solid proton conducting electrolytes: conduction...

Max-Planck-Institut für Festkörperforschung, StuttgartMax-Planck-Institute for Solid State Research

K.D. Kreuer, 2005

Solid Proton Conducting Electrolytes:Conduction Mechanism, Phenomenology and New Materials

for Fuel Cell Applications

K.D. Kreuer

Max-Planck-Institut für Festkörperforschung, Stuttgart

[email protected]


K.D. Kreuer, 2005

• A Fundamental Problemhydrogen bonding and long range proton transport

reorientation

transfer

two-step reaction mechanism

energies in the isolated dimer H5O2+

long range proton transport is typically related to extended ensembles in the liquid state

∆Ebarrier

- Ebond

Q

activationenthalpy ofproton mobilityin bulk water

Q / pm240 260 280 300 320

E /

eV0.0

0.5

1.0

1.5 rigid equilibriumconfiguration

stronghydrogen bond

weakhydrogen bond


K.D. Kreuer, 2005

formation ofhydrogen bond

breaking of hydrogen bond

Ab - initio - MD - Simulation (M.E. Tuckerman et al., J.C.P.103, 150 (1995)

H9O4+

(Eigen-ion)

H5O2+

(Zundel-ion)

H5O2+

(Zundel-ion)~10-13s

~10-12s

~10-13s

protonic charge follows the centre of symmetry of hydrogen bond - patternbond breaking and forming (reorientation) in weakly bond outer part of complexproton transfer in contracted central part of complexstrong coupling of both processes

revisited (K.D. Kreuer, Solid State Ionics 136-137, 149 (2000))

proton transport mechanism in watera liquid with solid-like properties

very high proton conductivity in the solid state? solids with liquid-like properties?


K.D. Kreuer, 2005

A few Technological Problems

1 nm

: -SO3

: protonic charge carrier

: H2O

-

-(CF2-CF2)n-CF-CF2- O-(CF2-CF-O)m-CF2-CF2-SO3H CF3

-

-NAFION ® hydration isotherme

conductivity

required high conductivity related to presenceof liquid water as a second phase(two phase effect)

Membranes for PEM - Fuel Cells

pH2O / pOH2O

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

λ =

[H2O

] / [S

O3H

]

0

2

4

6

8

10

12

14

16

18

20

22 NAFION 117(0.9 meq/g)

T = 300 K

λ = [H2O] / [-SO3H]

0 5 10 15 20 25 30 35

σ [ Ω

−1cm

-1]

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

T = 300 K


K.D. Kreuer, 2005

only conducting in the hydrated state

limited operation temperature (< 100°C)low activity of electrocatalyst (Pt)CO-poisoning of Ptexpensive heat management

high water/methanol "cross-over"(electroosmotic drag, permeation, chemical diffusion)expensive water/methanol management

Consequences for PEM - Fuel Cell Technology

separator membrane

PEM - FC

O2 (air)

H2O (CO2)

H+

T ~ 120°C

H2O CH3OH

CO2 / CO

H2 or H2 rich gas(humidified)


K.D. Kreuer, 2005

Separator Materials for SOFC

• high operation temperature

• high cathode overpotential

• mutual reactions of interconnect,electrodes and electrolyte

O2 (air)O2-

T = 800 - 1000°C

H2O / CO2

hydrocarbons

O2 (air)

H2O

H+

T = 500 - 700°C

CO2

hydrocarbons orH2-rich gas

oxide ion conductors

proton conductors

stable with highCO2 activities

stable with highwater activities

high protonicconductivity

• lower operation temperature

• metal - electrodes


K.D. Kreuer, 2005

• hydrated acidic polymers

• fully polymeric proton conductors based on heterocyclesand phosphonic acid as proton solvent

• proton conducting oxides

PEM-FC

SO-FC

⎫⎪⎪⎬⎪⎪⎭

from elementary reactions to new materials


K.D. Kreuer, 2005

• Hydrated acidic polymersrelation between microstructure and transport properties of hydrated acidic polymers

q / nm-1

0.1 1 10

(I / I

NV

) / n

m3

0.001

0.01

0.1

1

q-4 * const.

