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Aquivion Ionomer in Mixed Alcohol-Water Solution: Insights from Multi-Scale Molecular Modeling

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Aquivion Ionomer in Mixed Alcohol-Water Solution: Insights from Multi-Scale Molecular Modeling Aquivion Ionomer in Mixed Alcohol-Water Solution: Insights from Multi-scale Molecular Modeling Ali Malek a, b , Ehsan Sadeghi a, Jasna Jankovic c, Michael Eikerling a, d , Kourosh Malek a, b * a) Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada b) NRC-EME, 4250 Wesbrook Mall, Vancouver, BC, V6T 1W5, Canada c) Materials Science and Engineering Department, University of Connecticut, 97 North Eagleville Road, Storrs, CT, 06269-3136, USA c) Institute of Energy and Climate Research, IEK-13: Modelling and Simulation of Energy Materials, Forschungszentrum Jülich, 52425 Jülich, Germany * Corresponding author. Phone: +1 6042613000, email: [email protected] Abstract Short side-chain ionomers such as Aquivion are increasingly used in the fabrication of PEM fuel cells. Aquivion exhibits reduced gas crossover, enhanced glass transition temperature, better mechanical stability, and higher conductivity in comparison to Nafion, their long side-chain relative. We performed atomistic molecular dynamic simulations of Aquivion at varying water content and proportion of iso-propanol mixed into the solvent and examined the structure and dynamics of the system. Atomistic simulations are followed by coarse-grained simulations to develop a coarse- grained model applicable to both long and short side-chain ionomers. The density of Aquivion and Nafion membranes from simulations is calculated and compared with that from experiment. The Aquivion system higher diffusion coefficients for water molecules and hydronium ions than the Nafion system. The water network morphology is analyzed as a function of water content. 1 1. Introduction The core component of a polymer electrolyte fuel cell (PEFC) is an ionomer-based polymer electrolyte membrane (PEM) that conducts protons between anode and cathode.1-5 [references before or after punctuation???? Make it consistent.] The PEM exerts a vital impact on the performance characteristics of the cell. Relevant parameters include the molecular composition and structure of ionomer backbones and acid-terminated sidechains, grafting density of side chains along the backbone, and ion exchange capacity (IEC), the size of ionomer bundles and bundle connectivity,6, 7 water volume fraction, and pore space connectivity.8-11 Perfluorosulfonic acid (PFSA) ionomer membranes such as Nafion® from Dupont and Aquivion from Solvay currently offer the best combination of high proton conductivity and chemical, mechanical and thermal robustness. 5 While alternatives are emerging, Nafion remains the most extensively studied and commonly used PEM in PEFCs.5, 12, 13 Nafion ionomer consists of a polytetrafluoroethylene (PTFE) backbone, with hydrophilic perfluoropolyether sidechains that are terminated by sulfonic acid head groups (-SO 3H). Upon sufficient hydration, acid head groups dissociate and hydronium ions released form hydrated complexes that are vital for proton transport. Numerous structural models have been developed for PFSA membranes using data from small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and wide-angle X-ray diffraction (WAXD).11, 14-16 In the hydrated state, hydrophobic backbones self-aggregate into a network of bundles, surrounded by the connected water sub-phase. 6,7 Backbone bundles are lined on their surface by the sidechains exposing the dissociated head groups into the surrounding water phase, which conducts protons.5 The size and shape of these water domains varies with water content.17-19 Ionomer materials made by 3M, Solvay Specialty Polymers (i.e., Aquivion, formerly known as Dow or Hyfion), and Asahi Glass (i.e., Flemion) share the same backbone chemistry as Nafion, but have shorter side-chains and usually higher ion exchange capacity. 20, 21 These ionomers exhibit improved functionality 2 and robustness in PEFCs especially under higher cell temperature (> 80 °C ) and low relative humidity (RH). Short-side-chain (SSC) PFSA ionomers were originally developed by Dow Chemicals in the 1980s and their superior properties such as higher crystallinity and higher glass transition temperature compared to long side-chain (LSC) ionomers were subsequently demonstrated.22, 23 More recently, Solvay introduced a simplified synthesis route for the SSC monomeric sulfonic precursor, which facilitated its commercialization under the tradename Aquivion. 