Improved Performance of Ionic Liquid Supercapacitors by using Tetracyanoborate Anions

Abstract Supercapacitors are energy storage devices designed to operate at higher power densities than conventional batteries, but their energy density is still too low for many applications. Efforts are made to design new electrolytes with wider electrochemical windows than aqueous or conventional organic electrolytes in order to increase energy density. Ionic liquids (ILs) with wide electrochemical stability windows are excellent candidates to be employed as supercapacitor electrolytes. ILs containing tetracyanoborate anions [B(CN)4] offer wider electrochemical stability than conventional electrolytes and maintain a high ionic conductivity (6.9 mS cm−1). Herein, we report the use of ILs containing the [B(CN)4] anion for such an application. They presented a high maximum operating voltage of 3.7 V, and two‐electrode devices demonstrate high specific capacitances even when operating at relatively high rates (ca. 20 F g−1 @ 15 A g−1). This supercapacitor stored more energy and operated at a higher power at all rates studied when compared with cells using a commonly studied ILs.


Improved Performance of Ionic Liquid Supercapacitors by using Tetracyanoborate Anions
Vitor L. Martins, [a] Anthony J. R. Rennie, [a] Nedher Sanchez-Ramirez, [b] Roberto M. Torresi, [b] and Peter J. Hall [a] Supercapacitors are energy storage devices designed to operate at higher power densities than conventional batteries, but their energy density is still too low for many applications. Efforts are made to design new electrolytes with wider electrochemical windows than aqueous or conventional organic electrolytes in order to increase energy density. Ionic liquids (ILs) with wide electrochemical stability windows are excellent candidates to be employed as supercapacitor electrolytes. ILs containing tetracyanoborate anions [B(CN) 4 ] offer wider electrochemical stability than conventional electrolytes and maintain a high ionic conductivity (6.9 mS cm À1 ). Herein, we report the use of ILs containing the [B(CN) 4 ] anion for such an application. They presented a high maximum operating voltage of 3.7 V, and twoelectrode devices demonstrate high specific capacitances even when operating at relatively high rates (ca. 20 F g À1 @ 15 Ag À1 ). This supercapacitor stored more energy and operated at a higher power at all rates studied when compared with cells using a commonly studied ILs.
Supercapacitors, or electrochemical double-layer capacitors (EDLCs), are energy storage devices that are designed to operate at higher power densities than conventional batteries. EDLCs rely mainly on the charge stored in the double-layer formed at polarized electrode/electrolyte interfaces, so that the interactions of ions and electrode surface play a crucial role in the charge storage process. However, one consequence of this storage mechanism is that EDLCs have much lower energy densities than batteries, which limits their engineering applications. [1][2][3][4] The energy in EDLC is given by E = C · (V) 2 /2, E is the gravimetric or volumetric specific energy (J g À1 or J m À3 ) and C is the specific capacitance (F g À1 or F m À3 ). V (V) is the (operationally defined) maximum operating voltage, i. e. the maximum polarization voltage at which EDLCs can operate with tolerable loss of capacitance. Consequently, electrolytes that maximize the operating voltage are attractive because they maximize specific energy. In general, ionic liquids (ILs) operate at higher voltage than conventional aqueous or organic electrolytes and therefore fulfil this requirement. IL based EDLCs have not been developed commercially because they possess lower ionic conductivity values than conventional electrolytes, leading to higher internal resistance which results in reduced power density. [5][6][7][8][9][10] Over the past two decades, myriad ILs have been synthesized and characterized for use as EDLC electrolytes, reviewed by Salanne. [8] The ideal IL will maximize electrochemical stability, thermal stability, ionic conductivity and possess a liquid range to satisfy the engineering application requirements. N-butyl-Nmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Pyr 1,4 ] [Tf 2 N]) is one of the most commonly researched ILs because it possesses a good balance between these desirable properties [11][12][13][14][15][16][17][18][19][20][21][22] and acts as an informal reference against which other ILs can be compared.
