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C: Energy Conversion and Storage; Energy and Charge Transport
Effect of Polymer Architecture on the Nanophase Segregation, Ionic Conductivity, and Electro-Osmotic Drag of Anion Exchange Membranes Jibao Lu, Adam Barnett, and Valeria Molinero J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01165 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019
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Effect of Polymer Architecture on the Nanophase Segregation, Ionic Conductivity, and Electro-Osmotic Drag of Anion Exchange Membranes Jibao Lu,a Adam Barnett,a and Valeria Molinero* Department of Chemistry, The University of Utah, Salt Lake City, UT 84112-0850 Abstract. Anion exchange membranes (AEMs) are considered an attractive alternative to proton exchange membranes in fuel cell applications because they can operate with non-precious metal electrodes. However, widespread adoption of AEMs has been hampered by their insufficient ionic conductivity. Much of the growing body of research on AEMs focuses on designing new polymer chemistries and architectures that would increase their conductivity, while controlling the swelling of the membrane. It is, however, challenging to assess the separate effects of water content and polymer architecture on the phase segregation and molecular transport of the ions and water on the membrane, because changes in the chemistry of the polymer also impact the equilibrium water uptake. Here we use large-scale molecular simulations to study the distinct effects of water content and ionomer architecture on the nanophase segregation, anion and water diffusivity, ion conductivity, and water electro-osmotic drag coefficient of anion exchange membranes based on tetraalkylammonium-functionalized polyphenylene oxide (PPO/TMA). We find that the transport properties of the AEM are very sensitive to the water content but quite robust against changes in the architecture of the polymer electrolyte. Our analysis indicates that this insensitivity stems from the similarity in the structure of the hydrophilic domains for a given ratio of water to anions in the membrane. This is belied by the extreme differences in the structure factor of the different polymer electrolytes, and indicates that the structure factor alone is not appropriate to characterize the size of the hydrophilic channels that control the transport of ions and water in the AEM. Our results suggest that future efforts to design AEMs with improved conductivity should focus on elucidating the relationship between the polymer chemistry and the equilibrium water uptake of the membrane.
a Contributed equally to this work * corresponding author, email:
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1. Introduction. Fuel cells that directly convert chemical energy from fuel into electricity through an oxidation reaction are promising power sources for a variety of applications including portable devices, fuel cell vehicles, and primary and backup power for buildings.1 Among the different types of fuel cells, alkaline anion exchange membrane fuel cells (AAEMFCs) show advantages of low-cost with respect to acidic fuel cells because AAEMFCs can utilize non-noble metal oxygen reduction catalysts with air as oxidant.2 The non-noble metal catalysts can be used because of the more facile oxygen reduction reaction kinetics at the cathode and the low probability of corrosion of the catalysts in the alkaline environment.3-9 Polymeric anionexchange membranes (AEMs) overcome the problems of electrolyte leakage and carbonate precipitation that limit the use of traditional alkaline fuel cells with liquid electrolytes.2 The widespread use of AAEMFCs faces two major challenges. The first is that AAEMFCs are prone to chemical degradation, especially at low water contents.10 The second is to develop AEMs with conductivity comparable to that of proton exchange membranes such as Nafion.11 This has been demonstrated for AEMs with high water uptake, but not yet at the low water contents desirable for optimum mechanical and water management properties of the membrane.12 The polymers in AEMs have a hydrophobic backbone tethered with cationic organic groups.1-3 The combination of hydrophobic and ionic moieties in the same molecule results in nanophase segregation upon hydration. The ionic conductivity is determined by the diffusivity of the anions and their concentration; the later is given by the ion exchange capacity (IEC). While increasing the IEC improves the ionic conductivity, too high IEC causes excessive water uptake that results in significant swelling of the membrane.9,
13-15
Cross-linking of
polymers may suppress the membrane swelling,9 but it usually leads a decrease in the solubility of the polymers, which reduces the stability of ionomer solutions.9 Recent experimental studies indicate that changes in polymer chemistry can improve the conductivity of AEMs.1, 9, 16-23 A strategy that involves adding hydrophobic side chains to the polymers has been shown to be a promising way to efficiently improve the ionic conductivity of the membranes with moderate IEC, while keeping relatively low water uptake.7, 9, 16, 18, 24 For example, Hickner et al. observed improved hydroxide conductivity in comb-shaped quaternized poly(2,6-dimethyl phenylene oxide)s AEMs containing long alkyl side chains tethered on the
