Theoretical Studies of Pendant Effects on the Properties of Sulfonated

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Theoretical Studies of Pendant Effects on the Properties of Sulfonated Hydrocarbon Polymer Electrolyte Membranes Yuan-yuan Zhao, Yoong-Kee Choe, Eiji Tsuchida, Tamio Ikeshoji, and Akihiro Ohira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00710 • Publication Date (Web): 07 May 2015 Downloaded from http://pubs.acs.org on May 11, 2015

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The Journal of Physical Chemistry

Theoretical Studies of Pendant Effects on the Properties of Sulfonated Hydrocarbon Polymer Electrolyte Membranes Yuan-yuan Zhao,∗,† Yoong-Kee Choe,‡ Eiji Tsuchida,‡ Tamio Ikeshoji,†,‡ and Akihiro Ohira†,¶ Fuel Cell Cutting-Edge Research Center Technology Research Association (FC-Cubic), AIST Tokyo Waterfront Main Building, 2-3-26 Aomi, Koto-ku, Tokyo 135-0064, Japan, Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, Umezono 1-1-1, Tsukuba 305-8568, Japan, and Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan E-mail: [email protected]

Phone: +81 (0)29-861-2314. Fax: +81 (0)29-861-3171

∗

To whom correspondence should be addressed Fuel Cell Cutting-Edge Center Technology Research Association ‡ Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology ¶ Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology †

1

ACS Paragon Plus Environment


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Abstract Six model compounds of hydrocarbon polymer electrolyte membrane (PEM) with different neighboring pendants have been investigated using density functional theory (DFT). The effect of the neighboring pendant on the proton dissociation properties of the PEMs and on the chemical stability of the key adjacent bond containing a sulfonic group was evaluated. Results of the proton dissociation properties of the six model compounds indicate that the introduction of a strong electron-withdrawing group, such as CF, CF2 , or CN, on the neighboring pendant of the acid group can improve the proton dissociation properties of PEMs. The calculated pK a values confirm the relative acid strength of the six model compounds, whose properties are, to some extent, related to the proton conductivity. Our results demonstrate that a model compound containing a strong electron-withdrawing group in the neighboring pendant has stronger acid strength. DFT calculations on the C–S bond degradation reactions caused by OH or H radicals show that a –CF2 CF2 – group in the neighboring pendant improves the stability of the C–S bond against attack from a radical, while introduction of a –CH2 CH2 CH2 – or a CN group has little influence on the stability of the C–S bond.

Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are energy devices that use hydrogen as a fuel, and are expected to have applications in stationary devices as well as in vehicles. 1,2 An ionomeric polymer is an important component of PEMFCs, whose role is to separate the electrodes and to form a membrane electrode assembly. Current popular polymer electrolyte membranes (PEMs) used in PEMFCs are perfluorinated polymers, such as Nafion, although this polymer has several drawbacks, such as a high cost of production and a low operating temperature. A promising alternative for perfluorinated membranes is aromatic PEMs, whose production costs are expected to be lower than those of perfluorinated polymers. Thus, aromatic PEMs have attracted much attention. 3–8 Park et al., 6 Peckham et al., 7 and 2


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Li et al. 8 reviewed synthetic methods and properties of various membranes based on aromatic copolymers, such as statistical copolymers, block or graft copolymers, and super acidic fluoroalkyl copolymers. They illustrated the relationship between structure, morphology and ion transport property of these copolymers and elucidated that understanding this relationship is critical for developing better PEMs as well as anion exchange membranes (AEMs). In PEMs, the proton conductivity and material stability are two key properties that need to be optimized when these polymers are used in PEMFCs because these properties directly affect the performance and the lifetime of a PEMFC. 9,10 Therefore, many studies on the durability and proton conductivity of aromatic PEMs have been carried out. 11–13 The current situation regarding aromatic PEMs is that their proton conductivity is comparable to that of Nafion under high humidity conditions, while their proton conductivity is very poor under low humidity conditions. For the optimal operation of PEMFCs, low humidity conditions are favored because the mechanical property of PEMs deteriorates rapidly with high water uptake. Although much effort has been devoted towards improving the proton conductivity of aromatic PEMs at low humidity, there is still room for improvement. The proton conductivity of a PEM depends on its proton dissociation, which is one of its intrinsic properties, together with the transport of protons along the network of hydrogen-bonded water molecules. Many studies have been carried out by Paddison’s group to help understand the relationship between the degree of hydration and the proton dissociation behavior of PEMs. 14–16 Clark et al. 17 and Idupulapati et al. 18 have studied the proton dissociation properties of imide and sulfonic acid ionomers using density functional theory (DFT). Choe et al. have investigated the nature of proton dynamics in Nafion and sulfonated polyethersulfone (SPES). 19,20 Considering the close relationship between the structure of a polymer and its properties, the effect of molecular structure, especially the neighboring pendant functional group of acid group (sulfonic group), on the proton dissociation property of polymers deserves a study. Recently, two interesting experimental studies have been reported on aromatic PEMs. 21,22

