phthalate anions

phthalate anions

Available online at www.sciencedirect.com Solid State Ionics 179 (2008) 516 – 521 www.elsevier.com/locate/ssi Physicochemical properties of ionic li...

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Available online at www.sciencedirect.com

Solid State Ionics 179 (2008) 516 – 521 www.elsevier.com/locate/ssi

Physicochemical properties of ionic liquids based on imidazolium/ pyrrolidinium cations and maleate/phthalate anions Ye Song ⁎, Lin Liu, Xufei Zhu, Xinlong Wang, Hongbing Jia, Xuemei Xiao, Huadong Yu, Xiuli Yang School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu Province 210094, China Received 20 November 2007; received in revised form 20 March 2008; accepted 20 March 2008

Abstract Ionic liquids (ILs) based on imidazolium/pyrrolidinium cations and maleate/phthalate anions can be used as excellent electrolyte materials for electrolytic capacitors. In this study, we synthesized four ILs of this family and investigated their thermal behaviors, ionic conductivities and sparking voltages. The four ILs have high thermal stability for capacitor requirements. The conductivities of imidazolium ILs are slightly higher than those of pyrrolidinium analogs and the conductivities of maleate anion-based ILs are higher than those of corresponding phthalate anionbased ILs. Besides, the long-term thermal stability of imidazolium ILs in conductivity is superior to that of pyrrolidinium analogs. Whereas the long-term thermal stability of phthalate anion-based ILs is better than that of corresponding maleate anion-based ILs. The influence of cationic structure of the ILs on conductivity was analyzed. The temperature dependence of conductivity was also discussed in this work. The Vogel– Tammann–Fulcher (VTF) equation accurately describes the temperature dependence of conductivity for the ILs. In addition, the result of sparking voltage measurement shows that neither Ikonopisov nor Albella model is valid for the ILs. © 2008 Elsevier B.V. All rights reserved. Keywords: Ionic liquid; Conductivity; Thermal property; Sparking voltage

1. Introduction Ionic liquids (ILs), which are often referred to as room temperature or ambient temperature molten salts, have been investigated extensively because of both applied and fundamental interest in the last few decades. Especially, ILs can be used as safe and stable electrolyte in a variety of electrochemical devices, including electrochemical capacitors, high energy density batteries, solar cells and electrochromic windows [1– 4]. These ILs usually consist of inorganic or organic anions and nitrogen-containing organic cations such as alkylammonium, N, N'-dialkylimidazolium, N,N-dialkylpyrrolidinium and N-alkylpyridinium [5–8], etc. Whereas the most commonly used ionic liquid anions include halide ions, tetrachloroaluminate (AlCl4−), tetrafluoroborate (BF4−), hexafluorophosphate (PF6−), and bis ⁎ Corresponding author. Tel.: +86 25 84315949; fax: +86 25 84276082. E-mail address: [email protected] (Y. Song). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.03.035

(trifluoromethane sulfonyl)imide anion (CF3SO2)2N− (also known as bistriflyl imide (Tf2N−)) [1,6,9,10]. In recent years ILs have also been developed as novel electrolyte materials for aluminum electrolytic capacitor applications due to their unique properties like chemical and thermal stability, non-flammability, high ionic conductivity, and negligible vapor pressure [11,12]. It is well known that aluminum electrolytic capacitor is one of the most widely employed electronic components. Recent development trend of the capacitors is to achieve longer life, smaller size, broader operating temperature ranges, higher reliability, improved thermal stability and safety. The above-stated properties of ILs meet this trend exactly. Therefore, it is not surprising that the interest in ILs developed for aluminum electrolytic capacitors has increased dramatically. It has been reported in many patents that imidazolium or pyrrolidinium ILs dissolved in various organic solvents can be used in aluminum electrolytic capacitors due to their good solubility, high reliability and

