Title: MEA Manufacturing How We Make MEAs and Why
1MEA 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
2OUTLINE
- 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?
3Introduction
Basic Process Fuel Cell
Cathode
Anode
4Introduction
- 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)
5Fuel 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
6Introduction
Larminie and Dicks, 2003
7Introduction
Larminie and Dicks, 2003
8Introduction
Larminie and Dicks, 2003
9Introduction
Larminie and Dicks, 2003
10Introduction
- Fuel Cell Membrane Electrode Assembly (MEA)
- Basic building block
Larminie and Dicks, 2003
11Introduction
- 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,
12OUTLINE
- 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?
13How Are MEAs Manufactured?
- PEMFC Unit Cell Components
From Hirschenhofer et al., 2002
14How 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)
15How 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)
16How 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
17How 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
18How 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
19How are MEAs Manufactured?
- PEMFC Membrane
- Nafion electrolyte structure (contd)
From US Fuel Cell Council, 2005
20How 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
21How 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
22How 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
23How 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
24How 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
25How 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
26How 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
27How 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
28How 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
29How 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)
30How 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
31How Are MEAs Manufactured?
- Various methods for manufacturing GDL or backing
layers
From Mathias et. al., Handbook of Fuel Cells,
2003
32How 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
33How 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
34How 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
35How 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
36How 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
37How 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
38How 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
39How 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
40How Are MEAs Manufactured?
- Various means of putting together the membrane
electrode assembly (MEA)
From S.S. Kocha., Handbook of Fuel Cells, 2003
41How 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)
42How 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
43How 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
44How 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
45OUTLINE
- 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?
46Why 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
47Why 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
48Why are MEAs Manufactured This Way?
- Irreversible processes affect observed
performance - Representations of physical, chemical, and
electrochemical processes
49Why 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
50Why 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
51Why 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
52Why 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)
53Why are MEAs Manufactured This Way?
Electric field
Excess negative charge
Excess positive charge
54Why 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?
55Why 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
56Why 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
57Why 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
58Why 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)
59Why 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
60Why are MEAs Manufactured This Way?
reduction
oxidation
61Why 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
62Why 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)
63Why 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
64Why 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
65Why 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
66Why 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
67Why 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
68Why are MEAs Manufactured This Way?
- Calculated Properties of Nafion
- Water Content Profile Local
Conductivity Profile across membrane
across membrane
From OHayre et al., 2006
69Why 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
70Why are MEAs Manufactured This Way?
- Mass Transport
- Convection vs. Diffusion
- Convection (dP/dx driver) Diffusion (dc/dx
driver)
From OHayre et al., 2006
71Why are MEAs Manufactured This Way?
- Mass Transport - Flow Channel and GDL During
Operation
From OHayre et al., 2006
72Why are MEAs Manufactured This Way?
- Mass Transport in a Typical Fuel Cell Electrode
From OHayre et al., 2006
73Why are MEAs Manufactured This Way?
- Concentration Dynamics due to Coupled Mass
Transport and Reaction
From OHayre et al., 2006
74Why are MEAs Manufactured This Way?
- Concentration Polarization
- Limiting Current Density case when
concentration at the TPB goes to zero
From OHayre et al., 2006
75Why are MEAs Manufactured This Way?
- Schematic of 2-D Mass Transport Model in Fuel
Cell (Diffusion and Convection)
From OHayre et al., 2006
76Why are MEAs Manufactured This Way?
- Mass Transport
- Major Types of Flow Channel Configurations
From OHayre et al., 2006
77Why are MEAs Manufactured This Way?
- Modes of mass transport within anode and cathode
compartments
From OHayre et al., 2006
78Why are MEAs Manufactured This Way?
- Simultaneously promote in bulk and microscopic
scales
Electrochemical Kinetics
Charge Transport
Mass Transport
79OUTLINE
- 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?
80What 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
81What 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
82What 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
83What 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
84What 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
85What 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)
86What 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
87What are Some Issues and Challenges?
- Temperature affects membrane degradation
Temperature Range
90C
30C
Knights, Ballard, FCS, 2006
88What 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
89What 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
90What 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
91What 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
92What 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
93What 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
94What 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
95What 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
96What 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
97What 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
98What 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
99What 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
100What 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)
101THANKS for Your Attention!
Additional Questions?
102(No Transcript)
103How 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