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Comparing the differential capacitance of two ionic liquid electrolytes: Effects of specific adsorption
Electrochemistry Communications 38 (2014) 44–46
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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Comparing the differential capacitance of two ionic liquid electrolytes: Effects of specific adsorption Qian Zhang, Yining Han, Yonglong Wang ⁎, Shihai Ye, Tianying Yan Tianjin Key Laboratory of Metal- and Molecule-Based Material Chemistry, Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Institute of New Energy Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 4 September 2013 Received in revised form 15 October 2013 Accepted 28 October 2013 Available online 8 November 2013 Keywords: Ionic liquid Differential capacitance Electric double layer Specific adsorption Potential of zero charge
a b s t r a c t The differential capacitance/potential curves of two ionic liquid (IL) electrolytes, 1-butyl-3-methyl-imidazolium + − hexafluorophosphate (BMIM+/PF− 6 ) and N-butyl-N-methyl-pyrrolidinium hexafluorophosphate (Pyr14/PF6 ) on a glassy carbon (GC) electrode were measured experimentally. The differential capacitance of BMIM+/PF− 6 /GC is − higher in the negative polarization, while the differential capacitance of Pyr+ 14/PF6 /GC is higher in the positive polarization, although both ILs are composed of common anions, with cations of similar ionic structures and diameters. Such an opposite trend may be understood in terms of the specific adsorption between BMIM+ and the GC electrode, caused by the π-stacking interaction between the aromatic imidazolium ring and the sp2 graphite surface. The specific adsorption effectively shortens the electric double layer (EDL) thickness on the negatively charged electrode but elongates the EDL thickness on the positively charged electrode. Such an effect is manifested in the differential capacitance, with a higher value on the negative polarization branch than on the positive polarization branch. The impact of the specific adsorption is also seen from the positive shift of the + − potential of zero charge of BMIM+/PF− 6 /GC in comparison with that of Pyr14/PF6 /GC. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Because of their excellent properties, including being nonflammable, environment friendly, having wide electrochemical windows, good thermostability, and relatively low melting temperatures (below 100 °C), compared to high temperature molten salts, room-temperature ionic liquids (ILs) have attracted considerable attention in recent years [1]. One of the practical applications of ILs is as the electrolyte in electrochemical capacitors, or supercapacitors [2]. Toward the goal of functionalized ILs as designer solvents, it is crucial to understand EDL structure for this new type of electrolyte on the electrode interface. Experimentally, the EDL structure is often detected by the differential capacitance [3]. Since ILs are composed of asymmetric bare ions, the differential capacitance is also asymmetric, with higher value on the electrode containing smaller counter ions. This has been clearly demonstrated by the mean-field theory (MFT) [4,5], the density functional theory (DFT) [6,7], molecular dynamics (MD) simulations [8–10], and experimentally [11]. This can be qualitatively understood in the frame work of the Helmholtz model, i.e., C = εε0/d, in which ε is the dielectric constant of the electrolyte, ε0 is the permittivity of the vacuum, and d may be understood as the potential dependent effective thickness of the EDL [9,12]. In the simplest picture, d may be qualitatively understood as the radius of the counter ion, with the smaller ion giving the ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.10.027
higher differential capacitance. On the other hand, experiments [13–15] and simulations [16,17] have shown that the differential capacitance is more complicated in many cases. A well-known issue is the effect of the specific adsorption [6,13,14,16–19], which makes the differential capacitance beyond the theoretical prediction and modifies the position of the potential of zero charge (PZC) [8,14,18,20]. In our previous MD simulation of 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF− 6 ) on a graphite electrode, we found that the asymmetric differential capacitance may be attributed to the specific adsorption of cation on the graphite surface [16]. Considering the similar ionic diameters, i.e., 5.70 Å for BMIM+ and 4.76 Å for PF− 6 [21], it is somehow surprising to observe that the differential capacitance is much higher in the negative polarization branch than in the positive polarization branch, as found in the results from both experiment [14] and simulation [16], as well as the integral capacitance from MD simulation [22]. The above differential capacitance was conjectured to be caused by the π-stacking interaction between the aromatic imidazolium ring and the sp2 graphite surface [16,23–27]. If this is indeed the case, a cation with a similar structure as BMIM+, but without the π-stacking interaction with the graphite surface, may give completely different differential capacitance. Based on the above consideration, we choose N-butyl-Nmethyl-pyrrolidinium (Pyr+ 14), which has a diameter of 5.81 Å [21] and a similar structure to BMIM+, yet lacks an aromatic ring, making the π-stacking interaction with graphite unavailable. A comparison of differential capacitances of two ILs of different cations with a common
Q. Zhang et al. / Electrochemistry Communications 38 (2014) 44–46 + − anion, i.e., BMIM+/PF− 6 and Pyr14/PF6 on the GC electrode, is performed to study the effects of the specific adsorption. Discussions on the EDL structure as well as PZC are also made.