Y. Song et al. / Solid State Ionics 179 (2008) 516–521

thermal stability [13,14]. In our earlier work, we also reported a new electrolyte system based on these ILs for chip type aluminum electrolytic capacitors [15]. It needs to be pointed out that the anionic components of the ILs employed for aluminum electrolytic capacitors are usually carboxylate anions such as maleate and phthalate ones. Because those “aggressive” anions such as halide ions will do damage to anodic oxide films acting as the dielectric of the capacitors [16], an electrolyte for aluminum electrolytic capacitors can only contain a number of anions of the weak acids. Although the ILs based on imidazolium or pyrrolidinium cations and carboxylate anions have been successfully used in the field of aluminum electrolytic capacitors, the study of their physicochemical properties is lagging behind. Moreover, until recently the information about the ILs whose anionic components are carboxylate is still quite insufficient [17,18]. Clearly, it is important to comprehend the structure–property relationship for assessing the suitability of ILs for specific applications, as well as the design of new ILs. Thus, in this study, we have synthesized four ILs based on imidazolium/pyrrolidinium cations and maleate/phthalate anions for the capacitor applications. The physicochemical properties of these ILs, including the thermal property, ionic conductivity and sparking voltage, were investigated systematically. 2. Experimental

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Table 1 H NMR data for the synthesized ILs

1

ILs

δ/ppm

DMPr+M−

2.08(4H, m, CH2CH2) 2.99(6H, s, CH3NCH3) 3.36(4H, m, CH2NCH2) 6.14(2H, s, CO–CH=CH) 1.99(4H, m, CH2CH2) 2.89(6H, s, CH3NCH3) 3.26(4H, m, CH2NCH2) 7.44(4H, m, φ-H) 1.33(3H, t, NCH2CH3) 3.75(3H, s, NCH3) 4.08(2H, q, NCH2) 7.57(1H, s, CHCHN3) 7.65(1H, s, CHCHN1) 8.58(1H, s, NCHN) 5.85(2H, s, CO–CH=CH) 1.31(3H, t, NCH2CH3) 3.73(3H, s, NCH3) 4.06(2H, q, NCH2) 7.55(1H, s, CHCHN3) 7.62(1H, s, CHCHN1) 8.56(1H, s, NCHN) 7.36(4H, m,φ-H)

DMPr+P− EMIm+M−

EMIm+P−

scanned on heating from −60 °C to +150 °C, followed by cooling scans from +150 °C to −60 °C at a scanning rate of 10 °C min− 1 under a flow of nitrogen. The melting point was determined from the onset of an endothermic peak on heating and the freezing point was from the onset of an exothermic peak on cooling. The glass transition temperature was determined from the midpoint of a small heat capacity change on heating from the amorphous glass state to a liquid state. Thermogravimetric (TG) measurements of the samples were conducted using open Al pans on a Shimadzu TGA-50 from room temperature to 450 °C at a heating rate of 20 °C min− 1 under a nitrogen atmosphere.

2.1. Synthesis 2.3. Conductivity measurements Four ILs based on imidazolium/pyrrolidinium cations and maleate/phthalate anions, i.e. N,N-dimethylpyrrolidinium hydrogen maleate (DMPr+M − ), N,N-dimethylpyrrolidinium hydrogen phthalate (DMPr+P−), 1-methyl-3-ethylimidazolium hydrogen maleate (EMIm+M−) and 1-methyl-3-ethylimidazolium hydrogen phthalate (EMIm+P−) were synthesized by a twostep process. First, the N,N-dimethylpyrrolidinium or 1-methyl3-ethylimidazolium methyl carbonate salts were prepared by alkylation of N-methylpyrrolidine or N-ethylimidazole with dimethyl carbonate [19]. Then the target imidazolium/pyrrolidinium salts were prepared by neutralization of imidazolium/ pyrrolidinium methyl carbonate with maleic/phthalic acid, where methanol and CO2 are only byproducts. The detailed synthetic procedure and chemical structures of the ILs have been described elsewhere [20]. The structure of the resultant ILs was confirmed by 1H NMR spectroscopy (Bruker DM 300). The 1H NMR data for the synthesized ILs are summarized in Table 1. All chemicals used in this work were analytical purity and used without further purification, which were purchased from Nanjing Chemical Reagent Company (China). The prepared ILs were dried under vacuum at 60 °C for more than two days before use. 2.2. Thermal analysis Differential scanning calorimetry (DSC) tests (Pyris I, PerkinElmer) were done in Al pans sealed in the dry box. The sample was quenched initially to −60 °C from ambient temperature, then

