Highly Durable, Self-Standing Solid-State Supercapacitor Based on

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Highly Durable, Self-Standing Solid-State Supercapacitor Based on an Ionic Liquid-Rich Ionogel and Porous Carbon Nanofiber Electrodes Silas K. Simotwo,† Parameswara Rao Chinnam,*,‡ Stephanie L. Wunder,‡ and Vibha Kalra*,† †

Department of Chemical and Biological Engineering, Drexel University, 3141 Chestnut Street, 19104 Philadelphia, Pennsylvania, United States ‡ Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *

ABSTRACT: A high-performance, self-standing solid-state supercapacitor is prepared by incorporating an ionic liquid (IL)-rich ionogel made with 95 wt % IL (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and 5 wt % methyl cellulose, a polymer matrix, into highly interconnected 3D activated carbon nanofiber (CNF) electrodes. The ionogel exhibits strong mechanical properties with a storage modulus of 5 MPa and a high ionic conductivity of 5.7 mS cm−1 at 25 °C. The high-surface-area CNF-based electrode (2282 m2 g−1), obtained via an electrospinning technique, exhibits hierarchical porosity generated both in situ during pyrolysis and ex situ via KOH activation. The porous architecture of the CNF electrodes facilitates the facile percolation of the soft but mechanically durable ionogel film, thereby enabling intimate contact between porous nanofibers and the gel electrolyte interface. The supercapacitor demonstrates promising capacitive characteristics, including a gravimetric capacitance of 153 F g−1, a high specific energy density of 65 W h kg−1, and high cycling stability, with a capacitance fade of only 4% after 20 000 charge− discharge cycles at 1 A g−1. Moreover, device-level areal capacitances for the gel IL cell of 122 and 151 mF cm−2 are observed at electrode mass loadings of 3.20 and 5.10 mg cm−2, respectively. KEYWORDS: self-standing electrodes, ionogel, carbon nanofibers, porosity, solid state, mass loading, life cycle, conductivity

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INTRODUCTION Electric double-layer capacitors (EDLCs) are a class of electrochemical capacitors/supercapacitors energy storage mechanism of which entails a reversible electrostatic accumulation of charges at the electrode/electrolyte interface.1 EDLCs are generally characterized by fast charge−discharge kinetics, high power density, and long life cycle performance. These features make EDLCs suitable for applications in fields such as portable electronics, medical devices, and electric automobiles where they complement batteries for instant power delivery. Despite their exceptionally high power capability, applications of EDCLs in high energy demand areas are limited by their inadequate specific energy density (SED) (typically below 10 W h kg−1). Given that the energy stored by a capacitor is proportional to its capacitance and to the square of the operating voltage window (OVW), the energy density of EDLCs can be enhanced by improving the capacitance and/or potential window of the electrodes. The use of carbon-based nanomaterials such as graphene with hierarchical porous morphologies provides a feasible alternative to boost the capacitance of the EDLC electrodes. The voltage window can be enhanced via two strategiesadoption of nonaqueous electrolytes such as organic and ionic liquids (ILs) to replace © XXXX American Chemical Society

aqueous electrolytes and utilization of asymmetric cell configurations.2−4 Adoption of IL-based electrolytes promises to be the superior alternative in the development of highvoltage supercapacitors because of their nonflammability, negligible volatility, and exceptionally high thermal and electrochemical stability.5 The absence of solvents enables ILs to operate over a wide potential window spanning from 3 to 5 V.6 Such a wide potential window not only improves the energy storage capability but also aids in leveling the voltage range between supercapacitors and batteries in applications where these devices complement each other. The application of liquid IL electrolytes in electrochemical capacitors is hampered by potential environmental hazards due to leakage, corrosion, and packaging issues intrinsic to liquid electrolytes. Therefore, there is currently a marked interest in developing ionogel-based supercapacitors to overcome such challenges. However, a significant number of solid-state supercapacitors developed thus far are mostly based on aqueous electrolytes which possess a limited OVW.7−9 Recent Received: May 26, 2017 Accepted: September 5, 2017

