactured?
  • Three main procedures
  • Indirect-Decal Catalyst is transferred to the
    membrane by brushing onto a film and then
    transferred from the film to the membrane by
    pressing
  • GDL-based Catalyst is painted or sprayed on the
    GDL and followed by hot pressing
  • Membrane-based Catalyst is sprayed directly on
    the membrane rather than the GDL and followed by
    hot pressing

36
How Are MEAs Manufactured?
  • Variations in the Procedures
  • Applying the catalyst layer by different methods
    (e.g., brushing, screen printing, decal method,
    etc.)
  • Various temperature, pressure, chemical cleansing
    cycles
  • Composition variations in Pt/Carbon/Ionomer
    ratios
  • Cathodes and anodes are generally fabricated
    identically, but, anode kinetics are much faster
    allowing anode construction with less Pt

37
How Are MEAs Manufactured?
  • Catalyst Ink Manufacturing
  • Various strategies and formulations that depend
    upon ink application method
  • Typically mix carbon supported catalyst with
    dilute ionomer solution, surfactants, binders
    and/or other compounds
  • Use a volatile compound for transfer/spraying
    that will eventually evaporate (e.g., alcohol,
    volatile hydrocarbon)
  • Desire a homogeneous and smooth ink with good
    dispersion and viscosity

38
How Are MEAs Manufactured?
  • Membrane Pre-Treatment
  • Modify the membrane surface to promote adhesion,
    contacting, stability,
  • One method promotes cation exchange in the
    membrane to Na form

39
How Are MEAs Manufactured?
  • Ink Application Strategies
  • Airbrush air-blast or -assist spray ink onto
    membrane.
  • Painting - brush or hand paint ink onto membrane.
  • Dry Roller - spray a dry catalyst powder directly
    onto the membrane and then hot roll/press the
    membrane with GDL
  • CNC Sprayer/X-Y Plotter use sprayer mounted on
    a 2-D motion table with computer numerical
    control (CNC)
  • Inkjet - use inkjet or other electronic printing
    device to print ink onto membrane
  • Wet Roller - apply a thin film of ink onto
    engraved rolling plates to continuously apply ink
    to a moving membrane
  • Screen printer squeegee ink through a precision
    screen onto membrane

40
How Are MEAs Manufactured?
  • Various means of putting together the membrane
    electrode assembly (MEA)

From S.S. Kocha., Handbook of Fuel Cells, 2003
41
How Are MEAs Manufactured?
  • Hot Pressing MEA
  • MEA hot pressing method parameters
  • Pressure (500-1500 psi)
  • Time (2-5 min.)
  • Temperature (100-160 oC)
  • These hot-pressing conditions affect cell
    performance
  • Excessive hot-pressing temperature can decrease
    exchange current density, io (slowing kinetics,
    increasing activation polarization) because the
    active layer becomes too embedded in the
    electrolyte membrane
  • High hot-pressing temperature and pressure can
    also increase ohmic losses (cell resistance)

42
How Are MEAs Manufactured?
  • Hot Pressing Temperature, Pressure, Time
  • Ideal Too Much Too Little

No backing layer contact
Backinglayer crushed
Backinglayer
No membrane contact
Membrane thinned
Membrane
Catalyst particle detached
Catalyst layer embedded
Catalyst layer
43
How Are MEAs Manufactured?
  • MEA Post-Treatment
  • Treat MEA after pressing to clean, stablize,
    convert to desired form,
  • E.g., Cation exchange proton exchange membrane
    from previous Na to desired H form

44
How Are MEAs Manufactured?
  • Many MEA manufacturing options available
  • Purchase catalyst support, catalyst, backing
    layer, membrane and ancillary components /
    supplies
  • make supported-catalyst, make ink, treat
    membrane, apply ink, make MEA, press MEA,
    post-treat
  • Purchase supported catalyst, backing layer,
    membrane and ancillary components / supplies
  • make ink, treat membrane, apply ink, make MEA,
    press MEA, post-treat
  • Purchase electrode (supported catalyst on backing
    layer), membrane and ancillary components /
    supplies
  • treat membrane, apply electrodes, press MEA,
    post-treat
  • Purchase MEA

