The Synergistic Effect of Nitrile and Ether Functionalities for Gel

Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan...
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The Synergistic Effect of Nitrile and Ether Functionalities for Gel Electrolytes Used in Supercapacitors Mei-Fang Hsueh,† Cheng-Wei Huang,† Ching-An Wu,† Ping-Lin Kuo,† and Hsisheng Teng*,†,‡ †

Department of Chemical Engineering and Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan ‡ Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan S Supporting Information *

ABSTRACT: This study examines the linear triblock copolymer design of poly(acrylonitrile)-b-poly(ethylene glycol)-b-poly(acrylonitrile) (PAN-b-PEG-b-PAN) for a gel polymer electrolyte (GPE) swollen with dimethylformamide dissolving LiClO4. The study demonstrates the synergistic effect of the nitrile and ether functionalities in facilitating ion transport in the carbon films of electric double-layer capacitors (EDLCs). A GPE with a tuned AN/EG ratio exhibits ionic conductivity at approximately 10−2 S cm−1. The linear configuration incorporates the GPE border into the carbon electrodes. The PAN chain promotes ion solvation and transport into the carbon interior, and the PEG chain coordinates the solvent molecules to form ion motion channels. The synergistic effect of the PAN and PEG blocks enables a GPE EDLC delivering more energy and power than EDLCs with a liquidphase electrolyte. The GPE EDLC delivers 20 Wh kg−1 (approximately 10 Wh L−1) at a high power of 10 kW kg−1 (approximately 5 kW L−1) when using a high-porosity carbon electrode derived from mesophase pitch activation. facilitate polymer swelling and ion transport.29 A polymeric framework consisting of linear copolymer chains of connected PAN and PEG blocks behaves similarly to a sponge in trapping the LE solution because PEG blocks are soluble in the solvent molecules but PAN blocks are not.30,31 The copolymer becomes gelled (that is, plasticized) after association with solvent molecules. The strong solvating ability of PAN allows the polymeric framework, which serves as a GPE host, to effectively facilitate ion transport through embedded solvent channels and polymer chains with segmental motion.31 This structure also prevents polymer-chain crystallization, increasing ion mobility.32 A triblock copolymer, PAN-b-PEG-b-PAN, was used in a GPE swollen with a dimethylformamide (DMF) solution of LiClO4 salt.19 The GPE showed superior adjustable mechanical integrity, making the roll-to-roll assembly of GPE-based EDLCs scalable to industrial levels. This study examines the synergistic effect of nitrile and ether functionalities to show how the copolymer block length and composition affect the ionic conductivity, carbon surface compatibility, and ion transport behavior of the carbon electrode. The results are discussed according to the PAN-to-PEG block chain length ratio, nitrile and ether content, and specific chemical arrangement. Triblock copolymers are incorporated with a high-porosity-activated

1. INTRODUCTION In the rapidly growing field of electric double-layer capacitors (EDLCs), novel electrochemical systems (such as novel nanoscale electrode materials, a gel electrolyte, or both) may enable EDLCs to deliver high power density, high cycle efficiency, and a long cycle life.1,2 EDLCs store charge by using surface ion adsorption at the interface between a carbon electrode with a large surface area and an electrolyte.3−7 The ion storage and release enabled by this charge storage process are easily reversed.8−13 EDLCs mainly use aqueous and organic liquid electrolytes (LEs). Replacing LEs with gel polymer electrolytes (GPEs) would improve EDLC safety by solving electrolyte leakage and corrosion problems.14−20 Developing GPEs with high ionic conductivity and a network compatible with carbon electrodes is important for designing stable and flexible EDLCs, which have more applications because they are easily fabricated into desired shapes and sizes. Polymers with a linear configuration allow penetration of the carbon electrode to facilitate ion transport.21 Because most solid polymer electrolytes (SPEs) exhibit relatively low conductivity, their applicability for EDLCs is limited. GPEs exhibit higher conductivity than solvent-free SPEs and are effective at retaining solvents to prevent leakage and evaporation.22−25 Of the many types of GPEs, polymer chains containing poly(acrylonitrile) (PAN) segments exhibit excellent ion-solvating abilities and high electrochemical stability.26−28 Poly(ethylene glycol) (PEG) segments in a polymer are associated with solvent molecules, and they © XXXX American Chemical Society

