Investigation on capacitive behaviors of porous Ni electrodes in ionic liquids

Electrochimica Acta 105 (2013) 455–461

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Investigation on capacitive behaviors of porous Ni electrodes in ionic liquids Naoya Kobayashi a , Takeaki Sakumoto b , Shigeyuki Mori b , Hiroki Ogata b , Ki Chul Park c,∗ , Kenji Takeuchi c , Morinobu Endo b,c a

Samsung Yokohama Research Institute, 2-7 Sugasawa-cho Tsurumiku, Yokohama 230-0027, Japan Department of Electrical and Electronic Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan c Institute of Carbon Science and Technology (ICST), Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan b

a r t i c l e

i n f o

Article history: Received 26 February 2013 Received in revised form 13 April 2013 Accepted 3 May 2013 Available online 17 May 2013 Keywords: EDLC Porous metal electrode Ionic liquids Capacitance Cyclic voltammetry

a b s t r a c t We previously reported some findings on the applicability of porous Ni materials to the electrodes of electric double layer capacitors (EDLCs) [1]. It was found that the porous Ni material prepared via alkalileaching of Ni–Al alloys and dry process without heating and air-contact provides pseudocapacitance as well as electric double-layer (EDL) capacitance in organic electrolyte solution, TEA·BF4 /PC. The pseudocapacitance is ascribed to the electrochemically active surface and bulk state of the porous Ni with low crystallinity. In TEA·BF4 /PC, porous Ni materials were found to provide lower volumetric total capacitance than the values of the commercial activated carbons, due to the large difference of specific surface areas (i.e., porous Ni: 43 m2 /g, the activated carbons examined: 1508–2164 m2 /g). However, a significant point was the high value of EDL capacitance normalized by the surface areas (CSA ), i.e., 10.2 ␮F/cm2 , which was beyond 3.6–6.6 ␮F/cm2 of the activated carbons. In this study, the volumetric total capacitance and CSA of porous Ni materials have been further enhanced by using ionic liquids as electrolytes. The volumetric total capacitance has reached 67.4 F/cm3 (three-electrode evaluation) in EMIm·BF4 ionic liquid, which approaches 79.3 F/cm3 of a high-capacitance-type activated carbon, MSP-20. The total capacitance is affected by the class of ionic liquids due to the difference of the viscosity and conductivity, whereas the pure EDL capacitance is dependent on the ion sizes rather than the physical properties of ionic liquids. Furthermore, the difference of either anions or cations affects the capacitive behaviors in both positive and negative electrodes. Significantly, the CSA value of porous Ni electrodes has increased from 10.2 ␮F/cm2 in TEA·BF4 /PC to 16.6 ␮F/cm2 in EMIm·BF4 , which is much higher than 7.0 ␮F/cm2 of MSP-20. Furthermore, the electrochemical stabilization of porous Ni materials has been achieved by heat treatment under vacuum, resulting in an excellent cycle performance caused by the exclusion of pseudocapacitance. More noteworthy is that the high CSA can be retained even after the stabilization. The results of this study further emphasize the potential of porous Ni materials as EDLC electrodes. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction An electric double-layer capacitor (EDLC) has some unique characters of high power, quick charge/discharge, long cycle life and high safety. The storage principle of EDLCs is based on the physical adsorption of electrolyte ions onto the electrode material surface. Therefore, porous carbons (e.g., activated carbons) with high surface areas and favorable electric conductivity have been used as the electrode materials. However, the storage mechanism based

∗ Corresponding author. Present address: Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology (TIT), 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8550, Japan. Tel./fax: +81 3 5734 3088 (TIT). E-mail addresses: [email protected], [email protected] (K.C. Park). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.05.017

on electrode/electrolyte interfacial phenomenon inherently suffers from the issue that the energy density is 1–2 order lower than that of other storage devices exploiting the electrochemical reactions of the electrode materials [2]. Therefore, the enhancement of energy density without losing the inherent characters of EDLCs has been focused on as an attractive and challenging work. The edge-plane direction of graphite with high carrier density provides far higher electric double-layer (EDL) capacitance than the basal-plane direction (Cbasal : 1–3 ␮F/cm2 , Cedge : 70 ␮F/cm2 ) [3–5]. The differential capacitance of the semiconductor-like basal graphite with low carrier concentrations is limited by the much lower capacitance of a space charge region inside the basal layers than those of Helmholtz and diffusion layers in high electrolyte concentrations [6,7]. Therefore, a high carrier-density material is one potential candidate for EDLC electrodes to enhance the