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DOI: 10.1021/acsami.7b07479 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Fabricated Solid-State Self-Standing 3.5 V Symmetric Device with IL-Rich Ionogel and Activated Carbon Nanofiber Electrodes

(a-PCNF)-based electrode is highlighted by its 3-D interconnected porous nanofibers with a high surface area of 2282 m2 g−1. Such an electrode framework permits seamless infusion of the gel electrolyte, leading to intimate contact at the porous carbon/gel electrolyte interface and therefore co-continuous networks of electrons and ion-conducting pathways. Furthermore, the facility with which the gel IL is integrated within the porous electrodes enabled the usage of a high electrode mass loading ranging from 3.20 to 5.10 mg cm−2. A capacitance of 153 F g−1 at 20 mV s−1 was obtained for the cell (based on the mass of one electrode), corresponding to a SED of 65 W h kg−1 (based on the total mass of both electrodes) at a 3.5 V window. Such an SED capability compares admirably with that of 69 W h kg−1 obtained for neat IL at the same scan rate. The merits of the high electrode mass loading are reflected by high devicelevel areal capacitances of 122 and 151 mF cm−2 obtained at loadings of 3.20 and 5.10 mg cm−2, respectively. A marginal capacitance fade of only 4% was observed for the cell over 20 000 cycles at 1 A g−1. Postmortem characterizations of the electrode using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and X-ray diffraction (XRD) show that the electrode/gel electrolyte system remains relatively unperturbed during the course of cycling.

efforts have been focused on immobilizing ILs within the frameworks of organic materials (e.g., polymers/copolymers) and/or inorganic compounds (e.g., silica) without compromising the integral properties of the pure IL for the preparation of advanced, solid-state supercapacitors.10−14 In spite of the promising electrochemical stability of ionogels over voltage ranges similar to those exhibited by neat ILs,5 shortcomings regarding the high content of nonconductive/electrochemically inert host materials required to fabricate mechanically stable gel IL film (usually >20 wt %) remain to be resolved.15,16 The high content of IL immobilizers leads to reduced ionic conductivity and consequently low electrochemical performance. Moreover, majority of carbon-based electrode materials used in such cell configurations require binders, thus further elevating the content of inactive materials in the overall cell configuration.17 Additionally, the resulting electrode architectures lack welldefined pathways which would allow facile electrolyte diffusion throughout the network. The poor permeability of such electrodes by gel electrolytes has led to reports of unrealistically low electrode mass loadings. Hence, to further advance the work on solid supercapacitors, there is a need to develop mechanically durable ion gel films at low content of the host material as well as engineer electrodes with an appropriate macrostructure to permit uniform infusion of gel electrolytes. In this work, we have prepared a solid-state supercapacitor based on IL-rich ionogels and freestanding, highly porous 3-D carbon nanofibers (CNFs). The ion gel was prepared by the entrapment of 95% (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) in a matrix of 5% biofriendly and nature-abundant methyl cellulose (MC). The EMImTFSI/MC film is characterized by high flexibility, an ionic conductivity of 5.7 mS cm−1, high mechanical resiliency with a storage modulus of 5 MPa at 25 °C, and a wide thermal stability range (−15 to ∼300 °C). The freestanding CNF-based electrodes were prepared by the electrospinning technique. The nanofibers exhibit a range of micro- and mesopores generated both in situ during pyrolysis and ex situ via KOH activation as previously reported in our work.18 The activated porous CNF

