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PEMFC Electrodes

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Cross Section of Proton Exchange Membrane Fuel CellCross Section of Proton Exchange Membrane Fuel Cell

Anode Cathode

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Typical PEMFC

• Electrodes:- Anode – Hydrogen Oxidation

- Pt – Ru / C

- Cathode – Oxygen reduction- Pt / C

Pt is alloyed with Ru to enhance CO tolerance of Anode electrocatalyst

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Blacks vs. supported catalysts• Pt black – large particle sizes – difficult to

prevent agglomeration- Leads to low surface area (SA)

• Supported Pt:- High SA carbon used as support- Smaller Pt particle size- Better dispersion- Larger SA

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Dispersion• Ratio of surface atoms to bulk atoms

- higher dispersion, smaller particle size- larger SA

• Assuming spherical particles:

SA = 6 / (ρpt x d) (1)

• Where ρpt = density of Pt, d = particle diameter

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Pt particle size vs. SA

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Anode Catalysis• H2 electrooxidation in absence of impurities

VEeHH o 0;222 =+→ +

(2)

Net reaction at anode

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Proposed mechanism

• In acidic electrolytes, a two step mechanism is thought to occur

1. Dissociative adsorption of hydrogen2. Charge transfer (oxidation)

H2 adsorption is the rate limiting step

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Mechanism

adsHPtPtH −→+ 222(3)

eHPtHPt ads 2222 ++→− +

(4)

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Characteristics• High exchange current density

(io ~ 1mA/cm2) on Pt single crystals• Therefore – no significant overpotential

upon increased load• Reaction proceeds efficiently with very low

Pt loadings – 0.05 mg/cm2 is sufficient

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Problems?• Pure H2 anode feed – unrealistic• Typically – at least 10 ppm (upto

20,000ppm) of CO present in feed stream• CO – acts as a poison:

– adsorbs on Pt– reduces active sites for H2 adsorption– Increases anode overpotential

CO Tolerant Fuel Cells – Discussed in separate lecture

Data obtained at LANL 12

Effect of CO in Reformate on Performance

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Cathode electrolysis• Oxygen reduction reaction

OHeHO 22 244 →++ +

(5)

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Characteristics• Lower exchange current density (10-4 to 10-6

mA/cm2) on Pt• Much higher overpotential (compared to pure

H2 oxidation) for a given current density• Higher loadings (0.4 mg/cm2) needed• Kinetics – strong function of oxygen partial

pressure (0.7 – 1 order)

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Effect of oxygen concentration

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Components of an oxygen reduction electrode

- Pt catalyst- Carbon support- Ionomer (to facilitate proton conduction)- Teflon (optional) – to create hydrophobic

channels – permits gas diffusion through unwetted pores

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Need for ionomer contact with electrocatalyst

• Protons need to:

(a) get to anode / membrane interface from anode electrocatalyst oxidation site

(b) get to the cathode Pt site from membrane / cathode interface

Ionomer / catalyst contact is essential – to minimize resistive losses

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Oxygen transport• Reactant interaction with Pt – crucial (recall

strong kinetic dependence on concentration of Oxygen)

• Thus viable path for oxygen transport needed

• Places limitation on electrode thickness and composition

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The three phases

Catalyst

Ionomer Reactant

Intimate 3 phase contact – essential for low overpotential

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Typical modelCatalyst Particle

Ionomeric Skin

Reactant diffuses through the skin, and interacts with the catalyst

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Optimization• Factors to be considered include:

- catalyst loading- supported / unsupported- ionomer loading- electrode thickness- electrode porosity- ratio of hydrophobic / hydrophilic pores

Note – each optimized electrode will only be optimal at a given operating condition –

different conditions result in different optima!

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Interesting tradeoffs• Catalyst loading:

- Kinetics vs. Mass transport

• Catalyst Ionomer loading:- Catalytic activity vs. mass transport - Ionic resistance vs. mass transport- Electronic vs. ionic resistance

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Kinetics vs. transport• Larger catalyst loading – thicker electrode (for

a given catalyst):- Decrease in kinetic losses – to a certain

loading (thickness)- Increase in mass transport losses (increased

thickness) - above a certain thickness (loading)

Increasing catalyst loading – only works until mass transport effects negate gains in kinetics

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A caveat• Recall – catalysts are typically Pt supported

on carbon• Increasing the % of Pt/C – higher Pt density

per gram of catalyst• For given Pt loading (and ionomer content):

- lower electrode thickness with increasing Pt content in Pt/C catalyst

Note that Pt surface area decreases with increasing Pt loading in Pt/C - lower catalyst utilization

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Ionomer loading• Note that:

- Ionomer presence is essential- Homogeneous distribution required- Must intertwine with Pt particles

• Also recall:- Good ionomer – does not permit large reactant crossover

- Low oxygen permeability through ionomer - Diffusional problems!- Good ionomer – does not conduct electrons

