The PEM Fuel Cells. Frick Laboratory, Princeton University Catalyst Layer Pt/C with Proton...

The PEM Fuel Cells

Frick Laboratory, Princeton University

0 200 400 600 800 1000 1200 1400 1600 18000.0

0.2

0.4

0.6

0.8

1.0 Nafion 115 80oC 130

oC

Silicon Oxide/Aciplex 1004 130oC

Ce

ll P

ote

ntia

l / V

Current Density / mA cm2

Nafion Thermal Behavior


§The composite typically contains 3-6 wt% metal oxide.

§TGA indicates the same water content and dehydration temperature for pure Nafion and the composite.

§The conductivity of the composite measured in a mechanically unconstrained environment is the same or slightly worse than the conductivity of pure Nafion.

The metal oxide is not simply providing a water retentive or hydrated interface.

Is the Metal Oxide Phase Water Retentive?


If it’s not a question of direct dehydration, then what is occurring?

• First, we will seek a molecular picture.

• Then, we will attempt to make connections between our understanding of the molecular structure and bulk materials properties.


0 500 1000 15000.2

0.4

0.6

0.8

1.0

130oC (Degussa-Huls)

TiO2; 21nm; 50 m2 /g (R - 0.18)

SiO2; 20nm; 90 m2

/g (R - 0.21)

Al2O3; 13 nm; 100 m2

/g (R - 0.76)

Recast Nafion Control (R - 0.5)

Cel

l Pot

entia

l / V

Current Density / mA cm-2

Effect of Effect of Metal Oxide Identity on Membrane Metal Oxide Identity on Membrane PerformancePerformance


0 200 400 600 800 1000 1200 1400 1600

0.2

0.4

0.6

0.8

1.0TiO2

(AA)/Recast Nafion; 130˚C

unmodified (R - 0.50) silylated (R - 0.29) H2SO4, HNO3, "degreased" (R - 0.25)

Cel

l Pot

entia

l / V


Interfacial Chemistry is Critical


The Effect of Relative Humidity on Recast Nafion

0 200 400 600 800 1000 1200 1400 16000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

cell potential(V)

current density (mA/cm 2)

Control membrane(H2-O

2)

1300C 30 psig 100% RH 88% RH 75% RH


75% Relative Humidity

0 200 400 600 800 1000 1200 14000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

cell potential(V)

current density (mA/cm 2)

1300C 75% RH Control Recast Degussa Huls SiO

2

Alfa Aesar SiO2

Degussa Huls TiO2

Alfa Aesar TiO2

125µ Film

3 atm pressure

40 ml/min


----CF2-CF2---

OH OH

Metal Oxide

-----CF-----

O=S=O

OH

Metal Oxide

O

Ti

Metal Oxide

HO

-----CF-----

SO3-

Potential Chemical Interactions


Thermal decomposition of Nafion

SO2

1st step

CFO+

2nd step C3F5+

H2O

H2O

0 100 200 300 400 500-70

-60

-50

-40

-30

-20

-10

0

Weight loss/ %

Temperature / °C

m/z 64 SO2

MS abundance / a.u.

m/z 47 COF

m/z 131 C3F

5

m/z 18 H2O

H2O SO2

C3F5+

CFO+

　

-

Temperature Programmed Decomposition (TG-MS) of Nafion 117


Thermal decomposition of Nafion

SO2

1st step

CFO+

3rd stepC3F5

+

H2O

m/z 18 H2O

m/z 64 SO2

MS abundance / a.u.

m/z 47 COF

m/z 131 C3F

5

0 100 200 300 400 500-70

-60

-50

-40

-30

-20

-10

0

Weight loss/ %

Temperature / °C

H2O

SO2C3F5

+

CFO+

2nd step

-

HO

TiO2

TG-MS Profile of Nafion/TiO2 Composite Membranes


TPD-MS profiles of Nafion/Inorganic composite membranes

-

200 250 300 350 400

Intensity / a.u.

Temperature / °C

117Nafion + 3% Nafion TiO2

+ 3% Nafion SiO2

+ 3% Nafion Al2O3

+ 3% Nafion ZrO2

200 250 300 350 400

Intensity / a.u.

Temperature / °C


+ 3% Nafion SiO2

+ 3% Nafion Al2O3

+ 3% Nafion ZrO2

SO2 (m/z 64)

H2O (m/z 18)

CFO (m/z 47)

200 250 300 350 400

Intensity / a.u.

Temperature / °C


+ 3% Nafion SiO2

+ 3% Nafion Al2O3

+ 3% Nafion ZrO2

200 250 300 350 400

Intensity / a.u.

Temperature / °C


+ 3% Nafion SiO2

+ 3% Nafion Al2O3

+ 3% Nafion ZrO2

C3F5 (m/z 131)


MOx

•Crosslinking controls the mechanical properties of the polymer•Glass transition temperature•Bulk rigidity – better water retention under stress load

Molecular Model


Dependence of Nafion Glass Transition on Metal Oxide


SAXS Studies


Heat

Self Assembled Disordered

Crystalline

Order-Disorder Transition


Ionic inclusions swell with water uptake, requiring the membrane to push the electrodes apart.

