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Vibrational spectroscopic study of ionic association in poly(ethylene oxide)–NH4SCN polymer electrolytes
Spectrochimica Acta Part A 71 (2009) 1927–1931
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Vibrational spectroscopic study of ionic association in poly(ethylene oxide)–NH4 SCN polymer electrolytes Hucheng Zhang ∗ , Jianji Wang School of Chemistry & Environmental Science, Henan Normal University, 46 Jianshe, Xinxiang, Henan 453007, PR China
a r t i c l e
i n f o
Article history: Received 11 November 2007 Received in revised form 15 July 2008 Accepted 15 July 2008 Keywords: PEO NH4 SCN IR Ionic association Polymer electrolyte Proton conductor
a b s t r a c t The polymer–ammonium complexes are an important class of proton conducting polymer electrolytes. In this work, poly(ethylene oxide) (PEO)–NH4 SCN electrolytes were prepared over a large range of the salt content, and their FT-IR spectra were measured at room temperature. Based on the assignments of each band in the spectral envelope of SCN−1 , their relative intensities are determined by the use of FT-IR technique. Following the experimental results and spectral analyses, this paper reports the interactions, the various ionic associations, the changes of the ionic association with NH4 SCN content, and the characteristics of structure in PEO–NH4 SCN electrolytes. It is shown that the hydrogen bonds of PEO and NH4 SCN exert the great effect to the ionic association, the interactions of PEO with NH4 SCN, and PEO crystallinity, in particular, under the condition of high NH4 SCN content. In addition, the differences of ionic association among PEO–NaSCN, PEO–KSCN and NH4 SCN electrolytes are also compared in this paper. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Solid polymer electrolytes have been a field investigated intensively in academia and industry due to their advantages over liquid electrolytes, including light in weight, the required flexibility in design, free of leakage problems of liquid electrolytes and a good contact with electrode even under the changes of volume and other stress. Especially, alkali metal-based polymer electrolytes have been widely investigated for the reason that they can be potentially applied to solid-state rechargeable batteries and other electrochemical devices. However, relatively little attention has been devoted to proton conducting polymer electrolytes. As a matter of fact, such proton conductors are equally an important class of polymer electrolyte owing to the low operating temperature and possibilities of applications in fuel cells, sensors, and electrochromic displays. This is also the reason why proton conducting polymer electrolytes have been becoming the focus of a wide variety of fundamental and application-oriented investigations in recent years [1–3]. One class of proton conducting polymer electrolytes is the polymer–ammonium complexes, in which NH4 + is believed to be responsible for the ionic conduction. With the academic and applied intentions, many complexes of ammonium salt with various polymers have been investigated by the use of
∗ Corresponding author. Tel.: +86 373 3325805; fax: +86 373 3326544. E-mail address: [email protected] (H. Zhang). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.07.018
electrical impedance spectroscopy, differential scanning calorimetry, X-ray diffraction, etc. Commonly, the interested ammonium salts in those complexes include NH4 NO3 [1,5], NH4 SCN [2–4], NH4 PF6 [6], (NH4 )4 H2 (SeO4 )3 [7], NH4 HSO4 [8,9], NH4 SO3 CF3 [10], NH4 ClO4 [11], and otherwise. It is noticeable that protons in polymer–ammonium complexes are covalently bound to the nitrogen, and consequently can form hydrogen bonds with other electronegative atom. This situation results in the complicacy of the conductive mechanism. In order to understand the conductive mechanism and the structure of polymer–ammonium complexes, and further to improve the ionic conductivity, various spectroscopic techniques have been widely used to explore the characteristics and ionic association in the proton conductors [2,3,7]. However, the mechanism of ion transport is still controversial, and needs to be further addressed. In our previous work [12–16], it is shown that the stretching modes of SCN−1 have the characteristic vibrational frequencies, and are very sensitive to the changes of microenvironment and structure in poly(ethylene oxide) (PEO)-based polymer electrolytes. Using the spectral characteristics of SCN−1 , we have studied the effects of several factors on the structure and ionic association in PEO–NaSCN polymer electrolytes. In this work, the proton conductors, PEO–NH4 SCN electrolytes, were prepared, and their FT-IR spectra were collected at room temperature. Following the assignments of the bands of SCN−1 stretching and the calculation of their relative intensities, we focus the attention on the interactions of ion–ion and ion–polymer in PEO–NH4 SCN electrolytes, and the unique features as compared with PEO–NaSCN and PEO–KSCN
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electrolytes. It is expected that such a study will provide useful information about the structure and properties of the PEO–NH4 CN electrolytes. 2. Experimental PEO (MW = 6 × 106 ) was a special gift from Liansheng Co. and was dried under vacuum at ∼60 ◦ C for 24 h to remove traces of water from the sample. NH4 SCN (99.5%, Luoyang Reagent Co.) was twice recrystallized in anhydrous ethanol, and dried in a vacuum oven at 40 ◦ C for 48 h. PEO and given amounts of NH4 SCN were mixed with anhydrous acetonitrile (99.9%, Beijing Reagent Co.) and stirred vigorously for 24 h until a homogenous solution was formed. For infrared measurements, the gelatinous polymer solution was directly cast on KBr windows, and vacuum-dried at 40 ◦ C for 2 h. The composition of PEO–NH4 SCN electrolyte is represented as P(EO)n NH4 SCN or denoted as [EO]:[NH4 + ] = n:1, where n refers to the molar ratio of ethylene oxide repeating unit and salt. Infrared absorption spectra were collected in the range 4000–400 cm−1 at room temperature, on a computer-interfaced Digilab Bio-Rad FTS-40 FT-IR spectrometer with a resolution of 2 cm−1 . Dry nitrogen was used for purging purpose in order to exclude the infrared active H2 O and CO2 in the atmosphere of the sample chamber. No significant absorption of water and acetonitrile could be detected in the IR spectra, indicating that their content in the all samples was negligible. Fig. 1. FT-IR spectrum of CN stretching modes in P(EO)60 NH4 SCN electrolyte.
