Electrochimica Acta 184 (2015) 164–170
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Effects of anion on the electric double layer of imidazolium-based ionic liquids on graphite electrode by molecular dynamics simulation Xiaohong Liu, Yuanyuan Wang, Shu Li, Tianying Yan* School of Materials Science and Engineering, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300350, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 June 2015 Received in revised form 13 October 2015 Accepted 13 October 2015 Available online 17 October 2015
Effects of anion on the electric double layer (EDL) structure and differential capacitance (Cd) of ionic liquid (IL), 1-butyl-3-methyl-imidazolium bis(fluorosulfonyl)imide (BMIM+/FSI ) and 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM+/Tf ) on graphite electrode were studied by molecular dynamic simulations. It is found that ILs with FSI and Tf anions show substantially different EDL structures and Cd features. The potential-dependent capacitance curve of the BMIM+/FSI is asymmetric camel-shaped, with the potential of zero charge at the local minimum, while the BMIM+/Tf shows much weaker Cd dependence on potential between 0 to 1.1 V. Such feature of BMIM+/Tf indicates formation of Helmholtz-like EDL structure, which consists of a parallel alignment of imidazolium-ring in BMIM+ and the planar graphite electrode. A higher Cd of the both ILs at positive polarization can be attributed to the thinner effective EDL caused by the smaller size of anions compared to the cations. Apart from that, the slightly higher capacitance of BMIM+/Tf compared to BMIM+/FSI at positive polarization and the reverse trend at negative polarization is associated with the effectively shortened EDL thickness that resulted from the smaller size of Tf in contrast to FSI . ã 2015 Published by Elsevier Ltd.
Keywords: differential capacitance electric double layer ionic liquids molecular dynamic simulations
1. Introduction Room-temperature ionic liquids (ILs), as the “designer solvent”, has received intensive interest due to their low melting point and volatility, relatively high ionic conductivity, and wide electrochemical window [1], which can enhance charge storage capacity of supercapacitors, given the square dependence of the energy density on potential windows. Properties of IL are tunable through various combinations of organic cations and organic/inorganic anions, as well as the modification of cations and anions [1,2]. Therefore, IL is one of promising electrolyte of supercapacitors that can deliver a high power density and an energy density comparable to the lead-acid battery [3,4]. Although ILs possess favorable properties for use as electrolytes in supercapacitors, the electric double layer (EDL) structure at molecular level is still not well understood [5]. EDL is generally complicated by various conformations of ILs [6], ionic correlations [5], specific adsorption [7,8], and temperature [9–11],etc.. Differential capacitance (Cd) provides important insight into the potential dependent EDL both from theoretical and
* Corresponding author. E-mail address:
[email protected] (T. Yan). http://dx.doi.org/10.1016/j.electacta.2015.10.064 0013-4686/ ã 2015 Published by Elsevier Ltd.