Received: March 29, 2013 Revised: July 25, 2013

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performance. The EDLC cell was composed of two facing electrodes sandwiching a piece of GPE film. The cell was assembled under stress to ensure close contact at the carbon− carbon and carbon−Ti foil interfaces. Cyclic voltammetric characterization of the EDLC cell was conducted within the stable potential window at varying scan rates using an electrochemical analyzer (Solartron Analytical, Model 1470E, UK). An ac impedance spectroscopy analyzer (Zahner-Elektrik IM6e, Germany) was used to measure the impedance behavior of the capacitor cells at 0 V at an ac potential amplitude of 5 mV and a frequency range of 10 mHz to 100 kHz. Cell performance was recorded using the electrochemical analyzer by measuring the galvanostatic charge−discharge of symmetric cells within the stable potential window with a current density range of 0.125−75 A g−1. All electrochemical measurements were performed at room temperature (approximately 25 °C). A cell with a liquid-phase electrolyte was also assembled using the same method, which was used for the GPE cells. However, a piece of 30 μm thick cellulose paper replaced the gel film to separate the facing electrodes. The cell was soaked in a concentrated LiClO4/DMF solution (approximately 4 M, which gives optimal ionic conduction according to our auxiliary experiments) during electrochemical measurements.

mesophase pitch (aMP) to assess the compatibility of the GPE structure with carbon electrodes for high-performance EDLCs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of GPE Films. The PAN-b-PEG-b-PAN copolymer was prepared by redox copolymerization of acrylonitrile (Fluka, Switzerland) with PEG (Mn = 5000 g mol−1; Alfa Aesar, UK) at feed molar ratios (AN/(equivalent OH of PEG)) of 30, 70, 150, and 300. Polymerization was conducted by mixing acrylonitrile with an 8.6 wt % PEG aqueous solution in a nitrogen atmosphere at 40 °C for 6 h with ceric ammonium nitrate (Showa, Japan) as an initiator (the molar ratio of the Ce(IV)/equivalent OH of PEG was 2).33,34 The polymerization product was precipitated and purified by consecutively mixing with deionized water (200 mL) and acetone (200 mL). This precipitation−purification process was conducted for three times. The precipitated product was dried in vacuum at 80 °C for 48 h. To obtain a GPE film, a homogeneous mixture was formed by heating a mixture of a 0.02 g of PAN-b-PEG-b-PAN copolymer, 0.08 g of LiClO4, and 0.8 g of DMF in a sealed bottle at 80 °C for 12 h. The LiClO4/PAN-b-PEG-b-PAN/ DMF mixture (0.1 g) was cast onto an aluminum substrate (1.5 cm × 1.5 cm), evaporating some of the DMF at 80 °C for 1 h. The evaporation time controlled the DMF content of the GPE. The content was determined by measuring the weight of the cast GPE film. The resulting GPE film had a DMF/polymer weight ratio of 3.1:1 and a thickness of 50 μm. It was plastic, transparent, and yellowish. 2.2. Characterization of GPE Films. High-resolution nuclear magnetic resonance (NMR) spectroscopic measurements of the PAN-b-PEG-b-PAN copolymer, dissolved in deuterated dimethyl sulfoxide (also known as dimethyl-d6 sulfoxide, Aldrich), were performed using a Bruker AV-500 spectrometer (Germany) with a 1H resonance frequency at 500 MHz. Dimethyl-d6 sulfoxide shows no peaks in NMR spectra. The 1H chemical shifts were referenced relative to tetramethylsilane at 0.0 ppm. Raman spectra of the specimens were recorded at room temperature using a Bayspec Raman spectrometer with a laser line of 1064 nm. The Raman measurement resolution was 4 cm−1. GPE films were sandwiched between two stainless steel electrodes for an ac impedance spectroscopy (Zahner-Elecktrik IM6e, Germany) measurement to determine their ionic conductivity. A conductivity measurement was conducted at 0 V with an ac potential amplitude of 5 mV and a frequency range of 10 Hz−100 kHz. Electrolyte synthesis and device measurements were performed in a nitrogen-purged environment to eliminate interference from moisture. 2.3. EDLC Assembly and Electrochemical Measurements. The EDLC electrodes consisted of a 1 cm2 aMPpowder film, approximately 2 mg in weight and 40 μm thick, and a titanium-foil current collector. The aMP powder was derived from KOH activation of the mesophase pitch (China Steel Chemical Co., Taiwan) at 800 °C. This was then milled with 5 wt % multiwall carbon nanotubes (Aldrich, France; 10− 15 nm in diameter and 1−10 μm in length).35−43 Before any characterizations or measurements, the carbon powder was heated in an argon atmosphere at 900 °C to remove surface oxides. The aMP surface area, calculated using the Brunauer− Emmett−Teller (BET) isotherm (see Figure S1 in the Supporting Information), was 3100 m2 g−1. A symmetric twoelectrode capacitor cell was used to examine the capacitive