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Yoshimura et al. 21 synthesized a polymer having poly(arylene ether sulfone) in the main chain and −CF2 CF2 OCF2 CF2 SO3 H as a side chain (PES-PSA) and revealed that PES-PSA had a higher α relaxation temperature than Nafion. Chang et al.

22

synthesized poly(arylene

ether sulfone)-based ionomers with the sulfonate groups having various neighboring groups (e.g., perfluoroalkyl sulfonate, aryl sulfonate, and alkyl sulfonate groups) and concluded that the primary influence on the proton conductivity of these randomly sulfonated copolymers was the acid strength. These results suggest that the neighboring pendant of an acid should play an important role in determining the degree of acidity of the compounds. Furthermore, Ding et al. 23 and Nieh et al. 24 reported highly fluorinated comb-shaped polymers, containing −(C6 F4 )2 − group and −C(CF3 )2 − group in the structure, where the polymers exhibit high proton conductivities with low dimensional changes. A commercial sulfonated bisphenol monomer (C10 H5 (OH)2 SO3 H) has been used to synthesize sulfonated poly(arylene ether)-type polymers by Guiver and co-workers. 25,26 A series of reports 26–28 showed that the introduction of nitrile groups into aromatic polymers (−C6 H3 CN−) is an effective way to improve the performance of PEMs. Considering these experimental results, we selected six model compounds based on those membranes mentioned above, which included six different functional groups (C6 H5 −, −CH2 −, −CF2 −, −C6 F4 −, −C6 (OH)4 −, and −C6 (CN)4 −) in the neighboring pendant of sulfonic group to investigate their effects on the properties of PEMs. To gain an insight into the effect of the neighboring pendant, in this study, we applied DFT in conjunction with the continuum solvation model to calculate the pKa values of model compounds based on aromatic PEMs to estimate their acid strength. Besides the proton conductivity, the structure of copolymers has an obvious effect on its chemical stability. Our previous work 29 on the degradation of hydrocarbon copolymers showed that a chemical bond adjacent to a sulfonic acid group (the C–S bond connecting the sulfur atom of a sulfonic acid group to the carbon atom of a phenyl group) was one of the weakest sites against attack from radicals with very low barrier heights, -0.23 kJ · mol−1 and 6.79 kJ · mol−1 for

4


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proton-dissociated and proton-undissociated forms, respectively, as shown in Figure 1a) and 1b). Therefore, we also investigated the effect of those selected neighboring pendants on the chemical stability. Our results are expected to provide a guide towards improving the chemical stability of the C–S bond used in common aromatic PEMs. R=

a) R

b) R

O

CH2

_

SO3

CH2

+ OH

SO3H + OH

CH2

CH2 O

[

SO3

R

[

H

_

O SO3H

R

]

H

]

R

R

OH + SO3

OH + SO3H

_

SO2

SO2

Ea= -0.23 kJ mol-1

Ea= 6.79 kJ mol-1

Figure 1: Degradation mechanisms of OH radical attack on C–S bond in the sulfonic group. 29

The remainder of the paper is organized as follows. The details of our computational approach are described in Section 2. The proton dissociation and chemical stability of six model compounds based on aromatic PEMs are compared in Section 3, and our conclusions are given in the final section.

Computational Details Six model compounds of aromatic PEMs, denoted as M1–M6, were selected to study the effect of different neighboring pendants of the sulfonic acid group on their proton dissociation and chemical stability. M1 is an acid domain usually found in various aromatic PEMs, such as sulfonated poly(ether ether ketone) (SPEEK) and SPES, shown in Figure 2(a), while M2 is a model of an alkyl sulfonated aromatic PEM (–CH2 CH2 CH2 SO3 H), as shown in Figure 2(b). M3 and M4 are fluorinated compounds, as shown in Figures 2(c) and 2(d), respectively. In M3, four fluorine atoms replaced the hydrogen atoms of the phenyl ring in M1, while in M4, a perfluoroalkyl group (–CF2 CF2 –) was used to connect the phenyl ring and the sulfonic acid group. M5 and M6 are model compounds in which OH groups (in 5



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SO3H

H2 H 2 H 2 C C C SO3H

M1

(a) F

F

F2 C

M3

M4

(d)

OH

NC SO3H

HO

F2 C SO3H

F

(c) HO

M2

(b)

SO3H F

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CN

M5

SO3H NC

OH

M6

CN

(f)

(e)

Figure 2: The chemical structures of six selected model compounds based on aromatic PEMs, M1–M6.