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capacitance. From such viewpoint, we have studied the applicability of porous metals for EDLCs on the basis of the high carrier concentrations (1022 –1023 cm−3 ) and high electrical conductivity (>103 −1 m−1 ) of metals. In addition, the usage of porous metal electrodes has an advantage to increase volumetric capacitance, due to the higher true density than carbon materials. The enhancement of volumetric capacitance has more significant aspect of facilitating the loading to industrial products with limited space. We previously reported that porous Ni materials provide high volumetric capacitance irrespective of not so high specific surface area (SSA) of 43 m2 /g, compared with commercial activated carbons with 1508–2164 m2 /g [1]. The capacitance originates from an EDL capacitance and faradaic pseudocapacitance. Noteworthy is that the estimated EDL capacitance afforded higher capacitance per unit surface area (unit-area capacitance, CSA ) of 10.2 ␮F/cm2 than 3.6–6.6 ␮F/cm2 of the activated carbons. In porous Ni materials, the predominant mesopores lead to superior rate performance in comparison with the activated carbons mainly with micropores (<2 nm) suffering from high ion-transfer resistance. In organic electrolyte solution, e.g., tetraethylammonium tetrafluoroborate in propylene carbonate (TEA·BF4 /PC) commonly used for EDLCs, PC-solvated TEA+ cations and BF4 − anions within pore space are considered to be adsorbed on the respective counter-charged pore surfaces, as is modeled by electric double-cylinder capacitors [8]. The adsorption of the solvated ions within the limited pore space and areas would be unfavorable in terms of interfacial ion charge densities and the distance between the electrode surface and the ion charge [9–11]. As for porous metal electrode, furthermore, there is a possibility that the adsorption of solvated ions do not sufficiently compensate the high charge carrier density of the metal interface. In this case, the use of solvation-free ions such as ionic liquids is expected to further increase EDL capacitance. In this study, we examined the effect of ionic liquids on the capacitance of porous Ni electrodes and discuss the capacitive behavior in comparison with a commercial activated carbon.

2. Experimental 2.1. Preparation and characterization of porous Ni materials Porous Ni was prepared by alkali-leaching method of a Ni–Al alloy (R20, Nikko Rika Co.) with the mass ratio of Ni/Al = 50/50. The alloy powders were immersed in 20% NaOH aqueous solution at 65 ◦ C for 3 h, and then suction-filtered with repeated washing (distilled water) until pH of the filtrate reached 7. Caution is that the resulting porous Ni is so active to react with air accompanying dull red heat. In addition, as we reported previously, the oxidation of porous Ni surface enhances the electrochemical resistance to deteriorate the performance as capacitor electrodes [1]. Therefore, after washing, the wet porous Ni powders were immediately put into a glass tube equipped with a PTFE screw-valve, and dried under vacuum at room temperature. The resulting porous Ni material corresponds to pNi-7 in our previous paper. Furthermore, the heat treatment of porous Ni was conducted under vacuum. Then, the dried powders (or vacuum heat-treated powders) retained in the glass tube was placed in Ar-filled glove box (VAC 102282OMNILAB) controlling the concentration of moisture (<0.15 ppm) and O2 (<0.5 ppm). The fabrication of the electrodes and the electrochemical evaluation of the capacitor cell were carried out in the glove box in no contact with air. The SSA and pore volume were determined by N2 -adsorption/desorption analysis at 77 K on ASAP2020 (Micrometrics Co.). The crystallographic structure was investigated by X-ray diffraction (XRD) analysis. The XRD patterns were recorded

