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Liubing Dong, Chengjun Xu,* Yang Li, Changle Wu, Baozheng Jiang, Qian Yang, Enlou Zhou, Feiyu Kang,* and Quan-Hong Yang Flexible supercapacitors are important energy storage devices used for wearable and smart electronics. Various flexible supercapacitors have been prepared, including fiber supercapacitors,[1–10] paper-like supercapacitors,[11–15] 3D aerogel materials based supercapacitors, and textile supercapacitors.[4,16–23] During them, fiber supercapacitors can be utilized in microdevices, while textile supercapacitors are suitable for large-sized energy storage. The key to fabricating flexible supercapacitors lies in the acquirement of flexible electrodes, e.g., flexible fiber electrodes and textile electrodes. Generally, each fiber electrode contains a fibrous structural support (plastic fiber, metal wire, carbon nanotube (CNT) yarn, graphene fiber, carbon fiber bundle, etc. They are highly flexible and mechanically strong) and electrochemically active materials (activated carbon particles, CNTs, graphene sheets, MnO2, polyaniline, polypyrrole, etc.).[1–10] Textile electrodes are constructed by porous body materials (e.g., cotton cloth) and fillers of electrochemically active materials (as listed above).[18–23] Notice that in previously reported fiber electrodes and textile electrodes, fibrous structural supports and body materials often have a high weight and/or volume fraction, but their electrochemical performances are not satisfactory, which seriously undermines the specific capacitances of entire electrodes and supercapacitors.[18] Flexible fiber electrodes can be woven into textile electrodes, and this widens their application foreground; however, directly knitting fiber electrodes into textiles is very difficult.[4]
L. Dong, Dr. C. Xu, C. Wu, B. Jiang, Q. Yang, Prof. F. Kang, Prof. Q.-H. Yang Graduate School at Shenzhen Tsinghua University Shenzhen 518055, China E-mail: [email protected]; [email protected] L. Dong, Prof. F. Kang School of Materials Science and Engineering Tsinghua University Beijing 100084, China Y. Li College of Chemistry and Chemical Engineering Xiamen University Xiamen 361005, China E. Zhou Jiangnan Graphene Research Institute Changzhou 213149, China
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Simultaneous Production of High-Performance Flexible Textile Electrodes and Fiber Electrodes for Wearable Energy Storage
In turn, it is also hard to use textile electrodes to get fiber electrodes, because when textile electrodes are cut to microsize, their structures may collapse (at least, fabrication of fiber electrodes/supercapacitors from textile electrodes has rarely been reported). Therefore, we have to separately produce fiber electrodes and textile electrodes using different machines and/ or methods, potentially pushing the cost higher of flexible supercapacitor products. So it is reasonable to believe that simultaneous productions of flexible textile electrodes and fiber electrodes in one process will make it easier and cheaper for producing various flexible supercapacitors. Furthermore, scalable fabrication of high-performance flexible fiber electrodes, on the basis of simple preparation methods and inexpensive raw materials, is an another challenging issue.[2] Herein, we proposed a novel strategy for the simultaneous productions of high-performance flexible textile electrodes and fiber electrodes. We choose activated carbon fiber cloth (ACFC; it is a kind of good electric double-layer capacitor (EDLC) electrodes.[18,23,24]) as body materials to design ACFC/CNTs and ACFC/MnO2/CNTs composites. The composites can not only be directly used as textile electrodes, but also be easily dismantled into fiber bundles as miniaturizing fiber electrodes. Relatively high electrochemical activity of ACFC body material, excellent electrical conductivity of CNTs, and ultrahigh theoretical capacitance of MnO2 endow the fabricated textile electrodes with exceptional electrochemical properties, having an areal capacitance of 2542 mF cm−2, an energy density of 56.9 µWh cm−2, and a power density of 16287 µW cm−2. Furthermore, high electrical conductivity and porous structure of the prepared composite textiles allow us to assemble thicker textile electrodes with a larger energy output of up to 88.5 µWh cm−2. Moreover, fiber electrodes dismantled from the as-prepared composite textiles also exhibit attractive electrochemical properties: capacitance, energy density, and power density are as high as 640 mF cm−2, 11.1 µWh cm−2, and 8028 µW cm−2, respectively, much higher than those of many reported microelectrodes. In addition, both the textile electrodes and the fiber electrodes display outstanding cycling performance and structural flexibility. The developed strategy, being composed of a simple preparation method, low-cost raw materials, and high-performance electrode products, will effectively promote the commercialization of flexible energy storage devices. Scanning electron microscope (SEM) images of ACFC textiles and activated carbon fiber bundles (ACFBs) are displayed in Figure 1a,b. ACFC textiles are woven from ACFBs, and the latter ones are formed by thousands of twisted activated carbon
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Figure 1. SEM images of a) ACFC and b) ACFB. ACFC based textiles are c) bent and d) wound on a 8.5 mm diameter plastic tube. e) ACFB based fiber bundles with various lengths, easily getting from textiles, can be distorted, knotted, and woven into a simple textile-like framework.
