Computational Study of Microhydration in Sulfonated DielsâAlder Poly

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A: Molecular Structure, Quantum Chemistry, and General Theory

Computational Study of Micro-Hydration in Sulfonated Diels Alder Poly(Phenylene) Polymers Todd M. Alam J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01354 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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The Journal of Physical Chemistry

Computational Study of Micro-Hydration in Sulfonated Diels Alder Poly(Phenylene) Polymers Todd M. Alam* Department of Organic Material Science, Sandia National Laboratories, Albuquerque, NM 87185 USA * Corresponding Author: [email protected], Fax: (505) 844-2974

ABSTRACT: The nature of micro-hydration in sulfonated Diels Alder poly(phenylene) (SDAPP) polymer membranes is explored using ab initio and density functional theory (DFT) electronic structure calculations. The impact of the aromatic poly(phenylene) structure, including cooperative effects between multiple spatially adjacent sulfonic groups, on the hydration environment is addressed using a series of DFT B3LYP/6-311** optimized structures for different SDAPP•nH2O clusters. In addition, larger SDAPP polymer fragments, along with selected hydrophilic domain structures extracted from molecular dynamic (MD) simulations, are also evaluated using ONIOM HF/PM6 semi-empirical calculations. The SDAPP clusters reveal that spontaneous proton dissociation occurs at low levels of hydration to form sulfonic-acid associated H3O+ contact ion pairs (CIP), which then evolve into solvated CIP at higher hydration levels. For multiple sulfonic acid groups located on the poly(phenylene) sidechains, the hydration energies are a function of the relative acid location and backbone configuration. Variations in the phenylene backbone torsional angles allow remote sulfonic acids to adopt an optimal separation to produce an extended hydrogen bonded network of waters between the SDAPP acids groups. These calculations provide a baseline to help describe the proton transport and hydration behavior of SDAPP membranes.

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1. INTRODUCTION Polymer exchange membranes (PEM) are used in and directly impact the performance of energy conversion and storage devices including fuel cells, electrolyzers, and redox flow batteries.1-6 These membranes maintain physical isolation and charge separation between the anode and cathode, while providing high charge carrier transport. Perfluorosulfonic acid (PFSA) polymers, including the commercially available Nafion (DuPont), remain the benchmark PEM material in current applications because of good chemical stability, low permeability to reactant species, and high conductivity. Due to the desire for operations at high temperatures and low hydration levels (conditions where Nafion underperforms), there is continued interest in developing alternative fluorinated, as well as non-perfluorinated hydrocarbon polymers and composites.7-11 Proposed alternative PEMs include membranes derived from sulfonated polyether ether ketone (SPEEK), sulfonated polyether sulfone (SPES), polybenzimidazole (PBI), sulfonated poly(arylene ether sulfone) (BPSH), sulfonated poly(arylene thioether sulfone) (PATS), sulfonated polyimide (sPI), sulfonated poly(phenylene) sulfone (sPSO2), and sulfonated poly(phenylene) (SPP) polymers, among others. In this work, we continue to pursue the development of the aromatic sulfonated Diels Alder poly(phenylene) polymer (SDAPP, Figure 1),12-13 and related derivatives,14 for use as PEMs. In SDAPP, the aromatic nature and lack of hetero-atoms within the polymer backbone leads to high thermal stability (> 100 °C), improved chemical durability under strong acidic, basic and oxidative conditions, excellent permeability barrier to fuels, and low production costs. While SDAPP membranes have shown promising results in fuel cells and vanadium redox flow batteries,15-18 a basic understanding of the aromatic polymer properties that lead to the high conductivity observed under different environmental conditions (e.g. temperature, humidity, fuel type) is still lacking. For SDAPP polymers, initial molecular


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dynamics (MD) simulations,19 and only a handful of experimental structural characterization studies, including IR and NMR,20 SAXS,21 and double-quantum (DQ) NMR spin diffusion studies of domain sizes,22-23 have been reported. Basic unanswered questions remain for SDAPP, including “What is the atomic-level behavior of the sulfonated aromatic acidic protons in extended hydrogen bonded networks?”, and “What morphologies are formed during hydration-induced nanophase segregation?” Factors controlling PEM proton conduction are related to the i) complexity, ii) connectivity and iii) cooperativity of the proton environment.24-27 Complexity encompasses the initial dissociation of the acidic hydrogen, formation of a hydronium cation in direct contact with the conjugate base, subsequent solvation and lifetime of this hydronium ion contact pair,28 and related hydrogen and water diffusion.26 Connectivity factors range from the local atomic level hydrogen bond network (water clustering), continuity of the hydrogen bond network,28 to the mesoscale percolation of hydrophilic domains through the polymer membrane, while cooperativity effects include enhanced transport due to sidechain dynamics along with the amphoteric behavior of water.24-25 The connectivity/cooperativity also includes the sulfonated-mediated (or trimethyl ammonium mediated) “ion passing mechanism” between overlapping hydration spheres recently proposed for PEMs and anion exchange membranes (AEMs).29-30 Ultimately an understanding of the hydration and transport properties in these sulfonated aromatic polymers will allow the identification and contribution of the different factors controlling ion conductivity. A variety of computational methods have been used to explore the local structure and proton transport mechanisms in conducting materials including PEMs, crystalline solid acids and proton-conducting solid oxides.31-32 For PEMs, the majority of these investigations have concentrated on PFSA polymers, and include ab initio and density functional (DFT) studies of


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representative clusters,33-39 MD simulations,27-28 multi-state empirical valence bond (MS-EVB) MD simulations,30, 40 and ab initio molecular dynamics (AIMD) simulations.41-42 For aromatic based polymer systems, DFT investigations of sulfonated polyether ether ketone (SPEEK) and related fragments,38, 43-44 MD simulations on sulfonated polysulfone,45-46 and the more recent MD simulations of SDAPP polymers,

19

have been published. In this paper, we present DFT and ab

initio calculations describing the initial hydration of SDAPP polymers, and explore the impact that the rigid aromatic phenyl group backbone and sidechains have on the hydration behavior. These results can be compared to previous investigations of sulfonated aromatic polymers. The optimized structures and hydration energies for a series of SDAPP clusters with a complexity ranging from small clusters containing a single sulfonic acid fragment to larger hydrophilic domain structures containing multiple sulfonic acids and hydration waters are described.

