High Performance Symmetric Supercapacitor Constructed Using

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Surfaces, Interfaces, and Applications

High Performance Symmetric Supercapacitor Constructed Using Carbon Cloth Boosted by Engineering Oxygen-containing Functional Groups Zhenyu Miao, Yuan Huang, Jianping Xin, Xiaowen Su, Yuanhua Sang, Hong Liu, and Jian-Jun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04426 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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High Performance Symmetric Supercapacitor Constructed Using Carbon

Cloth

Boosted

by

Engineering

Oxygen-containing

Functional Groups Zhenyu Miaoa, Yuan Huanga, Jianping Xina, Xiaowen Sua, Yuanhua Sanga, Hong Liua, b, *, and Jian-Jun Wanga, * a State

Key Laboratory of Crystal Material, Shandong University, Jinan 250100, China

b Institute

for Advanced Interdisciplinary Research (IAIR), University of Jinan, Jinan 250022, China

* E-mail addresses: [email protected] (H. Liu), and [email protected] (J. Wang).

ABSTRACT Carbon materials display appealing physical, chemical and mechanical properties, and have been extensively studied as supercapacitor electrodes. The surface engineering further allows us to tune their capability of adsorption/desorption and catalysis. Therefore, a facile and inexpensive chemical acid-etched approach has been developed to activate the carbon cloth as an electrode for supercapacitor. The capacitance of the acid-etched carbon cloth electrode can approach 5310 mF cm-2 at a current density of 5 mA cm-2 with remarkable recycling stability. The all-solid-state symmetric supercapacitor delivered a high energy density of 4.27 mWh cm-3 at a power density of 1.32 W cm-3. Furthermore, this symmetric supercapacitor exhibited outstanding mechanical flexibility, and the capacity kept nearly unchanged after 1,000 bending cycles. KEYWORDS: Carbon cloth, Chemical acid-etching, Oxygen-containing functional groups, Symmetric supercapacitor, Flexible supercapacitor. INTRODUCTION The increasingly demanding for energy has accelerated the consuming of traditional fossil fuels and lead to seriously environmental issues including greenhouse gas and pollutants emissions. Thus, it is of significant importance to explore various types of eco-friendly and sustainable energy sources. The past decades have witnessed great progress of the development of renewable energy including solar and wind energy. However, the intermittent feature of these sustainable energy has prompted the evolution of diverse energy storage systems.1 As a sort of widely used power sources, electrochemical energy storage systems covering batteries, supercapacitors and hybrid

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capacitors have gained intensive investigation in the past years owing to their good recycle stability, high energy density, and desirable safety. Rechargeable batteries such as lithium-ion batteries can display high energy density, however, deliver relatively poor power density, while recently supercapacitors outperform in some respects due to several merits including fast recharge ability, high power density, good stability.2-3 The enhancement of the performance largely depends on the active materials employed and has stimulated unremitting exploration of new counterparts by material and structural tuning and engineering.4-6 Due to the unique geometric arrangement and bonding of the carbon atoms, carbon materials exhibit some appealing physical, chemical and mechanical merits including prominent conductivity, mechanical stiffness, high surface area.7 Furthermore, carbon-based materials can be easily tailored and functionalized into the desirable morphology and structures along with multifunctional features, which makes them extensively studied as supercapacitor electrodes in the field of electrochemical conversion and storage.8-15 A number of studies on various carbon materials for supercapacitors have been reported in the past decades, ranging from activated carbon, carbide derivatives, carbon nanotubes to graphene, carbon aerogel and the like. Owing to low-cost and flexible characteristic, carbon cloth (CC) is a promising electrode material for supercapacitor and lithium batteries, in particular the flexible feature for plenty of potential applications including wearable electronics and self-powered sensors.16-22 For instance, Jost et al.22 knitted activated carbon-based supercapacitor devices showing an outstanding capacitance of 0.51 F cm-2. Evanoff and coworkers23 used a carbon nanotube (CNT) based fabric coated by Si layer to fabricate a high capacity flexible anode. Modification of carbon cloth with active materials has been regards as an effective way to promote the transport of charge carriers in flexible electrodes. Xu et al.24 fabricated Co3O4 nanosheet on carbon cloth which can achieve a capacitance of 1.18 F cm-2 at a scan speed of 1 mV s-1. Wang et al.25 decorated carbon cloth with hierarchical Fe2O3/PPy Nanoarrays, and an areal capacitance of 382.4 mF cm-2 was obtained at a current density of 0.5 mA cm-2. Although these methods can obtain a good areal capacitance, the preparation method is complicated and costly. Lately, Wang et al.26 have proposed a new electrochemical activation strategy to activate carbon cloth and a decent capacitance of 756 mF cm-2 can be achieved at the current density of 6 mA cm-2. Alternatively, a chemical activation strategy has been invented to functionalize carbon cloth for supercapacitors, but the resulted capacitance is moderate.27 Introducing surface functional groups has been demonstrated as a powerful way to boost the capacitive performance constructed with other carbon materials as well. Béguin et al.28 have enhanced the capacitive performance of carbonaceous material by carbonization of a biopolymer due to the presence of oxygen-containing