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MATERIALS AND METHODS

Fabrication of a-PCNFs. Detailed preparation of the a-PCNFs is reported here.19 Briefly, a 30/70 w/w blend of polyacrylonitrile (PAN) (MW = 150 000 g mol−1, Sigma-Aldrich) and Nafion powder (obtained by drying LIQUION 1115) was dissolved in dimethylformamide (DMF) under gentle heating and stirring for 4 h, followed by electrospinning. The polymer blend comprised 21 wt % of the DMFbased solution. The resultant nanofiber mat was then stabilized at 280 °C in air for 5 h, followed by carbonization at 1000 °C for 60 min under a nitrogen atmosphere. During the carbonization process, Nafion was thermally decomposed out to generate predominantly mesoporous nanofibers. Chemical activation of the electrospun CNFs was achieved by soaking the nanofiber mat in 30 wt % KOH solution overnight and blotting it with lint-free paper upon removal. The B


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Figure 1. Comparison of the neat IL and gel IL: log σ vs 1000/T (a), second heating DSC thermograms (b), TGA thermograms (c), and storage and loss moduli as a function of frequency for the gel IL at 25 °C (d). sample was then heat-treated to and kept at 800 °C for 30 min under nitrogen flow. Preparation of the Electrolyte Gel. The preparation procedure for the ionogel is similar to that of our previously reported work.20 Briefly, for preparing 95/5 EMImTFSI/MC ionogel, 30 mg of MC was dissolved in 2 mL of DMF and 570 mg of EMImTFSI was dissolved in 1 mL of DMF separately at room temperature (RT). EMImTFSI is readily soluble in DMF, but MC takes ∼1 h for complete dissolution. The two solutions were added together and left to stir overnight at RT, then heated to 90−100 °C for 45 min, and cooled to RT, during which time (approximately 20 min) a gel (comprised of EMImTFSI/MC/ DMF) formed. Note that the vial containing the gel was tightly sealed and kept in a glovebox for later use to make film and/or infuse into the a-PCNF electrodes. To make thin films, the gels were reheated to 90− 100 °C (liquefied) and cast onto Teflon sheets, where upon cooling they reformed a gel. The thin gel films were left overnight at RT under N2, during which time the DMF evaporated, leaving a clear film. These films were further dried in vacuum at 120 °C for 24 h to remove residual DMF. Scheme 1 represents the fabrication of the gel IL-based supercapacitor. Material Characterization. Differential scanning calorimetry (DSC) (Hi-Res TA 2920) was used to obtain glass-transition (Tgs), crystallization (Tc), and melting (Tm) temperatures of the ionogels at 10 °C min−1 under N2 purge. The sample was scanned from 25 °C to 100 °C, 100 °C to −100 °C, and −100 °C to 100 °C with the second heating scans reported. Thermogravimetric analysis (TGA) (Hi-Res TA 2950) was employed to obtain the wt % of MC and EMImTFSI in the ionogels during which the samples were scanned at 10 °C min−1 from 25 to 800 °C. Dynamic mechanical analysis (TA Instruments DMA Q800) was used to measure the mechanical properties and glasstransition temperatures of the gel IL with a preload force of 2.0 mN, a constant frequency of 1 Hz, and an amplitude of 15 μm. Ionogel film samples, ∼12 mm length × 6.3 mm width × (0.01−0.1 mm) thickness, were equilibrated at 0 °C for 5 min. Measurements were taken from 0 to 250 °C at a heating rate of 2 °C min−1. Frequency sweep data were obtained on the gel IL at 25 °C over the frequency range 1−100 Hz. SEM (Zeiss Supra 50VP) was employed to characterize the surface

