The competitive adsorption of Nafion functional groups induce complex potential dependencies (Stark tuning) of vibrational modes of CO adsorbed (CO(ads)) on the Pt of operating fuel cell electrodes. Operando infrared (IR) spectroscopy, polarization modulated IR spectroscopy (PM-IRRAS) of Pt-Nafion interfaces, and attenuated total reflectance IR spectroscopy of bulk Nafion were correlated by density functional theory (DFT) calculated spectra to elucidate Nafion functional group coadsorption responsible for the Stark tuning of CO(ads) on high surface area fuel cell electrodes. The DFT calculations and observed spectra suggest that the side-chain CF3, CF2 groups (i.e., of the backbone and side chain) and the SO3(-) are ordered by the platinum surface. A model of the Nafion-Pt interface with appropriate dihedral and native bond angles, consistent with experimental and calculated spectra, suggest direct adsorption of the CF3 and SO3(-) functional groups on Pt. Such adsorption partially orders the Nafion backbone and/or side-chain CF2 groups relative to the Pt surface. The coadsorption of CF3 is further supported by Mulliken partial charge calculations: The CF3 fluorine atoms have the highest average charge among all types of Nafion fluorine atoms and are second only to the sulfonate oxygen atoms.

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Cluster and periodic density functional theory (DFT) of carbon monoxide adsorbed atop on Pt (COads) show that ruthenium alloying weakens both the COads internal and C−Pt bonds and reduces the COads adsorption energy. A new theoretical model based on the π-attraction σ-repulsion is used to explain the above results. This model correlates (1) Mulliken population, (2) density-of-states analysis of the COads orbitals, (3) the individual interaction of these orbitals with the metal lattice bands, and (4) their polarizations within the COads molecule. In this study, the σ interaction has both attractive and repulsive components via electron donation to the metal bands and Pauli repulsion, respectively. Cluster DFT shows that the overall weakening of the COads internal bond upon alloying is due to the dominance of reduced σ donation to the metal (which weakens the COads internal bond) over increased π bonding between the carbon and oxygen. However, periodic DFT calculations show that both the σ donation and the COads internal π bonding are simultaneously reduced. The C−Pt bond weakening upon alloying is primarily due to increased exchange repulsion between the adsorbate and the substrate. The adsorbing Pt atom sp/d z 2 orbitals population increase upon alloying for both calculations.

The time‐dependent IR spectra during dehydration of fully hydrated Nafion show the reversible disappearance of the 1061 cm−1 and 969 cm−1 concurrent with the emergence of peaks at ∼928 cm−1 and ∼1408 cm−1. The first pair of group modes is associated with a dissociated exchange group (sulfonate) with a local C3V symmetry. The C3V group modes shift with state‐of‐hydration: The 969 cm−1 peak completely vanishes and the 1061 cm−1 is reduced to a small shoulder at 1070 cm−1 at end of dehydration. The C3V group modes are replaced by the pair of group modes of an associated exchange group (sulfonic acid) with C1 local symmetry. The density functional theory normal mode analysis confirms that the sulfonic acid/sulfonate site plays a dominant role in the C1 and C3V group modes, respectively. This work clarifies the importance of assigning fluoropolymers peaks as group modes rather than traditional single functional group assignments as is often the case with the ∼1061 cm−1 and ∼969 cm−1 C3V group modes. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1329–1334

Operando Raman micro-spectroscopy of the membrane electrode assembly (MEA) of a fully operating hydrogen/oxygen Nafion electrolyte fuel cell is described. Coarse depth profiling of the fuel cell system enabled appropriate positioning of the microspectroscopy laser focal point for MEA catalytic layer spectroscopy. An increase in the ionomer state-of-hydration, from oxygen reduction at the cathode, transitions ion exchange sites from the sulfonic acid to the dissociated sulfonate form. Visualization of density functional theory calculated normal mode eigenvector animations enabled assignments of Nafion side-chain vibrational bands in terms of the exchange site local symmetry: C 1 and C 3V modes correlate to the sulfonic acid and sulfonate forms respectively. The gradual transition of the MEA spectra from C 1 to C 3V modes, from the fuel cell open circuit voltage to the short circuit current respectively, demonstrate the utility of vibrational group mode assignments in terms of exchange site local symmetry.