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of the Copolymer. The synthesized triblock copolymers exhibited identical PEGblock lengths and various PAN lengths. The 1H NMR spectrum of the PAN-b-PEG-b-PAN copolymers (Figure 1) shows a PEG resonance peak at 3.3−3.6 ppm, and the PAN CH2 and CH spectra show resonance peaks at 2.0−2.3 and 3.0−3.2 ppm,

Figure 1. 1H NMR spectrum of the PAN-b-PEG-b-PAN copolymers synthesized at varying AN/(equivalent OH in PEG) ratios: (a) PEG5k-PAN10; (b) PEG5k-PAN20; (c) PEG5k-PAN24; (d) PEG5kPAN53. B

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C−O−C stretching vibration peak at 1094 cm−1 (Figure S1).45 Figure 2 shows that the GPE peak shifts to a higher wavenumber at 1114 cm−1 because the added lithium salt or DMF plasticizer interacts with the polymer chains, changing the conformations of the polymer matrix and the C−O−C bond electronic cloud. The highly polar DMF can penetrate the bonds between the ether groups and then swell PEGcontaining polymers. The strong band at 900−970 cm−1 in the GPE Raman spectra corresponds to the ClO4− anion signal in different ionic environments.46,47 Figure 3a shows the magnification of this

respectively. Calculating the integrated areas of the PEG and PAN segments in Figure 1a shows that the composition of the PAN-b-PEG-b-PAN copolymer is (AN)579-(EG)117-(AN)579. This corresponds to an AN/EG chain length ratio of 10/1. The total molecular weight of the copolymer was 66 414 g mol−1, and the molecular weight of the PEG polymer chain accounted for 5000 g mol−1. This copolymer is referred to as PEG5kPAN10. The same calculation was used to obtain the chain lengths and molecular weights of the remaining copolymers, with their spectra shown in Figure 1b−d. Table 1 shows a summary of the calculated results for the copolymers in Figure 1. Table 1. Chemical Formulas of the PAN-b-PEG-b-PAN Triblock Copolymers Synthesized at Varying AN: (Equivalent OH in PEG) Compositions polymer code

AN/EG ratio in polymer

polymer formula

AN:OH in synthesis

PEG5k-PAN10 PEG5k-PAN20 PEG5k-PAN24 PEG5k-PAN53

10 20 24 53

(AN)579-(EG)117-(AN)579 (AN)1135-(EG)117-(AN)1135 (AN)1396-(EG)117-(AN)1396 (AN)3128-(EG)117-(AN)3128

30:1 70:1 150:1 300:1

Figure 2 shows the Raman spectra of the GPEs prepared by incorporating the LiClO4/DMF electrolyte solution with the

Figure 3. (a) Raman spectra of the ClO4− anion band (solid lines) and the constituting peaks obtained after spectrum decomposition (dotted lines) for different GPEs. (b) The CN band for different GPEs.

