Hydrocarbon Fuel Cell Membranes Containing Perfluorosulfonic Acid Group

Contact: baec@rpi.eduCenter for Future Energy System Annual Conference (01/25/2013)

Ying ChangYing Chang and and Chulsung BaeChulsung BaeDepartment of Chemistry & Chemical BiologyDepartment of Chemistry & Chemical Biology

Rensselaer Polytechnic Institute, Troy, NY 12180Rensselaer Polytechnic Institute, Troy, NY 12180

•• GuiseppeGuiseppe F. F. BrunelloBrunello, Jeffrey Fuller, , Jeffrey Fuller, Seung Soon JangSeung Soon Jang ((Georgia Tech)Georgia Tech)•• Marilyn Hawley, Marilyn Hawley, Yu Seung KimYu Seung Kim ((Los Alamos National Laboratory)Los Alamos National Laboratory)•• Melanie Melanie DisabbDisabb--Miller, Miller, Michael A. HicknerMichael A. Hickner (Pennsylvania State University)(Pennsylvania State University)

Collaborators

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Fuel Cell Applications

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Fuel Cells: PEMFC and AEMFC

• Higher power density• Automobile applications• PEM: insufficient conductivity at low RH

high cost of Nafion• Catalyst: expensive Pt

• Lower power density than PEMFC• Portable power applications• AEM: insufficient conductivity

poor stability against OH-• Catalyst: non-noble metals, Ni, Co. etc

Reaction at Anode: H2 2 H+ + 2 e- H2 + 2 OH- 2 H2O + 2 e-

Reaction at Cathode: ½ O2 + 2 H+ + 2 e- H2O ½ O2 + H2O + 2 e- 2 OH-

Overall Reaction: H2 + ½ O2 electricity + H2O H2 + ½ O2 electricity + H2O

Reaction at Anode: H2 2 H+ + 2 e- H2 + 2 OH- 2 H2O + 2 e-

Reaction at Cathode: ½ O2 + 2 H+ + 2 e- H2O ½ O2 + H2O + 2 e- 2 OH-

Overall Reaction: H2 + ½ O2 electricity + H2O H2 + ½ O2 electricity + H2O

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Proton Exchange Membrane (PEM) Fuel Cells

PV power capacity

Anode Cathode

• Key component in fuel cells– Transport H+ (and H2O)

• Acid-containing (-SO3H) polymer materials• Nafion from Du Pont

– Separate H2 and O2

• Ideal PEM– Acidic (sulfonated) polymers that are

• Easy to synthesize• Inexpensive• Highly proton-conductive (even at low RH)• Stable (chemical, thermal, mechanical)• Low swelling in water• Broad temp range (-20 to 120 oC)

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Nafion vs. Sulfonated Hydrocarbon PEMs

• Perfluorinated flexible backbone• Perfluorinated side-chain (15 mol% SO3H)• IEC = 0.9 mmol/g• Very high acidity (Superacid, pKa = -14)• -SO3H at flexible side chain• Distinct nano-scale phase separation• High water diffusion• Difficult to modify structure & property• High cost

• Stiff aromatic main-chain backbone• No side-chain • IEC = 1.5 – 2.0 mmol/g for good conductivity• Strong acidity (pKa = -6.5)• -SO3H at rigid aromatic main chain • Phase separation difficult (random copolymer)• Low water diffusion• Easy to modify structure & property• Low cost

Strong acidity & Favorable Morphology

Low proton conductivity at low RHGuiver, Holdcroft, Ding, Adv. Funct. Mater. 2006, 16, 1814

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Morphology Difference of Nafion & HC PEMs

Kreuer, K. D. J. Membr. Sci.2001, 185, 29Hickner, PivovarFuel Cells 2005, 5, 213

• Close packing of ionic groups• Wide channels & good connectivity• Good phase-separated morphology• Promotes loosely bound water• Good water (& H3O+) transport

• Narrow hydrophilic domain channels• Highly branched & dead-end channels• Lower degree of phase separation• More tightly bound water• Decreased water (& H3O+) transport

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Strategy to Improve Proton Conductivity at Low RHvia Hydrophilic-Hydrophobic Multi-Block Copolymers

1. To improve transport of [H2O]• Create favorable morphology via Hydrophobic-hydrophilic

sulfonated block copolymers• Continuous H3O+/H2O pathways via self-assembled microstructures• Better transport of H2O at low RH, but still high one-dimensional swelling

Proton conductivity depends on [H3O+] = [H2O] x [H+]

Kim, McGrath, Guiver, PivovarChem. Mater. 2008, 20, 5636

Sulfonated random copolymerIEC = 1.53 meq/g

Sulfonated multiblock copolymerIEC = 1.51 meq/g

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Strategy to Improve Proton Conductivity at Low RHvia Fluoroalkyl Sulfonic Acid (Superacid)

2. To increase [H+]• More –SO3H to PEM Higher IEC, Higher WU, Excessive swelling Mechanical failure of PEM

• Increase acidityIntroduce F to –SO3H

-CF2CF2SO3H

Better dissociation to increase [H+] Reduce water uptake Prevent excessive swelling Synthetic challengeaEstimated by the Ho method, In Superacids;

G. A. Olah, G. K. S. Prakash, J. SommerbIn Advanced Organic Chemistry; 5th Ed.; M. B. Smith and J. MarchcIn Organic Chemistry; 7th Ed.; J. McMurry

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PEMs with Different Sulfonic Acid Groups:Acidity Effect on PEM Properties

Flexibility

Acidity

• Role of acidity in proton conductivity?• Any relationship among acidity, morphology, water property?• Molecular level understanding of structure-property relation Develop low-cost high-performance hydrocarbon PEM

Objectives

Acidity

Nafion as benchmark PEM Nano-scale phase separation Multiblock HC ionomers Strong acidity (superacidic) ???

