1

Visualizing Chemical and Electrochemical Reactions, and

Crossover Processes in a Fuel Cell by Spin Trapping ESR Methods

Shulamith Schlick, Marek Danilczuk and Frank D. Coms

Department of Chemistry, University of Detroit Mercy and General Motors Fuel Cell Research Lab

Advances in PEM Fuel Cell SystemsAsilomar, 15-18 February 2009

2

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

Kms in nine cities CUTE, Barcelona

FC bus – Project CUTE London 2006Driving the GM Equinox - 2008

3

Reactions in Fuel Cells

CathodeFour-electron reduction of oxygen: O2 + 4H+ + 4e- 2H2O

AnodeOxidation of hydrogen: 2H2 4H+ + 4e-

ComplicationsTwo-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

Radicals can be detected by Direct ESR or by Spin Trapping

4

Electron Spin Resonance Experiment

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

=E

____________________________________________________

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

E= hv = gβeH0

5

Fluorinated PEMs

(CF2CF2)mCF2CF

OCF2CF2SO3H

(CF2CF2)mCF2CF

OCF2CF2CF2CF2SO3H

Nafion Dow, Solvay-Solexis 3M

(CF2CF2)mCF2CF

OCF2CFOCF2CF2SO3H

CF3

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

6

Microphase Separation in Ionomers: Nafion Aggregates

(CF2CF2)mCF2CF

OCF2CFOCF2CF2SO3H

CF3

The fibrillar structure of Nafion: (A) An aggregate. (B) Bundles of elongated aggregates made of polymeric chains surrounded by ions and water molecules.

Reactions in the membrane are typical of microheterogeneous systems___________________________________________________________• Szajdzinska-Pietek, E.; Schlick, S.; Plonka, A. Langmuir 1994, 10, 1101-1109.• Rubatat, L.; Rollet, A.L.; Gebel, G.; Diat, O. Macromolecules 2002, 35, 4050-4055. • van der Haijden, P.C.; Rubatat, L.; Diat, O. Macromolecules 2004, 37, 5327-5336.

(A) (B)

aggregate

bundle

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.

_______________

Curtin, D.E.; Losenberg, R.D.; Henry, T.J.; Tangeman, P.C.; Tisack, M.E. J. Power Sources 2004, 131, 41.

Healy, J.; Hayden, C.; Xie, T.; Olson, K.; Waldo, R.; Brundage, A.; Gasteiger, H.; Abbott, J. Fuel Cells 2005, 5, 302.

Zhou, C.; Guerra, M. A.; Qiu, Z.-M.; Zawodzinski, T. A.; Schiraldi, D. A. Macromolecules 2007, 40, 8695-8707.

8

• Direct ESR Detection: Nafion Membranes / Photo-Fenton Reaction

• Spin Trapping of Radicals: Model Compounds

• Visualizing Chemical Reactions and Crossover Processes in a Fuel Cell Inserted in the ESR Resonator (in situ)

• Unresolved Issues and Follow-up Studies

Plan of Lecture Objectives and Approach Results

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

Generating Reactive Oxygen Species in the Laboratory

Fenton ReactionH2O2 + 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 UVH2O2 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

CHC CH3N

O CH3

CH3

N

H3C

H3CH

O PBN DMPO MNP(-Phenyl-tert-butylnitrone) (5,5-Dimethylpyrroline-N-oxide) Methyl-nitroso-propane

Detection of Radical Intermediates: (1) Direct ESR and (2) Spin Trapping

C CH3NO

CH3

CH3

(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.

12

How it Works: DMPO

N

O

H N

O

H

R+ R•

• 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 (CCR).

Spin Trap 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.

13

Membranes / Direct ESR

14

3000 3200 3400 3600

77 K

Nafion/Fe(III)

Nafion/Fe(II)/H2O

2

RCF2CF

2

.

(2F, 2F)

77 K

Simulated

Experimental

Magnetic Field / G

The Chain End Radical in Nafion/Fe(II) /H2O2 and Nafion/Fe(III): RCF2CF2

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

n(Fα)=2Azz(Fα) = 222 GAxx(Fα)= Ayy(Fα) = 18 Gaiso(Fα) = 86 G

n(Fβ)=2 Azz(Fβ) = 30 G

Axx(Fβ)= Ayy(Fβ) = 38 G

aiso(Fβ) = 35 GThe simulation was based on planar geometry around Cα in the RCβF2CαF2

• radical

• 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

2005 Paper Revisited: Automatic Fitting + DFT

g-tensor:2.0030, 2.0023, 2.0023 giso = 2.0025 (fixed)n(Fα)=2222, 18, 18 G, aiso(Fα) = 86 G (fixed) n(Fβ)=2Fβ-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

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

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

3000 3100 3200 3300 3400 3500 3600 3700

Best fit

Magnetic Field / G

Experimental

All tensors coaxial

Planar around C

Pyramidal around Ca

Or C

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.

