Highly Flexible and Conductive Cellulose-Mediated PEDOT:PSS

Mar 28, 2017 - Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China. ACS Appl. Ma...
1 downloads 0 Views 2MB Size
Download PDF
Research Article www.acsami.org

Highly Flexible and Conductive Cellulose-Mediated PEDOT:PSS/ MWCNT Composite Films for Supercapacitor Electrodes Dawei Zhao,† Qi Zhang,† Wenshuai Chen,† Xin Yi,† Shouxin Liu,† Qingwen Wang,† Yixing Liu,† Jian Li,† Xianfeng Li,*,‡ and Haipeng Yu*,† †

Key Laboratory of Bio-Based Material Science and Technology, Ministry of Education, Northeast Forestry University, Harbin 150040, P. R. China ‡ Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China S Supporting Information *

ABSTRACT: Recent improvements in flexible electronics have increased the need to develop flexible and lightweight power sources. However, current flexible electrodes are limited by low capacitance, poor mechanical properties, and lack of cycling stability. In this article, we describe an ionic liquidprocessed supramolecular assembly of cellulose and 3,4ethylenedioxythiophene for the formation of a flexible and conductive cellulose/poly(3,4-ethylenedioxythiophene) PEDOT:poly(styrene sulfonate) (PSS) composite matrix. On this base, multiwalled carbon nanotubes (MWCNTs) were incorporated into the matrix to fabricate an MWCNT-reinforced cellulose/PEDOT:PSS film (MCPP), which exhibited favorable flexibility and conductivity. The MCPP-based electrode displayed comprehensively excellent electrochemical properties, such as a low resistance of 0.45 Ω, a high specific capacitance of 485 F g−1 at 1 A g−1, and good cycling stability, with a capacity retention of 95% after 2000 cycles at 2 A g−1. An MCPP-based symmetric solid-state supercapacitor with Ni foam as the current collector and PVA/KOH gel as the electrolyte exhibited a specific capacitance of 380 F g−1 at 0.25 A g−1 and achieved a maximum energy density of 13.2 Wh kg−1 (0.25 A g−1) with a power density of 0.126 kW kg−1 or an energy density of 4.86 Wh kg−1 at 10 A g−1, corresponding to a high power density of 4.99 kW kg−1. Another kind of MCPP-based solid-state supercapacitor without the Ni foam showed excellent flexibility and a high volumetric capacitance of 50.4 F cm−3 at 0.05 A cm−3. Both the electrodes and the supercapacitors were environmentally stable and could be operated under remarkable deformation or high temperature without damage to their structural integrity or a significant decrease in capacitive performance. Overall, this work provides a strategy for the fabrication of flexible and conductive energy-storage films with ionic liquid-processed cellulose as a medium. KEYWORDS: cellulose, carbon nanotubes, conducting polymers, electrodes, flexibility, supercapacitors

1. INTRODUCTION

PEDOT-based electrodes have been prepared by vapor-phase or electrochemical deposition methods, which require hours or even days, and relatively high temperatures are needed in these manufacturing processes. Although chemical polymerization of PEDOT in the solution phase has shown possibilities for upscaled preparation, the low porosity and low conductivity make it difficult to achieve high capacitances with PEDOT. Other challenges include the low solubility of PEDOT in common solvents and relatively poor mechanical performance, which hinder its use in flexible SCs. Novel efficient and scalable approaches for the fabrication of flexible conducting PEDOT film materials with a high capacitance are therefore greatly required.6 Generally, PEDOT can hardly be directly used as an electrode because it swells and shrinks during the intercalation/deintercalation process, which leads to a poor electro-

The emergence and rapid development of miniaturized and portable consumer electronics requires the development of lightweight and flexible power sources.1 Conventional electrochemical energy-storage devices, including batteries and capacitors, are usually too rigid and bulky for incorporation into flexible electronics.2 Therefore, the development of novel, flexible energy-storage devices that do not require complex operation but show high performance is required.3,4 The key challenges for the successful fabrication of such energy-storage devices involve finding suitable materials and appropriate methods for construction.3 Poly(3,4-ethylenedioxythiophene) (PEDOT) is an attractive pseudocapacitive polymeric conducting material. Recently, PEDOT has received considerable interest as a promising electrode material for supercapacitors (SCs), especially in combination with other conductive materials, owing to its suitable conjugated backbone, high conductivity, large electroactive potential window, and high charge-storage capacity.5−7 However, one problem with PEDOT is that the majority of © 2017 American Chemical Society

Received: February 7, 2017 Accepted: March 28, 2017 Published: March 28, 2017 13213