3. Results and discussion 3.1. Ionic association of NH4 SCN in PEO SCN− belongs to point group symmetry C∞V and has three vibrational modes associated with CN stretching, CS stretching and doubly degenerate SCN bending, respectively. The CN stretching modes have high absorption intensity, high sensitivity to its ionization states and little overlap with the FT-IR spectrum of PEO. Therefore, SCN− is a good candidate for studying ion association in PEO-based polymer electrolytes by vibrational spectroscopy. Usually, the FT-IR spectrum of the CN stretching in PEO-based polymer electrolytes appears as one envelope in the region of 2150–2000 cm−1 , and an example for P(EO)60 NH4 SCN electrolyte is given in Fig. 1. In order to investigate the ion association in the electrolytes, the spectral envelope of SCN− is curve-fit to a straight base line and one Gaussian–Lorentzan product function for each band using the Win-IR software, a program of Bio-Rad spectrometer. Fig. 1 shows also the curve-fitting results for P(EO)60 NH4 SCN electrolyte. Obviously, the spectral envelope of SCN− consists of four bands. The band at 2055 cm−1 is ascribed to the contact ionpairs and solvent-separated dimers [17]; the band at 2080 cm−1 is attributed to the triple ion aggregates [17,18]; the band at 2104 cm−1 is an indication for the formation of the higher ion aggregates [19]; the band at 2043 cm−1 is assigned to the contact ion-pair dimers [19,20]. The FT-IR spectra of PEO–NH4 SCN electrolytes with different salt content are measured and analyzed as done for P(EO)60 NH4 SCN electrolyte. Fig. 2 shows the obtained spectra of CN stretching in PEO–NH4 SCN electrolytes, indicating the significant changes of spectral envelope with the increase of NH4 SCN content. Following the results of curve-fitting data, the relative intensity of each band (in percent) in a spectral envelope is determined, and Fig. 3 shows the dependences of relative intensities of the bands on salt content in PEO–NH4 SCN electrolytes. It is seen that the ionic association varies impressively with the composition of PEO–NH4 SCN electrolytes, and exhibits several features that are significantly different
from those observed in PEO–NaSCN electrolytes. For the sake of the convenience of analyzing the ionic association, the interested range of salt content is divided into two sections: one is the electrolytes in which the salt content is less than that in P(EO)20 NH4 SCN, the other is those with salt content more than that in P(EO)10 NH4 SCN.
Fig. 2. IR spectra of P(EO)n NH4 SCN solid electrolytes in the region of 2140–2000 cm−1 . The compositions of the electrolytes were indicated on each curve.
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Fig. 3. Relative intensity of each band in the spectral envelope of CN stretching as a function of the composition of P(EO)n NH4 SCN electrolytes. 䊉 2043 cm−1 ; 2055 cm−1 ; 2073 cm−1 ; 2080 cm−1 ; and 2104 cm−1 .
