Presentation on theme: "1 Visualizing Chemical and Electrochemical Reactions, and Crossover Processes in a Fuel Cell by Spin Trapping ESR Methods Shulamith Schlick, Marek Danilczuk."— Presentation transcript:
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 Kms in nine cities CUTE, Barcelona FC bus – Project CUTE London 2006 Driving the GM Equinox - 2008
3 Reactions in Fuel Cells Cathode Four-electron reduction of oxygen: O 2 + 4H + + 4e - 2H 2 O Anode Oxidation of hydrogen: 2H 2 4H + + 4e - Complications Two-electron reduction of oxygen: O 2 + 2H + + 2e - H 2 O 2 Also expected HO· + H 2 O 2 HO 2 · + H 2 O and, in neutral solutions, HO 2 · + H 2 O O 2 · + H 3 O + ) HO·, HO 2 ·, and O 2 · 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=E ____________________________________________________ P. Atkins, Physical Chemistry, W.H. Freeman; New York, 1998 E= hv = gβ e H 0
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 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 CF 2 group in each step. (a) R F -CF 2 COOH + HO· R F -CF 2 · + CO 2 + H 2 O (b) R F -CF 2 · + HO· R F -CF 2 OH R F -COF + HF (c) R F -COF + H 2 O R F -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 Reaction H 2 O 2 + Fe(II) Fe(III) + HO + HO Fe(II) + O 2 ↔ Fe(III) + O 2 HO + H 2 O 2 HOO + H 2 O Photo-Fenton Reaction (UV Irradiation) Fe(III) + H 2 O Fe(II) + H + + HO Fe(II) + O 2 ↔ Fe(III) + O 2 O 2 + H + ↔ HOO Peroxide Decomposition by Heat or UV H 2 O 2 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 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 (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 + 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 14 N 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.
14 The Chain End Radical in Nafion/Fe(II) /H 2 O 2 and Nafion/Fe(III): RCF 2 CF 2 g zz = 2.0030, g xx = g yy = 2.0023 g iso = 2.0025 n(Fα)=2 A zz (Fα) = 222 G A xx (Fα)= A yy (Fα) = 18 G a iso (Fα) = 86 G n(Fβ)=2 A zz (Fβ) = 30 G A xx (Fβ)= A yy (Fβ) = 38 G a iso (Fβ) = 35 G The simulation was based on planar geometry around C α in the RC β F 2 C α F 2 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 g iso = 2.0025 (fixed) n(Fα)=2 222, 18, 18 G, a iso (Fα) = 86 G (fixed) n(Fβ)=2 Fβ-1: 34,3, 25,5, 15.0 G, a iso = 24.9 G Fβ-2: 29.3,23.4, 29.9 G, a iso = 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 Or C
16 DFT Results Based on two model structures: CF 3 OCF 2 CF 2 (R SC, radical on side chain) and CF 3 CF 2 CF 2 CF 2 (R MC, 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 Nafion Fragmentation “Quintet” “Quartet” RCF 2 CF 2 Side-Chain Radical The radical fragments suggest attack at or near the pendant chain
18 Model Compounds CH 3 COOH (acetic acid, AA) CF 2 HCOOH (difluoroacetic acid, DFAA) CF 3 COOH (trifluoroacetic acid, TFAA) CF 3 SO 3 H (trifluorosulfonic acid, TFSA) CF 3 CF 2 OCF 2 CF 2 SO 3 H (perfluro-(2-ethoxyethane)sulfonic acid, PFEESA) HO was generated by UV-irradiation of H 2 O 2 __________________________________________________________________ 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 – CF 3 SO 3 H Adducts 294 K (ESR and irradiation) pH=1.12, in situ irrad, 10 min Adductg iso a N / Ga H / G CCR OH Degrad 2.0054 2.0052 15.8 14.85 14.1 22.8 14.85 Adducts of carbon-centered radicals were detected in all model compounds.
20 CF 3 CF 2 OCF 2 CF 2 SO 3 H /DMPO/H 2 O 2 CF 2 CF 3 or CF 2 CF 2 SO 3 - ? The CCR2 adduct appears after longer irradiation time DMPO/CCR1: a N = 15.8 G, a H = 22.6 G DMPO/CCR2: a N = 14.8 G, a H = 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.