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

bulk density of 0.12 g cm−3, a pores per inch value of 110 ppi, an active specific surface area of 0.9 m2 g−1, and an electrical conductivity of 8.62 × 103 S cm−1. 2.2. Preparation of the Cellulose/PEDOT:PSS Films. [Bmim] Cl (25 g) was added to a three-necked flask and heated at 85 °C for 10 min. Cellulose (0.375 g) was then added to the flask and stirred until a homogeneous, transparent [Bmim]Cl/cellulose system was obtained. Increasing amounts of EDOT (0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.75, and 1.25 g) were dissolved in the [Bmim]Cl/cellulose system and yielded [Bmim]Cl/cellulose/EDOT solutions, which contained 0.4, 0.6, 0.8, 1.2, 1.6, 2.0, 3.0, and 5.0 wt % EDOT, respectively. The mixtures were then degassed in a vacuum oven at a pressure of 0.01 MPa at 65 °C for 1 h to remove bubbles and then at 85 °C for 30 min to decrease the viscosity of the solution. The resulting gels were then cast onto silicon wafers using a VTC-100 Vacuum Spin Coater (Kejing Instrument Co. Ltd., Shenyang, China) and then immersed in distilled water. After soaking for 1 h, white hydrogels were obtained and then impregnated with 200 mL of aqueous solution that contained PSS (3 g) as the doping agent. After the PSS solution diffused through the hydrogels at 35 °C for 60 min, APS (4.5 g) as a polymerization initiator was added. The polymerization proceeded for 3 h, and then, any unreacted PSS was replaced using distilled water to yield the cellulose/PEDOT:PSS hydrogels. The hydrogels were dried at 60 °C for 8 h to give the cellulose/PEDOT:PSS films. 2.3. Preparation of the MCPP Films. The preparation of the MCPP films was similar to that of the cellulose/PEDOT:PSS films, except that 0.258, 0.546, 0.773, 1.0312, 1.289, 1.547, and 1.805 g of MWCNTs were initially added to [Bmim]Cl (25 g) and heated at 85 °C for 90 min, respectively. The resulting MCPP films are referred to as MCPP-n, where n (= 1, 2, 3, 4, 5, 6, and 7) corresponds to the mass percentage of MWCNTs to [Bmim]Cl. 2.4. Preparation of the MCPP-Based Electrodes and SCs. A three-electrode configuration was used to evaluate the electrochemical properties of the MCPP-based electrodes, with a Pt wire counter electrode, Ag/AgCl reference electrode, and a 3 mol L−1 NaOH electrolyte solution. The working electrode was prepared by pressing an MCPP film (10 × 10 × 0.008 (thickness) mm3) onto a Ni foam current collector (10 × 10 mm2) at a pressure of 10 MPa for 5 min and then drying at 60 °C in a vacuum oven (0.05 MPa) for 4 h. The symmetric SC consisted of two pieces of MCPP-7 films and a quasisolid-state PVA/KOH gel as the electrolyte. The PVA/KOH electrolyte was prepared by dissolving PVA (6 g) and KOH (6 g) in water (60 mL) at 85 °C with mechanical stirring for 60 min. Two pieces of MCPP-based electrodes were immersed in the PVA/KOH gel for 15 min and were then assembled together and dried in an oven at 45 °C for 8 h. 2.5. Characterization. The micromorphology of the MCPPs was characterized using scanning electron microscopy (SEM) with a JSM7500F microscope (Hitachi, Tokyo, Japan) at an operating voltage of 10 kV. The samples were coated with platinum using a vacuum sputter coater before observation. A Tecnai G2 transmission electron microscope (TEM; FEI, Hillsboro, OR), with an accelerator voltage of 80 kV, was used for TEM imaging. Raman spectroscopy was performed on a Renishaw Via-Reflex spectrometer at an excitation wavelength of 532 nm. The surface areas and pore volumes of MCPP were determined using Brunauer−Emmett−Teller (BET) theory (Barrett−Joyner−Halenda method). N2 adsorption/desorption was performed using an ASAP 2020 instrument (Micromeritics). The electrical resistivity of the film was measured using an RTS-8 fourpoint probe system (Rthy Bo Gen Applied Materials Technology Co., Ltd., Xi’an, China) with copper probes arranged in a straight line (spacing of 1 mm), and the conductivity (σ) was calculated using the formula σ = 1/ρ (ρ is the electrical resistivity). Thermogravimetric (TG) analysis was performed using an STA 6000 analyzer (PerkinElmer Inc., Waltham) at temperatures ranging from 45 to 850 °C, with a heating rate of 20 °C min−1 in a nitrogen atmosphere (40 mL min−1). The tensile test was performed with an Instron 5569 universal testing machine at a cross-head speed of 1 mm min−1. The sample size for testing was 20 × 5 × (0.03−0.06) mm3 (length/width/ thickness). Each sample was subjected to five independent tests.

chemical cycling performance.8 Hence, a stable PEDOT-based electrode material should be designed and fabricated through an efficient and scalable approach. Cellulose, the most abundant biopolymer on earth, has attractive properties, including environmental friendliness, polymeric processability, chemical reactivity, and thermal and mechanical stability.9 It has been reported that cellulose can be combined with conducting polymers, carbon materials, or other active materials to prepare flexible energy-storage devices.4−6 Conventionally, cellulose/ conducting polymer electrodes for SCs can be fabricated by means such as direct mixing, soak polymerization, and vacuum filtration, but these methods suffer from problems such as heterogeneous distribution, polymer agglomeration, and limited interfacial bonding, which result in seriously decreased stability, conductivity, capacitance, and cycling performance.10−14 Considering these problems, it is necessary to find a new strategy for the synthesis of cellulose/conducting polymer composites for high-performance electrodes. Ionic liquid-processed cellulose has been revealed to exhibit excellent self-assembly characteristics,15,16 which would facilitate the construction of a cellulose−3,4-ethylenedioxythiophene (EDOT) supramolecular structure. On the basis of the ionic liquid processes, the EDOT monomers can easily diffuse in or out of the cellulose dendrimer host. The interactions between them may include (i) hydrogen bonding, (ii) host−guest interactions, (iii) electrostatic interactions, and (iv) coordination-driven self-assembly. To the best of our knowledge, the use of an ionic liquid to facilitate the supramolecular assembly of cellulose and EDOT toward the synthesis of a highly conductive composite for flexible SC electrodes has not yet been reported. Moreover, the role of the ionic liquid in the in situ polymerization of cellulose−EDOT and how optimal performance may be obtained remain poorly understood. In this paper, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) was chosen as a cosolvent to homogeneously dissolve both cellulose and EDOT and provide a surrounding environment for their supramolecular assembly. The obtained cellulose−EDOT supramolecular structure guided the polymerization of PEDOT and poly(styrene sulfonate) (PSS) along its supramolecular chains, resulting in a flexible cellulose/ PEDOT:PSS composite matrix. Multiwalled carbon nanotubes (MWCNTs), which possess high electrical conductivity (102− 10 5 S cm −1 ) and exceptional physical and chemical stability,17−20 were incorporated into the cellulose/PEDOT:PSS matrix to further enhance its structural strength and conductivity. The obtained MWCNT-reinforced cellulose/ PEDOT:PSS (MCPP) composite was expected to provide multiple pathways for electron and ion transport and exhibited increased capacitance and cycling durability. MCPP-based electrodes and symmetric SCs were assembled, and their flexibility, conductivity, stability, capacitance, cycling durability, energy density, and power density were studied.