M5) and CN groups (in M6) replace the hydrogen atoms of the phenyl ring in M1, as shown in Figures 2(e) and 2(f), respectively. M5 may also have potential applications in AEMs. M2–M6 are model compounds with a novel sulfonic acid domain containing fluorine atoms or other functional groups in the neighboring pendant. All six model compounds were hydrated by water molecules, whose number, n, was systematically increased from 1 to 4, and were optimized using DFT employing the B3LYP functional. 30,31 Initial geometry optimizations were performed at the B3LYP/6-31+G(d) level of theory and the structures obtained were refined at the B3LYP/6-311++G(2d, 2p) level of theory. For the crucial structures, we also confirmed the minimum energy structure at the M06-2X/6-311++G(2d,2p) level of theory. 32 The zero-point energies were included in the energy comparisons. Calculation of the pKa values was carried out to estimate the acid strength of the six model compounds using the thermodynamic cycle shown in Figure 3. In the thermodynamic cycle, a water molecule was introduced to the reactants for convenient calculation of the free 6


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HA(gas) + H2O(gas) ΔG0solv(HA)

ΔG0gas

H3O+(gas) + A-(gas) ΔG0solv(H3O+)

ΔG0solv(H2O) ΔG0solution

HA(solv) + H2O(solv)

ΔG0solv(A-)

H3O+(solv) + A-(solv)

Figure 3: Thermodynamic cycle for the calculation of pKa .

energy of the solvation of a proton. Using equations (1), (2), and (3), the value of ∆G0solution could be calculated. ∆G0solution = ∆G0gas + ∆∆G0solv

(1)

∆G0gas = G0gas (H3 O+ ) + G0gas (A− ) − G0gas (H2 O) − G0gas (HA)

(2)

∆∆G0solv = ∆G0solv (H3 O+ ) + ∆G0solv (A− ) − ∆G0solv (H2 O) − ∆G0solv (HA)

(3)

The relationship between pKa and ∆G0solution was elucidated using equations (4) and (5). 33,34 ∆G0solution = −2.303RT log(Ka /[H2 O])

(4)

pKa = (∆G0solution − 2.38)/1.364

(5)

All the free energies in the gas phase, including ∆G0gas (H3 O+ ), ∆G0gas (A− ), ∆G0gas (H2 O), and ∆G0gas (HA) were calculated at the B3LYP/6-31+G(d), B3LYP/6-311++G(2d, 2p), and M06-2X/6-311++G(2d,2p) levels of theory, respectively. The corresponding free energies of solvation were calculated at the same theoretical levels using a new continuum solvation model, called the SMD model, based on the quantum mechanical charge density of a solute molecule interacting with a continuum description of the solvent. 35 We used an experimental value of –110.2 kcal · mol−1 for the solvation free energy of H3 O+ , G0sol (H3 O+ ). 36 In addition, possible degradation reactions of the C–S bond in the six model compounds

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(M1–M6) and their proton dissociated forms denoted by star (M1*–M6*) from interactions with radicals were calculated using DFT. All the geometries of reactants, transition states, and products were optimized at the B3LYP/6-311++G(2d,2p) level of theory. The polarizable continuum model (PCM) was used to take into account the effect of the solvent . 37–39 The vibrational frequencies were calculated at the corresponding levels of theory to confirm that the transition state had only one imaginary frequency and that the local minimum had no imaginary frequency. All the calculations in this work were carried out using the Gaussian09 software package. 40

Results and discussion Proton dissociation properties The optimized structures of the six model compounds with hydration number n = 3 and 4 are shown in Figure 4. All six model compounds were undissociated when hydrated by one or two water molecules, while three or four water molecules are the key hydration numbers for proton dissociation. This is consistent with our previous studies on the proton dissociation of Nafion, SPES, and SPEEK . 20,41 Nafion has a better proton dissociation property than that of SPES (or SPEEK) and its proton can be spontaneously dissociated when hydrated by three water molecules (see Figure S1 in Supporting Information). Figure 4(a) shows that the proton is undissociated for M1, M2 and M5 for a hydration number equal to 3. The corresponding proton dissociated structures were also considered, and the calculated energies were higher than the undissociated structures by 2-3 kJ · mol−1 for the three levels of theory mentioned in Section 2, except that M2+3H2 O and M2+5H2 O were lower in energy in the dissociated structure by approximately 0.5 kJ · mol−1 compared with its undissociated structure calculated at the M06-2X/6-311++G(2d,2p) level of theory. The energy profiles obtained at the B3LYP/6-311++G(2d,2p) level of theory for proton dissociation of M1, M2 and M5 are shown in Figure S2. The small energy difference between the undissociated and 8


