Structural Characteristics of Hydrated Protons in Ion Conductive

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Structural Characteristics of Hydrated Protons in Ion Conductive Channels: Synergistic Effect of Sulfonate Group and Fluorine Studied by Molecular Dynamics Simulation Yuechun Song, Jun Huo, Ning Zhang, Junjiang Bao, Xiaopeng Zhang, Xuehua Ruan, and Gaohong He J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11020 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Structural Characteristics of Hydrated Protons in Ion Conductive Channels: Synergistic Effect of Sulfonate Group and Fluorine Studied by Molecular Dynamics Simulation Yuechun Song, Jun Huo, Ning Zhang*, Junjiang Bao, Xiaopeng Zhang, Xuehua Ruan, Gaohong He* State Key Laboratory of Fine Chemicals, School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin, 124221, China *Corresponding author.

Tel.: +86-427-84708774. Fax: +86-411-84708460. E-mail: [email protected], [email protected]

Abstract: The performance of proton exchange membrane depends on several factors including membrane backbone and side chain, channel size and connectivity, temperature, pressure, electric field, hydration level, etc. However, it is impossible to separately investigate the independent effect of each factor on proton transfer in the membrane. The synergistic relationship between the proton conductive channel environment and the hydrated proton structure plays a decisive role in understanding the mechanism of proton transfer through proton exchange membrane. In this paper, classical molecular dynamics simulation is adopted to investigate the independent effects of sulfonate group and fluorine on the confined hydrogen bond network in the proton conductive channel, which is modeled using single-walled carbon nanotubes decorated with sulfonate groups and fluorine atoms. Free energy profile and hydrogen bond arrangement suggest that the aggregated sulfonate groups help trap hydrated protons, and fluorination facilitates the proton dissociation in the proton conductive channel. This is further verified by coordination number of sulfonate group, hydronium dissociation. Fluorination also maintains the continuous proton transfer by stabilizing the confined hydrogen bond connectivity. These findings provide the understanding of the synergistic effects of sulfonate group and fluorine on proton transfer along the proton conductive channel of Nafion. 1

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1. Introduction Due to the high energy density and efficiency with zero emissions1-4, proton exchange membrane fuel cell (PEMFC) has potential applications in portable electronics, electric vehicles and residential power generators.5 The perfluorinated sulfonate (PFSA) membrane Nafion is the priority to be used in PEMFC.3, 6It is still necessary to increase the proton conductivity of the membrane7-10, which could be effectively achieved on the basis of the well understanding of the mechanism of proton transfer in the nanochannel of PEM.11-12 The sulfonate group and fluorine of Nafion have great contributions to the microphase segregation in the hydrated Nafion membrane13, the hydrophilic phase of which provides continuous proton conductive channel (PCC)14. It is important to investigate the relationship between the deformed hydrophilic phase and the interface of the hydrophobic phase. However, the relationship is time-dependent on the factors of temperature, pressure, electric field, hydration level, etc.15-18 Furthermore, the microphase segregation could not exist in the membrane without sulfonate group or fluorine. Therefore, it is impossible to separately study the effects of sulfonate group and fluorine on proton transfer in the PCC of Nafion by either experiment or simulation. One way to solve the problem is to construct a simplified model with adjustable concerned factors and fixed unconcerned factors. Recently, single-walled carbon nanotubes (CNTs) are modified with function groups to mimic the PCC of Nafion18-21. Clark II et al.21-22 also employed AIMD to investigate the effects of confinement and fluorine on the structural and dynamical properties of hydrogen bond network comprised of the confined molecules. It is necessary to employ classical molecular dynamics (CMD) simulation to have a longer observation of the nanoscale properties of the hydrogen bond network and the dynamic motions of water and proton in the PCC. Our recent CMD studies19-20 shows that confinement and fluorination have cooperative effect on the structure of the hydrogen bond network in the fluorinated CNT with fixed size. Appropriate combination of confinement and fluorination produces a spiral-like hydrogen bond network with few bifurcated hydrogen bonds in 2


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the central region, which is beneficial to unidirectional proton transfer along the channel without random movement. Sulfonate group plays an important role in forming the hydrophilic phase and facilitating proton transfer in the PCC23-27. It is of significant importance to investigate the independent effect of sulfonate group on proton transfer. Habenichit et al.18 carried out ab initio molecular dynamics (AIMD) simulation to study the dissociation and hydrated structure of proton inside the CNTs functionalized with – CF2SO3H group and fluorine. The –SO3H distribution in the channel section shows significant influence on the confined hydrogen bond network and the proton dissociation from the sulfonate group. During the simulation run, the unfixed structure of the CNTs could leads to a fluctuating confinement effect on the results. The interactions of neighboring SO3H groups have also been investigated by DFT calculations in the previous work28. It was found that the first hydration shell (3 H2O) of a sulfonate group remains stable even in cases where two sulfonate groups were placed very close to each other. Hence, there is still much to understand the independent effect of each factor on the dissociation of proton and the confined hydrogen bond network inside the PCC of PFSA. We have previously studied the separate effects of channel size and fluorination on the confined hydrogen bond network. In this study, the independent effect of sulfonate group is mainly investigated by CMD simulation. It has been reported that the hydrogen bond network inside the fully fluorinated CNT(10, 10) is beneficial to proton transfer19. Herein, the PCC backbone is modeled with CNT(10, 10), which is fixed during the simulation run in order to maintain the confinement effect. The CNTs are functionalized by one or two sulfonate groups with different spacings to compare the interplay between sulfonate groups. The inner surfaces of some CNTs are also functionalized with fluorine atoms to investigate the differences of confinement in the environment similar to PFSA membranes with high hydrophobic backbones and polymers with aromatic backbones such as the sulfonated poly(p-phenylene sulfone) membrane29. Free energy profile of the hydrated proton along the channel axis for each system is firstly analyzed to intuitively present the effect of the function groups 3



