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Molecular modeling assisted design of new monomers utilized in fuel cell proton exchange membranes
Journal of Membrane Science 401–402 (2012) 56–60
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Molecular modeling assisted design of new monomers utilized in fuel cell proton exchange membranes Patrick Laflamme a , Alexandre Beaudoin a , Thomas Chapaton b , Claude Spino a , Armand Soldera a,∗ a b
Department of Chemistry, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 General Motors R&D Center, Warren, MI 48090-9055, United States
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 21 January 2012 Accepted 23 January 2012 Available online 2 February 2012 Keywords: Molecular modeling Infrared spectra Nafion Deprotonation Triflic acid DFT
a b s t r a c t An extensive understanding of the proton dissociation mechanism that occurs in Nafion® is of great importance in the development of an improved proton exchange membrane for use in fuel cells (PEMFC). As the proton leaves the sulfonic acid group, structural changes within the Nafion® side-chain take place. To visualize such a process, molecular modeling is particularly useful. From an experimental viewpoint, changes that occur in bonds and atomic environment can be characterized by a judicious analysis of the normal modes of vibration. Using quantum chemical modeling of the infrared spectra of Nafion® , it was shown that a model system consisting of two triflic acid (TfOH) molecules accurately predicts the process of deprotonation in Nafion® involving the addition of water molecules. This model system allows the visualization of the deprotonation events by monitoring the changes in selected frequencies. We thus observed that only the sulfonic acid groups containing the departing proton undergoes structural modification before the first proton dissociation occurs. In turn, we used this information to design new monomers that respond to these particular changes resulting from the electronegativity of fluorine atoms. The rigidity of the proposed architecture should also exhibit improved mechanical properties. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The perfluorinated polymer Nafion® is extensively used as a proton exchange membrane (PEM) [1]. This copolymer possesses a polytetrafluoroethylene (PTFE) backbone and randomly distributed fluorinated ether side-chains each terminated with a sulfonic acid group. The PTFE backbone is extremely hydrophobic, while the fluorinated side-chain has a high electron affinity that promotes the dissociation of the proton of the sulfonic acid, making it a superacid. The particular structure and morphology of the Nafion® membrane precludes operation at low humidity and/or at temperatures above 100 ◦ C; this has fostered the design of new polymers to replace it [2–4]. However, in order to improve the membrane’s physical and proton transport properties, one must gain a thorough understanding of the interaction of Nafion® with water molecules. Understanding the mechanism of deprotonation in water may enable the design of new polymers structures that will promote proton dissociation properties in a low water content, ultimately leading to improved proton exchange membranes [5].
∗ Corresponding author. Tel.: +1 819 821 7650; fax: +1 819 821 8017. E-mail address: [email protected] (A. Soldera). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.01.027
The hydrophilic component of the Nafion® side-chains controls the transport of protons through the membrane while its structural stability is governed by the PTFE backbone [6–9]. Water absorption, influenced mainly by the intramolecular interactions between the sulfonic acid groups, can also be moderated by other effects, such as the work associated with the membrane’s swelling and the intermolecular hydrogen bonding between sulfonic acid groups within the hydrophilic pores. Those two effects were not considered in our design, which was based solely on the architecture of the end sidechain sulfonic group upon proton dissociation. For this purpose, all structural changes that occur during the deprotonation process have to be precisely understood and depicted in order to be accurately reproduced in the newly designed monomers. We recently published the infrared spectra of triflic acid (TfOH) with different water uptake [10]. Knowledge of the vibrational modes of TfOH and Nafion® was then used to probe structural changes of the sulfonic acid pair as the deprotonation occurred. An accurate depiction of the normal vibrational modes of the TfOH infrared spectrum not only revealed structural changes during proton dissociation, but also uncovered the strength of the bonds responsible for the actual vibrations. Evolution of the overall structure is herein disclosed by a series of snapshots at various levels of water content. Structural parameters that specifically characterize proton dissociation were thus extracted and used for the design of new monomers, which structures satisfy all or part of these characteristics.
P. Laflamme et al. / Journal of Membrane Science 401–402 (2012) 56–60
57
Scheme 1. System of minimum energy containing two TfOH [18].