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functional groups, which are considered as active components for the contribution to the pseudocapacitance. Wang et al.29 have improved the specific capacitance by introduction of functional groups including -OH and -COOH to the surface of CNT. In spite of these remarkable and encouraging efforts, the achieved areal capacitance is still limited. Therefore, engineering the surface of the carbon cloth, tailoring the ratio of the functional species and identifying the correlation of the functional species and the capacitive performance is crucial to enhance their performance of supercapacitors. Herein, we developed a facile, cost-effective and easily-scale-up chemical acid-etched approach to tune functional species on carbon cloth. The modified carbon cloth decorated by oxygen-containing functional groups, showed an outstanding areal capacitance of 5310 mF cm-2 at a current density of 5 mA cm-2, to our knowledge, which is the highest among all of reported carbon-based electrode materials and better than couples of widely investigated transition metal oxide materials. Moreover, the fabricated electrode shows good reversibility with capacitance loss of less than 7% after 5,000 cycles. More importantly, a flexible symmetric supercapacitor made from the optimized carbon cloth electrodes approaches a highly attractive energy density of 4.27 mWh cm-3 at a power density of 1.32 W cm-3. EXPERIMENTAL SECTION Preparation of modified carbon cloth Chemicals. All regents are of analytical grade without further purification. Carbon cloth (WOS1002, 360 μm in thick) was purchased from Carbon Energy of Taiwan, China, and cleaned by ultrasonication in acetone and ethanol successively before use. Preparation of acid-etched carbon cloth (AECC): At first, CC was immersed in a mixture of concentrated HNO3 (10 mL) and H2SO4 (20 mL) for 5min. Subsequently, KMnO4 (1.5 g) was added to the mixture and incubated at 30℃ for n hours (denoted as AECC-n). Afterwards, 100 mL distilled water was introduced to the container slowly with stirring for one more hour. Then, H2O2 solution (3 mL) was added to the mixture, resulting in a large number of bubbles, and the solution was kept for another hour. Finally, the treated CC was flushed using distilled water and dried at 60℃. For comparison, the acid-etched CC without KMnO4 sample was prepared with the same procedure (incubated at 30℃ in an oil bath for 12 hours). Material Characterization X-ray diffraction (XRD), transmission electron microscopic (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and the electrochemical measurement were performed as our previous report.30-31 Raman measurements were

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carried out on a Jobin-Yvon HR 800 spectrometer with an excitation laser of 532 nm. The all-solid-state supercapacitor devices were fabricated and tested with the PVA/ H2SO4 slurry as a solid electrolyte, prepared as following: PVA (4 g) was dissolved in 1.0M H2SO4 (40mL) solution and heated at 90 ℃, until they were completely dissolved and formed a slurry. RESULTS AND DISCUSSIONS

Figure 1 (a) Schematic illustration of the chemical acid-etched results. (b, c) typical SEM images of the untreated CC and the AECC-12. (d, e) typical TEM images of the untreated CC and the AECC-12 sample. The dashed line indicates the extent of the activation layer.