on a Rigaku RINT 2200V/PCSV diffractometer with a Cu K␣-source operated at 40 kV and 20 mA. 2.2. Electrochemical evaluation In the preparation of electrodes, porous Ni powders were mixed with 5 wt.% carbon black as conductive additives and 5 wt.% PTFE binder. After being ground, the mixture was pressed to a 10-mm diameter of disk-shape electrode. The symmetrical capacitor cell was assembled by sandwiching a cellulose-type separator with the two disk-shape electrodes, which were supported by two PTFE-plate holders equipped with a Ag wire. The electrochemical evaluation was conducted using 1 mol/L TEA·BF4 /PC (Tomiyama Pure Chemical Industries Ltd.) and four kinds of ionic liquids; 1-ethyl-3-methyl-imidazorium·tetrafluoroborate (EMIm·BF4 , Kouei Chemical Co.), EMIm·bis(fluorosulfonyl)imide (EMIm·FSI, Daiichi-Kogyo Seiyaku Co.), 1-methyl-1-propyl-pyrrolidinium·FSI (MPPy·FSI, Daiichi-Kogyo Seiyaku Co.), 1-methyl-1-propylpiperidinium·FSI (MPPi·FSI, Daiichi-Kogyo Seiyaku Co.). The water impurities quoted by the manufacturers are 5 ppm in EMIm·BF4 and <30 ppm in EMIm·FSI, MPPy·FSI and MPPi·FSI. The presence of small amount of water decreases the viscosity of not only hydrophilic but also hydrophobic ionic liquids [12]. Furthermore, water impurity narrows the electrochemical potential window [13]. To avoid water contamination, therefore, the storage and handling of all the ionic liquids and the electrochemical measurement of porous Ni electrodes in the ionic liquids were conducted in the Ar-filled glove box mentioned above. The cyclic voltammetry (CV) was measured on VSP potentiostat/galvanostat (BioLogic Inc.) at the potential range of −1.25–1.25 V in a three-electrode system (pseudoreference electrode: Ag wire). For comparison, a high-capacitance-type activated carbon, i.e., MSP-20 (Kansai Netsukagaku Co.) was also evaluated in the same CV method. The capacitance values were calculated by dividing the integrals of the discharge currents of the CV curves with the applied voltage window (2.5 V) and potential sweep rates, and furthermore, the obtained capacitances were converted to gravimetric and volumetric capacitances by dividing with the mass and volume of working electrodes, respectively. The CSA was calculated by dividing gravimetric capacitances with the specific surface areas of the electrode materials. 3. Results and discussion As we reported previously [1], the porous Ni prepared without heating and air-contact provided the total capacitance of 31.9 F/cm3 in TEA·BF4 /PC (measured at potential sweep rate of 1 mV/s in a three electrode system). The total capacitance is based on EDL capacitance and the faradaic pseudocapacitance originating the active surface and bulk state of porous Ni. It was found that the faradaic currents in the CV curve, which showed dependence on the potential sweep rate, almost disappeared at 20 mV/s to make it possible to observe EDL capacitive currents. The calculated volumetric EDL capacitance at 20 mV/s was 10.8 F/cm3 , which were much lower than the maximum EDL capacitances (measured at 1 mV/s) of the commercial activated carbons (40.3–78.0 F/cm3 ). As shown in Table 1, however, the porous Ni material has far smaller SSA than those of the activated carbons. Therefore, the CSA of the porous Ni (10.2 ␮F/cm2 ) was proved to be higher than those of the commercial activated carbons (3.6–6.6 ␮F/cm2 ). In addition, the porous Ni is comprised mainly of mesopores with the average pore diameter (PD) of 7.2 nm, which is different from microporous activated carbons (PD < 1.9 nm, Table 1). The larger mesopores of porous Ni contribute to facile ion transport, leading to higher rate performance compared with the microporous activated carbons.

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Table 1 Specific surface areas (SSA), pore volumes and average pore diameters of porous Ni material and commercial activated carbons determined by N2 adsorption/desorption analysis. Sample I.D.

SBET a [m2 /g]

Vtotal b [cm3 /g]

Vmicro c [cm3 /g]

Vmeso d [cm3 /g]

Vmeso /Vtotal [%]

Avg. PDe [nm]

Porous Ni MSP-20f RP-15g RP-20g YP-17h

43.1 2370 1607 1870 1508

0.0636 1.009 0.637 0.792 0.628

0.00480 0.855 0.536 0.642 0.452

0.0588 0.154 0.101 0.150 0.176

92.5 15.3 15.9 18.9 28.0

7.20 1.70 1.67 1.79 1.89

a b c d e f g h

BET specific surface area. DFT total pore volume. DFT micropore volume. DFT mesopore volume. Average pore diameter. Phenol-resin-derived alkali-activated carbon (Kansai Netsukagaku Co.) and the pore parameters listed here were based on the remeasured data for this study. Phenol-resin-derived steam-activated carbons (Kuraray Chemical Co.). Coconut-shell-derived steam-activated carbon (Kuraray Chemical Co.).

