Slide 1

Degradation of Fuel Cell Membranes
Using ESR Methods: In Situ and Ex
Situ Experiments
“A Dream of Hydrogen” *
Shulamith Schlick
Department of Chemistry, University of Detroit Mercy,
Detroit, Michigan, USA
Polymers in Membrane Technology Symposium
238 ACS National Meeting, Washington DC
16-20 August 2009
* New York Times Editorial, 9 August 2009

1


Slide 2

“Water is the fuel of the future”
Jules Verne, 1874

Driving the GM Equinox - 2008

FC bus – Project CUTE
London 2006
2


Slide 3

Electricity

H2 Gas-Anode

O2 Gas-Cathode

Heat
Water
Electrolyte
(Nafion, PEM)

The fuel cell is a reactor with strong oxidizing
power, capable of reducing the durability of
proton exchange membranes (PEMs)
3


Slide 4

Reactions in Fuel Cells
Anode
Oxidation of hydrogen: 2H2  4H+ + 4e-

Cathode
Four-electron reduction of oxygen: O2 + 4H+ + 4e-  2H2O

Complications
Two-electron reduction of oxygen: O2 + 2H+ + 2e-  H2O2
Also expected HO· + H2O2  HO2· + H2O and, in neutral
solutions, HO2· + H2O  O2· + H3O+)
 HO· , HO2· , and O2· are lethal reactive intermediates
Early events can be detected by Direct ESR or Spin Trapping
4


Slide 5

Electron Spin Resonance Experiment

E= hv =
gβeH0
=E

Resonance is achieved
when the frequency of the
incident radiation is the
same as the frequency
corresponding to the
energy separation, E

____________________________________________________
P. Atkins, Physical Chemistry, W.H. Freeman; New York, 1998

5


Slide 6

Fluorinated PEMs
(C F 2 C F 2 ) m C F 2 C F

(C F 2 C F 2 ) m C F 2 C F
OCF 2 CFOCF

2 CF 2 SO 3 H

OCF 2 CF 2 SO 3 H

CF 3

Nafion

Dow, Solvay-Solexis

(C F 2 C F 2 ) m C F 2 C F
OCF 2 CF 2 CF 2 CF 2 SO 3 H

3M

Degradation and possible stabilization of PEMs are
major problems that must be studied before the transition
to the hydrogen economy
6


Slide 7

Statement of the Problem
• Recent ideas on membrane degradation: main chain unzipping
due to chain-end impurities (COOH): loss of one CF2 group in each
step.
(a) RF-CF2COOH + HO·  RF-CF2· + CO2 + H2O
(b) RF-CF2· + HO·  RF-CF2OH  RF-COF + HF
(c) RF-COF + H2O  RF-COOH + HF  Further attack, unzipping

•

This mechanism is well documented, and the progress of
degradation is measured by following the concentration of fluoride
ions, F–.

•

Problem with this approach: Membranes degrade even when the
concentration of the chain-end impurities is negligible.

__________________________________________________________________

1. Curtin, D.E.; Losenberg, R.D.; Henry, T.J.; Tangeman, P.C.; Tisack, M.E. J. Power Sources 2004, 131, 41.
2. Healy, J.; Hayden, C.; Xie, T.; Olson, K.; Waldo, R.; Brundage, A.; Gasteiger, H.; Abbott, J. Fuel Cells 2005,
5, 302.
3. Zhou, C.; Guerra, M. A.; Qiu, Z.-M.; Zawodzinski, T. A.; Schiraldi, D. A. Macromolecules 2007, 40, 86958707.
8


Slide 8

Plan of Lecture
 Objectives and Approach
 Results
• Direct ESR Detection: Nafion Membranes /
Photo-Fenton Reaction (ex situ)

• Spin Trapping of Radicals: Model Compounds
(ex situ)
• Visualizing Chemical Reactions and Crossover
Processes in a Fuel Cell Inserted in the ESR
Resonator (in situ)
• Unresolved Issues and Stabilization
9