2. THEORETICAL CALCULATIONS Constituent fragments of the SDAPP polymer were selected to determine the local hydration environment structures and water adsorption energies. The smaller SDAPP clusters in the gas phase were optimized and hydration energies calculated using DFT with Becke’s three parameter exchange functional, the LYP correlation functional (B3LYP) and the 6-311** (i.e. 6311(d,p)) basis set as implemented in the Gaussian 09 suite of programs47 (Gaussian Inc., Wallingford, CT). Multiple starting structures with randomly placed hydration water were optimized. For some of these clusters obtaining convergence was challenging, and required an initial Hartree-Fock (HF) optimization using a lower level basis sets prior to the final B3LYP/6311** optimization. The inclusion of diffuse functions to the basis set, or changing the exchange


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correlational functional, was previously found to have minimal impact on the hydrogen bonding network in perfluorosulfonic acids and sulfonated poly(phenyl sulfone) ionomers, and were not addressed in the current study.44, 48 Two types of SDAPP clusters with a single sulfonic acid (SA) were studied, toluene sulfonic acid (Tol-SA) and diphenyl sulfonic acid (DIP-SA) clusters, with the SA group attached to either the ortho, meta and para position with respect to the bonded methyl or phenyl group (i.e. o-Tol-SA, m-Tol-SA, p-Tol-SA, o-DIP-SA, m-DIP-SA, and p-DIP-SA). The micro-hydrated Tol-SA•n(H2O) and DIP-SA•n(H2O) (n = 1 - 6) clusters were optimized and the H O water adsorption energies ∆E Ads determined using 2

H2O ∆E Ads = ( Tol-SA ) E Opt ( Tol-SA ⋅ nH 2O ) − E Opt ( Tol-SA ) − nE Opt ( H 2O ) H2O ∆E Ads = ( DIP-SA ) E Opt ( DIP-SA ⋅ nH 2O ) − E Opt ( DIP-SA ) − nE Opt ( H 2O )

(1)

where the first terms are the energies of the gas-phase optimized Tol-SA•n(H2O) or DIP-SA• n(H2O) clusters, with the remaining terms being the energy of optimized Tol-SA, DIP-SA and H2O clusters. Using Eqn. 1, ∆E Ads includes the energy required for the SA and H2O to adjust from their optimized gas-phase configurations to the final structure occurring in the micro-hydrated clusters. In this definition ∆E Ads < 0 is an exothermic adsorption process. No basis set superposition errors (BSSE) were included in the reported energies. In these clusters containing a single SA, n is equivalent to λ which is defined as the number of waters per sulfonic acid group. For the larger SDAPP repeat unit clusters, we utilized the ONIOM (Our own N-layered Integrated Molecular Orbital and Molecular Mechanics) method,49 as implemented in Gaussian 09 with two levels of computational complexity (even though Gaussian can incorporate three levels of theory). The single SDAPP polymer repeat unit (Figure 1) was extracted and truncated with hydrogen to satisfy the aromatic carbon valence. For these calculations, the high quantum level


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used DFT B3LYB/6-311** methods, and was chosen to correctly describe the hydrogen bond formation and sulfonic acid proton dissociation in the hydrated clusters (as represented by the ball and stick portion in Figure 3). The low level ONIOM layer describes the remainder of the aromatic polymer backbone (wire frame portion in Figure 3), and used the semi-empirical PM6 (parametric method number 6) method.50 These repeat unit micro-hydrated clusters were denoted as SDAPP1A•n(H2O) and SDAPP-2A•n(H2O), where 1A (S = 1) and 2A (S = 2) correspond to one or two sulfonic acid groups attached to the DAPP repeat unit, respectively, located at the para position of the terminal phenyl sidechains (Figure 1). In addition, the influence of different relative SA positions in SDAPP-2A clusters was also explored. Clusters where the two acid groups are spatially “remote” from each other with non-interacting hydration spheres, or clusters where the acid groups are “adjacent” on neighboring terminal phenyl rings allowing some overlap of the hydration spheres were evaluated. We also simulated SDAPP-2A clusters were the two SA are located on non-adjacent phenyl groups within the polymer repeat unit. For these clusters, changes in the phenyl backbone “torsional” angles ( ε , ξ ) allow for a more complete overlap of the acid group hydration spheres (Figure 1). To address the influence of even larger and extended hydrated hydrophilic domains, two representative clusters were extracted from recent molecular dynamics (MD) simulations of the SDAPP polymer (Figure 5).19 The details of the MD simulations have been previously described, and are not reproduced here.19 For the MD simulations, water was treated using the rigid 4-site TIP4P/2005 model,51 while the hydronium cation used a flexible 4-site TIP4P model.52 The TIP4P models do not allow for the transfer of protons between the water and hydronium species, such that these types of proton dynamics are not captured during the MD simulations. Single point energies were obtained using HF/6-311G methods, where the aromatic polymer phenyl backbone


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in the MD clusters was truncated and removed, retaining only the terminal sulfonated phenyl groups. For the relaxation analysis of the hydrophilic domain SA/H2O hydrogen bonded network, the spatial positions of the terminal phenyl groups were frozen, while the sulfonate, water and hydronium positions were reoptimized. Due to the size of the MD hydrophilic domains, a slight reduction in the level of theory (HF/6-311G ) became computationally necessary compared to the smaller cluster energetics described above. The small hydrophilic domain extracted from the S = 1, λ = 3 (per SDAPP repeat unit) MD simulation contained 8 phenyl-SA, 17 H2O and 8 H3O+ groups with an average radius of gyration of Rg = 7.6 Å. Here λ defines the number of H2O molecules or hydronium (H3O+) ions per sulfonate group. The larger hydrophilic domain extracted from the S = 4, λ = 10 MD simulations contained 11 phenyl-SA, 42 H2O, and 11 H3O+ groups with Rg = 7.7 Å. Because these hydrophilic domains were extracted from classical MD simulations,

the proton was already dissociated from the SA, and the hydronium cation retained a distinct Eigen structure. Separate adsorption hydration energies for the H2O and H3O+ species ( ∆E Ads ) were defined using

(

)

H 2O = ∆EAds (i ) E ( Cluster [ m H 2O] ) − E Cluster ( m − 1) H 2O(i )  − E Opt ( H 2O )

(

)

(

)

H 3O ( j ) E Cluster  n H 3O +  − E Cluster ( n − 1) H 3O + (j )  − E Opt ( H 3O + ) = ∆EAds +

(2)

and represent the energies required to remove a single individual H2O or H3O+ from the MD-based structure. Single point energies were evaluated after the removal of each individual H2O or H3O+. The resulting m-1 clusters (H2O) and n-1 clusters (H3O+) were not reoptimized to help distinguish the adsorption energies of H2O or H3O+ from energy changes due to hydrogen bond network rearrangement during optimization. This definition also avoided optimization issues of replacing the confining polymer backbone (truncated and removed) with a vacuum in the extracted domains.


