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(1)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 42

Direct Methanol Fuel Cells

Direct Methanol Fuel Cells

(2)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 43

Rationale for Direct Methanol Operation Rationale for Direct Methanol Operation

• Greatly simplified system design

• Readily available fuel infrastructure

• High fuel energy density – lowered system weight and volume

• Ideal for mobile applications such as laptops, cellular phones

• Also of great interest to the military – to power individual soldiers’ electronics

(3)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Gottesfeld 44

(4)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Gottesfeld 45

(5)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 46

Comparison of Performance between H

Comparison of Performance between H22 PEMFC and PEMFC and DMFC with Nafion 117 at 60

DMFC with Nafion 117 at 60ooC and 1atm C and 1atm

Current density(mA/cm2)

0 200 400 600 800 1000 1200

Cell Voltage(V)

0.0 0.2 0.4 0.6 0.8 1.0

DMFC-4mg/cm2 catalyst 1M MeOH/Air

H2 PEMFC-0.4mg/cm2 catalyst H2/Air

(6)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 47

Principle Challenges in DMFC Operation Principle Challenges in DMFC Operation

• Sluggish methanol oxidation (anode) kinetics:

- 6 electron transfer as opposed to 2 electron transfer for H2 oxidation

- formation of CO as an intermediate in the multi-step methanol electrooxidation mechanism – poisoning of catalyst

• Large methanol crossover through the membrane:

- linked to the electro osmotic drag

- has detrimental effect on fuel efficiency - may poison the cathode

- creates mass transport problems at cathode layer by

wetting hydrophobic gas channels, leading to increased flooding.

• CO2 removal at anode

(7)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 48

Breakup of DMFC Losses Breakup of DMFC Losses

EEcellcell = = EEcathodecathode -- EEanodeanode= 1.23 = 1.23 -- 0.046= 1.2 V0.046= 1.2 V (Thermodynamic) (Thermodynamic)

(8)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 49

Anode Kinetics Anode Kinetics

• Thermodynamically, methanol oxidation and hydrogen oxidation occur at nearly the same potential (0.046 V and 0 V respectively)

• However, hydrogen oxidation is a 2 electron process, while methanol oxidation is a 6

electron process.

• It is very unlikely that all 6 electrons are transferred at the same time

• Therefore, transfer occurs step by step, leading to the formation of intermediates

(9)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 50

Proposed Methanol Oxidation Mechanisms Proposed Methanol Oxidation Mechanisms

Pt + MeOH = Pt-MeOH = Pt-COads (methanol adsorption through a series of steps, see figure)

Pt + H2O = Pt-OHads + H+ + e- (generation of hydroxyl groups on catalyst)

Pt-COads + Pt-OHads = 2Pt +CO2 + H+ + e- (Oxidation of CO to CO2 - similar to CO oxidation in direct hydrogen systems)

(10)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 51

Postulated Methanol Adsorption Mechanism Postulated Methanol Adsorption Mechanism

Note:

Note: Final stage is CO adsorbed on Pt sitesCO adsorbed on Pt sites

(11)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 52

• Below 450 mV, Pt surface – entirely poisoned by CO

• No further methanol adsorption

• Further adsorption – requires CO electrooxidation – induces overpotential of 450 mV or greater for Pt catalysts

• Thus, Ecell = 1.23 - ~0.45 = ~ 0.8 V max. even at low currents!

Effect of anode overpotential

Effect of anode overpotential –– contributes to contributes to poor methanol performance seen in

poor methanol performance seen in performance curve

performance curve

(12)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 53

Breakup of DMFC Losses Breakup of DMFC Losses

Ecell = Ecathode - Eanode= 1.23 - 0.45= 0.8 V (small currents)

(13)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 54

Direct Methanol vs. Direct Hydrogen Direct Methanol vs. Direct Hydrogen

Direct Methanol Direct Methanol

• Pt-CO formation due to

adsorption of methanol and subsequent intermediate formation

• Pt-CO inhibits further methanol adsorption

• Large currents – requires CO electrooxidation to free catalyst sites for further

methanol adsorption-–

thereby inducing large overpotentials

Direct Hydrogen Direct Hydrogen

• Pt-CO formation is due to the adsorption of CO from the feed stream

• Pt-CO inhibits further hydrogen adsorption

• Large currents – requires CO electrooxidation to free catalyst sites for further

hydrogen adsorption–

thereby inducing large overpotentials

End Result

End Result identical identical POOR ANODE KINETICSPOOR ANODE KINETICS

(14)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 55

Improving Methanol Oxidation Kinetics Improving Methanol Oxidation Kinetics

• Similar approaches to those taken for H2/ CO operation:

- Better electrocatalysts for enhanced efficacy of CO electrooxidation – surface hydroxyls

generated at lower potential (see figure)

- High temperature operation (> 100 oC) for

improved anode kinetics – Note, this approach also pays significant dividends by reducing

methanol crossover (cathode kinetics???)

