Research Article www.acsami.org
Metallic Fabrics as the Current Collector for High-Performance Graphene-Based Flexible Solid-State Supercapacitor Jianhui Yu, Jifeng Wu, Haozong Wang, Anan Zhou, Chaoqiang Huang, Hua Bai, and Lei Li* College of Materials, Xiamen University, Xiamen 361005, People’s Republic of China S Supporting Information *
ABSTRACT: Flexible solid-state supercapacitors attract more and more attention as the power supply for wearable electronics. To fabricate such devices, the flexible and economical current collectors are needed. In this paper, we report the stainless steel fabrics as the current collector for highperformance graphene-based supercapacitors. The stainless steel fabrics have superior properties compared with the widely used flexible current collectors. The flexible supercapacitors show large specific capacitance of 180.4 mF/cm2, and capacitance retention of 96.8% after 7500 charge−discharge cycles. Furthermore, 96.4% of the capacitance is retained after 800 repeating stretching-bending cycles. The high performance is related to the excellent conductivity, good mechanical flexibility, and high electrochemical stability of the stainless steel fabrics. The achievement of such high-performance and flexible supercapacitor can open up exciting opportunities for wearable electronics and energy storage applications. KEYWORDS: metallic fabric, chemically converted graphene, flexible, solid-state, supercapacitor
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are too rigid for flexible devices. However, they are unstable in aqueous electrolytes, which restricts the lifetime of the devices. Polymer films, such as polyimide,23 are suffering from low stability when in contact with acidic or alkaline electrolyte. Therefore, a suitable flexible current collector with both high conductivity, good stability and proper mechanical properties is desirable for the further development of flexible energy storage devices. Herein, we report the usage of stainless steel fabrics (SSFs) as the current collector for graphene-based flexible solid-state supercapacitor. The SSFs are cost-efficient porous fabrics made of stainless steel fibers with high mechanical strength and excellent flexibility, which have been widely used as filtration membranes in industry.24 In addition, stainless steel proves stable in acidic electrolyte, and it has been used as the current collector in commercially available supercapacitors.25 SSFs also have the same electrochemical stability as stainless steel foil. Therefore, it is expected that the SSFs are suitable candidates for current collector in flexible devices. Here, we will demonstrate that using SSFs as the current collector, the flexible solid-state supercapacitor with chemically converted graphene (CCG) as the active material shows large specific capacitance, long cyclic lifetime, and high stability during repeating bending, which significantly pushes the flexible devices toward practical applications.
INTRODUCTION With the proliferation of microsized energy storage devices, the need for small, flexible, wearable power supplies is evidently increasing. Potential candidates for flexible energy storage devices include Li-ion batteries and supercapacitors, and the like. Among these devices, the solid-state supercapacitors, which are combined with long cycle life, high power density, good environmental friendliness, and high safety, afford a very promising option for flexible energy storage applications.1−7 In past decades, numerous efforts have been devoted to fabricating flexible supercapacitors, and great progress has been made in promoting the overall performance of the devices. For example, recent studies showed that graphene-based flexible supercapacitor electrode had a specific capacitance of 310 F/g at 1 A/g.8 In another work, flexible solid-state devices based on the pseudocapacitance of PANI exhibited a high specific capacitance of 1079 F/g at a specific energy of 1.73 A/g.9 In general, flexible solid-state supercapacitor devices consist of flexible current collectors and electrode materials, as well as solid-state electrolyte.10 Most of the previous researches focused on the development of active electrode materials and solid state electrolytes.11−16 However, the current collectors available for flexible supercapacitors are still very limited. The most widely used flexible current collector in literature is carbon fabrics, which is woven from carbon fibers.17,18 Carbon fabrics have high mechanical strength and good flexibility, but their conductivity is not satisfying. Other reported flexible current collectors include metal foils and polymer films with conductive coatings.19,20 For instance, Ti foil and Ni foil have been used as conductive substrates for solid state supercapacitors.21,22 These metal foils are highly conductive, but they © 2016 American Chemical Society
Received: December 14, 2015 Accepted: February 2, 2016 Published: February 2, 2016 4724
DOI: 10.1021/acsami.5b12180 ACS Appl. Mater. Interfaces 2016, 8, 4724−4729
Research Article
ACS Applied Materials & Interfaces
Figure 1. (A) Schematic illustration of fabrication process of the flexible solid-state supercapacitor. (B) Digital image and (C) SEM image of SSF. (D) Digital image of GHG monoliths with different sizes and shapes. (E) SEM image of the lyophilized GHG. (F) Digital images of CCG@SSF electrode. (G) Cross-sectional SEM image of the compressed CCG film on the CCG@SSF electrode.