fibers. Mechanically strong and flexible carbon fibers endow ACFC based textiles and ACFB based fiber bundles with certain strength and excellent flexibility: the composite textiles (Figure 1c,d) are able to be bent or wound on a plastic tube, and the composite fiber bundles (Figure 1e) can be distorted, knotted, or woven into a simple textile-shaped framework (however, the weaving process is very difficult because of the rough surface of fiber electrodes). In fact, the displayed composite fiber bundles are directly obtained by dismantling composite textiles. This offers a possibility for simultaneous productions
of flexible textile electrodes and fiber electrodes in one process, and also means that laboriously knitting fiber electrodes into textile electrodes becomes needless. Micromorphologies of “dipping and free-drying” method prepared ACFC/CNTs composite (noted as FxC, where x stands for the CNT concentration in aqueous solutions used for preparing composites) in Figure 2a,b and Figure S1a–e (Supporting Information) show that some CNTs cover fiber surfaces and the others disorderly disperse in the space between fibers. The CNTs connect carbon fibers together to form conductive
Figure 2. SEM images of a) F2C and b) F3C. CV curves of c) ACFC, d) F2C, and e) F3C supercapacitors. f) Areal capacitances of the textile electrodes.
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enhanced remarkably and specific surface area still reaches up to 721 m2 g−1 (Figure S2 and Table S1, Supporting Information). Electrochemical properties of ACFC/MnO2 and ACFC/MnO2/ CNTs composite textile electrodes based supercapacitors are given in Figure 3d–l and Figure S7 (Supporting Information). For F1M and F2M textile electrodes, MnO2 enhances electrochemical capacity at low scan rates, and a higher MnO2 content causes a larger supercapacity, benefiting from the ultrahigh theoretical capacitance of MnO2.[9,20] However, poor electrical property of MnO2 brings bad rate capability to F1M and F2M electrodes (Figure 3f). Furthermore, lowest electrical conductivity and specific surface area result in the worst capacitances of F3M at each scan rate (Figure S7, Supporting Information). Compared with F2M textile, F2M2C exhibits superior electrochemical performance (Figure 3d–f), especially rate capability. Moreover, F2M3C textile electrode is also fabricated, but it is not better than F2M2C in electrochemical behavior (Figure S7c–e, Supporting Information). Equivalent series resistance (ESR) values of ACFC, F2M, and F2M2C supercapacitors, calculated based on voltage drop (IR drop) in galvanostatic charge–discharge (GCD) curves (Figure 3g), are 4.4, 9.2, and 1.8 Ω cm2, respectively, quantitatively revealing that coated MnO2 on fiber surface markedly increases internal resistance of textile supercapacitors, while the presence of CNTs is conducive to reducing the resistance (in supercapacitors, textile electrodes are compressed. This leads to better electric contacts for CNTs,[19] thereby making F2M2C electrode more conductive than ACFC electrode). Electrochemical impedance spectroscopy (EIS) characteristics (Figure 3h) are consistent with above discussions. Large areal capacitance and exceptional rate performance allow F2M2C supercapacitor to provide a high energy density of 56.9 µWh cm−2 and a power density of up to 16287 µW cm−2 (Figure 3i). F2M2C textile electrode has a long operation life: only 4% loss in the areal capacitance is observed after 1500 cycles (Figure 3j). When F2M2C electrode is straight or bent at 90° and 180°, its capacitance fluctuation is less than 6% (Figure 3k); besides, after bending for 100 cycles, the electrode basically remains in its original appearance and shows a 93% capacitance retention (Figure 3l). In contrast, capacitance retention of CNTs/polyaniline film electrode is ≈92% after 50 cycles of bending,[25] while CNTs/ordered mesoporous carbon hybrid fiber electrode and CNTs/polyaniline coated plastic fiber electrode show ≈100% and 94% capacitance, respectively, when the cycle number of bending reaches 1000.[10,26] Supercapacitors possessing high energy densities are required in some fields. Preparation of high-energy supercapacitors needs thick electrodes with high loading of active materials.[22,27,28] Porous activated carbon fiber felt (2.5–3 mm thick) has been attempted to be used as such an electrode, but its electrochemical performance at high scan rates was very poor (Figure S8a, Supporting Information). By contrast, the ACFC/CNTs and ACFC/MnO2/CNTs composite textiles have high capacitances at high scan rates, but their thickness is relatively small (0.4–0.5 mm). A simple method is applied herein to prepare thicker electrodes: two layers of the same textiles were overlaid as a single electrode. The obtained thick electrodes are noted as ACFCt, FxCt, FyMt, and FyMxCt. Electrochemical performances of the thick electrodes are displayed in Figure 4 and Figure S8b (Supporting Information). For ACFCt, F2Ct, and F3Ct, a better electrical property of textiles corresponds
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networks, thus improving the electrical behavior of ACFC textiles. A higher CNT content leads to a better electrical conductivity of ACFC/CNTs composite textiles as intended (Figure S2, Supporting Information). When the ACFC/CNTs textiles are used as EDLC electrodes, their specific surface area and pore structure are as important as electrical property.[15,18,20,23] Table S1 (Supporting Information) suggests that introduction of CNTs decreases specific surface area, and a higher CNT content corresponds to a smaller specific surface area value. Meanwhile, the presence of CNTs affects total pore volume and average pore size, but as a whole, ACFC and ACFC/CNTs textiles have similar pore structures (Figure S3, Supporting Information). Figure 2c–e and Figure S4 (Supporting Information) give the cyclic voltammetry (CV) curves of ACFC and ACFC/CNTs composite textiles. These curves possess a rectangular shape at low scan rates. But for pure ACFC textile electrode, its closed CV curve measured at a high scan rate of 100 mV s−1 displays a narrow area, reflecting its poor rate capability.