Mixed membrane matrices (MMMs) based on Nafion® pristine/defected-UiO-66(Zr) MOFs: assessment of the effects of dopants on cluster morphology

Abstract

Cluster formation within proton exchange membrane fuel cells (PEMFCs) considerably affects the fuel cell's proton transfer performance. A hybrid membrane of solid-state proton conductors (Zr-MOF)-doped Nafion® shows a much superior proton transfer as compared to pure Nafion®, particularly for operating under low humidity. A series of molecular dynamics simulations were conducted to understand the conduction mechanism in the proposed hybrid membrane (Zr-MOF/Nafion®) to evaluate the improvement of fuel cell performance at the atomistic scale. Different hybrid PEMFCs were simulated by two levels of doping agent concentrations (2 and 5 wt%) of pristine and defected UiO-66 with a variety of hydration levels (λ = 3 to 15, the ratio of water molecules to Nafion® sulfonate groups). Through the determination and analysis of structural and dynamical parameters, we identified that the presence of UiO-66 plays an important role in modifying the cluster structures. We found that water molecules are more populated around the polymer atoms but are strongly bound to Lewis acid sites of zirconium nodes. We revealed the behavior of water clusters in defected MOF-doped polymers, demonstrating the stronger segregated spherical domains. Furthermore, the localized semicrystalline structure and statistical analysis of water clusters in the hybrid membrane were found to be useful to understand the improved proton transport mechanism.