1. Chapter 13. Direct Methanol Fuel Cells

2. Application of Fuel Cells
PEMFC
DMFC
SOFC
Micro fuel cell
MCFC mW 3W
50W
2 kW
70 kW MW
Micro-chip
Cellular phone
Notebook-PC
RPG
Vehicles
Power Plant
DMFC:
Easy handling of liquid fuel => suitable for portable power source

3. Recent products of DMFCs
MSI (2003)
MTI (2004)
Motorola (2005)
Toshiba (2005)
Polyfuel, Inc. (2003)
Samsung (2005)

4. 13.1 Introduction 1. Half-cell reaction for the anodic oxidation
CH3OH + H2O  CO2 + 6H+ + 6e-
2. Half-cell reaction for the cathodic reduction
6H+ + 3/2O2 + 6e-  3H2O
3. Free energy associated with the overall reation at 25 oC and 1 atm:
ΔG = -686 kJ/mol CH3OH; ΔE = 1.18 V

5. DMFC operation
A schematic of DMFC operation with a polymer electrolyte membrane as the electrolyte.

6. Advantages and disadvantages of DMFCs
Advantages by using methanol as a fuel:
1) Low cost
2) Simplicity of design
3) large availability
4) easy handling and distribution
Drawbacks:
1) Low activity of catalysts for methanol oxidation and oxygen reduction
2) Low performance by methanol crossover

7. 13.2 Anodic Oxidation of Methanol
Anode reaction:
CH3OH + H2O -> CO2 + 6H+ + 6e-
Various reaction intermediates are formed
Some intermediates are irreversibly adsorbed on the surface of electrocatalyst -> reduce power
Finding new electrocatalysts:
1) inhibit the poisoning
2) increase the rate of electro-oxidation

8. Pt electrocatalysts Dehydrogenation steps
CH3OH + Pt -> Pt-CH2OH + H+ + 1e-
Pt-CH2OH + Pt -> Pt-CHOH + H+ + 1e-
Pt-CHOH + Pt -> PtCHO + H+ + 1e-
Surface rearrangement
PtCHO -> Pt-C≡O + H+ + 1e- or
PtCHO -> Pt -> Pt2-C=O + H+ + 1e-

9. In the absence of a promoting element
=> water discharge occurs at high anodic overpotentials on Pt
Pt + H2O -> PtOH + H+ + 1e-
Final step
PtOH + PtCO -> 2Pt + CO2 + H+ + 1e-
Principal by-products:
Formaldehyde & formic acid

10. Pt-Ru binary alloy electrocatalysts
At suitable electrode potentials (0.2 V vs. RHE), water discharging occurs on Ru:
Ru + H2O -> Ru-OH + H+ + 1e-
Final step:
Ru-OH + Pt-CO -> Ru + Pt + CO2 + H+ + 1e-

11. Optimum composition of Pt:Ru
Mass and specific activities for methanol electro-oxidation on unsupported Pt-Ru catalysts at 130 oC
=> Pt:Ru of 1:1 is optimum
at 130 oC
At low temp.,
Low content of Ru is optimum
(at 60 oC, 33 % Ru is optimum)
Ref. H. A. Gasteiger et. al., J. Electrochem. Soc., 141 (1994) 1795

12. Finding new catalysts by high-throughput screening (HTS) for DMFC
Optical Screening
Electrochemical Screening
IR Thermography Screening
Scanning Electrochemical Screening

13. High-throughput screening using acid-base indicators
CH3OH + H2O  CO2 + 6H+ + 6e-
6H+ + 3/2O2 + 6e-  3H2O
Ni2+-PTP complex, pKa = 1.5
(quinine pKa=4.7, acridine pKa=5.6)
Phloxine B, pKa = Eosin Y, pKa = 2.7

14. Preparation of combinatorial array as a methanol oxidation catalyst
16cm
“275 combinatorial array
: Pt, Ru, Mo and W”
Pt 80%
60%
5cm
Pt 90%
70%
50%
: optical image before cyclic voltammetry
S. I. Woo et al, Catalysis Today 74 (2002)

15. Methanol oxidation reaction by fluorescence test
“lower over-potential
(≈ 0.4 V vs. RHE)“
: fluorescence image after 20th cyclic voltammetry
 The spot nearly composd of Pt(73)Ru(21)Mo(2)W(4) composition showed the brightest image at low overpotential
“higher over-potential
(≈ 0.46 vs. RHE)“
: fluorescence image after 20th cyclic voltammetry

16. Physico-chemical properties of the prepared catalysts & commercial catalyst
Catalyst Surface area [m2/g] Crystallite size [nm]
(by X-ray line broadening)
(A) Pt(77)Ru(17)Mo(4)W(2)
(B) Pt(74)Ru(20)Mo(4)W(2) (C) Pt(50)Ru(50), J.M. Cata
(D) Pt(82)Ru(18)
Methanol electro-oxidation activities ay 0.45V; 2M CH3OH in 0.5 M H2SO4 at 25oC.
Methanol electro-oxidation activities of catalyst materials; 2 M in 0.5 M H2SO4 at 25oC.

