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. 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 crossover. 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 λ = 3 to 19, where λ (= mol H 2O/mol SO 3 ) is the number of water molecules per sulfonic acid head group. It was assumed that all side chains are dissociated at any value of λ, in line with DFT-based acid dissociation studies. 8, 45 In order to generate simulation cells with varying water content, water molecules were removed in 10 sequential steps (480 water moleculs removed in each step), starting from the simulation cell with λ = 19. At λ = 19, the cell

+ − contained 4560 H 2O, 240 H 3O and 240 SO 3 .

2. 2 Atomistic MD

Details of atomistic MD simulation can be found in Supplementary Information.

The initial structure of one Aquivion-like oligomer chain and hydronium ions was prepared using the

Avogadro package.46, 47 The improved generalized AMBER force field (GAFF) was used to simulate the Aquivion polymer. The total potential energy is the sum of bonding (bond stretching, bond angle bending and dihedral torsion) and non-bonding interactions, as described in Ref. [48] and employed in

Ref. [49]. The nonbonding interactions are the sum over all pairs of van der Waals interactions, included in the 6-12 Lennard-Jones (LJ) potential, and Coulomb interactions . Similar force fields had been used previous simulations of hydrated Nafion ionomers.50-52 Geometry optimizations at the DFT level were carried with Gaussian09 using the B3LYP functional and the 6-31+G(d,p) basis set.53 Partial atomic charges along Aquivion chains were obtained using the chelpG method.54 In order to study both long and short side chain ionomers, we modified our previously developed structural and force field models. 42, 49

Following single chain optimization studies, we performed classical MD simulations of hydrated

+ 48 Aquivion using GROMACS 5.0.6, with SPC/E models for H 2O molecules and H 3O ions. We employed the smooth particle mesh Ewald method 55 to calculate electrostatic interactions with a cut off distance 10 Å, and performed energy minimization to equilibrate the system. Subsequently, we performed MD simulation up to 1 ns in the constant NVT ensemble at 298.15 K with 2 fs time steps and saved the configuration every 0.2 ps. Then we performed MD simulation for 10 ns in the constant NPT ensemble at 298.15 K with 2 fs time steps. After equilibration, the density of hydrated Aquivion was in good agreement with the known experimental density of Aquivion membranes (1.7 g.cm -3). 56 The range of λ values was chosen to examine the evolution of membrane morphology with increasing hydration level.

2. 3 Coarse-Grained MD

Details of coarse-Grained MD simulations can be found in Supplementary Infmation.

3. Results and Discussion

We characterize equilibrated structures of Aquivion at different water contents in terms of phase- segregation, mass density, site-site radial distribution functions (RDFs), and self-diffusion coefficients of water molecuels and hydronium ions.

3.1 Morphology of Hydrated Aquivion

Structures of hydrated Aquivion obtained from atomistic and coarse-grained MD simulations at different water contents and 298.15 K are shown in Figs. 1 and S3. The 3D snapshots were taken from the last frames of the respective MD simulations of the entire structure. For each frame, we show the entire system in panel a as well as structures with water molecules hidden in panel b and the water network only in panel c.

As can be seen in Figs. 1-c and Fig. 3-c the water network is disconnecred at λ = 3 and highly interconnected at λ = 19 , consistent with literature studies on Nafion membranes.57 With increasing water content, water clusters grow in size and their connectivity increases. The percolation transition to a sample spanning network of water channels, which is important for the transport properties and especially hydronium ion conductivity, occurs at a water content of λ = 6 or slightly below. At high water contents, most of the hydronium ions are located outside of the clusters within second hydration layers, which indicates they are interacting with the negatively charged sulfonate groups of the ionomers.

Fig. 2 shows the morphology of Aquivion at different volume fractions of IPA. The connectivity of water clusters decreases with increasing IPA volume fraction. On the other hand, an increase in the IPA volume fraction results in a more packed and aggregated structure of Aquivion ionomers.