3.2. PEO–NH4 SCN electrolytes with low salt content For the fist situation, in which the salt content in electrolytes is lower than that in P(EO)20 NH4 SCN, it can be seen that the relative intensity of the band of the contact ion-pairs and solvent-separated dimers increases with increasing the salt content, whereas the relative intensities of the bands of the triple ion and the higher ion aggregates decrease under the same condition. These experimental phenomena mainly result from the solvation of PEO toward NH4 SCN. It is well known that PEO at room temperature consists of crystalline and amorphous phases, and the inorganic salts can well dissolve in the amorphous PEO. When NH4 SCN is introduced into PEO, the large SCN−1 , the contact ion-pairs and solvent-separated dimers can play a role of “plasticizer”, and transform PEO form crystal to amorphous phase. Therefore, the microenvironment of solvation in the electrolytes is improved due to the addition of salt, and leads to the decrease of the relative intensity of the bands of the triple ion and higher ion aggregates. The curve (a) in Figs. 4 and 5 shows the FT-IR spectrum of pure PEO. The bands at 1360 and 1343 cm−1 are attributed to the asymmetric and symmetric CH2 wagging modes; the bands at 1242 and 1235 cm−1 are ascribed to the asymmetric CH2 twisting modes; the bands at 963 and 948 cm−1 are assigned to the symmetric and asymmetric CH2 rocking modes [21,22]. With the introduction of NH4 SCN into PEO, the band of CO stretching shifts from 1114 cm−1 in pure PEO to 1109 cm−1 in P(EO)20 NH4 SCN electrolyte (shown in Fig. 6), suggesting the occurrence of the solvation between NH4 SCN and ether oxygen of PEO backbone. In order to describe the changes of PEO conformation due to the solvation, a convenient method is to use the conformer ratio of gauche and trans arrangements along polymer backbone. Moreover, it has been shown that the bands at 1343, 1242 and 963 cm−1 are related to gauche arrangement, while the bands at 1360, 1235 and 948 cm−1 are associated to trans arrangement of PEO backbone [23]. As shown by curves (b) and (c) in Figs. 4 and 5, the solvation of PEO toward NH4 SCN causes the increase of the relative intensities of bands at 1343, 1242 and 963 cm−1 . This implies that the increase of the gauche/trans conformer ratio of PEO chains, and hence PEO backbones appear as gauche arrangements in PEO–NH4 SCN complexes. SCN−1 is an ambident anion, and can form N-bonding, Sbonding or/and bridge complexes through the coordination of the S or/and N atom, resulting in the complicacy of ionic association in PEO–NH4 SCN electrolytes. Based on hard/soft-acid/base theory,
Fig. 4. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 1550–1200 cm−1 . The compositions of the electrolytes were indicated on each curve.
the hard Lewis acid, N atom in SCN−1 , can interact with the hard Lewis base, NH4 + , and hence produces N-bonding complex in the PEO–NH4 SCN electrolytes. Usually, the ease in the formation of ion pairs causes the occurrence of the dominant band at 2055 cm−1
Fig. 5. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 1000–800 cm−1 . The compositions of the electrolytes were indicated on each curve.
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Fig. 6. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 1220–1000 cm−1 . The compositions of the electrolytes were indicated on each curve.
in PEO–NH4 SCN complexes. Notwithstanding its weak relative intensity, noticeably, the band of the contact ion-pair dimers at 2043 cm−1 can be detected in the PEO–NH4 SCN electrolytes with low salt content. Although the band at 2043 cm−1 is also found in PEO–NaSCN or PEO–KSCN electrolytes, it occurs in the electrolytes with the salt content more than that in P(EO)10 NaSCN or P(EO)10 KSCN [19]. However, the band of the contact ion-pair dimers can occur in the PEO–NH4 SCN electrolytes with low salt content less than that in P(EO)80 NH4 SCN. This implies that NH4 SCN in PEO very easily forms the contact ion-pair dimers due to the hydrogen bonds between the contact ion pairs. 3.3. PEO–NH4 SCN electrolytes with high salt content If the NH4 SCN content is more than that in P(EO)10 NH4 SCN electrolyte, the ionic association changes dramatically when compared with those in PEO–NH4 SCN electrolytes with low salt content (as shown in Fig. 3). It is observed that the bands at 2104 and 2080 cm−1 disappear in the PEO–NH4 SCN electrolytes, at the same time, the relative intensity of band at 2055 cm−1 decreases rapidly with increasing salt content. However, the relative intensity of band at 2043 cm−1 increases with the increase of NH4 SCN content in PEO. Moreover, a new band at 2073 cm−1 is detected, and its relative intensity exhibits the same growing tends as the band at 2043 cm−1 . The band at 2073 cm−1 is not observed in PEO–NaSCN electrolytes, but can be detected in polyethylene glycol (PEG)–NaSCN electrolytes [19,20]. This suggests that the –OH groups of PEG, namely the hydrogen bonds between PEG and SCN−1 , are responsible for the occurrence of the band at 2073 cm−1 . As shown by curves (e) and (f) in Figs. 4 and 7, the band of NH stretching at 3187 cm−1 and the band of NH deformation at 1410 cm−1 shift respectively to 3159 and 1403 cm−1 , and synchronously become significantly broadened with increasing the relative intensities of bands at 2043 and 2073 cm−1 . These are typical spectral features of the formation of hydrogen bonds, suggesting the band at 2073 cm−1 has the same
Fig. 7. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 3500–2500 cm−1 . The compositions of the electrolytes were indicated on each curve.