3.1. Electrochemical evaluation of porous Ni electrodes in ionic liquids Fig. 1 shows the CV curves of the symmetrical capacitor cells using porous Ni electrodes and an activated carbon, MSP-20, in TEA·BF4 /PC and EMIm·BF4 , which were measured at 1 mV/s in three electrode system (pseudoreference electrode: Ag wire). As seen in Fig. 1(a), the faradaic anodic and cathodic peaks (0.91 V and −0.42 V, respectively), which are caused by the electrochemical modification of the electrodes, can be observed in both system of TEA·BF4 /PC and EMIm·BF4 . The whole current area is more broadened in the ionic liquid than in PC-solvated ion system, which

indicates the enhancement of EDL capacitance. The calculated volumetric total capacitance is 67.4 F/cm3 , which is more than twice as high capacitance as in TEA·BF4 /PC (31.9 F/cm3 ). We have confirmed that the faradaic currents observed in this study also disappear at 20 mV/s (not shown here), similar to in TEA·BF4 /PC. The calculated volumetric EDL capacitance from the CV curve at 20 mV/s is 16.8 F/cm3 , which is higher than 10.82 F/cm3 in TEA·BF4 /PC. On the other hand, MSP-20 showed typical capacitive CV profiles of butterfly shape (Fig. 1(b)), and only slight increase of the current area in the ionic liquid (the calculated volumetric capacitance; 78.0 F/cm3 in TEA·BF4 /PC, 79.3 F/cm3 in EMIm·BF4 ). Although the volumetric total capacitance of porous Ni approaches the high capacitance of MSP-20, the EDL capacitance excluding pseudocapacitance is much lower than that of MSP-20. However, note that the calculated CSA of porous Ni in the ionic liquid is 16.6 ␮F/cm2 , far higher than 7.0 ␮F/cm2 of MSP-20. For reference, carbide-derived carbons (CDC), which develop extremely large capacitance by the effect of subnanopores comparable to the ion size, provided ca. 13.3 ␮F/cm2 for EMIm·trifluoromethane-sulfonyl imide (TFSI) ionic liquids at 60 ◦ C [14], although it should be noted that the anion size is larger than BF4 − . The CSA value of porous Ni in the ionic liquid (16.6 ␮F/cm2 ) significantly increases compared with the CSA in TEA·BF4 /PC (10.2 ␮F/cm2 ), which is in sharp contrast to the slight increase for MSP-20 (6.6 ␮F/cm2 in TEA·BF4 /PC, 7.0 ␮F/cm2 in EMIm·BF4 ). The difference of CSA between the porous Ni and the activated carbon would be ascribed, partly, to the difference in the utilization efficiency of mesopores and micropores [15]. Table 2 shows the bare and PC-solvated ion sizes of TEA·BF4 and the ion sizes of ionic liquids employed in this study. The mesopores of porous Ni (average PD; 7.2 nm) is quite larger than the PC-solvated ions and the ionicliquid ions. The larger pore size ensures the utilization of pore space and surface. Therefore, the significant increase of the EDL capacitance and CSA for the porous Ni would be attributed to the increase of ion charge density on the pore surface with a high charge carrier density, due to the smaller sizes of the ionic-liquid ions. On the other hand, the micropores of MSP-20 (average PD; 1.7 nm) have

Table 2 Ion diameters of organic electrolyte and ionic liquids employed in this study.

Fig. 1. CV curves of (a) porous Ni electrode prepared without heating and air-contact and (b) a high-capacitance-type activated carbon, MSP-20, in TEA·BF4 /PC (blue line) and EMIm·BF4 ionic liquid (red line). The CV curves were measured at 1 mV/s in a three-electrode system (reference electrode: Ag wire). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Ion species

Bare ion diametera [nm]

PC-solvated ion diametera [nm]

TEA+ EMIm+ MPPy+ MPPi+ BF4 − FSI−

0.67 0.57 0.63 0.66 0.46 0.53

1.96

a

Calculated using Car–Parrinello method.