practical points of view. The EDL structure of ILs is often experimentally probed by the potential-dependent capacitance (Cd–U) curve using electrochemical impedance spectroscopy (EIS) [12]. The available Cd data of various ILs provide invaluable information about EDL of this new kind of electrolyte. In terms of anionic effects, the trend of experimental Cd with variation of the anion size and the chemical structure is controversial. Lockett et al. [11] found the Cd decreases as the anion size increases from Cl to I in 1-butyl-3-methyl-imidazolium BMIM+/Y (Y = Cl , Br , I ) ILs on a glassy carbon electrode by EIS. However, Lust et al. [13] and Silva et al. [14] reported an opposite trend of ILs. Specifically, Lust et al. [13] found that the Cd of 1-ethyl-3-methyl-imidazolium EMIM+/TCB (tetracyanoborate) is higher than that of EMIM+/BF4 (tetrafluoroborate) on bismuth electrode using EIS. The Cd of BMIM+/TFSI (bis(trifluoromethylsulfonyl) imide), as studied by Silva et al. [14], is higher than that of BMIM+/PF6 (hexafluorophosphate) on both gold (Au) and platinum (Pt) electrodes. Similar contradiction also exists in the computer simulations of ILs/electrode interfacial properties. Qiao et al. [15] studied the EDL capacitance of two ILs at planar graphite electrode by using a combination of molecular dynamic (MD) simulations and density functional theory. They found that the Cd of BMIM+/Cl is higher than that of BMIM+/PF6 at the positive polarization. Feng et al. [16] studied the EDL of 1-hexyl-3-methyl-imidazolium
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(C6mim)22+-based ILs: (C6mim)22+/(BF4)22 and (C6mim)22+/ 2 (TFSI)2 on onion-like carbon electrode via MD simulations. They found that (C6mim)22+/(BF4)22 yielded a higher Cd in contrast to (C6mim)22+/(TFSI)22 . On the other hand, Vatamanu et al. [17] investigated the EDL of N-methyl-N-propylpyrrolidinium pyr13+/FSI (bis(fluorosulfonyl) imide) and pyr13+/TFSI ILs by performing MD simulations and found the smaller size of the FSI compared to TFSI did not result in an increase of the Cd at the positive polarization. Subsequently, they studied the EDL of BMIM+/BF4 and BMIM+/PF6 on planar graphites [18] and observed very similar EDL structures for both ILs with an average Cd of 5 mF cm 2. The authors concluded that the difference in the anion structure is not important for defining Cd on the planar graphite surface. Thus, there is still a discrepancy on the trend of anion-dependent capacitance in both experiments and computer simulations, due to the complexity of the ILs/electrode interfaces. The target of this work is to study the effects of anion on EDL structure of BMIM+-based ILs by MD simulations. It is found that the Cd–U curve of BMIM+/FSI is asymmetric camel-shaped with a higher Cd at positive polarization, which is in good agreement with a previous MD study [19]. However, a weak dependence of Cd on potential window of 0 to 1.1 V, i.e., a nearly flat Cd–U curve is observed for BMIM+/Tf . The potential of zero charge (PZC) of BMIM+/FSI is –0.077 V, while that of BMIM+/Tf is 0.071 V. The value of Cd at PZC decreases as the anion changes from Tf to FSI . On the other hand, the Cd of BMIM+/Tf is slightly higher than that of BMIM+/FSI at the positive polarization and such trend is reverse at the negative polarization, due to the more efficent screening of Tf with the smaller size in contrast to FSI . Apart from that, the decrease in Cd for both ILs at high polarizations was associated with the gradually increase in the effective thickness of EDL from the overcrowded counterions. Detailed analysis of EDL structure based on anatomy of the number density profiles and orientational ordering are described below. 2. Computational methods We performed MD simulations of BMIM+/FSI and BMIM+/Tf ILs between two oppositely charged electrodes, composed of frozen graphene layers on both sides of the simulation cell. The separation between the two inner graphene layers was set to be 100 Å, allowing the EDL and the differential capacitance on the two oppositely charged walls to be studied independently [20]. The structure of BMIM+, FSI and Tf are showed in Fig. 1a, b and c. The simulation process is similar to our previous studies [7,10,21]. Briefly, the BMIM+/FSI and BMIM+/Tf bulk phases contain 448 and 469 cation-anion pairs, respectively, for all systems and