origin as the band at 2043 cm−1 . That is, the band at 2073 cm−1 in PEO–NaSCN electrolytes results from the hydrogen bonds between PEO and NH4 SCN. In addition, the band at 750 cm−1 is assigned to the CS stretching, and the bands at 485 and 471 cm−1 are attributed to the SCN−1 bending [24]. These bands can be observed in the spectrum of pure NH4 SCN, but are barely detected when the band at 2055 cm−1 is the dominant one in PEO–NH4 SCN electrolytes with low salt content. However, these bands become significant for PEO–NH4 SCN electrolytes with high NH4 SCN content (as shown in Fig. 8), implying a microenvironment that is roughly similar to that in pure NH4 SCN. Therefore, it is reasonable to assign the band at 2073 cm−1 to contact ion-pair aggregates, which results from the hydrogen bonds between NH4 + and ether oxygen, and presumably the wrap of hydrogen bonds toward the contact ion pairs. Basically, this assignment is in agreement with that reported in the literature [2]. The PT-IR spectrum of PEO changes also significantly due to the transformation of ionic association, as shown by curves (d–f) in Figs. 4–6. Because of the broadened bands of PEO in PEO–NH4 SCN electrolytes, the bands, at 1467 and 1455, 1360 and 1343, 1242 and 1235, 963 and 948 cm−1 , join together to form the bands at 1453, 1351, 1250, 952 cm−1 , respectively. Synchronously, those at 1281 and 1114 cm−1 in PEO shift, respectively, to 1298 and 1099 cm−1 . All these experimental results are indicative of the characteristics of highly amorphous state in PEO–NH4 SCN electrolytes. It is known that ionic aggregates with large volume can play a role of good plasticizers. Therefore, the formation of hydrogen bonds, contact ion-pair dimers, and contact ion-pair aggregates is able to ruin completely the PEO crystalline phase. As a result, PEO takes the spectral features as its molten state when NH4 SCN content is enough great in PEO. Additionally, PEO–NH4 SCN electrolytes are very different from PEO–NaSCN electrolytes, in which the crystalline complex, P(EO)3 NaSCN, can be detected. However, PEO–NH4 SCN electrolytes
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aggregates. Moreover, the variable tends of these ionic associations with the NH4 SCN content in PEO are similar to those in PEO–NaSCN and PEO–KSCN electrolytes. However, in contrast to PEO–NaSCN and PEO–KSCN electrolytes, the remarkable particularity in PEO–NH4 SCN electrolytes is the formation of hydrogen bonds, which exert an important effect to ionic association. Because of the occurrence of hydrogen bonds, the contact ion-pair dimers can be observed in very low content of NH4 SCN in PEO, and the contact ion-pair aggregates can be detected when NH4 SCN content is more than that in P(EO)10 KSCN electrolyte. As a result, PEO–NH4 SCN electrolytes can appear as the highly amorphous state, and the crystalline complex cannot occur in the electrolytes. Acknowledgements The authors wish to acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 29973009), the Innovation Foundation of Henan Education Department and the Program for New Century Excellent Talents in University of Henan Province. References
Fig. 8. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 800–450 cm−1 . The compositions of the electrolytes were indicated on each curve.
are a little similar to PEO–KSCN electrolytes due to both spectral features of amorphous state under the condition of high salt content. It is possibly the reason that Na+ has least ion radius, but K+ and NH4 + have larger and almost equal ion radius. The large ion radius demands more ether oxygen atoms of PEO for wrapping around it, and can form a liquid-like environment in the electrolytes [19]. It should be noted that PEO–NH4 SCN electrolytes exhibit in greater extent the amorphous characteristics than PEO–KSCN electrolytes due to the hydrogen bonds between PEO and NH4 SCN. 4. Conclusions As shown in PEO–NaSCN and PEO–KSCN electrolytes, the ionic association in PEO–NH4 SCN electrolytes produces the contact ion-pairs, solvent-separated dimers, triple ion and higher ion
[1] L.S. Ng, A.A. Mohamad, J. Power Sources 163 (2006) 382. [2] C.S. Ramya, S. Selvasekarapandian, T. Savitha, G. Hirankumar, P.C. Angelo, Physica B 393 (2007) 11. [3] A. Awadhia, S.L. Agrawal, Solid State Ionics 178 (2007) 951. [4] S. Selvasekarapandian, R. Baskaran, M. Hema, Physica B 357 (2005) 412. [5] S.R. Majid, A.K. Arof, Physica B 355 (2005) 78. [6] J.P. Sharma, S.S. Sekhon, Mater. Sci. Eng. B 129 (2006) 104. [7] M. Polomska, J. Wolak, B. Hilczer, L. Szczepanska, Solid State Ionics 118 (1999) 261. [8] A.M.M. Ali, N.S. Mohamed, A.K. Arof, J. Power Sources 74 (1998) 135. [9] M.F. Daniel, B. Desbat, J.C. Lassegues, Solid State Ionics 28–30 (1988) 632. [10] M. Stainer, L.C. Hardy, D.H. Whitmore, D.F. Shriver, J. Electrochem. Soc. 131 (1984) 277. [11] S.A. Hashmi, A. Kumar, K.K. Maurya, S. Chandra, J. Phys. D 23 (1990) 1307. [12] H. Zhang, J. Wang, H. Zheng, K. Zhuo, Y. Zhao, J. Phys. Chem. B 