Nafion : Hydration, Microstructure and Schroeder’s paradox Viatcheslav Freger Maria Bass, Amir Berman (BGU) Oleg Konovalov, Amarjeet Singh (ESRF) Technion.

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