Higheracidity

Traditional Functionalizations of Arene

Electronic property of substituent already present in the aromatic ring determinesreactivity and selectivity

• Efficient biaryl C-C bond formation• Good functional group tolerance

FG1 = Alkyl, ortho-, para-directing activatingFG1 = NO2, meta-directing deactivating

2010 Nobel Prize in Chemistry

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Direct Borylation of Aromatic C-H Bonds

• Iridium-catalyzed aromatic C-H bond activation/borylation • Boron substitutes only aromatic C–H bonds selectively• Mixture of meta and para-borylated products

Miyaura & Hartwig, J. Am. Chem. Soc. 2002, 124, 390Smith, J. Am. Chem. Soc. 2000, 122, 12868

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Synthetic Applications of Borylated Arene: Intermediate for Functionalized Arenes

Smith, J. Am. Chem. Soc. 2003, 125, 7792 Miyaura, Tetrahedron 2008, 64, 4967Hartwig, Org. Lett. 2007, 9, 761

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New Sulfonation Method of Aromatic Polymers via Borylated Polymer

• C-H Borylation: control of sulfonate concentration • Suzuki coupling: change of sulfonate structures• Characterization of mol% (Bpin, SO3H) by 1H NMR

Entry [B2pin2]/[sPS]sPS-Bpin

Mn PDI (Mw/Mn) Bpin (%)1 0.03 132 2.37 2.52 0.05 116 2.74 5.93 0.07 116 2.53 9.94 0.1 90.0 2.50 16.45 0.2 124 2.40 23.66 0.4 97.0 2.55 34.2

Macromolecules 2007, 40, 8600; Macromolecules 2011, 44, 8458

Proton Conductivity, Water Uptake, Hydration Number vs. RH

Water uptake and hydration number by Michael Hickner at Penn State

Macromolecules 2011, 44, 8458

80 oC

40 mol% sulfonated

IEC

(mmol/g)

1.64

2.29

2.00

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Origin of Improved Proton Conductivity of Superacid PEM:Pair Correlation Function & AFM Morphology

S (sulfonate) – O (water)S (sulfonate) – O (hydronium)

-CF2SO3H• More aggregation of H2O near sulfonate• Solvation effect

-CF2SO3H• Better dissociation sulfonate to H3O+• Acidity effect

40-sPS-S1-(CF2)2SO3H

(20 wt% water)

40-sPS-S3-(CH2)3SO3H

(20 wt% water)

34.3A34.2A

Collaboration with Seung Soon Jang (Georgia Tech)Macromolecules2011, 44, 8458

Yu Seung Kim (LANL)15

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Extension of Aromatic C-H Borylation to Polysulfone

• Excellent Stabilities

(1) Good mechanical properties due to the rigid polymer chain

(2) Excellent resistance of water, acid, base, and oxidants

(3) Good thermal and hydrolytic stability (Tg = 190 oC)

• Controlled incorporation of functional group into polysulfone willbroaden its membrane application in a variety of different fields

(Fuel Cells, Water Purification, Gas Separation, etc) Functionalized polysulfone is highly desired

Sulfone part Bisphenol A part

FGFG

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Sulfonated Polysulfone for High Temp PEMs

135%-SO3H 160%-SO3H 200%-SO3H

Polym. Chem. 2013, 4, 272

J. Am. Chem. Soc. 2009, 131, 1656

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Water Absorption Properties & Proton Conductivity of Sulfonated Polysulfones

M.A. Hickner@ 100 oC

IEC0.891.942.572.29

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Morphology Study of Sulfonated PSUs with TEM & SAXS

TEM by L. MaExperimental SAXS (dry)by M. A. Hickner

2.7 nm

2.0 nm2.8 nm

Calculated SAXS (20% water) by S. S. Jang

2.3 nm

2.9 nm

No obvious difference in morphologyamong sulfonated PSUs

Summary

Ion-conducting aromatic polymers with different sulfonate structuressynthesized by combination of C-H borylation & Suzuki coupling:PSU-S1 (-CF2CF2-SO3H), -S2 (-C6H4-SO3H), -S3 (-CH2CH2CH2SO3H)

• Convenient controls of structurestructure & concentrationconcentration of sulfonic acid group

• Hydrophobic ponytail side chains reduce water uptake: PSU-S1 & -S3 < -S2

• Different sulfonic acids induce different proton conductivities at low RH: PSU-S1 > -S2 , -S3

• Morphology and molecular dynamics studies suggests that superacidic character of PSU-S1 induces better dissociated hydronium ions than PSU-S3 and enhanced proton conductivity at low RH

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Acknowledgment

Current and Past Group Members• Dr. Ying Chang • Angela Adams-Mohanty• Bhagyashree Date• Sarah Park

Collaborators• Georgia Tech: Seung Soon Jang (Molecular Dynamics)• Los Alamos National Lab: Yu Seung Kim, Marilyn Hawley (AFM, Morphology)• Penn State: Michael Hickner (Water Absorption, SAXS/SANS)

$$$• NSF (CAREER Award) • Ministry of Knowledge & Economy (S. Korea)• Rensselaer Polytechnic Institute

•Jihoon Shin (Ph.D)• Se Hye Kim (MS)

• ACS-PRF• Nevada Renewable Energy

Consortium