Molecular Dynamics Simulations of Hydroxide Solvation and Transport in Anionic Exchange Membranes

Anionic fuel cells are a promising energy technology that maintain many of the benefits of protonic fuel cells with additional fuel versatility beyond H2 and without requiring expensive platinum catalysts. Despite the advantages, anionic fuel cells have not replaced protonic fuel cells primarily because of low ionic conductivity, poor chemical stability and carbonate formation. This work attempts to understand hydroxide solvation and transport in anionic fuel cells by atomic level modeling and suggest improved design strategies from that microscopic information. First we will present simulations of a series of aqueous alkane ammonium cations. We have considered tetramethylammonium, tetraethyl-ammonium and benzyltrimethylammonium cations at temperatures between 300 K and 350 K and concentrations between 0.5 and 4.5 M. Standard molecular mechanics force fields define the bonding topology at the beginning of the simulation and retain that topology throughout the simulation. Using the multi-state empirical valence bond (MS-EVB) method with a hydroxide model we recently developed, we are able to efficiently and accurately describe bond making and breaking processes, and capture Grotthuss shuttling of the hydroxide ion in the membrane. The MS-EVB algorithm identifies a collection of possible bonding topologies around the reactive species and defines the solvation structure as a linear combination of these topologies, the relative weights for which are determined at each step. These simulations are an effort to systematically categorize the effect of cation on anion properties. This methodology can be extended to other cations or non-ammonium cations, to further judge the relative effect a cation has on the transport of hydroxide and potential suitability in anion exchange membranes. Additionally we will present work with benzyltrimethylammonium cations grafted to an alkane backbone on two different length-scales. We are using the previously described MS-EVB hydroxide model with in anion exchange membrane systems of four polymer chains with 10 monomer subunits per chain. These simulations reveal small-scale solvation and transport of hydroxide in the membrane. We are also employing largescale, standard atomistic simulations of the same anion exchange membrane. These systems are selected to be as representative as possible of experimental membrane conditions by containing 200 polymer chains each 100 monomer subunits in length built with a Monte Carlo based scheme of generating polymer configurations. These large-scale simulations will help to uncover the connection between the nanoscale membrane morphology and local atomic structure. Results have revealed interesting details about the microscopic structure of these membranes. As illustrated in Figure 1, hydroxide ions have been found to typically reside between ammonium cations in the membrane. This is supported by a strong peak in the radial distribution function (Figure 2) and the plateau of about 2 hydroxide ions near 5 A in the integrated coordination number for the nitrogen-oxygen distribution. This work will present methodology based on fundamental physics that advances the understanding of ion transport in anion exchange membranes. Work is ongoing to understand temperature and hydration effects, the role of morphology on membrane properties and the relationship between Grotthuss shuttling and standard vehicular diffusion in the transport of charge. This research was supported by the DOD Multidisciplinary University Research Initiative through the Army Research Office (58161-CH-MUR).