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
Dec 26, 2015
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 Engineering
Haifa, Israel
Nafion and Its Uses
Fuel Cells
Membrane electrolysisSensors
Catalysis
An ionomer developed by DuPont in 70s
Unique Microstructure: Microphase separation and 2D Micelle Morphology
Schmidt-Rohr and Chen, Nat Mater., 2008
Gebel, Diat et al, Macromolecules, 2002, 2004
Gebel, Polymer, 2000
Hsu and Gierke, JMS, 1983
2D Morphology: Transport vs. Hydration
Conductivity
VF et al., JMS, 1999Kreuer, JMS, 2001
0.001
0.01
0.1
1
0.01 0.1 1
water volume fraction XVD/
Dw
Blum et al.SPEPEEKKNafion
3D
2D
Water self-diffusion (NMR)
Schroeder’s Paradox: Two Isotherms?
Bass and Freger, 2008
0
10
20
30
0 0.2 0.4 0.6 0.8 1
water activity
l
vapor
liquid
Li-Nafion
Sample SampleOsmotic stressor
solution
Schroeder’s Paradox and Water Transport
If the thermodynamic potential of water is ill-defined, how
does one model water transport and “water management”?
51~
Hw
www J
RT
CDJ
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.
Fixing the Model: Structure and Equilibrium
Four terms are the minimal set
osmotic “inflation” interface “corona”
20( ) ( ) /o eff g G BR
1
34
5
2
)( ge vv
R
VF, Polymer, 2003; JPC B, 2009
Minimize g = f – lto getl
Chemical Equilibrium as Balance of Pressures
2
2/3
(1 )s
out sin d
RR R
g
”’
l”l’
Pressures:out , in - osmoticd - inflation (transient)s - interfacial-elastic (“Laplace”)
VF, JPC B, 2009
The interfacial tension is zero, but the “Laplace” pressure is not unless = 1.
Surface Equilibrium
Two more equilibrium conditions at the surface:
Balance of 3 tensions (Neumann construction)
Equilibrium between polymer bulk and surface
vapor
matrix (2)
an ionic group
liquid (1)12
12a b
c d e
VF, JPC B,2009
Surface Equilibrium: Interim Summary
In vapor water gets buried under surface; s ≥ 0.
In liquid micelles are inverted and s = 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.
2)1( Rs
normal-type micelles(“spaghetti”)surface-aligned
bundle (“macaroni”)
water
Examining the Surface Structure: GISAXS
keV 8for 2.0 c
nm 3~pd
Rubatat and Diat, Macrmolecules, 2007
(bulk SANS)
ESRF and ID10B
Nafion Surface in Vapor (GISAXS)
0.001
0.01
0.1
1
0.01 0.1 1 10Qxy , A-1
Qxy
*I,
A-1
a.u.
0.110.170.20.25
100 nm thick Nafion film spin-cast on a Si waferT = 30 C, RH ~ 97%Beam 8 keV
Bass et al., JPC B, 2010
GISAXS: Going Under Water
water vapor
C18-capped Si substrate
Nafion film
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).
Dry = 96.4 ± 1.2hydrophobic
Vapor RH=97% = 94.5 ± 1.1hydrophobic
water
Air bubble
Water drop
Air Water drop
Air
Liquid water = 25.4 ± 0.25
hydrophilic
Hydrophilic vs. Hydrophobic Substrate
OTS on Si: = -59 mV, = 130o (Yang & Abbott, Langmuir, 2010)
Dura et al., Macromolecules, 2009 (NR)
C18-capped Si substrate
Nafion film
Native Si substrate (SiO2)
Nafion film
Micelle Orientation at Interfaces
C18-capped Si substrate
a micelle bundle
Vapor
Native Si (SiO2) substrate
Water
Nafion film Micellebundles
bundlesbreaking up
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
2)1( Rs
Vapor Nafion Liquid
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 enhanced and stable transport characteristics.
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)
Thanks
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/p – 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”
0
1
2
3
4
5
6
0 0.5 1 1.5
Linear expansion
Mic
rosc
opic
sw
ellin
g
D=3
D=2
D=2 var
1
10
100
0.01 0.1 1
p
dm
ax, n
m
constantvariable
1
10
0
23
11 11
n for D=2 theoretical initial slope is 7 (exp 6)
n D
gDp w
g g
d dn
d D D