Random and Block Sulfonated Polyaramides as Advanced Proton

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Random and Block Sulfonated Polyaramides as Advanced Proton Exchange Membranes Corey L Kinsinger, Yuan Liu, Feilong Liu, Yuan Yang, Daniel Michael Knauss, Andrew M Herring, and C. Mark Maupin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06857 • Publication Date (Web): 09 Oct 2015 Downloaded from http://pubs.acs.org on October 10, 2015

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The Journal of Physical Chemistry

Random and Block Sulfonated Polyaramides as Advanced Proton Exchange Membranes Corey L. Kinsinger,1,† Yuan Liu,1,†Feilong Liu,2,3, Yuan Yang,3 Daniel M. Knauss,3 Andrew M. Herring,1,* C Mark Maupin1,* 1

Chemical and Biological Engineering Department, Colorado School of Mines, Golden, CO 80401, USA 2 Metallurgical and Materials Engineering Department, Colorado School of Mines, Golden, CO 80401, USA 3 Chemistry and Geochemistry Department, Colorado School of Mines, Golden, CO 80401, USA † *

Authors contributed equally to this work Corresponding authors [email protected], [email protected]

Keywords: Ionomer Morphology, Molecular Dynamics, Perfluorosulfonic Acid Ionomers, Proton Transport

ACS Paragon Plus Environment


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ABSTRACT Presented here is the experimental and computational characterization of two novel copolyaramide proton exchange membranes (PEMs) with higher conductivity than Nafion™ at relatively high temperatures, good mechanical properties, high thermal stability, and the capability to operate in low humidity conditions. The random and block copolyaramide PEMs are found to possess different ion exchange capacities (IEC) in addition to subtle structural and morphological differences, which impact the stability and conductivity of the membranes. SAXS patterns indicate the ionomer peak for the dry block copolymer resides at q=0.1 Å-1, which increases in amplitude when initially hydrated to 25% relative humidity, but then decrease in amplitude with additional hydration. This pattern is hypothesized to signal the transport of water into the polymer matrix resulting in a reduced degree of phase separation. Coupled to these morphological changes, the enhanced proton transport characteristics, and structural/mechanical stability for the block copolymer is hypothesized to be primarily due to the ordered structure of ionic clusters that create connected proton transport pathways while reducing swelling upon hydration. Interestingly, the random copolymer did not possess an ionomer peak at any of the hydration levels investigated indicating a lack of any significant ionomer structure. The random copolymer also demonstrated higher proton conductivity than the block copolymer, which is opposite to the trend normally seen in polymer membranes. However, it has reduced structural/mechanical stability as compared to the block copolymer. This reduction in stability is due to the random morphology formed by entanglements of polymer chains and the adverse swelling characteristics upon hydration. Therefore, the block copolymer with its enhanced proton conductivity characteristics, as compared to Nafion™, and favorable structural/mechanical stability, as compared to the random copolymer, represents a viable alternative to current proton exchange membranes.


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INTRODUCTION Electrochemical energy conversion devices, such as fuel cells, have received considerable attention due to their many favorable properties such as high efficiency and low pollution.1-2 In particular, proton exchange membranes (PEMs) are becoming increasing popular due to their high power density, system versatility, quick startup, and suitability for portable applications.3 Several perfluorosulfonic acid (PFSA) polymers have been developed primarily as permselective separators in chlor-alkali electrolyzers4 (e.g. Nafion™ and 3M).5-7 These PFSAs perform well as PEMs because they exhibit excellent chemical stability, high proton conductivity, and are capable of maintaining mechanical integrity under harsh operating conditions, which has led to their use as benchmark materials in many fuel cell studies.8-10 However, the application of fluorinated PEMs in fuel cells is hindered by their potential high-cost and environmental concerns related to their manufacture.9,

11

The loss of water at elevated temperatures (above

100 °C) in PFSAs also leads to a dramatic decrease in conductivity.12-14 Operating fuel cells at higher temperatures has several benefits including enhanced tolerance to carbon monoxide, and a significantly simplified heat exchange system.15-19 Therefore, the development of less expensive and more environmentally friendly alternative PEMs that are capable of operation at higher temperatures has become a major effort in fuel cell research. Sulfonated aromatic hydrocarbon PEMs, such as sulfonated poly ether ether ketone (SPEEK), are promising materials that have the potential to meet these requirements.20-27 To design superior PEMs it is essential to understand their inherent transport, mechanical, and morphological properties. To this end there has been a tremendous amount of work carried out over the last 3 decades.28-36 These studies have been greatly facilitated by the use of small angle X-ray and neutron scattering techniques that have enhanced our structural understanding of