Figure 2. Raman spectra of GPEs in the region of 600−3300 cm−1: (a) GPE10, (b) GPE20, (c) GPE24, and (d) GPE53.

band, including contributions from free ClO4− ions at 930 cm−1, solvent-shared Li+−solvent−ClO4− ions at 939 cm−1, Li+ClO4− contact ion pairs at 948 cm−1, and multiple clusters of [Li+ClO4−]n ions at 955 cm−1.48−52 Lorentzian curve fitting was used to deconvolute the 900−970 cm−1 band into its four constituting peaks. The dotted lines in Figure 3a show the simulation results from least-squares optimization. Table 2 shows the composition of the ClO4− band based on the simulated spectral band area. The composition of free ClO4− ions increases with the AN/EG ratio to a maximal value of 64% at AN/EG = 24 and then decreases as the ratio increases further. The increase in free-ClO4− composition indicates that the nitrile group has an enhanced solvation of Li+ cations. Further increases of the PAN chain length may result in nitrile group association and suppress the solvation ability of the copolymer.

copolymers. The nomenclature of these gel electrolytes uses the term “GPE”, followed by the AN/EG ratio of the embedded copolymer. For example, the GPEs embedded with copolymers PEG5k-PAN10 and PEG5k-PAN53 are called GPE10 and GPE53, respectively. The symmetric and asymmetric PAN C− H stretching vibrations create Raman peaks at 2960 and 3000 cm−1, respectively (see Figure S2 in the Supporting Information for the copolymer Raman spectra).44 Figure 2 shows that the symmetric C−H stretching band at 2960 cm−1 disappeared when an electrolytic salt was introduced. The Li+ ions cause electron delocalization near the nitrile groups to form −C(δ+)− CN(δ−). The ClO4− anions disturb the adjacent C(δ+)−H electronic cloud, resulting in the peak disappearance at 2960 cm−1. The PEG crystalline segments in the copolymer exhibit a C

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Table 2. Compositions (in %) of ClO4− Anions in Different Ionic Environments: Free ClO4− Ions, Solvent-Shared Li+− Solvent−ClO4− Ions, Li+ClO4− Ion Pairs, and [Li+ClO4−]n Ion Clustersa electrolyte code

free ClO4−

Li+−solvent− ClO4−

GPE10 GPE20 GPE24 GPE53

50.0 58.6 64.4 56.7

40.0 36.2 29.5 30.4

intercepting the Re(Z) axis at high frequencies. The ionic conductivity (σ) of the electrolytes was determined using

σ = RI−1S −1d

where RI is the intercept at the real axis in the impedance Nyquist plot, S is the geometric area of the electrolyte− electrode interface, and d is the distance between the two electrodes. Figure 4b shows a summary of the ionic conductivities of the LE and GPEs. The GPEs produced higher conductivities than the LE. For example, the ionic conductivities of GPE24 and the LE were 1.1 × 10−2 and 1.6 × 10−3 S cm−1, respectively. Ion conduction in GPEs occurs on two paths: the faster conduction path through the free volume phase and the slower conduction path through the swollen polymer-chain phase.54,55 Figure 4b shows that the GPE conductivity variation corresponds with the free-ClO4 − percentage trend shown in Table 2. This indicates that the high degree of LiClO4 dissociation by the copolymer nitrile groups caused the high GPE conductivity. The strong affinity between the ether linkage and solvent molecules promoted the swelling of the polymeric framework. The lower conductivity of GPE53 reflects the ether linkage shortage, facilitating aggregation of the PAN chains. 3.3. Capacitive Performance of Resulting Cells. Figure 5 shows the configuration of a symmetric EDLC cell with a

Li+ClO4− [Li+ClO4−]n 4.2 2.9 3.6 4.6

(1)

5.8 2.3 2.5 8.3

a

The percentages were determined from the decomposed Raman spectra shown in Figure 3a.