on the local structure of hydrated protons. The confined interaction between sulfonate group and the hydrated proton is characterized by hydrogen bonding properties including coordination number, hydrogen bond connectivity, and hydrogen bond lifetime. Specially, hydrogen bond lifetime is estimated to describe the ability of proton to dissociate from sulfonate group. The idealized models hardly provide comparable results with experiments but perhaps provoke further development in PEM.

2. Computational methods Each system is comprised of one functionalized CNT and two graphite sheets separating two water reservoirs with the size of 35 Å × 35 Å × 25 Å, as shown in Fig. 1. The idealized PCCs are modeled by functionalizing the single-walled CNT with the chirality of (10, 10) with sulfonate group and fluorine atom. Fig. 2 depicts the representative snapshots of the CNTs functionalized in different types. The functionalized sulfonate groups are arranged in line along the channel axis. Fluorination is achieved by adding fluorine atom to every alternate carbon atom as uniformly as possible.

Fig. 1 Initial structure of the half fluorinated system, the spheres with different colours denote the atoms of different types, where gray - carbon, yellow - fluorine, red - oxygen, white – hydrogen, orange- sulfur and blue-hydronium ion. This colour scheme is used through the paper. 4


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For the sake of clearness in the following discussion, each system is named with one capital (i.e. ‘N’ or ‘F’) to denote nonfluorinated or fluorinated and one number (i.e. 0 to 3) to denote the functionalization type of sulfonate group. The number 0 indicates the functionalization with only one sulfonate group, and the numbers 1 to 3 indicate the functionalization with two spaced sulfonate groups with the spacings of 2.38, 4.76 and 7.14 Å, respectively. The functionalized two spaced sulfonate groups characterize one simple form of the aggregation of sulfonate groups in Nafion. Thus, the CNT system that is functionalized with fluorine atoms and two sulfonate groups with the spacing of 7.27 Å is named as F2. The sulfonate group at the fixed position of 4.16 Å of all systems is named as S1, and the sulfonate group altered in position is named as S2. The detailed compositions of the systems are given in Table 1.

Fig. 2 Representative snapshots of the profiles of different channels. The red dashed lines between two molecules represent the hydrogen bonds. Classical molecular dynamics simulations were performed using the package NAMD 2.930 to study the properties of water and hydronium ion confined in the one-dimensional PCC. The molecules in each system were characterized with 5



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CHARMM 27 force field31. The transferable intermolecular potential 3-point (Tip3p) model32 was adopted to describe the water molecules. Hydronium ion (H3O+)33 was used to model the excess protons in water. The bond length and partial charge of the function groups were characterized by the parameters of Nafion reported by Tomonori34. The length of the C-S, O-S and C-F bonds were set to 1.888, 1.479 and 1.352 Å, respectively. The partial charges of sulfur, sulfonic oxygen, and the sulfur bonded carbon were set to 0.9244, -0.5607 and -0.2423 e. Other sets of the systems are referred to our previous study19-20.

Table 1 Configurations of the CNT systems

CNT system

S1-S2 distance/Å

Nsulfonate

Nwater

Nfluorine

Ncarbon

N0

0

1

1728

0

1096

N1

2.38

2

1728

0

1096

N2

4.76

2

1728

0

1096

N3

7.14

2

1728

0

1096

F0

0

1

1728

159

1096

F1

2.38

2

1728

158

1096

F2

4.76

2

1728

158

1096

F3

7.14

2

1728

158

1096

Simulations were executed in the isothermal-isobaric (NPT) ensemble at 300 K under atmospheric pressure. The Langevin dynamics and the Langevin piston Nosé-Hoover methods were applied to control pressure and temperature, respectively. For each system, the carbon atoms of CNT and graphene were fixed during the simulation run. Rigid bonds were applied to all molecules. Periodic boundary conditions were imposed in three-dimensions. Particle Mesh Ewald (PME) method was used to calculate the full electrostatics interaction. Each system was initially relaxed for 15 ns to reach equilibration. Then an additional simulation run for 40 ns was conducted for sampling with the frequency of once per 1000 time steps. Time step was set to 2 fs.