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nitrogen-centered cations at relatively low water uptake with respect to that of the AEMs without pendant side chains.16 Zhuang et al. achieved high hydroxide conduction at elevated temperatures (comparable to the H+ conduction in Nafion at the similar temperature) in the quaternary ammonia polysulfone membranes with the hydrophobic side chains attached onto the polysulfone backbone but separated from the cations.7, 18 Li et al. obtained enhanced hydroxide conductivity in the polyolefin copolymers with the hydrophobic poly(4-methyl-1-pentene) side chains and side-chain quaternary ammonium groups.24 Those experiments highlight the importance of the chemical architecture of the polymer in developing highly conductive AEMs. However, changes in polymer chemistry and architecture also have a significant effect on the water uptake of the membranes.25 The separate effects of water content and polymer architecture on the transport of water and ions in AEMs have not yet been elucidated. Here we use molecular simulations to investigate the independent effects of polymer architecture, ion exchange capacity, and water content on the mobility of water and ions in a model AEM. We focus on tetraalkylammonium-functionalized polyphenylene oxide (PPO/TMA) anion exchange membranes to elucidate the role of the chemical architecture and phase separation in determining the ionic transport. PPO/TMA has been shown to be a promising material for high-performance AEM.16, 26-30 We investigate chemical architectures of polymers with different blockiness, degrees of functionalization (DF), and positions of the alkyl chains. Our aim is to determine how the position of the hydrophobic fragments (either in the backbone or in the cation) affects the molecular transport for a wide range of water contents in the membrane, while keeping the IEC relatively unchanged. We model these polymeric membranes in chloride form and investigate the relationship between nanophase segregation, diffusion of ions and water molecules, ionic conductivity, and electro-osmotic drag coefficient of water in the membranes.
2. Models and computational methods 2.1. Polymer architectures. The membranes of this study are built from a combination of cationic fragments (X, Y and Z in Figure 1) and neutral, hydrophobic ones (A and B in Figure 1). These fragments are combined to produce 11 polymer architectures, listed in Table 1. This results in polymers with various blockiness, degree of functionalization, and position and length of alkyl chains on the
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polymer electrolyte. The blockiness describes how the TMA functionalized monomeric PPO units (T-PPOs) are distributed in the polymer: a dispersed polymer has the T-PPOs evenly spaced, while a blocky one has all the T-PPOs together in a block. The degree of functionalization is defined as the ratio between the number of T-PPOs and the total number of monomeric PPO units in a polymer. The “hydrophobic fragments” encompass the CH2 and CH3 groups in the alkyl chains, the aromatic rings, and also the ether oxygen between the aromatic rings.
Figure 1. The five building blocks we use in to create the oligomers (Table 1) that assemble to form the membranes.
Table 1. Polymer architectures built with the building blocks of Table 1, listing their ion exchange capacity (IEC), water content (l), and simulation cell size (L, or XL). Composition
(AX)10
A10X10
(A2X)7
A14X7
(AY)10
A10Y10
(BX)10
B10X10
(AZ)10
(XY)10
X10Y10
DF
50%
50%
33%
33%
50%
50%
50%
50%
50%
100%
100%
D
B
D
B
D
B
D
B
D
D
B
3.22
3.22
2.42
2.42
2.67
2.67
2.37
2.37
2.90
4.16
4.16
5
L
L
L
-
L
L
L
L
L
L
L
10
L, XL
L, XL
XL
XL
L, XL
L, XL
L
L
L
L
L
15
L, XL
L, XL
XL
-
L
L
L
L
L
L
L
20
L, XL
L
-
-
L
L
L
L
L
-
-
Block vs Disp IEC (mol/kg)
𝜆
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The number of carbon atoms in the alkyl chains of building blocks B, Y, and Z are 9, 9 and 6, respectively. We design the architectures to have IECs in four ranges – around 2.4, 2.9, 3.22 and 4.16 (Table 1). Equilibrations of membranes with a lower IEC of 1.5 and 1.2 resulted in a large fraction of ionic groups buried in the hydrophobic domains, hence their results are not shown. Architectures with DF 100% have too high IEC for practical applications in fuel cell membranes; nevertheless we investigate them for comparison with the other polymers for a reduced set of water contents. In addition, when creating the polymer (AZ)10, in which the cation has a large hydrophobic tail, we find that 14% of TMA groups end up buried within the hydrophobic domains after annealing of the membrane, a situation that would be thermodynamically unfavorable in the experiments, and results from the short-range nature of all interactions in the model. For comparison, the other architectures display less than 1% of buried TMA. For this reason, the (AZ)10 membrane was not studied in depth and its properties are not reported in the present study Water content is also an important factor for determining the properties of AEMs. We found that ions buried in the hydrophobic core also when modeling membranes with water contents l