18

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.

• Danilczuk, M.; Coms, F.D.; Schlick, S. Fuel Cells 2008, 8(6), 436-452.

19

DMPO – CF3SO3H Adducts

3320 3340 3360 3380 3400

33

2 2 2

11111

0.2%

9.8%

90%

Adducts

2

Magnetic Field / G

Exp

Sim

DMPO/CCR (1)

DMPO/OH (2)

DMPO/Degrad (3)

1

3

294 K (ESR and irradiation)

pH=1.12, in situ irrad, 10 min

Adduct giso aN / G aH / G

CCROH

Degrad

2.00542.0052

15.8 14.85

14.1

22.8 14.85

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

20

CF3CF2OCF2CF2SO3H /DMPO/H2O2

CF2CF3 or CF2CF2SO3- ?

The CCR2 adduct appears after longer irradiation time

3325 3350 3375 3400

Magnetic Field / G

Exp

Sim

DMPO/OH

DMPO/CCR 1

DMPO/CCR 2

DMPO/CCR1: aN= 15.8 G, aH = 22.6 G

DMPO/CCR2: aN= 14.8 G, aH = 20.6 G

Relative Conc. (%)Assignment

N

Me

Me

H

OH

O

DMPO/OH 20

DMPO/CCR1 40

DMPO/CCR2 40

N

Me

Me

H

R

O

We cannot determine the exact structure of CCR1 and CCR2.

21

MNP as a Spin Trap

MNP (2-methyl-2-nitrosopropane) MNP/R

____________________________________________________

• 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.

MNP is bought as a dimer, and dissociates in solution

(CH3)3C N O + R (CH3)3C N O

R

(CH3)3C N N C(CH3)3

O O

(CH3)3C N O2

•

R is close to 14N, therefore we can deduce details on its structure

22

CF3CF2OCF2CF2SO3H(0.1 M)/MNP/H2O2

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

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

Relative Conc. Tentativeassignment

89%

3330 3340 3350 3360 3370 3380 3390

Magnetic Field / G

Exp

Sim

1 1 1 1 1 1 1 1 1

22

2

pH = 7, UV and ESR at 300 K

(H3C)3C N CF2CF2R

O

____________________________________

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

(H3C)3C N C(CH3)3

O

11%

23

CF3CF2OCF2CF2SO3H (2 M)/MNP/H2O2

3330 3340 3350 3360 3370 3380 3390

Magnetic Field / G

2 2 2

1 1 1 1 1 1

Exp

SimMNP/F

pH = 7, UV and ESR at 300 K

(H3C)3C N F

O

1:MNP/F:aN = 16.6G,aF = 21.8 G

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

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

24

Model Compounds

Summary of Radicals Detected as Spin Adducts in Experiments with DMPO and MNP as the Spin Traps

Model CompoundSpin Trap*

DMPO MNP

CH3COOH (AA) CCR(1) •CH2COOH

CF3COOH (TFAA) CCR(1) F•OCF2R and •OCF2CF2R

CF3SO3H (TFSA) CCR(1) F•OCF2R and •OCF2CF2R

CF2HCOOH (DFAA) CCRs(2) •CF2COOH•OCF2R

CF3CF2OCF2CF2SO3H (PFEESA)

CCRs(2) F•OCF2R and •OCF2CF2R

* Numbers in parentheses indicates the numbers of spin adducts

25

Attack of HO Radicals on Carboxylic and Sulfonic Acid Groups

?

N ON

O•

CF3

•CF3

CO2

CF3CO2• CF3CO2H + •OH

H2O

CF3CO2- + •OH

H2O

H+

CF3CO2•

CO2

N

O•

OCF2OHMNP

N

O•

F[ox]

MNP

C O•F

HO

F

●OH

CF2O + HF

●OH

CF3OH+ H2O2●CF3

●OHH2O2

CHF3 + ●OOH

CF3SO3HH2O2

hn CF3SO3

SO3

CF3 CF3OH CF2O

HFH2O2

CHF2CO2

CO2

CHF2 CHF2OH OH

H2O

CF2OH CF2O

H2O2

Formation of CF3 radicals in

TFAA

From CF3 to an oxygen-

centered radical (OCR) and corresponding MNP adduct, and the MNP/F adduct

Formation of an oxygen-centered radical (OCR) in TFSA, and the OCR and F adducts

Formation of an oxygen-centered radical (OCR) in DFAA. The MNP/F adduct is not expected, and was not detected experimentally

26

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 Nearthe Ether Group

27

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.

