Modeling gas permeation through membranes by kinetic Monte Carlo: applications to H2, O2, and N2 in hydrated Nafion®.

ABSTRACT

We present a simulation tool in order to predict gas permeation through heterogeneous, microphase separated structures. The method combines dissipative particle dynamics (DPD) with kinetic Monte Carlo (KMC). Morphologies obtained from DPD are mapped onto a high density grid on which gas diffusion takes place. Required input parameters for the KMC calculations are the gas solubility and gas diffusion constant within each of the pure phase components. Our method was tested and validated for permeation of H(2), O(2), and N(2) gasses through hydrated Nafion membranes at various temperatures and water contents. We predict that membranes that contain an equal volume fraction of water, those with the highest ion exchange capacity exhibit the largest N(2) and O(2) permeation rates. For membranes of the same ion exchange capacity the H(2), O(2), and N(2) and permeability increases approximately linearly with Bragg spacing. We also predict that O(2) gas permeation depends much more on bottleneck phenomena within the phase separated morphologies than H(2) gas permeation. Overall, the calculated H(2) and O(2) permeability is found to be slightly lower than experimental values. This is attributed to the robustness of DPD resulting in ∼7% larger Bragg spacing as compared with experiment and∕or increased gas solubility within the polymer phase with water uptake.