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these systems. Over the years models representing a phase-separated morphology have become generally accepted as the structure of PFSAs; however, the exact morphology as it pertains to the shape, size, and distribution of the constituent nanostructures is not fully understood.37 Several inter-particle models have been proposed such as the cluster network model by Gierke et al., which adequately explained the high-q maxima (i.e. ionomer peak) observed in small angle scattering studies.7 Other inter-particle models attribute the ionomer peak to the inter-particle origin, such as the proposed lamella model by Litt et al., and the parallel cylindrical model by Schmitt et al.38-39 In addition the study of structural evolution at different hydration levels has indicated that the shape of the nano domains changes upon hydration.40 Molecular dynamics simulations have found that side chain length and the equivalent weight of the polymer can affect the morphology as well.41-42 While PFSAs have been extensively studied the evaluation of related compounds has been under explored. Some research has been done on sulfonated aromatic hydrocarbon PEMs such as SPEEK, which contain sulfonic side groups that serve as the charge sites for proton transport.43 It was found that these PEMs also form hydrophilichydrophobic phase separated morphology, but to a lower degree as compared to PFSAs. This morphological difference indicates that these alternate materials possess narrower ion conducting channels than the traditional PFSAs.17 To improve the ion conductivity and mechanical properties, several block copolymers with enhanced phase separated morphology have been explored, although to-date a replacement for PFSAs has not been realized.44-45 In our previous work, we found that the connectivity of hydrophilic domains has a large impact on the PEM’s transport properties.46 In this work, we investigated the morphology evolution of newly developed copolymers of m-phenylene diamine, 5-sulfoisophthalic acid, and isophthalic acid upon hydration, in an effort to relate their morphology to the proton


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conductivity, water uptake behavior, and to the chemical, thermal, and mechanical stability. The polymers investigated here contain amide groups in the backbone chain, which prevents the chains from forming a linear geometry, and so avoids the problem of polymer/liquid phase separation. In addition to the morphology the ability of the PEMs to uptake water is an important property, as it directly impacts the proton conductivity of the membrane. Because the water molecules in PEMs act as proton carriers, a PEM with lower water uptake will generally have lower conductivity; however, too much water uptake can result in excessive swelling of the membrane. In this study the properties of random and block copolymers are evaluated identifying morphologies conducive to enhanced conductance while providing reduced water uptake characteristics. METHODS Materials Random copolymer sulfonated poly (m-phenylene isophthalamide) [MPDA-SIPA40] and block copolymer [(MPDA-SIPA40)-b-(MPDA-IPA)] at a degree of sulfonation (DS) of 40% (Figure 1) were provided by Dr. Knauss and were synthesized as outlined in Liu and Knauss.47

Figure 1 Structure of random [MPDA-SIPA40] where A and B are random numbers, and block [(MPDA-SIPA40-b(MPDA-IPA)] C is a different random number for ACS Paragon where Plus Environment each segment D.


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NMR The self-diffusion coefficient of protons at maximum humidity was obtained through pulsed gradient spin-echo (PGSE) NMR measurements carried out on a Bruker AVANCEIII NMR spectrometer operating at 400 MHz using a 5 mm Bruker single-axis DIFF60L Z-diffusion probe, and a nine-interval stimulated-echo pulse sequence.45-46 Films were equilibrated in the sealed NMR tubes for 48 h, with a droplet of water added to the bottom of the tube. The “Stejskal and Tanner” equation was used to obtain the diffusion coefficient.48 Due to equipment limitations the temperature range analyzed was relatively small. Additionally, due to high levels of noise from the polymer backbone hydrogens, only maximum humidity diffusion readings were possible. SAXS SAXS experiments were performed using X-rays of 18 keV from a synchrotron radiation source at the Advanced Photon Source (APS) beamline (12ID C-D), Argonne National Laboratory, (Argonne, IL). The detailed experimental setup is shown in our previous work.35 SAXS measurements were performed under 0%, 25%, 50%, 75% and 95% RH at 60 °C. A onehour equilibration time was set for each humidification step up to a maximum of 95% RH, with X-ray shots taken for 3 samples and 1 Kapton background at the end of each equilibrating step. During the equilibration, multiple dynamic shots were performed on one sample. The Kapton background was subtracted using Argonne SAXS software package for Igor Pro. Molecular Dynamics Simulations The atomistic MPDA polymer system consisted of water, classical hydroniums, and several polymer chains. The hydronium cations were treated classically as an H3O+ molecule using the force field reported in our earlier publications,46, 49 while the monomers of the MPDA polymer