2. Materials and methods All ILs (99% with water less than 0.04%) were purchased from Lanzhou Institute of chemical physics and were pretreated to remove trace water under high vacuum at 50 °C for 72 h. All electrochemical measurements were conducted in a sealed three-compartment glass cell under argon atmosphere at 100 °C, with the GC electrode (Φ 1 mm, spectroscopic purity, Gaoss Union) as the working electrode, a graphite rod (Φ 3 mm, analytically purity, Gaoss Union) as the counter electrode, and a silver wire (99.999%, Gaoss Union) as the reference electrode. The GC electrode was mechanically polished by alumina powder (0.03–0.05 μm) on nylon pads and then ultrasonically treated in deionized water and ethanol, repeated three times. Cyclic voltammetry (CV) was measured using the ZahnerIM6-ex electrochemical workstation. Differential capacitance was also measured at the ZahnerIM6-ex with a constant frequency f = 1000 Hz and an AC signal of 5 mV and was calculated by Cd = −(2πƒZ″)−1 [13,14,19,20,28,29], in which Z″ is the imaginary part of impedance. The electrode potential was scanned from the open-circuit potential (OCP) to a negative potential of − 1.5 V and then scanned to the positive potential of 2.0 V, after waiting at the open-circuit potential for 1 h [13]. The delay time between the potential steps (10 mV) was 30 s. The geometric area (0.00785 cm2) of the working electrode was used as the real surface area to calculate the differential capacitance value [13,14]. It is notable that recent electrochemical impedance spectroscopy (EIS) studies demonstrate that there are fast and slow capacitive processes at IL/ electrode interfaces, with a temperature independent activation energy for the slow process [30–32]. In our case, the high temperature of 100 °C guarantees the single frequency measurement of the differential capacitance due to the overlap of the fast and slow process at such high temperature [30]. 3. Results and discussion Fig. 1 illustrates the differential capacitance and the CV of the two ILs, + − BMIM+/PF− 6 and Pyr14/PF6 , on the GC electrode at 100 °C. It is shown
Fig. 1. Differential capacitance and CV (inset) on the glassy carbon electrode of BMIM+/PF− 6 − (black dashed line) and Pyr+ 14/PF6 (red dashed line) at 100 °C. PZC is assigned according to the minimum of the camel-shaped differential capacitance. The ionic structures of BMIM+ and Pyr+ 14 are depicted on the individual differential capacitance.