The Raman band at 2245 cm−1, with magnified spectra in Figure 3b, is the PAN CN stretching vibration.53 The CN stretching vibration is highly sensitive to any interaction. The GPE24 CN band exhibits a prominent shoulder at 2300 cm−1, indicating extensive interaction between the CN group and Li+ cations in GPE24. 3.2. Effect of the Polymer Network on Ionic Conductivity. The ac impedance method was used to analyze the ionic conductivity of the GPEs with various AN/EG ratios. The performance of the 4 M LiClO4/DMF LE with a cellulose separator was also analyzed. Figure 4a shows the impedance spectra of the electrolytes on a Nyquist complex plane. All the electrolyte spectra show inclined lines with similar slopes, indicating that the electrolytes have similar affinities toward the electrode surface. The inset in Figure 4a shows a magnification of the high-frequency spectra, exhibiting inclined lines

Figure 5. Schematic configuration of the carbon/GPE/carbon symmetric capacitors.

GPE. The carbon electrodes were wetted with the DMF solvent before cell assembly. The border of the GPE film was incorporated into the wetted carbon films under pressure. The carbon prewetting resulted in the diffusion of the polymer into the carbon film, assuring full utilization of the carbon electrode mass in the double layer charge storage. The center zone of the GPE film maintained its mechanical integrity, successfully separating the facing electrodes. The inset reflects the conduction paths through the free-volume and swollen polymer-chain domains. Figure 6 shows cyclic voltammograms of the EDLCs assembled with the LE (panel a) and GPEs (panels b−d) at various scan rates. The cells behaved similarly to an ideal double-layer capacitor at low scan rates, reflected by the rectangular voltammograms. The voltammograms became more distorted with an increasing scan rate, showing that the

Figure 4. (a) Nyquist impedance plots for the LE and the GPEs with varying AN/EG ratios. The measurements were conducted by sandwiching the electrolytes with titanium-foil current collectors and applying a potential of 0 V with frequency ranging from 10 Hz to 100 kHz. The inset shows a magnified view of the high-frequency region of the impedance spectra. (b) Ionic conductivities of the LE and GPEs determined by the impedance analysis and the percentages of free ClO4− in the GPEs. D

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the ether groups, the GPEs exhibited a higher dielectric constant compared to the LE, creating a larger charge-storage capacity at the electrolyte−electrode interface. The linear configuration of the copolymer for intimate contact with the carbon surface can strengthen this effect on capacitance promotion. The GPE24 cell delivered the highest capacitance value because it exhibited the highest degree of LiClO4 dissociation. The high ionic conductivity of GPE24 resulted in superior capacitance retention at high discharge rates. However, the capacitance retention of the LE cell was better than that of most of the GPE cells. The thinner separator film of the LE cell (approximately 30 μm) reduces resistance and may cause better capacitance retention. Doubling the separator thickness resulted in a LE cell with capacitance retention poorer than all the GPE cells. The IR drop of the cells was estimated using the sudden voltage drop at the beginning of the galvanostatic discharge. This voltage drop is associated with the overall resistance (Rt) of the cell. Figure 8 shows the IR drop variation with the Figure 6. Cyclic voltammograms of symmetric two-electrode capacitors assembled with the LE and GPEs: (a) LE, (b) GPE10, (c) GPE24, and (d) GPE53.

charge transport resistance controls the charge storage process at high scan rates. The currents were significantly higher for the cell assembled with GPE24 (Figure 6c), which produced the highest ionic conductivity. The high currents indicate the high capacitance of the GPE24 cell. Galvanostatic charge−discharge measurements were performed to appraise the cell capacitance values. The specific discharge capacitance (C) of the electrodes in the EDLCs was calculated using C = 4Ccell = 4Itd /(M ΔV )

Figure 8. Variation of the IR drop with the discharge current of the LE and GPE cells.