109 (2005) 2610. [13] H. Zhang, X. Xuan, J. Wang, H. Wang, Spectrochim. Acta A 61 (2005) 347. [14] H. Zhang, X. Xuan, J. Wang, H. Wang, J. Phys. Chem. B 108 (2004) 1563. [15] H. Zhang, J. Wang, X. Xuan, K. Zhuo, Electrochim. Acta 51 (2006) 3244. [16] H. Zhang, Y. Zhao, J. Wang, H. Zheng, J. Phys. Chem. C 111 (2007) 5382. [17] D. Saar, P. Petrucci, J. Phys. Chem. 90 (1986) 3326. [18] P. Bacelon, J. Corset, C. de Loze, J. Solut. Chem. 9 (1980) 129. [19] H. Zhang, X. Xuan, J. Wang, H. Wang, Solid State Ionics 164 (2003) 73. [20] M. Xu, E.M. Eyring, S. Petrucci, Solid State Ionics 83 (1996) 293. [21] M.A.K.L. Dissanayake, R. Frech, Macromolecules 28 (1995) 5312. [22] R. Frech, W. Huang, Macromolecules 28 (1995) 1246. [23] J. Wang, H. Zhang, H. Zheng, X. Xuan, Chem. Phys. 325 (2006) 538. [24] C. Liu, D. Teeters, W. Potter, B. Tapp, M.H. Sukkar, Solid State Ionics 86–88 (1996) 431.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Vibrational spectroscopic study of ionic association in poly(ethylene oxide)–NH4 SCN polymer electrolytes Hucheng Zhang ∗ , Jianji Wang School of Chemistry & Environmental Science, Henan Normal University, 46 Jianshe, Xinxiang, Henan 453007, PR China
a r t i c l e
i n f o
Article history: Received 11 November 2007 Received in revised form 15 July 2008 Accepted 15 July 2008 Keywords: PEO NH4 SCN IR Ionic association Polymer electrolyte Proton conductor
a b s t r a c t The polymer–ammonium complexes are an important class of proton conducting polymer electrolytes. In this work, poly(ethylene oxide) (PEO)–NH4 SCN electrolytes were prepared over a large range of the salt content, and their FT-IR spectra were measured at room temperature. Based on the assignments of each band in the spectral envelope of SCN−1 , their relative intensities are determined by the use of FT-IR technique. Following the experimental results and spectral analyses, this paper reports the interactions, the various ionic associations, the changes of the ionic association with NH4 SCN content, and the characteristics of structure in PEO–NH4 SCN electrolytes. It is shown that the hydrogen bonds of PEO and NH4 SCN exert the great effect to the ionic association, the interactions of PEO with NH4 SCN, and PEO crystallinity, in particular, under the condition of high NH4 SCN content. In addition, the differences of ionic association among PEO–NaSCN, PEO–KSCN and NH4 SCN electrolytes are also compared in this paper. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Solid polymer electrolytes have been a field investigated intensively in academia and industry due to their advantages over liquid electrolytes, including light in weight, the required flexibility in design, free of leakage problems of liquid electrolytes and a good contact with electrode even under the changes of volume and other stress. Especially, alkali metal-based polymer electrolytes have been widely investigated for the reason that they can be potentially applied to solid-state rechargeable batteries and other electrochemical devices. However, relatively little attention has been devoted to proton conducting polymer electrolytes. As a matter of fact, such proton conductors are equally an important class of polymer electrolyte owing to the low operating temperature and possibilities of applications in fuel cells, sensors, and electrochromic displays. This is also the reason why proton conducting polymer electrolytes have been becoming the focus of a wide variety of fundamental and application-oriented investigations in recent years [1–3]. One class of proton conducting polymer electrolytes is the polymer–ammonium complexes, in which NH4 + is believed to be responsible for the ionic conduction. With the academic and applied intentions, many complexes of ammonium salt with various polymers have been investigated by the use of
∗ Corresponding author. Tel.: +86 373 3325805; fax: +86 373 3326544. E-mail address: [email protected] (H. Zhang). 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.07.018
electrical impedance spectroscopy, differential scanning calorimetry, X-ray diffraction, etc. Commonly, the interested ammonium salts in those complexes include NH4 NO3 [1,5], NH4 SCN [2–4], NH4 PF6 [6], (NH4 )4 H2 (SeO4 )3 [7], NH4 HSO4 [8,9], NH4 SO3 CF3 [10], NH4 ClO4 [11], and otherwise. It is noticeable that protons in polymer–ammonium complexes are covalently bound to the nitrogen, and consequently can form hydrogen bonds with other electronegative atom. This situation results in the complicacy of the conductive mechanism. In order to understand the conductive mechanism and the structure of polymer–ammonium complexes, and further to improve the ionic conductivity, various spectroscopic techniques have been widely used to explore the characteristics and ionic association in the proton conductors [2,3,7]. However, the mechanism of ion transport is still controversial, and needs to be further addressed. In our previous work [12–16], it is shown that the