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from the CV that the electrochemical windows are 4.5 V for BMIM+/ + − PF− 6 and 5.0 V for Pyr14/PF6 , which is in agreement with a previous study [33]. As no obvious faradic current can be observed within the electrochemical windows, the electrode is considered as ideally polarizable, which means that any current is attributed to the EDL charging. It can also be clearly seen that both ILs display camel-shaped differential capacitance due to the finite ionic sizes [4]. The butyl side chain behaves as “latent void” and thus accounts for the high compressibility [9]. The overall trend of the differential capacitance of BMIM+/PF− 6 is in agreement with the experimental result from the same IL on the GC electrode at 23 °C [14]. The apparent higher differential capacitance at the higher temperature in this study may be attributed to the thinning of the EDL, weakening the ion association with increasing temperature [13,14,34] or the acceleration of the EDL structure reorganization at higher temperature [30–32]. A distinct feature in Fig. 1 is that the trend of the asymmetric differential capacitance for the two ILs is completely reversed. It is notable that the differential capacitance of BMIM+/PF− 6 /GC is higher in the negative polarization than in the positive polarization, and for Pyr+ 14/ PF− 6 /GC, the trend is reversed. Specifically, the differential capacitance peak of 34.4 μF/cm2 at − 1.13 V vs. Ag wire for BMIM+/PF− 6 /GC is − 2 much higher than that of Pyr+ 14/PF6 /GC (13.9 μF/cm at − 1.14 V vs. Ag wire) on the negative polarization. On the other hand, the differential capacitance peak is similar for both ILs, i.e., 27.0 μF/cm2 at 1.35 V 2 vs. Ag wire for BMIM+/PF− 6 /GC and 27.6 μF/cm at 1.05 V vs. Ag wire + − for Pyr14/PF6 /GC. At a potential far from the PZC, the lattice saturation effect makes the ionic layers thicker to compensate for the excess charge on the surface of the electrode, and the differential capacitance is diminished in this potential range [4,9]. Because of the common anion, PF− 6 , for both ILs, it is reasonable to observe similar peaks in the differential capacitance on the positive polarization. However, considering the similar cationic diameter, i.e., 5.70 Å for BMIM+ and 5.81 Å for Pyr+ 14 [21], the large difference in the differential capacitance in the negative polarization indicates that the cation cannot be treated as a spherical ion. Thus, it is of interest to closely inspect the structure of the two cations. As depicted in Fig. 1, both cations are composed of a pentagon ring with butyl and methyl side chains of similar ionic structures. For Pyr+ 14, the positive charge is localized on the sp3 hybridized nitrogen atom in the center of the tetrahedron. On the other hand, the positive charge is delocalized on the flat aromatic ring on the π-bond for BMIM+. For BMIM+, there is specific adsorption between the aromatic imidazolium ring and the sp2 graphite surface, attributed to the π-stacking interaction [16,23–26]. The effect of specific adsorption on the differential capacitance is significant on the negative polarization, the PZC and the positive polarization, as discussed below: (1) As depicted in Fig. 2a, the π-stacking interaction between BMIM+ and the GC electrode causes parallel alignment between the imidazolium ring and the graphite surface and brings them in close contact [16,23], so that the effective EDL thickness is reduced. On the other hand, the lack of such an interaction between Pyr+ 14 and the graphite surface, altogether with the fact that the positive charge center on the sp3 hybridized nitrogen atom is surrounded by the bulky alkyl groups, causes longer effective EDL thickness, as depicted in Fig. 2d. The net result is that the differential capacitance of BMIM+ is much higher than that of Pyr+ 14 over the whole negative polarization, starting from PZC, as shown in Fig. 1. Thus, the higher differential capacitance for BMIM+/PF− 6 /GC may be attributed to the specific adsorption of BMIM+ on the GC electrode, due to the π-stacking interaction between the imidazolium ring and the graphite surface. (2) The above-mentioned shortening of the effective EDL length of BMIM+/PF− 6 /GC also enhances the differential capacitance at the PZC − comparing to Pyr+ 14/PF6 /GC, as shown in Fig. 1. Apart from that, the specific adsorption of BMIM+ on the GC electrode is also reflected in the positive shift of the PZC [8], located at the minimum of the camel− shaped differential capacitance, i.e., from −0.44 V for Pyr+ 14/PF6 toward
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Q. Zhang et al. / Electrochemistry Communications 38 (2014) 44–46
surface, has been discussed in previous studies [16,23–26]. In the current study, the effect of the specific adsorption on the EDL and the PZC was studied by comparing the experimental differential capacitance of two IL electrolytes of a common anion, BMIM+/PF− 6 and − Pyr+ 14/PF6 , with the GC electrode. Although both ILs are composed of cations with a similar structure and ionic diameter, the lack of an aromatic ring in Pyr+ 14 makes it lack specific adsorption via π-stacking interactions with the GC electrode. Indeed, the specific adsorption of BMIM+ effectively shortens the EDL thickness on the negatively charged electrode but elongates the EDL thickness on the positively charged electrode. Such an effect is manifested in the differential capacitance with a higher value on the negative polarization branch than that on the positive polarization branch. The impact of the specific adsorption is also seen from the positive shift of the PZC of BMIM+/PF− 6 /GC − in comparison with that of Pyr+ 14/PF6 /GC.