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system were described by the Generalized Amber Force Field (GAFF) with modified charges.50 The charges were obtained by optimizing the molecules at the B3LYP/6-311G(2d,d,p) level of theory and basis set within the Gaussian 09 program.51 The optimized structures were then subjected to a single point calculation at the HF/6-311G(2d,d,p) level of theory and basis set, and point charges determined by means of the restricted electrostatic potential (RESP) method.52 The entangled polymer systems were created by first making 20 chains containing 70 monomers for the random MPDA and 20 chains containing 60 monomers for the block MPDA with the final system containing 40% sulfonic acid containing monomers (i.e. DS of 40%). The chains for each system were then combined using an in-house dynamic polymer system construction code53 that creates a representative entangled polymer system. The addition of solvent to the dry entangled polymer systems was accomplished through the use of the Packmol54 program. Each system (i.e. random and block) was initially constructed with one hydronium molecule for every sulfonic acid group in the system (i.e. λ = 1). The TIP3P water molecules55 were then added to the systems with a hydration of λ > 1 such that the desired hydration level was obtained. In total twelve systems were created, six with the random polymer and six with the block polymer at varying hydration levels, λ = 1, 3, 4, 6, 8, and 10. All systems were simulated with the software package Amber 12.56-58 Systems were initially minimized for 10,000 steps using the conjugate gradient algorithm, and then cycled between the isothermal isobaric (NPT) ensemble (500 ps at 1 atm and 298 K) and the canonical (NVT) ensemble (500 ps at 450 K). The cycling was repeated until there was no significant change in the volume of the systems during the NPT simulations. The number of cycles needed to equilibrate the systems increased with increasing hydration levels and were typically between 3 and 12 total cycles. Post equilibration simulations consisted of 5 ns in the NVT ensemble at 298 K, followed by 20 ns at 298 K in the microcanonical (NVE)



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ensemble. All simulations were conducted with a time step of 1 fs, and at four different temperatures (298 K, 323 K, 348 K, and 373 K). The process for the elevated temperature simulations was identical to that described for the 298 K simulations. Analyses of the systems were carried out with in-house codes, AmberTools 12, VMD, and Fiji.59-60 Fiji, an added feature distribution for the ImageJ program, was used with VMD to analyze water clustering in the various systems. RESULTS and DISCUSSION Ion Exchange Capacity (IEC) and Water Uptake The IEC is an important property that can significantly impact proton conductivity and water uptake of a polymer. Typically, PEMs with higher IEC can achieve higher conductivity, as the higher levels of ionic functional groups in the membrane can provide a greater number of free protons and proton conducting pathways as compared to a system with a lower IEC. However, too high of an IEC can lead to swelling and a decrease in the film’s mechanical properties. It has been reported47 that the IEC and the DS can be controlled through adjusting the molar ratio of the hydrophobic monomer and the hydrophilic monomer, isophthalic and isophthalic acid respectively. The synthesis and characterization conducted by Liu and Knauss indicates that, at room temperature, the DS increases as the SIPA block is increased. In addition, it was found that that water uptake was high when the SIPA content was high (~ 50 mol%), and the water uptake was low when the SIPA monomer ratios were small. It was hypothesized by Liu and Knauss that the increase in water uptake was due to a change in the morphology of the membrane when the polymer has more hydrophilic sulfonated acid groups. The increase in hydrophilic groups resulted in more attraction between water and the backbone of the polymer through hydrogen bonding interactions. This result was in contrast to the low DS region where the hydrogen bonds


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1.7 λ4 1.5 λ6 1.6 λ8 1.5 λ10

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0.6 0.4 0.2 0

Figure 2 Sulfur-Sulfur (top) and Ring-Ring (bottom) radial distribution function results at different RH values for MPDA-SIPA40: λ = 4 (dotted), λ = 6 (dot-dashed), λ = 8 (dashed), and λ = 10 (solid), which cover the RH range from 40 to 100%. The coordination number for the first peak of the g(r)S-S is indicated in the top right.

can be formed more readily between the polymer backbones. Based on the mechanical stability, water uptake, and conductance measurements it was proposed that a DS of 40% would provide the optimal characteristics for a PEM. In order to evaluate Liu and Knauss's hypothesis pertaining to the polymer stability at lower DS values radial distribution functions depicting polymer-polymer interactions at various hydration levels were calculated from the MD simulations. The simulation results (Figure 2) indicates that the proximity of several key polymer structures (i.e. sulfonic-sulfonic group proximity and aromatic ring-ring proximity), are not strongly impacted by large increases in RH when DS is 40% for MPDA-SIPA40. These results support the hypothesis that the polymer has a



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Figure 3 Dynamic SAXS curves of (MPDA-SIPA40)-b(MPDA-IPA): (top) light to dark brown: equilibrating from dry to 25%RH; (bottom) light to dark green: equilibrating at 50%, 75% and 95%RH.

relatively high degree of structural stability (e.g. resists membrane swelling) at lower DS values. Based on the results of water uptake and favorable structural stability,47 in addition to the molecular-level insights from MD simulations it has been concluded that polyaramides with lower DS values warrant further investigation. SAXS The SAXS results of the block copolymer are found in Figure 3, and indicate a maxima around q=0.09 Å-1 for each of the different hydration curves. SAXS patterns were also obtained for films exchanged with different cations (Figure 4, top), which yielded maxima at similar q


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values as the proton membrane. The SAXS amplitude of the cation exchanged films was observed to increase with larger cation electron density, i.e. Na

Random and Block Sulfonated Polyaramides as Advanced Proton

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