[18,20] Introduction of CNTs in composite textiles leads to a larger area of the CV curves tested at 100 mV s−1, suggesting optimized capacitive properties at high scan rates. The scan rate dependence of the areal capacitance of ACFC based textiles is shown in Figure 2f. Areal capacitance of all textile electrodes decreases with increasing scan rates, but each kind of textiles has different capacitance retention at higher scan rates, which is a reflection of inherent rate capability of electrode materials.[6,10,18] Clearly, F2C and F3C possess good rate capability, being ascribed to their outstanding electrical conductivity.[18] At relatively low scan rates, F0.5C displays the largest areal capacitances, due to its large specific surface area, appropriate pore structure, and optimized electrical conductivity. Seriously reduced specific surface area results in the modest capacitive performances of F3C, even though its electrical conductivity is the highest. Electrical conductivity and specific surface area, etc., of F2C, fall between those of F0.5C and F3C, and so does the capacity at relatively low scan rates. MnO2 is introduced into ACFC textiles through the chemical reaction between 0.1 M KMnO4 aqueous solution and carbon fibers (synthesized ACFC/MnO2 composite textiles are marked as FyM, where y is the reaction time (unit: minute)). Large specific surface area of activated carbon fibers permits a high loading of MnO2. In F2M textile, area density of deposited MnO2 is 1.99 mg cm−2. Figure 3a,b and Figure S1f,g (Supporting Information) show that flake-like MnO2 uniformly coats on carbon fiber surface. Energy dispersive spectrometer and X-ray photoelectron spectroscopy characteristics (Figure S5, Supporting Information) directly indicate that the deposited manganese oxide on fiber surface is MnO2 (corresponding to 4+ oxidation state for Mn). Introduction of MnO2 causes decay of electrical conductivity, specific surface area, total pore volume, and average pore size of textiles (Figures S2 and S6 and Table S1, Supporting Information). This is more obvious upon a higher MnO2 content. In order to optimize the electrical property, CNTs modified F2M composite textiles were synthesized (denoted as F2MxC, where x has the same meaning as that mentioned above). Taking F2M2C textile as an example, its SEM micromorphology is given in Figure 3c and Figure S1h,i (Supporting Information). Electrical property of F2M2C is
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Figure 3. SEM images of a,b) F2M and c) F2M2C textiles. CV curves of d) F2M and e) F2M2C supercapacitors. f) Areal capacitances of the textile electrodes. g) GCD curves at 20 mA cm−2 and h) EIS of the supercapacitors. i) Ragone plots of our and some previously reported supercapacitors. The data noted as R1, R2, R3, R4, and R5 are from refs.[2,5–7], and[3], respectively. j) Cycling behavior of F2M2C textile electrode. Capacitance retention of F2M2C: k) under different bending states and l) after bending for various cycles (tested by CV at 2 mV s−1).
to a higher capacity and a better rate capability. Likewise, even though F2M2Ct electrode has very high areal capacitances at low scan rates, modest electrical conductivity makes it show very poor rate performance. F2M3Ct is similar to F2M2Ct in electrochemical behavior. Above all, F3Ct is a kind of promising electrodes for producing high-energy and high-power supercapacitors. Its specific capacitance is as large as 3416 mF cm−2 at 2 mV s−1 and 1620 mF cm−2 at 100 mV s−1. When charge– discharge current density increases from 10 to 75 mA cm−2 in GCD tests (Figure 4e gives GCD curves at 10 mA cm−2), energy density of F3Ct electrodes based symmetric supercapacitor
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changes from 88.5 to 41.6 µWh cm−2, and the power density enhances from 2172 to 13087 µW cm−2 (Figure 3i). Excellent electrical property, on the basis of porous structure, not only ensures the rapid movement of electrons and ions inside F3Ct electrode but also gives the electrode favorable cycling performance (Figure 4f and Figure S8c, Supporting Information). Another attractive feature of our high-performance ACFC based textiles is that they can be easily dismantled into numerous individual fiber bundles used as flexible fiber electrodes. F2M2C textile exhibits excellent supercapacitive properties, therefore, it was utilized to prepare ACFB/MnO2/CNTs
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fiber electrodes. As exhibited in Figure 1e, the obtained ACFB/ MnO2/CNTs composite fiber bundles are very flexible. In the meantime, the bundles display impressive electrochemical performances when they are used as fiber electrodes in symmetric supercapacitors (Figure 5a–d). Volumetric, length, areal, and gravimetric capacitance can reach 66 F cm−3, 78 mF cm−1, 640 mF cm−2, and 192 F g−1, respectively. According to GCD curves tested at current densities of 4–37 mA cm−2 (typical GCD curves are displayed in Figure 5c), energy density of
the fiber electrode based symmetric supercapacitor is calculated to be as high as 11.1 µWh cm−2 at a power density of 933 µW cm−2 and even when power density increases to 8028 µW cm−2, energy density keeps a high level of 6.3 µWh cm−2 (Figure 3i). The performances are significantly higher than many previously reported results of fiber electrodes/supercapacitors (detailed comparison is summarized in Supporting Information Table S2 and Figure 3i).[13,14] An important reason is that the applied fibrous structural support, i.e., ACFB, in our
Figure 5. Electrochemical performance of ACFB/MnO2/CNTs fiber electrode: a) CV curves; b) volumetric, length, areal, and gravimetric capacitances; c) typical GCD curves; d) cycling behavior; capacitance retention: e) under different bending states and f) after bending for various cycles (tested by CV at 2 mV s−1).