45
OUTLINE
  • Introduction to Important Concepts
  • Fuel Cell Operation
  • General Manufacturing Considerations and
    Techniques
  • How are MEAs Manufactured?
  • Overall Manufacturing Processes
  • Backing Layers
  • Catalyst Layers
  • Full Membrane Electrode Assemblies (MEA)
  • Why are MEAs Manufactured This Way?
  • What are Some Issues and Challenges?

46
Why are MEAs Manufactured This Way?
  • Reactant/Product Transport Mass Transport
  • Reaction Kinetics Electrochemical/Chemical
    Reactions
  • Ion/Electron Transport in electrodes/electrolyte
    Charge Transport

E. Stuve, 2002
47
Why are MEAs Manufactured This Way?
  • Irreversible processes affect observed
    performance
  • Many fundamental physical, chemical
    electrochemical mechanisms involved to actual FC
    operation
  • reactant transport - reactant dissolution
  • double layer penetration - double layer transport
  • adsorption - pre-electrochemical reaction
    kinetics
  • surface migration - electrochemical charge
    transfer
  • post-electrochemical reaction kinetics
  • post-electrochemical surface migration
  • desorption - product evolution
  • product transport

48
Why are MEAs Manufactured This Way?
  • Irreversible processes affect observed
    performance
  • Representations of physical, chemical, and
    electrochemical processes

49
Why are MEAs Manufactured This Way?
  • Electrode Electrolyte Interface
  • The interface between two dissimilar materials is
    electrified
  • Almost all surfaces carry an excess electric
    charge
  • Two phases (electrode/electrolyte) come together
    - charge separation at the interface called
    charge double layer

Electric charge double layer set up at
electrodeelectrolyte interface Electro-neutrali
ty appliesin bulk phases
50
Why are MEAs Manufactured This Way?
  • Electrode Electrolyte Interface
  • The charge double layer presents itself as a
    strong electric field at the interface
  • Simple approximate models have been proposed to
    describe the properties of the electrified
    interface
  • Helmholtz compact layer model
  • Gouy-Chapman diffuse layer model
  • Stern model
  • A useful conceptualization involves representing
    the interfacial structure as an electrical
    equivalent circuit
  • a single capacitor or series of capacitors

51
Why are MEAs Manufactured This Way?
  • Characterization of the Interface
  • Simplified equivalent circuit for interface
  • Ideal polarizable interface RCT ? 8
  • No charge leaks across interface
  • Ideal non-polarizable interface RCT ? 0
  • Charge transfer occurs across interface
  • Electrolyte acts as a resistance

CDL
iDL Double Layer Charging Current
iDL
i
RE
iF
iF Faradaic Current
RCT
i iDL iF
52
Why are MEAs Manufactured This Way?
  • Current at an electrode/electrolyte interface
    reflects both
  • Charging of electric double layer non-Faradaic
    charging current iDL
  • ET across electrode/electrolyte interface
    Faradaic current iF
  • We focus on the Faradaic component
  • The Faradaic current iF in turn can have
    components
  • rate determining interfacial ET (at low
    potentials)
  • mass transport (MT) limitations due to diffusion
    mechanisms (at high potentials)
  • These components can be quantified in terms of
    characteristic rate constants k0(cm/s) for ET
    and kD(cm/s) for MT (at high potentials)

53
Why are MEAs Manufactured This Way?
  • Electro-active area

Electric field
Excess negative charge
Excess positive charge
54
Why are MEAs Manufactured This Way?
  • Equilibrium
  • Measurements of redox potentials (and voltage
    potentials) gives a quantitative estimate of the
    reaction tendency to proceed (equilibrium)
  • No kinetic information is derived from these
    measurements
  • Kinetics
  • Need to know if the reactions (electron transfer)
    will proceed fast enough to make them useful
  • We desire the rate of electron transfer (ET)
    that occurs at the electrode-electrolyte
    interface for given conditions
  • How can kinetic information about ET processes
    be derived?