PEEKK 65% sulfonated, Φw = 0.41

Nafion 117, Φw = 0.40

characterisic separation length d

internal interface Σ

hydrophobic/hydrophilic nano-separation (especially in the presence of water)

microstructure from SAXS

: scattering invariant

: volume fraction of absorbed water

Σ Φ Φ= ⋅ ⋅ ⋅ −→∞

lim q w w

q I qINV

4

1( )

( )π

dqq)q(IINV 2

0⋅= ∫

∞

Φw

[(CF2 CF2)n (CF2 CF)]x

SO3HCF2CF2

O

O

CF3CFCF2 OO

O (SO3H)x

ONafion S-PEK

q2d π

=

M. Ise; Dissertation, Uni-Stuttgart (2000)K.D. Kreuer, J.Membrane Science 185, 29 (2001)


K.D. Kreuer, 2005

0.0 0.2 0.4 0.6 0.8 1.

1

2

3

4

5

6

7

8

9

dist

ance

s [n

m]

solvent volume fraction Φs

d: lattice constant

a: channel diameter

Nafion

S-PEEKK

(S-PEK)

COV

separation length and channel diameter

a

d


K.D. Kreuer, 2005

formation of protonic defects

K.D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster Chemical Reviews 104, 4637 (2004)


K.D. Kreuer, 2005

proton conductivity ac impedance

self-diffusion PFG-NMRpulsed magnetic field

gradient NMR

electroosmotic drag E-NMR electrophoretic NMR

transport matrix

phenomenologywith Lij = Lji

driving force method

112LF=σ

33OHCH

OHCH2

OHCHOHCH

22OH

OH2

OHOH

La ln d

dc

cRTD

La ln d

dc

cRTD

3

3

33

2

2

22

=

=

11

1313

2

OHCH

11

1212

2

OH

LL

LFK

LLLF K

3

2

=σ

=

=σ

=

Φ∇ F

Φ∇ F

RT

++

⎛ ⎞∇µ⎛ ⎞ ⎜ ⎟⎛ ⎞⎜ ⎟ ∇µ⎜ ⎟⎜ ⎟=⎜ ⎟ ⎜ ⎟⎜ ⎟ ∇µ⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎝ ⎠⎜ ⎟⎝ ⎠ ∇⎝ ⎠

2

2

HH 11 12 13 14

H OH O 21 22 23 24

MeOH31 32 33 34MeOH

total

j L L L Lj L L L L

L L L LjP

permeation24 34L ,L

∇ totalP permeation cell

i i Fµ = µ + Φ


K.D. Kreuer, 2005

connections to rf amplifier/receiver, temperature controller, power supply for constant current (E-NMR)

bifilar wound ohmic heater (temperatures from RT to 500 °C)

glass dewar

sample (thermally and mechanically isolated from gardient coil)

curre

nt(P

F G)

T -C

o ntro

l

air

wa t

er( fo

r coo

ling)

gas diffusion electrode (1 mg/cm2 Pt on C)

membranes slices or rolls, constant T and solvent content due to sealed container, water and air-tight

E-NMR probe

Pt-electrodes

superconducting magnet B0(max) = 8 T(inhomogeneity ca. 1 ppm)

tuning and matching capacitors

rf saddle coil on quarz tube

anti-Helmholtz gradient coilGmax = 0.5 T/cm(water-cooled)

measuring electroosmotic drag by E-NMR


K.D. Kreuer, 2005

transport coefficients as a function of water content and temperature(proton mobility (L11) and water self-diffusion(L22))

(1000/T) / K-1

2.6 2.8 3.0 3.2 3.4

D /

cm

2 s-1

10-6

10-5

10-4

T / °C

20406080100

NafionDH2O (bulk water)

n = 10

n = 5

n = 3

n = 16

DH2O

Dσ


K.D. Kreuer, 2005

10 -6

10 -5

10 -4

D

[ cm

2/ s

]