28

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 (a gift from Cortney Mittelsteadt, Giner Electrochemical Systems)

• V = 600-800 mV

• Operating time: up to 6 h

• Gas flows O2: 2 cm3/minH2 and D2 : 4 cm3/min

(Homage to Emil Roduner)

• Danilczuk, M.; Coms, F.D; Schlick, S., to be submitted.

29

ESR Spectra of DMPO Adducts, Cathode

3300 3320 3340 3360 3380 3400 3420

Magnetic Field / G

CCV

OCV

DMPO/OH

DMPO/OOH

Cathode side/H2/20 min

• DMPO/OH (CCV) and DMPO/OOH (OCV).

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

H• + O2 → HOO•

(chemical formation of HOO•) or by chemical formation of H2O2.

• Can detect separately adducts at cathode and anode.

30

ESR Spectra of DMPO Adducts, Cathode, CCV, H2

3360 3380 3400 3420 3440 3460

(A) 0-360 min

Magnetic Field / G

0 min

120 min

240 min

360 min

3360 3380 3400 3420 3440 3460

(B) 360 min

Magnetic Field / G

Exp

Sim

DMPO/H

DMPO/OOH

DMPO/CCR20%

44%

36%

H• adduct

Carbon-centered radical adduct (CCR)

H atoms

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

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

HO· + H2O2 → HOO· + H2O electrochemically

H· + O2 → HOO· chemically

HO• adduct

31

ESR Spectra of DMPO Adducts, Cathode, CCV, D2

3300 3320 3340 3360 3380 3400 3420

Magnetic Field / G

1 1 1 1

2 2

2

1 1 11

2 22

3 3

33

4 45 5

0 min

120 min

240 min

360 min

(A) 0-360 min

3300 3320 3340 3360 3380 3400 3420

15%

25%

10%

(B) 360 min

Magnetic Field / G

Exp

Sim

DMPO/OOH

DMPO/CCR

DMPO/H

DMPO/D

50%

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

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

32

ESR Spectra of DMPO Adducts, Anode, H2 vs D2

3300 3320 3340 3360 3380 3400 3420

2222

11 1111

1: DMPO/OOH2. DMPO/HOCV

CCVDMPO/H

(A) Anode side/H2/20 min

Magnetic Field / G

3300 3320 3340 3360 3380 3400 3420

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

Magnetic Field / G

CCV

OCV

Exp

sim

DMPO/H

DMPO/D

1: DMPO/OOH2: DMPO/H

1 1

11

22 2 2 22 2 2

2

1 1

• 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 oxygen to

produce HOO· • 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 experiments.

33

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

Results Processes

HO•/CCV/cathode

HOO•/CCV/cathode

Leading to H• or D• OCV/anode

H•,D•/CCV/cathode/anode

HOO•/OCV/cathode

O2 + 2H+ + 2e- H2O2 (electrochemical H2O2 formation) (1)

H2O2 2 HO• (electrochemical HO• formation) (2)

H2O2 + HO• HOO• + H2O(electrochemical HOO• formation) (3)

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

HO• + H2 (D2) → H2O + H• (D•)(Chemical H• and D• formation) (5)

H• + O2 → HOO• (chemical HOO• formation) (6)

Crossover Processes

34

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

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

36

In Situ Studies: Abstraction of Fluorine Atom by H•

(CF2CF2)mCF2CFCF2CF2

OCF2CFOCF2CF2SO3H

CF3

+ H• → CF2CF2CF2C.

OCF2CFOCF2CF2SO3H

CF3

CF2CF2

+ HF

↓

___________________________________________

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

• Attack of sulfonic groups (by HO• radicals and Fe(III))

• Main chain and side chain scission (by H• ) → HF

•

__________________________________________

• Coms, F.D. ECS Transactions 2008, 16(2) 235-255.

↓

37

UDM Group 2008

38

National Science Foundation (Polymers, Instrumentation, International Programs)

Fuel Cell Activities of General Motors

US Department of Energy

Ford Motor Company

Support