3352
Macromolecules 2010, 43, 3352–3358 DOI: 10.1021/ma1001386
Ranking the Stability of Perfluorinated Membranes Used in Fuel Cells to Attack by Hydroxyl Radicals and the Effect of Ce(III): A Competitive Kinetics Approach Based on Spin Trapping ESR Marek Danilczuk, Andrew J. Perkowski,† and Shulamith Schlick* Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols Road, Detroit, Michigan 48221. †NSF Research Experience for Undergraduate (REU) Fellow. Received January 19, 2010; Revised Manuscript Received February 13, 2010
ABSTRACT: The stability of Nafion, stabilized Nafion (StNafion), and 3M and Aquivion membranes to attack by the hydroxyl radical, HO•, was compared in their aqueous dispersions at 300 K. HO• radicals were generated by UV irradiation of hydrogen peroxide (H2O2), and radicals were detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Two types of adducts were detected in all dispersions: DMPO/ OH and DMPO/CCR, an adduct of carbon-centered radicals (CCRs) derived from the polymers. The competitive kinetics (CK) approach that has been adapted for ranking the polymer stability leads to the determination of their reaction rate constant with hydroxyl radicals. The experiments consisted of measuring the concentration of the DMPO/OH adduct in the presence of the polymers, which are considered to be “competitors” that react with HO• radicals and compete with the spin trapping reaction HO• þ DMPO f DMPO/OH. The results indicated an improved stability in StNafion compared with Nafion. The largest effect was, however, detected for the 3M and Aquivion polymers, which were significantly more stable. Examination of the hyperfine splittings of DMPO/CCR and of published reports suggests that the absence of the ether group and of the tertiary carbon in the side chain appears to be responsible for the greater stability of these ionomers. The addition of Ce(III) to the polymer dispersions led to a lower total concentration of the adducts DMPO/OH and DMPO/CCR. This result provided evidence for scavenging of HO• radicals by Ce(III) that also leads to a lower concentration of the DMPO/CCR adduct. The CK approach was used to determine the reaction rate constant of Ce(III) with the HO• radicals: kCe = 2.8 108 M-1 s-1, in good agreement with the literature value of 3 108 M-1 s-1. The present experiments reaffirmed the centrality of HO• radicals as an aggressive oxygen species involved in membrane degradation, the important effect of the side chain structure on membrane stability, and the stabilizing presence of Ce(III).
Introduction Ionomeric membranes perform a central role in the operation of fuel cells: by separating the anode and cathode sections, by allowing hydrogen cations to diffuse from the anode to the cathode, and by blocking the flow of reduced oxygen species from the cathode to the anode.1 The “traffic light” role of these proton exchange membranes (PEMs) is efficient when the integrity of the membrane molecular structure is maintained under the strongly oxidizing conditions of a fuel cell (FC). Whereas perfluorinated ionomers such as Nafion have held center stage in the operation of FCs, sulfonated hydrocarbon membranes have recently become valid alternatives.2 The durability of PEMs is a much studied topic in the current literature, and the major goals are to determine the nature of the attacking species and the site of attack.3 Whereas initial studies on the degradation of perfluorinated membranes containing side chains terminated by sulfonic groups (PFSA) have concluded that the degradation mechanism is by the unzipping of the membrane backbone due to attack of hydroxyl radicals on endchain -COOH groups,4,5 recent studies have provided evidence for the vulnerability of the Nafion side chain to degradation.6,7 Our studies have focused on early events in the degradation of PFSAs and model compounds by detection of the initially generated free radicals using direct electron spin resonance (ESR) *Corresponding author. Tel: 1-313-993-1012. Fax: 1-313-993-1144. E-mail:
[email protected]. pubs.acs.org/Macromolecules
Published on Web 02/19/2010