Slide 9

Objectives and Approach
• In situ vs ex situ experiments: What are the
mechanistic differences ?
• Beyond Curtin: Other degradation paths ?
• Our approach:
Membrane degradation
Model compounds
In situ experiments

10


Slide 10

Generating Reactive Oxygen
Species in the Laboratory
Fenton Reaction
H2O2 + Fe(II)  Fe(III) + HO + HO
Fe(II) + O2 ↔ Fe(III) + O2
HO + H2O2  HOO + H2O
Photo-Fenton Reaction (UV Irradiation)
Fe(III) + H2O  Fe(II) + H+ + HO
Fe(II) + O2 ↔ Fe(III) + O2
O2 + H+ ↔ HOO
Peroxide Decomposition by Heat or UV
H2O2  2 HO
________________________________________________

•
•
•
•

Walling, C. Acc. Chem. Res. 1975, 8, 125.
Freitas, A.R.; Vidotti, G.J.; Rubira, A.F.; Muniz, E. C. Polym. Degrad. Stab. 2005, 87, 425.
Bednarek, J.; Schlick, S. J. Phys. Chem. 1991, 95, 9940.
Bosnjakovic, A.; Schlick, S. J. Phys. Chem. B 2004, 108, 4332.
11


Slide 11

Detection of Radical Intermediates:
(1) Direct ESR and (2) Spin Trapping
(1) In direct ESR: vary T in order to
increase the stability of radicals.
(2) In spin trapping: transform short-lived
radicals into stable nitroxides.
O
CH



N

CH 3
C

CH 3

H 3C
H
H 3C

CH 3



N

CH 3

O

N

C

CH 3

CH 3

O

PBN
(-Phenyl-tert-butylnitrone)

DMPO
(5,5-Dimethylpyrroline-N-oxide)

MNP
Methyl-nitroso-propane
12


Slide 12

How it Works: DMPO
R

+ R• 
N

O

Spin Trap

H

N

H

O

Spin Adduct

Spin adducts exhibit hyperfine splittings from 14N nucleus and Hß
proton. It is easy to decide if a short-lived radical is present, and
more of a challenge to identify the radical.
• DMPO is the spin trap of choice for HO radicals.
• Hyperfine splitting from Hβ is <20 G for oxygen-centered
radicals (OCR), and ≥20 G for carbon-centered radicals 13
(CCR).


Slide 13

Membranes / Direct ESR

14


Slide 14

The Chain End Radical in Nafion/Fe(II)
/H2O2 and Nafion/Fe(III): RCF2CF2•
R C F 2C F 2

.

(2 F  , 2 F  )

gzz = 2.0030, gxx = gyy = 2.0023
giso = 2.0025

E x p e rim e n ta l

n(Fα)=2
Azz(Fα) = 222 G
Axx(Fα)= Ayy(Fα) = 18 G
aiso(Fα) = 86 G

77 K
N a fio n /F e (II)/H 2 O 2

77 K
N a fio n /F e (III)

n(Fβ)=2
Azz(Fβ) = 30 G
Axx(Fβ)= Ayy(Fβ) = 38 G
aiso(Fβ) = 35 G

S im u la te d

3000

3200

3400

3600

The simulation was based on
planar geometry around Cα in
the RCβF2CαF2• radical

M a g n e tic F ie ld / G

• Kadirov, M.V.; Bosnjakovic, A.; Schlick, S. J. Phys. Chem. B 2005, 109, 7664-7670.
• Roduner, E.; Schlick, S. In Advanced ESR Methods in Polymer Research, S. Schlick,
Ed.; Wiley: Hoboken, NJ, 2006; Chapter 8, pp 197-228.