2. MATERIALS AND METHODS 2.1. Materials. Cellulose with an α-cellulose content of approximately 90% was purchased from Tianjin Haojia Cellulose Co., Ltd. (Tianjin, China). The degree of polymerization was 1484, with an average molecular weight of 240 kDa. [Bmim]Cl, EDOT, MWCNT, PSS, ammonium persulfate (APS), potassium hydroxide (KOH), sodium hydroxide (NaOH), and polyvinyl alcohol (PVA, molecular weight of 88 kDa) were purchased from Aladdin Chemistry Co., Ltd. (Shanghai, China). All chemicals were of analytical grade and were used as received. Nickel foam (KX-NI1101) was purchased from Kuangxun Electronics Co., Ltd. (Kunshan, China). The Ni foam had a 13214

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic representation of the preparation of the cellulose/PEDOT:PSS films by (a) the conventional dipping polymerization method and (b) the present [Bmim]Cl-processed cellulose−EDOT supramolecular assembly method. (c) SEM images of the cellulose/PEDOT:PSS film prepared using the present method. (d) Dependence of electrical conductivity on EDOT weight percentage (w/w of water (200 mL)). The inset shows the conductivity of materials from previous reports. 2.6. Electrochemical Testing. The electrochemical properties of the electrodes and SC devices were investigated using a CHI 660D electrochemical workstation (Chenhua Instrument Co., Shanghai, China). Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy experiments were performed. The specific capacitance (C, F g−1) of the electrodes for the threeelectrode system was estimated from the GCD profiles according to eq 121 C=

I × Δt mΔV

3. RESULTS AND DISCUSSION 3.1. Properties of the Cellulose/PEDOT:PSS Films. The [Bmim]Cl-processed cellulose−EDOT supramolecular assembly used to fabricate the cellulose/PEDOT:PSS films is illustrated in Figure 1b. The proposed approach is quite different from the commonly used dipping and polymerization method, shown in Figure 1a. The [Bmim]Cl ionic liquid easily dissolved the cellulose and EDOT, generating a homogeneous mixed solution (Figure S1 in the Supporting Information (SI)). When [Bmim]Cl was substituted with water, a white hydrogel consisting of the cellulose−EDOT supramolecular structure (Figure S1b) was generated. The cellulose−EDOT structure provided a strong stereoscopic skeleton that enabled the polymerization of PEDOT:PSS along the supramolecular structure.23,24 The white hydrogel grew darker over time, which demonstrated that mild polymerization had proceeded, which could prevent excessive accumulation or structural collapse (Figure S2). After drying, the obtained cellulose/ PEDOT:PSS films exhibited a smooth morphology with massive mesopores (Figure 1c). The films exhibited a conductivity of up to 30 S cm−1, which was higher than that of most conducting films prepared using the dipping and polymerization method (Figure 1d).13,25−31 The conductivity of the cellulose/PEDOT:PSS films was significantly influenced by the EDOT content. The lowest amount of EDOT in [Bmim]Cl was determined to be 1.2 wt %, which corresponded to an optimal polymerization time of 3 h (Figure S3). 3.2. Properties of the MCPP Films. MWCNTs were incorporated into the cellulose/PEDOT:PSS matrix to improve

(1)

where m is the mass (0.00321 g) of the active materials in a single electrode and I, Δt, and ΔV represent the discharging current, discharging time (s), and discharging potential range (V), respectively. The energy density (E, Wh kg−1) and power density (P, W kg−1) against two electrodes in the SC device were estimated using eqs 2−421,22 CSC =

4 × I × Δt mΔV

E=

CSC × ΔV 2 4×2

P=

E Δt

(2)

(3)

(4) −1

where CSC is the specific capacitance (F g ) of the SC, m is the total mass (0.0152 g) of the active materials in two electrodes for the SC device, ΔV is the voltage drop on discharge (excluding Vdrop), and Δt is the time for a full discharge. 13215

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Schematic illustration showing the preparation route of the MCPP films. The optical photographs show the flexibility of the MCPP films. (b, c) TEM images show the MWCNTs in the cellulose/EDOT system. (d) Raman spectra. (e, f) SEM images of the MCPP film. (g) Poresize distribution of the MCPP film using nitrogen adsorption.

films could be folded, bent, and sanded without destroying their conductivities (Figure 3b),38 indicating excellent flexibility and holistic conductivity. The conductivity was stable even after the films were soaked in water for 24 h or heated at 150 °C for 60 min (Figure 3c). In addition, the MCPP films were thermally stable below 200 °C (Figure 3d,e). The MCPP-7 film exhibited an ultimate stress of 95.8 MPa and a Young modulus of 3.51 GPa (Figure 3f), which were higher than those of the cellulose nanofiber/PEDOT composite and the cellulose paper/ graphene composite.39,40 From the above results, it can be inferred that the MWCNTs contained in such a matrix can act as a structural reinforcing material and a highly conductive skeleton (Figure S8). However, the mass fraction of the MCWNTs was further increased and the MCPP films became mechanically deteriorated and easily breakable, although their conductivities showed improvement. 3.3. Electrochemical Properties of the MCPP Electrodes. The electrochemical properties of the MCPP as an electrode material were investigated using a three-electrode configuration (Figures 4a and S9). In our preliminary experiments, both alkaline and acidic solutions were used as electrolytes for electrochemical property evaluations.41,42 The NaOH solution was found more suitable for the MCPP than acidic solutions (Figure S10). The OH− in KOH solution is smaller than Cl− and SO42− and so can better wet the MCPP film and ensure full use of the mesopores in MCPP. The CV curves of the MCPP-based electrodes exhibited a rectangular shape, with gradual expansion with increasing MWCNT content (Figures 4b and S11). The CV curves did not display a distinct pseudocapacitive behavior, which was consistent with the findings of previous reports on conducting polymer-based