The relative advantages and disadvantages of using ILs compared to organic electrolytes can be seen in Table 1. For example, [Pyr 1,4 ][Tf 2 N] possess a maximum operating voltage of 3.7 V compared to 3.0 V for PC based electrolytes. Different operating voltages for organic solvents based electrolytes can be found in literature, since it greatly depends on electrode materials employed. Considering a constant capacitance, the difference in voltage mentioned above would give an increase in specific energy density of over 50 %. In addition, ILs have higher thermal stability which is a great advantage. However, the downside of using [Pyr 1,4 ][Tf 2 N] is that its ionic conductivity is 2.7 mS cm À1 , much lower than 12.2 mS cm À1 for PC based electrolyte. This would result in a much lower power density. [2] Wolff et al. [19] have shown that is possible to increase the power density by using ILs containing the dicyanamide anion ([N(CN) 2 ]). Table 1 shows that the ionic conductivity of [Pyr 1,4 ] [C(CN) 2 ] is 12 mScm À1 , which is much larger than the [Pyr 1,4 ] [Tf 2 N] analogue and comparable to PC based electrolyte. However, the determined operating voltage is only 2.5 V which is much lower than the 3.7 V used for [Pyr 1,4 ][Tf 2 N]. [19] In an earlier work, [20] we have studied the tricyanomethanide ([C(CN) 3 ]) anion and found similar results, obtaining higher power but reduced energy densities in comparison with [Pyr 1,4 ] [Tf 2 N]. [20] The operating voltage usually depends on the cationic resistance to reduction and the anionic resistance to oxidization, [23] nevertheless, it has been found that both cations and anions play important roles in the definition of positive and negative limits. [24] Using molecular modelling, Dhungana et al. [25] have shown that in the IL 1-ethyl-3-methylimidazolium tetracyanoborate [Im 1,2 ][B(CN) 4 ], the cation is more likely to oxidize than the anion, resulting in an IL with greater electrochemical stability than other ILs with cyano based anions. [25] Lust et al. investigated the use of this IL and show that the anion composition and its charge distribution have significant influence on resultant power densities. [18] Moreover, [B(CN) 4 ] anions may undergo polymerization at high overpotentials, thereby acting as a safety mechanism. [26] ILs containing [B(CN) 4 ] anions are known to possess a wide electrochemical stability window when an inert electrode material, (such as glassy carbon) [27] is employed. However, it is important to determine an ILs stability when combined with the activated carbon material used in EDLC electrodes, which presents much higher surface area and can catalytically decrease the ILs electrochemical stability. In this work, we present the results of EDLCs  Figure 1a (the physicochemical characteristics of these ILs are described elsewhere [27] ).
The method used to determine maximum operating voltage is based on the method proposed by Weingarth et al. [33] and is described in detail elsewhere. [13,20] Basically, the difference in coulombic charge between the discharge and charge processes (Dq = q charge Àq discharge ) of cyclic voltammograms (CVs) recorded at 5 mV s À1 over different voltage windows were determined. The highest stability was chosen as the voltage when the d 2 Dq/dV 2 exhibits a sharp increase, indicating a lower charge-discharge efficiency due to electrolyte decomposition. Figure 1b shows the maximum operating voltage determination for both ILs, [Pip 1,4 4 ], from 0.5 V up to 3.0 V for positive determinations, and from À1.0 V down to À3.0 V for the negative determinations. A maximum operating voltage of 3.7 V was determined for both ILs which is much higher than the values found for other ILs containing anions based on the cyano group, such as [N(CN) 2 ] (2.5 V) [19] and [C(CN) 3 ] (3.0 V) [20] , and similar to that shown by the IL [Pyr 1,4 ][Tf 2 N]. [12] It is worth noting that other approaches to enhance the maximum operating voltage of EDLCs were also described in literature. For example, Shen and Hu [34] used carbon black in the positive electrode and activated carbon in the negative electrode and obtained a device operating at 3.0 V using conventional electrolyte (1 mol L À1 TEABF 4 in PC), how-ever, the lower surface area of carbon black compared to activated