ME 295/320 Fuel Cell Engg.- J. M. Fenton 42
Direct Methanol Fuel Cells
Direct Methanol Fuel Cells
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
ME 295/320 Fuel Cell Engg.- J. M. Fenton Gottesfeld 44
ME 295/320 Fuel Cell Engg.- J. M. Fenton Gottesfeld 45
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
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
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)
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
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)
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
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
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)
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
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???)
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
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.
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)
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
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
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
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
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
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
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
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
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
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!
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)
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)
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
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
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
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
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.
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
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
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
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
ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 80
DMFC Summary (Anode)
DMFC Summary (Anode)
ME 295/320 Fuel Cell Engg.- J. M. Fenton Hoogers 81
DMFC Summary (Cathode)
DMFC Summary (Cathode)
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)