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these systems were run at 450 K under 1 bar. Table 1 summarizes the total number of ions and carbon atoms present in each simulated system containing IL/carbon interfaces. The force field parameters are taken from Pádua's work [22–24], and the carbon atoms of the graphite (0001) surface interact with BMIM+/FSI and BMIM+/Tf via the Lennard-Jones potential corresponding to the sp2 hybrid carbon atoms in the AMBER force field [25]. The van der Waals and the real space electrostatic interaction cutoff distance was set to be 12 Å, and the smoothed particle-mesh Ewald (SPME) algorithm [26] elongated up to 700 Å in z direction (i.e. the direction perpendicular to the graphite electrode surface) was used to handle the long-range electrostatic interactions in reciprocal space, whilst a slab correction [27] is induced along the z direction for such an essential two-dimensional periodic system along the xy-directions. For each system, 14 MD runs were performed with a fixed charge density s on the two inner graphene layers in the internal from –15.6 to +15.6 mC cm 2, with an increment of 1.2 mC cm 2. Charges with opposite signs were put on the carbon sites of the two inner graphene layers in contact with the BMIM+/ FSI and BMIM+/Tf , so that the whole system was charge neutral. For each simulation, after initial annealing from 1000 to 450 K within 20 ns, a trajectory of 50 ns was gradually generated at 450 K, coupled to a Nosé-Hoover-chain thermostat [28,29] to generate a converged EDL because dynamics of IL was slow [30,31]. The integration time step was 2 fs with a SHAKE/RATTLE algorithm [32,33] applied on constraining all the C-H bonds. The simulation was performed with a home-made MD package and the image charges were not considered in the current work. 3. Results and discussion 3.1. Influences of anion on Cd The simulated results of the electrode surface charge density (s ) and the applied electrode potential (U) of BMIM+/FSI /graphite and BMIM+/Tf /graphite is shown in Fig. 2a. The s –U curves were interpolated with B-spline interpolation [36]. Subsequently, Cd U curves were obtained by differentiating the interpolated s –U curves in Fig. 2a, Cd = ds /dU, which are depicted in Fig. 2b. Several features can be observed from the Cd–U curves, i.e., (1) The overall trend of Cd–U curves of BMIM+/FSI and BMIM+/Tf are asymmetric camel-shaped, with a higher Cd at positive polarization. Such trend of Cd–U curves can be ascribed to the thinner effective EDL caused by the smaller size of anions compared to the cations. The Cd–U curve of BMIM+/FSI on planar graphite electrode is in excellent agreement with a previous MD study of the same system [19]. Also, the camel-shaped Cd–U curve of BMIM+/Tf on planar
Fig. 1. Structures of BMIM+ (a), FSI (b) and Tf (c), the geometrical center of the im-ring of BMIM+, the butyl head carbon of BMIM+, denoted as gc and C4. Carbon (C), nitrogen (N), hydrogen (H), sulfur (S), oxygen (O), and fluorine (F) atoms are displayed in cyan, blue, white, yellow, red, and chartreuse spheres, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
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Table 1 Total number of ions and carbon atoms in each of the BMIM+/FSI (experimental values are compared in parentheses). System
systems and respective mass density after equilibration for the pure ionic liquids
r/g cm
Number of ionic and atomic species +
BMIM+/FSI /graphite BMIM+/Tf /graphite
and BMIM+/Tf
BMIM
FSI
Tf
carbon
448 469
448 –
– 469
2040 2040
graphite electrode agrees well with the negative-running camelshaped Cd–U curve of BMIM+/Tf on Au electrode [37]. Interestingly, the Cd of BMIM+/Tf shows much weaker dependence on potential between 0 to 1.1 V potential window than that of BMIM+/ FSI , indicating the formation of Helmholtz-like EDL structure that consists of a parallel alignment of im-ring (positive charge center) in BMIM+ and the planar graphite electrode. This distinct nearly flat Cd–U curve of BMIM+/Tf between 0 to 1.1 V will potentially inspire the design of supercapacitors with stable capacitive performance [38]; (2) The value of Cd at PZC decreases as the anion changes from Tf to FSI . This trend is caused by the smaller size of Tf in constrast to FSI , and is consistent with the results reported by Lockett et al. [11]. The PZC of BMIM+/FSI (–0.077 V) and BMIM+/ Tf (0.071 V) are both near the local minimum, as pointed by the arrows in Fig. 2b. The negative value of PZC indicates the affinity of anions toward an electrode while the positive PZC suggests preferential specific adsorption of BMIM+ cations on neutral graphite electrode; (3) The slightly higher Cd of BMIM+/Tf at positive polarization compared to that of BMIM+/FSI is related to the smaller size of Tf compared to FSI . Apart from that, the higher Cd of BMIM+/FSI than BMIM+/Tf at potentials more negative than –0.63 V, is due to the more effective screening of
Fig. 2. (a) Surface charge density (s ) versus electrode potential (U) for BMIM+/FSI and BMIM+/Tf . An increment of 6 mC cm 2 was successively added to the data of BMIM+/Tf to distinguish them as guide to eyes. The lines in (a) are interpolated to s –U relation for BMIM+/FSI and BMIM+/Tf , (b) Cd of BMIM+/FSI and BMIM+/Tf ILs as a function of electrode potential.