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fiber electrode has high specific capacitances (Figure S9, Supporting Information), and introduction of MnO2 and CNTs further optimizes electrochemical properties of the fiber electrode, as discussed in our textile electrodes. ACFB/MnO2/CNTs fiber electrode exhibits exceptional electrochemical stability over a very large cycle number of 25 000 cycles (Figure 5d). Owing to highly ordered arrangement of carbon fibers in ACFB bundles, electrical conductivity of ACFB based fiber electrodes is better than that of ACFC based textile electrodes.[6] This is why ACFB/MnO2/CNTs fiber electrode is superior to F2M2C textile electrode in cyclic performance. Besides, improvement in wetting of electrode materials by electrolyte causes an initial increase of electrode capacitances upon cycling (Figures 3j and 5d),[19] but at the same time, cyclic mechanical stress during charge/discharge cycles tends to attenuate electrode capacitances.[8] Consequently, the electrode capacitances increase firstly and then decrease. Compared with F2M2C textile electrode, relatively better electrical conductivity of ACFB/MnO2/ CNTs fiber electrode postpones appearance of capacitance decrease stage (2000 cycles for ACFB/MnO2/CNTs versus 100 cycles for F2M2C). But it is worth mentioning that test environment (e.g., a slight fluctuation of temperature during cyclic performance evaluation) also affects the cyclic behaviors of electrodes to some extent.[29] When the fiber electrode is bent at 90° and 180° (Figure 5e), its capacitance increases by 0.8% and 11.1%, respectively, compared with that of the electrode at original state. Specific capacitance of the fiber electrode gradually reduces with increasing cycle number of bending (Figure 5f). After bending, appearance of the fiber electrode does not change, but in the view of microscopic (Figure S10, Supporting Information), we can find that some carbon fiber breaks, and meanwhile, MnO2 and CNTs drop off from fiber surface in local regions. These are believed to be the main reasons why capacitances of ACFB/MnO2/CNTs fiber electrode and F2M2C textile electrode decrease after bending. In summary, an effective strategy was proposed for the simultaneous productions of high-performance flexible textile electrodes and fiber electrodes. The prepared textile electrodes are ACFC/CNTs and ACFC/MnO2/CNTs composite textiles. During them, F2M2C textile electrode/supercapacitor displays outstanding areal capacitances and energy densities, and highly conductive F3C textile is suitable for fabricating thicker electrode for supercapacitors requiring very high energy output. Scalable preparation of fiber electrodes was easily realized by dismantling ACFC based textiles. The obtained ACFB/MnO2/CNTs fiber electrode exhibits attractive electrochemical performance. In addition, our textile electrodes and fiber electrodes possess a long operation life and outstanding flexibility. Electrochemical properties of our electrodes may be further improved by regulating the morphologies of MnO2, using high-performance CNTs, and introducing electrochemically active polymers. Despite this, the present work is promising to promote the development of flexible electrodes and flexible energy storage devices.