stretching modes of SCN−1 have the characteristic vibrational frequencies, and are very sensitive to the changes of microenvironment and structure in poly(ethylene oxide) (PEO)-based polymer electrolytes. Using the spectral characteristics of SCN−1 , we have studied the effects of several factors on the structure and ionic association in PEO–NaSCN polymer electrolytes. In this work, the proton conductors, PEO–NH4 SCN electrolytes, were prepared, and their FT-IR spectra were collected at room temperature. Following the assignments of the bands of SCN−1 stretching and the calculation of their relative intensities, we focus the attention on the interactions of ion–ion and ion–polymer in PEO–NH4 SCN electrolytes, and the unique features as compared with PEO–NaSCN and PEO–KSCN
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electrolytes. It is expected that such a study will provide useful information about the structure and properties of the PEO–NH4 CN electrolytes. 2. Experimental PEO (MW = 6 × 106 ) was a special gift from Liansheng Co. and was dried under vacuum at ∼60 ◦ C for 24 h to remove traces of water from the sample. NH4 SCN (99.5%, Luoyang Reagent Co.) was twice recrystallized in anhydrous ethanol, and dried in a vacuum oven at 40 ◦ C for 48 h. PEO and given amounts of NH4 SCN were mixed with anhydrous acetonitrile (99.9%, Beijing Reagent Co.) and stirred vigorously for 24 h until a homogenous solution was formed. For infrared measurements, the gelatinous polymer solution was directly cast on KBr windows, and vacuum-dried at 40 ◦ C for 2 h. The composition of PEO–NH4 SCN electrolyte is represented as P(EO)n NH4 SCN or denoted as [EO]:[NH4 + ] = n:1, where n refers to the molar ratio of ethylene oxide repeating unit and salt. Infrared absorption spectra were collected in the range 4000–400 cm−1 at room temperature, on a computer-interfaced Digilab Bio-Rad FTS-40 FT-IR spectrometer with a resolution of 2 cm−1 . Dry nitrogen was used for purging purpose in order to exclude the infrared active H2 O and CO2 in the atmosphere of the sample chamber. No significant absorption of water and acetonitrile could be detected in the IR spectra, indicating that their content in the all samples was negligible. Fig. 1. FT-IR spectrum of CN stretching modes in P(EO)60 NH4 SCN electrolyte.
3. Results and discussion 3.1. Ionic association of NH4 SCN in PEO SCN− belongs to point group symmetry C∞V and has three vibrational modes associated with CN stretching, CS stretching and doubly degenerate SCN bending, respectively. The CN stretching modes have high absorption intensity, high sensitivity to its ionization states and little overlap with the FT-IR spectrum of PEO. Therefore, SCN− is a good candidate for studying ion association in PEO-based polymer electrolytes by vibrational spectroscopy. Usually, the FT-IR spectrum of the CN stretching in PEO-based polymer electrolytes appears as one envelope in the region of 2150–2000 cm−1 , and an example for P(EO)60 NH4 SCN electrolyte is given in Fig. 1. In order to investigate the ion association in the electrolytes, the spectral envelope of SCN− is curve-fit to a straight base line and one Gaussian–Lorentzan product function for each band using the Win-IR software, a program of Bio-Rad spectrometer. Fig. 1 shows also the curve-fitting results for P(EO)60 NH4 SCN electrolyte. Obviously, the spectral envelope of SCN− consists of four bands. The band at 2055 cm−1 is ascribed to the contact ionpairs and solvent-separated dimers [17]; the band at 2080 cm−1 is attributed to the triple ion aggregates [17,18]; the band at 2104 cm−1 is an indication for the formation of the higher ion aggregates [19]; the band at 2043 cm−1 is assigned to the contact ion-pair dimers [19,20]. The FT-IR spectra of PEO–NH4 SCN electrolytes with different salt content are measured and analyzed as done for P(EO)60 NH4 SCN electrolyte. Fig. 2 shows the obtained spectra of CN stretching in PEO–NH4 SCN electrolytes, indicating the significant changes of spectral envelope with the increase of NH4 SCN content. Following the results of curve-fitting data, the relative intensity of each band (in percent) in a spectral envelope is determined, and Fig. 3 shows the dependences of relative intensities of the bands on salt content in PEO–NH4 SCN electrolytes. It is seen that the ionic association varies impressively with the composition of PEO–NH4 SCN electrolytes, and exhibits several features that are significantly different
from those observed in PEO–NaSCN electrolytes. For the sake of the convenience of analyzing the ionic association, the interested range of salt content is divided into two sections: one is the electrolytes in which the salt content is less than that in P(EO)20 NH4 SCN, the other is those with salt content more than that in P(EO)10 NH4 SCN.
Fig. 2. IR spectra of P(EO)n NH4 SCN solid electrolytes in the region of 2140–2000 cm−1 . The compositions of the electrolytes were indicated on each curve.