Figure 1a shows the process of the chemical acid-etching carbon cloth. Typically, cleaned CC was immersed in a mixture solution of H2SO4, HNO3 and KMnO4 solution for n hours. SEM was utilized to analyze the morphology of the samples. Typical SEM images as shown in Figure 1 indicate that the surface of carbon cloth has been substantially modified. The untreated CC has a smooth surface, while the surface of

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carbon cloth treated for 12 hours became rough with a numerous wrinkles and bulges, especially at the edge of carbon fiber. It should be mentioned that the overall structure of the carbon fiber has not been destroyed. The microstructure of the samples was further examined by TEM (Fig. 1d, e). For the untreated sample, only negligible thin activation layer was observed. After chemical acid-etched, an activation layer of ~100 nm formed. This structure consolidates the results of SEM, further confirming the modification of the surface of carbon cloth by chemical acid-etching. The change of the surface is similar to that of carbon fiber treated by electrochemical activation method observed after treatment. XRD was carried out to detect if there were any possible crystal phases introduced during the treatment process. The XRD patterns presented in Figure S1 indicate that no by-product with crystal structure was observed and confirm that the adherent (wrinkles and bulges) on the surface are activated carbon product.

Figure 2 Core level XPS spectra. (a) normalized C1s spectra of the untreated CC, acid-etched CC without KMnO4 and AECC-12 samples. Deconvoluted (b) C1s and (c) O1s spectra of the AECC-12 sample. (d) Raman spectra of untreated CC, acid-etched CC without KMnO4 and AECC-12 sample.

Apart from the morphology change, the chemical species on carbon cloth were characterized using XPS and Raman spectroscopy. From the survey spectra of XPS (Figure S2), C and O signals dominated the spectra, while obvious S and N signal

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(Figure S3) were detected from the treated samples, which originated from H2SO4 and HNO3. This result agrees well with the element mapping measurement, indicating the uniform distribution of introduced functional groups (Figure S4). The high resolution XPS spectra of C 1s for different samples are shown in Figure 2a. Compared to the untreated CC, the acid-etched CC (AECC-12) exhibited two obvious shoulder peaks, suggesting the introduction of functional groups. The curve in Figure 2b was fitted with six component peaks at 283.7, 284.6, 285.6, 286.9, 288.7, 292.3 eV, which can be ascribed to sp2 and sp3 carbons of the substrate and the introduced functional groups (-C-OH, -C=O, -COOH) as well as the π–π shake-up satellite peak, respectively.10, 32 In contrast, the peak of C 1s from the acid-etched CC without KMnO4 was slightly broadened without obvious shoulder peaks, implying that KMnO4 is indispensable and plays a vital role in the modification. The core level O 1s spectrum of AECC-12 (Figure 1c) can be deconvoluted into three peaks locating at 531.8, 532.8, 533.7eV, which were ascribed to -C=O, -C-OH, -COOH, agreeing well with the spectrum of C 1s. The samples were further characterized by Raman spectroscopy. The D band and G band of carbon at 1342 and 1587 cm-1 were observed (Figure 2d). In general, the D-band referenced to defects and dangling bonds of carbon structures, while the G-band manifests the presence of graphitic carbons.33 Thus, the smaller the ID/IG ratio is, the greater the degree of graphitization and the larger the ordered structure is. The ID/IG ratio of EACC-12 (1.101) was the largest among the three samples. This result implies that more defects and dangling bonds accompanied with the introduction of oxygencontaining functional groups.

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Figure 3 (a) Cyclic voltammetry (CV) curve at 5 mVs-1 of the AECC-12 electrode tested in threeelectrode system with a wide potential range from -0.7 V to 1.2 V vs. Ag/AgCl. (b) CV curves recorded at a scan speed of 10 mV s-1. (c) Galvanostatic charge/discharge (GCD) curves with a current density of 10 mA cm-2. (d) Nyquist plots of different samples. The inset of (d) shows the enlarged high-frequency part. (e) Cycling performance of the AECC-12 electrode collected at a current density of 50 mA cm-2.