3
at 400 K
1.33 (1.43 at 300 K [34]) 1.26 (1.29 at 300 K [34], 1.25 at 363 K [35])
BMIM+ induced by a drastic decrease of distribution of FSI at interface, where the “latent voids”, i.e., the neutral ‘tails’ of ions [39], provided by the alkyl chain in BMIM+ are limited. Apart from that, the capacitance decreases sharply at high polarization of the both systems, which is due to the thicker EDL that results from the overcrowded counterions. It is desirable to take a close inspect on the effects of anion on EDL structure at both positive and negative polarizations, as will be discussed below. Notably, the electronic capacitance (CQ) is important for graphite [40–42]. For a capacitance model that takes into account CQ, the total capacitance (CT) is represented as a series of CQ and double layer capacitance (Cd), that is, 1/CT = 1/CQ + 1/Cd, in which Cd is given in Fig. 2b. It is demonstrated in Ref. 40-42 that CQ is a U-shaped curve, with the minimum located at PZC, and CQ increases dramatically with elevated potential. Thus, the CT is dominated by Cd at high potential, with CQ much larger than Cd, while the CQ dominates near the PZC and gives rise to a pronounced minimum. For the U-shaped Cd near the PZC of both the BMIM+/FSI and BMIM+/Tf ILs in this study, the contribution of the graphite electrode is expected to be small and does not alter the overall Cd curve in a fundamental manner. 3.2. Density profiles of BMIM+/FSI and BMIM+/Tf Effects of anion on the EDL and Cd of BMIM+/FSI and BMIM+/Tf ILs are also manifested in the number density profiles, r(z)'s, as depicted in Fig. 3, which shows the geometrical center of the imring of BMIM+, the butyl head carbon of BMIM+ (cf. gc and C4 denoted in Fig. 1a), and the center-of-mass (COM) of FSI and Tf at surface charge densities of s = 0 mC cm 2 (Fig. 3a, c, and e) and 4.8 mC cm 2 (Fig. 3b, d, and f). Clearly, the oscillations in both the cation and anion density profiles near the electrode surface indicate that both the BMIM+/FSI and the BMIM+/Tf ILs exhibit long-range, charge-ordered layering structures, which extend over 20 Å to the bulk. Also, the specific adsorption of BMIM+ to the uncharged electrode surface, caused by the p-stacking interaction between the aromatic im-ring and the sp2 graphite surface, can be clearly seen in the current two systems, as described in our previous computational [7,10,21,43] and experimental studies [8]. A distinct feature is that the geometrical center of the im-ring remains close contact to the neutral and negatively charged electrode surface, as depicted in Fig. 3a and b. This is reasonable because the positive charge on the BMIM+ cation is largely distributed on the im-ring. Upon charging from s = 0 mC cm 2 to s = –4.8 mC cm 2, the first peak of the geometrical center of the im-ring shifts from z = 3.5 Å to 3.4 Å, with r(z) increases from approximately 0.012 Å 3 to 0.020 Å 3 for BMIM+/FSI and 0.013 Å 3 to 0.018 Å 3 for BMIM+/Tf , respectively. The overall increased accumulation of the first peak of r(z) is around 1.7 and 1.4 for BMIM+/FSI and BMIM+/Tf , respectively. It can be seen from Fig. 3b that the BMIM+ cations are persistently adsorbed on the surface even at s = 4.8 mC cm 2, while the first peaks of r(z) of the both systems are repelled to be located at 3.6 Å with much lower r(z) of 0.003 Å 3 and 0.009 Å 3 for BMIM+/FSI and BMIM+/ Tf , respectively. On the other hand, r(z) of C4 atom of BMIM+ do
