Scavenging of Hydroxyl Radicals by Ceria Nanoparticles: Effect of

Mar 8, 2016 - Ford Research and Advanced Engineering, Ford Motor Company, MD 3179, ... hydroxyl radicals as a function of their size and concentration...
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Scavenging of Hydroxyl Radicals by Ceria Nanoparticles: Effect of Particle Size and Concentration Shulamith Schlick,*,† Marek Danilczuk,† Andrew R. Drews,‡ and Ratandeep S. Kukreja§ †

Department of Chemistry and Biochemistry, University of Detroit Mercy, 4001 West McNichols, Detroit, Michigan 48221, United States ‡ Ford Research and Advanced Engineering, Ford Motor Company, MD 3179, P.O. Box 2053, Dearborn, Michigan 48121, United States § General Motors Technical Center, 30500 Mound Road, Warren, Michigan 48090, United States ABSTRACT: The most commonly used proton exchange membranes (PEMs) for hydrogen and direct methanol fuel cells are the perfluorosulfonic acid ionomers (PFSA) such as Nafion, which are based on a Teflon-like backbone with side chains containing sulfonic groups. These materials exhibit good mechanical, chemical, and thermal stability in both oxidative and reductive media; however, the fuel cell is a reactor with strong oxidizing power, capable of reducing the durability of the PEMs. The centrality of hydroxyl radicals, HO·, in the degradation mechanism of PFSA is well documented in numerous papers, including our work. Therefore, membrane durability remains a critical issue for further development of fuel cell applications. We present the use of ceria nanoparticles (NPs) for mitigating the chemical degradation process and describe their effectiveness for scavenging hydroxyl radicals as a function of their size and concentrations. The hydroxyl radicals were generated by UV-irradiation of hydrogen peroxide, and the radicals were detected by spin trapping electron spin resonance (ESR) with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trap. The size of the ceria NPs was in the range 7.1−14.7 nm, as determined by X-ray diffraction (XRD), and the molar concentration of ceria was varied between 0 and 200 μM. The ESR results indicated that larger NPs are more effective scavengers of the HO· radicals. Transmission electron microscopy (TEM) data measured in the scanning TEM (STEM) mode showed pronounced agglomeration of the smaller NPs. Consideration of both TEM and ESR results suggests that the agglomerated particles do not contribute fully to radical scavenging. This conclusion is due to the short diffusion distances of short-lived species such as hydroxyl radicals, typically 4−6 nm, compared to 10−20 nm for stable species. The results of this study indicated that scavenging of hydroxyl radicals is more effective for larger NPs.

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INTRODUCTION The proton exchange membrane (PEM) is an essential component of a fuel cell (FC), and its role is crucial: to separate the cathode and anode compartments, to allow efficient proton transport from the anode to the cathode, and to block the flow of partially reduced oxygen species from the cathode to the anode.1 The most commonly used PEMs for hydrogen and direct methanol fuel cells are perfluorosulfonic acid ionomers (PFSA) based on a Teflon-like backbone with side chains containing sulfonic groups; examples include Nafion, Aquivion, and 3M membranes, as seen in Chart 1, where EW is the mass that contains 1 mol of sulfonic groups. These materials exhibit good mechanical, chemical, and thermal stability in both oxidative and reductive media; however, the FC is a reactor with strong oxidizing power, capable of reducing the durability of the PEMs.2 Therefore, membrane durability remains a critical issue in fuel cell systems and is usually assessed by a combination of ex situ methods such as the Fenton test3,4 and in situ measurements of accelerated lifetime at high temperature, typically 370 K, low humidity (50% or less), and open circuit voltage (OCV) operating conditions.5 © XXXX American Chemical Society

Chart 1. Nafion, 3M, and Aquivion Membranes

In situ experiments at 300 K in a FC operating with Nafion and inserted in the resonator of the electron spin resonance (ESR) spectrometer have the ability to monitor chemical processes leading to the generation of reactive oxygen intermediates, hydrogen atoms, and carbon-centered radicals (CCRs) resulting from membrane fragmentation. The identity Received: January 13, 2016 Revised: February 22, 2016