morphology of the nanofibers and to observe the integration of the gel electrolyte within the nanofiber mat pores. XRD (Rigaku SmartLab, Xray diffractometer, Cu Kα, scanning range 10−50°, and step size of 0.02°) was used to probe the chemical structure, composition, and crystallinity of the gel IL and a-PCNFs. The specific surface area of the CNFs was measured using the nitrogen sorption isotherm at 77 K (Autosorb-1, Quantachrome). All samples were initially degassed at 200 °C under vacuum for 24 h prior to the measurements. The pore size distribution was calculated based on adsorption−desorption curves using the quenched solid density functional theory with the assumption of slit-shaped pores. Electrode Assembly and Testing. The a-PCNF mat (thickness of 100−180 μm) was punched into freestanding electrodes with 3/8 in. diameters (Figure S1a). The mass loading of the electrodes used was 3.20 mg cm−2 for both the electrodes. The two symmetrically punched out electrodes were placed in separate Petri dishes in a conventional oven preheated to 100 °C. The aforementioned EMImTFSI/MC/DMF gel was also preheated to 100 °C under gentle stirring for ∼10 min (the vial containing the gel was tightly sealed during the heating process to prevent the premature evaporation of DMF). The gel (80−100 μL) was swiftly loaded onto each electrode, and the Petri dishes were sealed to slow down the rate of DMF evaporation during the infusion process. The electrodes were allowed to soak for 20 min, quickly assembled together using a Celgard 3501 separator, and pressed between two Teflon plates using a binder clip (Figure S1b). The assembly was placed back in the vacuum oven at ∼100 °C. After 1 h, the two Teflon plates were removed and the freestanding gel IL cell (Figure S1c) was allowed to dry at 90 °C for 48 h under vacuum to remove residual DMF and any moisture present. The cell was then transferred to an argon-filled glovebox which is equipped with electrical feedthroughs to connect the cell to an external potentiostat. A two-way Swagelok cell and stainless steel current collectors were used during electrochemical testing (see the Supporting Information, Figure S1d). The electrochemical behavior of the cell was investigated using cyclic voltammetry (CV), electrochemical impedance (100 kHz to 5 mHz), and galvanostatic C


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Figure 2. SEM micrographsgel IL film (a), a-PCNFs (b), a-PCNFs filled with the gel IL (c), and electrode/separator/electrode cross section filled with the gel IL (d). The inset scale bars are 200 nm (b) and 1 μm (c). charge−discharge. The specific capacitance (SC) of the cell was calculated from CV using Expression 1.

Cs =

1 ∫ I dV 2v×m×V

The conductivity of the gel IL is only 1.5−1.8 times lower than that of the neat IL, which is reasonable for supercapacitor applications because of the small difference in the equivalent series resistance (ESR) values of the gel IL (5 Ω) and neat IL (2 Ω) as discussed below. Figure 1b shows the comparative DSC curves for the gel IL and neat IL in which the gel IL shows similar but weaker crystallization/melting phenomena compared with the neat IL as we have observed previously.20 This weaker crystallization may be due to the interaction of the IL and the MC nanofibers. We have previously shown that butyl-N-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide (PYR14TFSI) is microphaseseparated (trapped in the semicrystalline MC nanofibers) in MC.20 With IL used in this study, there appears to be a greater interaction between EMImTFSI and MC so that swelling of the MC nanofibers may occur, as indicated by the decrease and broadening of the glass-transition temperature (Tg) compared with the neat MC. In general, low thermal stability of the electrolyte limits the high-temperature operation of supercapacitor devices. Figure 1c shows the comparative TGA curves for the gel IL and the neat IL. The decomposition temperature (Td) of the gel IL is limited by the MC decomposition (red) at around 375 °C which corresponds to a 5% loss of MC. The major weight loss peak (95%) at around 420 °C originates from the IL and matches the neat IL weight loss profile (blue). The high-temperature thermal stability of the gel IL is promising for high-temperature supercapacitor applications. The storage and loss moduli at 25 °C between 1 and 100 Hz (Figure 1d) for the gel IL clearly show a solidlike behavior in this frequency range. From Figure 1d, the storage modulus of the gel IL is 5 MPa at 25 °C, which is the highest value reported using 95% liquid trapped in a polymer network. Moreover, the gel IL is soft and binds to and maintains good contact with the electrodes. The compatibility of the gel IL and the electrode may be due to the soft film (but mechanically strong) with a very high loading of IL (95%), which facilitates high electrochemical performance. Electrode Preparation and Incorporation of the Gel Electrolyte. The electrospinning technique was employed to