6


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3. Results and discussion 3.1 Free energy profile Free energy profile of the hydronium ions along the axis of each CNT is measured with the potential mean force (PMF) method, defined as Eq. (1)

 ρ( z )   EPMF ( z ) = − k BTln   ρ0 

(1)

where kB is the Boltzmann constant, T denotes temperature, ρ0 is the reference density acquired by calculating the bulk density outside the CNT, ρ(z) is the local density at the position z along the channel axis, and EPMF(z) represents the local free energy at the position z. Fig. 3 shows the free energy profiles of hydronium ions along the channel axis in different systems. The plots for |z| < 8.9 Å refer to the free energy profiles inside the CNTs, and the plots |z| > 8.9 Å correspond to the free energy profiles of the bulk outside the CNTs. It is shown that the free energy profiles inside the CNTs fluctuates more than that outside the CNTs, and there are two obvious valleys close to the CNT ends. The energy minimum near S1 could increase the possibility of trapping the hydronium ions. The energy minimum of hydronium ion in N0 locates at the position about 3.46 Å away from the sulfur atom of S1. The distance discrepancy between the minimum position and S1 position may include the S-OS bond of sulfonate group, the H-OH bond of hydronium ion, and the hydrogen bond OS···H-OH. In the N1, N2 and N3 systems, the two sulfonate groups produce the comparable potential well at the position between them, which is deeper than that in N0. The hydronium ions trapped in the potential well of the systems N1-N3 are more stable than that of the N0 system. Hence, it is easy for the sulfonate groups of N1, N2 and N3 to trap the hydrated protons into the deep potential well, which makes it difficult for the trapped proton to dissociate from the sulfonate group. As S2 moves close to S1, the energy minimum also moves right. The energy 7



minima of hydronium ion in N1, N2 and N3 locate at the positions of 2.9, 1.7 and 0.5 Å, respectively. Besides, the position of the energy minimum inside the channel of N3 system is similar to that of N0 at 0.7 Å. Interestingly, the midpoint positions between the two sulfonate groups in N1, N2 and N3 are estimated at 2.97, 1.78 and 0.59 Å, which are similar to the corresponding positions of the energy minimum. Hence, the two sulfonate groups have a joint attraction to the surrounding hydronium ions, which helps to produce the structure of one hydronium ion simultaneously hydrogen bonded to S1 and S2. Indeed, the structure (SOିଷ ⋯ HଷOା ⋯ SOିଷ ) is found in the systems except N0 and F0. This is consistent with the previous study on the existence of the structure ( SOିଷ ⋯ Hଷ Oା ⋯ SOିଷ )18. The energy minimum between S1 and S2 is gradually weakened with the increasing S1-S2 distance. It indicates that the joint effect of the aggregated sulfonate groups on trapping the hydronium ion is decreased as the S1-S2 distance increases. Comparing with the nonfluorinated CNTs, the overall free energy profile of hydronium ion in each fluorinated CNT is lowered by the functionalized fluorine atoms. This is in accordance with our previous results19 that fluorination lowers the potential barrier for hydronium ion along the fluorinated CNT. Especially for the N1, N2 and N3 systems, the depth of the energy minimum is decreased by the modified fluorine atoms. This will weaken the ability of the sulfonate groups to trap the hydronium ions, thus enhance the dissociation of the trapped hydronium ions. For the N2 and N3 systems, fluorination could increase the fluctuation frequency of the free energy profile with smaller amplitude compared with that in the nonfluorinated systems. It implies that proper cooperation of fluorination and sulfonation benefits forming the wave-like potential pattern with high fluctuation frequency and small amplitude and depth. Proton transfer could present a movement wavelength by wavelength along the energy surface inside the fluorinated channel. It is believed that the wave-like pattern of energy profile helps a steady proton transfer. Interestingly, F1 and N1 have the most aggregated sulfonate groups and thus show the strongest attraction to hydronium ion. As a result, the depth and width of the deepest potential well in F1 and N1 are larger than that of the other systems. In F1, the modified 8


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fluorine atoms help to deepen the neighbour potential wells near the aggregated sulfonate groups, increasing the attraction scope of the channel surface.

Fig. 3 Free energy profiles of hydronium ion along the channel axis of different systems. The sulfonate groups placed in the positions corresponds to the positions in the nanochannels. From the above analysis of free energy profile, it is known that aggregated sulfonate groups could produce a deep and wide potential well to trap the hydrated proton, and it is difficult for the trapped proton to dissociate from the sulfonate groups. The presence of fluorine atoms shallows the potential well, facilitating the escape of proton from the trap of the sulfonate groups. Reasonable combination of sulfonate 9



group and fluorine could produce a wave-like potential pattern with high fluctuation frequency and small amplitude and depth, promoting steady proton transfer wavelength by wavelength. Therefore, sulfonate group and fluorine have synergistic effect on the passing proton along the PCC.