the conductivity and toughness of the resulting MCPP films (Figure 2a). The incorporation of MWCNTs using the present method helped overcome the problems of a usually inadequate interface and poor bonding between the polymer matrix and MWCNTs (Figures 2b and S4). The cellulose/PEDOT:PSS matrix adhered to the MWCNTs, and the MWCNTs were observed to be dispersed uniformly throughout the matrix and in a 3D complex system (Figures 2c,e and S5). The Raman spectra, Fourier transform infrared spectra, and X-ray diffraction patterns of the MCPP exhibited the characteristic peaks of MWCNTs (i.e., D-band at 1356 cm−1 and G-band at 1572 cm−1), cellulose, and PEDOT:PSS (Figures 2d and S6). Massive evenly distributed mesopores (∼20 nm) were found in the MCPP (Figures 2f,g and S7). The BET specific surface area of the MCPP was 140 m2 g−1, significantly higher than that of PEDOT:PSS (0.70 m2 g−1) and also superior to that of graphene oxide/polypyrrole (PPy)/cellulose paper (9.3 m2 g−1), PPy@nanocellulose (53 m2 g−1), CNT/c-EVA (76.1 m2 g−1), and PEDOT nanopaper (137 m2 g−1).10,14,32,33 The high surface area and plentiful mesoporous channels gave the MCPP better contact with the electrolyte and provided abundant channels for ion transportation and diffusion, which should result in improved electrochemical properties. The addition of MWCNTs to the MCPP enhanced its conductivity from 26.8 to 275 S cm−1 (Figure 3a). This conductivity was higher than that of similar composite materials, including cellulose/graphite/PPy (0.55 S cm−1),31 SWCNT/polyaniline (125 S cm −1 ), 34 polycarbonate/ MWCNT/PEDOT:PSS (10−100 S cm−1),35 Li4Ti5O12/ cellulose nanofiber/CNT (30−35 S cm−1),36 and nanocellulose/CNT (200 S cm−1).37 Furthermore, the MCPP 13216

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Conductivity of the MCPP films as a function of the MWCNT content. The inset shows the brightness of a lamp using the MCPP film as the conductor. (b) Conductivity of the MCPP-7 film under various deformation conditions. (c) Conductivity of the MCPP-7 film as a function of temperature. (d, e) TG and derivative TG curves, and (f) stress−strain curves of the MCPP films. The inset in (f) shows a comparison of Young’s modulus and ultimate tensile strengths.

reversible. The contribution of current density to the specific capacitance of the MCPP-based electrodes was investigated by comparing the specific capacitances observed at various current densities. The MCPP-7 electrode exhibited the highest contribution (Figure 4f). As the current density was increased from 1 to 20 A g−1, the specific capacitance of the MCPP-7 electrode dropped from 485 to 350 F g−1, which was still nearly 72% of the initial value. The MCPP-7 electrode showed a superior specific capacitance and rate capability to those of some PEDOT-based electrodes reported previously (Table S1). A Nyquist plot of the MCPP electrodes within a frequency range of 0.01−100 kHz at 5 mV is shown in Figure 4g. The plot was almost vertical, with phase angles close to −90° within the low-frequency region, which further suggested that the electrodes had an ideal capacitive behavior. The absence of a semicircular feature in the plot indicated that the electrochemical behavior was not affected by electron-transfer limitations.44 The equivalent series resistance of the MCPP-7 electrode was 0.45 Ω, which was significantly lower than that of

composites, indicating that the response of the MCPP electrode to redox reactions was fast and its resistance was rather low.8 The CV curves of the MCPP-7 electrode exhibited a relatively rectangular mirror image at scan rates of 2−100 mV s−1 (Figure 4d), which indicated good capacitive behavior. However, those at high scan rates of 200 and 300 mV s−1 were distorted (Figure S12), which can be attributed to a limited diffusion rate constant.43 The GCD profiles of the MCPP electrodes within a voltage window of −0.4 to 0.1 V and at a current density of 1 A g−1 are shown in Figure 4c. The profiles reveal that the Coulombic efficiency of the MCPP electrodes was closely associated with the mass loading of the MCWNTs; a low mass fraction of MCWNTs resulted in a low Coulombic efficiency. The GCD profiles of the MCPP-7 electrode at different current densities are shown in Figure 4e. The profiles displayed a typical triangular shape. The symmetry of the charge and discharge profiles between current densities of 2 and 20 A g−1 indicated a relatively high Coulombic efficiency, suggesting that the charge transfer in the electrodes was highly 13217

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

Figure 4. Electrochemical experiments performed on the MCPP-based electrodes. (a) Optical photographs of the flexible electrodes. (b) CV curves recorded at 5 mV s−1. (c) GCD profiles at a current density of 1 A g−1. (d) CV curves of the MCPP-7 electrode at scan rates of 2−100 mV s−1. (e) GCD profiles of the MCPP-7 electrode taken at different current densities. (f) Capacitance values at different current densities. (g) Nyquist plot of MCPP electrodes. An enlargement of the low-frequency region is shown in the top right. (h) Cycling performance of the MCPP-7 electrode at a current density of 2 A g−1, showing the long-term cycling stability. The inset shows the GCD profiles corresponding to the 1st, 1000th, and 2000th cycles. (i) CV curves of the MCPP-7 electrode at a current density of 50 mV s−1 before and after 1000 and 2000 cycles.