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dissociated structures indicates that a small number of protons can dissociate in solution in case of M1, M2 and M5 when hydrated by three water molecules. In contrast, the protons become dissociated in case of M3, M4, and M6 for n=3, and no corresponding undissociated structures were found, which indicated that complete proton dissociation occurred. Figure 4(b) shows that when hydrated by four water molecules, the protons become dissociated for all six model compounds. Two selected atomic distances are shown in Figure 4, rOs−H (the distance between the proton donor oxygen atom of the sulfonic group and a proton) and rOw−H (the distance between the proton acceptor oxygen atom of water and a proton) and these show the changes experienced by the proton when n increases from 3 to 4, especially for M1, M2, and M5, while only slight changes were observed for M3, M4, and M6. Comparing the atomic distances, rOs−H , to that of Nafion, we found that all rOs−H of M1-M6 are shorter than that of the model compound of Nafion (CF3 CF2 SO3 H), 20 which implies the better proton dissociation property of Nafion. According to our previous studies, 20,41 M1, M2, and M5 have proton dissociation properties similar to those of SPES and SPEEK, while M3, M4, and M6 have proton dissociation properties similar to those of Nafion. A common characteristic of M3, M4, and M6 is that they all have a strong electron-withdrawing group, either CF or CF2 or CN, in the neighboring pendant of the sulfonic acid group, which allows them to release a proton easily on hydration. This suggests that the neighboring pendant of the acid group has a significant effect on the proton dissociation properties of a PEM. The pKa values of the six model compounds were calculated using the approach discussed in Section 2 to estimate their relative acid strength. The pKa values evaluated at different levels of theory are listed in Table 1. Although these calculated pKa values may not be at the level of accuracy required for a comparison with experimentally measured pKa values, they can be used to estimate the relative acid strength of the six model compounds qualitatively. The obtained pKa values show that the order of acid strength was similar, regardless of the levels of theory employed, i.e., M6 > M4 > M3 > M5 > M1 ≃ M2. Evidently, the model compounds having strong electron-withdrawing groups (CF, CF2 , and CN) in the

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(b) n=4

(a) n=3

M1

M1

M2

rOs-H=1.06 rOw-H=1.44

M3

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M2

rOs-H=1.06

rOs-H=1.44

rOs-H=1.42

rOw-H=1.44

rOw-H=1.05

rOw-H=1.06

M3

M4

M4

rOs-H=1.47

rOs-H=1.50

rOs-H=1.48

rOs-H=1.50

rOw-H=1.05

rOw-H=1.04

rOw-H=1.04

rOw-H=1.03

M5

M5

M6

M6

rOs-H=1.08

rOs-H=1.56

rOs-H=1.45

rOs-H=1.55

rOw-H=1.40

rOw-H=1.02

rOw-H=1.05

rOw-H=1.02

Figure 4: The optimized structures and interatomic distances (in Å) of M1–M6 and water molecules: (a) for hydration number n=3 and (b) for hydration number n=4.

neighboring pendant of the sulfonic acid group, i.e., M3, M4, and M6, have more negative pKa values, indicating a stronger degree of acidity, while model compounds M1 and M2, with CH or CH2 electron-donating groups have less negative pKa values, indicating a weaker acidic nature. This result is in agreement with our calculations on the proton dissociation properties discussed above. The calculated pKa value for M5 is in the range of –7 to – 9. Such a value is very close to that of M3, which implies that M5 is a relatively strong acid. However, the calculations on the proton dissociation properties of M5 showed that four water molecules are required for its proton dissociation, while M3 requires only three water molecules. This disagreement may be explained by the effect of intramolecular hydrogen bonds on its proton dissociation. We found an isomer of M5 denoted as M5’. Its energy is about 10 kJ · mol−1 higher than that of M5. In the structure of M5’, two hydrogen atoms form hydrogen bonds with the oxygen atoms of sulfonic group, as compared with M5 in Figure S3. The calculations on the proton dissociation property show that the proton in M5’ is spontaneously dissociated at the B3LYP/6-31+G(d) and M06-2X/6-311++G(2d,2p) 10