55
Why are MEAs Manufactured This Way?
  • Basic Kinetic Concepts for Interfacial ET
    process
  • Current flow is proportional to reaction flux
    (rate)
  • Reaction rate is proportional to interface
    reactant concentration
  • Similar to homogeneous reaction chemical kinetics
  • constant of proportionality between reaction rate
    ? (mol/cm2/s) and reactant concentration c
    (mol/cm3) is the rate constant k (cm/s)
  • All chemical and electrochemical reactions are
    activated processes
  • Activation energy barrier that must be overcome
    for reactions to proceed
  • Energy must be supplied to surmount the
    activation energy barrier
  • Energy may be supplied thermally or also (for ET
    processes at electrodes) via application of a
    potential to the electrodes

56
Why are MEAs Manufactured This Way?
  • Basic Kinetic Concepts for Interfacial ET process
    (contd)
  • Applying a potential to an electrode generates an
    electric field at the electrode/electrolyte
    interface that reduces the magnitude of the
    activation energy barrier increasing the ET
    reaction rate
  • Electrolysis works on this principle
  • An applied potential acts as a driving force for
    the ET reaction
  • Expect that current should increase with
    increasing driving force
  • Catalysts act to reduce the magnitude of the
    activation energy barrier

57
Why are MEAs Manufactured This Way?
  • Electrochemical reactions are usually complex
    multi-step processes involving the transfer of
    more than one electron
  • Single outer sphere electron transfer
  • Metal deposition / phase transformation
  • Gas evolution
  • Metal dissolution
  • Oxide layer formation
  • Metal oxidation
  • Oxygen electro-reduction in porous gas diffusion
    electrode
  • In this lecture we will only consider a simple
    single step ET processes involving the transfer
    of a single electron
  • The kinetics of simple ET processes can be
    understood using the activated complex theory of
    chemical kinetics

58
Why are MEAs Manufactured This Way?
  • Gibbs Free Energy (G) Typical Spontaneous
    Reaction
  • Galvanic Cell (self driving, energy producing)

G
Activation Energy G Gr
Gr
Gibbs Free Energy (G)
DG Gp Gr
Gp
ereactants
eproducts
e
Reaction progress variable (e)
59
Why are MEAs Manufactured This Way?
  • Consider a simple ET process in which bonds are
    not broken or made and one electron is
    transferred (neglecting mass transport
    limitations)
  • Oxidation and Reduction processes are
    microscopically reversible
  • Net current at interface representsthe balance
    between iox and ired
  • Symmetry factor, b, determines how much of the
    input energy affects the activation energy
    barrier of the redox process (0 lt b lt 1)
  • Results in Butler-Volmer Equation

oxidationcomponent
reductioncomponent
60
Why are MEAs Manufactured This Way?
  • Butler-Volmer Equation

reduction
oxidation
61
Why are MEAs Manufactured This Way?
  • Approximations to the Butler-Volmer equation
  • The BV equation reduces to the Tafel equation
    when the overpotential, h, is large (typically h
    gt 120 mV)
  • At high overpotentials, the forward ET reaction
    occurs at a much higher rate than the reverse
    reaction allowing us to neglect the reverse
    reaction
  • This results in a logarithmic relationship
    between current and overpotential
  • A plot of ln(i) vs h is linear, which is called a
    Tafel plot
  • The slope of the linear Tafel region gives the
    symmetry factor, ß
  • Exchange current, io, is obtained from the
    intercept at h 0
  • NOTE Tafel analysis is not valid for low
    overpotentials

62
Why are MEAs Manufactured This Way?
  • Approximations to the Butler-Volmer equation
    Tafel analysis
  • If h gtgt 0, then i ? iox , net electron loss
    (oxidation)
  • If h ltlt 0, then i ? ired , net electron gain
    (reduction)