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Fig. 3. Number density profiles of the im-ring geometrical center of BMIM+ (a,b), the butyl head carbon of BMIM+ (c,d), and the center-of-mass position of FSI (black lines) and Tf (red lines) (e,f) at the surface charge densities of 0 mC cm 2 (a, c, e) and 4.8 mC cm 2 (b, d, f). The two electrodes locate at –50 Å (neutral or positively charged) and +50 Å (neutral or negatively charged), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
not undergo such drastic enhancement or depression as surface charge density switches from s = 0 mC cm 2 to s = 4.8 mC cm 2, as depicted in Fig. 3c and d. It is of interest to note that the first peak of r(z) of C4 atom remains located at 3.5 Å upon charging to s = 4.8 mC cm 2. Fig. 3e and f show number densities of the COM of FSI and Tf . It can be found that both the first peak of r(z) of FSI and Tf gets closer to the electrode from neutral electrode (3.9 Å for FSI and 4.4 Å for Tf ) to s = 4.8 mC cm 2 (3.8 Å for FSI and 3.9 Å for Tf ), as expected, with a similar value of r(z) of FSI (0.024 Å 3) and Tf (0.020 Å 3). Apart from that, FSI is repelled from neutral electrode to s = –4.8 mC cm 2, with the first peak shifts from 3.9 Å to 4.1 Å, while that of Tf (4.4 Å) is unchanged, as shown in Fig. 3e and f. It is infered that the negatively charged atoms in FSI ions is more disperse than in Tf , thus, the FSI ions suffered weaker repulsion than the Tf at negative polarization and was closer to the electrode surface. The depression of the first peak of r(z) of FSI and Tf as s switches from 0 mC cm 2 to –4.8 mC cm 2 is 0.013 Å 3 to 0.005 Å 3 and 0.015 Å 3 to 0.012 Å 3, respectively. The much lower density of FSI than that of the Tf in the second layer after the first BMIM+ layer at negative polarization, combined with the much larger accumulation of BMIM+ in BMIM+/FSI (Fig. 3b), allows more effective screening of cations to the negative surface charge, verified by a higher Cd of BMIM+/FSI at negative polarization in Fig. 2b.
3.3. Orientational distribution of BMIM+/FSI and BMIM+/Tf To disclose more details of the interfacial structure of BMIM+/ FSI and BMIM+/Tf on the graphite surface, the orientational ordering parameter, P2(u ), which is defined as the ensemble average of the second Legendre polynomial, i.e., P2(u) = <(3cos2(u)1)>/2 [21], where u is the angle between the im-ring normal direction vector (cf. Fig. 1a) and the electrode surface normal (z) at s = 0 mC cm 2 (Fig. 4a) and 4.8 mC cm 2 (Fig. 4b), or between the sulfur-sulfur vector (S–S, cf. Fig. 1b) in FSI and z, or the carbonsulfur vector (C–S, cf. Fig. 1c) in Tf and z at s = 0 mC cm 2 (Fig. 4c) and 4.8 mC cm 2 (Fig. 4d) was calculated, as shown in Fig. 4. As can be seen from Fig. 4a, the im-ring of BMIM+ in the first layer at s = 0 mC cm 2 oriented preferentially parallel to the electrode surface in both BMIM+/FSI and BMIM+/Tf ILs, accompanied by the values of P2(u) approximating unit. As the first r(z) of the gc of the BMIM+ and the C4 in BMIM+ peaks at the same location to the neutral electrode surface (cf. Fig. 3a and c), as well as the parallel alignment of im-ring (Fig. 4a), it can be deduced that the BMIM+ in BMIM+/FSI and BMIM+/Tf tends to lie flat on uncharged electrode. This is consistent with results reported in earlier MD simulations of BMIM+/PF6 electrolyte [7,10,15,21,43]. Such flat configuration of BMIM+ in both systems is expelled to be slightly slant at the surface charge density of 4.8 mC cm 2, because the