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levels of theory, and it is undissociated only at the B3LYP/6-311++G(2d,2p) level of theory. The calculated pKa value for M5’ is in the range of –8 to –10, which is slightly lower than that of M5. In addition, we notice that OH groups play different roles when they substitute different positions on a phenyl ring. To make this point clear, additional computations on two derivative model compounds of M5, M5-O (two OH groups positioned at the ortho-position of sulfonic group), and M5-M (two OH groups positioned at the meta-position of sulfonic group), have been carried out. Results of the computations, as presented in Figure S4, show that M5-O has properties analogous to those of M5, which indicates that the two OH groups substituted at the meta-position have a negligible effect on its proton dissociation property and acid strength (see Supporting Information for details). In summary, the introduction of an electron-withdrawing group in the neighboring pendant of the sulfonic acid group is advantageous for improving its proton dissociation properties, which can affect the proton conductivity. The obtained proton dissociation properties and acid strength order for M1, M2, and M4 show a similar trend to the experimentally achieved proton conductivity, namely M4 > M1(M2). 22 Table 1: The pKa values of M1–M6 obtained at different levels of theory. Method B3LYP/6-31+G(d) B3LYP/6-311++G(2d,2p) M06-2X/6-311++G(2d,2p)

M1 M2 M3 M4 M5 M6 -4.3 -2.9 -9.4 -11.9 -7.7 -12.0 -4.6 -4.7 -8.9 -11.4 -7.3 -12.8 -5.4 -4.1 -10.1 -12.5 -8.4 -13.0

Chemical stability In addition to the proton dissociation properties and acid strength, we also considered the effect of the pendant on the stability of the chemical bond adjacent to the sulfonic acid group. Two oxidative species, OH and H radicals, were used as an initiator of the degradation reactions. All the details of the proposed reactions, including reaction initiator, degradation site, and barrier heights obtained at different levels of theory are summarized in Table 2. Unless otherwise noted, all the barrier heights mentioned in the following text were the 11



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energy difference between the transition states and the reactants. For the reactions with negative barrier heights, there is an intermediate complex, (not presented in figures), in between reactants and transition states along a reaction coordinate where energetics of the complex is lower than that of the reactants. All the energy profiles of the following proposed reaction mechanisms are shown in Figures S5-S10 of Supporting Information. M1 can be considered as a model compound for SPEEK or SPES, which are well known representative aromatic PEMs. We considered M1 to be a reference system and compared its C–S bond stability with the other model compounds. Figure 5a)-5d) shows four possible C–S bond degradation reaction paths of the undissociated form of M1 and the dissociated form, M1*. In all these four simple reactions, the OH or H radical attacks the carbon atom adjacent to the sulfonic acid group, resulting in a breaking of the C–S bond. M1 and radicals firstly formed a complex of M1 · · · OH(H) as an intermediate between the reactants and the transition state on the reaction coordinate. The degradation products are phenol or benzene, and a sulfonic acid radical. The calculated barrier heights are –1.4 kJ · mol−1 for the OH radical attack on the carbon atom in M1 and 4.1 kJ · mol−1 for the OH radical attack on carbon atom in M1*, which indicates that OH radical attack can easily cause breakage of the C–S bond with no, or an extremely low, energy barrier. When the H radical attacks the same carbon atom in M1 and M1*, the barrier heights are calculated to be 29.5 kJ · mol−1 and 21.5 kJ · mol−1 , respectively, whose values are higher than those from OH radical attack. These results indicate the less aggressive nature of H radials compared with OH radicals. For reactions involving direct H radical attack on the S atom, which also results in breakage of the C–S bond, the calculated energy barrier heights are 71.1 kJ · mol−1 for M1 and 44.0 kJ · mol−1 for M1*. These values are significantly higher than those of the four reactions discussed above. These results are in agreement with our previous studies on the degradation of hydrocarbon copolymers. 29 Only few PEMs are known to have a structure similar to M2. The proton dissociation studies showed that its dissociation properties were similar to those of M1. Possible degra-

12


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OH 1.99

a)

SO3H

+ OH

SO3H

OH

1.82

+ HSO3

(No barrier)

H 1.87

b)

SO3H

+ H

SO3H

1.80

+ HSO3 (Ebarr=29.5 kJ.mol-1)

OH

c)

SO3-

2.04

+ OH

SO3-

1.83

d)

SO3-

OH

+ SO3- (E =4.1 kJ.mol-1) barr

H + H

1.88 SO3-

+ SO3-

(Ebarr=21.5 kJ.mol-1)

1.82

Figure 5: The proposed degradation reaction mechanisms of M1 from OH or H radical attack and the optimized structural parameters (in Å) of the corresponding transition states shown in square brackets.

