Effect of hydration on the mechanical properties of

anion exchange membranes Benjamin R. Caire, Melissa A. Vandiver, Andrew M. Herring, and Matthew W. Liberatore

Colorado School of Mines and University of Toledo

MURI: #W911NF-10-1-0520, DURIPs: #W911NF-11-1-030, #W911NF-11-1-0462

Images credits: GM and CellEra

What is a fuel cell? Electrochemical device with potential to provide low/zero carbon emission power

Grid back-up power Small portable devices Automobiles

2

Anode Cathode

Alkali Anion Exchange Membrane

Load

Fuel: CH3OH O2 +

H2O

e-

CO2 + H2O

e-

e- e-

e-

e-

OH-

OH-

OH-

Solid polymer electrolyte membranes that transport anions in electrochemical devices

Anion Exchange Membranes (AEMs)

3

Advantages over PEMs: Increased kinetics of alkali media Non-Pt based catalysts Reduction in methanol crossover

Challenges: Lower ionic conductivity compared to PEMs Chemical and mechanical stability

N

n m

OH

Collaboration for robust AEM

4

Composites Processing

Ordered Structures

Solutions Surfactants

Synthesis Knauss

Yan Coughlin

Optimized Film

Theory Voth Witten

Characterization Herring

Liberatore

Anionic    Transport    in    Organic    Media  

M  U  R  I  

1.  High ionic (OH-) conductivity

2.  Selective permeability to transport ions and prevent fuel/air crossover

3.  Adequate water sorption without significant dimensional swelling

4.  Chemical stability (backbone and ionic site)

5.  Mechanical stability

Requirements of an AEM

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1.   High ionic (OH-) conductivity §  Thin membranes, to reduce ionic resistance

2.  Selective permeability to transport ions and prevent fuel/air crossover

3.   Adequate water sorption without significant dimensional swelling

4.  Chemical stability (backbone and ionic site)

5.   Mechanical stability §  Durable and thin membranes

Film durability critical for device lifetime

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Variations in humidity can lead to failure

Quantify the interplay of swelling and mechanical performance

Patankar et al., J. Polym Sci, Poly Phys., 48, (2010)

50 µm 20 µm

Mathias et al., Electrochemical Society Interface 14 (2005).

TA ARES rheometer with SER fixture for extensional tests and film/fiber tension fixture for DMA

Small sample size 20 mm (L) X 5 mm (W)

Custom sample chamber to control temperature/humidity

Benchmarked with LDPE and Nafion

Mechanical testing w/ controlled RH

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Advanced RH/T Control

Oven

DMA (SER) Elongation and Tensile

Screw down clamps with silicone rubber holds film securely in place

Testing below the melting temperature

Characterizes 10-100 micron thick films

SER <5% of material needed for tensile tester

Modified SER for thin film testing

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10 mm

Temperatures up to 150°C

T & RH conditions available

9

Investigate transitions as function of water content

Ramping T or RH

10

Two mechanical tests

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Young’s Modulus (Elasticity) Stress at Break (Strength)

Strain at Break (Elongation)

Extensional Testing Dynamic Mechanical Analysis

E* – Complex Modulus E’ – Elastic component (Stiffness) E’’ – Viscous component (Energy Loss) Menard, K.P., Dynamic Mechanical Analysis, A Practical Introduction, 2nd ed., CRC Press, Inc., Boca Raton 2008.

Rubbery, polyethylene based, diblock to improve elasticity and flexibility

Solution casting large (300 cm2), uniform 10 μm films

Robust, conductive, and thin AEM

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N Br

PE-b-PVBTMA[Br-] 1.2 mmol/g Provided by

Daniel M. Knauss

PE Diblock has modest conductivity, given lower IEC

Minimal dimensional swelling from water = 8 ± 3%

PE diblock is conductive

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PE-b-PVBTMA – 1.2 mmol/g

Good strength & elasticity at dry conditions

At hydrated conditions films soften significantly

PE diblock has “good” properties

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Wanted: Moderate elasticity dry & wet

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Too Stiff (Brittle)