1.71

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Fig. 2. Potential-sweep-rate dependence of volumetric capacitance of porous Ni electrode in TEA·BF4 /PC (blue circle) and EMIm·BF4 ionic liquid (red circle). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

comparable sizes to the PC-solvated TEA+ and BF4 − . The observed slight change of EDL capacitance and CSA for MSP-20 suggests that the ion-adsorption utilization of pore space and surface is in almost the same level for both PC-solvated ions and ionic liquid, despite the smaller ion sizes of the ionic liquid than the PC-solvated ions. Some previous works reported the experimental results strongly suggesting that ionic-liquid ions are inserted into micropores comparable to the isolated ion size [14,16]. However, the CV data in our applied potential range (−1.25–1.25 V) suggests that the EDL formation would be derived from reorientation of ionic pairs within the micropores of almost the same size as utilized in PC-solvated TEA+ /BF4 − ions. This might be related to the EDL structures of ionic liquids and molten salts, that is, the cations and anions form alternating layered structure near the charged carbon surface due to the strong correlation between cations and anions [17–24]. Fig. 2 shows the potential sweep-rate dependence of the volumetric total capacitances calculated from the CV curves for the porous Ni in TEA·BF4 /PC and EMIm·BF4 . In the ionic liquid, the abrupt decrease of capacitance has occurred by a small variation of low sweep-rate region, in sharp contrast to the gentle decline observed in TEA·BF4 /PC. This result indicates the higher resistance in the ionic liquid. We previously reported the higher rate performance of the porous Ni than that of commercial activated carbons. The larger mesopores of porous Ni contributes to lower the iontransfer resistance, which is remarkable for microporous activated carbons. Nevertheless, the retention percentages of capacitance, i.e., (C100-mV/s /C1-mV/s ) × 100, in the ionic liquid decrease to 11.2%, which is much lower than 28.1% in TEA·BF4 /PC. The small difference of capacitance of pNi-7 at the high sweep-rate region in the two electrolyte systems implies that the electrode resistance (or electrochemical activity to increase surface and bulk resistance) is similar level in both electrolyte systems. Therefore, as one reason for the abrupt capacitance decrease in the low sweep-rate region, the high viscosity of ionic liquids would be responsible for the inferior rate performance due to low ion mobility [25]. As another, there might be a strong adsorption of ionic-liquid ions without solvation on the pore surface.

Fig. 3. XRD spectra of vacuum heat-treated porous Ni materials. (a) Heat-treated at 65 ◦ C, (b) 200 ◦ C, (c) 300 ◦ C, (d) 380 ◦ C.

electrochemical stability of porous Ni electrodes, there is a necessity to increase the crystallinity of porous Ni. Fig. 3 shows the XRD patterns of the porous Ni materials heat-treated up to 380 ◦ C under vacuum. The intense diffraction peaks observed at 2 = 44.6◦ , 51.9◦ and 76.5◦ are ascribed to (1 1 1), (2 0 0) and (2 2 0) reflections of facecentered cubic (fcc) Ni metals, respectively [JCPDS, no. 04-0850]. The full width at half maximum (FWHM) of each diffraction peak has become narrow with the increase of heat-treatment temperature, indicating the improvement of crystallinity. Fig. 4 shows the CV curves of the capacitor cells using vacuum heat-treated porous Ni electrodes in EMIm·BF4 (potential sweep rate: 1 mV/s). The faradaic currents have almost disappeared for the samples heat-treated at more than 300 ◦ C, and the CV profiles have become close to the rectangular shape characteristic of EDL capacitive currents. The correlation of the calculated volumetric capacitance with the FWHM of Ni(1 1 1) diffraction peak is shown in Fig. 5. The increase of crystallinity has led to the decrease of volumetric total capacitance due to the disappearance of

3.2. Vacuum heat-treatment stabilization of porous Ni materials The porous Ni prepared without heating and air-contact is so active that the surface and bulk resistances are significantly enhanced by the electrochemical modification with the repetition of charge/discharge cycle, resulting in the decrease of the total capacitance. The increase of resistance is considered to result from a microstructural change of the low-crystallinity porous Ni and a crucial rearrangement of the pore structure [1]. To increase the