24 The molecular structures of both Nafion and Aquivion are shown in Fig. S1. Side chains of Aquivion are shorter since the etheric group and the tertiary carbon have been removed. This chemical modification has a significant impact on the water retention capability, thermal stability, and ion transport properties of the membrane, particularly at low RH.25 The higher glass transition temperature of Aquivion ionomer due to its higher degree of crystallinity is beneficial during hot-pressing in the fabrication of membrane electrode assemblies (MEAs) for PEFCs.26, 27 Moreover, the enhanced crystallinity improves the thermomechanical membrane stability during fuel cell operation. SSC structures exhibit lower gas crossover compared to LSC structures, indicating smaller ionic clusters or a denser packaging of polymeric chains. This property is vital under PEFC operation, where gas- crossover is a performance-limiting factor. Furthermore, SSC ionomers could be made with ion- exchange capacity (IEC) upto1.66 mmol/g IEC by virtue of the preserved residual crystallinity. In the case of LSC ionomers, residual crystallinity is preserved only upto ~1.25 mmol/g IEC, rendering formation of stable LSC membranes with higher IEC practically impossible. Extending the range of stability to higher IEC is a major asset of SSC ionomers, as it enables higher ionic conductivity. 23 The high glass transition temperature of the SSC ionomer membranes enhances the fuel cell operating temperature by around 40 °C compared to LSC ionomers, thus exceeding the target value of 120 °C for automotive applications. 27, 28 The higher operating temperature implies higher tolerance of Pt-based catalyst to poisoning species and contaminants.29 PEM based on SSC ionomer are also less prone to 3 radical attack because of their higher crystallinity and reduced swelling less, which suppress gas cross- over. Moreover, SSC ionomers do not contain the ether linkages in the side chain, which render LSC ionomers highly susceptible to radial attack. 28, 30, 31 While macroscopic properties and fuel cell performance of PFSA membranes are well documented, a detailed understanding of how these properties are linked to their chemical architecture and microscale morphology is more difficult to unveil, albeit essential for the development of high performance PEMs. An important characteristic of PFSA membranes is the relationship between the size and connectivity of hydrophilic domains and the membrane water content.32-34 In spite of higher crystallinity and higher water absorption of SSC compared to LSC PFSA, SSC ionomer membranes possess lower gas permeability due to less interconnected ionic clusters. 24 Therefore, high water uptake does not directly result in improved transport properties and performance. For this reason, a PEM that can operate stably across a wide RH range is desirable. Aquivion exhibits reduced sensitivity to changes in water content, making it a promising candidate for high-T and low RH operation. 35 Understanding the differences in structure and properties of various ionomer membranes is an essential prerequisite for being able to explain complex performance aspects under various scenarios of membrane use and operation. For this purpose, we should be able to assess and compare mechanisms of self- assembly of rod-like ionomer molecules into bundle-like aggregates,7, 36, 37 organization of bundles into superstructures housing porous water-filled domains,18 and interconnecting of bundles and pores to form percolating networks of water-filled pathways.38 Simulations based on first principles and molecular dynamics have become essential tools in this regard.10, 13, 39-42 Using atomistic MD simulations, the main goal of this study is to elucidate how the chemistry of the side chain affects the aggregation of ionomer molecules and their physical properties at different hydration levels and in solutions containing iso-propyl alcohol (IPA) and water in mixture ratios of 0.2-1.0. The properties of ionomer solutions are vital in the fabrication of catalyst layer inks. An IPA/water mixture is the common solvent used during preparation of catalyst ink solutions.43, 44 Furthermore, we conducted 4 coarse-grained (CG) molecular dynamics (MD) simulations for systems priorly considered in atomistic MD simulations. Results of atomistic MD and CGMD simulations are compared with each other as well as with other MD and experimental data available. 2. Atomistic Model and Simulaton Details 2. 1 Model System We considered an Aquivion ionomer chain consisting of 20 monomer units. The chemical structure of the repeat monomer of Aquivion is shown in Fig. S1-b (See Supporting Information). Side chains are separated by seven tetrafluoroethylene units in the backbone to allow comparison with Nafion 117 at a comparable value of the IEC, chosen at 0.87 mmol.g -1. The IEC of Aquivion ionomers in our simulated system was close to 1.02 mmole.g -1. MD simulations were started from the initial configuration comprising 12 Aquivion chains (i.e., a total + of 240 monomers) and water, with 240 hydronium ions (H3O ) added for electroneutrality. Water − molecules were added to give water contents from
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