(15)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 56

Enhanced Activity of Alloy Catalysts Enhanced Activity of Alloy Catalysts

Clearly, for a given Specific Activity

Clearly, for a given Specific Activity (current density at a (current density at a high voltage / unit active catalyst area)

high voltage / unit active catalyst area), Alloy catalysts have , Alloy catalysts have lower overpotentials for methanol oxidation

lower overpotentials for methanol oxidation

(16)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 57

Anode kinetics

Anode kinetics alone does not account for the alone does not account for the significant performance loss seen in DMFC significant performance loss seen in DMFC

systems systems The effect of

The effect of methanol crossovermethanol crossover is equally is equally important and affects parameters such as fuel important and affects parameters such as fuel efficiency, cathode kinetics, and mass transport in efficiency, cathode kinetics, and mass transport in

the cathode layer. Methanol crossover, and the cathode layer. Methanol crossover, and

techniques to limit it are discussed in the following techniques to limit it are discussed in the following

slides slides

Another important aspect of DMFCs (not discussed Another important aspect of DMFCs (not discussed

in this lecture) is the efficiency of CO

in this lecture) is the efficiency of CO22 removal at removal at the anode.

the anode.

(17)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 58

Methanol Crossover Methanol Crossover

• Ionomeric membranes have a complex water distribution during operation:

PEM

O2/Air H2

(or)

Methanol

Electro osmotic Drag H+(H2O)n

Water Back Diffusion

H2→2 H+ + 2 e- (or) CH3OH + H2O

CO2+6H+ +6e-

Anode Cathode

O2 + 4 H+ +4 e- 2H2O (water generation)

(18)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 59

Why (& How) does Methanol Crossover?

Why (& How) does Methanol Crossover?

• As water moves from anode to cathode (osmotic drag), methanol migrates along with the water

• On the cathode side, it is adsorbed onto the cathode electrocatalyst, thereby reducing efficiency

• The flux of methanol across the membrane depends upon:

- methanol concentration in fuel stream - current density

- membrane selectivity (ratio of protonic

conductivity to methanol permeability; property of membrane)

- membrane thickness

(19)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 60

Detrimental Effects of Methanol Crossover Detrimental Effects of Methanol Crossover

• Reduced fuel efficiency

• Cathode mixed potential (due to competing methanol oxidation and oxygen reduction) – lowering of open circuit voltage

• Cathode poisoning – CO adsorption on cathode catalyst, lowered cathode activity (see figure)

• Mass-Transport limitations at cathode (especially for air based applications) –

methanol wets the hydrophobic gas channels, and permits flooding – thereby increasing

diffusional resistance

(20)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 61

Effect of Methanol on Oxygen Reduction Effect of Methanol on Oxygen Reduction

(Cathode) Kinetics

(Cathode) Kinetics

(21)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 62

Effect of Methanol Concentration and Membrane Effect of Methanol Concentration and Membrane

Thickness on Crossover Thickness on Crossover

Methanol crossover(mA/cm2 )

0 50 100 150 200

3.3mil 1.7mil

0.5M

1M

2M

Evidently, crossover increases as methanol Evidently, crossover increases as methanol concentration increases and as membrane concentration increases and as membrane

thickness decreases thickness decreases

(22)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 63

Why the Concentration and Thickness Effect?

Why the Concentration and Thickness Effect?