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This definition of the adsorption energy used for these MD domains (Eqn. 2) is different than the adsorption energy used for the smaller clusters (Eqn. 1), but introduces only a minor change in the magnitude of the adsorption energies per water determined.


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Figure 1. Structure of the SDAPP polymer repeat unit (left) with a 3D representation (right). This repeat unit is functionalized with three sulfonic acid groups (i-iii) in the para position of pendant phenyl side chains corresponding to S = 3 (IEC = 3.0 meq/g). The torsional angles (α, β, ε, ξ) control the conformation of the polymer sidechains and backbone. The relative orientation of the sulfonic acid groups is dominated by changes in the backbone angles ε and ξ, as discussed in the text.


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3. RESULTS AND DISCUSSION para-Toluene and para-Diphenyl Sulfonic Acid. A basic building block of the SDAPP polymer is the pendent phenyl group sulfonated in the para position, and is the starting point for discussion of water environments that make up the hydrophilic domains. The DFT optimized, minimum energy structures of the micro-hydrated para-toluene sulfonic acid [p-Tol-SA•n(H2O)] and paradiphenyl sulfonic acid [p-DIP-SA•n(H2O)] clusters (n = 0 to 6) are shown in Figure 2 (p-DIP-SA) and Figure S1 (p-Tol-SA, supplemental material). Hydration adsorption energies ( ∆E Ads ) defined by Eqn. (1) for these clusters are summarized Table 1 for both (p-DIP-SA) and (p-Tol-SA). Optimized structures and adsorption energies for p-Tol-SA have been previously presented multiple times,24, 31, 38, 53 but are given here for direct comparison in the current discussion. Ab initio and DFT studies of p-Tol-SA have been used as the benchmark for understanding proton dissociation and hydration energetics in sPEEK, as well as addressing the electron-withdrawing impact of sulfonated aromatic systems in comparison with the trifluoro group of triflic acid. While the extended aromatic system of SDAPP (Figure 1) is not expected to have any significant impact on the electronegativity of the oxygen atoms in the sulfonic acid groups, we elected to still directly compare the micro-hydration behavior of p-DIP-SA and the p-Tol-SA. Both the structure of the water hydrogen bonding environment (Figure 1) and ∆E Abs (Table 1) for these two hydrated aromatic clusters are almost identical, confirming that there is minimal impact of the extended DIP aromatic ring on the electronegativity of the sulfonate group. As previously observed,53 no spontaneous dissociation of the sulfonic acid proton occurs until the n = 3 hydration level is reached. In the p-DIP-SA•n(H2O) clusters addition of the first two hydration waters results in a gradual lengthening of the SO-H bond (see Figure 1) from 0.984 Å (n = 0) to 1.05 Å (n = 2). This bond length then increases significantly to 1.55 Å at n = 3 signaling proton dissociation to produce


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a distorted hydronium cation (H3O+) that is coordinated to the sulfonate group via a contact ion pair (CIP). For all hydration levels n ≥ 3, H3O+ formation is favored as the lowest energy configuration. The low energy symmetric n = 4 clusters revealed a “solvated” H3O+ CIP where the hydronium is no longer directly hydrogen bonded to the sulfonate group, but instead is entirely hydrogen bonded to waters residing in the immediate hydration sphere of the sulfonate. The optimized n = 5 cluster shows H3O+ once again as a direct CIP to the sulfonate group with hydrogen bonding to two adjacent H2O molecules that are in turn hydrogen bonded to the remaining sulfonate oxygens. The direct CIP and solvated CIP are very similar in energies and can easily interconvert between the different H3O+ configurations in both the p-Tol•n(H2O) and pDIP•n(H2O) clusters. The O-O bond distance between the H3O+ CIP and the sulfonate oxygen is ~ 2.59 Å, which compared to the O-O bond distance for the Zundel (H5O2+, 2.45 Å)54 and the Eigen (H9O4+, 2.6 Å) cations of water. For n ≥ 6 the H3O+ once again becomes solvated and is separated from the sulfonate anion, existing as a true Eigen cation with an asymmetric O-O separation from the sulfonate oxygen of 4.18 and 3.92 Å. These results are consistent with previous reports that for high hydration levels the direct H3O+-sulfonic acid CIP is no longer the lowest energy configuration. Instead the cluster evolves to the hydrated H3O+ CIP configuration being the most stable. The observed spontaneous hydrogen dissociation in SDAPP occurs at the same hydration level (n = 3, λ = 3) reported for PFSA polymers. In PFSA clusters at n = 3 to 5, the H3O+ forms a strong CIP with the sulfonate anion, and becomes a solvated CIP for n ≥ 6. Equivalent DFT studies for hydrated SPEEK clusters showed that the spontaneous proton dissociation did not occur until n = 6 with the formation of a H3O+ CIP.43 For SPEEK the solvated environment was approximately -8 kcal/mol lower in energy than a H3O+ CIP configuration at the n = 6 hydration. Comparing these simulations suggests that SDAPP will have proton dissociation and subsequent


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CIP solvation at significantly lower hydration levels than SPEEK. The similarity between SDAPP and PFSA hydration behavior suggests that their conductivities will be comparable; consistent with experimental observation.15 Analogous studies of the polymer hydration environment in SPES also predicted spontaneous proton dissociation at n = 3, with the solvated H3O+ CIP also being the low energy configuration at the n = 4 hydration levels. For SPES, the energy difference between the H3O+ CIP and the solvated H3O+ configuration was small (~0.8 kcal/mol), leading to easy isomerization between these two H3O+ environments.46 If proton dissociation was the dominant factor controlling conductivity at low hydration levels, SPES and SDAPP should have similar conductivity; but as discussed in later sections, the nanophase structure of the hydrophilic domains can easily overshadow small differences in the H3O+ solvation energetics observed in these small clusters. The predicted ∆E Ads for the [p-Tol-SA•n(H2O)] and [p-DIP-SA•n(H2O)] (n = 1 to 6) clusters are essentially equivalent (Table 1), with the average hydration adsorption energy (

∆E Ads / n ) ranging from approximately -16.6 to -19.3 kcal/mol. There is a -1.4 kcal/mol decrease between n = 1 and n = 2, reflecting both H2O-H2O and H2O-SA hydrogen bonding contributions with increasing hydration. ∆E Ads / n showed a maximum at n = 4, and then decreased slightly with continued hydration, consistent with the formation of the solvated H3O+ at n = 4. These DFT results for [p-DIP-SA•n(H2O)] confirm the general assumption that local electrostatics, proton dissociation behavior, and hydration structures of SDAPP are indeed similar to other sulfonated aromatic polymers, and that the extended phenyl backbone has a minimal effect on the local hydration structure. It should be noted that we do not argue that these optimized micro-hydrated structures represent the exact hydrogen bond network, but rather serve to provide a comparison


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between systems and functionality, along with an estimate of the differences in hydration energies occurring in these sulfonated aromatic systems.

