Understanding the factors that control microstructure formation in catalyst layers (CLs) of polymer electrolyte fuel cells is of vital importance for improving the operation of these cells. Here, we employ, for the first time, coarse-grained molecular dynamics simulations to perform a structural analysis of the microphase segregation occurring during the fabrication process of CLs. Our mesoscale simulations provide insights into the structural correlations and dynamical behavior of different phases in the catalyst layer composite. This versatile computational study, moreover, rationalizes how the solvent used in catalyst layer fabrication influences the evolution of stable agglomerated conformations. In this realm, we evaluate dispersion media with distinct dielectric properties in view of capabilities for controlling the sizes of carbon/Pt agglomerates and ionomer domains and the resulting pore network topology. These insights are highly valuable for the structural design of catalyst layers with optimized performance and stability.
Reported results of coarse-grained molecular dynamics simulations rationalize the effect of water on the phase-segregated morphology of Nafion ionomers. We analyzed density maps and radial distribution functions and correlated them with domain structures, distributions of protogenic side chains, and water transport properties. The mesoscopic structures exhibit spongelike morphologies. Hydrophilic domains of water, protons, and anionic side chains form a random three-dimensional network, which is embedded in a matrix of hydrophobic backbone aggregates. Sizes of hydrophilic domains increase from 1 to 3 nm upon water uptake. At low water content, hydrophilic domains are roughly spherical and poorly connected. At higher water content, they convert into elongated cylindrical shapes with high connectivity. Further structural analysis provides a reasonable estimate of the percolation threshold. Radial distribution functions from coarse-grained and atomistic molecular dynamics models exhibit a good agreement. Water cluster size distributions from coarse-grained molecular dynamics and dissipative particle dynamics are consistent with small angle x-ray scattering data. Moreover, we calculated the water diffusivity by molecular dynamics methods and corroborated the results by comparison with pulsed field gradient NMR.
Gas diffusion electrodes ͑GDEs͒ containing a graded distribution of Nafion were prepared and characterized, and their performance as fuel cell cathodes compared to GDEs possessing a uniform distribution of Nafion. Cyclic voltammetry, electrochemical impedance spectroscopy ͑EIS͒, and porosimetry are used to characterize the variations in electrochemical properties, ionic conductivity, and microstructures. The cathodic performance was improved over uniform electrodes at intermediate and high levels of polarization when the Nafion content in the GDE was higher toward the catalyst layer/membrane interface and lower toward the catalyst layer/carbon paper interface since this maximizes proton transport in the GDE in the region of greatest ion flux and maximizes porosity in the region of greatest gaseous flux, respectively. Fuel cell performance is much poorer when the gradient of Nafion content is reversed, i.e., highest at the catalyst layer/carbon paper interface since this distribution disfavors proton and gas transport in the regions where they need to be maximized.
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