other PEDOT-based electrodes.5 The relaxation time of the MCPP-7 electrode was 0.57 s (Figure S13), smaller than that of graphene-based aerogel electrodes (0.73 s)45 and comparable to that of graphene nanomesh electrodes (0.47 s)46 and graphene framework electrode (0.49 s).47 A short time constant can significantly enhance the ion-transfer kinetics in an electrode and improve its electrochemical performance. The cycling performance of the MCPP-7 electrode at a current density of 2 A g−1 is shown in Figure 4h. The specific capacitance slightly decreased from 485 to 448 F g−1 after cycling at 2 A g−1 for 2000 cycles, which represents 95% retention of the initial value. The CV curves after 1000 and 2000 cycles almost overlapped, further demonstrating the good capacitance and cyclic stability of the electrode.48 3.4. Electrochemical Properties of the MCPP-7-Based SCs. A symmetric SC was assembled by joining two MCPP-7 films in a PVA/KOH electrolyte, with Ni foam as the collector (Figure 5a).40,49 CV curves of the SC at scan rates of 2−100 mV s−1 exhibited quasirectangular shapes, within a potential window of 0−1.0 V, and the GCD curves displayed an approximately linear relationship between discharge/charge voltage and time (Figure 5b,c), which indicates that the SC had

good capacitive properties and a fast charge−discharge ability.50 The specific capacitance of the SC was estimated to be 380 F g−1 at 0.25 A g−1 (Figure 5d). Even at a high current density of 10 A g−1, it still had a high specific capacitance of 140 F g−1. The Nyquist plot displayed a diffusion-controlled Warburg capacitive behavior (Figure 5e).51 The equivalent series resistance was only 0.8 Ω. The relaxation time constant of 7.15 s was significantly less than the 10 s time constant of common activated carbon-based SCs.52 In addition to its good efficiency, the SC also exhibited good cycling performance. After 1000 charge−discharge cycles at 1 A g−1, the capacitance remained almost 90% of its initial value (Figure 5f). The SC could be operated under deformation, even up to 180°, during which its specific capacitance remained almost unchanged (Figure S14). The SC could even work under high temperatures. As the temperature was increased from 30 to 80 °C, the charge−discharge profiles at 1 A g−1 remained stable and the specific capacitance of the SC exhibited a slight increase (Figure S15). From the Ragone plot shown in Figure 6a, the MCPP-7based SC achieved a maximum energy density of 13.2 Wh kg−1 (0.25 A g−1) with a power density of 0.126 kW kg−1 and was 13218

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

Figure 5. (a) Structure of the MCPP-7-based SC. (b) CV curves of the SC at different scan rates within a voltage window of 0−1.0 V. (c) GCD profiles of the SC taken at different current densities. (d) Capacitance of the SC at different current densities. (e) Nyquist plots of the SC. (f) Cycling performance of the SC at a current density of 1 A g−1. The insets show the GCD profiles of the first three and last three charge−discharge cycles.

Figure 6. (a) Ragone plots of the MCPP-7-based SC. (b) CV curves at a scan rate of 50 mV s−1 of two SCs in a series connection. (c) GCD profiles at 1 A g−1 of two SCs in a parallel connection. (d) Self-discharging duration of two SCs connected in series. The insets in (d) show two SCs connected in series used to power a red LED lamp.

able to achieve an energy density of 4.86 Wh kg−1 at a high current density of 10 A g−1, corresponding to a high power density of 4.99 kW kg−1. The energy-storage performance of the MCPP-7-based SC appeared to be better than that of some similar SCs based on conducting polymers and/or carbon

materials, including PPy−cladophora cellulose,53 aramid nanofiber/PEDOT:PSS,54 MnO2/graphene,55 nanofibrillar PEDOT:PSS,56 graphene/MnO2/CNT,57 and reduced graphene oxide−polyaniline.58 In addition, the SCs could be connected in series or in parallel to provide a high power output. As 13219

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces

Figure 7. Electrochemical performance of the MCPP-7-based SC without the Ni foam. (a) GCD profiles at current densities of 0.05−1.0 A cm−3. (b) CV curves at scan rates of 10−300 mV s−1. The insets in (a) show the flexibility of the SC.

expected, two SCs connected in parallel provided an output current of 8 A g−1 (Figure 6b), whereas two connected in series provided an output potential of 2.0 V (Figure 6c). When the two SCs were connected in series and used to power a lightemitting diode (LED) lamp (2.0 V, 20 mA), the brightness of the lamp was maintained for longer than 30 min (Movie S1). An open-circuit self-discharging test revealed a capacity retention of approximately 80% after self-discharging for 3000 s (0.83 h) (Figure 6d). Another MCPP-7-based SC was assembled without using Ni foam as the current collector (Figure 7). This SC also showed excellent flexibility and a highest volumetric capacitance of 50.4 F cm−3 at a current density of 0.05 A cm−3.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Y.). *E-mail: [email protected] (X.L.). ORCID

Xianfeng Li: 0000-0002-8541-5779 Haipeng Yu: 0000-0003-0634-7913 Author Contributions

4. CONCLUSIONS

H.Y. supervised the project. D.Z. designed the research plan and performed most of the experiments. Q.Z. and X.Y. participated in the experiments. W.C. and X.L. were involved in the discussion of the results. H.Y., D.Z., and X.L. co-wrote the paper. S.L., Q.W., Y.L., and J.L. provided some useful suggestions.

In summary, this work has demonstrated an ionic liquidprocessed cellulose−EDOT supramolecular self-assembly strategy as an effective and scalable method for the synthesis of highly flexible conducting films. [Bmim]Cl provided a cosoluble basis for the preparation of homogeneous composites formulated from cellulose/PEDOT:PSS and MWCNTs. The resulting MCPP film exhibited a high conductivity and improved mechanical properties when the mass loading of MWCNTs in [Bmim]Cl was increased to 7%. The ternary components formed a connected conductive network that ensured a low resistance and high conductivity of the MCPP film. Electrodes fabricated from the MCPP exhibited a desirable electrochemical performance, with a capacitance of up to 485 F g−1 (at 1 A g−1). The capacitance remained approximately 95% of its initial value after charge−discharge at 2 A g−1 for 2000 cycles. An MCPP-7-based symmetric SC exhibited a high specific capacitance of 380 F g−1 at 0.25 A g−1 and an energy density of 4.86−13.2 Wh kg−1, corresponding to a power density of 0.126−4.99 kW kg−1. Another MCPP-7-based SC assembled without the Ni foam showed a volumetric capacitance of 50.4 F cm−3 at a current density of 0.05 A cm−3. The MCPP films fabricated using the present approach exhibited desirable mechanical and electrochemical properties and showed great promise for use in electrodes.