3.2 Coordination number of sulfonate group The abovementioned free energy profiles of hydronium ions confined in the nanochannels characterize the environment of the PCCs. It is necessary to investigate the local structure of sulfonate group to describe the hydrogen bonding interaction attraction between channel surface and the confined molecules (i.e. hydronium ion and water), which has great effect on the dissociation of hydronium ion from sulfonate group. Thus, the coordination number of sulfonate group for each system is investigated in this section. Coordination number (CN) of sulfonate group is defined as the average number of hydrogen bonds formed by the sulfonate group as both acceptor and donor.18 Herein, sulfonate-hydronium and sulfonate-water hydrogen bonds are formed by the sulfonate group. Hydrogen bond is usually identified by the geometric criterion, which is employed in this work. It is necessary to determine the thresholds of the distance O···O, hydrogen bond length H···O, and the angle H—O···O for the two hydrogen bonds by radial density distribution (RDF) and intermolecular angle distribution35, where the symbol “—” stands for the covalent bond, and the symbol “···” represents non-bonded interaction. RDFs of the oxygen-oxygen (OS-OW and OS-OH) pairs and oxygen-hydrogen

(OS-HW and OS-HH) pairs are calculated from the simulation trajectory. The subscripts “s”, “w” and “h” are the abbreviations for sulfonate group, water and hydronium ion, respectively. The first minimum positions of the RDFs are taken as the thresholds of the corresponding distance and hydrogen bond length. The intermolecular angle αO···O-H is the angle between the O-H covalent bond axis of water or hydronium ion and the vector from the oxygen atom to the oxygen of 10


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sulfonate group. For simplicity, the results of the RDFs of gOs-Oh(r), gOs-Ow(r), gOs-Hw(r) and gOs-Hh(r) and the angle αO···O-H distributions are appended in the Supporting Information (Fig. S1–S3). The geometric criteria are identified as ROO﹤ 3.1 Å, ROH﹤2.2 Å and α﹤35º for the sulfonate-hydronium hydrogen bond, and ROO ﹤3.3 Å, ROH﹤2.4 Å and α﹤35º for the sulfonate-water hydrogen bond.

Fig. 4 Average coordination numbers of sulfonate group with water (CNW) and hydronium ion (CNH) for different systems. According to the defined hydrogen bond criteria, the coordination numbers of sulfonate group with water and hydronium ion are shown in Fig. 4. Comparing the systems with one sulfonate group (i.e. N0 and F0) and with two sulfonate groups (i.e. N1-N3 and F1-F3), it is found that aggregation of sulfonate groups increases the CN of sulfonate group with hydronium. This is consistent with the results that the functionalized S2 lowers the surrounding potential barrier for hydronium ion, as shown in Fig. 3. Thus, in comparison with N0, the aggregated sulfonate groups in N1 attracts more hydronium ions in place of the surrounded water molecules. As the S1-S2 distance increases, the value of CNH presents a decreasing followed by an increasing tendency. Fig. 3 shows that the free energy well near the aggregated sulfonate groups of F1 or N1 is wider than that of other systems. The wide energy well presents strong attraction to hydronium ions, resulting in high CNH. It is found 11



ି ା that the average numbers of hydronium ion in the structure of SOି ଷ ⋯ Hଷ O ⋯ SOଷ

are observed to be 1.3, 0.4 and 0.3 for F1, F2 and F3, and 1.0, 0.9 and 0.5 for N1, N2 ି ା and N3, respectively. The highly stable hydronium ions in SOି ଷ ⋯ Hଷ O ⋯ SOଷ help

to maintain high CNH value. The joint attraction by S1 and S2 decreases as the S1-S2 distance increases, which lowers the value of CNH. Nevertheless, if the S1-S2 distance gets large enough, the space between S1 and S2 will accommodate more hydronium ions and increase the value of CNH. Moreover, as fluorination further lowers the potential barrier, thus CNH in the fluorinated channel is larger than that in the nonfluorinated channel. For the nonfluorinated systems, enhancing the sulfonate aggregation facilitates the formation of sulfonate-water hydrogen bond, thus increases the value of CNW. For the fluorinated systems, the negative partial-charge surface of the channel has more attraction to proton, thus it is difficult for water to be close to the sulfonate group. Fluorination could stabilize the interaction between proton and sulfonate group. This further verifies the dependence of CNH on fluorination. It is also found that the value of (CNH+CNW) for the fluorinated system is larger than the corresponding nonfluorinated system. Proton has larger size than water. The increasing number of protons around sulfonate group will dispel more water molecules. As a result, fluorination also weakens the hydration structure of sulfonate group.

3.3 Hydronium dissociation Dissociation of proton from sulfonate group is an important process during the proton transfer along the PCC of PEM36. Break of the hydrogen bond between hydronium ion and sulfonate group is the key step in the process of dissociation. Thus, hydrogen bond lifetime is adopted to characterize the dissociation of hydronium ion from the functionalized sulfonate group inside the nanochannels. Long hydrogen bond lifetime implies that it is difficult for hydronium ion to dissociate from sulfonate group. Hydrogen bond lifetime is estimated by means of the continuous autocorrelation function of the hydrogen bond number37-38 confined in the 12


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functionalized nanochannel, which is defined as follows.