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in the mitigation mechanism provided by Ce(III). The in situ FC was operated at 300 K under CCV and OCV conditions, with a membrane-electrode assembly (MEA) based on Nafion 117 and Pt as a catalyst (referred to as MEA/Ce). The results were compared with in situ results in a FC based on Nafion 117 in the acid form, MEA/H, which served as control. The differences between the experiments performed with the two types of membranes were significant: (a) Spin trapping indicated the presence of different radicals: HO· for MEA/H and HOO· for MEA/Ce. (b) DMPO/CCR, an adduct of a carbon-centered radical (CCR) generated by membrane fragmentation, was absent for the MEA/Ce FC but was detected in the MEA/H FC under similar operating conditions. (c) The intensity of all adducts of reactive intermediates was much lower in the MEA/Ce FC. The absence of DMPO/OH for the MEA/Ce was explained by Ce(III) scavenging of the aggressive HO· radicals, a process that also generates Ce(IV). The presence of DMPO/OOH for MEA/Ce was rationalized by Ce(IV) oxidation of H2O2, leading to the formation of HOO·. The absence of DMPO/CCR is a result of HO· scavenging and the presence of the less aggressive oxidant, HOO·, in MEA/Ce. The low intensity of all adducts is also a result of HO· scavenging by Ce(III). The conclusion from this experiment was that Ce(III) is an effective and reversible HO· scavenger because of the Ce(III)/Ce(IV) couple redox chemistry, as shown in the following proposed mechanism.

of oxygen radicals such as hydroxyl and hydroperoxyl radicals, HO· and HOO·, respectively, and other intermediates was studied by spin trapping ESR using 5,5-dimethyl-1-pyrroline-Noxide (DMPO) as a spin trap, as shown in Scheme 1.6,7 Spin Scheme 1. Spin Trapping by 5,5-Dimethyl-1-pyrroline-Noxide (DMPO)

trapping transforms short-lived radical intermediates into more stable nitroxide radicals. An additional advantage of these in situ experiments is the ability to detect separately intermediates at the cathode and anode sides. The DMPO/OH adduct was detected only at the cathode, for closed circuit voltage (CCV) conditions, suggesting that the major source of HO· is its electrochemical formation by the two-electron reduction of oxygen. The DMPO/H adduct was detected in this study for the first time: at the cathode for CCV conditions and at the anode for both CCV and OCV conditions. The DMPO/OOH adduct, also detected in this study for the first time in an operating FC, appeared at the cathode for CCV and OCV operations and at the anode for OCV operation. Adducts of CCR were detected at the cathode and indicated that membrane fragments are generated at temperatures lower than the typical operating temperature of FCs, ≈350 K. Our ex situ and in situ experiments show that the hydroxyl radical, HO·, is an aggressive oxygen radical intermediate that may attack both the main chain and the side chain in PEMs. Taken together, these experiments have formulated three main degradation paths for the PEMs: main chain unzipping and side chain attack (both by hydroxyl radicals attack) and main chain and side chain scission by hydrogen atoms at the tertiary carbon atoms.3−7 Several recent studies have demonstrated that small amounts of Ce(III) and Mn(II) ions, as well as metal oxides added to the membrane, can significantly reduce the rate of degradation.8−13 Endoh showed that the perfluorinated membranes used in FCs, when partially neutralized by Ce(III) and Mn(II) ions, have higher chemical stability in a wide range of temperatures, up to 120 °C, and concluded that the cationic additives increased the membrane stability toward the aggressive hydroxyl radicals.8 Coms et al. demonstrated that the degradation rates of Nafion membranes were dramatically reduced by incorporating Ce(III) or Mn(II) into the membrane, and both additives functioned as scavengers of hydroxyl radicals; the authors also suggested that Ce(III) is more effective compared to Mn(II) due to its faster rate of reaction with HO· radicals.9 Trogadas et al. showed that the degradation rate of Nafion membranes containing ceria nanoparticles (NPs) was more than 1 order of magnitude lower than in their absence10−12 and that the conductivity of Nafion membranes containing these NPs was similar to that of the native membranes. Banham et al. studied the effect of ceria NPs and doped ceria NP size on membrane stability.13 The mechanism by which Ce(III) mitigates the degradation was studied by in situ experiments in a FC inserted in the resonator of the ESR spectrometer during operation with a Nafion membrane that was 10% neutralized by Ce(III).14 The main objective of these experiments was to identify early events

Ce(III) + HO· + H+ → H 2O + Ce(IV)