The conductivities of ILs were measured with a conductivity meter (CM-20S, TOA electronics Ltd.) by using a pair of Ptblack electrodes. Variable temperature data were obtained in a MINI-SUBZERO environmental chamber (MC-810, TABAI ESPEC Co.). To test the long-term stability in conductivities of the ILs, an appropriate amount of ILs was sealed in a glass tube and then was stored in an oven at 125 °C for 500 h. After the thermal treatment, the conductivity of the ILs was measured at 30 °C. 2.4. Sparking voltage measurement The method of sparking voltage measurement is the same as described in our previous papers [20,21]. Sparking voltage values of the ILs were obtained by the appearance of voltage oscillation in the voltage–time curves as criterion for identifying the sparking voltage. 3. Results and discussion 3.1. Thermal properties The thermal behaviors of the four ILs were characterized by DSC and TG. The melting and freezing points of these ILs were determined by DSC and the data are tabulated in Table 2. For the two imidazolium ILs, a melting transition or glass transition was observed on heating and no phase transition was detected

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Table 2 Thermal properties of the synthesized ILs

on cooling within the temperature range studied. In the case of the pyrrolidinium ILs, melting and freezing transitions were observed. Fig. 1(a) and (b) shows the DSC traces of the pyrrolidinium and imidazolium ILs, respectively. Interestingly, DMPr+M− exhibited two distinct exothermic transitions at 46.5 °C and 42.8 °C in the cooling scan, which implied a solid– solid phase transition. This was attributed to the existence of polymorph of the crystalline state in this salt. Both DMPr+P− and DMPr+M− show substantial supercooling as the freezing points of the samples are significantly lower than the melting

points (Table 2). It is worthwhile to note that the cation of the pyrrolidinium ILs having melting and freezing points is symmetric, which may be favorable in crystal formation. Whereas the cation of the imidazolium ILs exhibiting no freezing transitions is unsymmetric. Fig. 2 shows TG traces of the four ILs. It can be seen that the imidazolium and pyrrolidinium ILs exhibit different thermal decomposition patterns. The imidazolium ILs start to lose weight at ca. 150 °C, while the pyrrolidinium analogs at ca. 200 °C. And their temperature regions of thermal decomposition are different. The thermal decompositions of the imidazolium ILs proceed over wider temperature regions than those of the pyrrolidinium analogs. Nevertheless, for these two types of ILs, the decomposition temperatures corresponding to the maximum rate of weight loss are close to each other. These ILs are all stable up to 180 °C and can meet the capacitor demand as a whole. In addition, the phthalate anion-based ILs have greater thermal stability than the corresponding maleate anion-based ILs (Fig. 2). It is generally accepted that the decomposition temperatures depend primarily on the nucleophilicity of the anion [6,22]. Those ILs containing weakly nucleophilic anions have greater thermal stability. Our findings are also in accordance with this trend. Interestingly, char yields of maleate

Fig. 1. DSC traces of the ILs.

Fig. 2. TGA traces for the ILs investigated. (a) Pyrrolidinium. (b) Imidazolium.

Tm a/°C

ILs + −

EMIm P EMIm+M− DMPr+P− DMPr+M−

22.6 ND 87.7 73.7

Tfr b/°C e

ND ND 41.5 46.5

Tg c/°C

ΔS d/J K− 1 mol− 1

ND − 47.1 ND ND

– – 43.6 56.1

a

Melting point. Freezing point. c Glass transition temperature. d Entropy of melting (ΔSm = ΔHm / T, where ΔHm is the enthalpy of melting at temperature T (K) and measured by DSC). e Not detected. b

Y. Song et al. / Solid State Ionics 179 (2008) 516–521 Table 3 Conductivity data of the four ILs at 30 °C Conductivity σ/mS cm− 1 σ V/mS cm− 1 a (σ - σ V) σ/% a

ILs EMIm+P−

EMIm+M−

DMPr+P−

DMPr+M−

0.87 0.86 1.15

3.75 3.52 6.13

0.61 0.59 3.28

3.53 3.24 8.22

Conductivities measured after the thermal treatment (125 °C, 500 h).