63
Why are MEAs Manufactured This Way?
  • Bulk Activation Polarization
  • Activation Polarization Loss can be estimated for
    most operating conditions by the Tafel expression
    (h gt 120mV)
  • a is the Electron Transfer Coefficient (nature
    of activated state)
  • R is the universal gas constant
  • T is temperature
  • F is Faradays constant
  • i is current
  • io is the Exchange Current Density
  • io is the most important parameter in Tafel
    expression related to rate with which ET takes
    place with zero net current flow catalysts are
    often presumed to increase io

64
Why are MEAs Manufactured This Way?
  • Bulk Activation Polarization (contd)
  • Activation polarization depends upon
  • Nature of the electrode material
  • Ion-ion interactions
  • Ion-solvent interactions (acidic aqueous solution
    membranes)
  • Characteristics of the electric double layer at
    the electrode-electrolyte interface (TPB)
  • Activation polarization can be reduced by
  • Increasing the operating temperature
  • Increasing the electrodes active surface area
  • Increasing activity of the electrodes through the
    use of catalysts

65
Why are MEAs Manufactured This Way?
  • PEMFC Charge Transport
  • Chemical structure of Nafion Microscopic view
    of protonwith Teflon (PTFE) backbone conduction
    in Nafion

From OHayre et al., 2006
66
Why are MEAs Manufactured This Way?
  • Ion Transport in PEMFC Membrane
  • Membrane swells with water due to hydrophilicity
    of SO3H groups
  • Ionic transport depends upon water and loading of
    SO3H groups

Backbone
Water
HO3 S-CF2-CF2-mO-CF-CF2-O- ?
CF3
Backbone
H3O
-O-CF2-CF-Om-CF2-CF2-SO3 H
CF3
H O3 S-CF2-CF2-mO-CF-CF2-O-
? CF3
H3O
-O-CF2-CF-Om-CF2-CF2-SO3H CF3
HO3 S-CF2-CF2-mO-CF-CF2-O- ?
CF3
-O-CF2-CF-Om-CF2-CF2-SO3 H
CF3
H3O
-O-CF2-CF-Om-CF2-CF2-SO3H CF3
H O3 S-CF2-CF2-mO-CF-CF2-O-
? CF3
67
Why are MEAs Manufactured This Way?
  • PEMFC Charge Transport
  • Membrane water content and temperature affect
    conductivity

Water content
Water contentvs. activity
Temperature
From OHayre et al., 2006
68
Why are MEAs Manufactured This Way?
  • Calculated Properties of Nafion
  • Water Content Profile Local
    Conductivity Profile across membrane
    across membrane

From OHayre et al., 2006
69
Why are MEAs Manufactured This Way?
  • Mass Transport
  • 3 main processes
  • Convection mass transport by hydrodynamic flow
  • Diffusion mass transport due to concentration
    gradient
  • Migration mass transport due to potential
    gradient
  • Diffusion and Convection are the most important
    in electrochemistry since electro-migration is
    usually suppressed in experiments
  • Steady state mass transport

diffusion migration convection
70
Why are MEAs Manufactured This Way?
  • Mass Transport
  • Convection vs. Diffusion
  • Convection (dP/dx driver) Diffusion (dc/dx
    driver)

From OHayre et al., 2006
71
Why are MEAs Manufactured This Way?
  • Mass Transport - Flow Channel and GDL During
    Operation

From OHayre et al., 2006
72
Why are MEAs Manufactured This Way?
  • Mass Transport in a Typical Fuel Cell Electrode

From OHayre et al., 2006
73
Why are MEAs Manufactured This Way?
  • Concentration Dynamics due to Coupled Mass
    Transport and Reaction

From OHayre et al., 2006
74
Why are MEAs Manufactured This Way?
  • Concentration Polarization
  • Limiting Current Density case when
    concentration at the TPB goes to zero