Experimental Section ACFC/CNTs textiles were prepared using the “dipping and freezedrying method.”[18] ACFC/MnO2 textiles were synthesized by immersing
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ACFC into 0.1 M neutral solution of KMnO4 for 1–3 min. ACFC/MnO2/ CNTs textiles were fabricated by immersing ACFC/MnO2 composites in CNT solutions followed by the freeze-drying process. Various fiber electrodes were obtained by directly dismantling the corresponding textile electrodes into individual fiber bundles. Brunauer–Emmett– Teller analyzer, four-point probe instrument, and SEM were used to characterize the samples. Electrochemical performances of the textile electrodes and fiber electrodes were tested in the form of symmetric supercapacitors (Figure S11, Supporting Information) with electrolyte of 6 M KOH aqueous solution. CV test, GCD technique, and EIS (5 mV AC amplitude and a frequency range of 100 kHz to 10 mHz) were exploited to estimate electrochemical behaviors of these electrodes at an Im6e (Zahner) electrochemical station. Cyclic performances of electrodes were evaluated by CV testes at 50 mV s−1.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors would like to thank NSFC (Grant No. 51102139), the National Key Basic Research (973) Program of China (Grant No. 2014CB932400), and Shenzhen Technical Plan Projects (Grant Nos. JC201105201100A and JCYJ20130402145002425) for financial support. The authors also appreciate the financial support from Guangdong Province Innovation R&D Team Plan (Grant No. 2009010025). Received: September 26, 2015 Revised: November 11, 2015 Published online: December 17, 2015
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Liubing Dong, Chengjun Xu,* Yang Li, Changle Wu, Baozheng Jiang, Qian Yang, Enlou Zhou, Feiyu Kang,* and Quan-Hong Yang Flexible supercapacitors are important energy storage devices used for wearable and smart electronics. Various flexible supercapacitors have been prepared, including fiber supercapacitors,[1–10] paper-like supercapacitors,[11–15] 3D aerogel materials based supercapacitors, and textile supercapacitors.[4,16–23] During them, fiber supercapacitors can be utilized in microdevices, while textile supercapacitors are suitable for large-sized energy storage. The key to fabricating flexible supercapacitors lies in the acquirement of flexible electrodes, e.g., flexible fiber electrodes and textile electrodes. Generally, each fiber electrode contains a fibrous structural support (plastic fiber, metal wire, carbon nanotube (CNT) yarn, graphene fiber, carbon fiber bundle, etc. They are highly flexible and mechanically strong) and electrochemically active materials (activated carbon particles, CNTs, graphene sheets, MnO2, polyaniline, polypyrrole, etc.).[1–10] Textile electrodes are constructed by porous body materials (e.g., cotton cloth) and fillers of electrochemically active materials (as listed above).[18–23] Notice that in previously reported fiber electrodes and textile electrodes, fibrous structural supports and body materials often have a high weight and/or volume fraction, but their electrochemical performances are not satisfactory, which seriously undermines the specific capacitances of entire electrodes and supercapacitors.[18] Flexible fiber electrodes can be woven into textile electrodes, and this widens their application foreground; however, directly knitting fiber electrodes into textiles is very difficult.[4]
L. Dong, Dr. C. Xu, C. Wu, B. Jiang, Q. Yang, Prof. F. Kang, Prof. Q.-H. Yang Graduate School at Shenzhen Tsinghua University Shenzhen 518055, China E-mail: [email protected]; [email protected] L. Dong, Prof. F. Kang School of Materials Science and Engineering Tsinghua University Beijing 100084, China Y. Li College of Chemistry and Chemical Engineering Xiamen University Xiamen 361005, China E. Zhou Jiangnan Graphene Research Institute Changzhou 213149, China
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Simultaneous Production of High-Performance Flexible Textile Electrodes and Fiber Electrodes for Wearable Energy Storage
In turn, it is also hard to use textile electrodes to get fiber electrodes, because when textile electrodes are cut to microsize, their structures may collapse (at least, fabrication of fiber electrodes/supercapacitors from textile electrodes has rarely been reported). Therefore, we have to separately produce fiber electrodes and textile electrodes using different machines and/ or methods, potentially pushing the cost higher of flexible supercapacitor products. So it is reasonable to believe that simultaneous productions of flexible textile electrodes and fiber electrodes in one process will make it easier and cheaper for producing various flexible supercapacitors. Furthermore, scalable fabrication of high-performance flexible fiber electrodes, on the basis of simple preparation methods and inexpensive raw materials, is an another challenging issue.[2] Herein, we proposed a novel strategy for the simultaneous productions of high-performance flexible textile electrodes and fiber electrodes. We choose activated carbon fiber cloth (ACFC; it is a kind of good electric double-layer capacitor (EDLC) electrodes.[18,23,24]) as body materials to design ACFC/CNTs and ACFC/MnO2/CNTs composites. The composites can not only be directly used as textile electrodes, but also be easily dismantled into fiber bundles as miniaturizing fiber electrodes. Relatively high electrochemical activity of ACFC body material, excellent electrical conductivity of CNTs, and ultrahigh theoretical capacitance of MnO2 endow the fabricated textile electrodes with exceptional electrochemical properties, having an areal capacitance of 2542 mF cm−2, an energy density of 56.9 µWh cm−2, and a power density of 16287 µW cm−2. Furthermore, high electrical conductivity and porous structure of the prepared composite textiles allow us to assemble thicker textile electrodes with a larger energy output of up to 88.5 µWh cm−2. Moreover, fiber electrodes dismantled from the as-prepared composite textiles also exhibit attractive electrochemical properties: capacitance, energy density, and power density are as high as 640 mF cm−2, 11.1 µWh cm−2, and 8028 µW cm−2, respectively, much higher than those of many reported microelectrodes. In addition, both the textile electrodes and the fiber electrodes display outstanding cycling performance and structural flexibility. The developed strategy, being composed of a simple preparation method, low-cost raw materials, and high-performance electrode products, will effectively promote the commercialization of flexible energy storage devices. Scanning electron microscope (SEM) images of ACFC textiles and activated carbon fiber bundles (ACFBs) are displayed in Figure 1a,b. ACFC textiles are woven from ACFBs, and the latter ones are formed by thousands of twisted activated carbon
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Figure 1. SEM images of a) ACFC and b) ACFB. ACFC based textiles are c) bent and d) wound on a 8.5 mm diameter plastic tube. e) ACFB based fiber bundles with various lengths, easily getting from textiles, can be distorted, knotted, and woven into a simple textile-like framework.