(2)

discharge current for the GPE and LE cells. The IR drop increases linearly with the discharge current for each cell. The slope of the linear relationship corresponds to the Rt value of the cell. A large Rt reduces the power performance of the EDLC. Table 3 shows that the Rt values of the GPE cells

where Ccell is the total capacitance of the cell, I is the discharge current, t is the discharge time, M is the total mass of the two symmetric electrodes, and ΔV is the discharge voltage difference excluding the IR drop. Figure 7 shows the specific

Table 3. Resistance Components of the Symmetric Capacitor Cells Assembled with Different Electrolytesa electrolyte code

Rt/Ω

Res/Ω

Rw/Ω

f k/Hz

LE GPE10 GPE20 GPE24 GPE53

4.27 4.64 5.16 2.79 4.22

1.71 1.95 1.67 1.13 1.02

1.27 0.81 0.88 0.44 1.03

1.44 2.11 3.02 8.12 2.11

a Overall resistance (Rt), equivalent series resistance (Res), and Warburg resistance (Rw). The knee-frequency (f k) corresponds to the ac frequency for transition between the Warburg and capacitancedominating regions during impedance measurements (Figure 9).

Figure 7. Variation of the specific electrode capacitance with the discharge current of the LE and GPE cells.

decrease as GPE conductivity increases. The LE cell produces a lower Rt value than most of the GPE cells, although the conductivity of the LE is low. The low Rt of the LE cell may be caused by its thin separator film because doubling the separator thickness significantly increased the Rt value. EDLC performance, especially power density performance, is closely related to the resistance elements of the cells. Ac impedance spectroscopy, which distinguishes between device resistance and capacitance, shows the resistance of the capacitor

capacitance values at various discharge currents. The LE cell delivered an ultimate capacitance value of 183 F g−1 at the smallest discharge rate of 0.125 A g−1. GPE cells GPE10, GPE20, GPE24, and GPE53 exhibited higher ultimate capacitance values of 187, 191, 224, and 199 F g−1, respectively. Because the PAN-b-PEG-b-PAN copolymers improved the dissociation of the lithium salt with the nitrile groups and improved the segregation of the solvent molecule clusters with E

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cells in the double-layer formation on the carbon electrodes.56,57 Figure 9 shows the Nyquist-type impedance spectra of

Figure 10. Ragone plots of the LE and GPE cells with a potential window 2.1 V. Data obtained from the galvanostatic discharge measurement at a current density range of 0.125 and 62.5 A g−1.

Figure 9. Nyquist impedance plots of the LE and GPE cells at a frequency range 10 mHz to 100 kHz and an applied potential of 0 V. The inset shows a magnified view of the high-frequency region of the impedance spectra.

values. The synergistic effect of the PAN and PEG blocks enabled the GPE24 cell to deliver more power and energy than the LE cell. In GPE24, the PAN block provides intimate contact between the polymer chains and the carbon surface for facile ion penetration into the carbon pores while the PEG blocks carry the solvent molecules into the carbon film. The specific energy of the GPE24 cell was as high as 20 Wh kg−1 at a power density of 10 000 W kg−1. The density of the aMP electrode was 0.48 g cm−3, which is higher than that of conventional activated carbon (approximately 0.4 g cm−3).42,43 The aMP film was assembled at 2 mg cm−2, which corresponded to a film thickness of 42 μm. The aMP film and the GPE24 electrolyte produced a volumetric capacitance of 108 F cm−3 (at a geometric-area capacitance of 0.454 F cm−2), an extraordinarily high value for a carbon material in an organic electrolyte system.5 The GPE24 cell delivered a high volumetric energy density of 17 Wh L−1 and maintained 10 Wh L−1 at a high power of 5 kW L−1 (Figure 10). The excellent cell performance indicates that the synergistic effect of PAN and PEG provides a platform for high-porosity carbon materials to efficiently store and deliver electricity in an organic electrolyte system.