2. Methodology The 6-31+G(d,p) basis set was used with the hybrid density functional Becke 3 term with Lee, Yang, Parr exchange (B3LYP) in the Gaussian 09 [11] environment to determine equilibrium geometries. Such a basis set includes both polarization and diffuse functions, which are required for accurate calculations involving acid–water networks. The presence of both diffuse and polarization functions tends to minimize the basis set superposition error (BSSE) [12], which results from the error induced by the mixing of basis sets from bonded atoms. This basis set represents a compromise between accuracy and efficiency. All optimized geometries were free of imaginary frequencies. 3. Results and discussion The studied system consists of two TfOH molecules, combined with 0–6 water molecules. is then introduced; it describes the number of water molecules per sulfonic acid group, so varies from 0 to 3. This simple system aims to mimic the structural end of the Nafion® side chains in a water environment during the proton dissociation process [13–15]. Isolated sulfonic acid groups allow for the study of the acid deprotonation event free of the entropic effects and steric hindrance within the rest of the polymer. The structural changes within this pair of sulfonic acids, occurring during proton dissociation were monitored as water molecules were successively added to the system. An excellent correlation between the computed and experimental infrared spectra was observed, while the computer simulation unveiled the different steps of the deprotonation event [10,16]. Since there are two acidic protons, the following notation was used: a (1) or a (2) after the name of a sulfonic group, refers to the site of the first or the second proton dissociation, respectively. 3.1. No water molecules in the system ( = 0) Before adding water molecules, energy minimization of the dimeric system ( = 0) reveals that the position of the two sulfonic acid groups tends to maximize hydrogen bonding, causing the two C S bonds to be approximately symmetrical, as shown in Scheme 1 [10,17]. Two structural parameters were monitored with particular attention during the dissociation event: the S· · ·S distance as well as the angle between the oxygen atom linked to the departing hydrogen, the departing hydrogen atom, and the oxygen atom of · ·O, where n is 1 or 2. For = 0, the the second sulfonic group, OH(n)· · ·O angle ˚ and the OH(n)· S· · ·S distance was calculated to be 4.3 A, 175.6◦ .
Fig. 1. Free binding energy for the dimeric system with respect to the number of water molecules (), using the 6-31G+(d,p) basis set. The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
3.2. Before the first dissociation ( ∈ [0,1.5]) Free binding energy was computed as water molecules were added to the dimeric system. It represents the stability gain as one free water molecule is added to the system. The O H bond distance, accompanied by an energy well, was used as the indication of deprotonation. A first energy well is observed at = 0.5 (Fig. 1), but it occurs before the dissociation of the proton from the protogenic group. The first dissociation actually occurs when = 1.5. The appearance and vanishing of calculated normal modes of vibrations during the simulated deprotonation were found to be in good agreement with experimental data. Table 1 shows the symmetric stretching of the SO H bond in both sulfonic groups. (SO H)s (1) steeply decreases from 3191 cm−1 for = 0–1439 cm−1 for = 1, while (SO H)s (2) slowly changes from 3266 cm−1 for = 0 to 3002 cm−1 = 1. Following Badger’s rule, a change in the stretching frequency reveals that a modification of the architecture took place [19]. The observed frequency changes indicate that SO3 H (2) is not greatly affected by the addition of water molecules, while the structure of SO3 H (1) undergoes important changes. The following structural parameters were further inspected: the S· · ·S distance slightly increases from = 0 to = 0.5, but returns to its initial value at = 1.5, i.e. at the first proton dis· ·O angle (Fig. 3) steeply sociation (Fig. 2). However, the OH(1)· · ·O angle remains decreases from = 0 to = 1.0, while the OH(2)· roughly constant. The first water molecules are thus absorbed on the protogenic site (1) without breaking the O H covalent bonds of the spectator sulfonic acid group (2), as is experimentally observed with Nafion® [20]. The increased sharing of the departing hydrogen atom between the protogenic group SO3 H (1) and the water environment accounts for the significant red shift of the computationally determined (SO H)s (1), which indicates that the strength of the O H (1) bond diminishes until = 1.5 (Table 1). Accordingly, as water molecules are added to the dimeric sulfonic acid system, they affect the structure of sulfonic acid group (1) more than the
Table 1 Computed frequencies of SO H.