Acknowledgement This work is supported by NSFC (21373118, 21073097, and 51102137), 973 (2009CB220100), Natural Science Foundation of Tianjin (12JCYBJC13900), and NCET-10-0512. We are grateful to Dr. Jacqui Cloud for polishing the English of the manuscript.
References [1] [2] [3] [4] [5] [6] + − Fig. 2. Schematic diagrams of the EDL structure of BMIM+/PF− 6 (a, b, and c) and Pyr14/PF6 (d, e, and f) on the negatively charged, PZC, and positively charged electrode.
−0.03 V for BMIM+/PF− 6 vs. Ag wire, as shown in Fig. 1. The above effect is depicted in Fig. 2b and e. (3) On the positive polarization branch, the differential capacitance + − of BMIM+/PF− 6 /GC is quickly suppressed by that of Pyr14/PF6 /GC. As shown in Fig. 1, the trend switches at 0.32 V vs. Ag wire. This can be attributed to the specific adsorption of BMIM+ on the electrode, which increases the surface charge on the positively charged electrode and thus depresses the differential capacitance on the positive polarization. The strong adsorption of BMIM+ makes it remain in contact with the electrode surface at low positive potentials, as depicted in Fig. 2c. In addition, the specific adsorption of BMIM+ prevents a closer approach of PF− 6 to the electrode surface and further results in a thicker EDL with a lower differential capacitance on the positive polarization. Such a finding is also supported by previous theoretical simulations of BMIM+ on the graphite electrode [16,22]. On the other hand, the lack of π-stacking interactions between Pyr+ 14 and the graphite surface makes it displaced easily from the electrode surface and thus replaced by PF− 6 at low positive potentials, as depicted in Fig. 2f. Thus, the effect of the specific adsorption of BMIM+ on the graphite surface causes the positive shift of the differential capacitance peak, i.e., 1.35 V for + − BMIM+/PF− 6 /GC and 1.05 V vs. Ag wire for Pyr14/PF6 /GC, although the peak values of the differential capacitance are similar.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
4. Conclusion The specific adsorption of BMIM+ on the GC electrode, caused by the π-stacking interaction between the imidazolium ring and the graphite
[31] [32] [33] [34]
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Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Comparing the differential capacitance of two ionic liquid electrolytes: Effects of specific adsorption Qian Zhang, Yining Han, Yonglong Wang ⁎, Shihai Ye, Tianying Yan Tianjin Key Laboratory of Metal- and Molecule-Based Material Chemistry, Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Institute of New Energy Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e
i n f o
Article history: Received 4 September 2013 Received in revised form 15 October 2013 Accepted 28 October 2013 Available online 8 November 2013 Keywords: Ionic liquid Differential capacitance Electric double layer Specific adsorption Potential of zero charge
a b s t r a c t The differential capacitance/potential curves of two ionic liquid (IL) electrolytes, 1-butyl-3-methyl-imidazolium + − hexafluorophosphate (BMIM+/PF− 6 ) and N-butyl-N-methyl-pyrrolidinium hexafluorophosphate (Pyr14/PF6 ) on a glassy carbon (GC) electrode were measured experimentally. The differential capacitance of BMIM+/PF− 6 /GC is − higher in the negative polarization, while the differential capacitance of Pyr+ 14/PF6 /GC is higher in the positive polarization, although both ILs are composed of common anions, with cations of similar ionic structures and diameters. Such an opposite trend may be understood in terms of the specific adsorption between BMIM+ and the GC electrode, caused by the π-stacking interaction between the aromatic imidazolium ring and the sp2 graphite surface. The specific adsorption effectively shortens the electric double layer (EDL) thickness on the negatively charged electrode but elongates the EDL thickness on the positively charged electrode. Such an effect is manifested in the differential capacitance, with a higher value on the negative polarization branch than on the positive polarization branch. The impact of the specific adsorption is also seen from the positive shift of the + − potential of zero charge of BMIM+/PF− 6 /GC in comparison with that of Pyr14/PF6 /GC. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Because of their excellent properties, including being nonflammable, environment friendly, having wide electrochemical windows, good thermostability, and relatively low melting temperatures (below 100 °C), compared to high temperature molten salts, room-temperature ionic liquids (ILs) have attracted considerable attention in recent years [1]. One of the practical applications of ILs is as the electrolyte in electrochemical capacitors, or supercapacitors [2]. Toward the goal of functionalized ILs as designer solvents, it is crucial to understand EDL structure for this new type of electrolyte on the electrode interface. Experimentally, the EDL structure is often detected by the differential capacitance [3]. Since ILs are composed of asymmetric bare ions, the differential capacitance is also asymmetric, with higher value on the electrode containing smaller counter ions. This has been clearly demonstrated by the mean-field theory (MFT) [4,5], the density functional theory (DFT) [6,7], molecular dynamics (MD) simulations [8–10], and experimentally [11]. This can be qualitatively understood in the frame work of the Helmholtz model, i.e., C = εε0/d, in which ε is the dielectric constant of the electrolyte, ε0 is the permittivity of the vacuum, and d may be understood as the potential dependent effective thickness of the EDL [9,12]. In the simplest picture, d may be qualitatively understood as the radius of the counter ion, with the smaller ion giving the ⁎ Corresponding author. E-mail address: [email protected] (Y. Wang). 1388-2481/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2013.10.027