Workable Film (75 – 450 MPa)

Too soft, “gel like”

60°C PS

Dib

lock

PI-ran-PS x-link

PI-ran x-link

PE D

iblo

ck

Fum

asep

ATM

PP

Surr

ey

Naf

ion

N11

5 Team AEMs of Study Benchmarks PEM

Bre

xar4

3 B

rexa

r70

PPO

dib

lock

PCO

E tr

iblo

ck

Bre

xar2

0

1

10

100

1000E'

(MPa

)

90705030Temperature (°C)

Difference between dry/hydrated states warranted testing at intermediate humidities

Dynamic thermomechanical response

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1

10

100

1000E'

(MPa

)

90705030Temperature (°C)

1

10

100

1000E'

(MPa

)

90705030Temperature (°C)

1

10

100

1000E'

(MPa

)

90705030Temperature (°C)

PE-b-PVBTMA[Br]

Humidification causes sharp softening transition at ~65%RH

Transition is reversible, but displays hysteresis

Restiffening during dehumidification ~45%RH

Dynamic hygromechanical response

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60°C PE-b-PVBTMA[Br]

Slight hysteresis between sorption and desorption

DMA can be normalized to lambda to account for water content

Water content varies with humidity

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60°C PE-b-PVBTMA[Br]

λ = WU

m(H2O) ⋅IEC= Waters

Cation

Normalizing by lambda narrows hysteresis window

But hysteresis in transition remains

Hysteresis in narrow lambda range

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60°C PE-b-PVBTMA[Br]

Conductivity increases above λ~9, suggesting mechanical softening correlates with ion conduction

Conductivity increases after softening

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60°C PE-b-PVBTMA[Br]

Mechanical properties are directly related to chemistry, IEC, and water sorption

Cationic functionalities increase stiffness, but plasticization due by water can severely soften and weaken membranes

Chain flexibility due to hygromechanical softening may enhance ion conduction

Conclusions

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M.A. Vandiver, B.R. Caire, T.P. Pandey, Y. Li, S. Seifert, A. Kusoglu, D.M. Knauss, A.M. Herring, M.W. Liberatore. “Effect of hydration on the mechanical properties and ion conduction in a polyethylene-b-poly(vinylbenzyl trimethylammonium) anion exchange membrane”, Journal of the Membrane Science, 497 (2016): 67-76.

B.R. Caire, M.A. Vandiver, M.W. Liberatore. “Mechanical testing of small, thin samples in a humidity controlled oven.” Rheologica Acta, 15 (2015): 253-261

M.A. Vandiver, B.R. Caire, Z. Poskin, Y. Li, S. Seifert, D.M. Knauss, A.M. Herring, M.W. Liberatore. “Durability and performance of polystyrene-b-poly(vinylbenzyl trimethylammonium) diblock copolymer and equivalent blend anion exchange membranes.” Journal of Applied Polymer Science, 132 (2015): 41596

M.A. Vandiver, B.R. Caire, J.R. Carver, K. Waldrop, M.R. Hibbs, J.R. Varcoe, A.M. Herring, and M.W. Liberatore. “Mechanical Characterization of Anion Exchange Membranes by Extensional Rheology under Controlled Hydration.” Journal of The Electrochemical Society, 161 (2014): H677-H683

R. Janarthanan, J.L. Horan, B.R. Caire, Z.C. Ziegler, Y. Yang, X. Zuo, M.W. Liberatore, M. R. Hibbs and A. M. Herring. “Understanding anion transport in an aminated trimethylpolyphenylene with high anionic conductivity.” Journal of Polymer Science Part B:Polymer Physics, 51 (2013): 1743-1750.

Y. Liu, J.L. Horan, G.J. Schlichting, B.R. Caire, M.W. Liberatore, S.J. Hamrock, G.M. Haugen, M.A. Yandrasits, S. Seifert, and A.M. Herring. “A Small-Angle X-ray Scattering Study of the Development of Morphology in Films Formed from the 3M Perfluorinated Sulfonic Acid Ionomer.” Macromolecules, 45 (2012): 7495-7503.

Membrane Publications

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