dation reactions of M2 and M2* caused by OH or H radicals are shown in Figure 6i-a), 6i-b) and 6ii). One path is an abstraction reaction of the H atom by one (OH or H) radical, which produces an alkane radical in the first step. Subsequently, the alkane radical produced degrades further, resulting in breakage of the C–S bond. Details of the reaction mechanisms and selected structural parameters of transition states are shown in Figure 6. We found that Reaction i-a), which is initiated by an OH radical, has a much lower barrier energy, 1.4 kJ · mol−1 , than that of reaction i-b), which is initiated by an H radical, 23.7 kJ · mol−1 , again indicating the less aggressive nature of H radicals relative to OH radicals in the degradation reaction of aromatic PEMs. However, the barrier heights of the two reactions are similar in magnitude, being 23.3 kJ · mol−1 and 23.7 kJ · mol−1 , respectively. These values are higher than those of an OH radical attack on M1 or M1*, and are close to those of an H radical attack on M1 or M1*. Such a result indicates that the insertion of an alkane chain between the phenyl group and the sulfonic acid group improves the chemical stability of the C–S bond to some extent. Another possible degradation reaction is the direct attack by an H radical on the S atom of the sulfonic acid group, which would result in the breakage of the C–S bond. The barrier height of the reaction was calculated to be similar to the case of M1. Both M3 and M4 are model compounds that have fluorine atoms in the neighboring 13



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1.55 OH

H

1.14

i-a)


H2 H H2 C C C SO3H

+ OH

0.98 H

H2 H C C

H2 C SO3H

+ H 2O


H

1.30

i-b)

ii)


H2 H C C

H2 C SO3H

H2 H H2 C C C SO3H

+ H

H2 C C H

H2 H C C

2.38 C H2

H2 C C H

SO3H

H2 C SO3H

CH2

+ H2

+ HSO3

(Ebarr=23.7 kJ.mol-1) (Ebarr=23.3 kJ.mol-1)

Figure 6: The proposed degradation reaction mechanisms of M2 caused by OH or H radicals and the optimized structural parameters (in Å) of the corresponding transition states shown in square brackets.

pendants. The proposed reaction mechanisms for M3 and M4 are shown in Figures 7 and 8, respectively. As shown in Figures 7a)–7d), the degradation reactions of M3 are similar to those of M1 because of their structural similarity. As can be seen in these figures, an OH or H radical directly attacks the carbon atom in the C–S bond of M3 or M3*, resulting in breakage of the C–S bond. The calculated barrier heights are 18.2 kJ · mol−1 for an OH radical attack on M3, 25.3 kJ · mol−1 for an H radical attack on M3, 6.4 kJ · mol−1 for an OH radical attack on M3*, and 20.9 kJ · mol−1 for an H radical attack on M3*. These energy barrier heights are similar to those of M1, which suggests that the introduction of fluorine atoms in the neighboring phenyl group does not significantly improve the chemical stability of the C–S bond. H radicals exhibit a less aggressive nature than OH radicals do in these reactions. M4 is a model compound having a fluorocarbon moiety (–CF2 CF2 –) in the neighboring pendant of the sulfonic acid group, and PEMs having such a structure are reported to have a good proton conductivity by Yoshimura et al.

21

and Chang et al.. 22 As shown in Figures 8a)–

8c), three possible degradation mechanisms were proposed from our computational results. Since the carbon atoms in a fluorocarbon group are sp3 hybridized, an OH radical cannot attack these carbon atoms directly. Moreover, in our previous study, the reaction path where OH radical attacks the sulfonic acid group, resulting in a breakage of the C–S bond, has been shown to have a high energy barrier height . 29 Therefore, only the reactions initiated 14