3.2 Density

We validated force filed parameters by calculating densities of Aquivion-water mixtures from atomistic and coarse-grained MD simulations at λ = 3, 5, 7, 9, 11, 13, 15, 17 and 19 at 298.15 K. Results displayed

7 in Fig. 3 show that the density of hydrated Aquivion and Nafion gradually decrease as λ increases. This trend agrees with other MD studies.39, 56, 58, 59 The results for the density using atomistic MD are in agreement with values from Sunda et al .60 , and Hristov 39 , who reported density values of 1.68 and 1.67 g/cm 3 for λ = 3. Density data for Aquivion ionomer are scarce in the literature. The closest study is an atomistic MD simulation by Devanathan and Dupuis. 61 The values of density in their simulations lie between those obtained from our atomistic and coarse-grained MD studies. Generally, the density values from coarse-grained MD are slightly higher than the values from atomistic MD, due to differences in simulation set-up and parametrization of force-field. These differences are largely negligible for water content higher than 10. Overall, it can be seen that density values of all Aquivion membranes gathered from the literature 58, 61 lie between our atomistic and coarse-grained values with different IEC and sizes, with some values closer to coarse-grained and some closer to atomistic results. In addition, the calculated density of Nafion is higher than that of Aquvion over the range of water content and the discrepancy increases with the increase in λ. The simulated values of Nafion density were verified against reported experimental and MD studies.37, 62

The densities of the IPA/water mixture with and without Aquivion ionomer in solution with λ = 30 are calculated with atomistic MD simulations. Fig. 7 shows the variations in the density of the IPA/water mixture with and without Aquivion ionomer against the IPA volume fraction. The calculated density of the IPA/water mixture in the absence of Aquivion ionomer agrees with the experimental result of Ngo et al . . 43

As the IPA content of the IPA/water mixture increases, the density decrease from 1.43 to 0.935 g/cm 3 for the volume fraction of IPA varying from 0.0 to 0.83. Interestingly, the experimental density of the

IPA/water mixture does not change linearly with IPA volume fraction. Water and IPA are polar substances; the self-association and inter-molecular association through the hydrogen bonding of these two compounds influences the density of the mixed solvent. The experimental density data suggests a strong hydrogen bonding interaction between IPA and water molecules, resulting in larger density values

8 of the mixture than those obtained by linear combination of pure IPA and water densities.

3.3 Self-diffusion

We determine the diffusion coefficients, D, for the classical model of water molecules and hydronium ions as a function of λ, using the Einstein-Smoluchowski equation,

1  2 Dlim | rtri () i (0)| , (2) t  6t where t is the diffusion time and r(t) is the center of mass position of the vector of oxygen in water molecules and hydronium ions. The diffusion coefficients of water molecules and hydronium ions calculated from the linear regime of the MSD curves are illustrated in Fig. 5. The water diffusion coefficient is in good agreement with experimental data and MD values reported in the literature, particularly at lower water contents. 63

The discrepancies in diffusion coefficient values between experimental and MD results at medium to high λ can be attributed to variations in membrane preparation and processing as well as experimental methods and conditions.64 For Aquivion at λ = 5, the diffusion coefficients of water molecules and

+ -7 2 -1 -8 2 -1 hydronium ions (D H2O and D H3O ) are calculated as 7.1 x 10 cm s and 9.8 × 10 cm s , respectively.

For comparison, Berrod et al . 63 , have measured the diffusion coefficient of water using the PFG-NMR method and obtained a value 8 × 10 -7 cm 2 s-1 at λ = 5 , which is comparable with the value calculated here from atomic MD. The diffusion coefficient of hydronium ions is thus approximately 4-5 times lower than that of water molecules, even at low λ. Hydronium ions are immobolized by strong Columb interactions with charged sulfonate groups. These diffusion coefficients are slightly higher in Aquivion

+ than in Nafion, which can be attributed to more H 3O ion and H 2O molecules in the vicinity of the

Aquivion side-chain compared to that in Nafion.

The diffusion of water molecules is faster in Aquivion than in Nafion with similar IEC. 58, 61 The higher

9 diffusion coefficient of H2O molecules in Aquivion compared to Nafion is consistent with the experimental observations of Li et al . 24 and MD simulations in Karo et al . 40 . In addition, the diffusion coefficient found from CG simulations does not agree well with atomistic simulations at low and high λ.