(1) −1

where Cs, v, I, m, and V represent the capacitance (F g ), the scan rate, the current (A), the mass of one electrode (g), and the voltage window (V), respectively. Similarly, the SC was obtained from charge−discharge curves using Expression 2

Cs =

4 × Δt × I ΔV × m

(2)

where I is the constant discharge current, m is the mass of one electrode, Δt is the discharge time, and ΔV is the potential window. The specific energy (E) and power density (P) of the cells were calculated as follows

E=

1 Cs × V 2 8

P = E /Δt Ca =

m × Cs 4×A

(3) (4) (5)

where Ca is the area (A)-normalized capacitance.

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RESULTS AND DISCUSSION Ionic Conductivity and Thermal and Mechanical Properties of the Gel IL Electrolyte. A series of EMImTFSI/MC gel electrolytes at various compositions of MC ranging from 5 to 40 wt % were prepared, and their conductivity and thermal and mechanical properties were investigated as shown in the Supporting Information (Figure S2a−d). The 95/5 EMImTFSI/MC was used for the supercapacitor application because it had the highest ionic conductivity and still had reasonable mechanical properties. Henceforth, the 95/5 EMImTFSI/MC composition will be referred to in the paper as the gel IL. Figure 1a shows the temperature dependence of the ionic conductivities of the gel IL compared with those of the neat IL. D


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Figure 3. EIS data for the liquid IL cell and gel IL cell (a) and corresponding CV curves at 100 mV s−1 (b). Note that all gel IL cells were made using the a-PCNF electrodes.

generate PAN/Nafion nanofibers with 30/70 weight fraction. The resulting nanofiber mats were stabilized in air at 280 °C for 5 h and further pyrolized at 1000 °C for 60 min under nitrogen flow. During this pyrolysis process, simultaneous carbonization of PAN and thermal decomposition of Nafion occurred (at 90 °C) for a period of 20 min in an enclosed setup (to minimize DMF evaporation and prevent increase in viscosity during infusion). The cell/capacitor was then rapidly assembled using a Celgard 3501 separator and pressed between two Teflon plates. This setup was kept in a vacuum oven at 90 °C for 1 h prior to disassembly where further infusion of the gel within the electrodes was expected to occur (although with a gradual loss of DMF). Infusion of the EMImTSFI/MC/DMF at an elevated temperature was necessary because the solution was a solid gel at RT and would not be able to diffuse within

the hierarchical pores of the activated nanofibers. Figure 2c,d provides the top-view and cross-sectional SEM images of the cell, respectively, for the a-PCNF-based electrodes filled with the gel electrolyte. The gel is uniformly filled within the interfiber macropores, giving an intimate electrode/electrolyte contact for efficient transport of ions and electrons to the electrode/electrolyte gel interface. The gel IL-based cell showed a good mechanical integrity, with both the electrodes remaining firmly adhered to the separator even under mechanical duress during handling and electrochemical testing. The electrochemical activity in the liquid IL-cell (control) and gel IL-cell (both using symmetric a-PCNF electrodes) was first evaluated using electrochemical impedance spectroscopy (EIS) technique. Figure 3a shows the Nyquist plots for the liquid IL cell (blue) and gel IL cell (red). Both plots exhibit the characteristic features of porous electrodes: a semicircle at a higher frequency attributed to the charge-transfer resistance, a 45° line in the midfrequencies assigned to the Warburg diffusion regime, and a near vertical line in the lower frequencies corresponding to ion diffusion within the electrodes. The ESR is higher for the gel IL cell (6.3 Ω) relative to that for the liquid IL cell (3.1 Ω). The higher ESR is attributed to the slightly diminished conductivity of the EMImTFSI ions due to their confinement within the semicrystalline MC nanofiber network. Moreover, as shown in the SEM image in Figure 2c, the gel IL forms a thin layer over the CNF surface which is likely to increase the electron-transfer resistance at the current collector/electrode interfacenote that the gel IL is a poor electron conductor. Nonetheless, the ESR value reported herein for the solid electrolyte is on the lower range compared to most values reported in the literature.22 The charge-transfer resistance is also higher for the gel IL cell (5.2 Ω) compared to that of the liquid IL cell (4.1 Ω) as expected because of the slower ion mobility to the gel IL/electrode pore interface. Only a small increase in charge-transfer resistance for the gel IL (∼1.1 Ω) is a confirmation of the seamless contact at the nanofiber/gel IL interface, which permits quick charge transfer. Knee frequency/characteristic frequency ( fo) reflects the point where the impedance of a cell starts to be dominated by a capacitive behavior.23 It is the highest frequency where most of the capacitance of a cell is retained and is inversely related to the cell time constant (to). From complex power analysis,24 the characteristic frequencies of the liquid IL cell and gel IL cell were ascertained to be 0.32 and 0.18 Hz, respectively (see the Supporting Information Figure S4a,b and eqs 1−5). This represents the time constants of 3.1 and 5.5 s, respectively. E