and spin trapping ESR.8-13 In an important development in our studies, we have studied the radical species in an FC inserted in the resonator of the ESR spectrometer and detected separately at the anode and at the cathode the presence of hydroxyl radicals (HO•), hydroperoxyl radicals (HOO•), hydrogen and deuterium atoms (H• and D•), and carbon-centered radicals (CCRs) derived from Nafion.14 These species were detected at 300 K by spin trapping with DMPO (Scheme 1). The capture of short-lived radicals by the diamagnetic spin trap leads to the formation of a paramagnetic spin adduct with higher stability: a nitroxide radical. In most cases, the identification of the trapped radical is based on simulating the experimental ESR spectra, determining the hyperfine splitting (hfs) from 14N (aN) and from Hβ protons (aHβ), and comparing the results with the literature, including the Spin Trapping Database.15 The centrality of hydroxyl radicals in the degradation mechanism of PFSAs was a major conclusion from our results: Hydrogen peroxide (H2O2) is formed in FCs in small concentrations but is not considered to be a main damaging agent. HO• is the most reactive of the reactive oxygen species and in most cases the only attacking species.16 This latter idea was a major conclusion of our recent study on the effect of Ce(III) as a neutralizing cation for a Nafion-based membrane-electrode assembly (MEA/Ce) in an FC inserted in the ESR resonater. The stabilizing effect of Ce(III) was clearly documented: in contrast with results of the study of the MEA/H FC,14 no HO• radicals and no membrane-derived CCRs were detected. We note that the DMPO/OOH was r 2010 American Chemical Society
Article
Macromolecules, Vol. 43, No. 7, 2010
Scheme 1. Spin Trapping by 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO)
Chart 1. Nafion, 3M, and Aquivion Membranes
300 K in situ using the Oriel Xe-free UV source equipped with the UG-11 bandpass optical filter. Electron Spin Resonance Measurements. These measurements were carried out at 300 K using a Bruker X-band EMX spectrometer operating at 9.7 GHz and 100 kHz magnetic field modulation and equipped with an ER 4105DR double rectangular resonator and ER 4111 VT variable temperature controller. The ESR spectra were simulated using the WinSim software package,22 which also calculates the relative intensity of each spectral component when the ESR spectrum consists of a superposition of contributions from several spin adducts. Typical margin of error in the determination of relative intensities is about (5%. Competitive Kinetics (CK). The rate constant for the reaction between the perfluorinated ionomers and hydroxyl radicals was determined by measuring the intensity of the DMPO/OH adduct as a function of UV irradiation time first in the absence and then in the presence of the “competitor, C” whose reaction rate constant is determined. The major kinetic steps in the early stages of the reaction are HO• þ DMPO f DMPO=OH
ðrate constant kDMPO Þ
HO• þ C f C• or other products
detected, thus proving that HOO• is a less aggressive oxidant. Several recent studies have demonstrated that small amounts of Ce(III) and Mn(II) ions as well as metal oxides to the membrane can significantly reduce the rate of degradation.17-20 Our study of the MEA/Ce FC has illuminated the in situ degradation mechanism of membrane stabilization by cationic additives. In the present study, we focus on the effect of PFSA structure on the rate of reaction with HO• radicals for Nafion, stabilized Nafion (StNafion), and 3M and Aquivion membranes as aqueous dispersions at 300 K. The hydroxyl radicals were generated by UV irradiation of H2O2, and the radicals were detected by spin trapping with DMPO. Nafion, 3M, and Aquivion membranes are shown in Chart 1. StNafion is a chemically stabilized Nafion, in which the concentration of -COOH end groups was reduced. The reactivity of the four membranes was compared by a kinetic approach based on the competition for HO• radicals between the spin trap and the membrane. As described below, this study has evidenced the much higher stability of the 3M and Aquivion membranes compared with Nafion and StNafion. Experimental Section Materials. High purity DMPO from Alexis Biochemicals was used as received. Aqueous solutions of H2O2 (3%) were from Fisher Scientific. Methanol 99.8% and CeCl3 were purchased from Sigma-Aldrich. The Nafion dispersion (N117, 10 wt %) was also from SigmaAldrich. The stabilized Nafion aqueous dispersion (StNafion 117, D 1021, 10 wt %) was obtained from Ion Power, Inc. The 3M membrane aqueous dispersion (F12217, EW 825, 18 wt %) was from Steven Hamrock of 3M Co. The Aquivion dispersion (D83-20B, 20 wt %, EW 830) was supplied by Ajedium Films, Newark, DE (a division of Solvay-Solexis). Deionized “ultra pure” water with low conductivity and