15


Slide 15

2005 Paper Revisited: Automatic Fitting
+ DFT
Experimental

Or C

All tensors coaxial

Planar around C

Pyramidal around Ca

Best fit
3000 3100 3200 3300 3400 3500 3600 3700

Magnetic Field / G

•The simulation indicated an angle of 12°
between the largest principal values of the
two Fα nuclei: a pyramidal geometry

g-tensor:
2.0030, 2.0023, 2.0023 giso = 2.0025 (fixed)
n(Fα)=2
222, 18, 18 G, aiso(Fα) = 86 G (fixed)
n(Fβ)=2
Fβ-1: 34,3, 25,5, 15.0 G, aiso = 24.9 G
Fβ-2: 29.3,23.4, 29.9 G, aiso = 27.5 G

16
• Lund, A.; Macomber, L.D.; Danilczuk, M.; Stevens, J.E.; Schlick, S. J. Phys. Chem. B 2007, 111, 9484-9491.


Slide 16

DFT Results
Based on two model structures:
CF3OCF2CF2• (RSC, radical on side chain) and
CF3CF2CF2CF2• (RMC, radical on main chain),
results suggest side chain radical formation.
This mechanism is supported by recent NMR
results (“the pendant side chains of the ionomers are more
affected than the main chain”) .
___________________

• Ghassemzadeh, L.; Marrony, M.; Barrera, R.; Kreuer, K.D.; Maier, J.;Müller,
K. J. Power Sources 2009, 186, 334-338.
17


Slide 17

Model Compounds
•
•
•
•
•

CH3COOH (acetic acid, AA)
CF2HCOOH (difluoroacetic acid, DFAA)
CF3COOH (trifluoroacetic acid, TFAA)
CF3SO3H (trifluorosulfonic acid, TFSA)
CF3CF2OCF2CF2SO3H
(perfluro-(2-ethoxyethane)sulfonic acid, PFEESA)

HO was generated by UV-irradiation of H2O2
__________________________________________________________________
• Schlick, S.; Danilczuk, M. Polym. Mat. Sci. Eng. (Proc. ACS Div. PMSE) 2006,
95, 146-147.
19
• Danilczuk, M.; Coms, F.D.; Schlick, S. Fuel Cells 2008, 8(6), 436-452.


Slide 18

DMPO – CF3SO3H Adducts

1

1

1

1

2

2
3

1

1

3

294 K (ESR and irradiation)

2

2

Exp

3

S im

pH=1.12, in situ irrad, 10 min

Adduct

giso

aN / G

aH / G

CCR
OH
Degrad

2.0054
2.0052

15.8
14.85
14.1

22.8
14.85

A d d u c ts
90%

D M P O /C C R (1 )

9 .8 %

D M P O /O H (2 )

0 .2 %

D M P O /D e g ra d (3 )

3320

3340

3360

3380

M a g n e tic F ie ld / G

Adducts of carbon-centered
radicals were detected in all
model compounds.

3400

20


Slide 19

MNP as a Spin Trap
R
(CH 3 ) 3 C

N

•
O + R

(CH 3 ) 3 C

MNP (2-methyl-2-nitrosopropane)

N

O

MNP/R

R is close to 14N, therefore we can deduce details on its structure
MNP is bought as a dimer, and dissociates in solution
(CH 3 ) 3 C

N

N

C(CH 3 ) 3

2 (CH 3 ) 3 C

N

O

O O
____________________________________________________
• Madden, K.; Taniguchi, H. J. Am. Chem. Soc. 1991, 113, 5541.
• Kojima,T.; Tsuchiya,J.; Nakashima, S.; Ohya-Nishiguchi, H.; Yano, S.; Hidai, M. Inorg. Chem.
1992, 31, 2333.
22


Slide 20

CF3CF2OCF2CF2SO3H(0.1 M)/MNP/H2O2
2

2

pH = 7, UV and ESR at 300 K

Exp
2
1

1 1

1

1 1

1 1

1:MNP/R: aN = 16.57G,aF = 11.52G(2F),
aF = 0.5G(2F)

1

2: Di-tert-butyl nitroxide (DTBN), aN = 17.1 G

Tentative
assignment

S im

Relative Conc.
O

(H 3 C) 3 C

N

CF 2 CF 2 R

89%

O
(H 3 C) 3 C

N

C(CH 3 ) 3

11%

____________________________________
3330

3340

3350

3360

3370

M a g n e tic F ie ld / G

3380

3390

•

Pfab, J. Tetrahedron Letters 1978, 19 (9), 23
843.