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Fig. 4. Orientational ordering, P2(u), of the BMIM+ im-ring normal, along the surface normal direction z of the electrode in BMIM+/FSI (black lines) and BMIM+/Tf (red lines), under the electrode surface densities of 0 mC cm 2 (a) and 4.8 mC cm 2 (b); and P2(u), between the S–S vector in FSI (red), the C–S vector in Tf (black) and z at s = 0 mC cm 2 (c) and s = 4.8 mC cm 2 (d), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
values of P2(u ) vary to 0.859 and 0.864 at the first r(z) peak of the geometrical center of the BMIM+ in BMIM+/FSI and BMIM+/Tf , respectively. Also, a closer contact of C4 (3.5 Å) and the positively charged electrode of 4.8 mC cm 2 was obtained, while the gc of the im-ring of BMIM+ locates at 3.6 Å (cf. Fig. 3b), indicating a C4-gc pattern of BMIM+ with C4 in closer contact with the positively charged electrode. The im-ring of BMIM+ in both BMIM+/FSI and BMIM+/Tf ILs tends to lie flat upon charging to s = –4.8 mC cm 2 to achieve more effective screening to the negatively charged surface, with the values of P2(u) at the first r(z) peak of the geometrical center of the BMIM+ in BMIM+/FSI and BMIM+/Tf approximating unit (Fig. 4b). However, the BMIM+ cations in BMIM+/FSI and BMIM+/Tf switches to a slant configuration at s = –4.8 mC cm 2, with the gc of BMIM+ locates at 3.4 Å (cf. Fig. 3b) and the C4 of BMIM+ locates at 3.5 Å (cf. Fig. 3d), suggesting a gc-C4 pattern of BMIM+ with gc in closer contact with the negatively charged electrode. As can be seen from Fig. 4c, the S–S vector of FSI tends to be parallel to the neutral electrode, with the value of P2(u) at the first peak of r(z) (cf. Fig. 3c) about –0.5, whilst the C–S vector of in the first layer slants to the surface with a P2(u ) value of 0.36. The S–S vector of FSI and the C–S vector of Tf upon charging to s = 4.8 mC cm 2 tends to be parallel to the electrode, accompanied by the values of P2(u ) approximating –0.5, while only the C–S vector of Tf switches to a slant configuration upon charging to s = –4.8 mC cm 2, accompanying the P2(u) value around 0.40 (Fig. 4d).
electrode due to the strong specific adsorption between im-ring of BMIM+ and graphite surface by p-stacking interaction, as reported in our previous studies [7,8,10,21,43]. Such flat configuration of BMIM+, which can also be concluded through the position of the first number density peak of gc and the C4 of the BMIM+ in Fig. 3a and c, as well as the P2(u ) of BMIM+ (cf. Fig. 4a), switches to a slant configuration when electrodes are charged to s = 4.8 mC cm 2, as shown in Fig. 5c and d. On the other hand, the smaller size of Tf compared to FSI generates a thinner EDL structure of BMIM+/Tf than that of BMIM+/FSI at positive polarization (Fig. 5c and d), and
3.4. EDL structure of BMIM+/FSI and BMIM+/Tf on planar graphite electrode Based on the above inspections of Fig. 3 and Fig. 4, it is important to note that the change in orientation of the ions in the innermost layer determines the EDL structure of BMIM+/FSI and BMIM+/Tf . Fig. 5 displays EDL of BMIM+/FSI and BMIM+/Tf at surface charge densities of 0 mC cm 2 (Fig. 5a and b) and 4.8 mC cm 2 (Fig. 5c and d), respectively. The BMIM+ ions in the innermost layer of both systems orient predominantly parallel to neutral
Fig. 5. Electric double layer(EDL) structure of BMIM+/FSI (a, c) and BMIM+/Tf (b, d) on the neutral (a, b) and charged electrodes of s = 4.8 mC cm 2 (c, d).