Membrane　Mechanical　Properties　Affect　Cell　Response


0 20 40 60 80 100 120 140 160 180 2000.0

5.0x1051.0x1061.5x1062.0x1062.5x1063.0x1063.5x1064.0x1064.5x1065.0x1065.5x106

Metal Oxide Composite Nafion 112

Str

ess

(N

/m2 )

Strain(%)

Stress-Strain Response


Too Much of a Good Thing is Bad

Applied Pressure by Current Collector Plates

Swelling Pressure ofPolymer Membrane

50

55

60

20000 22000 24000 26000 28000 30000

Time (s)

Current (mA)

30

30.5

31

31.5

32

32.5

33

33.5

34

34.5

35

Power (mW)


Membrane Swelling

(c)Additional pressure further increases the membrane/catalyst contact. However, the larger pressure forces water out of the membrane.

(a)The membrane is in contact with the catalyst support particles.

(b)Applied pressure enhances the membrane/catalyst contact.

mem

bra

ne

Carbon support


Hydrogen Crossover

800

850

900

950

1000

Open　　Circuit　Voltage　(mV)

0.0

1.0

2.0

3.0

4.0

Crossover　Current　(m

A/cm2)

125µm 40µm 40µm Composite

Membrane


•Increased Tg allows maintenance of hydrated proton conduction paths at elevated temperatures.

•Improved mechanical rigidity allows for dimensional stability under conditions where water content of the membrane may be changing.

•Maintains good catalyst contact on deswelling

•Eliminates water loss on swelling.

What Role Does the Metal Oxide Play?


0 200 400 600 800 1000 1200 1400 16000.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 Nafion 115 - 80o

C Pt Anode w/o CO w/100 ppm CO

TiO2 - 130

oC Pt/Ru Anode

w/100 ppm CO w/500 ppm CO

Cel

l Pot

entia

l / V


Carbon Monoxide Tolerance


Summary

High Temperature Nafion Based PEM Fuel Cells overcome

several limitations associated with current cell design

Addition of a metal oxide phase affects the mechanical properties of the membrane:

Increased Tg

Improved gas barrier

Mitigation of swelling/deswelling effects

Bonus Material (It’s not electrochemistry, but it is interesting)

So, How Does One Store Hydrogen on the Run?


Storage Issues Safety For mobile applications range & power should be maintained.

5-10Kg of H2 needed for a 65-75kW engine.

H2 feed rate is ~1000 liters/minute Weight Effective Density of Hydrogen Volume Requirements

Size Geometry

Refill Availability Recharge rate.

Cost


Hydrogen Storage Phases

Limiting　Densities　of　Molecular　Hydrogen

Hydrogen Phase Moles HydrogenAtom/cm3 Weight Percent

Solid Hydrogen (4.2K) 0.088 100%

Liquid Hydrogen (20K) 0.070 100%

Hydrogen Gas 200bar (300K) 0.016 ~1%


Storage Options Standard steel tanks (2000-5000psi)

Known technology. Good Safety Record

Subject to hydrogen imbrittlement Forms projectiles if structure is breached

Tanks are challenging to fill because hydrogen heats upon expansion Heavy

Storage capacity is only 0.5-1% by weight Poor volumetric storage due to non-ideality of hydrogen:

P +n2aV2

⎛

⎝ ⎜

⎞

⎠ ⎟ V −nb( ) =nRT

a=0.2444 b=0.02661

~20% volumetric expansion at 5000psi


Storage Options

Composite Tanks (~10,000psi) High storage capacity: Light weight Can store 7% H2 by weight! Does not fragment upon failure Cost


Storage Options

Generation on the fly: in-situ or ex-situ reforming of hydrocarbon fuels using an on-site reformer. Energy density of gasoline Easy access to fuel (gasoline stations) Systems integration is poor No carbon mitigation.

Solid-state storage by intercalation (metal hydrides, carbons) Safe Heavy Expensive Chemical thermodynamics and kinetics are difficult

Significant heating is required to release the hydrogen∆H losses up to 30% are typical with operating temperatures of 200-300C. Tank filling is very exothermic Chemical kinetics are a difficult to handle


Hydride Storage Capacity

Metal　Hydride　Storage　Systems

Hydrogen Phase Moles HydrogenAtom/cm3 Weight Percent

LaNi5H6 0.091 1.37%

TiFeH2 0.10 1.89%

Mg2NiH4 0.098 3.6%

MgH2 0.11 7.6%


Storage Options Chemical Hydrides

“Hydrogen on Demand” (Sodium Borohydride) Not flammable High Effective hydrogen pressure (~7000psi) Low Volume Simple system Chemical Safety Recyclable Cost??

NaBH4 + Aqueous Base Catalyst ⏐ → ⏐ ⏐ H2 + NaBO2 •2H2O

The PEM Fuel Cells. Frick Laboratory, Princeton University Catalyst Layer Pt/C with Proton...

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