Ce(IV) + H 2O2 ⇄ Ce(III) + HOO· + H+

On the basis of these results, we were encouraged to explore the performance of ceria nanoparticles (NPs) as HO· scavengers. These NPs are extensively used in the production and purification of hydrogen, in catalytic converters, and in other catalytic applications.15 Although “ceria” is commonly referred to CeO2, (Ce(IV)), experiments suggest that the cerium in the ceria NPs is multivalent. Their efficiency in increasing the membrane stability is based on the relative ease with which cerium can switch oxidation states between Ce(III) and Ce(IV) in the NPs, as shown above in the proposed mechanism.14 A major conclusion from numerous studies is that the properties of ceria NPs are expected to depend on the particle size, which determine the concentration of Ce(III) ions, and that the concentration of Ce(III) relative to Ce(IV) increases as particle size decreases.16 In the present study we measured the intensity of hydroxyl radicals, HO·, in the presence of ceria NPs, with focus on the effect of the size and concentration of the NPs. The hydroxyl radicals were generated by UV-irradiation in an aqueous suspension of ceria NPs containing hydrogen peroxide and DMPO spin trap. The scavenging ability of the NPs was determined by measuring the ESR signal from the DMPO/OH adduct. The size of the ceria NPs was measured by X-ray diffraction (XRD). Contrary to expectations, the ESR results indicated that larger NPs are more effective scavengers of the HO· radicals. To understand these results, we examined the NPs agglomeration by transmission electron microscopy (TEM), as described in Chart 1 and Scheme 1.

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EXPERIMENTAL SECTION Materials. Hexamethylenetetraamine (HMT), cerium(III) nitrate hexahydrate (CeN), hydrogen peroxide (H2O2, 3%), and DMPO were purchased from Sigma-Aldrich. All reagents, B

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with a Cs corrector and operated at an acceleration voltage of 200 kV in the STEM mode. A high angle annular dark (HAADF) detector was used to capture dark field STEM images of the NPs.

of analytical grade, were used as received. Ultrapure Type I deionized (DI) water was supplied by the Direct-Q3 UV Millipore water purification system and used in the preparation of all aqueous samples. Synthesis. The larger ceria NPs (14.7 and 13.7 nm) were obtained by mixing equal volumes of solutions containing 0.0375 M cerium(III) nitrate hexahydrate (CeN) and 0.5 M HMT at ambient temperature for 12 h, followed by centrifugation in order to separate the NPs.17 The yellowish ceria precipitate obtained after centrifugation was washed with water and ethanol, filtered, and oven-dried at 375 K for 30 min. The smaller NPs (9.5 and 7.1 nm) were prepared by mixing the reagents at 365 K for 1 h, followed by extracting the NPs. Before use, the ceria NPs solutions were sonicated for several hours to break up the agglomerates. Sample Preparation. Immediately before the ESR measurements, each sample was prepared by mixing hydrogen peroxide (1.8 mM), DMPO (0.1 mM), and ceria NPs in water at neutral pH and exposing the mixture to UV-irradiation at ambient temperature in order to generate the HO· radicals. ESR 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, equipped with an ER 4105DR double rectangular resonator, and the ER 4111VT variable temperature controller. Typical acquisition parameters for the ESR spectra were sweep width 150 G, time constant 10.24 ms, conversion time 20.48 ms, 2048 points, modulation amplitude 2 G, and receiver gain 1.0 × 104. ESR spectroscopy has been used for radical scavenging studies in biological systems18,19 where the stability of the DMPO/OH adduct was recorded, which is not representative of scavenging. In the present study the generation of DMPO/OH was recorded as well as the effect of the ceria NPs on its intensity. The ESR spectrum of the DMPO/OH adduct consists of four lines with relative intensities 1:2:2:1; the height of the second low field peak of the DMPO/OH adduct was monitored for the purpose of determining the HO· concentration as a function of UV irradiation time. X-ray Diffraction (XRD). XRD measurements were performed on a Rigaku Mini Flex II Desktop diffractometer using Cu Kα radiation generated at 30 kV and 15 mA. Specimens were prepared by dusting ground powdered samples onto an off-axis Si substrate. Diffracted intensities were collected at a scan rate of 3°/min, a step size of 0.01°, and in the angle range 10° ≤ 2θ ≤ 90°. Diffraction patterns were fit based on refinement of the cubic fluorite structure of CeO2. The XRD patterns for all nanoparticles were analyzed by the PDXL software, and the sizes of NPs were calculated using Halder−Wagner method, as given by eq 1. Using the peak widths (β) corrected for instrumental broadening, the linear slope of a plot of β2/tan2 θ as a function of β/(tan θ × sin θ) was determined by fitting and used to calculate the crystallite size L (slope = (Kλ)/L). β2 β Kλ = + 16e 2 2 L tan θ sin θ tan θ