H. Zhang, J. Wang / Spectrochimica Acta Part A 71 (2009) 1927–1931
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Fig. 3. Relative intensity of each band in the spectral envelope of CN stretching as a function of the composition of P(EO)n NH4 SCN electrolytes. 䊉 2043 cm−1 ; 2055 cm−1 ; 2073 cm−1 ; 2080 cm−1 ; and 2104 cm−1 .
3.2. PEO–NH4 SCN electrolytes with low salt content For the fist situation, in which the salt content in electrolytes is lower than that in P(EO)20 NH4 SCN, it can be seen that the relative intensity of the band of the contact ion-pairs and solvent-separated dimers increases with increasing the salt content, whereas the relative intensities of the bands of the triple ion and the higher ion aggregates decrease under the same condition. These experimental phenomena mainly result from the solvation of PEO toward NH4 SCN. It is well known that PEO at room temperature consists of crystalline and amorphous phases, and the inorganic salts can well dissolve in the amorphous PEO. When NH4 SCN is introduced into PEO, the large SCN−1 , the contact ion-pairs and solvent-separated dimers can play a role of “plasticizer”, and transform PEO form crystal to amorphous phase. Therefore, the microenvironment of solvation in the electrolytes is improved due to the addition of salt, and leads to the decrease of the relative intensity of the bands of the triple ion and higher ion aggregates. The curve (a) in Figs. 4 and 5 shows the FT-IR spectrum of pure PEO. The bands at 1360 and 1343 cm−1 are attributed to the asymmetric and symmetric CH2 wagging modes; the bands at 1242 and 1235 cm−1 are ascribed to the asymmetric CH2 twisting modes; the bands at 963 and 948 cm−1 are assigned to the symmetric and asymmetric CH2 rocking modes [21,22]. With the introduction of NH4 SCN into PEO, the band of CO stretching shifts from 1114 cm−1 in pure PEO to 1109 cm−1 in P(EO)20 NH4 SCN electrolyte (shown in Fig. 6), suggesting the occurrence of the solvation between NH4 SCN and ether oxygen of PEO backbone. In order to describe the changes of PEO conformation due to the solvation, a convenient method is to use the conformer ratio of gauche and trans arrangements along polymer backbone. Moreover, it has been shown that the bands at 1343, 1242 and 963 cm−1 are related to gauche arrangement, while the bands at 1360, 1235 and 948 cm−1 are associated to trans arrangement of PEO backbone [23]. As shown by curves (b) and (c) in Figs. 4 and 5, the solvation of PEO toward NH4 SCN causes the increase of the relative intensities of bands at 1343, 1242 and 963 cm−1 . This implies that the increase of the gauche/trans conformer ratio of PEO chains, and hence PEO backbones appear as gauche arrangements in PEO–NH4 SCN complexes. SCN−1 is an ambident anion, and can form N-bonding, Sbonding or/and bridge complexes through the coordination of the S or/and N atom, resulting in the complicacy of ionic association in PEO–NH4 SCN electrolytes. Based on hard/soft-acid/base theory,
Fig. 4. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 1550–1200 cm−1 . The compositions of the electrolytes were indicated on each curve.
the hard Lewis acid, N atom in SCN−1 , can interact with the hard Lewis base, NH4 + , and hence produces N-bonding complex in the PEO–NH4 SCN electrolytes. Usually, the ease in the formation of ion pairs causes the occurrence of the dominant band at 2055 cm−1
Fig. 5. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 1000–800 cm−1 . The compositions of the electrolytes were indicated on each curve.
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Fig. 6. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 1220–1000 cm−1 . The compositions of the electrolytes were indicated on each curve.
in PEO–NH4 SCN complexes. Notwithstanding its weak relative intensity, noticeably, the band of the contact ion-pair dimers at 2043 cm−1 can be detected in the PEO–NH4 SCN electrolytes with low salt content. Although the band at 2043 cm−1 is also found in PEO–NaSCN or PEO–KSCN electrolytes, it occurs in the electrolytes with the salt content more than that in P(EO)10 NaSCN or P(EO)10 KSCN [19]. However, the band of the contact ion-pair dimers can occur in the PEO–NH4 SCN electrolytes with low salt content less than that in P(EO)80 NH4 SCN. This implies that NH4 SCN in PEO very easily forms the contact ion-pair dimers due to the hydrogen bonds between the contact ion pairs. 3.3. PEO–NH4 SCN electrolytes with high salt content If the NH4 SCN content is more than that in P(EO)10 NH4 SCN electrolyte, the ionic association changes dramatically when compared with those in PEO–NH4 SCN electrolytes with low salt content (as shown in Fig. 3). It is observed that the bands at 2104 and 2080 cm−1 disappear in the PEO–NH4 SCN electrolytes, at the same time, the relative intensity of band at 2055 cm−1 decreases rapidly with increasing salt content. However, the relative intensity of band at 2043 cm−1 increases with the increase of NH4 SCN content in PEO. Moreover, a new band at 2073 cm−1 is detected, and its relative intensity exhibits the same growing tends as the band at 2043 cm−1 . The band at 2073 cm−1 is not observed in PEO–NaSCN electrolytes, but can be detected in polyethylene glycol (PEG)–NaSCN electrolytes [19,20]. This suggests that the –OH groups of PEG, namely the hydrogen bonds between PEG and SCN−1 , are responsible for the occurrence of the band at 2073 cm−1 . As shown by curves (e) and (f) in Figs. 4 and 7, the band of NH stretching at 3187 cm−1 and the band of NH deformation at 1410 cm−1 shift respectively to 3159 and 1403 cm−1 , and synchronously become significantly broadened with increasing the relative intensities of bands at 2043 and 2073 cm−1 . These are typical spectral features of the formation of hydrogen bonds, suggesting the band at 2073 cm−1 has the same
Fig. 7. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 3500–2500 cm−1 . The compositions of the electrolytes were indicated on each curve.