carbon decreases the overall device specific capacitance. Moreover, Balducci and collaborators [35][36][37][38] have employed different organic solvents or ILs as conductive salts in organic solvent to increase the electrolyte stability. A maximum operating voltage of 3.5 V was obtained when 1 mol L À1 of [Pyr 1,4 ][Tf 2 N] in PC [35] or TEABF 4 in 3-cyanopropionic acid methyl  [13,20] [Pyr 1,4 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57 ester were used. [36] The [B(CN) 4 ] ILs still show greater maximum operating voltage than these alternatives. The CVs at the determined limits can be seen in Figure 1c, they show the expected rectangular shape for capacitor behavior and no faradaic peaks are observed. [Pip 1,4 4 ], however it is also 0.1 V lower on the positive side; thus, overall, they have the same operating voltage. To ensure charge distribution is symmetrical for both electrodes in the EDLCs when operating at high rates (since 3.7 V at 5 mV s À1 would discharge in 740 s), the ratio of the charge quantities in the discharge steps of negative (q -) and positive (q + ) scans were analyzed at different scan rates at the determined window for each electrode (positive and negative). [39] Figure S1 (in Supporting Information (SI)) shows the mass ratio of positive to negative electrode required to balance the charge (m + /m À = q À /q + ). This ratio increases with scan rate, and in order to have an extended life the mass ratio at 100 mV s À1 was used for EDLC design which was determined to be 2.  1,4 ] counterpart. This is a likely consequence of its higher ionic conductivity (6.9 vs 3.7 mS cm À1 , see Table 1) and smaller cation size; these features combined facilitate mass transport into the smaller pores, resulting in an increased specific capacitance. It is worth noting that this feature was also observed when the [C(CN) 3 ] anion was used with the same cations. [20] Figure 2b shows that higher capacitance is retained over a broad range of scan rates when the [Pyr 1,4  Galvanostatic charge-discharge (GCD) cycling was also performed. EDLCs were cycled 20 times at each current rate from 0.5 to 10 A g À1  4 ] results in a device with lower ESR, while the other two liquids produce similar ESRs as they also have similar viscosity and ionic conductivity, as showed in Table 1 4 ] EDLC delivered any energy at 15 A g À1 , and it is worth noting that the ESR increased to 15.2 W, which is still lower than the other EDLCs at lower rates. This sudden increase indicates that the energy storage process must be close to the mass transfer limit, raising the ESR. It is worth noting that EDLCs containing the same electrode composition and 1 mol L À1 TEABF 4 in PC as electrolyte showed an ESR of 16 4 ], consolidating the superior performance of the latter. The cell design clearly needs optimization in order to decrease the high ESR found in these EDLCs compared to values found in literature, but the comparison among the devices showed in  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57 this study support the superior performance of EDLC containing [Pyr 1,4

][B(CN) 4 ] as electrolyte.
In order to better understand the behavior of ILs in EDLCs, electrochemical impedance spectroscopy (EIS) was performed on the devices at their rest voltage. Figure S2 ( 6 and 5.7 W, respectively). The depressed semi-circle is a consequence of the impedance distribution in the carbon pores with different microstructure [18] and possibly due to heterogeneous adsorption of ions in the carbon surface being a determinant step at high frequencies. [40] Other possibilities for the observed high frequency shape are the formation of a passivating film on the current collector (i. e. the stainless-steel coin cell spacer) or an increased resistance between electrodes and electrolyte/separator. [41] Nonetheless, the ionic resistance (R i , the semi-circle diameter) observed for [Pip 1,4 1,4 ][Tf 2 N] are 5.7, 3.6 and 7.5 W, respectively. The resistances were found with the best fit of EIS data using the model R1-(R2 CPE1)-CPE2-C1 (details are shown in Figure S2 and Table S1, in SI). The devices specific capacitances obtained from the following equation  N], which is in agreement with the results from GCD since these two ILs also showed similar capacitance at low rate.