Figure 2. Optimized clusters for the para-diphenyl•n(H2O) clusters (n = 0 - 6) obtained using DFT B3LYP/6-311** methods. Select O-H distances between the sulfonic acid oxygen and the hydrogen bonded water/hydronium protons are shown. Spontaneous proton dissociation from the sulfonic acid to form a H3O+ contact ion pair (CIP) occurs at n = 3, with a solvated H3O+ being observed at n = 4 and 6 hydration levels.


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Table 1. Micro-hydration Adsorption Energies of Optimized para-Toluene Sulfonic Acid and para-Diphenyl Sulfonic Acid Fragments. p-Tol-SA p-DiP-SA n ΔEAds ΔEAds/ n n ΔEAds ΔEAds/ n H2 O (kcal/mol)a (kcal/mol) H2 O (kcal/mol)a (kcal/mol) 1

-16.6

-16.6

1

-16.6

-16.6

2

-36.0 (-19.4)

-18.0

2

-36.0 (-19.4)

-18.0

3

-56.0 (-20.0)

-18.7

3

-56.4 (-20.4)

-18.8

4

-77.1 (-21.1)

-19.3

4

-77.5 (-21.1)

-19.4

5

-93.6 (-16.5)

-18.7

5

-94.1 (-16.6)

-18.8

6

-112.5 (-18.9)

-18.8

6

-112.8 (-18.7)

-18.8

Values in parentheses are the adsorption energy gains per additional water with ΔEAds calculated using Eqn. (1). a

Impact of Sulfonic Acid Linkage Location. The general synthetic method used to sulfonate the DAPP polymer employs treatment with chlorosulfonic acid or trimethylsilyl chlorosulfonate, which produce primarily para-substituted sulfonic acids (with respect to the aromatic backbone linkage) on the pendant phenyl groups.12-14 More elaborate synthetic schemes could be used to sulfonate in the ortho and meta position of the phenyl side chains, but this modification has not been synthetically pursued to date. It is therefore interesting to evaluate whether the location of the sulfonic acid impacts or influences the acid proton dissociation behavior, changes the microhydration structure, or changes the energy of water adsorption. Calculated hydration energies

∆E Abs are presented in Table 2 for the DIP-SA•n(H2O) clusters with different SA binding positions, and in Table S1 for the corresponding Tol-SA•n(H2O) clusters. Figure S2 (supplemental) shows the optimized low energy structures found for the hydrated ortho-, meta- and para-DIPSA•n(H2O) clusters (and can be compared to Figure 2). The resulting hydrogen bonded network


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structure is identical at low hydration, but a slight anisotropy in the water cluster shape is noted for the ortho- DIP-SA•n(H2O) n = 5 and 6 clusters due to interaction of the water with the nearby aromatic ring (Figure S2). The differences in hydration ∆E Abs for the different SA locations are minimal, with the hydrated ortho-DIP-SA clusters having slightly increased exothermic adsorption energy (< 0.2 kcal/mol). These results predict that the location of isolated sulfonic acid at different positions on the phenyl sidechain will produce almost no change to the local proton/water hydrogen bonding behavior, until overlap of different adjacent sulfonic acid hydration spheres occurs (see additional discussion below). This invariance to linkage location will be especially prevalent at low sulfonation levels where no neighboring sulfonic acids groups is expected to occur. At this juncture, the small differences predicted do not warrant the need to pursue additional synthetic control during sulfonation. Table 2: Micro-hydration Adsorption Energies of Optimized para-, meta- and orthoDiphenyl Sulfonic Acid Fragments. p-DiP-SA

m-DiP-SA

o-Dip-SA

n H2O

ΔEAds (kcal/mol)a

ΔEAds/ n (kcal/mol)

ΔEAds (kcal/mol)a


ΔEAds (kcal/mol)a


1

-16.6

-16.6

-16.5

-16.5

-16.9

-16.9

2

-36.0 (-19.4)

-18.0

-35.8 (-19.3)

-17.9

-35.9 (-19.0)

-18.0

3

-56.4 (-20.4)

-18.8

-56.2 (-20.4)

-18.7

-57.1 (-21.2)

-19.0

4

-77.5 (-21.1)

-19.4

-77.3 (21.1)

-19.3

-77.8 (-20.7)

-19.5

5

-94.1 (-16.6)

-18.8

-93.9 (-16.6)

-18.8

-94.2 (-16.4)

-18.8

6

-112.8 (-18.7)

-18.8

-112.1 (-18.2)

-18.7

-113.8 (-19.6)

-19.0

Values parentheses are the adsorption energy gains per additional water with ΔEAds calculated using Eqn. (1). a


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Role of Interaction Between Sulfonic Acids. Extensive computational studies into the impact of protogenic group spacing/distribution in perfluorosulfonic acid (PFSA) membranes have been performed.25, 33-34, 55 These studies revealed that for a “kinked” PFSA backbone, higher hydration adsorption energies occur, and that spontaneous deprotonation occurs at lower hydration levels results.34, 37 Similarly, the aromatic backbone configurations in sPSO2 polymers that leads to the interaction of adjacent sulfonic acid groups was shown to greatly improve the low hydration behavior.44 Cooperative interaction between protogenic groups and the initial waters of hydration are important factors for the deprotonation of the sulfonic acid and subsequent structure of the hydrogen bonding network of the formed H3O+. SDAPP polymers do not have the same conformational flexibility as PFSA materials because of the aromatic SDAPP backbone and reduced degrees of freedom (See torsional angles in Figure 1) in comparison to the PFSA CF2 backbone and side chain conformations. While sPSO2 polymers also contain an aromatic backbone, they are more flexible than SDAPP due to the presence of a sulfone group between the phenyl rings. Regardless, increased sulfonation on adjacent phenyl sidechains is predicted to impact hydration energies at low water content. These types of nearest-neighbor SA interactions will become more prevalent at the higher levels of sulfonation (S ~ 2 to 3.5) employed for SDAPP membranes. Figures 3 and 4 show representative optimized structures for a series of micro-hydrated SDAPP repeat unit clusters with different degrees of sulfonation and hydration. We explored systems containing either a single sulfonic acid per repeat unit (S = 1, SDAPP-1A), or two sulfonic acids per repeat unit (S = 2, SDAPP-2A). Table 3 summarizes the hydration adsorption energies