From OHayre et al., 2006
75
Why are MEAs Manufactured This Way?
  • Schematic of 2-D Mass Transport Model in Fuel
    Cell (Diffusion and Convection)

From OHayre et al., 2006
76
Why are MEAs Manufactured This Way?
  • Mass Transport
  • Major Types of Flow Channel Configurations

From OHayre et al., 2006
77
Why are MEAs Manufactured This Way?
  • Modes of mass transport within anode and cathode
    compartments

From OHayre et al., 2006
78
Why are MEAs Manufactured This Way?
  • Simultaneously promote in bulk and microscopic
    scales

Electrochemical Kinetics
Charge Transport
Mass Transport
79
OUTLINE
  • Introduction to Important Concepts
  • Fuel Cell Operation
  • General Manufacturing Considerations and
    Techniques
  • How are MEAs Manufactured?
  • Overall Manufacturing Processes
  • Backing Layers
  • Catalyst Layers
  • Full Membrane Electrode Assemblies (MEA)
  • Why are MEAs Manufactured This Way?
  • What are Some Issues and Challenges?

80
What are Some Issues and Challenges?
  • Membrane Degradation
  • Membrane failure has been a dominating failure
    mode under many fuel cell applications
  • The membrane may either thin and fail or may fail
    in discrete regions
  • Fluoride release rate can be used as a
    quantitative indicator of membrane degradation
    occurring

Membrane is thinning in discrete areas
Reduced physical strength leads to rupture
From Knights, Ballard, 2006
81
What are Some Issues and Challenges?
  • Membrane Degradation Mechanism
  • Peroxide generation in fuel cell
  • Peroxide radical production through reaction with
    Fentons catalysts, such as iron (Fe2)
  • Attack of membrane by radicals resulting in loss
    of material (thinning) and loss of mechanical
    strength
  • Rupture of membrane due to mechanical stresses

82
What are Some Issues and Challenges?
  • 1. Peroxide Generation
  • Peroxide is generated electrochemically from
    oxygen reduction reaction (ORR)
  • Oxygen reduction is split between two parallel
    mechanisms Between 0.5 V and 0.65 V
  • the 4 e- transfer mechanism to water
  • the 2 e- transfer mechanism to hydrogen peroxide
  • At lower potentials (lt 0.5 V), the mechanism
    shifts in favour of the hydrogen peroxide
    formation
  • Peroxide can also be produced by reactant
    cross-over
  • H2 reacting on cathode
  • O2 reacting on anode
  • Also caused by air bleed introduced directly on
    anode

83
What are Some Issues and Challenges?
  • 2. Radical Production/Fentons Chemistry
  • Radical initiation involving iron (example of
    Fenton catalysis)
  • initiation of hydroxyl hydroperoxyl radicals
  • In the absence of Fe2, radical initiation is
    slowed
  • It has been shown that degradation increases in
    presence of iron


From Pozio, et al., 2003
84
What are Some Issues and Challenges?
  • 3. Polymer Attack
  • Non-perfluorinated polymer end groups with
    residual H-containing terminal bonds are
    susceptible to chemical attack by peroxide
    radicals
  • There is some evidence that attack may occur in
    other locations as well, e.g., side chain
  • Improvement seen through reduction in polymer
    end group sites using ex-situ accelerated test
    protocols.

Curtin, et al., JPS, 2004
85
What are Some Issues and Challenges?
  • 4. Mechanical Degradation
  • Membrane mechanical failures may be based on
  • Swelling and dimensional change due to hydration
    changes
  • Creep
  • Fatigue
  • Dissolution

Mathias, et al., ECS Interface, 2005 (RH cycles)
86
What are Some Issues and Challenges?
  • Insufficient membrane water content results in
    early degradation and significant increases in
    gas crossover for a non-optimized cell/MEA design.