higher differential capacitance. On the other hand, experiments [13–15] and simulations [16,17] have shown that the differential capacitance is more complicated in many cases. A well-known issue is the effect of the specific adsorption [6,13,14,16–19], which makes the differential capacitance beyond the theoretical prediction and modifies the position of the potential of zero charge (PZC) [8,14,18,20]. In our previous MD simulation of 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIM+/PF− 6 ) on a graphite electrode, we found that the asymmetric differential capacitance may be attributed to the specific adsorption of cation on the graphite surface [16]. Considering the similar ionic diameters, i.e., 5.70 Å for BMIM+ and 4.76 Å for PF− 6 [21], it is somehow surprising to observe that the differential capacitance is much higher in the negative polarization branch than in the positive polarization branch, as found in the results from both experiment [14] and simulation [16], as well as the integral capacitance from MD simulation [22]. The above differential capacitance was conjectured to be caused by the π-stacking interaction between the aromatic imidazolium ring and the sp2 graphite surface [16,23–27]. If this is indeed the case, a cation with a similar structure as BMIM+, but without the π-stacking interaction with the graphite surface, may give completely different differential capacitance. Based on the above consideration, we choose N-butyl-Nmethyl-pyrrolidinium (Pyr+ 14), which has a diameter of 5.81 Å [21] and a similar structure to BMIM+, yet lacks an aromatic ring, making the π-stacking interaction with graphite unavailable. A comparison of differential capacitances of two ILs of different cations with a common
Q. Zhang et al. / Electrochemistry Communications 38 (2014) 44–46 + − anion, i.e., BMIM+/PF− 6 and Pyr14/PF6 on the GC electrode, is performed to study the effects of the specific adsorption. Discussions on the EDL structure as well as PZC are also made.
2. Materials and methods All ILs (99% with water less than 0.04%) were purchased from Lanzhou Institute of chemical physics and were pretreated to remove trace water under high vacuum at 50 °C for 72 h. All electrochemical measurements were conducted in a sealed three-compartment glass cell under argon atmosphere at 100 °C, with the GC electrode (Φ 1 mm, spectroscopic purity, Gaoss Union) as the working electrode, a graphite rod (Φ 3 mm, analytically purity, Gaoss Union) as the counter electrode, and a silver wire (99.999%, Gaoss Union) as the reference electrode. The GC electrode was mechanically polished by alumina powder (0.03–0.05 μm) on nylon pads and then ultrasonically treated in deionized water and ethanol, repeated three times. Cyclic voltammetry (CV) was measured using the ZahnerIM6-ex electrochemical workstation. Differential capacitance was also measured at the ZahnerIM6-ex with a constant frequency f = 1000 Hz and an AC signal of 5 mV and was calculated by Cd = −(2πƒZ″)−1 [13,14,19,20,28,29], in which Z″ is the imaginary part of impedance. The electrode potential was scanned from the open-circuit potential (OCP) to a negative potential of − 1.5 V and then scanned to the positive potential of 2.0 V, after waiting at the open-circuit potential for 1 h [13]. The delay time between the potential steps (10 mV) was 30 s. The geometric area (0.00785 cm2) of the working electrode was used as the real surface area to calculate the differential capacitance value [13,14]. It is notable that recent electrochemical impedance spectroscopy (EIS) studies demonstrate that there are fast and slow capacitive processes at IL/ electrode interfaces, with a temperature independent activation energy for the slow process [30–32]. In our case, the high temperature of 100 °C guarantees the single frequency measurement of the differential capacitance due to the overlap of the fast and slow process at such high temperature [30]. 3. Results and discussion Fig. 1 illustrates the differential capacitance and the CV of the two ILs, + − BMIM+/PF− 6 and Pyr14/PF6 , on the GC electrode at 100 °C. It is shown
Fig. 1. Differential capacitance and CV (inset) on the glassy carbon electrode of BMIM+/PF− 6 − (black dashed line) and Pyr+ 14/PF6 (red dashed line) at 100 °C. PZC is assigned according to the minimum of the camel-shaped differential capacitance. The ionic structures of BMIM+ and Pyr+ 14 are depicted on the individual differential capacitance.