This deviation can be attributed to the reduction in the degrees of freedom using coarse graining simulations. Simplified water beads are intrinsically different in view of their dynamic properties due to smoother non-bonding potentials and reduced number of interactions. Fig. S4 compares diffusion coefficients of hydronium ions and water for IPA/water mixtures. The diffusion coefficient of water is larger than that of hydronium ions by a factor of 3.5 to 5.6 over the range of IPA volume fractions considered. The minimum and maximum values of diffusion coefficient for water molecules occur at

IPA/water volume fractions of 0.75 and 0.46, respectively, while for hydronium ions these values occur at IPA/water volume fractions of 0.83 and 0.46.

3.4 Radial Distribution Functions

Radial distribution functions (RDF) are useful means of unravelling structural correlations in complex composite media. The RDF is calculated from the following equation,

, (3) =

2 where 4r dr is the volume of a spherical shell of thickness d r at radius r from particle A, nB is the number of B particles in the shell, V is the volume of the system, and NB is the number of B particles in the whole system. An RDF represents the probability of finding one type of object at a particular distance from another type of object.

The RDF model approximates the behavior of molecules within solvation shells found in a solution. A solvation shell is comprised of the nearest neighbor solvent molecules surrounding a particular solute

10 molecule. Numerical integration of the radial distribution function yields the coordination numbers of molecules of interest. The number of water and hydronium ion around the sulfonic acid group is coordination number in this study. This is shown in eq. (4), where n is the coordination number, r is the radius of the first solvation shell, and ρ is the density of a particular particle. For n, we use

, (4) = 4 employing gA-B (r) up to the first minimum of the RDF curve.

The interaction between sulfonate groups can be analysed from the S-S RDFs in Fig. 6a. The sulfonate- sulfonate interaction decreases with increasing hydration level in Aquivion due to the increase in the number of water molecules in the solvation shell around the sulfonate group. The S-S RDF profile seen at λ = 3 for Aquivion is similar to that for Nafion at λ = 9 studied in the work of Sunda et al . 60 , where a bi-fragmented peak at 5.5 Å is observed. This suggests that at low hydration of Aquivion (λ = 3) and at higher hydration of Nafion (λ = 9), there is proximity of water molecules around sulfonate anions.

The S-S RDF shows the first peak for λ = 3 at r = 5.1 Å. As the water content increases, peaks broaden and eventually flatten. The intensity of the first S-S RDF peak for Aquivion ionomer, gS-S(r) = 1.2, is

57 smaller than that for Nafion (gS-S(r) ≈ 3.1) at lower λ . This indicates a reduced tendency of sulfonate groups to aggregate in a short side-chain ionomer such as Aquivion compared to Nafion. With the shorter side-chains, electrostatic repulsion of charged anionic head groups that favours ionomer dispersion gains in strength compared to the effect of hydrophobicity of backbones and sidechain trunks that drives ionomer aggregation. As a consequence of the relative increase in electrostatic repulsion of charged ionic head groups, Aquivion chains are stiffer.

The influence of hydration on sulfonate-water interactions can be seen from the S-H2O RDFs in Fig.10- b. The peak intensity of sulfonate-water interactions decreases with increasing hydration level, exhibiting a particularly large drop between λ = 3 and 7. This indicates that water molecules are more loosely packed around sulfonate groups upon increasing λ. There is a distinct peak at r = 4 Å and a broad peak from r =

4.8 to 8 Å. These two peaks represent the first two hydration shells. Since most hydronium ions reside in these two solvation shells, the location of these two peaks represents the axial position of the two

+ hydration layers around the S-H3O .

The sulfur-hydronium ion interactions are displayed in RDFs in Fig. 6c. The peak positions are similar to those for sulfur-water RDFs, which indicates that the sulfonate groups attracts both water and hydronium ions. The peak intensities of sulfur-hydronium RDFs are larger than sulfur-water RDFs, indicating a stronger location of hydronium ions around the sulfonate anions. The intensity of the first peak decreases with increasing λ as water molecules mediate the sulfonate-hydronium ion interactions.