■

Figures and tables for TEM and SEM characterization and additional electrochemical data of the control materials (PDF) MCPP-7-based SCs powering an LED lamp in the initial state (AVI)

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 31622016) and the Natural Science Foundation of Heilongjiang Province of China (Grant No. JC2016002). This work was also sponsored by the National Program for Support of Top-Touch Young Professionals.

■

REFERENCES

(1) Kim, J.; Lee, J.; You, J.; Park, M. S.; Al Hossain, M. S.; Yamauchi, Y.; Kim, J. H. Conductive Polymers for Next-Generation Energy Storage Systems: Recent Progress and New Functions. Mater. Horiz. 2016, 3, 517−535. (2) Yan, J.; Wang, Q.; Wei, T.; Fan, Z. Recent Advances in Design and Fabrication of Electrochemical Supercapacitors with High Energy Densities. Adv. Energy Mater. 2014, 4, No. 1300816. (3) Wang, X.; Lu, X.; Liu, B.; Chen, D.; Tong, Y.; Shen, G. Flexible Energy-Storage Devices: Design Consideration and Recent Progress. Adv. Mater. 2014, 26, 4763−4782. (4) You, B.; Sun, Y. Hierarchically Porous Nickel Sulfide Multifunctional Superstructures. Adv. Energy Mater. 2016, 6, No. 1502333. (5) Nyholm, L.; Nyström, G.; Mihranyan, A.; Strømme, M. Toward Flexible Polymer and Paper-Based Energy Storage Devices. Adv. Mater. 2011, 23, 3751−3769.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01852. 13220

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces (6) Wang, Z.; Tammela, P.; Huo, J.; Zhang, P.; Strømme, M.; Nyholm, L. Solution-Processed Poly(3,4-ethylenedioxythiophene) Nanocomposite Paper Electrodes for High-Capacitance Flexible Supercapacitors. J. Mater. Chem. A 2016, 4, 1714−1722. (7) Yu, Z.; Li, C.; Abbitt, D.; Thomas, J. Flexible, Sandwich-like AgNanowire/PEDOT:PSS-Nanopillar/MnO2 High Performance Supercapacitors. J. Mater. Chem. A 2014, 2, 10923−10929. (8) Hu, L.; Pasta, M.; La Mantia, F.; Cui, L.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Stretchable, Porous, and Conductive Energy Textiles. Nano Lett. 2010, 10, 708−714. (9) Li, Y.; Liu, Y.; Chen, W.; Wang, Q.; Liu, Y.; Li, J.; Yu, H. Facile Extraction of Cellulose Nanocrystals from Wood using Ethanol and Peroxide Solvothermal Pretreatment Followed by Ultrasonic Nanofibrillation. Green Chem. 2016, 18, 1010−1018. (10) Wan, C.; Jiao, Y.; Li, J. Flexible, Highly Conductive, and FreeStanding Reduced Graphene Oxide/Polypyrrole/Cellulose Hybrid Papers for Supercapacitor Electrodes. J. Mater. Chem. A 2017, 5, 3819−3831. (11) Cheng, Y.; Lu, S.; Zhang, H.; Varanasi, C. V.; Liu, J. Synergistic Effects from Graphene and Carbon Nanotubes Enable Flexible and Robust Electrodes for High-Performance Supercapacitors. Nano Lett. 2012, 12, 4206−4211. (12) Zhao, M.-Q.; Ren, C.; Ling, Z.; Lukatskaya, M. R.; Zhang, C.; Van Aken, K. L.; Barsoum, M. W.; Gogotsi, Y. Flexible MXene/ Carbon Nanotube Composite Paper with High Volumetric Capacitance. Adv. Mater. 2015, 27, 339−345. (13) Yu, H.; Chen, P.; Chen, W.; Liu, Y. Effect of Cellulose Nanofibers on Induced Polymerization of Aniline and Formation of Nanostructured Conducting Composite. Cellulose 2014, 21, 1757− 1767. (14) Zhang, Y.; Tan, Y.; Yang, K.; Wu, Z.; Zhang, Z.; Zhang, M.; Mai, K. Molecular Chain Bonding Synthesis of Nanoporous, Flexible and Conductive Polymer Composite with Outstanding Performance for Supercapacitors. J. Mater. Chem. A 2016, 4, 10091−10097. (15) Liu, S.; Wang, F.; Dong, R.; Zhang, T.; Zhang, J.; Zhuang, X.; Mai, Y.; Feng, X. Dual-Template Synthesis of 2D Mesoporous Polypyrrole Nanosheets with Controlled Pore Size. Adv. Mater. 2016, 28, 8365−8370. (16) Zhou, Z.; Yan, X.; Cook, T. R.; Saha, M. L.; Stang, P. J. Engineering Functionalization in a Supramolecular Polymer: Hierarchical Self-Organization of Triply Orthogonal Non-Covalent Interactions on a Supramolecular Coordination Complex Platform. J. Am. Chem. Soc. 2016, 138, 806−809. (17) Zhang, L. L.; Zhao, X. S. Carbon-Based Materials as Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38, 2520−2531. (18) Lu, P.; Hsieh, Y. L. Multiwalled Carbon Nanotube (MWCNT) Reinforced Cellulose Fibers by Electrospinning. ACS Appl. Mater. Interfaces 2010, 2, 2413−2420. (19) Lee, K. Y. T.; Shi, H. H.; Lian, K.; Naguib, H. E. Flexible Multiwalled Carbon Nanotubes/Conductive Polymer Composite Electrode for Supercapacitor Applications. Smart Mater. Struct. 2015, 24, No. 115008. (20) Yang, Q.; Pang, S. K.; Yung, K. C. Study of MWCNTs/ PEDOT-PSS Composite and MWCNTs/PTFE Composite as Electrodes for Supercapacitors in Aqueous Solution. ECS Trans. 2014, 59, 9−15. (21) Liu, L.; Shen, B.; Jiang, D.; Guo, R.; Kong, L.; Yan, X. Watchband-Like Supercapacitors with Body Temperature Inducible Shape Memory Ability. Adv. Energy Mater. 2016, 6, No. 1600763. (22) Xu, J.; Tan, Z.; Zeng, W.; Chen, G.; Wu, S.; Zhao, Y.; Ni, K.; Tao, Z.; Ikram, M.; Ji, H.; Zhu, Y. A Hierarchical Carbon Derived from Sponge-Templated Activation of Graphene Oxide for High-Performance Supercapacitor Electrodes. Adv. Mater. 2016, 28, 5331. (23) Marcilla, R.; Ochoteco, E.; Pozo-Gonzalo, C.; Grande, H.; Pomposo, J. A.; Mecerreyes, D. New Organic Dispersions of Conducting Polymers using Polymeric Ionic Liquids as Stabilizers. Macromol. Rapid Commun. 2005, 26, 1122−1126.