0
0.5
1
1.5
2
2.5
3
3.5
SO H (1) SO H (2)
3191 3266
2394 3168
1436 3002
– 1923
– 2081
– 2330
– –
– –
58
P. Laflamme et al. / Journal of Membrane Science 401–402 (2012) 56–60
Scheme 2. System of minimum energy containing two TfOH and 2 water molecules, = 1 [18]. Fig. 2. Variation of the S· · ·S distance as a function of the number of water molecules () in the dimeric system. The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
structure of sulfonic acid group (2). An illustrative structure with = 1 is shown in Scheme 2. 3.3. After the first dissociation ( > 1.5) At = 2 (Scheme 3), after the first protonation has occurred, the · ·O angle (Fig. 3) does not change, while the S· · ·S distance OH(2)· increases (Fig. 1). An additional water molecule infiltrates the existing water network around the sulfonate group. At larger , there is a · ·O angle (Fig. 3). A modification in the overall change in the OH(2)· structure undoubtedly takes place since additional water molecules now have a greater impact on the structure of the remaining sulfonic acid group (i.e. sulfonic group (2)). The structure of the system for = 2.5 is shown in Scheme 4. A series of snapshots at different water contents of the bistrifluorosulfonic system clearly shows the changes in the overall structure. As water molecules are added inside the system, the O H
· ·O ( ) angles on the number · ·O ( ) and OH(2)· Fig. 3. Dependence of the OH(1)· of water molecules () in the dimeric system. The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
Scheme 3. System of minimum energy containing two TfOH and 4 water molecules, = 2 [18].
bond from one SO3 H group does not break until sufficient strength brought about by the water network is involved. The slight increase · ·O in the S· · ·S distance and the important changes in the OH(1)· angle, indicate that a new hydrogen bond is formed between the first departing hydrogen atom and the oxygen atom belonging to a water molecule. This initial hydration only affects the sulfonic acid groups undergoing the first SO H bond dissociation. Recent experimental infrared studies on Nafion® highlights the role of sulfonic acid group in retaining water and promote dissociation [20]. This hydration continues after the first SO H bond dissociation has
Scheme 4. System of minimum energy containing two TfOH and 5 water molecules, = 2.5 [18].
P. Laflamme et al. / Journal of Membrane Science 401–402 (2012) 56–60
59
Scheme 6. New molecules proposed based on structural analyses of proton dissociation in Nafion® .
Scheme 5. System of minimum energy containing two TfOH and 6 water molecules, = 3 [18].
occurred and until = 2. Beyond that, if one water molecule is added ( = 2.5), the overall structure is subjected to an important modification. The remaining hydrogen bond between the sulfonic acid (2) and the sulfonate group (1) is apparently redirected between the · ·O sulfonic group (2) and additional water molecules: the OH(2)· ◦ ◦ angle decreases from 176 to 135 (Fig. 3). At the second proton dissociation, i.e. = 3, a symmetric structure is obtained, in agreement with the proposed architecture of Zundel (see Scheme 5). From Fig. 1, it is worth noting that the S· · ·S distance of 4.3 A˚ in the initial structure with no water molecules ( = 0) as well as at the proton dissociation ( = 1.5), are roughly equivalent. The dissociation occurs after a rotation of the C S bond, which explains the relative constancy in the value of the S· · ·S distance and the impor· ·O angle. Questions then arise about tant changes in the OH(1)· the consequences of structurally imposing this S· · ·S distance to a molecule and letting the C S bonds parallel as in the bis-sulfonic structure without water molecules (Scheme 1). New structures having such a structural feature have been imagined and are shown in Scheme 6 [21,22]. The X and Y groups represent (bridge), O (ether), S (thio), or SO2 (sulfone). B represents functional groups able to bear polymerizable linkers. Among the structures that were optimized, the case in which Y = O, and X = SO2 was found to be of particular interest. It will be referred to as the cyclic structure. The S· · ·S distance of 4.3 A˚ (Scheme 7) value corresponds almost exactly to the S· · ·S distance calculated on the bis-trifluorosulfonic acid system up to the point of first proton dissociation. A comparison between the changes of · ·O (n = 1, 2) angles upon water uptake between the two the OH(n)·
· ·O angles on the number of water molecules () Fig. 4. Dependence of the OH(n)· in the dimeric system. Red circles for n = 1, and blue squares for n = 2. Filled for bissulfonic system and empty for the cycle system (Y = O, and X = SO2 of structure in Scheme 6). ( ) ( ). The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
systems also shows very good agreement (Fig. 4). It indicates that the first proton dissociation also occurs at = 1.5 for the cyclic system. Initial calculations reveal that the structural disposition of the two sulfonic acid groups in Scheme 7 corresponds to that of the assembly of the two TfOH without added water molecules. The rigid structure that supports the two sulfonic groups could possibly help reduce the water content necessary for the first proton dissociation and lead to a lower pKa. Ultimately, it may improve the mechanical properties of the ensuing membrane [5]. The synthesis of the
Scheme 7. Molecule of Scheme 6 with Y = O, and X = SO2 . The S· · ·S distance is also displayed [18].