C (t ) =

h(t ) ⋅ h(0) h(0)

2

(2)

where the variable h(t) is the instantaneous number of hydrogen bonds evolving with time. When the initially hydrogen bonded hydronium ion with sulfonate group remain hydrogen bonded from t = 0 to time t, the corresponding variable h(t) is defined as 1, otherwise it is zero. The autocorrelation function C(t) is the probability that the hydrogen bond between sulfonate group and hydronium ion remain hydrogen bonded for a time duration of t. Fig. 5 shows the time evolution of the hydrogen bond autocorrelation function between sulfonate group and hydronium ion. It is shown that the autocorrelation function of hydrogen bond in F1 decays the most slowly. It implies that the hydrogen bonds between sulfonate group and hydronium ion in F1 are more stable than that in any other systems. For comparing the hydrogen bond stability, Table 2 lists the lifetimes (τSH) of the confined sulfonate-hydronium hydrogen bonds in different systems, which are obtained by integrating the autocorrelation functions of hydrogen bond with respect to time.

13



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Fig. 5 Hydrogen bond autocorrelation function between sulfonate group and hydronium ion for different systems The hydrogen bond lifetime between hydronium ion and sulfonate group in the fluorinated CNT is shorter than that in the corresponding nonfluorinated CNT. It indicates that fluorination helps hydronium ion dissociate from the sulfonate group. This is consistent with the abovementioned conclusion that fluorination lowers the free energy barrier for hydronium ion transfer along the channel and facilitates the dissociation of hydronium ion from the sulfonate group. However, the hydrogen bond lifetime in F1 is the longest among the studied systems. This is due to strong joint attraction of the highly aggregated sulfonate groups to hydronium ions in F1, as stated in section 3.1. Table 2 Hydrogen bond lifetimes (τSH) between sulfonate group and hydronium ion in different systems. System

N0

N1

N2

N3

F0

F1

F2

F3

τSH / ps 0.077 0.073 0.038 0.055 0.013 0.131 0.022 0.014 As the S1-S2 distance increases, τSH shows a decreasing tendency. The sulfonate density of F3 presents the relatively short hydrogen bond lifetime between sulfonate and hydronium as well as high CNH. The results are similar to the previous report that sulfonate groups assigned 8.6 Å away from each other is larger enough to avoid the trap states of proton18. There has been report that it is difficult for proton dissociation from the sequential sulfonate groups with close distance, which is adjusted by partially folding the backbone of the perfluorosulfonic acid membrane36. This finding further verifies the joint effect of the aggregated sulfonate groups on proton dissociation. Therefore, appropriate combination of sulfonate aggregation and fluorination will produce the free energy profile beneficial for sulfonate to trap proton as well as proton dissociation into the confined water phase.

14


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3.4 Hydrogen bond network It was previously reported that appropriate hydrogen bond network is the prerequisite for long-range and effective proton transfer19. This section focuses on the connectivity and deforming frequency of the hydrogen bond network confined in the nanochannels. The hydrogen bond network is comprised of the water-water and water-hydronium hydrogen bonds, which are identified by the previously defined hydrogen bond criteria19: (I) ROO﹤3.5 Å, ROH﹤2.4 Å and α﹤35ºfor the water-water hydrogen bond; (II) ROO﹤3.5 Å, ROH﹤2.2 Å and α﹤35º the water-hydronium hydrogen bond.

3.4.1 Hydrogen bond connectivity Hydrogen bond connectivity presents the hydrogen bond structure confined in the nanochannels, which has great effect on proton conductivity.39 The size and spanning length of the largest cluster are employed to characterize the hydrogen bond connectivity inside the nanochannels.19 A cluster is a group of molecules (including water and hydronium ion) which are simultaneously connected by hydrogen bonds. 40 Cluster size is defined as the number of molecules in one cluster. The largest cluster size is described by the ratio (Pn) of the largest cluster size to the molecule number confined in the nanochannels. The spanning length (Lc) of the largest cluster is defined as the length of the largest cluster along the channel axis.

15



Fig. 6 The ratio Pn of the largest cluster corresponds to the left black ordinate and the cluster spanning length Lc of the largest cluster corresponds to the right red ordinate. Fig. 6 shows the results of Pn and Lc for different systems. It is shown that both Pn and Lc in the fluorinated CNTs are less than that in the nonfluorinated CNTs. With the modification of fluorine to the CNTs, Pn has a maximum decrease from 66.8 to 47.6 %, while the corresponding Lc decreases from 13.9 to 11.6 Å. It implies that fluorination has a larger contribution to disrupting the branched hydrogen bonds. According to our previous studies19, 20, fluorination helps the entrance of proton into the nanochannel, and the confined protons benefit the formation of stable hydrogen bond network with less branch hydrogen bond in the central region of the channel with certain size. Thus the less branched hydrogen bond network is resulted from the cooperative effect of confinement and fluorination of the channel. It has been reported that the branched hydrogen bonds provide ineffective moves for proton, which delays the long-range conduction19, 21, 41. Hence, fluorination contributes to facilitating the long-range proton transfer through PCC. Moreover, sulfonate group density has a larger effect on the hydrogen bond connectivity in the nonfluorinated nanochannel than that in the fluorinated nanochannel. It indicates that fluorination help stabilize the hydrogen bond connectivity inside the PCC. 16


