I-014

Eugene S. Smotkin

e.smotkin@northeastern.edu

Dan J Donnelly III, Moon Young Yang, Nicholas Dimakis, Seung Soon Jang, William A. Goddard III

Northeastern University, USA

Reactive force field MD simulations of proton exchange membranes: Elucidation of operando and ex-situ vibrational spectra

Ion conductive polymer (ionomer) membranes are widely used as solid electrolyte separators in electrochemical devices such as fuel cells and electrolyzers. Perfluorinated sulfonic acids (PFSA) have dominated the ionomer market for decades, owing to their superior chemical-mechanical stability and high proton conductivity. Proton transport via diffusion and hopping mechanisms occurs upon hydration and ionization of sulfonic acid groups (i.e., exchange sites). Ionomer water content is typically characterized by λ (bulk H2O/SO3(H) ratio), while λ enumerates the number of waters within a predefined radius (6 Å in our work) of an individual exchange site. Our prior classical molecular dynamics (CMD) simulations of Nafion, revealed a broad distribution of Λ for non-zero λ values. These distributions were used to explain λ dependent changes to operando and ex situ PFSA infrared spectra. CMD simulations are governed by Newtonian mechanics which do not accommodate the dynamic exchange of protons between SO3H groups and water/hydronium. We now introduce the use of reactive force fields (ReaxFF) to fully atomistic MD simulations to model bond formation and breaking events (i.e., acid-base association-dissociation processes). We applied 2 ns of ReaxFF to classically equilibrated Nafion simulation cells (32 10-mers) at nine hydration levels from λ = 0 (~22000 atoms) to λ =  20 (~41000 atoms). Our simulations reveal that λ distributions are a sum of protonated (α) exchange site Λ distributions (i.e., αΛ) and deprotonated (β) site Λ distributions (i.e., βΛ). The ReaxFF MD simulations show that protonation states not only depend on Λ, but also the local dielectric environment, local charge distribution, and long range electrostatic effects.