(24) Kwon, J.-Y.; Kim, E.-Y.; Kim, H.-D. Preparation and Properties of Waterborne-Polyurethane Coating Materials Containing Conductive Polyaniline. Macromol. Res. 2004, 12, 303−310. (25) Huang, J.; Miller, P. F.; Wilson, J. S.; de Mello, A. J.; de Mello, J. C.; Bradley, D. D. C. Investigation of the Effects of Doping and PostDeposition Treatments on the Conductivity, Morphology, and Work Function of Poly(3,4-Ethylenedioxythiophene)/Poly(Styrene Sulfonate) Films. Adv. Funct. Mater. 2005, 15, 290−296. (26) Seyedin, M. Z.; Razal, J. M.; Innis, P. C.; Wallace, G. G. StrainResponsive Polyurethane/PEDOT:PSS Elastomeric Composite Fibers with High Electrical Conductivity. Adv. Funct. Mater. 2014, 24, 2957− 2966. (27) Khan, S.; Ul-Islam, M.; Khattak, W. A.; Ullah, M. W.; Park, J. K. Bacterial Cellulose−Poly(3,4-Ethylenedioxythiophene)−Poly(Styrenesulfonate) Composites for Optoelectronic Applications. Carbohydr. Polym. 2015, 127, 86−93. (28) Wu, X.; Lu, C.; Xu, H.; Zhang, X.; Zhou, Z. Biotemplate Synthesis of Polyaniline@Cellulose Nanowhiskers/Natural Rubber Nanocomposites with 3D Hierarchical Multiscale Structure and Improved Electrical Conductivity. ACS Appl. Mater. Interfaces 2014, 6, 21078−21085. (29) Hu, W.; Chen, S.; Yang, Z.; Liu, L.; Wang, H. Flexible Electrically Conductive Nanocomposite Membrane Based on Bacterial Cellulose and Polyaniline. J. Phys. Chem. B 2011, 115, 8453−8457. (30) Wang, S.; Wei, C.; Gong, Y.; Lv, J.; Yu, C.; Yu, J. Cellulose Nanofiber-Assisted Dispersion of Cellulose Nanocrystals@Polyaniline in Water and its Conductive Films. RSC Adv. 2016, 6, 10168−10174. (31) Chen, J.; Xu, J.; Wang, K.; Qian, X.; Sun, R. C. Highly Thermostable, Flexible and Conductive Films Prepared from Cellulose, Graphite and Polypyrrole Nanoparticles. ACS Appl. Mater. Interfaces 2015, 7, 15641−15648. (32) Wang, Z.; Tammela, P.; Zhang, P.; Strømme, M.; Nyholm, L. High Areal and Volumetric Capacity Sustainable All-Polymer PaperBased Supercapacitors. J. Mater. Chem. A 2014, 2, 16761−16769. (33) Lu, W.; Yuan, Z.; Zhao, Y.; Zhang, H.; Zhang, H.; Li, X. Porous Membranes in Secondary Battery Technologies. Chem. Soc. Rev. 2017. 10.1039/C6CS00823B. (34) Yao, Q.; Chen, L.; Zhang, W.; Liufu, S.; Chen, X. Enhanced Thermoelectric Performance of Single-Walled Carbon Nanotubes/ Polyaniline Hybrid Nanocomposites. ACS Nano 2010, 4, 2445−2451. (35) Zhou, J.; Lubineau, G. Improving Electrical Conductivity in Polycarbonate Nanocomposites using Highly Conductive PEDOT/ PSS Coated MWCNTs. ACS Appl. Mater. Interfaces 2013, 5, 6189− 6200. (36) Cao, S.; Feng, X.; Song, Y.; Liu, H.; Miao, M.; Fang, J.; Shi, L. In-Situ Carbonized Cellulose-Based Hybrid Film as Flexible PaperAnode for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2016, 8, 1073−1079. (37) Hamedi, M. M.; Hajian, A.; Fall, A. B.; Håkansson, K.; Salajkova, M.; Lundell, F.; Wågberg, L.; Berglund, L. A. Highly Conducting, Strong Nanocomposites Based on Nanocellulose-Assisted Aqueous Dispersions of Single-Wall Carbon Nanotubes. ACS Nano 2014, 8, 2467−2476. (38) Ng, K.; Gao, B.; Yong, K. W.; Li, Y.; Shi, M.; Zhao, X.; Li, Z.; Zhang, X.; Pingguan-Murphy, B.; Yang, H.; Xu, F. Paper-Based Cell Culture Platform and its Emerging Biomedical Applications. Mater. Today 2017, 20, 32−44. (39) Malti, A.; Edberg, J.; Granberg, H.; Khan, Z. U.; Andreasen, J. W.; Liu, X.; Zhao, D.; Zhang, H.; Yao, Y.; Brill, J. W.; Engquist, I.; Fahlman, M.; Wågberg, L.; Crispin, X.; Berggren, M. An Organic Mixed Ion−Electron Conductor for Power Electronics. Adv. Sci. 2016, 3, No. 1500305. (40) Weng, Z.; Su, Y.; Wang, D. W.; Li, F.; Du, J.; Cheng, H. M. Graphene−Cellulose Paper Flexible Supercapacitors. Adv. Energy Mater. 2011, 1, 917−922. (41) Zhong, C.; Deng, Y.; Hu, W.; Qiao, J.; Zhang, L.; Zhang, J. A Review of Electrolyte Materials and Compositions for Electrochemical Supercapacitors. Chem. Soc. Rev. 2015, 44, 7484−7539. 13221