origin as the band at 2043 cm−1 . That is, the band at 2073 cm−1 in PEO–NaSCN electrolytes results from the hydrogen bonds between PEO and NH4 SCN. In addition, the band at 750 cm−1 is assigned to the CS stretching, and the bands at 485 and 471 cm−1 are attributed to the SCN−1 bending [24]. These bands can be observed in the spectrum of pure NH4 SCN, but are barely detected when the band at 2055 cm−1 is the dominant one in PEO–NH4 SCN electrolytes with low salt content. However, these bands become significant for PEO–NH4 SCN electrolytes with high NH4 SCN content (as shown in Fig. 8), implying a microenvironment that is roughly similar to that in pure NH4 SCN. Therefore, it is reasonable to assign the band at 2073 cm−1 to contact ion-pair aggregates, which results from the hydrogen bonds between NH4 + and ether oxygen, and presumably the wrap of hydrogen bonds toward the contact ion pairs. Basically, this assignment is in agreement with that reported in the literature [2]. The PT-IR spectrum of PEO changes also significantly due to the transformation of ionic association, as shown by curves (d–f) in Figs. 4–6. Because of the broadened bands of PEO in PEO–NH4 SCN electrolytes, the bands, at 1467 and 1455, 1360 and 1343, 1242 and 1235, 963 and 948 cm−1 , join together to form the bands at 1453, 1351, 1250, 952 cm−1 , respectively. Synchronously, those at 1281 and 1114 cm−1 in PEO shift, respectively, to 1298 and 1099 cm−1 . All these experimental results are indicative of the characteristics of highly amorphous state in PEO–NH4 SCN electrolytes. It is known that ionic aggregates with large volume can play a role of good plasticizers. Therefore, the formation of hydrogen bonds, contact ion-pair dimers, and contact ion-pair aggregates is able to ruin completely the PEO crystalline phase. As a result, PEO takes the spectral features as its molten state when NH4 SCN content is enough great in PEO. Additionally, PEO–NH4 SCN electrolytes are very different from PEO–NaSCN electrolytes, in which the crystalline complex, P(EO)3 NaSCN, can be detected. However, PEO–NH4 SCN electrolytes
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aggregates. Moreover, the variable tends of these ionic associations with the NH4 SCN content in PEO are similar to those in PEO–NaSCN and PEO–KSCN electrolytes. However, in contrast to PEO–NaSCN and PEO–KSCN electrolytes, the remarkable particularity in PEO–NH4 SCN electrolytes is the formation of hydrogen bonds, which exert an important effect to ionic association. Because of the occurrence of hydrogen bonds, the contact ion-pair dimers can be observed in very low content of NH4 SCN in PEO, and the contact ion-pair aggregates can be detected when NH4 SCN content is more than that in P(EO)10 KSCN electrolyte. As a result, PEO–NH4 SCN electrolytes can appear as the highly amorphous state, and the crystalline complex cannot occur in the electrolytes. Acknowledgements The authors wish to acknowledge the financial supports from the National Natural Science Foundation of China (Grant No. 29973009), the Innovation Foundation of Henan Education Department and the Program for New Century Excellent Talents in University of Henan Province. References
Fig. 8. FT-IR spectra of P(EO)n NH4 SCN electrolytes in the region of 800–450 cm−1 . The compositions of the electrolytes were indicated on each curve.
are a little similar to PEO–KSCN electrolytes due to both spectral features of amorphous state under the condition of high salt content. It is possibly the reason that Na+ has least ion radius, but K+ and NH4 + have larger and almost equal ion radius. The large ion radius demands more ether oxygen atoms of PEO for wrapping around it, and can form a liquid-like environment in the electrolytes [19]. It should be noted that PEO–NH4 SCN electrolytes exhibit in greater extent the amorphous characteristics than PEO–KSCN electrolytes due to the hydrogen bonds between PEO and NH4 SCN. 4. Conclusions As shown in PEO–NaSCN and PEO–KSCN electrolytes, the ionic association in PEO–NH4 SCN electrolytes produces the contact ion-pairs, solvent-separated dimers, triple ion and higher ion
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