60
P. Laflamme et al. / Journal of Membrane Science 401–402 (2012) 56–60
molecule shown in Scheme 7 and further calculations are presently being pursued to test these conclusions. 4. Conclusion An accurate description of the structural changes occurring within the dimeric trifluorosulfonic acid system as monitored by observing the calculated shifts in the infrared spectra during proton dissociation was achieved. Analysis of the structural changes as a function of added water molecules reveals that only one sulfonic group, the protogenic group, is affected by the inclusion of water molecules before proton dissociation occurs. Such a conclusion stems from the relative constancy of the S S distance, around · ·O angle, and the modification of the OH(1)· · ·O ˚ the OH(2)· 4.3 A, angle. It was also noticed that the S S distance remains constant at its initial value (when no water molecules are present in the system) to the point when the first proton dissociation occurs. It was then argued that new bis-sulfonic acid molecular scaffolds possessing this optimal S S distance and a comparable behavior of the OHO angle, might perform as well as or better than Nafion® for a fraction of the cost to produce it. By calculation it exhibits the same structural behavior, as water molecules are added to the system, as the bis-sulfonic system. The synthesis of the proposed molecules, and others, are presently underway. Acknowledgments The present work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Université de Sherbrooke, and General Motors Canada. Computations have been made available thanks to the Réseau Québécoise de Calcul Haute Performance (RQCHP), and Compute Canada. Special thanks to Eric Schnieder, Jan Herbst, John Ulicny and Louis Hector of GM R&D for their technical input.
References [1] K.A. Mauritz, R.B. Moore, State of understanding of Nafion, Chem. Rev. 104 (2004) 4535–4585. [2] J.S. Wainright, J.-T. Wang, D. Weng, R.F. Savinell, M.H. Litt, Acid-doped polybenzimidazoles, J. Electrochem. Soc. 142 (1995) L121–L123. [3] S.-P.S. Yen, S.R. Narayanan, G. Halpert, E. Graham, A. Yavrouian, Polymer material for electrolytic membranes in fuel cells, in: 5,795,496, US, 1998.
[4] R. Nolte, K. Ledjeff, M. Bauer, R. Mülhaupt, Partially sulfonated poly(arylene ether sulfone) – a versatile proton conducting membrane materials for modern energy conversion technologies, J. Membr. Sci. 83 (1993) 211. [5] A. Soldera, Y. Qi, W.T. Capehart, Phase transition and morphology of polydispersed ABA’ triblock copolymers determined by continuous and discrete simulations, J. Chem. Phys. 130 (2009) 064902. [6] W.Y. Hsu, T.D. Gierke, Ion transport and clustering in Nafion perfluorinated membranes, J. Membr. Sci. 13 (1983) 307–326. [7] M.H. Litt, A reevaluation of Nafion morphology, Polym. Preprints 38 (1997) 80. [8] H.-G. Haubold, T. Vad, H. Jungbluth, P. Hiller, Nano structure of NAFION: a SAXS study, Electrochim. Acta 46 (2001) 1559–1563. [9] L. Rubatat, A.L. Rollet, G. Gebel, O. Diat, Evidence of elongated polymeric aggregates in Nafion, Macromolecules 35 (2002) 4050–4055. [10] P. Laflamme, A. Beaudoin, T. Chapaton, C. Spino, A. Soldera, Simulated infrared spectra of triflic acid during proton dissociation J. Comput. Chem., doi:10.1002/jcc.22950, in press. [11] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr, J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N.J. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, Gaussian, Inc., Wallingford, CT, 2009. [12] N.R. Kestner, J.E. Combariza, Basis set superposition errors: theory and practice, in: D.B.B. Kenny, B. Lipkowitz (Eds.), Reviews in Computational Chemistry, vol. 13, Wiley-VCH, 1999. [13] M. Eikerling, S.J. Paddison, L.R. Pratt, T.A. Zawodzinsky Jr., Defect structure for proton transport in a triflic acid monohydrate solid, Chem. Phys. Lett. 368 (2003) 108–114. [14] A. Roudgar, S.P. Narasimachary, M. Eikerling, J. Phys. Chem. 110 (2006) 20469–20477. [15] S.J. Paddison, T.A. Zawodzinski Jr., Molecular modeling of the pendant chain in Nafion(R), Solid State Ionics 113–115 (1998) 333–340. [16] G. Zundel, H. Metzger, Energy