∆E Ads for these different SDAPP-1A and SDAPP-2A clusters. In addition, we assessed the role of through space interactions between neighboring sulfonic acids. SDAPP clusters where the two


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sulfonic acids (and hydration spheres) do not interact are designated as “remote” (SDAPP2A:remote), while those clusters containing sulfonic acids on pendant phenyl rings attached to the same backbone phenyl ring (the sidechain linkages are ortho to each other, SA (i) and (ii) in Figure 1) are designated as “adjacent” (SDAPP-2A:adjacent). The rotation of the phenyl groups around the α and β torsional angles (Figure 1) will not alter the S-S distance (~ 8 Å in the n = 0 cluster) between these adjacent sulfonic acids since the para sulfonation position is coincident with the rotation axis. Systems where the two sulfonic acids are attached to remote phenyl sidechains on different sides of the backbone phenyl ring are designated as “torsional” clusters (SDAPP2A:torsional). For these systems, rotation around the ε and ξ angles results in a wide range of possible S-S distances (See Figure S3). With sulfonation possible on the six pendant phenyl groups in a SDAPP repeat unit, there are multiple sulfonic acid configurations possible that are a function of both the degree of sulfonation and the relative positions of the acid groups. It is predicted that these configurations generally fall into the three basic categories: isolated, adjacent and torsional. It should also be noted, because of the Diels Alder synthesis, the backbone linkage can exist in either a meta or para position. For the clusters evaluated we assumed only a para bonding arrangement, with the meta linkage still producing a range of S-S distances in “torsional” complexes. For the SDAPP-1A•n(H2O) clusters, the optimized structure of the hydrated domain and the adsorption energies ∆E Ads are equivalent to those seen in the p-DIP•n(H2O) clusters (Figure 2, Table 1) with the spontaneous deprotonation occurring at n = 3 and the formation of a hydrated H3O+ CIP at n = 4 hydration levels. This demonstrates that the contribution from long range polymer structure is minimal for isolated sulfonic acids that might occur at low sulfonation levels, and will not be discussed further. As expected, the SDAPP-2A•n(H2O):remote clusters have the


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same hydration structure and adsorption behavior as the SDAPP-1A•n(H2O) clusters if equivalent λ values (waters per sulfonic acid) are compared (see Table 3). These remote sulfonic acids have hydration spheres that do not interact or overlap up to the λ = 6 levels investigated, and are essentially individual SDAPP-1A configurations. For the SDAPP-2A•n(H2O):adjacent clusters differences were observed in both the hydration structure and adsorption energies compared to the SDAPP-1A and SDAPP-2A:remote configurations. The very first hydration water (n =1, λ =0.5) is hydrogen bonded between the two sulfonic acids with a ∆E Ads = -20.3 kcal/mol, which is -4 kcal/mol lower in energy than the first hydration water in the other SDAPP clusters. This single bridging H2O is seen for all n ≥ 1 low energy structures. Additional hydration produced waters that were hydrogen bonded to either the remaining sulfonic acid oxygens or the bridging H2O, with ∆E Ads and ∆E Ads / n being equivalent to the values for previously discussed clusters. With additional waters coordinating to either acid group, the general shape of the water domain tends to be elongated (Figure 3), most notably at the lower hydration levels. Spontaneous deprotonation to form a H3O+ CIP was not observed until n = 5, but this corresponds to λ = 2.5 which is actually lower than the hydration level for deprotonation observed in the p-DIP-SA, SDAPP-1A and SDAPP-2A:remote clusters (λ = 3). It appears that the interaction of the hydration water with two SA increases ∆E Ads at low hydration levels, but because the S-S distance is fixed and does not vary in this adjacent configuration, the interaction does not dominate ∆E Ads with increasing hydration.


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Figure 3. Representative optimized structures for the hydrated SDAPP repeat unit clusters with either one (SDAPP-1A) or two sulfonic acids (SDAPP-2A) at various hydration levels. For SDAPP-2A clusters, a remote and an adjacent sulfonic acid configuration are shown, corresponding to non-interacting and interacting hydration spheres around the individual SA groups.


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Figure 4: Optimized clusters for the torsional SDAPP-2A•nH2O clusters (n = 0 - 6) obtained from DFT B3LYP/6-311** calculations. Select S-S distances between the two sulfonic acids are included. Spontaneous proton dissociation from one of the sulfonic acids to form a H3O+ contact ion pair (CIP) occurs at n = 3, λ = 1.5. A solvated H3O+ was not observed in these optimized n = 1 to 6 hydration clusters.


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The hydrated torsional SDAPP-2A•n(H2O) clusters and ∆E Ads are shown in Figure 4 and Table 3, respectively. The ability of the S-S distance to vary with changing ε and ξ angles directly contributes to the observed behavior. For the non-hydrated cluster (n = 0) the sulfonic acids are hydrogen bonded to each other, with a S-S distance of ~ 4.4 Å. The torsional complex is -28.6 kcal/mol lower in energy than the SDAPP-2A:remote cluster (where the S-S distance cannot vary). This is a significantly stabilized structure and is reminiscent of other acid dimers observed in organic materials. The first hydration H2O now bridges the two sulfonic acids and disrupts the acid dimer (Figure 4), with the S-S distance relaxing to ~5.5 Å. The adsorption energy ∆E Ads for the first H2O is only -13.6 kcal/mol (Table 3), but this is in relation to the non-hydrated hydrogen bonded dimer, or -42.4 kcal/mol in relation to the non-hydrated remote SDAPP-2A. Subsequent hydration H2O produce changes in ∆E Ads similar to the other SDAPP-2A clusters, with only small additional variation of the S-S distance. Spontaneous proton dissociation from one of the sulfonic acids to form a H3O+ contact ion pair (CIP) occurs at n = 3, λ = 1.5, which is significantly lower than the λ = 3 levels observed in the other SDAPP clusters, while both acid protons showed spontaneous dissociation at n = 6 (λ =3). A solvated H3O+ was not observed in any of these optimized n = 1 to 6 (λ = 0.5 to 3) hydrated clusters. It is clear the ability of sulfonic acids and their hydration spheres to interact changes the hydration behavior significantly. Higher sulfonation content, and interactions between sulfonic acids groups from different repeat units in the same or different polymer chains, could also give rise to cooperative effects during hydration. These are explored in the MD hydrophilic domains section below.