17. Membrane-electrode Assembly Performances
Conditions for cell operation
Catalyst loading: 6mg/sq.cm
2MMeOH(2cc/min), O2(500ml/min)
Anode (0.5 atm), Cathode (14.7 psig)
Conditions for cell operation
Catalyst loading: 6mg/sq.cm
2MMeOH(2cc/min), O2(500ml/min)
Anode (0 atm), Cathode (0 psig)

18. Role of carbon support
Key issue: synthesis of a highly dispersed electrocatalyst phase in conjunction with ah high metal loading on a carbon support
Most carbon blacks
Acetylene black (BET area: 50 m2/g)
Vulcan XC-72 (BET area: 250 m2/g)
Ketjen black (BET area: 1000 m2/g)
Low surface area: low dispersion, high mass transport
High surface area: high dispersion, low mass transport (because of micropores)

19. 13.3 Cathodic Reduction of Oxygen
Oxygen reduction reaction
O2 + Pt -> Pt-O2
Pt-O2 + H+ + 1e- -> Pt-HO2
Pt-HO2 + Pt –(rds)-> Pt-OH + Pt-O
Pt-OH + Pt-O + 3H+ + 3e- -> 2Pt + 2H2O
Methanol oxidation
CH3OH + 3Pt -> Pt3-COH
Pt3COH -> Pt-CO + H+ + 2Pt
Pt + H2O -> Pt-OH + H+ + 1e-
Pt-OH + PtCO -> 2Pt + CO2

20. Non-noble Metal Based Electrocatalysts
Chevrel-phase type
Mo4Ru2Se8
Transition metal sulfides
MoxRuySz, MoxRhySz
Transition metal chalcogenides
Ru1-xMoxSeOz
=> Activity for oxygen reduction is significantly lower than for Pt
Unsupported or supported Pt are state-of-the-art catalyst for oxygen reduction

21. 13.4 Proton Conducting Membrane
Nafion membranes are currently used
In DMFC, methanol cross-over significantly reduce power
key issue:
high proton conductivity, low methanol cross-over
Requirements:
high ionic conductivity (5×10-2 ohm-1 cm-1)
low permeability to methanol (less than moles min-1 cm-2)

22. Methanol crossover
A schematic of methanol crossover in DMFCs with a polymer electrolyte membrane as the electrolyte

23. Effect of methanol crossover
Lower the fuel utilization
Crossover methanol at cathode with oxygen CH3OH + 3/2 O2 → CO2 2H2O =>lower oxygen concentration, aging of the cathode catalyst due to heat of reaction
Decreasing of potential difference

24. Methanol crossover measurement
Two different approaches
Amount of the methanol crossover (easy way)
Current efficiency (popular)
For typical DMFCs, Ucr = 20%
Icr : Crossover current
I : Actual current produced in the cell
For typical DMFCs, ηI = 80%

25. Methanol crossover with cell current drawn expression
Assumption Operation with the same pressure on both anode and cathode sides
dominant mechanism of methanol crossover:
diffusion electroosmotic drag of methanol
Linearly decrease

26. Diffusion is dominant in the electrode, so
Rate of methanol flow through electrode = rate of methanol reaction at electrode-membrane interface
N : methanol flow rate F : Faraday constant n : number of mole electrons per mole methanol H3OH+H20→6H++6e-+CO2
Diffusion is dominant in the electrode, so
A : electrode active area De : effective diffusion coefficient in anode–membrane interface CI : concentration of methanol at anode–membrane interface δe : thickness of anode electrode
In membrane (diffusion and electroosmotic drag effect)
Dm : effective diffusion coefficient in membrane ξ : electroosmotic coefficient for water CT : total concentration of methanol and water

27. With above three equations
Arrangement
k is small (diffusion > electroosmotic) rate of methanol crossover decrease with the cell current
k is large (electroosmotic > diffusion) crossover increase with the cell current

28. Influence of cell current density and methanol concentration in anode feed stream

29. How to decrease methanol crossover
Pd barrier by sputtering
(Pd: proton can pass through, while methanol can not)
Pd sputtered
Plasma etched
Unmodified
Plasma etched and
Pd sputtered
Ref. S.I. Woo et al., J. Power Sources, 96 (2001) 411