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3.4.2 Hydrogen bond rearrangement It is known that hydrogen bond network is produced by a time-dependent process of simultaneously breaking and forming of hydrogen bonds due to the fast librational and vibrational motions of the hydrogen bonded molecules in a short time interval. According to the Grotthuss and vehicular mechanisms41-42, hydrogen bond rearrangement has great effect on proton transfer through PCC. Hydrogen bond lifetime is employed to characterize the rearrangement frequency of the hydrogen bond network confined in the nanochannels. Table 3 shows the average lifetimes of the hydrogen bonds (i.e. water-water and water-hydronium hydrogen bonds) confined in different nanochannels, which are obtained as stated in section 3.3. The hydrogen bond network inside the nanochannel of F3 presents the shortest lifetime. It implies that the confined hydrogen bond network has the highest rearrangement frequency. Thus, the hydrogen bond network formed in the nanochannel of F3 benefits the proton transfer along the PCC. The hydrogen bond lifetimes in the fluorinated nanochannels (except F1) are shorter than that in the nonfluorinated nanochannels. As stated above, the sulfonate groups in F1 generate a deep and wide potential well with a strong attraction to the trapped molecules. Thus fluorination generally facilitates the hydrogen bond rearrangement in the PCC. Table 3. The average lifetimes (τ) of the hydrogen bonds confined in different nanochannels

CNT system τ / ps

N0

N1

N2

N3

F0

F1

F2

F3

0.066 0.061 0.060 0.065 0.062 0.064 0.056 0.055

4. Conclusion Single-walled carbon nanotubes were functionalized by sulfonate groups and fluorine atoms to produce one-dimension nanochannel with Nafion-like environment for hydrated protons. Several classical molecular dynamics simulations have been 17



performed to investigate the free energy profiles of hydronium ion, and the structures and dynamics of the confined hydrogen bond network. Sulfonate group and fluorine have synergistic effect on proton transfer in the proton conductive channel. High sulfonate group density produces deep and wide potential well for trapping hydronium ions. It is difficult for the trapped hydronium ions to dissociate from the potential well. Fluorination shallows the deep potential well to facilitate the hydronium dissociation, and stabilize the hydrogen bond connectivity to support the continuous proton transfer. Moreover, the confined hydrogen bond rearrangement could be enhanced by the presence of fluorination. This contribution is inferred to facilitate proton transfer by the Grotthuss and vehicular mechanisms41-42. Therefore, reasonable combination of sulfonate and fluorine produces wave-like potential pattern with small fluctuations to help steady transfer of hydronium ion along the proton conductive channel. Further studies are still needed to investigate the independent effects of the other factors and their synergistic effects on proton transfer confined in the proton conductive channel.

Supporting Information The gOs-Oh(r), gOs-Ow(r), gOs-Hw(r) and gOs-Hh(r) RDFs and the angle αO···O-H distributions for the sulfonate-water and sulfonate-hydronium pairs are shown in Figures S1–S3.

Acknowledgements This research has been supported by National Natural Science Foundation of China (Grant No. 21506019), the Fundamental Research Funds for the Central Universities (Grant No. DUT16QY43), the Program for Changjiang Scholars (T2012049).

Reference 1.

Xu, J. B.; Zhao, T. S., Cheminform Abstract: Mesoporous Carbon with Uniquely 18


Page 18 of 29

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Combined Electrochemical and Mass Transport Characteristics for Polymer Electrolyte Membrane Fuel Cells. Cheminform 2013, 44, 16-24. 2. Lee, H.; Han, M.; Choi, Y. W.; Bae, B., Hydrocarbon-Based Polymer Electrolyte Cerium Composite Membranes for Improved Proton Exchange Membrane Fuel Cell Durability. J. Power Sources 2015, 295, 221-227. 3. Ling, X.; Bonn, M.; Parekh, S. H.; Domke, K. F., Nanoscale Distribution of Sulfonic Acid Groups Determines Structure and Binding of Water in Nafion Membranes. Angew. Chem. Int. Ed. 2016, 55, 4011-4015. 4. Li, J.; Pan, M.; Tang, H., Understanding Short-Side-Chain Perfluorinated Sulfonic Acid and Its Application for High Temperature Polymer Electrolyte Membrane Fuel Cells. Rsc Adv. 2013, 4, 3944-3965. 5. Fujita, S.; Koiwai, A.; Kawasumi, M.; Inagaki, S., Enhancement of Proton Transport by High Densification of Sulfonic Acid Groups in Highly Ordered Mesoporous Silica. Chem. Mater. 2013, 25, 1584-1591. 6. Kreuer, K. D.; Portale, G., A Critical Revision of the Nano-Morphology of Proton Conducting Ionomers and Polyelectrolytes for Fuel Cell Applications. Adv. Funct. Mater. 2013, 23, 5390–5397. 7. Zhang, Z.; Chattot, R.; Bonorand, L.; Jetsrisuparb, K.; Buchmüller, Y.; Wokaun, A.; Gubler, L., Mass Spectrometry to Quantify and Compare the Gas Barrier Properties of Radiation Grafted Membranes and Nafion ®. J. Membr. Sci. 2014, 472, 55-66. 8. Choi, B. G.; Yun, S. H.; Park, Y. C.; Jung, D. H.; Hong, W. H.; Park, H. S., Enhanced Transport Properties in Polymer Electrolyte Composite Membranes with Graphene Oxide Sheets. Carbon 2012, 50, 5395-5402. 9. Burgaz, E.; Lian, H.; Alonso, R. H.; Estevez, L.; Kelarakis, A.; Giannelis, E. P., Nafion–Clay Hybrids with a Network Structure. Polymer 2009, 50, 2384-2392. 10. Tang, H. L.; Pan, M., Synthesis and Characterization of a Self-Assembled Nafion/Silica Nanocomposite Membrane for Polymer Electrolyte Membrane Fuel Cells. J. Phys. Chem. C 2008, 112, 11556-11568. 11. Hickner, M. A.; Pivovar, B. S., The Chemical and Structural Nature of Proton Exchange Membrane Fuel Cell Properties. Fuel Cells 2005, 5, 213-229. 12. Peighambardoust, S. J.; Rowshanzamir, S.; Amjadi, M., Review of the Proton Exchange Membranes for Fuel Cell Applications. Int. J. Hydrogen Energy 2010, 35, 9349-9384. 13. Kim, S.; No, K.; Hong, S., Visualization of Ion Transport in Nafion Using Electrochemical Strain Microscopy. Chem. Commun. 2015, 52, 831-834. 14. Soberats, B.; Yoshio, M.; Ichikawa, T.; Taguchi, S.; Ohno, H.; Kato, T., 3d Anhydrous Proton-Transporting Nanochannels Formed by Self-Assembly of Liquid Crystals Composed of a Sulfobetaine and a Sulfonic Acid. J. Am. Chem. Soc. 2013, 135, 15286-15289. 15. Wu, L.; Zhang, Z.; Ran, J.; Zhou, D.; Li, C.; Xu, T., Advances in Proton-Exchange Membranes for Fuel Cells: An Overview on Proton Conductive Channels (Pccs). PCCP 2013, 15, 4870-4887. 16. Hofmann, D. W. M.; Kuleshova, L. N.; D’Aguanno, B., Theoretical Simulations 19