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by H radicals were considered here. Reaction a) is a two-step reaction. The first step is an abstraction reaction of a fluorine atom by the H radical, producing a HF molecule and a fluorocarbon radical. This step requires a very high energy of 98.4 kJ · mol−1 for the reaction to occur, which may be because of the absence of a tertiary carbon atom in the fluorocarbon group of M4, because it has been considered as the weakest site against H radical attack in Nafion. 42,43 The higher energy barrier height of Reaction a-i) indicates the difficulty of the entire Reaction a) to occur, although the subsequent Reaction a-ii) has a lower energy barrier height of 29.5 kJ · mol−1 . Alternatively, H radicals can also attack the sulfonic acid group directly, as shown in Figures 8b) and 8c). The energy barrier heights were calculated to be 64.3 kJ · mol−1 for M4 and 60.0 kJ · mol−1 for M4*. These values are lower than those of Reaction a), but are still higher than the lowest energy barrier heights of the degradation reactions of the other model compounds. Obviously, this type of C–S bond in M4 exhibits a good chemical stability against OH or H radical attack, which implies that the introduction of a fluorocarbon group in the neighboring pendant would not only improve the proton dissociation properties of the PEM, but would also improve the chemical stability of the key C–S bond. M5 and M6 are model compounds where the former has OH groups (electron-donating effect at the ortho-position and electron-withdrawing effect at the meta-position) and the latter has CN groups (strong electron-withdrawing effect). Their structural features are similar to those of M1 and M3, respectively. The proposed degradation reaction mechanisms for M5 and M6 are shown in Figures 9a)–9d) and 10a)–10d), respectively. Because M1 and M3 have similar degradation routes as M5 and M6, we will not go into the details of the degradation reactions. The calculated energy barrier heights are 1.8 kJ · mol−1 for an OH radical attack on M5, 15.6 kJ · mol−1 for an H radical attack on M5, –11.4 kJ · mol−1 for an OH radical attack on M5*, and 14.5 kJ · mol−1 for an H radical attack on M5*. Comparing the energy barrier heights with those of M1, it was found that the chemical stability of the C–S bond was not improved by introducing OH groups onto the phenyl ring. For M6, the

15



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Page 16 of 27

OH F

F

a)

SO3H F

F

+ OH

F

F

2.06 SO3H

F

F

F

H

F

F

F SO3H

F

1.92

+ H

F

. -1 + HSO3 (Ebarr=25.3 kJ mol )

SO3H

1.81 F


OH

1.82 F

F

b)

F

F

F

F

F

F

F

F

F

OH F

F SO3-

c) F

F

F

F

F

F

+ OH

2.05 SO3-

1.87 F

F

F

F

OH F

+ SO3- (Ebarr=6.4 kJ.mol-1)

F

H SO3-

d) F

F

F

1.93

+ H

1.86 F

F

SO3-

F

+ SO3F


F


1.35 1.68

a-i)

a-ii)

F2 C

F2 C SO3H

F C

F2 C SO3H

F2 C

F2 C SO3H

F C

+ H

F C

F2 C

H

F

F2 C SO3H

2.57

F C

SO3H

F2 C SO3H

F C

+ HF

CF2 + SO3H

1.39 H

b)

+ H

F2 C

C F2

SO3H

F2 C

F2 C SO3-

+ H

F2 C

C F2


F2 C

CF2 + H2SO3


F2 C

CF2 + HSO3-


2.02

1.25

c)


H

SO3-

2.00

Figure 8: The proposed degradation reaction mechanisms of M4 caused by H radicals and the optimized structural parameters (in Å) of corresponding the transition states shown in square brackets.

16


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HO

OH

OH

a)

HO SO3H

HO

OH

HO

OH

b)

HO

HO

OH

OH

HO

OH


OH

OH

HO

1.89

OH


SO3H

1.81

HO

OH

OH

OH HO

OH

HO

2.08

OH

SO3-

+ OH

OH

1.86 HO

OH

HO

OH

HO

OH

+ SO3-

(No barrier)

H HO SO3-

d) HO

+ H HO

SO3OH

OH

1.81

H

OH

HO

OH

SO3H

HO

OH

c)

HO

2.08

+ OH HO

SO3H HO

OH

+ H HO

OH

1.96 SO31.84

+ SO3HO

OH


OH


obtained barrier heights were 21.8 kJ · mol−1 for an OH radical attack on M6, 23.2 kJ · mol−1 for an H radical attack on M6, 14.7 kJ · mol−1 for an OH radical attack on M6*, and 22.8 kJ · mol−1 for an H radical attack on M6*. The chemical stability of the C–S bond in M6 was found to be better than that of M1, and similar to that of M2. Therefore, the introduction of a strong electron-withdrawing group, CN, improves the proton dissociation properties significantly and enhances the chemical stability of the C–S bond. The results of the proton dissociation properties of the six model compounds described in the previous section indicate that, compared with M1, M4 exhibits a significant improvement in both proton dissociation properties and chemical stability. M6 shows a similar improvement in proton dissociation properties, but a less pronounced improvement in chemical stability. The results for M2 show only an improvement in chemical stability and M3 shows only an improvement in proton dissociation properties, while M5 shows no clear improvement in both properties. Therefore, the introduction of an electron-withdrawing group in neighboring pendant has a positive effect on the proton dissociation behavior, while its effect on chemical stability requires further study. Although it is an undesirable element, 17



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Page 18 of 27

OH NC

CN

a)

NC SO3H

NC

+ OH

CN

CN

NC

2.06

CN

SO3H

OH

1.84 NC

CN

NC


CN

H NC

CN

NC

CN

NC

1.97

b)

SO3H NC

+ H

CN

CN


SO3H

1.83 NC

CN

NC

CN

OH NC

CN

c)

NC SO3-

NC

CN

+ OH

CN

1.87 NC

NC

2.07

CN

SO3-

CN

OH NC

+ SO3- (Ebarr=14.7 kJ.mol-1)

CN

H NC

CN

d)

NC SO3-

NC

CN

+ H

CN

1.86 NC

NC

1.96

CN

SO3-

+ SO3-

CN

NC


CN


fluorine is present in M4, and it is still meaningful to improve the performance of aromatic PEMs significantly with the minimum use of fluorine atoms.

