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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.
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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
Nafion and Its Uses An ionomer developed by DuPont in 70s Fuel Cells Catalysis Sensors Membrane electrolysis
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
2D Morphology: Transport vs. Hydration Conductivity Water self-diffusion (NMR) VF et al., JMS, 1999 Kreuer, JMS, 2001
Schroeder’s Paradox: Two Isotherms? Osmotic stressorsolution Sample Sample Li-Nafion Bass and Freger, 2008
Schroeder’s Paradox and Water Transport If the thermodynamic potential of water is ill-defined, how does one model water transport and “water management”?
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.
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
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
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
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.
Examining the Surface Structure: GISAXS Rubatat and Diat, Macrmolecules, 2007 (bulk SANS)
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
Nafion film C18-capped Si substrate GISAXS: Going Under Water water vapor
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
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)
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
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
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)
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
Microscopic vs. macroscopic swelling • The model shows a good agreement with scattering data, provided a 2D morphology is “plugged in”