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immersed in the hot H2SO4/PVA blend solution (containing 8.3% H2SO4 and 8.3% PVA) for 15 min,28 and then picked out for airdrying at room temperature for 12 h. A polymer gel electrolyte layer forms on the each electrode. Afterward, the two electrodes were pressed together under a pressure of ∼1 MPa for 10 min, making the polymer gel electrolyte on each electrode merge into one thin separating layer, and the two electrodes tightly adhere to each other. All the components were assembled into a layered structure and tightly sealed by scotch tape (3M) for electrochemical measurements. The typical thickness of the whole device was around 0.20 mm. Characterization. The morphologies of SSF and GHG were characterized by SEM (Hitachi TM3000 scanning electron microscope). The sheet resistance of SSF was tested by RTS-9 four probes resistance meter. All the electrochemical experiments were carried out using CHI 660D electrochemical workstation. Before testing the device was activated by cyclic voltammetry cycling from 0 V to the work voltage of each device for 50 cycles.29 The electrochemical impedance spectroscopy measurements were performed at open circuit potential with a sinusoidal signal over a frequency range from 100 kHz to 10 mHz at an amplitude of 10 mV. The cycle life tests were conducted by galvanostatic charge−discharge (GCD) measurements. The specific capacitances (Cd) derived from galvanostatic discharge curves were calculated based on the following formula:30−32
EXPERIMENTAL SECTION
Chemicals and Materials. 316L stainless steel fabric (SSF) was provided by Longyan QLon Metal Fiber Co., Ltd. Before use, SSF was washed using acetone and ethanol for 10 min in an ultrasonic bath. Graphene oxide (GO) was prepared from natural graphite according to the modified Hummers’ method, as reported in our previous paper.26 Natural graphite powders (325 mesh) were bought from Qingdao Huatai Lubricant Sealing S&T Co., Ltd. Concentrated sulfuric acid (H2SO4, 98%) and hydrazine hydrate (80%) were purchased from Xilong Chemical Industry Incorporated Co., Ltd. Poly(vinyl alcohol) (PVA) (MW ∼ 1800 g/mol), acetone, and ethanol were supplied by Sinopharm Chemical Reagent Co., Ltd. Fabrication of CCG@SSF Electrode. The graphene hydrogel (GHG) was prepared by reducing GO hydrothermally and treating the product with hydrazine.27 Briefly, 10 mL GO aqueous dispersion (2 mg/mL) was sealed in a Teflon-lined autoclave and maintained at 180 °C for 12 h. After the autoclave cooled to room temperature, the black GHG block in the autoclave was taken out, immersed into an aqueous solution of hydrazine monohydrate (50%), and heated at 95 °C for 8 h. Finally, the resulting GHG was purified by dialysis overnight in ultrapure water. The as-prepared GHG was immersed in 1.0 M H2SO4 aqueous electrolyte for 12 h to exchange its interior water with electrolyte and cut into small cylindrical blocks (with a thickness of ∼1 cm and a diameter of ∼0.8 cm). Then the solvated GHG block was placed on a piece of SSF (1.8 × 0.8 cm), and compressed into thin films using hydraulic press. The sample (CCG@SSF) was kept under 30 MPa for 30 min to form well-adhered CCG film on the SSF, and was used as the electrode in the following investigations. The areal mass loading of the electrode was 4.4−5.6 mg/cm2 (efficient electrode area: 0.503 cm2), unless specified otherwise. Setup of Solid-State Supercapacitor Device. To fabricate solid-state supercapacitor device, two CCG@SSF electrodes were
Cd =
J=
Jt V − IR
I S
(1)
(2)
where J is the areal current density, I is the current applied on the device, S is the superficial area of two electrodes, t is the discharge time, V is the highest voltage in the GCD curves, and IR represents the voltage drop at the beginning of the discharge process, caused by the 4725
DOI: 10.1021/acsami.5b12180 ACS Appl. Mater. Interfaces 2016, 8, 4724−4729
Research Article
ACS Applied Materials & Interfaces internal resistance of the device. The energy density, E, and power density, P, of the electrode materials in device were calculated using the following formula:30−32 E=J
P=
∫t
t2
Vdt
1
(3)
E t 2 − t1
(4)
where t1 is the time at the beginning of the discharge process, t2 the time at the end of the discharge process, and J and V have the same definitions as in eqs 1 and 2. The specific capacitance (Cs) of the single electrode was calculated from the GCD curves measured in the threeelectrode system:30−32
Cs =
It SΔV
(5)
where S is the efficient area of one electrode (the area of the CCG film on the SSF), ΔV represents voltage drop upon discharging (excluding the IR drop), I and t have the same definitions as in eq 1 and 2.
