Chapter 13. Direct Methanol Fuel Cells

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

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

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

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

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

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

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-

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

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-

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

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

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 = 2.7 Eosin Y, pKa = 2.7

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) 235-240

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

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) 83.10 3.43 (B) Pt(74)Ru(20)Mo(4)W(2) 50.52 3.23 (C) Pt(50)Ru(50), J.M. Cata. 78.76 2.46 (D) Pt(82)Ru(18) 65.81 3.89 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.

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)

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)

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

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

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 10-6 moles min-1 cm-2)

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

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

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%

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

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

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

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

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

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

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

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

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.

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

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

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

Chronoamperometry curves of the electrodes recorded in 2 M CH3OH + 0 Chronoamperometry curves of the electrodes recorded in 2 M CH3OH + 0.5 M H2SO4 deaerated with ultra pure N2 at 25 oC during the potential-step up from 0.075 to 0.55 V vs. RHE. Anode polarization curves of the electrode recorded in 2 M CH3OH + 0.5 M H2SO4 deaerated with ultra pure N2 at 25 oC. The results of half cell test are in agreement with the combi test

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

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

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

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

13.7 Stack Development, Performance and Potential Application

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

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

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

Pt dissolution at 0.9 V ~ 1.2 V pH < 0 NAFION pH  -0.3

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

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

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

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)