the GPE and LE cells scanned at 0 V. The full-scale spectra in this figure contain only vertical lines, indicating that all cells produced good capacitive performance. The inset in Figure 9 shows the high-frequency region of the impedance spectra for the cells. Table 3 shows the equivalent series resistance (Res; the intercept at the Re(Z) axis) of the cells assembled with the LE and GPEs. The Res value is associated with the resistance created by ion migration in the carbon films and separator region. For GPE cells, if Res decreases with the PAN block length, this indicates that the linear PAN chain has a high affinity for the carbon film, reducing the Res value. GPE cells with a high AN/EG ratio produced smaller Res values than the LE cell. The inset in Figure 9 shows the presence of a transitional Warburg region in the high-frequency region of the Nyquist plots. The Warburg region is caused by hindered ion diffusion in the carbon−electrolyte interface region. The intercepts of the extrapolated low-frequency vertical lines at the Re(Z) axis correspond to the sum of Res and the Warburg resistance (Rw). Table 3 shows the Rw values calculated using this extrapolation method. The value of Rw decreases with the increasing percentage of free ClO4− ions, indicating that a higher degree of salt dissociation facilitates ion diffusion at the electrolyte− electrode interface. Table 3 shows that the GPE24 cell exhibits the smallest Rw value and, therefore, the highest knee-frequency (f k), where the Warburg region (characterized by the 45° sloping line) transforms into a region dominated by capacitive behavior (characterized by the vertical line).58,59 The f k value of the GPE24 cell was 8 Hz, an extraordinarily fast charge− discharge performance for a GPE EDLC.60−62 The galvanostatic charge−discharge data at 2.1 V were used to calculate the gravimetric power and energy densities of the GPE and LE cells by using the following equations:

Pcell = (I ΔV )/2M

4. CONCLUSIONS This study examines the synergistic effect of PAN and PEG in the linear polymeric GPE host used in EDLCs. The PAN nitrile group exhibits a high cation-solvating ability, and the PEG ether group effectively coordinates solvent molecules to swell the polymeric framework. GPEs derived from swelling PAN-bPEG-b-PAN triblock copolymers with a LiClO4/DMF solution exhibited ionic conductivities that varied according to PANand PEG-block length. Adjusting the AN/EG chain length ratio produced GPEs with a maximal ionic conductivity of 1.1 × 10−2 S cm−1, whereas the conductivity of LiClO4/DMF LE was only 1.6 × 10−3 S cm−1. When the GPEs and LE were incorporated with the high-porosity aMP powder to form EDLCs, the GPE cells exhibited higher specific capacitance values than the LE cell (187−224 and 183 F g−1, respectively). This was probably caused by the higher dielectric constant of the GPEs. The PAN chain had a high affinity with the carbon surface, and increasing the PAN block length facilitated ion penetration into the carbon pores. The ion diffusion rate in the Warburg region increased with the degree of salt dissociation. With a well-tuned AN/EG ratio of 24, the resulting GPE24 cell delivered more energy and power than the LE cell. Because of the high surface area and high density of the aMP carbon, the GPE24 cell

(3) 2

Ecell = Pcelltd = Ccell(ΔV ) /2

(4)

where Pcell is the specific power of a cell and Ecell is the specific energy of a cell. Figure 10 shows the Pcell and Ecell results as Ragone plots. All GPE cells delivered more specific energy than the LE cell at low power values because of the higher degree of salt dissociation in the GPEs. The GPE cells, except GPE24, delivered less power than the LE cell because of their large Res F

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delivered a high energy density of 34 Wh kg−1 (approximately 17 Wh L−1) and maintained 20 Wh kg−1 (approximately 10 Wh L−1) at a high power of 10 kW kg−1 (approximately 5 kW L−1). The synergistic effect of PAN and PEG in linear triblock copolymers significantly promotes ionic motion in the carbon film, making the copolymer a promising host for GPEs used in various applications.

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ASSOCIATED CONTENT

S Supporting Information *

N2 sorption isotherms of the aMP powder; the Raman spectra of a PAN-b-PEG-b-PAN triblock copolymer with various PAN and PEG block lengths; the complete author lists for refs 2, 8, 16, and 61. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel 886-6-2385371; e-mail [email protected] (H.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Science Council of Taiwan (101-2221-E-006-243-MY3, 101-2221-E-006-225-MY3, 102-3113-P-006-012, and 102-3113-E-006-002) and the “Aim for the Top-Tier University and Elite Research Center Development Plan” of National Cheng Kung University.

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REFERENCES

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