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from the CV that the electrochemical windows are 4.5 V for BMIM+/ + − PF− 6 and 5.0 V for Pyr14/PF6 , which is in agreement with a previous study [33]. As no obvious faradic current can be observed within the electrochemical windows, the electrode is considered as ideally polarizable, which means that any current is attributed to the EDL charging. It can also be clearly seen that both ILs display camel-shaped differential capacitance due to the finite ionic sizes [4]. The butyl side chain behaves as “latent void” and thus accounts for the high compressibility [9]. The overall trend of the differential capacitance of BMIM+/PF− 6 is in agreement with the experimental result from the same IL on the GC electrode at 23 °C [14]. The apparent higher differential capacitance at the higher temperature in this study may be attributed to the thinning of the EDL, weakening the ion association with increasing temperature [13,14,34] or the acceleration of the EDL structure reorganization at higher temperature [30–32]. A distinct feature in Fig. 1 is that the trend of the asymmetric differential capacitance for the two ILs is completely reversed. It is notable that the differential capacitance of BMIM+/PF− 6 /GC is higher in the negative polarization than in the positive polarization, and for Pyr+ 14/ PF− 6 /GC, the trend is reversed. Specifically, the differential capacitance peak of 34.4 μF/cm2 at − 1.13 V vs. Ag wire for BMIM+/PF− 6 /GC is − 2 much higher than that of Pyr+ 14/PF6 /GC (13.9 μF/cm at − 1.14 V vs. Ag wire) on the negative polarization. On the other hand, the differential capacitance peak is similar for both ILs, i.e., 27.0 μF/cm2 at 1.35 V 2 vs. Ag wire for BMIM+/PF− 6 /GC and 27.6 μF/cm at 1.05 V vs. Ag wire + − for Pyr14/PF6 /GC. At a potential far from the PZC, the lattice saturation effect makes the ionic layers thicker to compensate for the excess charge on the surface of the electrode, and the differential capacitance is diminished in this potential range [4,9]. Because of the common anion, PF− 6 , for both ILs, it is reasonable to observe similar peaks in the differential capacitance on the positive polarization. However, considering the similar cationic diameter, i.e., 5.70 Å for BMIM+ and 5.81 Å for Pyr+ 14 [21], the large difference in the differential capacitance in the negative polarization indicates that the cation cannot be treated as a spherical ion. Thus, it is of interest to closely inspect the structure of the two cations. As depicted in Fig. 1, both cations are composed of a pentagon ring with butyl and methyl side chains of similar ionic structures. For Pyr+ 14, the positive charge is localized on the sp3 hybridized nitrogen atom in the center of the tetrahedron. On the other hand, the positive charge is delocalized on the flat aromatic ring on the π-bond for BMIM+. For BMIM+, there is specific adsorption between the aromatic imidazolium ring and the sp2 graphite surface, attributed to the π-stacking interaction [16,23–26]. The effect of specific adsorption on the differential capacitance is significant on the negative polarization, the PZC and the positive polarization, as discussed below: (1) As depicted in Fig. 2a, the π-stacking interaction between BMIM+ and the GC electrode causes parallel alignment between the imidazolium ring and the graphite surface and brings them in close contact [16,23], so that the effective EDL thickness is reduced. On the other hand, the lack of such an interaction between Pyr+ 14 and the graphite surface, altogether with the fact that the positive charge center on the sp3 hybridized nitrogen atom is surrounded by the bulky alkyl groups, causes longer effective EDL thickness, as depicted in Fig. 2d. The net result is that the differential capacitance of BMIM+ is much higher than that of Pyr+ 14 over the whole negative polarization, starting from PZC, as shown in Fig. 1. Thus, the higher