bands of tunneling excess protons in liquid acids. Ir spectroscopic study of the nature of H5 O2 + groups, Z. Phys. Chem. (Frankfurt am Main) 58 (1968) 225–245. [17] R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto, A. Zecchina, Interaction of H2 O, CH3 OH, (CH3 )2 O,CH3 CN, and pyridine with the superacid perfluorosulfonic membrane Nafion: an IR and Raman study, J. Phys. Chem. 99 (1995) 11937–11951. [18] C.Y. Legault, CYLview, in: Sherbrooke, 2010, http://www.cylview.org. [19] J. Cioslowsky, G. Liu, R.A. Mosquera Castro, Badger’s rule revisited, Chem. Phys. Lett. 331 (2000) 497–501. [20] L. Grosmaire, S. Castagnoni, P. Huguet, P. Sistat, M. Boucher, P. Bouchard, P. Bébin, S. Deabate, Probing proton dissociation in ionic polymers by means of in situ ATR-FTIR spectroscopy, PCCP 10 (2008) 1577–1583. [21] W. Capehart, T. Chapaton, C. Spino, A. Soldera, G. Maier, M. Gross, in: 7,718,753 B2, US, 2010. [22] W. Capehart, T. Chapaton, C. Spino, A. Soldera, G. Maier, M. Gross, in: 7,863,402 B2, US, 2011.
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Molecular modeling assisted design of new monomers utilized in fuel cell proton exchange membranes Patrick Laflamme a , Alexandre Beaudoin a , Thomas Chapaton b , Claude Spino a , Armand Soldera a,∗ a b
Department of Chemistry, Université de Sherbrooke, Sherbrooke, Québec, Canada J1K 2R1 General Motors R&D Center, Warren, MI 48090-9055, United States
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 21 January 2012 Accepted 23 January 2012 Available online 2 February 2012 Keywords: Molecular modeling Infrared spectra Nafion Deprotonation Triflic acid DFT
a b s t r a c t An extensive understanding of the proton dissociation mechanism that occurs in Nafion® is of great importance in the development of an improved proton exchange membrane for use in fuel cells (PEMFC). As the proton leaves the sulfonic acid group, structural changes within the Nafion® side-chain take place. To visualize such a process, molecular modeling is particularly useful. From an experimental viewpoint, changes that occur in bonds and atomic environment can be characterized by a judicious analysis of the normal modes of vibration. Using quantum chemical modeling of the infrared spectra of Nafion® , it was shown that a model system consisting of two triflic acid (TfOH) molecules accurately predicts the process of deprotonation in Nafion® involving the addition of water molecules. This model system allows the visualization of the deprotonation events by monitoring the changes in selected frequencies. We thus observed that only the sulfonic acid groups containing the departing proton undergoes structural modification before the first proton dissociation occurs. In turn, we used this information to design new monomers that respond to these particular changes resulting from the electronegativity of fluorine atoms. The rigidity of the proposed architecture should also exhibit improved mechanical properties. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The perfluorinated polymer Nafion® is extensively used as a proton exchange membrane (PEM) [1]. This copolymer possesses a polytetrafluoroethylene (PTFE) backbone and randomly distributed fluorinated ether side-chains each terminated with a sulfonic acid group. The PTFE backbone is extremely hydrophobic, while the fluorinated side-chain has a high electron affinity that promotes the dissociation of the proton of the sulfonic acid, making it a superacid. The particular structure and morphology of the Nafion® membrane precludes operation at low humidity and/or at temperatures above 100 ◦ C; this has fostered the design of new polymers to replace it [2–4]. However, in order to improve the membrane’s physical and proton transport properties, one must gain a thorough understanding of the interaction of Nafion® with water molecules. Understanding the mechanism of deprotonation in water may enable the design of new polymers structures that will promote proton dissociation properties in a low water content, ultimately leading to improved proton exchange membranes [5].