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thus causes a higher Cd (cf. Fig. 2b) of BMIM+/Tf . It is notable that the parallel alignment of BMIM+ in the first layer of both ILs (Fig. 5a and b) switches to a slant alignment at negative polarization. Such alignment of BMIM+ cations, as well as its much larger size than the anions, results in a thicker EDL at negative polarization than that of the positive polarization, and thus induces a smaller Cd (cf. Fig. 2b) at negative polarization. Apart from that, the decrease of Cd of BMIM+/FSI and BMIM+/Tf at high potentials (cf. Fig. 2b), was due to the gradual increase in the effective thickness of EDL resulting from the overcrowded counterions, as shown in Fig. 5. 4. Conclusions Effects of anions on the interfacial EDL structure and Cd of BMIM+/FSI and BMIM+/Tf on graphite electrode were studied by MD simulations. Different EDL structures and Cd dependence of BMIM+/FSI /graphite and BMIM+/Tf /graphite were found. The Cd– U curve of BMIM+/FSI is asymmetric camel-shaped, while that of the BMIM+/Tf shows much weaker dependence on potentials between 0 to 1.1 V. Such weak dependence of BMIM+/Tf indicates the formation of Helmholtz-like EDL structure that constructed from a parallel alignment of im-ring of BMIM+ and the planar graphite electrode. A higher Cd of both ILs at positive polarization is related to the thinner effective EDL, which arises from the smaller size of anions than the BMIM+ cations. On the other hand, the higher Cd of BMIM+/Tf at positive polarization is attributed to the shortened effective thickness of EDL, induced by the smaller size of Tf compared to that of FSI . Apart from that, a higher Cd for BMIM+/FSI than for BMIM+/Tf at potentials more negative than –0.63 V is associated with the more effective screening of BMIM+ to the surface charge, and the decrease in Cd at high potentials of both systems is caused by the gradual increase in the EDL thickness. The insight into the EDL structure of ILs at molecular level can provide us with a rational design for the new electrolytes to fulfill the demands in supercapacitors. Acknowledgements This work was supported by NSFC (21373118, 21203100), the Natural Science Foundation of Tianjin (13JCQNJC06700), and the MOE Innovation Team (IRT13022) of China. References [1] M.V. Fedorov, A.A. Kornyshev, Ionic liquids at electrified interfaces, Chem. Rev. 114 (5) (2014) 2978–3036. [2] F.V. Rantwijk, R.A. Sheldon, Biocatalysis in Ionic Liquids, Chem. Rev. 107 (2007) 2757–2785. [3] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Carbon-based supercapacitors produced by activation of graphene, Science 332 (6037) (2011) 1537–1541. [4] L. Zhang, F. Zhang, X. Yang, G. Long, Y. Wu, T. Zhang, K. Leng, Y. Huang, Y. Ma, A. Yu, Y. Chen, Porous 3D graphene-based bulk materials with exceptional high surface area and excellent conductivity for supercapacitors, Sci. Rep. 3 (2013) 1408. [5] M.Z. Bazant, B.D., Storey, A.A. Kornyshev, Double layer in ionic liquids: overscreening versus crowding, Phys. Rev. Lett. 106 (4) (2011) 046102; Bazant, M.Z., Storey, B.D., Kornyshev, A.A., Double Layer in Ionic Liquids: Overscreening versus Crowding [Phys. Rev. Lett. 106, 046102 (2011)] Phys. Rev. Lett. 2012, 109 (14), 149903. [6] A.A. Kornyshev, Double-layer in ionic liquids: paradigm change? J. Phys. Chem. B 111 (2007) 5545–5547. [7] X.J. Si, S. Li, Y.L. Wang, S.H. Ye, T.Y. Yan, Effects of specific adsorption on the differential capacitance of imidazolium-based ionic liquid electrolytes, ChemPhysChem 13 (7) (2012) 1671–1676. [8] Q. Zhang, Y.N. Han, Y.L. Wang, S.H. Ye, T.Y. Yan, Comparing the differential capacitance of two ionic liquid electrolytes: Effects of specific adsorption, Electrochem. Commun 38 (2014) 44–46. [9] V. Lockett, R. Sedev, J. Ralston, M. Horne, Differential capacitance of the electrical double layer in imidazolium-based ionic liquids-Influence of potential cation size and temperature, J. Phys. Chem. C 112 (2008) 7486–7495.
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