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 R is close to 14 N, therefore we can deduce details on its structure
22 CF 3 CF 2 OCF 2 CF 2 SO 3 H(0.1 M)/MNP/H 2 O 2 1:MNP/R: a N = 16.57G,a F = 11.52G(2F), a F = 0.5G(2F) 2: Di-tert-butyl nitroxide (DTBN), a N = 17.1 G Relative Conc. Tentative assignment 89% pH = 7, UV and ESR at 300 K ____________________________________ Pfab, J. Tetrahedron Letters 1978, 19 (9), 843. 11%
23 CF 3 CF 2 OCF 2 CF 2 SO 3 H ( 2 M)/MNP/H 2 O 2 MNP/F pH = 7, UV and ESR at 300 K 1:MNP/F:a N = 16.6G,a F = 21.8 G 2: Di-tert-butyl nitroxide (DTBN), a N = 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 Compound Spin Trap* DMPOMNP CH 3 COOH (AA)CCR(1) CH 2 COOH CF 3 COOH (TFAA)CCR(1)F OCF 2 R and OCF 2 CF 2 R CF 3 SO 3 H (TFSA)CCR(1)F OCF 2 R and OCF 2 CF 2 R CF 2 HCOOH (DFAA)CCRs(2) CF 2 COOH OCF 2 R CF 3 CF 2 OCF 2 CF 2 SO 3 H (PFEESA) CCRs(2) F OCF 2 R and OCF 2 CF 2 R * Numbers in parentheses indicates the numbers of spin adducts
25 Attack of HO Radicals on Carboxylic and Sulfonic Acid Groups N O O C F 2 OH MNP N O F [ox] MNP CO F HO F ● OH CF 2 O + HF ● OH CF 3 OH + H 2 O 2 ● CF 3 ● OHH 2 O 2 CHF3 + ● OOH CF 3 SO 3 H H2O2H2O2 h n CF 3 SO 3 SO 3 CF 3 CF 3 OH CF 2 O HF H2O2H2O2 CHF 2 CO 2 CO 2 CHF 2 CHF 2 OH OH H2OH2O CF 2 OH CF 2 O H2O2H2O2 Formation of CF 3 radicals in TFAA From CF 3 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 CH 3 COOH : The site of attack by HO is the CH 3 group CF 2 HCOOH : The sites of attack by HO are H in the CHF 2 and COOH groups CF 3 COOH : The site of attack by HO are H in the COOH group CF 3 SO 3 H : The site of attack by HO are H in the SO 3 H group CF 3 CF 2 OCF 2 CF 2 SO 3 H: Probably H in the SO 3 H, and Near the 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 19 F 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 H 2 O 2 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/cm 2 (a gift from Cortney Mittelsteadt, Giner Electrochemical Systems) V = 600-800 mV Operating time: up to 6 h Gas flows O 2 : 2 cm 3 /min H 2 and D 2 : 4 cm 3 /min (Homage to Emil Roduner) Danilczuk, M.; Coms, F.D; Schlick, S., to be submitted.
29 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 O 2 and H atoms (OCV): H + O 2 → HOO (chemical formation of HOO ) or by chemical formation of H 2 O 2. Can detect separately adducts at cathode and anode.
30 ESR Spectra of DMPO Adducts, Cathode, CCV, H 2 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· + H 2 O 2 → HOO· + H 2 O electrochemically H· + O 2 → HOO· chemically HO adduct
31 ESR Spectra of DMPO Adducts, Cathode, CCV, D 2 Assignments: 1-DMPO/OOH, 2-DMPO/Degr, 3-DMPO/CCR, 4-DMPO/H, 5-DMPO/D. Both DMPO/H and DMPO/D adducts with D 2 at anode.
32 ESR Spectra of DMPO Adducts, Anode, H 2 vs D 2 H 2. 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· D 2. 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 ResultsProcesses HO /CCV/cathode HOO /CCV/cathode Leading to H or D OCV/anode H,D/CCV/cathode/anode HOO /OCV/cathode O 2 + 2H + + 2e - H 2 O 2 (electrochemical H 2 O 2 formation) (1) H 2 O 2 2 HO (electrochemical HO formation) (2) H 2 O 2 + HO HOO + H 2 O (electrochemical HOO formation) (3) H 2 + O 2 → 2HO (Chemical HO formation on catalyst) (4) HO + H 2 (D 2 ) → H 2 O + H (D ) (Chemical H and D formation) (5) H + O 2 → 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 H 2 and D 2 to the cathode and O 2 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 D 2 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 + 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 Transactions 2008, 16(2) 235-255. ↓