∗ Corresponding author. Tel.: +1 819 821 7650; fax: +1 819 821 8017. E-mail address: [email protected] (A. Soldera). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2012.01.027
The hydrophilic component of the Nafion® side-chains controls the transport of protons through the membrane while its structural stability is governed by the PTFE backbone [6–9]. Water absorption, influenced mainly by the intramolecular interactions between the sulfonic acid groups, can also be moderated by other effects, such as the work associated with the membrane’s swelling and the intermolecular hydrogen bonding between sulfonic acid groups within the hydrophilic pores. Those two effects were not considered in our design, which was based solely on the architecture of the end sidechain sulfonic group upon proton dissociation. For this purpose, all structural changes that occur during the deprotonation process have to be precisely understood and depicted in order to be accurately reproduced in the newly designed monomers. We recently published the infrared spectra of triflic acid (TfOH) with different water uptake [10]. Knowledge of the vibrational modes of TfOH and Nafion® was then used to probe structural changes of the sulfonic acid pair as the deprotonation occurred. An accurate depiction of the normal vibrational modes of the TfOH infrared spectrum not only revealed structural changes during proton dissociation, but also uncovered the strength of the bonds responsible for the actual vibrations. Evolution of the overall structure is herein disclosed by a series of snapshots at various levels of water content. Structural parameters that specifically characterize proton dissociation were thus extracted and used for the design of new monomers, which structures satisfy all or part of these characteristics.
P. Laflamme et al. / Journal of Membrane Science 401–402 (2012) 56–60
57
Scheme 1. System of minimum energy containing two TfOH [18].
2. Methodology The 6-31+G(d,p) basis set was used with the hybrid density functional Becke 3 term with Lee, Yang, Parr exchange (B3LYP) in the Gaussian 09 [11] environment to determine equilibrium geometries. Such a basis set includes both polarization and diffuse functions, which are required for accurate calculations involving acid–water networks. The presence of both diffuse and polarization functions tends to minimize the basis set superposition error (BSSE) [12], which results from the error induced by the mixing of basis sets from bonded atoms. This basis set represents a compromise between accuracy and efficiency. All optimized geometries were free of imaginary frequencies. 3. Results and discussion The studied system consists of two TfOH molecules, combined with 0–6 water molecules. is then introduced; it describes the number of water molecules per sulfonic acid group, so varies from 0 to 3. This simple system aims to mimic the structural end of the Nafion® side chains in a water environment during the proton dissociation process [13–15]. Isolated sulfonic acid groups allow for the study of the acid deprotonation event free of the entropic effects and steric hindrance within the rest of the polymer. The structural changes within this pair of sulfonic acids, occurring during proton dissociation were monitored as water molecules were successively added to the system. An excellent correlation between the computed and experimental infrared spectra was observed, while the computer simulation unveiled the different steps of the deprotonation event [10,16]. Since there are two acidic protons, the following notation was used: a (1) or a (2) after the name of a sulfonic group, refers to the site of the first or the second proton dissociation, respectively. 3.1. No water molecules in the system ( = 0) Before adding water molecules, energy minimization of the dimeric system ( = 0) reveals that the position of the two sulfonic acid groups tends to maximize hydrogen bonding, causing the two C S bonds to be approximately symmetrical, as shown in Scheme 1 [10,17]. Two structural parameters were monitored with particular attention during the dissociation event: the S· · ·S distance as well as the angle between the oxygen atom linked to the departing hydrogen, the departing hydrogen atom, and the oxygen atom of · ·O, where n is 1 or 2. For = 0, the the second sulfonic group, OH(n)· · ·O angle ˚ and the OH(n)· S· · ·S distance was calculated to be 4.3 A, 175.6◦ .