30. 13.5 Membrane and Electrode Assemblies
The performance of a DMFC is strongly affected by the fabrication procedure of the membrane –electrode assembly (MEA)
=> TEM micrograph of the catalyst-Nafion interface showing metal particles supported on carbon agglomerate and nafion ionomer micelles

31. Combinatorial optimization of MEA
Motivation
Optimization of multilayer electrode structure for improved performance.
MEA is composed of three components (PTFE, Nafion solution and electrode catalyst).
Hydrophilicity, electron transfer and proton transfer are issues.
Catalyst layers
Carbon paper

32. High-throughput optical screening
UV light
Fluorescence emission
(on the most active electrode)
Potentiostat
Reference electrode
Working electrode
Counter electrode

33. Combinatorial array of multilayer electrodes
Combinatorial array and screening results by fluorescence imaging. Active electrodes for methanol electro-oxidation are shown as bright spots. The potential was 0.45 V vs. RHE.

34. Different platinum concentrations in MEA
Best
Medium
Poor
Large
Large
Top
Bottom
Small
Small

35. Different Nafion concentrations in MEA
Best
Medium
Poor
Large
Large
Top
Bottom
Small
Small

36. Different PTFE concentrations in MEA
Best
Medium
Poor
Small
Small
Top
Bottom
Large
Large

37. Chronoamperometry curves of the electrodes recorded in 2 M CH3OH + 0
Chronoamperometry curves of the electrodes recorded in 2 M CH3OH M H2SO4 deaerated with ultra pure N2 at 25 oC during the potential-step up from to 0.55 V vs. RHE.
Anode polarization curves of the electrode recorded in 2 M CH3OH M H2SO4 deaerated with ultra pure N2 at 25 oC.
The results of half cell test are in agreement with the combi test

38. Summary
A combinatorial array of electrodes with novel multilayer structure used for direct methanol fuel cell was fabricated. Gradient of the concentrations of platinum catalyst, Nafion ionomer and PTFE were established from the outer surface of MEA to membrane.
High-throughput optical screening showed the structure of the best gradient. Pt
Nafion
PTFE
Large
Large
Small
Top
Bottom
Small
Small
Large
The results of half cell test are in agreement with the combi test

39. 13.6 Single cell Performance (polarization behavior)
Effect of cathode feed
Oxygen vs. Air
(Oxygen exhibits higher performance)
Oxygen
Air

40. Effect of methanol concentration, pressure and flow rates on DMFC performance
Generally, 1M methanol is preferable
Low methanol concentration: low cross-over, low diffusion rate
High methanol concentration: high crossover, high diffusion rate
To improve efficiency without increasing methanol crossover
1) High cathode backpressures
2) A high oxygen partial pressure
3) High cathode flow rates

41. Flow fields
Galvanostatic polarization data for a DMFC equipped with serpentine of interdigitated flow field at 130 0C in the presence of 2M methanol and an air feed

42. 13.7 Stack Development, Performance and Potential Application

43. 13.8 Life time of DMFC Life time targets Cars > 4,000 h
Stationary > 40,000 h
Portable > 1,500 h
Reasons for Performance Deterioration
Catalyst problems
Membrane problems
Bipolar plate problems -> Corrosion
Seal problems -> Corrosion
GDL problems

44. Catalyst problems 1. Catalyst particles show agglomeration
carbon
new after 150 h operation

45. 2. Carbon (catalyst support) is oxidized
3. Pt & Ru is dissolved
4. CO (CO2) acts as catalyst poison
5. S-compounds acts as catalyst poison
6. Hydrophobicity of catalyst layer is changed

46. Pt dissolution
at 0.9 V ~ 1.2 V
pH < NAFION pH  -0.3

47. Membrane problems 1. Metal cations (e.q. Na+, Mg2+, Ca2+, Cu2+, Cr3+)
RnNH4-n+ cations (R=H, CH3, C2H5, C3H7, C4H9, n = 0 … 4)
contaminate the membrane: substitution of H+
2. Radical ions degrade the membrane
=> Chemical degradation via –OH and –OOH radicals (generated from oxygen reduction)
3. Dry up of the membrane

48. Membrane Degradation 1.5 10-6 g F /cm² h membrane
new MEA MEA after 1000 h operation

49. GDL problems: Teflon degradation
Contact Angle
without Teflon
135°
9% Teflon
156°
23% Teflon
164°

50. 13.9 Future R&D
Improve membrane electrolyte to reduce methanol crossover.
Fast anode kinetics to reduce anode overpotential.
(fast methanol reaction & CO tolerance)
Fast cathode kinetics to reduce cathode overpotention.
(Fast oxygen reduction & methanol tolerance)