fibers. Mechanically strong and flexible carbon fibers endow ACFC based textiles and ACFB based fiber bundles with certain strength and excellent flexibility: the composite textiles (Figure 1c,d) are able to be bent or wound on a plastic tube, and the composite fiber bundles (Figure 1e) can be distorted, knotted, or woven into a simple textile-shaped framework (however, the weaving process is very difficult because of the rough surface of fiber electrodes). In fact, the displayed composite fiber bundles are directly obtained by dismantling composite textiles. This offers a possibility for simultaneous productions
of flexible textile electrodes and fiber electrodes in one process, and also means that laboriously knitting fiber electrodes into textile electrodes becomes needless. Micromorphologies of “dipping and free-drying” method prepared ACFC/CNTs composite (noted as FxC, where x stands for the CNT concentration in aqueous solutions used for preparing composites) in Figure 2a,b and Figure S1a–e (Supporting Information) show that some CNTs cover fiber surfaces and the others disorderly disperse in the space between fibers. The CNTs connect carbon fibers together to form conductive
Figure 2. SEM images of a) F2C and b) F3C. CV curves of c) ACFC, d) F2C, and e) F3C supercapacitors. f) Areal capacitances of the textile electrodes.
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enhanced remarkably and specific surface area still reaches up to 721 m2 g−1 (Figure S2 and Table S1, Supporting Information). Electrochemical properties of ACFC/MnO2 and ACFC/MnO2/ CNTs composite textile electrodes based supercapacitors are given in Figure 3d–l and Figure S7 (Supporting Information). For F1M and F2M textile electrodes, MnO2 enhances electrochemical capacity at low scan rates, and a higher MnO2 content causes a larger supercapacity, benefiting from the ultrahigh theoretical capacitance of MnO2.[9,20] However, poor electrical property of MnO2 brings bad rate capability to F1M and F2M electrodes (Figure 3f). Furthermore, lowest electrical conductivity and specific surface area result in the worst capacitances of F3M at each scan rate (Figure S7, Supporting Information). Compared with F2M textile, F2M2C exhibits superior electrochemical performance (Figure 3d–f), especially rate capability. Moreover, F2M3C textile electrode is also fabricated, but it is not better than F2M2C in electrochemical behavior (Figure S7c–e, Supporting Information). Equivalent series resistance (ESR) values of ACFC, F2M, and F2M2C supercapacitors, calculated based on voltage drop (IR drop) in galvanostatic charge–discharge (GCD) curves (Figure 3g), are 4.4, 9.2, and 1.8 Ω cm2, respectively, quantitatively revealing that coated MnO2 on fiber surface markedly increases internal resistance of textile supercapacitors, while the presence of CNTs is conducive to reducing the resistance (in supercapacitors, textile electrodes are compressed. This leads to better electric contacts for CNTs,[19] thereby making F2M2C electrode more conductive than ACFC electrode). Electrochemical impedance spectroscopy (EIS) characteristics (Figure 3h) are consistent with above discussions. Large areal capacitance and exceptional rate performance allow F2M2C supercapacitor to provide a high energy density of 56.9 µWh cm−2 and a power density of up to 16287 µW cm−2 (Figure 3i). F2M2C textile electrode has a long operation life: only 4% loss in the areal capacitance is observed after 1500 cycles (Figure 3j). When F2M2C electrode is straight or bent at 90° and 180°, its capacitance fluctuation is less than 6% (Figure 3k); besides, after bending for 100 cycles, the electrode basically remains in its original appearance and shows a 93% capacitance retention (Figure 3l). In contrast, capacitance retention of CNTs/polyaniline film electrode is ≈92% after 50 cycles of bending,[25] while CNTs/ordered mesoporous carbon hybrid fiber electrode and CNTs/polyaniline coated plastic fiber electrode show ≈100% and 94% capacitance, respectively, when the cycle number of bending reaches 1000.[10,26] Supercapacitors possessing high energy densities are required in some fields. Preparation of high-energy supercapacitors needs thick electrodes with high loading of active materials.[22,27,28] Porous activated carbon fiber felt (2.5–3 mm thick) has been attempted to be used as such an electrode, but its electrochemical performance at high scan rates was very poor (Figure S8a, Supporting Information). By contrast, the ACFC/CNTs and ACFC/MnO2/CNTs composite textiles have high capacitances at high scan rates, but their thickness is relatively small (0.4–0.5 mm). A simple method is applied herein to prepare thicker electrodes: two layers of the same textiles were overlaid as a single electrode. The obtained thick electrodes are noted as ACFCt, FxCt, FyMt, and FyMxCt. Electrochemical performances of the thick electrodes are displayed in Figure 4 and Figure S8b (Supporting Information). For ACFCt, F2Ct, and F3Ct, a better electrical property of textiles corresponds
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networks, thus improving the electrical behavior of ACFC textiles. A higher CNT content leads to a better electrical conductivity of ACFC/CNTs composite textiles as intended (Figure S2, Supporting Information). When the ACFC/CNTs textiles are used as EDLC electrodes, their specific surface area and pore structure are as important as electrical property.[15,18,20,23] Table S1 (Supporting Information) suggests that introduction of CNTs decreases specific surface area, and a higher CNT content corresponds to a smaller specific surface area value. Meanwhile, the presence of CNTs affects total pore volume and average pore size, but as a whole, ACFC and ACFC/CNTs textiles have similar pore structures (Figure S3, Supporting Information). Figure 2c–e and Figure S4 (Supporting Information) give the cyclic voltammetry (CV) curves of ACFC and ACFC/CNTs composite textiles. These curves possess a rectangular shape at low scan rates. But for pure ACFC textile electrode, its closed CV curve measured at a high scan rate of 100 mV s−1 displays a narrow area, reflecting its poor rate capability.