Knights, Ballard, FCS, 2006
87
What are Some Issues and Challenges?
  • Temperature affects membrane degradation

Temperature Range
90C
30C
Knights, Ballard, FCS, 2006
88
What are Some Issues and Challenges?
  • Pt Particle Growth/Agglomeration
  • Also called sintering, Oswald ripening,
    coalescence
  • Small Pt particles (2-6 nm) are thermodynamically
    unstable, they tend to join with the other
    particles to form more stable larger particles
    (gt7nm)
  • Pt particle growth is accelerated by a number of
    operational and design issues

Pt particles after 80C cycling to 1.2 V
Fresh cathode Pt catalyst particles
More and Reeves, DOE, 2005
89
What are Some Issues and Challenges?
  • Pt dissolution
  • Competing representative reactions at low pH and
    high potential (V)
  • Pt dissolution Pt ? Pt2 2 e- (gt0.9 V)
  • other ionic species may also be formed
  • Pt oxide formation Pt H2O ? PtO 2 H 2 e-
    (gt0.7 V)
  • Pt hydroxide formation may occur, e.g., Pt(OH)2
  • Oxide chemical dissolution PtO 2 H ? Pt2
    H2O (relatively slow)
  • Pt instability window occurs at potentials
    sufficient to dissolve Pt but insufficient to
    form a protective oxide layer, 0.95 to 1.2V
  • Transitions between these regions result in
    higher levels of Pt dissolution because slow
    oxide formation loses out to faster dissolution
    reactions

Darling and Meyers, J.ECS, 2003
90
What are Some Issues and Challenges?
  • Pt Migration
  • Pt is often found present in a band embedded in
    the membrane
  • Hypothesis
  • The dissolved Pt, such as Pt2, migrates through
    the membrane until it reaches a suitably reducing
    atmosphere due to diffusion of H2 from the anode
  • The Pt2 is reduced to Pt and precipitates in the
    membrane

Anode
Cathode
Pt band in membrane
Knights, Ballard, FCS, 2006
91
What are Some Issues and Challenges?
  • Effect of Voltage on Surface Area Loss
  • Pt surface area loss increases with cell
    voltage
  • Steady state test 2,000h H2/air
  • OCV (0.93V vs RHE)
  • 0.2A/cm2 (0.78-0.73V)
  • Mathias, et al., ECS Interface, 2005

92
What are Some Issues and Challenges?
  • Cathode Corrosion Mechanism
  • Main corrosion reaction
  • C 2H2O ? CO2 4H 4e- E0 0.207 V
  • This reaction is kinetically sluggish
  • Pt catalyses the reaction such that the carbon
    support begins to appreciably corrode at gt1.0 V
  • Normal FC operation at gt0.8 V creates milder
    oxidative conditions on the cathode (degradation
    over 1,000 to 10,000s of hours)
  • Occurrence of hydrogen/air front on the anode can
    drive local cathode potential as high as 1.8 V
  • Partial fuel starvation and stop/start processes
  • Damage in hours to 100s of hours

Knights, Ballard, FCS, 2006
93
What are Some Issues and Challenges?
  • Hydrogen/air front on anode results in
  • Oxygen present on anode driving increase in local
    potential
  • High conductivity in catalyst layer driving cell
    voltage to be maintained along cell

Corrosive region with high local potential
  • Cathode corrosion cell schematic

e-
2H2O ? O2 4H 4e-
Cathode
O2 4H 4e-? H2O
C 2H2O ? CO2 4H 4e-
Membrane
H
H
Anode
2H2 ? 4H 4e-
O2 4H 4e-? H2O
e-
  • Cathode Corrosion
  • Fuel rich region

Fuel depleted region
Knights, Ballard, FCS, 2006
94
What are Some Issues and Challenges?
  • Fuel Starvation Anode Corrosion
  • Fuel starvation occurs when there is insufficient
    hydrogen to sustain the current in any cell
    within a stack

