Fuel Cells -2Fuel Cells -2 -...

Fuel Cells - 2Fuel Cells Fuel Cells -- 22

Jong Hak KimChemical Engineering

Yonsei University

Jong Hak KimChemical Engineering

Yonsei University

Recent Research Trends in Recent Research Trends in Polymer Electrolyte MembranesPolymer Electrolyte Membranes

Fuel Cells Fuel Cells

PEMFC (Proton Exchange Membrane Fuel Cell)DMFC (Direct Methanol Fuel Cell)

Electrolyte

CurrentCollector

CH3OH

Anode

Water Electrical

Circuite-

Cathode

O2

CurrentCollector

Catalyst layer

Water

Electrolyte

CurrentCollector

CH3OH

Anode

Water Electrical

Circuite-e-

Cathode

O2

CurrentCollector

Catalyst layer

Water

H2, CH3OH/H2O H+

ContentsContents

Overview of proton conducting membranes- Structure, Synthesis - Chemical properties- Electrochemical properties- Applications to fuel cell (DMFC, PEMFC)

Membranes- Sulfonated aromatic polymers- Alkylsulfonated aromatic polymers- Acid-base polymer complexes- Block copolymer electrolytes (BCE)- Graft copolymer electrolytes (GCE)

Self-organizing !!!

Background Background -- 11

Perfluorinated polymer electrolytes- Satisfactory properties - High cost (~ $600/m2)- low H+ conductivity at high T & low humidity- high MeOH crossover

Background Background -- 22

Aciplex, Dow membrane- Short side chain - High ratio of SO3H/CF2

- High specific conductivity

GoreSelect membrane- Fine-mesh PTFE support impregnated with Nafion- high mechanical stability ~10μm

PTFE: poly(tetra fluoro ethylene)

Features of Features of PerfluorinatedPerfluorinated PEsPEs

Background Background -- 33

Features of hydrocarbon polymers- Cheap !!!- Low thermal/chemical stability- Life-time can be improved by catalyst, assembly technology

- Contain polar group: high water uptake- Decomposition can be depressed by molecular design

- easily recycled by conventional method

SulfonatedSulfonated Aromatic PolymersAromatic Polymers

Polysulfone (PSf)

Poly(styrene) (PS) Poly(ether ether ketone) (PEEK)

Poly(benz imidazole) (PBI)

Poly(phenylene sulfide) (PPS)Poly(ether sulfone) (PES)

AkylsulfonatedAkylsulfonated Aromatic PolymersAromatic Polymers

Alkylsulfonated PBI

Alkylsulfonated PSU

- Deprotonation of N in BI ring- Reaction of alkylsultone with PBI anion

J. Membr. Sci. 2004, 230, 61.

Macromolecules 1993, 26, 5633.

AcidAcid--Base Polymer ComplexesBase Polymer Complexes

PBI/H3PO4

PEO/strong acid PEI/strong acid

PAAM/strong acid PVA/strong acid

PEO: poly(ethylene oxide), PEI: poly(ethylene imine)PAAM: poly(acrylamide), PVA: poly(vinyl alchol)

Types of Block copolymer

Block CopolymersBlock Copolymers

Phase separated morphology

Block CopolymersBlock Copolymers

- Physical connection by covalent bond

- Immiscible pair: degree of segregation

- Order-disorder transition temperature (ODT)

- Self-assembly in nanometer length scales

Phase Separated Morphology

1/T

Block Copolymer Electrolytes

conducting

nonconducting

H+

H+

H+

H+

Advantages- Formation of ion conducting channels by microphase separation

- Hindering of swelling by surrounding non-sulfonated phase

- Lowering of MeOH permeability- High mechanical stability

High ionic conductivityGood mechanical property

PolystyrenePolystyrene--based BC (1)based BC (1)

Styrene-b-(ethylene-co-butylene)-b-styrene (SEBS)- 0.1 S/cm, bicontinuous structure

Interdomaindistance

~ ion cluster

cf. Nafion~5nm

Comparable conductivity SANS

Macromolecules 2002, 35, 5916

SANS: small angle neutron scattering

PolystyrenePolystyrene--based BC (2)based BC (2)

SEBS - Comparable H+ conductivity (~ 0.1S/cm)- Lower MeOH crossover

Δ: The ratio of conductivity to MeOH permeability

J. Memb. Sci. 2002, 207, 129.

PolystyrenePolystyrene--based BC (3)based BC (3)

Styrene-isobutylene-styrene - hexagonally packed PS cylinder- Upon increase of PS fraction, distance btncylinders unchanged, cylinder diameter change

Block copolymer Ionomer

Polymer, 2000, 41, 3205

Polystyrene based BC (4)Polystyrene based BC (4)

Crosslinked SBS- Structure fixation by UV crosslinking

Macromolecules 2003, 36, 3228

Φ: The ratio of conductivity to MeOH permeability

Membrane properties

Polyimide (1)Polyimide (1)

Block PI : successive step of polycondensation

• 1st step: 1,4,5,8-naphthalene tetracarboxylic dianhydride (NDTA) + 4,4’-diaminobiphenyl-2,2-disulfonic acid (BDSA)

• 2nd step: NDTA + 4,4’-oxydianiline (ODA)

Sep. Pur. Tech. 2001, 22, 681

Polyimide (2)Polyimide (2)

Block PI

Polymer, 2001, 42, 359

Aromatic diaminesNDTA BDSA

Polyimide (3)Polyimide (3)

Property: Conductivity, morphology

Polymer, 2001, 42, 359

• Low ionic conductivity ~ few mS/cm • Presence of large ionic domains by SANS

SANS

PolyaramidesPolyaramides

Polyaramide- Swelling is restricted by non-sulfonated domains - not stable at high T (135 oC) due to desulfonation, hydrolytic cleavage of amide bonds.