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ACS Applied Materials & Interfaces Figure 3b shows the CV plots obtained at 100 mV s−1, where the blue (dotted) and red lines represent the CV curves for the liquid IL cell and gel IL cell, respectively. An electrochemical potential window of up to 3.5 V was achieved for both the electrolyte system, indicating a high electrochemical stability of the gel IL. Both capacitors exhibit near-rectangular CV curves indicative of fast kinetics in both systems at such high scan rates. A drop in capacitance of 19% was observed for the cell based on the solid electrolyte compared to the cell based on the liquid electrolyte. The slight decay in capacitance for the solid capacitor is possibly due to the entrapment of IL ions within the MC network which would likely impede their diffusion rates/mobility to the double-layer region compared to the neat IL ions (see the time constants mentioned above). Furthermore, at all temperatures, the IL/MC/DMF solution is slightly more viscous than the liquid electrolyte and, as a result, might access less of the micropores within the electrode compared to the liquid electrolyte. Table 1 shows the

20 and at 100 mV s−1, respectively. The promising performance of the ionogel-based cell over the scan rate range can be attributed to the uniform percolation of the gel electrolyte within the 3-D framework of the a-PCNF electrode(s), leading to co-continuous pathways for electrons and ion transport to the electrolyte/electrode pore interface. Comparatively, the liquid IL cell achieved capacitances of 163 and 145 F g−1 at 20 and 100 mV s−1, respectively, operating at the same voltage window. From Expression 3, the energy density of the solid cell was calculated to be 65 W h kg−1 at a power density of 1150 W kg−1 based on the mass of both electrodes. This compares favorably with the energy density values obtained for the liquid IL cell (69 W h kg−1) at the same power density. Galvanostatic charge−discharge was further employed to study the electrochemical performance of the gel IL cell at various current densities. The charge−discharge profiles at various current densities shown in Figure 4b exhibit linear triangular profiles, indicating a high capacitance performance. The SC calculated using Expression 2 is shown in Figure 4d (closed circles). Capacitances of 162 and 118 F g−1 were obtained for the gel IL cell at 0.5 and 5 A g−1, respectively. These represent 72% retention in capacitance, at 10-fold magnitude increase in current density. Corresponding values for the liquid IL cell (Figure 4d, closed squares) were 161 and 152 F g−1 obtained at 0.5 and 5 A g−1, respectively. Therefore, a similar performance was observed at low current densities for the liquid- and solid-based cells. However, the liquid IL cell displays a better power handling capability at higher current density because of its slightly higher ionic conductivity and lower time constant in the liquid electrolyte as seen in the EIS data. The Ragone plot is shown in Figure S5a. One of the hallmarks of an EDL supercapacitor is its outstanding life cycle performance. The life cycle performance