differential capacitance for BMIM+/PF− 6 /GC may be attributed to the specific adsorption of BMIM+ on the GC electrode, due to the π-stacking interaction between the imidazolium ring and the graphite surface. (2) The above-mentioned shortening of the effective EDL length of BMIM+/PF− 6 /GC also enhances the differential capacitance at the PZC − comparing to Pyr+ 14/PF6 /GC, as shown in Fig. 1. Apart from that, the specific adsorption of BMIM+ on the GC electrode is also reflected in the positive shift of the PZC [8], located at the minimum of the camel− shaped differential capacitance, i.e., from −0.44 V for Pyr+ 14/PF6 toward
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surface, has been discussed in previous studies [16,23–26]. In the current study, the effect of the specific adsorption on the EDL and the PZC was studied by comparing the experimental differential capacitance of two IL electrolytes of a common anion, BMIM+/PF− 6 and − Pyr+ 14/PF6 , with the GC electrode. Although both ILs are composed of cations with a similar structure and ionic diameter, the lack of an aromatic ring in Pyr+ 14 makes it lack specific adsorption via π-stacking interactions with the GC electrode. Indeed, the specific adsorption of BMIM+ effectively shortens the EDL thickness on the negatively charged electrode but elongates the EDL thickness on the positively charged electrode. Such an effect is manifested in the differential capacitance with a higher value on the negative polarization branch than that on the positive polarization branch. The impact of the specific adsorption is also seen from the positive shift of the PZC of BMIM+/PF− 6 /GC − in comparison with that of Pyr+ 14/PF6 /GC.
Acknowledgement This work is supported by NSFC (21373118, 21073097, and 51102137), 973 (2009CB220100), Natural Science Foundation of Tianjin (12JCYBJC13900), and NCET-10-0512. We are grateful to Dr. Jacqui Cloud for polishing the English of the manuscript.
References [1] [2] [3] [4] [5] [6] + − Fig. 2. Schematic diagrams of the EDL structure of BMIM+/PF− 6 (a, b, and c) and Pyr14/PF6 (d, e, and f) on the negatively charged, PZC, and positively charged electrode.
−0.03 V for BMIM+/PF− 6 vs. Ag wire, as shown in Fig. 1. The above effect is depicted in Fig. 2b and e. (3) On the positive polarization branch, the differential capacitance + − of BMIM+/PF− 6 /GC is quickly suppressed by that of Pyr14/PF6 /GC. As shown in Fig. 1, the trend switches at 0.32 V vs. Ag wire. This can be attributed to the specific adsorption of BMIM+ on the electrode, which increases the surface charge on the positively charged electrode and thus depresses the differential capacitance on the positive polarization. The strong adsorption of BMIM+ makes it remain in contact with the electrode surface at low positive potentials, as depicted in Fig. 2c. In addition, the specific adsorption of BMIM+ prevents a closer approach of PF− 6 to the electrode surface and further results in a thicker EDL with a lower differential capacitance on the positive polarization. Such a finding is also supported by previous theoretical simulations of BMIM+ on the graphite electrode [16,22]. On the other hand, the lack of π-stacking interactions between Pyr+ 14 and the graphite surface makes it displaced easily from the electrode surface and thus replaced by PF− 6 at low positive potentials, as depicted in Fig. 2f. Thus, the effect of the specific adsorption of BMIM+ on the graphite surface causes the positive shift of the differential capacitance peak, i.e., 1.35 V for + − BMIM+/PF− 6 /GC and 1.05 V vs. Ag wire for Pyr14/PF6 /GC, although the peak values of the differential capacitance are similar.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
4. Conclusion The specific adsorption of BMIM+ on the GC electrode, caused by the π-stacking interaction between the imidazolium ring and the graphite
[31] [32] [33] [34]
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