Figure 2. Electrochemical properties of CCG@SSF electrode. (A) Cyclic voltammetry (CV) curves of CCG@SSF electrode at different scan rates. (B) GCD curves of CCG@SSF electrode at different constant current densities. (C) Plot of specific capacitance of CCG@ SSF electrode versus the discharge current density. (D) Nyquist plot of CCG@SSF electrode in the frequency range of 1 mHz−100 kHz.
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RESULTS AND DISCUSSION The fabrication process of the flexible supercapacitor and its architecture are schematically illustrated in Figure 1A. GHG monolith was compressed onto SSF, producing CCG@SSF electrode, which was then assembled into solid-state device using H2SO4/PVA gel electrolyte. Figure 1B shows the typical digital image of SSF, which is flexible but very tough. The SEM image in Figure 1C shows that the SSF is the nonwoven fabric, with long steel fibers densely packed and randomly oriented, forming a porous network structure. Such a network increases the surface roughness and offers large contact area for GHG. Due to the high conductivity of steel, the sheet resistance of SSF (40 μm) was measured to be as low as 81.4 ± 2.3 mΩ/□. This is much lower than that of carbon fabrics (342 μm, 732.3 mΩ/□; Figure S1 in the Supporting Information). If SSF is used as the current collector in supercapacitors, it can reduce the inner resistance of the device, which is very important for high power density devices. The as-prepared GHG synthesized via hydrothermal reduction of GO is a black monolith, as shown in Figure 1D (left). During the hydrothermal process, GO sheets were reduced and became less hydrophilic (see Figure S2 for the Raman spectrum of the GHG), thus they self-assembled into 3D porous network with the pore sizes ranging from submicrometers to several micrometers.33 The structure of the lyophilized GHG is shown in Figure 1E. GHG has large specific surface area and interconnected channels, thus is a suitable electrode material for supercapacitors. Moreover, GHG monolith can be tailored into desired shape with a knife and easily compressed into flexible film. As depicted in Figure 1F, after compression, the GHG monolith was converted into a CCG film and firmly attached on the SSF, producing CCG@ SSF electrode, which was highly flexible. The SEM image of CCG film (Figure 1G) reveals a compact layered structure. However, because the channels of GHG are filled with H2SO4, a small portion of H2SO4 solution remains between the CCG sheets after compression.34 Thus, there are still channels in the compressed CCG film, and H2SO4 is able to diffuse into the film. The electrochemical properties of CCG@SSF electrode were first investigated in a three-electrode system. As shown in Figure 2A, the CV curves show a pair of redox wave at around 0.4 V vs SCE, which is attributed to the redox reaction of the
residual oxygen-containing functional groups on CCG sheets. The current densities of the redox peaks increase linearly with the scan rates, indicating the fast diffusion of the electrolyte in the CCG film (Figure S3 in the Supporting Information). Figure 2B shows the GCD curves of the CCG@SSF electrode at different current densities. The GCD curves are nearly symmetric triangles, with a slight inflection at 0.4 V, corresponding to the redox wave in the CV curves.35 In addition, it is evident that CCG@SSF electrode exhibits a small ohmic drop, indicating the quite low internal resistance. The specific capacitance values calculated from their respective charge−discharge curves, using eq 5, are plotted as a function of the discharge current density and presented in Figure 2C. The areal specific capacitances of CCG@SSF electrode are calculated to be 730.8 ± 8.7 mF/cm2 (average of 3 samples, 182.7 ± 2.2 F/g) at 2 mA/cm2 (0.5 A/g) and 508.8 ± 3.4 mF/ cm2 (average of 3 samples, 127.2 ± 0.9 F/g) at 72 mA/cm2 (18 A/g), which are much higher than those of the graphene-based electrodes (graphene hydrogel/nickel foams electrodes, 41.1 mF/cm2 at 50.0 mA/cm2;36 graphene hydrogel/Zn foils supercapacitors, 33.8 mF/cm2 at 1 mA/cm2).37 We further used impedance spectroscopy to investigate