[18,20] Introduction of CNTs in composite textiles leads to a larger area of the CV curves tested at 100 mV s−1, suggesting optimized capacitive properties at high scan rates. The scan rate dependence of the areal capacitance of ACFC based textiles is shown in Figure 2f. Areal capacitance of all textile electrodes decreases with increasing scan rates, but each kind of textiles has different capacitance retention at higher scan rates, which is a reflection of inherent rate capability of electrode materials.[6,10,18] Clearly, F2C and F3C possess good rate capability, being ascribed to their outstanding electrical conductivity.[18] At relatively low scan rates, F0.5C displays the largest areal capacitances, due to its large specific surface area, appropriate pore structure, and optimized electrical conductivity. Seriously reduced specific surface area results in the modest capacitive performances of F3C, even though its electrical conductivity is the highest. Electrical conductivity and specific surface area, etc., of F2C, fall between those of F0.5C and F3C, and so does the capacity at relatively low scan rates. MnO2 is introduced into ACFC textiles through the chemical reaction between 0.1 M KMnO4 aqueous solution and carbon fibers (synthesized ACFC/MnO2 composite textiles are marked as FyM, where y is the reaction time (unit: minute)). Large specific surface area of activated carbon fibers permits a high loading of MnO2. In F2M textile, area density of deposited MnO2 is 1.99 mg cm−2. Figure 3a,b and Figure S1f,g (Supporting Information) show that flake-like MnO2 uniformly coats on carbon fiber surface. Energy dispersive spectrometer and X-ray photoelectron spectroscopy characteristics (Figure S5, Supporting Information) directly indicate that the deposited manganese oxide on fiber surface is MnO2 (corresponding to 4+ oxidation state for Mn). Introduction of MnO2 causes decay of electrical conductivity, specific surface area, total pore volume, and average pore size of textiles (Figures S2 and S6 and Table S1, Supporting Information). This is more obvious upon a higher MnO2 content. In order to optimize the electrical property, CNTs modified F2M composite textiles were synthesized (denoted as F2MxC, where x has the same meaning as that mentioned above). Taking F2M2C textile as an example, its SEM micromorphology is given in Figure 3c and Figure S1h,i (Supporting Information). Electrical property of F2M2C is
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Figure 3. SEM images of a,b) F2M and c) F2M2C textiles. CV curves of d) F2M and e) F2M2C supercapacitors. f) Areal capacitances of the textile electrodes. g) GCD curves at 20 mA cm−2 and h) EIS of the supercapacitors. i) Ragone plots of our and some previously reported supercapacitors. The data noted as R1, R2, R3, R4, and R5 are from refs.[2,5–7], and[3], respectively. j) Cycling behavior of F2M2C textile electrode. Capacitance retention of F2M2C: k) under different bending states and l) after bending for various cycles (tested by CV at 2 mV s−1).
to a higher capacity and a better rate capability. Likewise, even though F2M2Ct electrode has very high areal capacitances at low scan rates, modest electrical conductivity makes it show very poor rate performance. F2M3Ct is similar to F2M2Ct in electrochemical behavior. Above all, F3Ct is a kind of promising electrodes for producing high-energy and high-power supercapacitors. Its specific capacitance is as large as 3416 mF cm−2 at 2 mV s−1 and 1620 mF cm−2 at 100 mV s−1. When charge– discharge current density increases from 10 to 75 mA cm−2 in GCD tests (Figure 4e gives GCD curves at 10 mA cm−2), energy density of F3Ct electrodes based symmetric supercapacitor
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changes from 88.5 to 41.6 µWh cm−2, and the power density enhances from 2172 to 13087 µW cm−2 (Figure 3i). Excellent electrical property, on the basis of porous structure, not only ensures the rapid movement of electrons and ions inside F3Ct electrode but also gives the electrode favorable cycling performance (Figure 4f and Figure S8c, Supporting Information). Another attractive feature of our high-performance ACFC based textiles is that they can be easily dismantled into numerous individual fiber bundles used as flexible fiber electrodes. F2M2C textile exhibits excellent supercapacitive properties, therefore, it was utilized to prepare ACFB/MnO2/CNTs
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fiber electrodes. As exhibited in Figure 1e, the obtained ACFB/ MnO2/CNTs composite fiber bundles are very flexible. In the meantime, the bundles display impressive electrochemical performances when they are used as fiber electrodes in symmetric supercapacitors (Figure 5a–d). Volumetric, length, areal, and gravimetric capacitance can reach 66 F cm−3, 78 mF cm−1, 640 mF cm−2, and 192 F g−1, respectively. According to GCD curves tested at current densities of 4–37 mA cm−2 (typical GCD curves are displayed in Figure 5c), energy density of
the fiber electrode based symmetric supercapacitor is calculated to be as high as 11.1 µWh cm−2 at a power density of 933 µW cm−2 and even when power density increases to 8028 µW cm−2, energy density keeps a high level of 6.3 µWh cm−2 (Figure 3i). The performances are significantly higher than many previously reported results of fiber electrodes/supercapacitors (detailed comparison is summarized in Supporting Information Table S2 and Figure 3i).[13,14] An important reason is that the applied fibrous structural support, i.e., ACFB, in our
Figure 5. Electrochemical performance of ACFB/MnO2/CNTs fiber electrode: a) CV curves; b) volumetric, length, areal, and gravimetric capacitances; c) typical GCD curves; d) cycling behavior; capacitance retention: e) under different bending states and f) after bending for various cycles (tested by CV at 2 mV s−1).