Conclusions In the present study, we have investigated the chemical stability and proton dissociation properties of six model compounds, M1–M6. The results show that M3 (C6 F4 HSO3 H), M4 (C6 H5 CF2 CF2 SO3 H), and M6 (C6 (CN)4 HSO3 H) have better proton dissociation properties and stronger acid strength than M1 (C6 H5 SO3 H) , while M2 (C6 H5 CH2 CH2 CH2 SO3 H) and M5 (C6 (OH)4 HSO3 H) were found to have similar proton dissociation properties to M1. The introduction of an electron-withdrawing group in the neighboring pendant plays an important role in the improvement of proton dissociation properties and acid strength. In addition, we also compared the stability of the C–S bond, which was identified in our previous study as a vulnerable site against OH or H radical attack in five novel compounds. Various degradation reaction mechanisms have been proposed, and the corresponding energy barrier heights of

18


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Table 2: A summary of all the proposed degradation reaction mechanisms of M1–M6 and their energy barrier heights (in kJ · mol−1 ).

a

Model Initiator Attacking site ∆E a ∆E b ∆E c M1 OH · C -9.3 -5.7 -1.4 M1 H· C 27.6 28.6 29.5 M1* OH · C -14.3 -11.6 4.1 M1* H· C 18.6 19.4 21.5 M2 OH · C 19.4 25.1 23.3 M2 H· C 19.4 25.1 23.7 M3 OH · C 8.9 11.6 18.2 M3 H· C 21.5 24.1 25.3 M3* OH · C -18.2 -14.6 6.4 M3* H· C 14.6 16.7 20.9 M4 H· C 99.8 99.4 98.4 M4 H· S 52.3 62.0 64.3 M4* H· S 43.4 38.1 60.0 M5 OH · C -1.7 2.2 1.8 M5 H· C 11.2 14.4 15.6 M5* OH · C -34.5 -29.9 -11.4 M5* H· C 10.2 11.8 14.5 M6 OH · C 13.9 18.8 21.8 M6 H· C 23.6 26.4 23.2 M6* OH · C -15.6 -11.3 14.7 M6* H· C 15.2 18.5 22.8 Energetics obtained at the B3LYP/6-31+G(d) level of theory; b Energetics obtained at the B3LYP/6-311++G(2d,2p) level of theory; c Energetics obtained at the B3LYP(PCM)/6-311++G(2d,2p) level of theory.

19



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the reactions were obtained. The results show that the C–S bond in M4 is very robust against radical attack, while the C–S bond in M2 and M6 shows only moderate stability. Therefore, M4 has an ideal neighboring pendant, and its introduction improves both the proton conductivity and the stability significantly compared with M1, while M6 shows only a moderate improvement in both aspects. Concerning the other model compounds, M3 improved only the proton dissociation properties, while M5 showed properties similar to those of M1. Although the neighboring pendant in M4 involved unfavored fluorine atoms, it improved two key properties of aromatic PEMs, while having the minimum use of fluorine.

Acknowledgement We gratefully acknowledge financial support through the New Energy and Industrial Technology Development Organization (NEDO), and the anonymous reviewers for their helpful comments.

Supporting Information Available Structures and interatomic distances of model compound of Nafion with waters; energy profiles for the proton dissociation of M1, M2 and M5; calculations of the proton dissociation property and acid strength for M5’ and M5; calculations of the proton dissociation property and acid strength for M5-O and M5-M; energy profiles of degradation reactions for M1–M6. This material is available free of charge via the Internet at http://pubs.acs.org/.

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activity with Perfluorosulfonated Acid Ionomer. J. Am. Chem. Soc. 2013, 135, 15923– 15932.

26


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Table of Contents graphic

M1+3H2O

M4+3H2O

kJ mol-1

60.0

M4+H

-1.4

0.0

TS

M1+OH R INT

C--SO3H

P

27


Theoretical Studies of Pendant Effects on the Properties of Sulfonated

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