React. Funct. Polym. 2003, 57, 77

Poly(etherPoly(ether sulfonesulfone) (1)) (1)

Sulfonated BC- Silyl method in NMP/K2CO3 at 150 oC

Macromol. Symp. 2004, 210, 175

Hydrophobic Hydrophilic

TMS: trimethylsilyl

Poly(etherPoly(ether sulfonesulfone) (2)) (2)

Conductivity, DMFC performance - lower conductivity, but comparable cell performance

Macromol. Symp. 2004, 210, 175

Due to low MeOH crossover and lower thickness of membrane

PEEKPEEK

Nucleophilic aromatic substitution

J. Membr. Sci, 2004, 234, 75

high conductivity

low MeOH permeability

Block Copolymer Block Copolymer IonomerIonomer

Two-stage process: Nucleophilic substitution

J. Membr. Sci. 2004, 245, 147

Phase separation leads to physical crosslinking

End-functionalized hydrophobic blocks

Block Copolymer Block Copolymer IonomerIonomer

Cell performance

J. Membr. Sci. 2004, 245, 147

PEMFC DMFC

Good stability, acceptable performance during the operation

NitroxideNitroxide--mediated Living Radicalmediated Living Radical

Styrene-b-styrenesulfonic acid

Polymer 2002, 43, 3155

DEPN: 2,2,5,5-Tetramethyl-4-phosphono-3-azahexane-3-nitroxide

Polymerization of SSPEN or SSBu

Transformation to sulfonic acid

Block copolymerization

Graft CopolymerizationGraft Copolymerization

• Types of high energy radiations

1) Electromagnetic radiation- γ-ray, x-ray (Co-60; radioactive isotope, Cs-137; γ-ray source)

2) Particulate radiation (charged particle)- electron, swift heavy ions- from accelerator

• Radiation (γ-ray, electron beam)- chain grafts are covalently attached to main chain

• Polymer backbone is irradiated in the presence of monomer (vap. liq. bulk, solution)

• In air, N2 or under vacuum

Simultaneous Irradiation Method

1) Mutual recombination

DeactivationDeactivation

P· : primary radical of polymer backbone

2) Homopolymerization

1) Irradiation of polymer to form active radical2) Contact with monomer

PrePre--irradiation methodirradiation method

• Vac./inert atmosphere

• O2 atmosphere

RadiationRadiation--induced graftinginduced grafting

• Broad, promising technique

• Reaction parameter:- dose / dose rate- type or concentration of monomer- type of polymer film- type of additive(diluent, inhibitor, acid, x-agent)

- temperature

Graft CopolymerizationGraft Copolymerization

Grafting of PS onto PTFE

Prog. Polym. Sci. 2004, 29, 499

Graft Copolymer ElectrolytesGraft Copolymer Electrolytes

Membrane property

J. Membr. Sci. 2005, 251, 121

Base polymers

ETFE: poly(ethylene tetrafluoroethylene, PVDF: poly(vinylidene fluoride)LDPE: low density polyethylene

Basic grafting scheme

Graft Copolymer ElectrolytesGraft Copolymer Electrolytes

Superior DMFC performance

J. Membr. Sci. 2005, 251, 121

LDPE PVDF

Graft Copolymer ElectrolytesGraft Copolymer Electrolytes

Grafting of GMA onto PE

Prog. Polym. Sci. 2004, 29, 499J. Electrochem. Soc, 1996, 143, 2795

GMA: glycidyl methacrylate

CrosslinkedCrosslinked Graft CopolymerGraft Copolymer

Crosslinking with DVB

Crosslinking agent

J. Membr. Sci. 2003, 216, 27

(DVB)

PFA-g-PS/DVB-SO3H Membrane

PFA: poly(tetrafluoroethylene-co-perfluorovinyl ether)

CrosslinkedCrosslinked Graft CopolymerGraft Copolymer

Membrane properties

J. Membr. Sci. 2003, 216, 27

- High conc. of DVB reduce water uptake and ionic conductivity

- Thermal stability (decomposition of SO3H) is not affected by crossliking

Commercial Graft MembranesCommercial Graft Membranes

Commercial Membranes

• Permion: irradiation grafting of styrene onto FEP (or PTFE, ETFE)

• Raymion: pre-irradiation grafting of trifluorostyrene onto ETFE,

followed by sulfonation

J. Electrochem. Soc. 1998, 145, 780

• comparable performance with Nafion 117

• ~ 10,000h stability

FEP: poly(tetrafluoroethylene-co-hexafluoropropylene)PTFE: poly(tetrafluoro ethylene)ETFE: poly(ethylene tetrafluoroethylene)

Living Graft copolymerLiving Graft copolymer

PVDF-g-PVBC-PS by ATRP

J. Poly. Sci. Poly. Phys. 2002, 40, 591

PVBC: poly(vinylbenzyl chloride)

Electron-beam preirradiation grafting => atomic transfer radical polymerization (ATRP)

Promising materials for fuel cells !!!

Living Graft copolymerLiving Graft copolymer

TEMPO mediated polymerization

Macromolecules 2004, 37, 9909

TEMPO: 2,2,6,6-tetramethylpiperidinyl-1-oxy, PSSA: poly(styrene sulfonic acid)

PVDF-g-PSSA

Preliminary H2/O2 fuel cell tests show promise for PEM

Block Copolymer

Partially fluorinatedHydrocarbon

Closing RemarkClosing Remark

Graft Copolymer

Future Direction to Polymer Electrolytes

MATERIALS

STRUCTURE