Oxidant
Fuel
Normal reaction
2H2 ? 4H 4e-
Reactions in absence of H2
O2 4H 4e-? H2O
H
2H2O? O2 4H 4e-
C 2H2O ? CO2 4H 4e-
e-
e-
Knights, Ballard, FCS, 2006
Anode
Cathode
95
What are Some Issues and Challenges?
  • Typical Appearance of Corrosion
  • Water becomes darkly colored aftercorrosion
    occurs
  • Cathode catalyst layer thins andbrightens after
    corrosion

Knights, Ballard, FCS, 2006
Reiser et al., ESL, 2005
96
What are Some Issues and Challenges?
  • Aging of GDL
  • Electrochemical oxidation due to cathode
    potentials
  • High humidity, oxygen, temperature
  • Chemical oxidation from peroxide produced during
    fuel cell operation
  • Results in reduced hydrophobicity
  • Causes MEA flooding and increases in mass
    transport losses

Borup et al., DOE, 2005
97
What are Some Issues and Challenges?
  • General FC Manufacturing Considerations
  • Fuel cell output scales with active surface area
    versus volume for heat engines
  • Fuel cells are typically manufactured today using
    scaled up laboratory fabrication methods
  • Labor intensive
  • Repetitive measurements of components and
    repetitive connection are required to assure
    reliability
  • Needs
  • Standardization of fuel cell components and
    manufacturing processes to facilitate mass
    production
  • Transformation of laboratory fabrication methods
    to full-scale, high-volume processes
  • Methods for accurate measurement and process
    control
  • Development of supplier base and networks

98
What are Some Issues and Challenges?
  • General Manufacturing Challenges
  • Developing innovative, low-cost fabrication
    methods for new materials and applications
  • Adapting laboratory fabrication methods to
    low-cost, high volume production
  • Establishing and refining cost-effective
    manufacturing techniques while products are still
    evolving
  • Meeting customer requirements for systems and
    components
  • Addressing the diversity and size of industries
    in both manufacturing and energy sectors

99
What are Some Issues and Challenges?
  • PEMFC Developments
  • Nafion material is more robust and longer
    lasting
  • MEAs demonstrating remarkable power density
    longevity
  • Cross-linking and composite construction of
    electrolyte has led to greater durability and
    life
  • Understanding of the role of hydrogen peroxide
    (and resultant OH radicals) in membrane
    degradation
  • Understanding of proton conduction process and
    needs for humidification and water management
  • Development of new membrane materials
  • (1) poly-benzimidizole (PBI) doped with
    phosphoric acid or sulfonated side groups, (2)
    sulfonated poly-benzoxazoles, (3)
    poly-phosphazenes (hybrid inorganic-organic), (4)
    others

100
What are Some Issues and Challenges?
  • PEMFC Developments (contd)
  • Recent higher temperature PBI MEAs
  • operation in the 160C range
  • Higher operating temperature eliminates/reduces
    CO poisoning by reducing CO occlusion of the
    platinum sites
  • Operating temperature is better for stationary
    combined heat/power (CHP) and heat rejection
  • e.g., PBI requires significantly lower (or zero)
    water content to facilitate proton transport
  • easier water management
  • Temperature and pressure have a significant
    influence on cell performance (Nernst, kinetics,
    mobility)

101
THANKS for Your Attention!
Additional Questions?
102
(No Transcript)
103
How Are MEAs Manufactured?
  • Catalyst manufacturing
  • Why use a supported catalyst?
  • Lower catalyst loading
  • Prevent agglomeration/sintering
  • Collect current
  • Why use carbon?
  • Cost lower than other supports (e.g., alumina,
    silica)
  • Good thermal, mechanical, and chemical stability
    in PEMFC
  • Microstructure Various characteristics available
    (fibers, porous spheres, planar, )
  • Adhesion and gas diffusion can modify chemical
    nature of the surface to promote catalyst
    adhesion and control porosity
  • Good electrical conductivity
  • Recycling can recover catalyst by combustion of
    carbon support
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