Methanol flux ~ concentration gradient High concentration, low thickness

High concentration, low thickness –– maximum maximum concentration gradient (

concentration gradient (dCdC//dxdx))

Cleft

Cright

x

(23)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 64

Note…

Note…

• Cannot use very low concentrations of

methanol – this is because the energy density of the fuel goes down with dilution, and very low methanol concentrations will increase

system weight and volume

• Can however use neat methanol as fuel, and dilute using water tapped from the cell

(cathode) prior to injection into anode – approach adopted by LANL

(24)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Gottesfeld 65

Effect of Current Density and Concentration Effect of Current Density and Concentration

on Methanol Crossover

on Methanol Crossover

(25)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 66

• Current – plays two competing roles:

- increased faradaic electrooxidation of

methanol at anode (lowers concentration at anode, reduces flux) – positive effectpositive effect

- increased protonic current – increased water transport to cathode by electro

osmotic drag – larger methanol crossover negative effect

negative effect

Clearly, the former effect dominates at low Clearly, the former effect dominates at low methanol concentrations, and the latter at methanol concentrations, and the latter at

high methanol concentrations high methanol concentrations

(26)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 67

Effect of Temperature on Methanol Effect of Temperature on Methanol

Crossover Crossover

Temperature (oC)

0 20 40 60 80 100 120

Methanol Crossover (A/cm2 )

0.0 0.1 0.2 0.3 0.4 0.5

Resistance (Ohm-cm2 )

0.0 0.1 0.2 0.3 0.4 0.5

Methanol crossover Resistance

Liquid-fed MeOH Vapor-fed MeOH

(27)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 68

• Methanol crossover increases with temperature up to a temperature of ~ 100°C

• This is because the diffusion coefficient increases with temperature

• Above ~ 100°C, there is a precipitous drop in crossover

• This drop occurs due to reduced water transport

through the membrane as liquid water does not exist at ambient pressure – recall water uptake chart

Note

Note membrane resistance also increases with membrane resistance also increases with increasing temperature (decreasing relative increasing temperature (decreasing relative

humidity)

humidity)– therefore, a tradeoff exists!therefore, a tradeoff exists!

(28)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 69

High Temperature Membranes High Temperature Membranes

• Recall discussion on elevated temperature membranes

• These membranes can be used for high temperature DMFCs

• The liquid water (and hence methanol) transport rates through the membrane remain minimal

• However, the resistance at elevated temperatures is greatly reduced

• Comparative performance data – reveals that this technique permits superior performance. Reasons include:

- lower crossover (above discussion) - lower resistance (above discussion)

- improved anode kinetics (CO desorption favoured)

(29)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 70

Low Temperature DMFC Approaches Low Temperature DMFC Approaches

• Certain applications (cellular phones) – require operation at room temperature (or low

temperatures)

• The high temperature approach is clearly invalid under these conditions

• Lowering methanol concentration is achieved by tailoring the membrane microstructure

• Proven technique – using a polymer that does not permit methanol transport (e.g. PVDF)

(30)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 71

Unfortunately…

Unfortunately…

• Such polymers do not conduct protons well either!

• Therefore, it is important to look at the membrane selectivity

• A tradeoff clearly exists between eliminating crossover and retaining protonic conductivity

• Illustrated in the following diagrams

(31)

ME 295/320 Fuel Cell Engg.- J. M. FentonPVDF Content in Nafion-PVDF (% wt) Si 72

0 20 40 60

Proton Conductivity (S/cm)

0.001 0.01

0.1 20 oC

PVDF Content in Nafion-PVDF (% wt)

0 20 40 60 80 100 120

Methanol Permeability (cm2 /s)

1e-10 1e-9 1e-8 1e-7 1e-6

1e-5 Nafion-PVDF (10 micron)

Nafion Membrane (50 micron)

Methanol Methanol permeability permeability decreases with decreases with PVDF content PVDF content

So does protonic So does protonic

conductivity conductivity

Optimal PVDF Content Optimal PVDF Content

will determined will determined

experimentally, depends experimentally, depends

upon thickness upon thickness

(32)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 73

Alternate Design Alternate Design

• Use a thin layer of methanol barriers such as Nafion – PVDF blends sandwiched between proton conducting Nafion® layers

• Recall – resistance ~ thickness/conductivity

• Therefore, very small thickness of barrier – acceptable increase in resistance

• The thin layer is reasonably effective in keeping methanol out

• Thickness of barrier layer – determined experimentally

(33)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 74

Methanol Concentration Profile Through Methanol Concentration Profile Through

Layered membrane Layered membrane

Cleft

Nafion layer

Cright (<< Cleft)