Fig. 1. Free binding energy for the dimeric system with respect to the number of water molecules (), using the 6-31G+(d,p) basis set. The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
3.2. Before the first dissociation ( ∈ [0,1.5]) Free binding energy was computed as water molecules were added to the dimeric system. It represents the stability gain as one free water molecule is added to the system. The O H bond distance, accompanied by an energy well, was used as the indication of deprotonation. A first energy well is observed at = 0.5 (Fig. 1), but it occurs before the dissociation of the proton from the protogenic group. The first dissociation actually occurs when = 1.5. The appearance and vanishing of calculated normal modes of vibrations during the simulated deprotonation were found to be in good agreement with experimental data. Table 1 shows the symmetric stretching of the SO H bond in both sulfonic groups. (SO H)s (1) steeply decreases from 3191 cm−1 for = 0–1439 cm−1 for = 1, while (SO H)s (2) slowly changes from 3266 cm−1 for = 0 to 3002 cm−1 = 1. Following Badger’s rule, a change in the stretching frequency reveals that a modification of the architecture took place [19]. The observed frequency changes indicate that SO3 H (2) is not greatly affected by the addition of water molecules, while the structure of SO3 H (1) undergoes important changes. The following structural parameters were further inspected: the S· · ·S distance slightly increases from = 0 to = 0.5, but returns to its initial value at = 1.5, i.e. at the first proton dis· ·O angle (Fig. 3) steeply sociation (Fig. 2). However, the OH(1)· · ·O angle remains decreases from = 0 to = 1.0, while the OH(2)· roughly constant. The first water molecules are thus absorbed on the protogenic site (1) without breaking the O H covalent bonds of the spectator sulfonic acid group (2), as is experimentally observed with Nafion® [20]. The increased sharing of the departing hydrogen atom between the protogenic group SO3 H (1) and the water environment accounts for the significant red shift of the computationally determined (SO H)s (1), which indicates that the strength of the O H (1) bond diminishes until = 1.5 (Table 1). Accordingly, as water molecules are added to the dimeric sulfonic acid system, they affect the structure of sulfonic acid group (1) more than the
Table 1 Computed frequencies of SO H.
0
0.5
1
1.5
2
2.5
3
3.5
SO H (1) SO H (2)
3191 3266
2394 3168
1436 3002
– 1923
– 2081
– 2330
– –
– –
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Scheme 2. System of minimum energy containing two TfOH and 2 water molecules, = 1 [18]. Fig. 2. Variation of the S· · ·S distance as a function of the number of water molecules () in the dimeric system. The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
structure of sulfonic acid group (2). An illustrative structure with = 1 is shown in Scheme 2. 3.3. After the first dissociation ( > 1.5) At = 2 (Scheme 3), after the first protonation has occurred, the · ·O angle (Fig. 3) does not change, while the S· · ·S distance OH(2)· increases (Fig. 1). An additional water molecule infiltrates the existing water network around the sulfonate group. At larger , there is a · ·O angle (Fig. 3). A modification in the overall change in the OH(2)· structure undoubtedly takes place since additional water molecules now have a greater impact on the structure of the remaining sulfonic acid group (i.e. sulfonic group (2)). The structure of the system for = 2.5 is shown in Scheme 4. A series of snapshots at different water contents of the bistrifluorosulfonic system clearly shows the changes in the overall structure. As water molecules are added inside the system, the O H
· ·O ( ) angles on the number · ·O ( ) and OH(2)· Fig. 3. Dependence of the OH(1)· of water molecules () in the dimeric system. The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
Scheme 3. System of minimum energy containing two TfOH and 4 water molecules, = 2 [18].
bond from one SO3 H group does not break until sufficient strength brought about by the water network is involved. The slight increase · ·O in the S· · ·S distance and the important changes in the OH(1)· angle, indicate that a new hydrogen bond is formed between the first departing hydrogen atom and the oxygen atom belonging to a water molecule. This initial hydration only affects the sulfonic acid groups undergoing the first SO H bond dissociation. Recent experimental infrared studies on Nafion® highlights the role of sulfonic acid group in retaining water and promote dissociation [20]. This hydration continues after the first SO H bond dissociation has
Scheme 4. System of minimum energy containing two TfOH and 5 water molecules, = 2.5 [18].
P. Laflamme et al. / Journal of Membrane Science 401–402 (2012) 56–60
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Scheme 6. New molecules proposed based on structural analyses of proton dissociation in Nafion® .