The sulfur-hydronium RDFs at all λ show two distinct peaks, a sharp peak at about 3.9 Å and a broad peak at about 5.8 Å. The locations of the two peaks are related to the first two hydration shells in which hydronium ions reside. As λ increases, the intensity of the first sharp peak decreases significantly. This means the hydronium tends to be further away from sulfonate with increasing λ; but for the second peak we see the peak intensity increasing with the increase in λ. The intensity of the first peak for the S- hydronium RDF in Aquivion is 16.97, 12.32, 11.1, 10.6 for λ = 3, 7, 9, and 15, respectively. In a study done by Cui et al . [30], the intensity of the first peak for Nafion is 14.1, 6.9, 5.2, 4.1 for λ = 4.4, 6.4, 9.6,

12.8, respectively. This suggests that the hydronium ions are, on average, more likely to be found in close proximity of the sulfonate groups in Aquivion as compared to Nafion.

Figure 10-d shows the RDFs of oxygen-oxygen pairs of bulk water and confined water molecules within the Aquivion system at different λ. The peaks in the oxygen-oxygen RDFs reflects the local aggregation of water in Aquivion, which facilitates the formation of water clusters through which water and hydronium ions can migrate. 65 The first peak was positioned at 2.8 Å, corresponding to the closest contact of the two oxygen atoms (the experimental O-O distance for a water dimer is 2.73 Å). As λ increases, the intensity of the first peak decreases, which indicates that the number of water molecules surrounded by hydronium ions decreases.

The RDF between sulfur and water oxygen or hydronium oxygen implies clustering of sulfonate groups,

12 affecting the binding of H 2O and H 3O+ to SO 3- . The corresponding coordination numbers, n(r), as a function of distance are shown in Fig. S5. We have defined the first hydration shell at a cut-off distance of 4.4 Å between sulfur and water oxygen and hydronium oxygen, and the results are reported in Table

1. This cut-off value is chosen based on the fact that the n(r) curves are flat beyond this cut-off distance, as shown in Fig. S5a and Fig. S5b. The average number of water molecules around sulfonate groups increases as λ increases. This observation implies that at high λ, a large fraction of the water molecules behaves like bulk water molecules, while at low λ the water molecules are not bulk-like, as expected. As seen in Table 1, the coordination number of H 2O for λ = 3 is > 2, which indicates the number of ratio of

- H2O/SO 3 at this λ. This shows that with increasing λ more water molecules are shared among multiple

- - SO 3 and more water molecules enter the first hydration shell of SO 3 . The latter leads to the formation

+ - of a solvent-separated ion pair and movement of H 3O away from the SO 3 groups.

3.5 Dynamics of water and hydronium ions

The average number of hydronium ions around sulfonate groups reported in Table 1 is clearly decreasing

+ - as λ increases, which shows that H 3O ions move away from the SO 3 groups as water is added to the membrane. This can also be observed from the snapshots of Fig. 1. The average number of H 3O+ in the

- - first hydration shell of SO 3 is greater than unity, i.e., each SO 3 has at least one H 3O+ in its vicinity on average. When this number is observed to increase beyond one at lower water, a given H 3O+ will be

- interacting with multiple SO 3 groups. The H 3O+ is less likely to be free in Aquivion due to the greater

- - extent of SO3 clustering. The binding of H 3O+ by multiple SO 3 groups affects the mobility of H 3O+.

The interactions among sulfonate groups, water, IPA molecules, and hydronium ions are investigated by

RDFs at different volume fraction of IPA in the presence of Aquivion.

The RDF of sulfur-sulfur for different volume fractions is shown in Fig. 7a. Since the number of sulfur atoms in all systems is the same, the peak intensities can provide us with some structural information. As

13 shown in Fig. 7a, the intensity of the main peak increases with an increase in IPA volume fraction. The intensity of peak for the volume fraction of 1 is significantly larger than for other IPA volume fractions and its position is close to 4 Å, while for other IPA volume fractions, the peaks occur at the same position, at the radial distance of 5 Å. This huge discrepancy in the intensity of peaks can be related to the interaction of only IPA molecules with sulfonate groups while for other IPA volume fractions, we have a mixture of water and IPA molecules surrounding sulfonate anions. This indicates that at volume fraction of 1, the sulfonate anions are closer to each other compared to the condition where both IPA and water exist in the system. A second peak only appears at the presence of IPA. Increasing amount of IPA molecules in the system may result in the formation of sulfonate anions clusters.