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222

Research Article

ACS Applied Materials & Interfaces (42) Li, L.; Song, H.; Zhang, Q.; Yao, J.; Chen, X. Effect of Compounding Process on the Structure and Electrochemical Properties of Ordered Mesoporous Carbon/Polyaniline Composites as Electrodes for Supercapacitors. J. Power Sources 2009, 187, 268−274. (43) Zhu, M.; Huang, Y.; Deng, Q.; Zhou, J.; Pei, Z.; Xue, Q.; Huang, Y.; Wang, Z.; Li, H.; Huang, Q.; Zhi, C. Highly Flexible, Freestanding Supercapacitor Electrode with Enhanced Performance Obtained by Hybridizing Polypyrrole Chains with MXene. Adv. Energy Mater. 2016, 6, No. 1600969. (44) Nyström, G.; Strømme, M.; Sjödin, M.; Nyholm, L. Rapid Potential Step Charging of Paper-Based Polypyrrole Energy Storage Devices. Electrochim. Acta 2012, 70, 91−97. (45) Sun, X.; Cheng, P.; Wang, H.; Xu, H.; Dang, L.; Liu, Z.; Lei, Z. Activation of Graphene Aerogel with Phosphoric Acid for Enhanced Electrocapacitive Performance. Carbon 2015, 92, 1−10. (46) Wang, H.; Sun, X.; Liu, Z.; Lei, Z. Creation of Nanopores on Graphene Planes with MgO Template for Preparing High-Performance Supercapacitor Electrodes. Nanoscale 2014, 6, 6577−6584. (47) Xu, Y.; Lin, Z.; Zhong, X.; Huang, X.; Weiss, N. O.; Huang, Y.; Duan, X. Holey Graphene Frameworks for Highly Efficient Capacitive Energy Storage. Nat. Commun. 2014, 5, 4554. (48) Liu, S.; Gordiichuk, P.; Wu, Z. S.; Liu, Z.; Wei, W.; Wagner, M.; Mohamed-Noriega, N.; Wu, D.; Mai, Y.; Herrmann, A.; Müllen, K.; Feng, X. Patterning Two-Dimensional Free-Standing Surfaces with Mesoporous Conducting Polymers. Nat. Commun. 2015, 6, 8817. (49) Li, H.; Hou, Y.; Wang, F.; Lohe, M. R.; Zhuang, X.; Niu, L.; Feng, X. Flexible All-Solid-State Supercapacitors with High Volumetric Capacitances Boosted by Solution Processable MXene and Electrochemically Exfoliated Graphene. Adv. Energy Mater. 2017, 7, No. 1601847. (50) Kim, B. C.; Hong, J. Y.; Wallace, G. G.; Park, H. S. Recent Progress in Flexible Electrochemical Capacitors: Electrode Materials, Device Configuration, and Functions. Adv. Energy Mater. 2015, 5, No. 1500959. (51) Ding, J.; Wang, H.; Li, Z.; Cui, K.; Karpuzov, D.; Tan, X.; Kohandehghan, A.; Mitlin, D. Peanut Shell Hybrid Sodium Ion Capacitor with Extreme Energy−Power Rivals Lithium Ion Capacitors. Energy Environ. Sci. 2015, 8, 941−955. (52) El-Kady, M. F.; Strong, V.; Dubin, S.; Kaner, R. B. Laser Scribing of High-Performance and Flexible Graphene-Based Electrochemical Capacitors. Science 2012, 335, 1326−1330. (53) Razaq, A.; Nyholm, L.; Sjödin, M.; Strømme, M.; Mihranyan, A. Paper-Based Energy-Storage Devices Comprising Carbon FiberReinforced Polypyrrole-Cladophora Nanocellulose Composite Electrodes. Adv. Energy Mater. 2012, 2, 445−454. (54) Li, Y.; Ren, G.; Zhang, Z.; Teng, C.; Wu, Y.; Lu, X.; Zhu, Y.; Jiang, L. A Strong and Highly Flexible Aramid Nanofibers/ PEDOT:PSS Film for All-Solid-State Supercapacitors with Superior Cycling Stability. J. Mater. Chem. A 2016, 4, 17324−17332. (55) Li, Z.; An, Y.; Hu, Z.; An, N.; Zhang, Y.; Guo, B.; Zhang, Z.; Yang, Y.; Wu, H. Preparation of a Two-Dimensional Flexible MnO2/ Graphene Thin Film and its Application in a Supercapacitor. J. Mater. Chem. A 2016, 4, 10618−10626. (56) D’Arcy, J. M.; El-Kady, M. F.; Khine, P. P.; Zhang, L.; Lee, S. H.; Davis, N. R.; Liu, D. S.; Yeung, M. T.; Kim, S. Y.; Turner, C. L.; Lech, A. T.; Hammond, P. T.; Kaner, R. B. Vapor-Phase Polymerization of Nanofibrillar Poly(3,4-Ethylenedioxythiophene) for Supercapacitors. ACS Nano 2014, 8, 1500−1510. (57) Cheng, Y.; Lu, S.; Zhang, H.; Varanasi, C. V.; Liu, J. Synergistic Effects from Graphene and Carbon Nanotubes Enable Flexible and Robust Electrodes for High-Performance Supercapacitors. Nano Lett. 2012, 12, 4206−4211. (58) Zhang, J.; Jiang, J.; Li, H.; Zhao, X. S. A High-Performance Asymmetric Supercapacitor Fabricated with Graphene-Based Electrodes. Energy Environ. Sci. 2011, 4, 4009−4015.

13222

DOI: 10.1021/acsami.7b01852 ACS Appl. Mater. Interfaces 2017, 9, 13213−13222