of Proton Conductivity: Basic Principles for Improving the Proton Conductor. J. Power Sources 2010, 195, 7743-7750. 17. Zhu, Y.; Ruan, Y.; Wu, X.; Lu, X.; Zhang, Y.; Lu, L., Electric Field‐Responsive Nanopores with Ion Selectivity: Controlling Based on Transport Resistance. Comput. Theor. Chem. 2016, 39, 993-997. 18. Habenicht, B. F.; Paddison, S. J.; Tuckerman, M. E., The Effects of the Hydrophobic Environment on Proton Mobility in Perfluorosulfonic Acid Systems: An Ab Initio Molecular Dynamics Study. J. Mater. Chem. 2010, 20, 6342-6351. 19. Zhang, N.; Song, Y.; Ruan, X.; Yan, X.; Liu, Z.; Shen, Z.; Wu, X.; He, G., Structural Characteristics of Hydrated Protons in the Conductive Channels: Effects of Confinement and Fluorination Studied by Molecular Dynamics Simulation. PCCP 2016, 18, 24198-24209. 20. Zhang, N.; Song, Y.; Huo, J.; Li, Y.; Liu, Z.; Bao, J.; Chen, S.; Ruan, X.; He, G., Formation Mechanism of the Spiral-Like Structure of a Hydrogen Bond Network Confined in a Fluorinated Nanochannel: A Molecular Dynamics Simulation. J. Phys. Chem. C 2017, 121, 13840-13847. 21. Clark II, J. K.; Habenicht, B. F.; Paddison, S. J., Ab Initio Molecular Dynamics Simulations of Aqueous Triflic Acid Confined in Carbon Nanotubes. PCCP 2014, 16, 16465-16479. 22. Clark II, J. K.; Paddison, S. J., Ab Initio Molecular Dynamics Simulations of Water and an Excess Proton in Water Confined in Carbon Nanotubes. PCCP 2014, 16, 17756-17769. 23. Eikerling, M.; Kornyshev, A. A.; Kuznetsov, A. M.; Ulstrup, J.; Walbran, S., Mechanisms of Proton Conductance in Polymer Electrolyte Membranes. J. Phys. Chem. B 2002, 105, 3646-3662. 24. Paddison, S. J.; Paul, R.; Kreuer, K. D., Theoretically Computed Proton Diffusion Coefficients in Hydrated Peekk Membranes. PCCP 2002, 4, 1151-1157. 25. Choe, Y. K.; Tsuchida, E.; Ikeshoji, T.; Yamakawa, S.; Hyodo, S. A., Nature of Proton Dynamics in a Polymer Electrolyte Membrane, Nafion: A First-Principles Molecular Dynamics Study. PCCP 2009, 11, 3892-3899. 26. Wang, C.; Clark II, J. K.; Kumar, M.; Paddison, S. J., An Ab Initio Study of the Primary Hydration and Proton Transfer of Cf3so3h and Cf3o(Cf2)2so3h: Effects of the Hybrid Functional and Inclusion of Diffuse Functions. Solid State Ionics 2011, 199, 6-13. 27. Clark II, J. K.; Paddison, S. J., The Effect of Side Chain Connectivity and Local Hydration on Proton Transfer in 3m Perfluorosulfonic Acid Membranes. Solid State Ionics 2012, 213, 83-91. 28. Wang, C.; Paddison, S. J., Hydration and Proton Transfer in Highly Sulfonated Poly (Phenylene Sulfone) Ionomers: An Ab Initio Study. J. Phys. Chem. A 2013, 117, 650-660. 29. de Araujo, C. C.; Kreuer, K. D.; Schuster, M.; Portale, G.; Mendil-Jakani, H.; Gebel, G.; Maier, J., Poly(P-Phenylene Sulfone)S with High Ion Exchange Capacity: Ionomers with Unique Microstructural and Transport Features. PCCP 2009, 11, 3305-3312. 20