The RDF for oxygen of water and sulfur at different volume fractions of IPA is depicted in Fig. 7b. It can be observed that augmentation of IPA molecules affects the first shell of water molecules around sulfonate anions, making it broader. Such an effect makes sense based on the sulfur-sulfur RDF, as for the hydronium ion shell around sulfonate anions. It is worth noting the right shift of the second peak.

This shift is due to the insertion of IPA molecules instead of water molecules around sulfonate anions .

More IPA molecules will be around sulfonate anions, results in pushing the second layer of water molecules further from the sulfonate anions It is reasonable to see a decreasing trend in the number of water molecules around sulfonate anions due to reduced number of the water molecules. This can be gleaned from the coordination number in Fig. 6a and Table 1. While the number of IPA molecules is increasing for all systems under study, the number of neighbouring water molecules is more than the number of IPA molecules. In such a system, the number of water molecules around a sulfonate anions is approximately 3-5 times more than the number of IPA molecules. The water molecule is smaller and more polar than IPA molecules, which allows the continuous formation of water clusters, however, the

IPA molecules has a bigger size and has some polar and non-polar zones. Having a bigger size does not assist in aggregation, in contrast for water molecules that can form cluster in a constant volume.

Figure 12-c shows the RDFs of sulfur atom and oxygen atom of hydronium ions for different IPA volume

14 fractions. For all systems, the hydronium ions act as bridges between the sulfonate anions. The first peak of the RDF for all systems appears around 3.8 Å. The number of neighbouring hydronium ions surrounding a sulfur atom can be obtained from the coordination number of S-H3O+ shown in Fig. 6b.

As seen in this figure, increasing the IPA content does not change the number of neighbouring hydronium ions around a sulfur atom and such a coordination number is independent of the IPA content. For the volume fraction of 1 when we have only pure IPA in the system, the coordination number is about 2.2 times larger than the one for mixture of IPA/water. The second peak in Fig. 7c is shifted to the right and narrower with increasing proportion of IPA molecules. Such a behavior is expected on the same basis as that for S-S RDF. This RDF shows that an increase in IPA volume fraction leads to formation of the second layer of sulfonate anions and since hydronium ions serve as bridges between sulfonate anions, consequently formation of second layer of hydronium ions around sulfonate anions should be better ordered as well. The RDF of S-S shows that by increasing the volume fraction of IPA, the first peak is made wider, therefore, the main peak of the hydronium-sulfur RDF is expected to be wider.

The first peak in the RDF of O H2O -OH2O at different volume fraction of IPA occurs at 2.7 Å, as shown in

Fig. 7d which corresponds to the OH2O -OH2O distance in the first hydration shell. Fig. 7f displays the

RDFs of sulfur atoms of sulfonate anions toward the oxygen atom in the presence of IPA. The first peak occurs at 3.9 Å, and a comparison between the intensity of this peak and the peak in S-OH2O RDF, indicates that the intensity of the peak in S-OH2O is about two times greater than the peak in S-OIPA . It can be interpreted that in Aquivion membrane swollen with water-IPA mixture, solvent competes with water to surround sulfonate anions. Water molecules are more able to solvate the sulfonate anions and thereby a greater number of water molecules can be found in the solvation shell around sulfonate anions.

The average number of IPA molecules around sulfonate anions increases as the IPA volume fraction increases. The average number of IPA for φ IPA of 0.24, 0.46, and 0.66 is less than 1, which indicates less

- IPA molecules can be shared by multiple SO 3 . For φIPA of 0.83 and pure IPA, more IPA molecules will

- enter the first hydration shell of SO 3 . In comparison with water molecules, the average number of IPA

- molecules around SO 3 at φ IPA = 0.83 is about twice less than that for water molecules.