0.1 1

10 -7

water volume fraction Φw

Nafion

T = 300 K

transport coefficients as a function of water volume fraction(proton mobility (L11) and water self-diffusion(L22))

K.D.Kreuer, J.Membrane Science 185, 29 (2001)

DH2O ~ L22

decreasing percolation

retardation of waterdiffusion (confinement)

suppression ofintermolecularproton transfer

nano - scale molecular scale

Monte Carlo

n = 14 H2O / SO3H


K.D. Kreuer, 2005

10 -6

10 -5

10 -4

D

[ cm

2/ s

]

0.1 1

10 -7

water volume fraction Φw

Nafion

T = 300 K

transport coefficients as a function of water volume fraction(proton mobility (L11) and water self-diffusion(L22))


• mobility of protonic defects andwater diffusion related

• both coefficients dominatedby percolation(to some extend bywater/polymer interaction)

decreasing percolation

retardation of waterdiffusion (confinement)

suppression ofintermolecularproton transfer

nano - scale molecular scale

DH2O ~ L22

Dσ ~ L11

n = 14 H2O / SO3H

DH O2

~


K.D. Kreuer, 2005

transport coefficients as a function of water volume fraction(water permeation (L34))

0.1 1

10-7

10-6

10-5

10-4

D

/ cm

2 s-1

XV

NafionT = 300 K

3 5 10 20 50 100

n = [H2O] / [-SO3H]

DH O2

Dσ (H2O)

DH O2

P

water permeation has large hydrodynamic contribution


K.D. Kreuer, 2005

H3O+

lower limit given by primary hydration of proton (H3O+)

otherwise electroosmotic drag may be described as hydrodynamic process controlled by water/waterand water/polymer interaction, microstructure and swelling

electroosmotic drag:

increase water confinement

K.D. Kreuer, S.J. Paddison, E. Spohr, M. Schuster Chemical Reviews 104, 4637 (2004)

electroosmotic water/methanol drag

⎟⎟⎠

⎞⎜⎜⎝

⎛

11

13

11

12LL

,LL


K.D. Kreuer, 2005

• Fully polymeric proton conductors based on heterocyclesphosphonic acid as proton solvent

from similar hydrogen bond networks as water

transport coefficients similar to those of waterfor a given temperature relative to melting point

have proton donor and acceptor function (Ka; Kb > amphoteric)

K.D. Kreuer, A. Fuchs, M. Ise, M. Spaeth, J. Maier; Electrochim. Acta 43, 1281 (1998)


K.D. Kreuer, 2005

structure diffusion in liquid imidazole

K.D.Kreuer, A.Fuchs, M.Ise, M.Spaeth, J.Maier; Electrochim. Acta 43, 1281 (1998)

W.Münch, K.D.Kreuer, W.Silverstri, J.Maier, G.Seifert; Solid State Ionics 145, 295 (2001)

ab - initio - MD

mechanism similar to that in water


K.D. Kreuer, 2005

~ 30 ps

~ 0.3 ps ~ 0.3 ps

structure diffusion of excess protons in liquid imidazole

W. Münch, K.D. Kreuer,W. Silvestri, J. Maier, G. Seifert; Solid State Ionics 145, 437 (2001)


K.D. Kreuer, 2005

approach:

σ-bond in symmetrical position

teminated soft side-chains


hierachic architecture

model for spacer O

O

NH

N

NH

N

n Imi-n with n = 2, 3, 5

rigid polymerbackbone

soft spacer

liquid - likeproton solvent


K.D. Kreuer, 2005

how can heterocycles be immobilized without loosing the localliquid-like dynamics assisting "structure diffusion" ?