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Table 3. Micro-hydration Adsorption Energies of Optimized SDAPP-1A and SDAPP-2A Sulfonic Acid Fragments. SDAPP-1A

a

SDAPP-2A

SDAPP-2A

SDAPP-2A

(remote)

(Adjacent)

(Torsional)

n H2O

ΔEAds (kcal/mol)a


n H2O

ΔE (kcal/mol)


n H2O

ΔE (kcal/mol)


ΔE (kcal/mol)


1 (λ = 1) 2 (λ = 2) 3 (λ = 3) 4 (λ = 4) 5 (λ = 5) 6 (λ = 6)

-16.3

-16.3

-16.3

-16.3

-20.3

-13.6

-13.6

-18.1

-36.1 (-19.8)

-18.1

-37.4 (-17.1)

-18.7

-34.3 (-20.7)

-17.2

-56.5 (-20.4)

-18.8

-56.5 (-20.4)

-18.8

-57.7 (-20.3)

-19.2

-59.5 (-25.2)

-19.8

-77.4 (20.9)

-19.4

-77.4 (20.9)

-19.4

-77.4 (-19.7)

-19.4

-79.0 (-19.5)

-19.8

-94.0 (-16.6)

-18.8

-94.2 (-16.8)

-18.8

-93.6 (-16.2)

-18.7

-87.9 (-8.9)

-17.6

-112.1 (-18.1)

-18.7

-112.8 (-18.6)

-18.8

1 (λ = 0.5) 2 (λ = 1.0) 3 (λ = 1.5) 4 (λ = 2.0) 5 (λ = 2.5) 6 (λ = 3.0)

-20.3

-36.1 (-19.8)

2 (λ = 1) 4 (λ = 2) 6 (λ = 3) 8 (λ = 4) 10 (λ = 5) 12 (λ = 6)

-110.2 (-16.6)

-18.4

-111.0 (-23.1)

-18.5

Optimized structures and energies evaluated at the ONIOM B3LYP/6-311**:PM6 level with ΔEAds calculated using Eqn. (1).


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Larger Hydrophilic Domain Formation. Two different hydrophilic domains extracted from the MD simulations19 of the SDAPP polymer are shown in Figure 5. The small SDAPP domain (S = 1, λ = 3) has an anisotropic shape and has been described as “stringy”, with most of the waters in the cluster being hydrogen bonded to a sulfonic acid group (a total of 8 SA). The larger SDAPP domain (S = 4, λ = 10) is more spherical in shape, containing 42 H2O molecules and 11 H3O+ that are hydrogen bonded in a wide variety of different environments. The calculated adsorption energies ∆E Ads (Eqn. 2) corresponding to the removal of each individual H2O or H3O+ is presented in Figure 6. The H3O+ cations are all easily identified having a trigonal Eigen structure. For both MD-based SDAPP domains, ∆E Ads can be separated into two regions with ∆E Ads3 occurring H O+

H O between -200 and -100 kcal/mol, and ∆E Ads occurring between -40 and -10 kcal/mol. For the H2O 2

or H3O+ hydration environments, there is a distribution of ∆E Ads resulting from small differences in the local hydrogen bonding networks. For example, a H2O that is hydrogen bonded to two other H O waters gives ∆E Ads ~ -18.5 kcal/mol (Figure 6C), while a H2O hydrogen bonded to an adjacent 2

H O ~ -39.2 kcal/mol (Figure 6C). A H2O environment that involves one hydrogen H3O+ gives ∆E Ads 2

H O bond to H2O and one hydrogen bond to a sulfonic acid group has a ∆E Ads of ~ -20.0 kcal/mol 2

(Figure 6F), which is similar to the changes in ∆E Ads seen in Table 1S and Table 1 for the pTol•n(H2O) and p-DIP•n(H2O) clusters. In Figure 5, environment (II) highlights a H3O+ that is hydrogen bonded to two different sulfonic acids and to one hydration water. While these types of bridging networks were noted in the SDAPP-2A adjacent and torsional cluster (Figures 3 and 4), the presence of a H3O+ as the bridging species was not observed in those smaller SDAPP clusters. This extracted domain from the MD simulations also revealed a H3O+ environment completely coordinated to three different sulfonic acid groups (Figure 5, environment I) which also gave the


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largest ∆E Ads3 adsorption energies (Figure 6). This type of hydration environment simply was not H O+

captured in analysis of the smaller clusters because those clusters were limited to two sulfonic acids, whereas the relative position of the acids in the MD-extracted domains arise from acids in different repeat units or on other polymer chains.


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Figure 5. Hydrophilic domain clusters extracted from MD simulations of SDAPP polymers (see Ref. 19) used to calculate the individual adsorption energies for the H2O and H3O+ species in the domain (Eqn. 2). The small cluster (left) corresponds to a domain obtained from the S = 1, λ = 3 (per SDAPP repeat unit) MD simulation, with the entire cluster containing 8 sulfonic acids (SA), 17 H2O molecules and 8 H3O+ cations. The larger cluster (right) corresponds to a domain obtained from the S = 4, λ = 10 (per SDAPP repeat unit) MD simulation, with the entire cluster containing 11 SA, 42 H2O molecules, and 11 H3O+ cations. Multiple H2O and H3O+ bonding environments are present, including a few highlighted examples: (I) a H3O+ cation that is hydrogen bonded to three different SA groups as a contact ion pair (CIP), (II) a H3O+ cation bonded to two different SA groups and a H2O oxygen, and (III) a solvated H3O+ cation coordinated to three H2O molecules to form the classic hydrated Eigen H9O4+ cation.