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Catalytic activity vs. mass transport• Large ionomer loading:

- Good contacting with Pt particles- High catalyst utilization- High catalytic activity

• However:- Large ionomeric skin thickness (slide 20)- Enhanced diffusional losses

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Ionic resistance vs. mass transport• Large ionomer loading:

- Good ionomeric network between catalyst site and membrane electrode interface

- High proton conductivity / transport- Low resistive losses

• However:- Large ionomeric skin thickness (slide 20)- Enhanced diffusional losses

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Ionic vs. electronic resistance• Large ionomer loading:

- Good ionomeric network between catalyst site and membrane electrode interface

- High proton conductivity / transport- Low resistive losses

• However:- Extensive coating of carbon network with non –electron conducting ionomer

- Enhanced electronic resistance losses

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Complex interplay

Catalyst loading

Pt content in Pt/C Electrode thickness

Ionomer loading

Kinetic losses Ohmic losses Transport losses

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OptimizationExtremely challenging – difficult to manipulate

single parameters

Optimal compositions change with type of catalyst –even from manufacturer to manufacturer

Optimal compositions certainly change with operating conditions

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Typical catalyst ink manufactureCatalyst + solvent

(methanol) Stirring / sonication

• Additional steps may include:

- Drying and redissolution

- Heat treatment

- Addition of Teflon

Addition of ionomer

Stirring / sonication

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Added complexity• Variables in catalyst ink manufacture:

- Temperature of ink manufacture- Type / extent of agitation - Heat treatment- Solvent used……

Any change in above – results in different optimal compositions for any given condition

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Yet another parameter – MEA manufacturing technique

• Several techniques of applying catalyst to membrane

• Broad subdivision:- Catalyst applied to membrane- Catalyst applied to gas diffusion layer

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Catalyst applied to membrane

Spraying ontomembrane

Silk screening ontomembrane Decal transfer

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Spraying onto membranePreparation of Catalyst Ink:

low viscosity;good particle dispersion neededlow viscosity;good particle dispersion needed

Use of ultrasonic,magnetic stirring, homogenizers etc.

preferred dispersion techniques

Variables include dispersion time, solution viscosity,dispersion temperature, solvent used etc.

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Spraying onto membrane - setup

Membrane (active area exposed)

Spray gun (paint brush)

IR lamp

Carrier G

as (N2 )

• The lamp enables solvent evaporation

• The carrier gas propels the dispersion on to the membrane surface

• Caution – carrier gas pressure must be low enough to prevent hole formation

Can be done by hand.

Better control obtained using computer controlled X-Y recorder based spraying instrument

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Silk screeningPreparation of Catalyst Ink:

higher viscosity;good particle dispersion neededhigher viscosity;good particle dispersion needed

Use of ultrasonic,magnetic stirring, homogenizers etc.

preferred dispersion techniques –viscosity controlled by

external agents such as ethylene glycol

Variables include dispersion time, solution viscosity,dispersion temperature, solvent used etc.

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Silk Screening - setup

Substrate

Membrane

Masks – to expose desired active area

Brush – to spread the ink

Screen – known mesh size

Note – all pneumatically controlled!

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Decal transferPreparation of Catalyst Ink:

highest viscosity;good particle dispersion neededhighest viscosity;good particle dispersion needed

Use of ultrasonic,magnetic stirring, homogenizers etc.

preferred dispersion techniques –viscosity controlled by

external agents such as ethylene glycol

Variables include dispersion time, solution viscosity,dispersion temperature, solvent used etc.

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Decal transfer - methodology

Blank + ink

Paint (or silk screen / spray) ink onto blank–desired quantity – heat treat to evaporate

solvent

blank

membrane

Place painted blank onto membrane - Hot press Blank + ink

blankblank

membrane + ink+Peel off blank

membrane + inkRepeat procedure on other side – different blank

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Catalyst applied to GDL

Prepare catalyst ink –desired viscosity

Apply on to porous gas diffusion layer

Spraying Silk screening

Dry the ink – evaporate solvent

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Making the MEA

Catalyst coated GDL

Catalyst coated GDL

Membrane

Hot press

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Catalyst onto membraneMerits

•• High catalyst High catalyst utilizationutilization

•• Intimate interfacial Intimate interfacial contact

Demerits• Can be complicated• Can lead to wastage –

especially if spraying is used

• The other two techniques do not lead to wastage – but are difficult to use with thin membranes

contact

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Catalyst onto GDLDemerits

• Low catalyst utilization – catalyst lost in GDL pores

• Interfacial contact may be poor

• Risk of short circuiting – especially with thin membranes

Merits• Simple technique• Less wastage of

catalyst – especially if catalyst is screened onto GDL

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RemarksOn the whole, applying the catalyst onto the

membrane is the preferred technique for high performance

Decal transfer – with catalyst inks screened on to the surface of the blank – most elegant

technique – good reproducibility