N N

CH3

H(H)

N N

CH3

H(H)

effect of asymmetric bonding

∆E = 0

∆E ~ 20 meV

choice of spacerenergy variation with rotational coordinate

isolated chainS.J. Paddison, work in progress


K.D. Kreuer, 2005

2.2 2.4 2.6 2.8

10-9

10-8

10-7

10-6

D /

cm

2 s-1

(1000/T) / K-1

100150

decoupling of proton and oligomer transport(pure systems)

proton transport via structure diffusion

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.81E-9

1E-8

1E-7

1E-6

Imi-5 DH Dσ

Imi-3 DH Dσ

Imi-2 DH Dσ DF

D /

cm2 s

-1

(1000/T) / K-1

140 120 100 80 60 40 20 0

10~DD

H

σ

N

N

H

N

N

H

CH2 O (CH2 CH2O)2 CH2

Imi2:

Dσ

DHNHD(H )

CHD(H )

CH3

Si O

O

N

N

H

n

chemical shift [ppm]

incre

asing

magne

tic fie

ld gr

adien

tin

tens

ity

H


K.D. Kreuer, 2005

proton conduction mechanismaggregation and dynamic hydrogen bonding

W.Münch, K.D.Kreuer, J.Maier; Solid State Ionics, in preparation

+

+

21

2

G.R. Goward et al. J.Phys.Chem.B 106, 9322 (2002)

MD-simulation 1H-NMR


K.D. Kreuer, 2005

NH

N

NH

N

O O

NH

N NH

N

OO

O

NH

N NH

N

O

N

NH

m

OO

O

NH

N

CH3

Si O

O

NH

N

n

n = 4

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.810-8

10-7

10-6

10-5

10-4

10-3

10-2

σ [S

cm

-1]

1000 / T [K-1 ]

050100150200

limiting conductivity ~ 10-3 S cm-1

[°C]

conductivity limit (pure systems)from liquids to solids with liquid-like properties

colaboration with MPI-P, W.H. Meyer


K.D. Kreuer, 2005

(1000/T) / K-1

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

log

[σ /

Scm

-1]

-10

-9

-8

-7

-6

-5

-4

-3

-2

T / °C

020406080100120140

Imi-2 (pure)Imi-2 + 0.15 mol% CF3SO3H Imi-2 + 0.62 mol% CF3SO3H Imi-2 + 1.25 mol% CF3SO3H Imi-2 + 2.51 mol% CF3SO3HImi-2 + 5.40 mol% CF3SO3HImi-2 + 10.20 mol% CF3SO3H Imi-2 + 16.40 mol% CF3SO3HImi-5/2 + 10.10 mol% CF3SO3H

acid doped systems

0 5 10 150.0

1.0x10-3

2.0x10-3

3.0x10-3

σ [S

cm

-1]

mol% CF3SO3H

T = 120 °C


K.D. Kreuer, 2005

0 1x106 2x106 3x106

0.1

1

M(G

) / M

(0)

γ 2 I(G) [s cm -2 ]

D(Hε)

D(Hα−γ)

[Hz]

∆ν ~ 30 Hz

α,β

γδ

ε

OO

O

NH

N NH

Nαα

β β γγ

δ

δ

δ

δ ε

ε

1H NMR spectrum of Imi-2at T = 130 °Cν0 = 49.836 MHz

site-selective PFG-NMR

compound Haven ratio

H3PO4 1.7 Dippel

NaOH ~ 100 Spaeth

Imi ~3-15 Schuster

related observation:

cooperativity of proton transfer reactions

D(Hε)

D(Hα−γ)~ 2

σD

σ=

80100120

2.5 2.6 2.7 2.8 2.9

10-4

10-3

10-2

σ /

S c

m-1

(1000 / T) / K-1

σD

σ

T / °C


K.D. Kreuer, 2005 reorientation rate > separation rate0 50 100 150 200

0

10

20

30

40

50

60

70

D(H

) / D

σ

T [°C]

25ε =

rate ratio exp 2

0

e4 akTπεε

⎧ ⎫⎨ ⎬⎩ ⎭

proton diffusion without charge separation

n = 4

formation of contact ion pair

ring-reorientation

protons change their identity

local dissociation

back-transfer

annihilation of contact ion pair

. 10a 4 9 10 m−= ⋅

NN H HN N HN N HN N

NN H N N HN N HN NH +-

- +NN H N N HN N HN NH

NN H HN N HN N HN N

CH3

Si O

O

N

N

H

n


K.D. Kreuer, 2005

• increase of amphoteric character• increase of local dielectric constant• optimization of polymer architecture with respect todynamical hydrogen bonding:- aggregation by hydrogen bonding- fast hydrogen bond breaking and forming processes (Umlagerung)