the CCG@SSF electrode. Figure 2D depicts the Nyquist plot of CCG@SSF electrode. In the high frequency region, the Nyquist plot starts from the Z′-axis and progresses almost vertically to the Z″-axis at the low frequency, indicative of the ideal capacitive nature of the system.38,39 The series resistance of the CCG@SSF determined from the intercept of the Nyquist plot on Z′-axis is only 0.49 Ω, showing the high conductivity of the CCG@ SSF. In the high frequency region the CCG@SSF electrode exhibits a small semicircle, which is caused by the charge transfer reaction of the residual oxygen-containing functional groups on CCG sheets, as demonstrated by the CV curves. The above experimental results definitely reveal that CCG@SSF has high capacitive performance, and the SSF is a suitable current collector for supercapacitors. We then built the flexible all-solid-state supercapacitor with CCG@SSF as the electrode. The CV test from 0 to 1 V at different scan rates affords deformed rectangular curves (Figure 4726
DOI: 10.1021/acsami.5b12180 ACS Appl. Mater. Interfaces 2016, 8, 4724−4729
Research Article
ACS Applied Materials & Interfaces 3A). Also the current density increases linearly with the scan rate, showing the characteristic behavior of the capacitor
(Figure S4 in the Supporting Information). Meanwhile, the GCD curves show the nearly symmetric triangular shape with small voltage drops at the initial region of the discharge curve (Figure 3B). Both results demonstrate an ideal capacitive behavior and a very low equivalent series resistance in the CCG@SSF supercapacitor. According to eq 1 and 2, the specific capacitances of the CCG@SSF supercapacitor are calculated to be 180.4 ± 2.3 mF/cm2 (average of 3 samples, 45.1 ± 0.6 F/g) at a current density of 1 mA/cm2 (0.25 A/g), and 108.8 ± 2.3 mF/cm2 (average of 3 samples, 27.2 ± 0.6 F/ g) even at 16 mA/cm2 (4 A/g), which are much higher than those of the graphene-based supercapacitors (graphene hydrogel/Zn foils supercapacitors, 33.8 mF/cm2 at 1 mA/cm2;37 graphene hydrogel/Pt foils supercapacitors, 71.0 mF/cm2 at 1 mA/cm2),40 as shown in Figure 3C. Figure 3D shows the electrochemical impedance spectroscopy of the solid-state device. The Nyquist plot of the supercapacitor typically consists of a short 45° region, indicating the efficient electrolyte diffusion. At high frequency, the intercept of the Nyquist plot with the real axis is only 0.019 Ω, which suggests low series resistance of the supercapacitor, benefiting from the excellent conductivity of the SSF substrate. The internal resistance, R, determines the maximum output power, Pmax, of the device by following equation:30−32
Pmax =
V2 4R
(6)
where V is the voltage of the device. Thus, the low internal resistance ensures the high Pmax of the device. The cycling stability of the CCG@SSF solid-state supercapacitor was measured by GCD technique. Figure 3E represents the specific capacitance of CCG@SSF solid-state supercapacitor during the cycling GCD at a constant current density of 8 mA/cm2. The specific capacitance remains at 96.8% after 7500 cycles. The long cycle life of the device can be attributed to the good electrochemical stability of both the CCG sheets and the SSF. To further evaluate the performance of our CCG@SSF solid-state supercapacitor for practical applications, we calculated the energy and power density of the device. The corresponding Ragone plot is presented in Figure 3F. The energy density reaches as high as 19.2 W h/cm2
Figure 3. Electrochemical properties of CCG@SSF supercapacitor. (A) CV curves of CCG@SSF supercapacitor at different scan rates. (B) GCD curves of CCG@SSF supercapacitor at different constant current densities. (C) Plot of specific capacitance of CCG@SSF supercapacitor versus the discharge current density. (D) Nyquist plot of CCG@SSF supercapacitor in the frequency range of 1 mHz−100 kHz. (E) Variation in the relative and real capacitance of CCG@SSF supercapacitor as a function of the charge−discharge cycles at a constant current density of 8 mA/cm2. (F) Ragone plots of the solidstate flexible devices and (inset) an LED powered by the solid-state flexible device.