Slide 21

CF3CF2OCF2CF2SO3H (2 M)/MNP/H2O2
Exp

2

1

2

1 1

2

1

1

pH = 7, UV and ESR at 300 K
1

1:MNP/F:aN = 16.6G,aF = 21.8 G
2: Di-tert-butyl nitroxide (DTBN), aN = 17.1 G

O
(H 3 C) 3 C

N

F

S im

MNP/F

The MNP/F adduct is detected at higher
PFEESA concentration.

3330 3340 3350 3360 3370 3380 3390
M a g n e tic F ie ld / G

24


Slide 22

Sites of Attack
CH3COOH : The site of attack by HO• is the CH3 group
CF2HCOOH : The sites of attack by HO• are H in the
CHF2 and COOH groups

CF3COOH : The site of attack by HO• are H in the
COOH group
CF3SO3H : The site of attack by HO• are H in the
SO3H group
CF3CF2OCF2CF2SO3H: Probably H in the SO3H, and Near
27
the Ether Group


Slide 23

Conclusions (ex situ)
• DMPO: detection of spin adducts of carbon-centered radicals
(CCRs), and allowed the determination of the HO attack site.
• MNP has emerged as a sensitive method:
1. The identification of CCRs present as adducts, based on large
hyperfine splittings from, and the number of, interacting 19F nuclei.
2. The detection of the MNP/F adduct is related to the detection
of fluoride ions, F─, in the fuel cell product water in numerous studies.
3. The identification of oxygen-centered radicals (OCRs) as
adducts, and rationalized by reaction of the acid anions with HO, and
further reactions of the product with H2O2 and HO.
• Taken together, the results suggested:
Both sulfonate and carboxylate groups can be attacked by HO
radicals.
Confirm two possible degradation mechanisms in Nafion
membranes: originating at the end-chain impurity –COOH group and
at the sulfonic group of the side-chain.
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Slide 24

In Situ Studies: A Fuel Cell
Inserted in the ESR Spectrometer
• Closed circuit voltage (CCV)
and open circuit voltage (OCV),
300 K
• Pt-covered Nafion 117,
0.2 mg Pt/cm2
• V = 600-800 mV
• Operating time: up to 6 h
• Gas flows
O2: 2 cm3/min
H2 and D2 : 4 cm3/min

• Danilczuk, M.; Coms, F.D; Schlick, S. J. Phys. Chem. B 2009, 113, 8031-8042.
29


Slide 25

ESR Spectra of DMPO Adducts,
Cathode
Cathode side/H2/20 min
CCV
DMPO/OH

• DMPO/OH (CCV) and
DMPO/OOH (OCV).
• DMPO/OOH detected for the first
time in a FC, from crossover O2 and
H atoms (OCV):

OCV

DMPO/OOH

H• + O2 → HOO•
(chemical formation of HOO•)

• Can detect separately adducts at
cathode and anode.

3300

3320

3340

3360

3380

Magnetic Field / G

3400

3420

30


Slide 26

ESR Spectra of DMPO Adducts,
Cathode, CCV, H2
(A) 0-360 min

(B) 360 min

HO• adduct

Exp

0 min

H• adduct

Sim

120 min

36%

3360

3380

3400

3420

3440

3460

Magnetic Field / G

Carbon-centered
radical adduct
(CCR)

 CCR adduct is derived
from Nafion: fragmentation
even at 300 K

DMPO/H

 HOO· at the cathode can
be generated in two ways:

240 min

360 min

 H atoms

44%

DMPO/OOH

20%

DMPO/CCR

3360

3380

3400

3420

3440

Magnetic Field / G

3460

HO· + H2O2 → HOO· +
H2O electrochemically

H· + O2 → HOO·
chemically

31


Slide 27

ESR Spectra of DMPO Adducts,
Cathode, CCV, D2
(A) 0-360 min
1

1

1

2

2

2

2
1

3

(B) 360 min

1

1
2

1 2

3

Exp

0 min

1 3

120 min

3

240 min

Both DMPO/H and
Sim DMPO/D adducts with D2
at anode.
DMPO/OOH

50%

DMPO/CCR

10%
360 min

4 5

5 4

DMPO/H

25%

DMPO/D

15%
3300

3320

3340

3360

3380

Magnetic Field / G

3400

3420

3300

3320

3340

3360

3380

3400

3420

Magnetic Field / G

Assignments: 1-DMPO/OOH, 2-DMPO/Degr,
3-DMPO/CCR, 4-DMPO/H, 5-DMPO/D.
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Slide 28

ESR Spectra of DMPO Adducts,
Anode, H2 vs D2

DMPO/H

H2. Appearance of the
DMPO/H adduct on CCV
and OCV conditions, and of
the DMPO/OOH adduct only
on OCV conditions: H• may
be formed at the catalyst,
both CCV and OCV, and
reacts with crossover

DMPO/D

oxygen to produce HOO·

•

(A) Anode side/H2/20 min

(B) Anode side/D2/20 or 120 min
Exp

CCV
CCV

DMPO/H

sim

1: DMPO/OOH
2. DMPO/H

OCV

1

2

2
1

3300

3320

1

OCV

2

2

2

11 11 1

3340

3360

3380

Magnetic Field / G

3400

1

3420

3300

3320

2 2

3340

1
1

2 2 2
3360

1: DMPO/OOH
2: DMPO/H

1

2 2

2

3380

Magnetic Field / G

3400

3420

• D2. Appearance of both
DMPO/H and DMPO/D
adducts on CCV operation,
and the DMPO/OOH and
DMPO/H on OCV operation.
• Very weak CCR adducts
were also detected in some
33
experiments.


Slide 29

Table 1. Processes Suggested by the In Situ Fuel Cell
Experiments
Results

HO•/CCV/cathode

HOO•/CCV/cathode

Processes
O2 + 2H+ + 2e-  H2O2
(electrochemical H2O2 formation)

(1)

H2O2  2 HO•
(electrochemical HO• formation)

(2)

H2O2 + HO•  HOO• + H2O
(electrochemical HOO• formation)

(3)

Leading to H• or D•
OCV/anode

H2 + O2 → 2HO•
(Chemical HO• formation on catalyst) (4)

H•,D•/CCV/cathode/anode

HO• + H2 (D2) → H2O + H• (D•)
(Chemical H• and D• formation)

(5)

H• + O2 → HOO•
(chemical HOO• formation)

(6)

HOO•/OCV/cathode

34


Slide 30

Main Conclusions of In Situ FC
• Ability to examine separately processes at anode and
cathode.
• Obtain evidence for crossover of H2 and D2 to the
cathode and O2 to the anode.
• Reactions at the catalyst + crossover lead to the
formation of H and D atoms at both the cathode and
the anode.
• Unresolved Issues:
H· adduct with D2 at anode
• Question: What role can H and D atoms play?
35


Slide 31

In Situ Studies: Abstraction of
Fluorine Atom by H•
↓
(C F 2 C F 2 ) m C F 2 C FC F 2 C F 2

↓

+ H• →

.
C F 2 C F 2 C F 2 C CF 2 CF 2

OCF 2 CFOCF 2 CF 2 SO 3 H

OCF 2 CFOCF 2 CF 2 SO 3 H

CF 3

CF 3

+ HF
___________________________________________

Summary of attack sites:
• Main end-chain unzipping (by HO• radicals) → HF

• Attack of sulfonic groups (by HO• radicals or Fe(III))
• Main chain and side chain scission (by H• ) → HF
__________________________________________
• Coms, F.D. ECS Transactions 2008, 16(2) 235-255.

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Slide 32

UDM Group 2008

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Slide 33

Support
National Science Foundation (Polymers,
Instrumentation, International Programs)
Fuel Cell Activities of General Motors
US Department of Energy
Ford Motor Company

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