Scheme 5. System of minimum energy containing two TfOH and 6 water molecules, = 3 [18].
occurred and until = 2. Beyond that, if one water molecule is added ( = 2.5), the overall structure is subjected to an important modification. The remaining hydrogen bond between the sulfonic acid (2) and the sulfonate group (1) is apparently redirected between the · ·O sulfonic group (2) and additional water molecules: the OH(2)· ◦ ◦ angle decreases from 176 to 135 (Fig. 3). At the second proton dissociation, i.e. = 3, a symmetric structure is obtained, in agreement with the proposed architecture of Zundel (see Scheme 5). From Fig. 1, it is worth noting that the S· · ·S distance of 4.3 A˚ in the initial structure with no water molecules ( = 0) as well as at the proton dissociation ( = 1.5), are roughly equivalent. The dissociation occurs after a rotation of the C S bond, which explains the relative constancy in the value of the S· · ·S distance and the impor· ·O angle. Questions then arise about tant changes in the OH(1)· the consequences of structurally imposing this S· · ·S distance to a molecule and letting the C S bonds parallel as in the bis-sulfonic structure without water molecules (Scheme 1). New structures having such a structural feature have been imagined and are shown in Scheme 6 [21,22]. The X and Y groups represent (bridge), O (ether), S (thio), or SO2 (sulfone). B represents functional groups able to bear polymerizable linkers. Among the structures that were optimized, the case in which Y = O, and X = SO2 was found to be of particular interest. It will be referred to as the cyclic structure. The S· · ·S distance of 4.3 A˚ (Scheme 7) value corresponds almost exactly to the S· · ·S distance calculated on the bis-trifluorosulfonic acid system up to the point of first proton dissociation. A comparison between the changes of · ·O (n = 1, 2) angles upon water uptake between the two the OH(n)·
· ·O angles on the number of water molecules () Fig. 4. Dependence of the OH(n)· in the dimeric system. Red circles for n = 1, and blue squares for n = 2. Filled for bissulfonic system and empty for the cycle system (Y = O, and X = SO2 of structure in Scheme 6). ( ) ( ). The coloured regions correspond to the preparation of the first ( ) and second ( ) deprotonation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
systems also shows very good agreement (Fig. 4). It indicates that the first proton dissociation also occurs at = 1.5 for the cyclic system. Initial calculations reveal that the structural disposition of the two sulfonic acid groups in Scheme 7 corresponds to that of the assembly of the two TfOH without added water molecules. The rigid structure that supports the two sulfonic groups could possibly help reduce the water content necessary for the first proton dissociation and lead to a lower pKa. Ultimately, it may improve the mechanical properties of the ensuing membrane [5]. The synthesis of the
Scheme 7. Molecule of Scheme 6 with Y = O, and X = SO2 . The S· · ·S distance is also displayed [18].
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molecule shown in Scheme 7 and further calculations are presently being pursued to test these conclusions. 4. Conclusion An accurate description of the structural changes occurring within the dimeric trifluorosulfonic acid system as monitored by observing the calculated shifts in the infrared spectra during proton dissociation was achieved. Analysis of the structural changes as a function of added water molecules reveals that only one sulfonic group, the protogenic group, is affected by the inclusion of water molecules before proton dissociation occurs. Such a conclusion stems from the relative constancy of the S S distance, around · ·O angle, and the modification of the OH(1)· · ·O ˚ the OH(2)· 4.3 A, angle. It was also noticed that the S S distance remains constant at its initial value (when no water molecules are present in the system) to the point when the first proton dissociation occurs. It was then argued that new bis-sulfonic acid molecular scaffolds possessing this optimal S S distance and a comparable behavior of the OHO angle, might perform as well as or better than Nafion® for a fraction of the cost to produce it. By calculation it exhibits the same structural behavior, as water molecules are added to the system, as the bis-sulfonic system. The synthesis of the proposed molecules, and others, are presently underway. Acknowledgments The present work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Université de Sherbrooke, and General Motors Canada. Computations have been made available thanks to the Réseau Québécoise de Calcul Haute Performance (RQCHP), and Compute Canada. Special thanks to Eric Schnieder, Jan Herbst, John Ulicny and Louis Hector of GM R&D for their technical input.
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