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Shulamith Schlick , Marek Danilczuk and Frank D. Coms

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

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Shulamith Schlick , Marek Danilczuk and Frank D. Coms

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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 Systems Asilomar, 15-18 February 2009

  2. “Water is the fuel of the future”Jules Verne, 1874 FC bus – Project CUTE London 2006 Driving the GM Equinox - 2008 Kms in nine cities CUTE, Barcelona

  3. 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 expectedHO· + 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= hv = gβeH0 =E ____________________________________________________ P. Atkins, Physical Chemistry, W.H. Freeman; New York, 1998

  5. Fluorinated PEMs Nafion Dow, Solvay-Solexis 3M 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 bundle aggregate (A) (B) • 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. Macromolecules2004, 37, 5327-5336.

  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 Cells2005, 5, 302. • Zhou, C.; Guerra, M. A.; Qiu, Z.-M.; Zawodzinski, T. A.; Schiraldi, D. A. Macromolecules 2007, 40, 8695-8707.

  8. Plan of Lecture • Objectives and Approach • Results • 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

  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 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. B2004, 108, 4332.

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

  12. How it Works: DMPO + R•  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. • 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).

  13. Membranes / Direct ESR

  14. The Chain End Radical in Nafion/Fe(II) /H2O2 and Nafion/Fe(III): RCF2CF2• gzz = 2.0030, gxx = gyy = 2.0023 giso = 2.0025 n(Fα)=2 Azz(Fα) = 222 G Axx(Fα)= Ayy(Fα) = 18 G aiso(Fα) = 86 G n(Fβ)=2 Azz(Fβ) = 30 G Axx(Fβ)= Ayy(Fβ) = 38 G aiso(Fβ) = 35 G The simulation was based on planar geometry around Cαin the RCβF2CαF2• radical • Kadirov, M.V.; Bosnjakovic, A.; Schlick, S. J. Phys. Chem. B2005, 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 Or C 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 • The simulation indicated an angle of 12° between the largest principal values of the two Fα nuclei: a pyramidal geometry • Lund, A.; Macomber, L.D.; Danilczuk, M.; Stevens, J.E.; Schlick, S. J. Phys. Chem. B 2007, 111, 9484-9491.

  16. DFT ResultsBased 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 Sources2009, 186, 334-338.

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

  18. DMPO – CF3SO3H Adducts 294 K (ESR and irradiation) pH=1.12, in situ irrad, 10 min Adducts of carbon-centered radicals were detected in all model compounds.

  19. CF3CF2OCF2CF2SO3H /DMPO/H2O2 The CCR2 adduct appears after longer irradiation time CF2CF3 or CF2CF2SO3- ? DMPO/CCR1: aN= 15.8 G, aH = 22.6 G DMPO/CCR2: aN= 14.8 G, aH= 20.6 G Relative Conc. (%) Assignment DMPO/OH 20 DMPO/CCR1 40 DMPO/CCR2 40 We cannot determine the exact structure of CCR1 and CCR2.

  20. MNP as a Spin Trap • MNP (2-methyl-2-nitrosopropane) 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 • ____________________________________________________ • 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.

  21. CF3CF2OCF2CF2SO3H(0.1 M)/MNP/H2O2 pH = 7, UV and ESR at 300 K 1:MNP/R:aN = 16.57G,aF= 11.52G(2F), aF = 0.5G(2F) 2: Di-tert-butyl nitroxide (DTBN), aN = 17.1 G Tentative assignment Relative Conc. 89% 11% • ____________________________________ • Pfab, J. Tetrahedron Letters1978,19 (9), 843.

  22. CF3CF2OCF2CF2SO3H(2 M)/MNP/H2O2 pH = 7, UV and ESR at 300 K 1:MNP/F:aN = 16.6G,aF= 21.8 G 2: Di-tert-butyl nitroxide (DTBN), aN = 17.1 G MNP/F The MNP/F adduct is detected at higher PFEESA concentration.

  23. Model Compounds Summary of Radicals Detected as Spin Adducts in Experiments with DMPO and MNP as the Spin Traps * Numbers in parentheses indicates the numbers of spin adducts

  24. SO3 CO2 • H2O O HF ●OH H2O2 H2O2 H2O2 N MNP CHF2CO2 CF3SO3H CF3SO3 CHF2 CF3 CF3OH CHF2OH CF2OH CF2O F CF2O + H ●CF O CF OH CF O + HF n OH h 3 2 2 3 2 [ox] H O ●OH ●OH • 2 2 O F MNP F N C O H CHF3 + ●OOH • 2 H O C O O F Attack of HO Radicals on Carboxylic and Sulfonic Acid Groups 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

  25. 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 the Ether Group

  26. 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.

  27. 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/min • H2 and D2 : 4 cm3/min • (Homage to Emil Roduner) • Danilczuk, M.; Coms, F.D; Schlick, S., to be submitted.

  28. ESR Spectra of DMPO Adducts, Cathode • 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.

  29. ESR Spectra of DMPO Adducts, Cathode, CCV, H2 HO• adduct • 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 H• adduct Carbon-centered radical adduct (CCR)

  30. ESR Spectra of DMPO Adducts, Cathode, CCV, D2 Both DMPO/H and DMPO/D adducts with D2 at anode. Assignments: 1-DMPO/OOH, 2-DMPO/Degr, 3-DMPO/CCR, 4-DMPO/H, 5-DMPO/D.

  31. ESR Spectra of DMPO Adducts, Anode, H2 vs D2 • 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.

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

  33. 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?

  34. 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

  35. In Situ Studies: Abstraction of Fluorine Atom by H• ↓ + H• → ↓ + 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 Transactions2008, 16(2) 235-255.

  36. UDM Group 2008

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

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