Fig. 4. CV curves of vacuum heat-treated porous Ni electrodes in EMIm·BF4 . The CV curves were measured at 1 mV/s in a three-electrode system (pseudoreference electrode: Ag wire). The blue solid line shows the vacuum heat-treated sample at 65 ◦ C, and red, green and purple dashed lines correspond to the samples treated at 200 ◦ C, 300 ◦ C and 380 ◦ C under vacuum, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. The vacuum heat-treatment-temperature dependence of the volumetric capacitance (red square) of porous Ni electrodes in EMIm·BF4 and the FWHM (blue diamond) of Ni(1 1 1) XRD peak. The volumetric capacitances were calculated from the CV data of Fig. 4. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

pseudocapacitance. This result implies the improved electrochemical stability of porous Ni by the vacuum heat treatment. Furthermore, the volumetric capacitance of 380 ◦ C-treated sample is very close to the EDL capacitance (16.8 F/cm3 ) calculated from the CV data of untreated porous Ni at 20 mV/s. Therefore, the high CSA of porous Ni (16.6 ␮F/cm2 ) can be almost retained even after the vacuum heat-treatment stabilization. As shown in Fig. 6, the cycle test of the 380 ◦ C-treated sample by repetition of CV measurement at 100 mV/s in EMIm·BF4 has shown good reproducibility for EDL capacitive currents. Furthermore, the charge/discharge efficiency was kept constant at nearly 100% up to 1000 cycles while the volumetric EDL capacitance increased with cycles. As reported in our previous paper, the sample prepared without air-contact and heating (pNi-7) showed the capacitance decrease during charge/discharge cycles, which is caused by the increase of surface and bulk resistance originating from the electrochemical modification of low crystalline porous Ni electrode. In contrast, the results obtained here demonstrate the improved electrochemical stability of the vacuum heat-treated porous Ni electrode with a high capacitance surface, which is modified better with charge/discharge cycles. Considering this, the

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Fig. 7. CV curves of porous Ni electrodes in various ionic liquids; EMIm·BF4 (blue solid line), EMIm·FSI (red dashed line), MPPy·FSI (green dashed line) and MPPi·FSI (purple dashed line). The porous Ni heat-treated at 65 ◦ C under vacuum was used as the electrodes, and the CV curves were measured at 1 mV/s in a three-electrode system (pseudoreference electrode: Ag wire). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

ohmic behavior deviated from a capacitive rectangular shape (the inset in Fig. 6) would be attributed largely to the electrolyte resistance (high viscosity and low conductivity of ionic liquids) at the high potential sweep rate of 100 mV/s, rather than the electrode resistance. It was previously reported that microporous activated carbon fibers with mean micropore width of 0.66–0.74 nm showed a crucial irreversible adsorption of cations in EMIm·BF4 , resulting in the abrupt decline of capacitance within several cycles [16]. Furthermore, the irreversibility of ion adsorption is suggested to be improved by a facile ion transfer (short ion path length). Also in this view, mesoporous Ni will be advantageous to cycle performance. 3.3. Capacitive behaviors of porous Ni electrodes in ionic liquids with different ion sizes We examined the influence of different ionic-liquid ions on the capacitive behavior of porous Ni. Fig. 7 shows the CV curves of porous Ni electrodes in four kinds of ionic liquids including EMIm·BF4 (three-electrode measurement at 1 mV/s). The sizes of cation and anion of each ionic liquid are listed in Table 2. The viscosity and conductivity of the ionic liquids and the calculated volumetric total capacitance and CSA are summarized in Table 3. The difference of cation size in EMIm·FSI, MPPy·FSI and MPPi·FSI showed a simple ordinality of capacitance (cation-size order: MPPi+ > MPPy+ > EMIm+ , capacitance order: EMIm+ > MPPy+ > MPPi+ ). However, the larger capacitance in EMIm·FSI than in EMIm·BF4 composed of the same cation and Table 3 Physical properties of ionic liquids and volumetric total capacitance and CSA of porous Ni electrode in each ionic liquid.

Fig. 6. Charge/discharge efficiency (blue diamond) and volumetric capacitance (red square) of the 380 ◦ C-vacuum heat-treated porous Ni electrode by 1000-times repetition of CV measurement (inset figure) at 100 mV/s in EMIm·BF4 . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Ionic liquid

Viscositya [mPa s]

Conductivitya [mS/cm]

Volumetric total capacitanceb [F/cm3 ]

CSA c [␮F/cm2 ]