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fiber electrode has high specific capacitances (Figure S9, Supporting Information), and introduction of MnO2 and CNTs further optimizes electrochemical properties of the fiber electrode, as discussed in our textile electrodes. ACFB/MnO2/CNTs fiber electrode exhibits exceptional electrochemical stability over a very large cycle number of 25 000 cycles (Figure 5d). Owing to highly ordered arrangement of carbon fibers in ACFB bundles, electrical conductivity of ACFB based fiber electrodes is better than that of ACFC based textile electrodes.[6] This is why ACFB/MnO2/CNTs fiber electrode is superior to F2M2C textile electrode in cyclic performance. Besides, improvement in wetting of electrode materials by electrolyte causes an initial increase of electrode capacitances upon cycling (Figures 3j and 5d),[19] but at the same time, cyclic mechanical stress during charge/discharge cycles tends to attenuate electrode capacitances.[8] Consequently, the electrode capacitances increase firstly and then decrease. Compared with F2M2C textile electrode, relatively better electrical conductivity of ACFB/MnO2/ CNTs fiber electrode postpones appearance of capacitance decrease stage (2000 cycles for ACFB/MnO2/CNTs versus 100 cycles for F2M2C). But it is worth mentioning that test environment (e.g., a slight fluctuation of temperature during cyclic performance evaluation) also affects the cyclic behaviors of electrodes to some extent.[29] When the fiber electrode is bent at 90° and 180° (Figure 5e), its capacitance increases by 0.8% and 11.1%, respectively, compared with that of the electrode at original state. Specific capacitance of the fiber electrode gradually reduces with increasing cycle number of bending (Figure 5f). After bending, appearance of the fiber electrode does not change, but in the view of microscopic (Figure S10, Supporting Information), we can find that some carbon fiber breaks, and meanwhile, MnO2 and CNTs drop off from fiber surface in local regions. These are believed to be the main reasons why capacitances of ACFB/MnO2/CNTs fiber electrode and F2M2C textile electrode decrease after bending. In summary, an effective strategy was proposed for the simultaneous productions of high-performance flexible textile electrodes and fiber electrodes. The prepared textile electrodes are ACFC/CNTs and ACFC/MnO2/CNTs composite textiles. During them, F2M2C textile electrode/supercapacitor displays outstanding areal capacitances and energy densities, and highly conductive F3C textile is suitable for fabricating thicker electrode for supercapacitors requiring very high energy output. Scalable preparation of fiber electrodes was easily realized by dismantling ACFC based textiles. The obtained ACFB/MnO2/CNTs fiber electrode exhibits attractive electrochemical performance. In addition, our textile electrodes and fiber electrodes possess a long operation life and outstanding flexibility. Electrochemical properties of our electrodes may be further improved by regulating the morphologies of MnO2, using high-performance CNTs, and introducing electrochemically active polymers. Despite this, the present work is promising to promote the development of flexible electrodes and flexible energy storage devices.
Experimental Section ACFC/CNTs textiles were prepared using the “dipping and freezedrying method.”[18] ACFC/MnO2 textiles were synthesized by immersing
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ACFC into 0.1 M neutral solution of KMnO4 for 1–3 min. ACFC/MnO2/ CNTs textiles were fabricated by immersing ACFC/MnO2 composites in CNT solutions followed by the freeze-drying process. Various fiber electrodes were obtained by directly dismantling the corresponding textile electrodes into individual fiber bundles. Brunauer–Emmett– Teller analyzer, four-point probe instrument, and SEM were used to characterize the samples. Electrochemical performances of the textile electrodes and fiber electrodes were tested in the form of symmetric supercapacitors (Figure S11, Supporting Information) with electrolyte of 6 M KOH aqueous solution. CV test, GCD technique, and EIS (5 mV AC amplitude and a frequency range of 100 kHz to 10 mHz) were exploited to estimate electrochemical behaviors of these electrodes at an Im6e (Zahner) electrochemical station. Cyclic performances of electrodes were evaluated by CV testes at 50 mV s−1.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The authors would like to thank NSFC (Grant No. 51102139), the National Key Basic Research (973) Program of China (Grant No. 2014CB932400), and Shenzhen Technical Plan Projects (Grant Nos. JC201105201100A and JCYJ20130402145002425) for financial support. The authors also appreciate the financial support from Guangdong Province Innovation R&D Team Plan (Grant No. 2009010025). Received: September 26, 2015 Revised: November 11, 2015 Published online: December 17, 2015
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