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Figure 6. Hydration adsorption energies ∆E Ads (Eqn. 2) for the represenative small and large hydrohilic domains extracted from the SDAPP MD simulations. The overall energy distribution (A and D) can be seperated into the hydronium adsorption energy region ∆E Ads3 (B and E) and the H O water adsorption energy region ∆E Ads (C and F). The energy distributions reflect different local hydrogen bonding environments either to different numbers of adjacent sulfonic acids (SA), H3O+ or H2O molecules. H O+

2

The ∆E Ads3 energies revealed a wide distribution of different coordination environments (Figure H O+

6B and E and Table 4). Indeed, the hydronium coordination environments in the MD simulations were previously analyzed and statistics can be found in Fig. 3 of Ref. 19 for varying sulfonation and hydration levels. For the small MD-extracted hydrophilic domain studied here, the H3O+ are exclusively in CIP with the sulfonic acids. Of these most have two hydrogen bonds to sulfonic


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acids and one hydrogen bond to a hydrating water, but there are examples of H3O+ being coordinated by 3 acid groups (Environment I, Figure 5). For all the MD configurations at S = 1, λ = 3, the most common coordination environment is for the hydronium bonded to two SA groups and 1 H2O, followed by the hydronium bonded to 2 SA groups and 2 H2O, or to 3 SA groups and 1 H2O. There were no solvated H3O+ observed in this small hydrophilic domain (although solvated H3O+ were occasionally observed in the MD simulations19 of this SDAPP system). For the large domain, the ∆E Ads3 revealed a broader distribution (Table 4) with more H3O+ environments H O+

+

H O containing hydrogen bonds to H2O leading to the smaller ∆E Ads between -140 and -100 kcal/mol 3

(Figure 6E). Solvated H3O+ species were observed (Figure 5, III), and produced the strongest +

H 3O adsorption energies with ∆E Ads ranging between -190 and -200 kcal/mol. In the MD simulations

for this S = 4, λ = 10 system, the most common coordination environments are for the hydronium bonded to 1 SA group and 3 or 4 H2Os, or a solvated hydronium coordinated with 4 waters (and no SA groups). These results demonstrate that for the SDAPP hydrophilic domains at low hydration levels, H3O+ CIPs are the dominant structure formed, and in many cases the H3O+ is bonded to multiple sulfonic acids. With increasing hydration levels, the CIP evolve into H3O+ environments that include hydrogen bonding waters leading to the formation of solvated H3O+ CIP. The solvated CIP were only one hydration layer away from the sulfonic acids and can easily exchange for a directly bonded CIP. The broad range of H2O and H3O+ bonding environments, H O along with the corresponding adsorption energies ∆E Ads observed in the MD-extracted domains 2

could not be identified by only analyzing the low energy optimized structure discussed for the smaller SDAPP clusters (Figures 3, 4 and 5).


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It has been suggested that some of the differences observed between the smaller clusters and the larger MD-based domains is a result of the starting state of the sulfonic acid. The small cluster optimizations begin with neutral acid groups (protonated) that subsequently undergo spontaneous deprotonation to form H3O+ CIP at λ > 3, while in the MD simulations the hydronium ions already existed at the start of the simulation and are retained (no re-protonation) due to the classical force fields used. The use of the TIP4P model in the MD simulations also biases the H3O+ as an Eigen cation structure, with no Zundel cations H5O2+ being observed, because proton transfer between water and hydronium is not allowed in that model. This is clearly a limitation of using a classical MD-based structure for analysis of the hydrophilic domains, and to correctly model the proton dynamics would require either AIMD or reactive MD type simulations.56 Never the less, the goal of looking at these different size MD-based hydrophilic domains was to demonstrate that large distributions in the adsorption energies exist, are function of the domain size, and increasing the size of the hydrophilic domain decreases the average adsorption energies for both H2O and H3O+ (Table 4). Table 4: Average Adsorption Energies and Standard Deviation for Micro-Hydration in the Hydrophilic Domains Extracted from MD Simulations.

H 3O ∆E Ads

+

H O σ ( ∆E Ads ) +

3

H 3O ∆E Ads

+

H O σ ( ∆E Ads ) +

3

Small Domain (kcal/mol) -171.3

Large Domain (kcal/mol) -163.4

8.6

7.2

-24.3

-19.8

6.9

7.2


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We did explore the question of whether the sulfonate groups would become protonated if the protons in the MD-extracted domains would extensively allowed to relax under additional HF optimization. While there were small changes in hydrogen bond lengths and positions, along with HO H 2O ∆E Ads and ∆E Ads3 distributions following optimization (Figure S4, +

variations in the

supplemental), there was no major restructuring of the local hydration environments. Surprisingly, the spontaneous re-protonation of sulfonic acids did not occur during this subsequent HF optimization. This is most likely explained by the hydrogen bond network being in a local minimum, originally biased by the H3O+ Eigen structure imposed through use of the TIP4P water model during the MD simulation, along with the freezing of the sulfonic acid positions during the HF optimization which coordinate the majority of the H3O+ environments.

4. CONCLUSIONS The micro-hydration for a series of SDAPP clusters with different numbers of sulfonic acids over a range of hydration levels (λ = 1-6) were evaluated using DFT and ab initio computational methods. Increasing levels of complexity and cluster size were investigated allowing the impact of the extended aromatic structure in SDAPP and the role of spatially adjacent sulfonic acids groups to be determined. It was found that the aromatic structure of SDAPP has a minimal role on the hydration behavior, and is like other sulfonated aromatic polymers in that respect. The rigidity of the aromatic backbone in SDAPP does limit the degrees of conformational freedom of the phenyl rings and hence limits the spatial positioning of different sulfonic acids. Acid group interactions (clustering) within the same polymer repeat unit are limited, except for motion about the backbone torsional angles ε and ξ. More extensive interactions between sulfonic acids can


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occur for sulfonate groups attached to other repeat units or other polymer chains. The adsorption energies for hydration were determined and show variations only when sulfonic acid to sulfonic acid interactions can occur, thereby enhancing the adsorption process. It was shown that a wide distribution of local hydrogen bonding environments, and corresponding adsorption energies, occur with increasing hydrophilic domain size. The calculated adsorption energies for these different hydration environments may provide insight into how proton conductivity could be modulated in SDAPP membranes. Assuming the simplistic argument that the proton transport rate through the polymer membrane is controlled by the relative time it spends in “bulk-like” water environments, versus the time the proton resides in a surface acid environment, then increasing the bulk-like water fraction through the formation of very large domains would increase Grötthuss diffusion rates and improved conductivity. For SDAPP at low hydration levels, the H3O+ species were found to predominantly form CIPs with the sulfonic acid groups. In addition, the domain shapes observed in the DFT-optimized SDAPP clusters containing two sulfonic acids (SDAPP2A), as well as the MD-based hydrophilic domains at low hydration levels, were very anisotropic and elongated, and revealed almost no bulk-like water speciation. MD simulations predict hydrophilic domain shape changes with increasing hydration levels, becoming larger and more spherical.19 This would suggest that at low hydration levels SDAPP should have both a very slow diffusion constant (vehicular transport) due to strong CIP formation, and a disordered hydrogen bond network leading to disruption of Grötthuss diffusion. This should produce a very poor proton conductivity regardless of the degree of sulfonation, which contradicts experimental measurements. In contrast, one could envision the influence of the “sulfonate passing mechanism”29-30, 56 where the rate-limiting step is not the movement of the H3O+ CIP from the local sulfonate hydration sphere to bulk-like water, but instead the movement of the proton