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RESULTS AND DISCUSSION Figure 1 presents the XRD data, which indicate that the NPs have the typical cubic fluorite structure and crystallite sizes of

Figure 1. XRD pattern for ceria nanoparticles of size 14.7, 13.7, 9.5, and 7.1 nm.

14.7, 13.7, 9.5, and 7.1 nm. Here, we assume that the particle size is equal to the crystallite size. TEM imaging (discussed below) supports this assumption qualitatively. Scavenging of Hydroxyl Radicals by Ceria NPs. The inset of Figure 2 presents the ESR spectrum of the DMPO/OH

Figure 2. ESR line intensity of the DMPO/OH adduct in the presence of various concentrations for ceria NPs of size 13.7 nm, recorded after 30 s of UV-irradiation at ambient temperature. Arrows point to the ESR peak selected for determining the concentration of the DMPO/ OH adduct.

adduct, and the arrow points to the signal selected for recording the adduct intensity. The main spectrum shows the adduct intensity as a function of the concentrations of ceria NPs of size 13.7 nm, recorded after 30 s of UV-irradiation at ambient temperature. The control is the ESR spectrum in the absence of the ceria NPs. DMPO/OH adduct formation was studied in the presence of different concentrations of ceria NPs, in the range 0 (control) to 200 μM, as seen in Figure 2.

(1)

In eq 1, K is the Scherrer constant estimated as 1.05, λ is the wavelength of radiation, β is the corrected peak width at half height in radians, and θ is the Bragg angle of the reflection. Transmission Electron Microscopy (TEM). TEM measurements were performed at the General Motors Technical Center in Warren, MI, using a JEOL JEM 2100F microscope C

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Figure 3. ESR intensity of DMPO/OH adducts in the presence of various concentrations of ceria NPs, for NP sizes 14.7, 13.7, 9.5, and 7.1 nm. Contrary to expectations, the scavenging effect is more intense for the largest NP, size 14.7 nm, and decreases for the lower NPs sizes.

Figure 3 presents the ESR intensity of the DMPO/OH adducts as a function of UV irradiation time. We note that for maximum accuracy every line of scavenging shown in Figure 3 is an average of five separate measurements, as was also reported in our previous study.20 As seen in Figure 3, the growth rate of the ESR signal intensity for NPs sizes 14.7, 13.7, 9.5, and 7.1 nm decreases with increasing NP concentrations. Moreover, the growth rate of the signal intensity decreases depending on the NP size: For ceria concentrations of 200 μM, 85 s of UV-irradiation gave an ESR estimate of the DMPO/OH adduct concentration that is reduced to 0.50 of its initial value for 14.7 nm, 0.56 for 13.7 nm, 0.65 for 9.5 nm, and 0.70 for 7.1 nm. This increasing trend in the adduct concentration with decreasing NP size suggests that for equal ceria concentrations larger NPs are more effective at scavenging HO· radicals, despite their smaller available surface area (calculated surface area of 7.1 nm NPs is twice the calculated surface area of 14.7 nm NPs). We note that ref 16 predicted that the concentration of Ce(III) relative to Ce(IV) increases as the particle size decreases and that the scavenging ability is expected to be higher for smaller NPs. As clearly seen in Figure 3, however, in our system the smaller NPs are less ef fective scavengers, a result that is assigned to NPs agglomeration. The NPs agglomeration in the system presented here was investigated by STEM; the results are shown in Figures 4 and 5. STEM micrographs for the four sizes of the NPs are shown in Figure 4. Qualitatively, the STEM images show a trend in primary particle size that is consistent with the XRD crystallite size estimation. However, the smaller NPs (9.5 and 7.1 nm) appear to show a greater degree of agglomeration in comparison with the larger NPs (14.7 and 13.7 nm). Although agglomeration is observed in all four cases, a wider field image

Figure 4. STEM micrographs for ceria nanoparticles of size 14.7, 13.7, 9.5, and 7.1 nm.