Table 1. SC as a Function of Scan Rate for the Control and IL Gel-Based Cellsa SC (F g−1) scan rate (mV s−1)

liquid IL cell

gel IL cell

20 50 100

163 153 145

153 132 118

a

Solid ILs exhibit good rate capability when cycled at higher scan rate/ current density.

capacitance of the liquid IL cell and gel IL cell at various scan rates, and Figure 4a,b shows the corresponding CV plots. The gel IL cell delivered capacitances of 153 and 118 F g−1 at

Figure 4. CV curves at different scan rates for the gel IL cell (a) and liquid IL cell (b), charge−discharge curves for the gel IL cell (c), and capacitance at different current densities for the gel IL cell (red, closed circles) and liquid IL cell (blue, closed squares) (d). F


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Figure 5. Life cycle performance at 1 A g−1 for 20 000 cycles (a), EIS spectra before and after cycling (b), postmortem SEM analysis (c), and XRD patterns before and after cycling (d) for the gel IL-based cell.

of the solid-state supercapacitor reported herein was examined at a rate of 1 A g−1, as shown in Figure 5a. The cell exhibited a mere 4% loss in capacitance after 20 000 continuous cycles, after which the cycling was stopped for postmortem studies. The impedance data for the gel IL cell (Figure 5b) recorded prior to and after the cyclability test show a minimal change in impedance parameters, indicating that the hybrid gel IL suffered a negligible degradation during the course of cycling. Moreover, the intimate porous carbon/gel electrolyte contact was preserved during the life cycle test as observed by SEM (Figure 5c), which is attributed to the gel IL mechanical resiliency. Preservation of such a contact is paramount to ensure that the charge-transfer process to the double-layer region is not disturbed. The XRD pattern of the pristine gel IL film shows peaks at 2θ = 12.5° and 21° ascribed to the semicrystalline MC phase, as shown in Figure 5c. The XRD patterns of the gel IL/electrode before and after cycling show no observable alteration in the position of the diffraction peaks ascribed to the host polymer which further corroborates the stability of the gel IL/electrode system. The high SED of 65 W h kg−1 and long life cycle performance reported in this work are very competitive compared to those of other works on solid-state supercapacitors reported thus far. The promising electrochemical performance is a result of complementary combination of the desirable electrode architecture and the high IL loading of the electrolyte gel. The flexible gel IL binds to and maintains good contact with the a-PCNFs, which facilitates efficient transport of charges to and from the electrolyte/electrode interface as evidenced by the low charge-transfer resistance. The compatibility between the gel IL and the electrode, due to the soft but mechanically strong film, and the IL-rich gel electrolyte (95%) likely contributes to the stable cycling performance as well. The freestanding electrode characterized by hierarchical pores and interfiber porosity provides well-defined pockets to accom-

modate the gel IL. The macrolevel interfiber porosity within the 3-D electrode structure allows the percolation of the gel IL with relative ease. The impregnation of the gel IL in the electrodes with such a facility permits the usage of higher electrode mass loading (3−5 mg cm−2), as shown in Table 2. Many ionogelTable 2. Electrochemical Performance of the Gel IL Cell at Different Mass Electrode Loadingsa mass loading (mg cm−2)

gravimetric capacitance (F g−1)

areal capacitance (mF cm−2)

3.20 3.50 5.10

153 145 119

122 127 151

a Capacitances are based on the CV curves at 20 mV s−1. The SC (F g−1) is reported per weight of a single electrode, whereas the areal capacitance is reported per complete device.

based works reported in Table 3 employ electrodes devoid of clearly defined pathways for gel IL transport within such electrode morphologies, leading to poor electrolyte integration. To offset such an unsatisfactory electrolyte infusion, thin electrodes and/or impractically low electrode mass loadings are occasionally employed, as shown in Table 3. Additionally, electrochemically inert materials are introduced to both the electrodes (not freestanding) in the form of binders and the ionogels because of the high content of host materials (typically ≥20 wt %). The presence of a high content of dead weight materials in the cell configurations and low electrode loading often lead to low energy density of the cell when the total cell weight is considered.