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between adjacent sulfonate groups through a solvated H3O+ environment that is adjacent to the sulfonic acid (2nd hydration sphere). The SDAPP cluster calculations did indeed predicted the formation of solvated H3O+ species in optimized structures at high hydration levels. This H3O+ species would be involved in the Grötthuss proton transport mechanism.56 The work of Voth and co-workers have shown that these solvated H3O+ environments can readily transfer between adjacent sulfonate groups if their hydration spheres overlap. Differences in the relative contributions of vehicular versus Grötthuss mechanisms should be discernable by comparing the water diffusion rates measured using pulsed field gradient NMR, and the proton conductivity as a function of sulfonation and hydration. This experimental characterization is ongoing and will be reported in a subsequent paper. We also realize that simply reporting water adsorption energies ( ∆E Ads ) for SDAPP does not reflect any of the dynamics occurring within the hydrogen bonded

network of waters. Analysis of dynamic processes will require either AIMD or reactive MD type simulations to further identify the different contributions to the conductivity processes.


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■ ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publication website at DOI: 10.1021/XXXX. Four figures of related cluster structures and one table of adsorption energies (PDF).

■ AUTHOR INFORMATION Corresponding Author * Phone +1-505-844-1225; fax +1-505-844-2974; e-mail [email protected] Notes The author declares no competing financial interest. ■ ACKNOWLEDGMENTS The authors would like to thank Lauren Abbott for providing the MD snapshots from reference 19 from which the hydrophilic domain structures were extracted. This work was supported by the Sandia Laboratory Directed Research and Development (LDRD) program. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the U.S. Department of Energy or the United States Government. ■ REFERENCES 1. Parasuraman, A.; Lim, T. M.; Menictas, C.; Skyllas-Kazacos, M. Review of Material Research and Development for Vanadium Redox Flow Battery Applications. Electrochim. Acta 2013, 101, 27-40.


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2. Zhang, H.; Shen, P. K. Advances in the High Performance Polymer Electrolyte Membranes for Fuel Cells. Chem. Soc. Rev. 2012, 41, 2382-2394. 3. Ito, H.; Maeda, T.; Nakano, A.; Takenaka, H. Properties of Nafion Membranes Under PEM Water Electrolysis Conditions. Int. J. Hydrogen Energy 2011, 36, 10527-10540. 4. Li, X.; Zhang, H.; Mai, Z.; Zhang, H.; Vankelecom, I. Ion Exchange Membranes for Vanadium Redox Flow Battery (VRB) Applications. Energy Environ. Sci. 2011, 4, 1147-1160. 5. Maurya, S.; Shin, S.-H.; Kim, Y.; Moon, S.-H. A Review on Recent Developments of Anion Exchange Membranes for Fuel Cells and Redox Flow Batteries. RSC Adv. 2015, 5, 3720637230. 6. Smitha, B.; Sridhar, S.; Khan, A. A. Solid Polymer Electrolyte Membranes for Fuel Cell Applications — A Review. J. Membr. Sci. 2005, 259, 10-26. 7. Shin, D. W.; Guiver, M. D.; Lee, Y. M. Hydrocarbon-Based Polymer Electrolyte Membranes: Importance of Morphology on Ion Transport and Membrane Stability. Chem. Rev. 2017, 117, 4759-4805. 8. Miyake, J.; Miyatake, K. Fluorine-Free Sulfonated Aromatic Polymers as Proton Exchange Membranes. Polym. J. 2017, 49, 487. 9. Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Alternative Polymer Systems for Proton Exchange Membranes (PEMs). Chem. Rev. 2004, 104, 4587-4612. 10. Maier, G.; Meier-Haack, J., Sulfonated Aromatic Polymers for Fuel Cell Membranes. In Fuel Cells II, Scherer, G. G., Ed. Springer Berlin Heidelberg: Berlin, Heidelberg, 2008; pp 1-62. 11. Rikukawa, M.; Sanui, K. Proton-Conducting Polymer Electrolyte Membranes Based on Hydrocarbon Polymers. Prog. Polym. Sci. 2000, 25, 1463-1502. 12. Fujimoto, C. H.; Hickner, M. A.; Cornelius, C. J.; Loy, D. A. Ionomeric Poly(phenylene) Prepared by Diels−Alder Polymerization: Synthesis and Physical Properties of a Novel Polyelectrolyte. Macromolecules 2005, 38, 5010-5016. 13. Skalski, T. J. G.; Britton, B.; Peckham, T. J.; Holdcroft, S. Structurally-Defined, SulfoPhenylated, Oligophenylenes and Polyphenylenes. J. Am. Chem. Soc. 2015, 137, 12223-12226. 14. Adamski, M.; Skalski, T. J. G.; Britton, B.; Peckham, T. J.; Metzler, L.; Holdcroft, S. Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes. Angew. Chem. Int. Ed. 2017, 56, 9058-9061. 15. Hickner, M. A.; Fujimoto, C. H.; Cornelius, C. J. Transport in Sulfonated Poly(phenylene)s: Proton Conductivity, Permeability, and the State of Water. Polymer 2006, 47, 4238-4244. 16. Tang, Z.; Lawton, J. S.; Sun, C.-N.; Chen, J.; Bright, M. I.; Jones, A. M.; Papandrew, A. B.; Fujimoto, C. H.; Zawodzinski, T. A. Characterization of Sulfonated Diels-Alder Poly(phenylene) Membranes for Electrolyte Separators in Vanadium Redox Flow Batteries. J. Electrochem. Soc. 2014, 161, A1860-A1868. 17. Lawton, J. S.; Jones, A. M.; Tang, Z.; Lindsey, M.; Fujimoto, C.; Zawodzinski, T. A. Characterization of Vanadium Ion Uptake in Sulfonated Diels Alder Poly(phenylene) Membranes. J. Electrochem. Soc. 2016, 163, A5229-A5235. 18. Pezeshki, A. M.; Tang, Z. J.; Fujimoto, C.; Sun, C.-N.; Mench, M. M.; Zawodzinski, T. A. Full Cell Study of Diels Alder Poly(phenylene) Anion and Cation Exchange Membranes in Vanadium Redox Flow Batteries. J. Electrochem. Soc. 2016, 163, A5154-A5162. 19. Abbott, L. J.; Frischknecht, A. L. Nanoscale Structure and Morphology of Sulfonated Polyphenylenes via Atomistic Simulations. Macromolecules 2017, 50, 1184-1192.


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