][B(CN) 4 ], [Pyr 1,4 ][B(CN) 4 ] and [Pyr
An alternative analysis of EIS data is the calculation of complex capacitance, so the energy stored can be evaluated by the real contribution Þand the energy losses can be evaluated by considering the imaginary contribu- Þ . [42,43] Figure 4a-b show the real capacitance (C') and the imaginary capacitance (C'') as a function of frequency. The EDLCs only begin to store energy when low frequencies are reached, but as suggested by the capacitance retention by the [Pyr 1,4 ][B(CN) 4 ] EDLC in GCD (see Figure 3c), this device starts to store energy at higher frequencies than its counterparts. This effect is also evident considering the relaxation time constant calculated from the peak in Figure 4b Figure 4c, which shows the values of specific energy (E) and specific average power (P average ) that were calculated from GCD considering only the active mass of both electrodes using the following equations:  4 ] EDLC stores more energy than the [Tf 2 N] EDLC at each specific current. The average specific power delivered by these cells are considerably lower than reported for others ILs in different cell designs, [18,[44][45][46] further optimization in order to decrease the ESR is still necessary to improve the power delivery. The cycle life of both [B(CN) 4 ] EDLCs were also evaluated cycling between 0 and 3.7 V, and the specific capacitance evolution up to 50000 cycles at 2.0 A g À1 are shown in Figure 4d 4 ], respectively, and retained more than 75 % of initial capacitance after 50000 cycles. In addition, Figure S5, in SI, shows the coulombic efficiency of the cells with cycling. Both EDLCs have coulombic efficiency higher than 99.8 % over the 50000 cycles.
In summary, we investigated the performance of two ILs  higher powers and deliver more energy than the [Pyr 1,4

Experimental Section
The preparation of tetracyanoborate containing ILs has been described elsewhere. [27,32,47,48]  Electrodes were prepared using activated carbon (AC), conductive carbon (Super C45, Imerysys G&C) and PTFE binder (Teflon 30-N, 60 % suspension in water, Alfa-Aesar). AC physical properties can be seen in Figure S3, Figure S4 and Table S2 (SI). The AC has a specific surface area (S BET ) of 1,930 m 2 g À1 , arising mainly from micropores and has particles ranging from 1 to 10 mm, with an average size of 5.8 mm. The three materials were blended in a proportion of 80-10-10 by mass (AC-C45-PTFE) in ethanol and mixed using a spatula until a dough-like consistency was observed. The mixture was then compressed to the desired thickness using a calendering mill, punched into 12 mm discs and dried at 80 8C under vacuum overnight before weighing and cell assembly. Selfstanding electrodes used in EDLCs had thicknesses from 100 to 250 mm and mass loadings from 1.0 to 4.5 mg cm À2 . Highly asymmetrical electrodes used in maximum operating voltage determinations had thicknesses from 50 to 300 mm.
Coin cells (2016, stainless steel) were assembled inside the glovebox using stainless steel spacers, the electrodes and glass fiber separator (GF/F, Whatman) soaked with excess of IL. In order to improve electrolyte impregnation into electrodes, the cells were kept in the glovebox anti-chamber under vacuum at 50 8C for five minutes prior to crimping.
The maximum operating voltage of each IL were determined by cyclic voltammetry (CV) at 5 mV s À1 , using a Solartron Analytical 1470E Multichannel Potentiostat/Galvanostat. Positive and negative limits were determined using fresh cells, which contained a large counter-electrode (with same composition as the working electrode, m CE ! 20 m WE ). Five cycles were carried out from 0 to 0.5 V (vs OCV), for positive determination, or À1.0 V, for negative determination Then, four cycles were recorded before increasing the window by 0.1 V up to 3.0 V, for positive determination, or À3.0 V, for negative determination. The quantity of charge passed during the charge and discharge processes was calculated (q ¼ R iÁdt) for the last cycle at each window, and the second derivative of the difference of charge (d 2 Dq/dV 2 ) was used to determine both the positive and negative limits -a sharp increase in d 2 Dq/dV 2 indicates the presence of faradaic reactions and a decrease in efficiency.