(Figure 5) of the larger NPs shows that the majority of particles are in very small (1−5 particle) agglomerates. For the smaller NPs, the TEM images in Figure 4 show large, dense agglomerates composed of many particles (tens to hundreds). The TEM images suggest a possible mechanism for the lower scavenging ability of the smaller NPs: In forming large, dense agglomerates, a disproportionate fraction of their surface area is buried within the agglomerate and rendered inaccessible to D

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conductivities and hydrogen crossover compared to conventional Nafion. However, the fluoride emission rate (FER) from accelerated tests was lowered by more than 1 order of magnitude, suggesting that CeO2 nanoparticles can significantly improve membrane durability. The scavenging properties of ceria NPs can also be tuned by controlling the size of the NPs and incorporation of additional elements, for example Zr.12,23 Wang et al. investigated self-assembled Nafion/CeO2 nanocomposites, with ceria content of 1−5 wt %.24 At high relative humidity the membranes exhibited slightly reduced proton conductivity, and a Fenton test of the Nafion/CeO 2 membranes showed significantly reduced FER compared to Nafion. The dissolution of the NPs in the MEAs in a fuel cell environment has also been considered: Recent studies of CeO2−MEAs with CeO2-coated gas diffusion electrodes showed 6 times longer lifetime of the membrane and 40 times reduced FER in accelerated stress test compared to the MEA without cerium.25 An important result was that most CeO2 on the anode dissolves and ceria ions migrate to the membrane and mitigate the attack of hydroxyl radicals. The experiments presented in this study suggested that the size, dispersion, and agglomeration of the ceria NPs will be critical for their ability to scavenge hydroxyl radicals and to increase the membrane stability. Therefore, the real scavenging test will be to examine these parameters in a fuel cell environment.

Figure 5. Low-magnification STEM micrographs for ceria nanoparticles of size 14.7, 13.7, 9.5, and 7.1 nm.

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radicals diffusing during their lifetime. If the diffusion distance of an HO· radical during its lifetime is viewed as a shell surrounding each NP, any radical within the thickness of a shell has the opportunity to interact with the corresponding NP, but if a radical lies outside the shell of any NP, it is unlikely to interact with any NP. When agglomerates are generated, the interacting shells overlap, and buried NPs are unlikely to contribute to scavenging. In addition, formation of agglomerates depletes the remaining volume of the sample outside of the agglomerates of NPs and increases the distance a radical must diffuse before it encounters a NP. Both processes are significant if the diffusion distance of the radicals is small compared to the agglomerate size. Two studies on hydroxyl radicals in aqueous solutions in the presence of alcohols as scavengers report a lifetime value of ≈4 × 10−9 s and a corresponding diffusion distance of ≈4−6 nm.21,22 While these values are expected to depend on the type and concentration of the scavengers, we note that the reaction rate constant for DMPO trapping of hydroxyl radicals determined is 3.6 × 10−9 M−1 s−1, of the same order as the corresponding value determined for methanol, 9.7 × 10−8 M−1 s−1.20 Based on the STEM images, agglomerates for all NP sizes are about 10 times larger than the expected diffusion distances of radicals (50 nm vs 5 nm); therefore, we expect that the agglomeration may be effective at masking the activity of buried NPs. These results present the behavior of the system described in the above experiments. The mitigating effect of ceria NPs on the chemical degradation of PEMs used in fuel cells is an important question and is expected to be strongly dependent on the dispersion and the size of the NPs within the membrane. The mitigating effect of Ce ions and ceria NPs has encouraged numerous studies. Early events in the mitigation of a PEMFC by Ce(III) have been described in ref 14. Trogadas et al. used commercially available and synthesized CeO2 nanoparticles to prevent membrane degradation.10−12 The nanocomposite membranes exhibited similar proton