anion-based ILs are apparently larger than those of phthalate anion-based analogs (Fig. 2). 3.2. Ionic conductivity Ionic conductivity is one of the most important properties of ILs as electrolyte materials. The conductivities of the four ILs at 30 °C are reported in Table 3. It is found that the conductivity of maleate anion-based ILs is higher than that of corresponding phthalate anion-based ILs (EMIm+M− N EMIm+P−, DMPr+M− N DMPr+P−), which is mainly due to a difference in the anion size (M− b P−) [23]. An anion with a large size may lead to lower ion mobility and result in lower conductivity [1]. It can be also found from Table 3 that the conductivities of imidazolium ILs are higher than those of corresponding pyrrolidinium ILs (EMIm+M− N DMPr+M−, EMIm+P− N DMPr+P−). MacFarlane et al. [11,24,25] also reported the similar trend in conductivity of various ILs in a series of papers. They emphasized the importance of cation configuration in promoting conductivity. The planarity of the imidazolium ring is responsible for the high conductivity of imidazolium ILs. Moreover, the presence of the substituents above and below the plane of the pyrrolidinium ring clearly restricts the mobility of this ring compared to its more planar imidazolium relative. In fact, in addition to the difference of the mobilities of the cations, the delocalization of the charge in the imidazolium cation can result in an improvement in the degree of dissociation of ions compared with pyrrolidinium cation with localized charge. It should be pointed out that the opposite tendency of conductivity was observed for these ionic liquid

Fig. 3. Arrhenius plots of ionic conductivity (ln σ vs. 1 / T) for the ILs.

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solutions in an organic solvent in our recently reported work [20]. This suggests the importance of solvation in promoting the conductivity of electrolyte solutions. To assess the long-term thermal stability of the ILs, an endurance test was carried out. Table 3 also shows the change in conductivities of the four ILs after being stored at 125 °C for 500 h. Obviously, the imidazolium ILs are more stable in conductivity than the corresponding pyrrolidinium ILs after the thermal treatment, which is mainly ascribed to the aromaticity of the imidazolium cation. Generally speaking, the aromaticity may lead to good stability [26]. On the other hand, the phthalate anion-based ILs have better conductivity and durability than the corresponding maleate anion-based ILs. This result is in agreement with that of TG mentioned above. Arrhenius plots of the ionic conductivity for the ILs are shown in Fig. 3. For the sake of clarity, Fig. 3 only gives the temperature dependence of the conductivity for DMPr+M− and EMIm+P−. The conductivity for the ILs exhibits non-Arrhenius behavior, i.e. a distinct downward curvature at lower temperatures in the Arrhenius plot (Fig. 3). This is not surprising since this non-Arrhenius behavior is a characteristic of glass forming melts and the nonlinear variation with temperature of electrical conductivity for ILs is well-documented [27,28]. Therefore, the three-parameter Vogel–Tammann–Fulcher (VTF) equation was used to represent the temperature dependence of conductivity for the ILs:   A B r ¼ pffiffiffiffi exp  T  T0 T

ð1Þ

where A, B, T0 are constants, T is absolute temperature. T0 is referred to as the “ideal glass transition temperature” and is interpreted as the limiting value of Tg as the cooling rate becomes infinitely slow [29]. The conductivity data in Fig. 3 were refitted to the logarithmic form of the VTF equation. This was established by the linearity of ln σT1/2 vs. (T − T0)− 1plots (Fig. 4). The expected very straight lines show the goodness of the fit. The best-fit parameters of VTF equation, along with the

Fig. 4. VTF plots of ionic conductivity (ln σT1/2 vs. 1 / T − T0) for the ILs.

