Atomistic Simulation of Water Percolation and Proton Hopping in Nafion Fuel Cell Membrane

We have performed a detailed analysis of water clustering and percolation in hydrated Nafion configurations generated by classical molecular dynamics simulations. Our results show that at low hydration levels H2O molecules are isolated and a continuous hydrogen-bonded network forms as the hydration level is increased. Our quantitative analysis has established a hydration level (λ) between 5 and 6 H2O/SO3− as the percolation threshold of Nafion. We have also examined the effect of such a network on proton transport by studying the structural diffusion of protons using the quantum hopping molecular dynamics method. The mean residence time of the proton on a water molecule decreases by 2 orders of magnitude when the λ value is increased from 5 to 15. The proton diffusion coefficient in Nafion at a λ value of 15 is about 1.1 × 10−5 cm2/s in agreement with experiment. The results provide quantitative atomic-level evidence of water network percolation in Nafion and its effect on proton conductivity

Morphology of supported polymer electrolyte ultra-thin films: a numerical study

Morphology of polymer electrolytes membranes (PEM), e.g., Nafion, inside PEM fuel cell catalyst layers has significant impact on the electrochemical activity and transport phenomena that determine cell performance. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic/hydrophobic character of these surfaces on the structural formation of the films has not been sufficiently explored yet. Here, we report about Molecular Dynamics simulation investigation of the substrate effects on the ionomer ultra-thin film morphology at different hydration levels. We use a mean-field-like model we introduced in previous publications for the interaction of the hydrated Nafion ionomer with a substrate, characterized by a tunable degree of hydrophilicity. We show that the affinity of the substrate with water plays a crucial role in the molecular rearrangement of the ionomer film, resulting in completely different morphologies. Detailed structural description in different regions of the film shows evidences of strongly heterogeneous behavior. A qualitative discussion of the implications of our observations on the PEMFC catalyst layer performance is finally proposed

The (2)Pi(g) shape resonance in electron-acetylene scattering: an investigation using the dilated electron propagator method

The zeroth order (Sigma(0)), the second order (Sigma(2)), the diagonal two particle-one hole Tamm Dancoff approximation (Sigma(2ph-TDA)) and the corresponding quasi-particle decouplings of the dilated electron propagator have been used to investigate the (2)Pi(g) C2H2- shape resonance. The results compare favourably with the experimental and other theoretical results. A plot of the resonant Feynman-Dyson Amplitudes establishes that the capture of the impinging electron is indeed in the pi(g)* orbital of the acetylene molecule. The resonant orbital on the real line shows the attributes of the acetylene lowest unoccupied molecular orbital and for the optimal complex scaling parameter shows a depletion of electron density near the carbon nuclei. (C) 1998

Treatment of shape and Auger resonances using the dilated electron propagator

An overview of the dilated electron propagator method based on operators defined on biorthogonal orbitals from bivariationally obtained complex scaled SCF procedure is presented and discussed. Results from applications to atomic and molecular electron attachment and detachment resonances using different decouplings of the dilated electron propagator are analyzed to adduce the role of correlation and relaxation effects in the formation and decay of shape and Auger resonances. Feynman-Dyson amplitudes of the dilated electron propagator are examined to try and unravel the mechanistic underpinnings of these resonances. (C) 2002 Wiley Periodicals, Inc

An investigation of basis set effects in the characterization of electron-atom scattering resonances using the dilated electron propagator method

The effects of basis set variations on resonance attributes are investigated using systematically augmented basis sets by correlating the resulting changes in resonance energy and width with the alterations induced in the radial probability density profile of the resonant orbital. Applications to P-2 Be- and P-2 Mg- Shape resonances reveal that basis sets capable of describing both electron density accumulation near the target nucleus to facilitate resonance formation and sufficiently large electron density away from the target nucleus to provide for its decay are necessary for effective characterization of these resonances. A comparison of radial probability density profiles from the bivariational self-consistent field, the second-order, the diagonal two particle-one hole Tamm-Dancoff approximation and quasiparticle decouplings reveals that relaxation effects dominate in resonance formation

Higher order decouplings of the dilated electron propagator with applications to (PBe-)-P-2, (PMg-)-P-2 shape and (SBe+)-S-2 (1s(-1)) Auger resonances

The full third order (Sigma (3)), quasi-particle third order (Sigma (3)(q)) and Outer Valence Green's Function. decouplings of the bi-orthogonal dilated electron propagator have been implemented for the first time and results from their application to P-2 Be-, P-2 Mg- shape and S-2 Be+ (1s(-1)) Auger resonances are presented and compared with energies-and widths obtained using the zeroth order (Sigma (0)), quasi-particle second order (Sigma (2)(q)) and second order (Sigma (2)) decouplings. The energies and widths from third order decoupling for shape resonances are close to those obtained using second order self-energy approximants. The energy and width calculated using the third order decoupling for Auger resonances provide better agreement with experimental results, with the much more economic quasi-particle third order decoupling being just as effective. The differences between FDAs from different decouplings are analyzed to elicit the role of correlation and relaxation: in the formation and decay of shape and Auger resonances. (C) 200

Charge delocalization effects on Nafion structure and water /proton dynamics in hydrated environments

\u3cp\u3eIn this work, using molecular dynamics simulations, we examine the effect of atomic charge delocalization on the pendant side chain of Nafion membrane on the structural and dynamical properties in various hydrated environments. The sulfur-sulfur radial distribution functions suggest that the sulfonate groups of the pendant side chain have closer geometric proximity with an increase in charge delocalization. However, the interactions of the sulfonate groups with water molecules/hydronium ions show a slight change with the charge delocalization. The average water cluster size decreases significantly with charge delocalization, though the diffusion coefficients of water molecules (at medium and higher water concentration) increase initially and then decreases slightly with excessive charge delocalization. The diffusion coefficients of hydronium ions do not follow any particular trend with charge delocalization. A complex interplay between sulfur-sulfur, sulfur-water/hydronium interactions, and water cluster distribution plays an essential role in the magnitude of the diffusion coefficient of water molecules and hydronium ions.\u3c/p\u3

Probing translational and rotational dynamics in hydrophilic/hydrophobic anion based imidazolium ionic liquid-water mixtures

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Characterization of shape and auger resonances using the dilated one electron propagator method

Some representative results from applications of the second order, diagonal 2ph-TDA and quasi-particle decouplings of the biorthogonal dilated electron propagator based on a complex scaled bivariational SCF to the investigation of P-2 Mg-, (B2gC2H4-)-B-2 Shape and S-2 (1S(-1)) Be+ Auger resonances are presented. These results demonstrate the effectiveness of the dilated electron propagator in calculation of energies and widths as also in unravelling of the mechanistic details of resonance formation and decay. The Feynman-Dyson amplitudes are shown to effectively isolate the LUMO as the resonant orbital