Page 20 of 29

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30. Phillips, J. C.; Braun, R.; Wang, W.; Gumbart, J.; Tajkhorshid, E.; Villa, E.; Chipot, C.; Skeel, R. D.; Kalé, L.; Schulten, K., Scalable Molecular Dynamics with Namd. J. Comput. Chem. 2005, 26, 1781-1802. 31. MacKerell, A. D.; Banavali, N.; Foloppe, N., Development and Current Status of the Charmm Force Field for Nucleic Acids. Biopolymers 2000, 56, 257-265. 32. Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D.; Impey, R. W.; Klein, M. L., Comparison of Simple Potential Functions for Simulating Liquid Water. J. Chem. Phys. 1983, 79, 926-935. 33. Jang, S. S.; Molinero, V.; Çaǧın, T.; Goddard III, W. A., Nanophase-Segregation and Transport in Nafion 117 from Molecular Dynamics Simulations: Effect of Monomeric Sequence. J. Phys. Chem. B 2004, 108, 3149-3157. 34. Kawakami, T.; Shigemoto, I., Molecular Dynamics Studies on the Structures of Polymer Electrolyte Membranes and Diffusion Mechanism of Protons and Small Molecules. Polymer 2014, 55, 6309-6319. 35. Zhang, N.; Liu, Z.; Ruan, X.; Yan, X.; Song, Y.; Shen, Z.; Wu, X.; He, G., Molecular Dynamics Study of Confined Structure and Diffusion of Hydrated Proton in Hyfion ®; Perfluorosulfonic Acid Membranes. Chem. Eng. Sci. 2017, 158, 234-244. 36. Paddison, S. J.; Elliott, J. A., On the Consequences of Side Chain Flexibility and Backbone Conformation on Hydration and Proton Dissociation in Perfluorosulfonic Acid Membranes. PCCP 2006, 8, 2193-2203. 37. Xu, H.; Berne, B. J., Hydrogen-Bond Kinetics in the Solvation Shell of a Polypeptide. J. Phys. Chem. B 2008, 105, 11929-11932. 38. Zhang, N.; Li, W.; Chen, C.; Zuo, J.; Weng, L., Molecular Dynamics Study on Water Self-Diffusion in Aqueous Mixtures of Methanol, Ethylene Glycol and Glycerol: Investigations from the Point of View of Hydrogen Bonding. Mol. Phys. 2013, 111, 939-949. 39. Paddison, S. J.; Promislow, K. S., Device and Materials Modeling in Pem Fuel Cells; Springer New York, 2009. 40. Chen, C.; Li, W. Z.; Song, Y. C.; Weng, L. D.; Zhang, N., Formation of Water and Glucose Clusters by Hydrogen Bonds in Glucose Aqueous Solutions. Comput. Theor. Chem. 2012, 984, 85-92. 41. Zhang, N.; Shen, Z.; Chen, C.; He, G.; Hao, C., Effect of Hydrogen Bonding on Self-Diffusion in Methanol/Water Liquid Mixtures: A Molecular Dynamics Simulation Study. J. Mol. Liq. 2015, 203, 90-97. 42. Agmon, N., The Grotthuss Mechanism. Chem. Phys. Lett. 1995, 244, 456-462.

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Fig. 1 Initial structure of the half fluorinated system, the spheres with different colours denote the atoms of different types, where gray - carbon, yellow - fluorine, red - oxygen, white – hydrogen, orange- sulfur and blue-hydronium ion. This colour scheme is used through the paper. 289x168mm (95 x 95 DPI)



Fig. 2 Representative snapshots of the profiles of different channels. The red dashed lines between two molecules represent the hydrogen bonds. 208x247mm (96 x 96 DPI)


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Fig. 3 Free energy profiles of hydronium ion along the channel axis of different systems. The sulfonate groups placed in the positions corresponds to the positions in the nanochannels. 221x302mm (95 x 95 DPI)



Fig. 4 Average coordination numbers of sulfonate group with water (CNW) and hydronium ion (CNH) for different systems. 297x210mm (300 x 300 DPI)


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Fig. 5 Hydrogen bond autocorrelation function between sulfonate group and hydronium ion for different systems 297x210mm (300 x 300 DPI)



Fig. 6 The ratio Pn of the largest cluster corresponds to the left black ordinate and the cluster spanning length Lc of the largest cluster corresponds to the right red ordinate. 297x210mm (300 x 300 DPI)


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TOC Graphic 60x61mm (600 x 600 DPI)


Structural Characteristics of Hydrated Protons in Ion Conductive

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