EMIm·BF4 EMIm·FSI MPPy·FSI MPPi·FSI

32 17.0 39.3 87.0

13.0 16.5 8.3 3.7

49.2 61.9 38.0 25.7

16.2 13.8 6.9 2.5

25 ◦ C. The volumetric capacitance was calculated from the CV data (Fig. 7) recorded at 1 mV/s. c The CSA values were calculated using the CV data recorded at 20 mV/s. a

b

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the smaller size of anions implies not a simple size effect on capacitance. As seen from Table 3, the order of volumetric total capacitance has a good correlation to the ordinality of viscosity and conductivity of all ionic liquids. Therefore, it is considered that the viscosity and conductivity rather than ion sizes would contribute to the higher total capacitance (including pseudocapacitance). However, the CSA values are higher with the decrease of cation and anion sizes. These results indicate that the pseudocapacitive behavior (which is slow faradaic reaction) is affected by the charge-transfer resistance derived from the low viscosity and conductivity of ionic liquids, whereas the charge accommodation on the electrode/ionicliquid interface is dependent on ion sizes rather than the physical properties of ionic liquids. Furthermore, it should be noted that the CV shapes in EMIm·BF4 and EMIm·FSI are significantly varied not only in positive potential region but also in negative potential region. This suggests that the difference of anion has an impact on the charge/discharge manner of cation. Also in the different ionic liquids with the same FSI anion, the effect has spilled over the CV profiles corresponding to the charge/discharge of cations. Previous studies on molecular dynamics of ionic liquids show the electrode-potential dependence of distribution and orientation of cations and anions within their multiple layers constructed at the interface of the charged carbon electrodes [26,27]. In other work, it has been reported that the size of anions affects the distribution of cations near both positive and negative electrodes fundamentally due to the different attractions between cations and anions with different sizes [18]. Furthermore, the charge delocalization of ionic-liquid ions with complex shapes can affect the long-range electrostatic interactions in EDL, so that the mean electrostatic force determining the EDL structure depend on the ion orientation as well [28]. Considering these previous findings, not only the size but also shape of both cations and anions are significant factors to determine EDL structures near positive and negative electrodes. As outstanding examples, imidazolium cations with different lengths of alkyl chains have provided different or unique EDL structures with the chain length [29,30]. Therefore, we consider that, also in porous Ni electrodes, the strong correlation of cations and anions would affect the charge/discharge behaviors of both positive and negative electrodes regardless of the difference in either cations or anions.

4. Conclusions The porous Ni material of low crystallinity (prepared via dry process without heating and air-contact) has provided the high volumetric total capacitance (including pseudocapacitance) of 67.4 F/cm3 (three-electrode evaluation) in EMIm·BF4 ionic liquid, which approaches 79.3 F/cm3 of a high-capacitance-type activated carbon, MSP-20. The rate performance of the porous Ni electrode in the ionic liquid is more deteriorated than in the conventional TEA·BF4 /PC electrolyte solution, despite the advantageous mesoporous structure to ion transfer. The volumetric total capacitance of porous Ni electrodes is varied depending on the physical properties of ionic liquids, whereas the pure EDL capacitance is simply dependent on the cation an anion sizes of ionic liquids. Furthermore, the size and shape of cations or anions affect the charge/discharge behaviors in both positive and negative electrodes. Significantly, the EDL unit-area capacitance, CSA , of the porous Ni electrode has increased from 10.2 ␮F/cm2 in TEA·BF4 /PC to 16.6 ␮F/cm2 in EMIm·BF4 , which is much higher than 7.0 ␮F/cm2 of MSP-20. The heat treatment at 380 ◦ C under vacuum increases the electrochemical stability of porous Ni materials, resulting in the exclusion of pseudocapacitance and the charge/discharge efficiency of almost 100% up to 1000 cycles. Although the total capacitance of the stabilized porous Ni decreases due to the disappearance of

pseudocapacitance, the CSA can be retained at almost the same high level as that of the untreated porous Ni. In the present stage, the SSA of porous Ni materials stays in not so high value of 43 m2 /g. However, the high CSA value and improved stability of vacuum heat-treated porous Ni provide a certain positive prospect for the development of high-performance EDLC electrode materials, although there remains a necessity to further enhance the SSA of porous Ni materials. References [1] N. Kobayashi, H. Ogata, K.C. Park, K. Takeuchi, M. Endo, Investigation on capacitive behaviors of porous Ni electrodes for electric double layer capacitors, Electrochimica Acta 90 (2013) 408. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845. [3] J.P. Randin, E. Yeager, Differential capacitance study on the edge orientation of pyrolytic graphite and glassy carbon electrodes, Electroanalytical Chemistry and Interfacial Electrochemistry 58 (1975) 313. [4] R.J. 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