1 / 22

Nafion : Hydration, Microstructure and Schroeder’s paradox Viatcheslav Freger

Nafion : Hydration, Microstructure and Schroeder’s paradox Viatcheslav Freger Maria Bass , Amir Berman (BGU) Oleg Konovalov, Amarjeet Singh (ESRF) Technion – Israel Institute of Technology Wolfson Department of Chemical Engineering Haifa, Israel. Nafion and Its Uses.

curt
Download Presentation

Nafion : Hydration, Microstructure and Schroeder’s paradox Viatcheslav Freger

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Nafion: Hydration, Microstructure and Schroeder’s paradox Viatcheslav Freger Maria Bass , Amir Berman (BGU)Oleg Konovalov, Amarjeet Singh (ESRF) Technion – Israel Institute of TechnologyWolfson Department of Chemical EngineeringHaifa, Israel

  2. Nafion and Its Uses An ionomer developed by DuPont in 70s Fuel Cells Catalysis Sensors Membrane electrolysis

  3. 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

  4. 2D Morphology: Transport vs. Hydration Conductivity Water self-diffusion (NMR) VF et al., JMS, 1999 Kreuer, JMS, 2001

  5. Schroeder’s Paradox: Two Isotherms? Osmotic stressorsolution Sample Sample Li-Nafion Bass and Freger, 2008

  6. Schroeder’s Paradox and Water Transport If the thermodynamic potential of water is ill-defined, how does one model water transport and “water management”?

  7. Schroeder’s 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.

  8. 2 1 4 3 5 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

  9. g l” l’ s’ s” s 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

  10. g12 liquid (1) g2 g1 a b matrix (2) vapor an ionic group c d e 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

  11. normal-type micelles(“spaghetti”) surface-alignedbundle (“macaroni”) water Surface Equilibrium: Interim Summary • In vapor water gets buried under surface; ps ≥ 0. • In liquid micelles are inverted andps = 0 (Schroeder’s paradox). • Nafion should dissolve in water, but dissolution never happens (relaxation time ≥ 105 s). • However, (quasi-)dissolution may occur at the surface.

  12. Examining the Surface Structure: GISAXS Rubatat and Diat, Macrmolecules, 2007 (bulk SANS)

  13. ESRF and ID10B

  14. 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

  15. Nafion film C18-capped Si substrate GISAXS: Going Under Water water vapor

  16. Air Air Water drop Air bubble Water drop water 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 RH=97%q = 94.5 ± 1.1hydrophobic Liquid water q = 25.4 ± 0.25 hydrophilic Dry q = 96.4 ± 1.2hydrophobic

  17. Nafion film C18-capped Si substrate 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)

  18. Water Vapor Nafion film bundlesbreaking up a micelle bundle Micelle bundles Native Si (SiO2) substrate C18-capped Si substrate Micelle Orientation at Interfaces Bass et al., 2010 Some of these are metastable non-equilibrium structures! (non-relaxed elastic stress, relaxation time >105 s) Balsara et al, NanoLett, 2007

  19. Summary • Solid Nafion is a non-equilibrium structure. • Non-relaxed pressures in Nafion result in a non-thermodynamic behavior (Schroeder’s 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 enhancedand stable transport characteristics. Liquid Vapor Nafion

  20. Thanks ISFESRF Maria Bass Oleg Konovalov, Amarjeet Singh, Jiři Novak (ESRF, ID10B) Amir Berman, Yair Kaufman, Juergen Jopp (BGU) Special thanks: Emmanuel Korngold (BGU), Klaus-Dieter Kreuer, Martin Ise (MPI Stuttgart)

  21. 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

  22. Microscopic vs. macroscopic swelling • The model shows a good agreement with scattering data, provided a 2D morphology is “plugged in”

More Related