Charge delocalization effects on Nafion structure and water /proton dynamics in hydrated environments Rakesh Pant a , Soumyadipta Sengupta b , Alexey V. Lyulin b, 1 , ** , Arun Venkatnathan a, * a Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune, 411008, Maharashtra, India b Theory of Polymers and Soft Matter, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, 5600 MB, the Netherlands article info Article history: Received 1 May 2019 Received in revised form 30 September 2019 Accepted 1 October 2019 Available online 2 October 2019 Keywords: Molecular dynamics Nafion Charge delocalization Cluster distribution Diffusion coefficient abstract In this work, using molecular dynamics simulations, we examine the effect of atomic charge delocal- ization 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 delocal- ization. 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 dis- tribution plays an essential role in the magnitude of the diffusion coefficient of water molecules and hydronium ions. © 2019 Elsevier B.V. All rights reserved. 1. Introduction Polymer electrolyte membrane (PEM) fuel cells have been widely explored for several stationary and transportation applica- tions [1 ,2]. Perfluorosufonic acid (PFSA) membranes (e.g. Nafion) have been extensively studied [2e5] using a wide range of exper- imental techniques and theoretical methods and continues to be preferred choice due to their properties like high proton conduc- tivity and chemical stability. Experimental investigations on hy- drated Nafion from spectroscopy, microscopy, X-ray, and neutron scattering studies have mainly focused on morphological changes which occur with varying humidification and temperature [5]. The conductivity of Nafion depends on the extent of hydration which further influences the membrane morphology and the transport dynamics of protons and water molecules. The transport of protons is governed by a vehicular mechanism [6] (e.g. proton attached to water molecule) and structural diffusion [7] where protons can hop among water molecules. Several theoretical investigations using quantum chemistry calculations and computer simulations [4] (classical/reactive/ coarse-grained/ab initio molecular dynamics (MD) simulations) have also provided a wealth of data on membrane morphology, mechanism of proton transport, and diffusion coefficients in hy- drated Nafion environments. Paddison [8] reported that the ether oxygen atoms present in the pendant side chains of Nafion were not hydrophilic and attributed the same to the strong electron- withdrawing effect of the neighboring CF 2 groups. In another study, Paddison [9] examined hydrated model polymers and concluded that the excess electron density on the sulfonate group (due to the dissociation of a proton from the sulfonic acid group) is delocalized by the neighboring electron-withdrawing CF 2 groups on pendant side-chain Nafion. * Corresponding author. ** Corresponding author. E-mail addresses: a.v.lyulin@tue.nl (A.V. Lyulin), arun@iiserpune.ac.in (A. Venkatnathan). 1 Center for Computational Energy Research, Department of Applied Physics, Eindhoven University of Technology, Eindhoven, 5600MB, The Netherlands. Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid https://doi.org/10.1016/j.fluid.2019.112340 0378-3812/© 2019 Elsevier B.V. All rights reserved. Fluid Phase Equilibria 504 (2020) 112340