CONCLUSIONS Membrane durability remains a critical issue for further development of fuel cell applications. The centrality of hydroxyl radicals, HO·, in the degradation mechanism of perfluorosulfonic acid ionomers (PFSA) membranes is well documented. Ceria NPs have been shown to be effective scavengers of radicals without affecting the conductivity of Nafion membranes. In this study we present the behavior of ceria nanoparticles (NPs) for mitigating the chemical degradation process by measuring the scavenging of the hydroxyl radicals as a function of the size and concentrations of the NPs. The hydroxyl radicals were generated by UV-irradiation of hydrogen peroxide, and the radicals were detected by spin trapping ESR with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin trap. For ceria NPs in the range 7.1−14.7 nm, ESR measurements indicated that larger NPs were more effective scavengers of the HO· radicals compared to equivalent ceria concentrations of the smaller NPs. Although the smaller NPs are expected to have twice the surface area of the larger NPs, their relative activity is only about half of the activity of the larger NPs. Transmission electron microscopy (TEM) images indicated more extensive agglomeration of the smaller NPs. Consideration of both TEM and ESR results suggests that the agglomerated particles do not fully contribute to radical scavenging. This result is not surprising in view of the short lifetimes and diffusion distances of short-lived species such as hydroxyl radicals, typically 4 × 10−9 s and 4−6 nm, respectively. This study has revealed that the effectiveness of Ce NPs may depend not only on the initial particle size but also on particle dispersion and agglomeration. An additional important factor is the cerium oxidation state, which is outside the scope of this study.

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AUTHOR INFORMATION

Corresponding Author

*Tel 1-313-993-1012; e-mail [email protected] (S.S.). E

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Disclaimer: While this article is believed to contain correct information, Ford Motor Company (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of Ford. This disclaimer may not be removed, altered, superseded or modified without prior Ford permission. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Polymers Program of the National Science Foundation and by an unrestricted grant from the University Research Program of Ford Motor Company. We thank Admira Bosnjakovic for her initial involvement in the study of Ceria NPs. We are grateful to Mark Mathias, Craig S. Gittleman, and Frank D. Coms of General Motors Global Fuel Cell Activities in Pontiac, MI, for their comments and for facilitating the TEM measurements.

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REFERENCES

(1) Roduner, E.; Schlick, S. In Advanced ESR Methods in Polymer Research; Schlick, S., Ed.; Wiley: Hoboken, NJ, 2006; Chapter 8, pp 197−228. (2) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; et al. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107, 3904−3951. (3) Kadirov, M. K.; Bosnjakovic, A.; Schlick, S. Membrane-Derived Fluorinated Radicals Detected by ESR in UV-Irradiated Nafion® and Dow Ionomers: The Effect of Counterions and H2O2. J. Phys. Chem. B 2005, 109, 7664−7670. (4) Danilczuk, M.; Coms, F. D.; Schlick, S. Fragmentation of Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments, and Implications for Behavior of PEMs in Fuel Cells. Fuel Cells 2008, 8, 436−452. (5) Gittleman, C. S.; Coms, F. D.; Lai, Y. H. In Polymer Electrolyte Fuel Cell Degradation; Matthew, M., Mench, M., Eds.; Academic Press: Boston, 2012; pp 15−88. (6) Schlick, S.; Danilczuk, M.; Spulber, M. Using Spin Trapping Electron Spin Resonance for Determining the Degradation Mechanism of Membranes Used in Fuel Cells, in the Special Issue entitled Modern EPR Spectroscopy. Mol. Phys. 2013, 111, 2738−2745. (7) Danilczuk, M.; Coms, F. D.; Schlick, S. Visualizing Chemical Reactions and Crossover Processes in a Fuel Cell Inserted in the ESR Resonator: Detection by Spin Trapping of Oxygen Radicals, Nafion®Derived Fragments, and Hydrogen and Deuterium Atoms. J. Phys. Chem. B 2009, 113, 8031−8042. (8) Endoh, E. Development of Highly Durable PFSA Membrane and MEA for PEMFC Under High Temperature and Low Humidity Conditions. ECS Trans. 2008, 16, 1229−1240. (9) Coms, F. D.; Liu, H.; Owejan, J. E. Mitigation of Perfluorosulfonic Acid Membane Chemical Degradation Using Cerium and Manganese Ions. ECS Trans. 2008, 16, 1735−1747. (10) Trogadas, P.; Parrondo, J.; Ramani, V. Platinum Supported on CeO2 Effectively Scavenges Free Radicals Within the Electrolyte of an Operating Fuel Cell. Chem. Commun. 2011, 47, 11549−11551. F

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