4. Conclusions

We have implemented atomistic MD simulations to study the structural and dynamic properties of aqueous Aquivion ionomer dispersions. This study, for the first time, provides a comprehensive molecular description of dynamic properties of Aquivion at atomistic level. There is a distinct difference between dynamics of water and proton in Aquivion to those in Nafion solutions. We have introducds a novel coarse-grained model in order to compare structural properties of Nafion and Aquivion using the same coarse-graining strategy without altering the accuracy of the model and parameters therein. The latter is critical to demonstrate and understand differences between structural properties of Aquivion- and Nafion-based membranes at meso-scale.

Water contents ranging from 3 to19 water molecules per sulfonic acid head group were evaluated and mixtures of water and IPA with different relative volume fractions were considered. To facilitate the comparison with Nafion ionomer dispersions, we developed a coarse-grained MD model that can be used for both SSC and LSC ionomers. Simulation results show a good agreement of ionomer density, self- diffusivity of water, and water coordination number for Aquivion ionomer dispersions in water or water/IPA with those in the literature. The latter verifies the suitability of the developed force fields in this study.

The morphology of hydrated Aquivion at different water contents showed that the water cluster distribution is more dispersed at lower hydration levels while there is a higher tendency to form a dense connected water network at higher hydration levels. The simulations showed that by adding IPA molecules to the system, the connectivity of the water network decreases. The latter makes diffusion coefficient for hydronium ions to be 4-5 times smaller than that for water. The high diffusion coefficient of hydronium ions at λ = 19 in comparison to other hydration levels is a result of solvation stabilization

16 of hydronium ions at distances far away from sulfonate groups which can be confirmed with snapshots of structures and coordination numbers.

One should note that the atomistic MD study presented here was an effort to establish the foundation of force-field development for a new coarse-grained model. The new coarse-grained model has been verified against atomistic MD. It will be used in further MD studies of membrane and catalyst layers containing short or long side-chain ionomers.

5. Acknowledgements

The authors gratefully acknowledge the financial support from NSERC and Automotive Fuel Cell

Cooperation Corp. (AFCC). JJ would like to gratefully acknowledge Madhu Saha and Darija Susac for fruitfull discussions and insights on interpretation of modeling data and experimental results.

6. References

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Table 1. Coordination number at first hydration shell for water, hydronium ion, and IPA at 298.15 K

hydration number for different water content atomic types distance( Å) 3 7 11 15 19 S-O of H 2O 0-4.4 2.155 3.40 4.42 4.59 4.99 + S-O of H 3O 0-4.4 2.24 1.68 1.24 1.20 1.03

hydration number for different volume fraction of IPA 0.24 0.46 0.66 0.80 1.00 S-O of H 2O 0-4.3 4.966 4.268 3.75 3.08 - + S-O of H 3O 0-4.3 0.693 0.795 0.872 0.95 2.19 S-O of IPA 0-4.3 0.448 0.699 0.96 1.36 1.99

a) b) c)

Figure 1. Final snapshots of MD simulations of hydrated Aquivion ionomer at different water contents. The side-chain are shown in yellow color, hydrophilic domains (water and hydronium ions) are shown in cyan and purple colors, while hydrophobic domains (backbones) are shown in silver color; a) entire system, b) system with hidden water, and c) only water network

c) Figure 2. Final snapshots of MD simulations of Aquivion ionomer at different volume fractions of IPA. The side-chain, backbone, water, hydronium, and IPA are shown in yellow, silver, cyan, purple, and orange, respectively; a) entire system, b) system with hidden water and IPA, and c) only water network

Figure 3. Atomic and coarse-grained MD density of hydrated Aquivion in comparison with density of Nafion and literature data. All values are calculated under operating temperature 298.15 K and pressure 1 atm.

Figure 4. Simulated densities using Atomic MD for hydrated Aquivion at different φ of IPA. All values are calculated under operating temperature 298.15 K and pressure 1 atm.

Figure 5. Water and hydronium diffusion coefficients as functions of water content: results from present atomistic and coarse-grained simulations in comparison with the literature

Figure 6. Radial distribution functions (RDF) for a) S-S, b) S-OH2O, c) S-OH3O and d) O H2O -OH2O at four different water contents

Figure 7. Radial distribution functions (RDF) for a) S-S), b) S-OH2O , c) S-OH3O+, d) O H2O -OH2O , and f) S-OH IPA at different volume fraction of IPA