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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
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