Figure 4. (A) Schematic and digital images of the flexibility of the device with a total thickness of ∼0.20 mm. (B) The CV curves of the device at a scan rate of 50 mV/s at different bending diameter. (C) The relative capacitance of the CCG@SSF supercapacitor after different bending cycles. 4727
DOI: 10.1021/acsami.5b12180 ACS Appl. Mater. Interfaces 2016, 8, 4724−4729
ACS Applied Materials & Interfaces
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at a power density of 386.2 W/cm2 at a current density of 1 mA/cm2, while it can be still maintained at 7.4 W h/cm2 at a power density of 4531.8 W/cm2. An LED (rated voltage: 2 V) can be lit up using three series devices (Figure 3F, inset), showing the potential application of our solid-state supercapacitor as an ultrathin power supply. The SSF-based solid-state supercapacitor shows excellent mechanical flexibility. As shown in Figure 4A, a device can be reversibly bent to a small diameter of ∼2 mm. The CV curves of the device are found insensitive to the bending diameter, as depicted in Figure 4B, indicating the high mechanical flexibility of the device. Fatigue test reveals that after 800 stretchingbending cycles, the specific capacitance of the device maintained at 96.4% of the original value (Figure 4C). This antifatigue performance is higher than those of the most reported flexible supercapacitors (GH-PANI/GP supercapacitors: 98% after 100 bending;41 PANI/graphene supercapacitors: 97% after 100 bending).42 The morphology of the CCG@ SSF after Fatigue test was investigated by SEM, as shown in Figures S5 and S6 in the Supporting Information. No crack or deformation was observed in these SEM images, indicating that both SSF and CCG were not damaged by repeated stretchingbending. Obviously, the good mechanical strength and toughness of the SSF and CCG endow the device with excellent flexibility. Besides, the rough surface morphology of the SSF increases the contact area with the CCG film, also promotes the interfacial stability. Therefore, we can determine that SSF, with a good mechanical properties and high electrical conductivity, is an excellent current collector for flexible supercapacitor devices.
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CONCLUSION
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ASSOCIATED CONTENT
Research Article
AUTHOR INFORMATION
Corresponding Author
* Tel: +86-592-2186296. Fax: +86-592-2183937. E-mail: lilei@ xmu.edu.cn. Author Contributions
All authors contributed to the experimental design and data analyses. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51373143 and 21174116), the Natural Science Foundation of Fujian Province (No. 2014J0105) and the Fundamental Research Funds for the Central Universities (No. 2013SH003 and 201312G004).
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In summary, we have fabricated a high-performance solid-state flexible supercapacitor using SSF as the current collector. Experimental results show that the flexible CCG@SSF supercapacitor delivers a high energy density of 19.2 W h/ cm2 at even a high power density of 386.2 W/cm2, and after 7500 cycles at a high current density of 8 mA/cm2, the CCG@ SSF device also demonstrates an excellent capacitance retention of 96.8%. Moreover, the devices can be bent to a small diameter of 2 mm, and after 800 stretching-bending cycles, the capacitance of the device remains 96.4% of the original value, showing excellent flexibility and stability. We demonstrated that the SSF is emerging as a promising current collector for flexible energy storage systems owing to its excellent conductivity, good mechanical flexibility, and high electrochemical stability, and we believe such an all-solid-state supercapacitor with SSF as the current collector will boost the development of highly flexible and wearable electronics and integrated fabric power devices.
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12180. Digital image of carbon fabrics; the correlation between the redox peaks current densities vs scan rates of CCG@ SSF electrode and supercapacitor in CV curves; the SEM images of SSF and the compressed CCG film after bending cycles. (PDF) 4728
DOI: 10.1021/acsami.5b12180 ACS Appl. Mater. Interfaces 2016, 8, 4724−4729
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DOI: 10.1021/acsami.5b12180 ACS Appl. Mater. Interfaces 2016, 8, 4724−4729