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CONCLUSIONS In this study, a high-energy-density solid-state supercapacitor has been developed based on a gel IL with high IL content and G


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ACS Applied Materials & Interfaces Table 3. State-of-the-Art Literature on Ion Gels host material

electrolyte

this work PVDF-HFP*16 PVDF-HFP17 BNC*25 PVDF-HFP13 PVDF-HFP26 c-PV4Ph27 PILs*11 silica22 PVDF-HFP10 PILs28

EMImTFSI EMImBF4 BMIMBF4 EMImNTf2 BMIMBF4 EMIMFAP EMImTFSI PYR14TFSI EMImNTf2 EMIMTCB PYR14FSI

IL loading (%)

electrode loading (mg cm−2)

capacitance (F g−1)

energy density (W h kg−1)

95 50 80 96 83 80 78 60

3.2−5.10 2.5−3.5 0.5 0.8 2.7−3.5 2.0

153

65 (3.5 V) 53 (3.5 V) 33 (2.0 V) 16 (3.0 V) 76 (3.5 V) 17 (2.5 V) 72 (4.0 V) 32 (3.5 V) 41 (3.0 V) 3.5 (3.5 V) 36 (2.5 V)

80 60

242 51 190 76 172 100 135 34 150

2−4.0 0.2 2.5 2−4.0

(20 000) (5000) (12 000) (5000) (5000) (10 000) (1000)

97 (4000)

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ABBREVIATIONS EDLCs, electric double-layer capacitors; a-PCNFs, activated porous carbon nanofibers; CV, cyclic voltammetry; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; TEM, transmission electron microscopy; SEM, scanning electron microscopy; EIS, electrochemical impedance spectroscopy

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07479. Digital photographs for electrochemical testing setup and freestanding solid-state cell, additional electrolyte characterizations, and numerical analysis for obtaining cell time constant (PDF)

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the funding of this project. P.R.C. and S.L.W. would like to acknowledge the financial support of National Science Foundation, grant CBET-1437814. The authors are also grateful to Drexel University Centralized Research Facility for allowing them to use their characterization equipment.

3-D PCNF electrodes. The supercapacitor shows promising electrochemical performance, including high gravimetric capacitance, leading to energy densities comparable to those observed for liquid IL-based supercapacitors, high cycling stability of over 20 000 cycles with minimal capacitance loss, and a high device-level areal capacitance of up to 151 mF cm−2. Such an electrochemical performance is predicated upon (1) the low content of electrochemically inert materials that are necessary in other systems which includes a lower content of gelators in the ionogel and the absence of binders for the electrode materials. Such a low content of inert materials ensures unhindered ion and electron conductivity in the gel electrolyte and electrodes, respectively; (2) the presence of hierarchical porosity in the 3-D electrode network which facilitates thorough infusion of the soft gel IL even at higher electrode thickness/mass loading. Moreover, the intimate contact established at the nanofiber pore/gel IL interface leads to a minimal charge-transfer impedance for ion shuttling across the interface. The mutual advantages of the highly conductive gel IL and the 3-D electrode architecture make the system attractive for large-scale applications.

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capacitance retention (%) (cycle #)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.R.C.). *E-mail: [email protected] (V.K.). ORCID

Stephanie L. Wunder: 0000-0002-7193-4762 Vibha Kalra: 0000-0002-2630-1560 Author Contributions

S.K.S. and P.R.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.S. and V.K. would like to thank the National Science Foundation, grant #s CBET-1150528 and CMMI-1463170, for H


Research Article

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Highly Durable, Self-Standing Solid-State Supercapacitor Based on

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