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Table 4 VTF equation fitting parameters for conductivity data of ILs ILs

ln A/Ms cm− 1 K1/2

B/K

T0/K

rms deviation in ln σ VTF

Arrhenius

DMPr+M− DMPr+P− EMIm+M− EMIm+P−

10.90 ± 0.033 11.08 ± 0.048 11.86 ± 0.023 10.28 ± 0.017

1005.4 ± 54.9 1087.1 ± 65.7 1114.1 ± 82.9 781.9 ± 22.2

155.8 ± 3.2 175.6 ± 3.2 160.6 ± 3.5 200.3 ± 1.1

0.004 0.006 0.035 0.005

0.16 0.20 0.66 0.68

rms deviation for Arrhenius and VTF equations, are tabulated in Table 4. It can be concluded that the VTF interpretation is more valid than the Arrhenius type in describing the temperature dependence of conductivity for the ILs. 3.3. Sparking voltage It is well known that the dielectric breakdown of anodic oxide film will occur with visible sparking over the film surface when the formation voltage reaches a certain value during the anodizing of aluminum at constant-current density in an electrolyte solution. Sparking voltage is one of the most important properties of any electrolyte used in electrolytic

capacitors as it determines the working voltage of the capacitors. Previous studies of sparking voltage were focused on electrolyte solutions [20,30]. To the best of our knowledge, the anodization behavior of aluminum in neat ILs has never been reported. Because ILs are liquid at room temperature, their sparking voltage can be tested like those of common electrolyte solutions. Fig. 5 shows the anodic oxidation behavior of aluminum in the four ILs at room temperature during constantcurrent anodization. The sparking voltage values of the four ILs are also labeled in Fig. 5. Previous studies have indicated that sparking voltage of electrolyte solutions followed the Ikonopisov [31] or Albella [32] model. By assuming a mechanism of avalanche multiplication of electrons injected into the aluminum oxide by the Schottky mechanism, Ikonopisov has correctly predicted the dependence of sparking voltage Us on electrolyte resistivity ρ: Us ¼ a1 þ b1 log q

ð2Þ

where a1 and b1 are empirical constants. On the other hand, Albella et al. reported evidence that electrolyte species incorporated into oxides act as a source of avalanching electrons. This assumption has yielded the well-known

Fig. 5. Voltage–time behavior during anodizing of aluminum in the four ILs.

Y. Song et al. / Solid State Ionics 179 (2008) 516–521

logarithmic dependence of sparking voltage on electrolyte concentration C: Us ¼ a2  b2 log C

ð3Þ

where a2 and b2 are empirical constants. Compared with our recently reported data of the ionic liquid solutions in gammabutyrolactone (GBL) [20], the four ILs have an unusual characteristic of sparking voltage. The resistivities of the four neat ILs are greater than those of the corresponding ionic liquid solutions in GBL, however, the sparking voltages of DMPr+M− and DMPr+P− are lower than those of their solutions in GBL. This result is not in agreement with the Ikonopisov model (Eq. (2)). In addition, the neat ILs should be 100% concentration relative to their solutions. But the sparking voltages of EMIm+M− and EMIm+P− are greater than those of their solutions in GBL. This trend is also incompatible with the Albella model (Eq. (3)). The reasons behind these findings are not well understood at present. It is probably related to the strong Coulombic interactions in ILs. The information obtained from this study of sparking voltage for ILs seems to suggest that the theories of breakdown for anodic oxide film are incomplete. Fully understanding the phenomenon of sparking breakdown for anodic oxide film will require further study. 4. Conclusions We have synthesized four ILs based on imidazolium/ pyrrolidinium cations and maleate/phthalate anions. The ILs are attractive for potential applications in developing long-life electrolytic capacitors. The four ILs have high thermal stability. The conductivities of imidazolium ILs are slightly higher than those of corresponding pyrrolidinium ILs and the conductivities of maleate anion-based ILs are higher than those of corresponding phthalate anion-based ILs. The differences of conductivity of the ILs are attributed to different structures of pyrrolidinium and imidazolium cations as well as the size of anions. The planarity of the imidazolium ring is responsible for higher conductivity of imidazolium ILs. On the other hand, the long-term thermal stability of imidazolium ILs in conductivity is superior to that of pyrrolidinium ILs. Whereas the long-term thermal stability of phthalate anion-based ILs in conductivity is superior to that of maleate anion-based ILs irrespective of pyrrolidinium or imidazolium cation. The temperature dependences of conductivity for the ILs are well described by the VTF equation. In addition, the sparking voltage of the ILs obeys neither the Ikonopisov nor the Albella model. Our findings imply that the theories of breakdown for anodic oxide film are incomplete.

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