Barrier Layer – Nafion® / PVDF

(34)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 75

PVDF Content (% wt)

0 10 20 30 40 50 60 70

Proton Conductivity (S/cm)

0.0001 0.001 0.01 0.1

Tri-layer Membrane (50 micon)

Nafion-PVDF Membrane (10 micron, measured) Nafion-PVDF Membrane (10 micron, calculated )

Nafion

PVDF Content in Nafion-PVDF (% wt)

0 20 40 60 80 100 120

Methanol Permeability (cm2 /s) 1e-10 1e-9 1e-8 1e-7 1e-6

1e-5 Nafion-PVDF (10 micron)

Tri-layer Membrane (50 micron) Nafion Membrane (50 micron)

Proton Conductivity

Proton Conductivity of 50

µm Tri-layer Membrane Containing a 10 µm Nafion-

PVDF Barrier Layer as a Function of PVDF in the Barrier Layer.

Methanol Permeability

Methanol Permeability of 50 µm Tri-layer Membrane Containing 10 µm Nafion-

PVDF Barrier Layer as a Function of PVDF Content in Nafion-PVDF.

(35)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 76

Effect of Barrier Layer Thickness on Effect of Barrier Layer Thickness on

Methanol Crossover Methanol Crossover

PVDF Content (% wt)

0 25 50 75

Methanol Crossover (mA/cm 2 ) 0 100 200 300 400 500 600

0.5 M Methanol 1 M Methanol 2 M Methanol

PVDF Content (% wt)

0 25 50 75

Methanol Crossover (mA/cm 2 ) 0 100 200 300 400 500 600

10 micron Nafion-PVDF Layer

6 micron Nafion-PVDF Layer

Nafion117-1M MeOH Nafion117-1M MeOH

Note 1.

Note 1. – The

comparison is made against a Nafion®

membrane 175 µm thick, and for a 1M methanol concentration

Note 2.

Note 2. – for a thinner barrier layer, a greater fraction of methanol barrier component (namely PVDF) is required to attain a comparable crossover

(36)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 77

Effect of Barrier Layer Thickness on Proton Effect of Barrier Layer Thickness on Proton

Conductivity Conductivity

PVDF Content (% wt)

20 30 40 50 60 70

Proton Conductivity (S/cm)

0.000 0.025 0.050 0.075 0.100

Tri-layer with 6 micron Nafion-PVDF layer Tri-layer with 10 micron Nafion-PVDF Layer Nafion-PVDF

Note

Note – thinner barrier layer – larger

conductivity for a given PVDF content

(37)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Si 78

Performance with Layered Membranes Performance with Layered Membranes

Current density(mA/cm2)

0 100 200 300 400 500

Cell Voltage(V)

0.0 0.2 0.4 0.6

0.8 Nafion112 (50micron)

Tri-layer (50 micron) Nafion117 (175 micron)

Current density (mA/cm2)

0 100 200 300 400 500

Power density (mW/cm2 ) 0 20 40 60 80 100

Performance and Power Density with 50 µm Nafion112 and 50 µm Tri-Layer Membrane and 175 µm Nafion117 at 60 oC with 1M

Methanol/Air under 1atm Pressure.

The efficacy of the layered The efficacy of the layered

membrane approach is membrane approach is

evident evident

(38)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 79

Other Techniques to Reduce Crossover Other Techniques to Reduce Crossover

• Air/oxygen bleed – to oxidize the CO formed at the anode

• High oxidant flow rates:

- enhances oxidant crossover – oxidizes CO - may reduce methanol crossover by

inducing a suitable pressure profile

• Pressurized cathodes – pressure based disincentive for methanol crossover

(39)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 80

DMFC Summary (Anode)

DMFC Summary (Anode)

(40)

ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 81

DMFC Summary (Cathode)

DMFC Summary (Cathode)

(41)

ME 295/320 Fuel Cell Engg.- J. M. Fenton 82

Recap of Challenges

Recap of Challenges - - Electrodes Electrodes

• Operation of a fuel cell with Reformate (w / CO and CO2) and on direct methanol have similar challenges on the electrode front:

- better catalysts for efficient CO electro oxidation at low noble metal loadings - in-situ techniques for enhanced CO

electrooxidation and reduced CO adsorption - better materials to facilitate high temperature

operation (to permit lower CO adsorption)

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