Nafion: Hydration, Microstructure and Schroeder

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Nafion Hydration, Microstructure and
Schroeders paradox Viatcheslav Freger Maria
Bass , Amir Berman (BGU)Oleg Konovalov, Amarjeet
Singh (ESRF) Technion Israel Institute of
TechnologyWolfson Department of Chemical
EngineeringHaifa, Israel
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Nafion and Its Uses
An ionomer developed by DuPont in 70s
Fuel Cells
Catalysis
Sensors
Membrane electrolysis
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Unique Microstructure Microphase separation and
2D Micelle Morphology
Hsu and Gierke, JMS, 1983
Gebel, Diat et al, Macromolecules, 2002, 2004
Schmidt-Rohr and Chen, Nat Mater., 2008
Gebel, Polymer, 2000
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2D Morphology Transport vs. Hydration
Conductivity
Water self-diffusion (NMR)
VF et al., JMS, 1999
Kreuer, JMS, 2001
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Schroeders Paradox Two Isotherms?
Osmotic stressorsolution
Sample
Sample
Li-Nafion
Bass and Freger, 2008
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Schroeders Paradox and Water Transport
If the thermodynamic potential of water is
ill-defined, how does one model water transport
and water management?
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Schroeders paradox explained ?

• Choi and Datta (JES, 2003) were first to publish
  an explanation,

• but they assumed
• permanent pores
• hydrophobic pore walls (despite ionic groups)
• stability of surface structure and 3-phase line.

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Fixing the Model Structure and Equilibrium

• Four terms are the minimal set
• osmotic inflation interface
  corona

• Minimize g f ml to get m(l)

VF, Polymer, 2003 JPC B, 2009
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Chemical Equilibrium as Balance of Pressures
Pressures pout , pin - osmotic pd -
inflation (transient) ps -
interfacial-elastic (Laplace)
The interfacial tension is zero, but the
Laplace pressure is not unless f 1.
VF, JPC B, 2009
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Surface Equilibrium

• Two more equilibrium conditions at the surface
• Balance of 3 tensions (Neumann construction)
• Equilibrium between polymer bulk and surface

VF, JPC B, 2009
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Surface Equilibrium Interim Summary

• In vapor water gets buried under surface ps 0.
  
• In liquid micelles are inverted and ps 0
  (Schroeders paradox).
• Nafion should dissolve in water, but dissolution
  never happens (relaxation time 105 s).
• However, (quasi-)dissolution may occur at the
  surface.

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Examining the Surface Structure GISAXS
Rubatat and Diat, Macrmolecules, 2007 (bulk SANS)
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ESRF and ID10B
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Nafion Surface in Vapor (GISAXS)
100 nm thick Nafion film spin-cast on a Si
wafer T 30 C, RH 97 Beam 8 keV
Bass et al., JPC B, 2010
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GISAXS Going Under Water
water
vapor
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Vapor vs. Liquid Contact Angle and AFM

• CA Nafion surface is hydrophobic in vapor and
  hydrophilic in water
• AFM under water the surface gets rougher
  (surface tension drops).

Vapor RH97q 94.5 1.1hydrophobic

• Liquid water
• q 25.4 0.25
• hydrophilic

Dry q 96.4 1.2hydrophobic
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Hydrophilic vs. Hydrophobic Substrate
Nafion film
Native Si substrate (SiO2)
OTS on Si z -59 mV, q 130o (Yang Abbott,
Langmuir, 2010)
Dura et al., Macromolecules, 2009 (NR)
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Micelle Orientation at Interfaces
Bass et al., 2010
Some of these are metastable non-equilibrium
structures! (non-relaxed elastic stress,
relaxation time gt105 s)
Balsara et al, NanoLett, 2007
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Summary

• Solid Nafion is a non-equilibrium structure.
• Non-relaxed pressures in Nafion result in a
  non-thermodynamic behavior (Schroeders
  paradox) this needs to be accounted for in
  transport modeling.
• Interfaces affect the morphology and orientation
  of micelles in thin Nafion films this could be
  attractive for developing barriers with enhanced
  and stable transport characteristics.

Liquid
Vapor
Nafion
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Thanks
ISFESRF Maria Bass Oleg Konovalov, Amarjeet
Singh, Jiri Novak (ESRF, ID10B) Amir Berman,
Yair Kaufman, Juergen Jopp (BGU) Special
thanks Emmanuel Korngold (BGU), Klaus-Dieter
Kreuer, Martin Ise (MPI Stuttgart)
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Another old puzzle microscopic vs. macroscopic
swelling

• The relative change of Bragg spacing (d-do)/d
  (microscopic swelling) may be compared with the
  relative macroscopic linear expansion (1/fp
  1)1/3 calculated from l.
• Though for high l the relation is as for dilute
  2D micelles, for solid Nafion (small and moderate
  l) it is nearly linear, as if the structure is 1D
  (lamellae)

Gebel, 2000 Fujimura et al., 1981, 1982
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Microscopic vs. macroscopic swelling

• The model shows a good agreement with scattering
  data, provided a 2D morphology is plugged in