K.D. Kreuer, 2005

phosphonic acid functionalized polymers

P

O

OHOH

P

O

OHOH

O

POH OH

P

O

OHOH

OSi

O Si

OSi

O

Me

SiMeO

Me

Me

(CH2)n

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

10-6

10-5

10-4

10-3

10-2

50100150200

σ / S

cm

-1

(1000/T) / K-1

n = 2, 6, 11 n = 2 n = 2 n = 4 n = 6 n = 6

°C

dry

under 1 atm H2Oat 1 atm pH2O


K.D. Kreuer, 2005

formation of protonic defects

•O

xO

••O2 OH 2=O+V+OH

• proton conducting oxides

T / °C

0 200 400 600 800 1000

mol

% O

HO

0

2

4

6

8

10

10Y:BaZrO3

pH2O = 23 hPa·

protonic defects in theperovskite structure


K.D. Kreuer, 2005

rapid rotational diffusion of •OOH

QNS

µSR

quantum-MD

wavenumber / cm-1

1500200025003000350040004500

abso

rban

ce

IR spectra of single crystals

Y:BaCeO3

300 K

Y:BaZrO3

300K

strong hydrogen bonds

nature of hydrogen bonding?

mobility of protonic defects


K.D. Kreuer, 2005

average configuration

OH

transition hydrogen bond (schematical)

formation of dynamical hydrogen bonds with all 8 nearest neighbours

• strong bendhydrogen bonds

• lattice distortion

W. Münch; Solid State Ionics 125, 39 (1999)K.D. Kreuer; Solid State Ionics 125, 285 (1999)


K.D. Kreuer, 2005

dO/O / pm

220 240 260 280 300 320

∆A

/ eV

-0.7-0.6-0.5-0.4-0.3-0.2-0.10.00.10.20.30.40.50.60.70.80.9

O-O - coordinate

transient OH····O coordinate

structural O-O separation

BaCeO3

hydrogen bonding

lattice distortion

thermodynamics of hydrogen bond formation

extended variations of OH / O separation (local lattice softening)


K.D. Kreuer, 2005

dynamical hydrogen bond

t / ps0 2 4 6 8 10 12 14 16

q d / p

m

70

80

90

100

110

120

130

140

variation of hydrogen bond strength leads toconfigurations which favors bond breaking andsituations which favors proton transfer"liquid-like dynamics"


K.D. Kreuer, 2005

• balance hydrogen bond and covalent interaction

• maximize local symmerty


K.D. Kreuer, 2005

Instead of a Summary Iclosing the T-gap with new materials

(1000/T) / K-1

0.5 1.0 1.5 2.0 2.5 3.0 3.5

log σ

/ Ω-1

cm-1

-6

-5

-4

-3

-2

-1

0

T / °C

902005001000

Zr0.9Y0.1O3-δ

BaY0.2Zr0.8O3-δ

NAFION

Si

Me

(CH2)

PO(OH)2

O

2

3 - 5

oxid ion conductors proton conductors

SOFC LT-SOFC HT-PEM PEM

Poly-arylene-sulfone


K.D. Kreuer, 2005

• dynamical hydrogen bonding: the clue to the understanding of protonconductivity in the liquid- and solid state

• typical feature of bulky material

Instead of a Summary II


K.D. Kreuer, 2005

thank you !

MPI - FKF MPI -P MPI - MF

A. Fuchs W.H. Meyer hosting nmr

U. Traub M.F. Schuster

M. Ise G. Scharfenberger

M. Schuster Uni Göttingen

H.G. Herz St. Adams

A. Noda Uni Dresden

T. Rager G. Seifert

Motorola

W. Münch S.J. Paddison

J. Maier (head of department)

financial support

G. Frank, Hoechst

R. Bauer, FuMaTech

M. Waidhas, Siemens

DFG

BMBF

Energiestiftung BW

H+

solid proton conducting electrolytes: conduction...

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