To assess the feasibility of the AECC as an electrode material for supercapacitors (SCs), the electrochemical measurements were performed in a standard three-electrode cell. Figure 3a displays the CV curve of AECC-12 electrode within a wide potential range from -0.7 V to 1.2 V (vs. Ag/AgCl) to determine the suitable potential range for SCs. The generation of hydrogen and oxygen started at ~-0.5 V and 1.0 V (vs. Ag/AgCl), respectively. Thereafter, a potential range from -0.5 V to 1.0 V (vs. Ag/AgCl) was selected as the working potential window.34 Figure 3b shows the typical CV curves of untreated CC, acid-etched CC without KMnO4 and AECC-12 electrodes recorded at a scan speed of 10 mV s-1. As expected, the curve of the untreated CC nearly overlapped and gave minimum capacitive performance. On the contrary, the curve of the AECC-

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12 electrode displays a symmetric quasi-rectangle shape, suggesting the significantly enhanced capacitive performance of the AECC-12 electrode (Fig. S5). The symmetric feature indicated that the AECC-12 electrode can operate properly within the selected potential range. The acid-etched CC without KMnO4 exhibited minimum charge storage capability, further confirming adding KMnO4 is necessary for the activation of carbon cloth. Figure 3c presents the galvanostatic charge/discharge curves of acidetched CC without KMnO4 and AECC-12 electrodes collected at a current density of 10 mA cm-2. The discharge time of the acid-etched CC without KMnO4 electrode is 15.2 s, while the discharge time is elongated 45-folds for the AECC-12 electrode, confirming its excellent capacitive performance. Moreover, the capacitance of the AECC-12 electrode exhibited excellent long-term cycling stability indicated by the less than 7% loss after 5,000 cycles (Figure 3e). Electrochemical impedance spectroscopy (EIS) was employed to probe the charge transport process of the fabricated electrodes. The EIS curves of the untreated CC, acid-etched CC without KMnO4 and AECC-12 were collected within the frequency window from 0.01 Hz to 100 kHz. Generally, the electrolyte contact resistance (Rs) coupling with the charge transfer resistance (Rct) can be estimated from the intercepts of the Nyquist plots and the diameter of semicircle, respectively. The Rs for the AECC-12 electrode is ~4.13 Ω, slightly higher than that of the untreated CC (3.71 Ω) and acid-etched CC without KMnO4 electrode (3.82 Ω).8, 35 It is worth noting that the untreated CC and acid-etched CC without KMnO4 electrode showed no obvious semicircles, implying the fast charge transfer process. In contrast, a tiny semicircle was observed for the AECC-12 electrode, indicating the successful modification of functional species and its minim effect on the charge transfer process.

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Figure 4. (a) CV curves of acid-etched CC without KMnO4 and AECC-n electrodes at a scan speed of 10 mV s-1. (b) Areal capacitance versus the current density of acid-etched CC without KMnO4 and AECC-n electrodes. (c) Different acid-etched time carbon cloth oxygen-containing functional groups content. (d) Areal capacitance versus the current density of AECC-12 electrode and other previously reported electrodes.

The significantly enhanced capacitance of the AECC-12 electrode can be ascribed to the introduction of functional species onto the surface. To study the impact of functional species on the capacitive performance, the amount of functional species was tuned by treating samples for different time (1, 6, 12, to 24 h, named as AECC-n, n = 1, 6, 12 or 24). CV curves of different samples were collected at a scan speed of 10 mV s-1. Figure 4a depicts that all the CV curves displayed a quasi-rectangular shape with two redox peaks at ~ 0.1 and 0.6 V versus Ag/AgCl, which may be ascribed to the transformation between C=O and C-OH. This observation is similar to the studies by Montes-Moran and Béguin where the pseudocapacitive contribution was ascribed to the quinonehydroquinone pair or pyrone-like structures.11, 36-37 The capacitance was enhanced as the acid-etched time increased. The value approached the peak at the time of 12h and decreased with prolonged treatment time. The species on the surface were analyzed by XPS. From Figure 4c, S6, the ratio of -C-O groups increased, -COOH and -C=O groups approached a peak value at ~12h, and decreased as the acid-etched time was prolonged.

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As reported previously, the functional groups such as the ether and phenol (-C-OR), quinone and ketone (-C=O), and carboxylic groups (-O-C=O) are contributory factors for reversible pseudocapacitance.37 The growing of active groups (-C=O, -C-OH and COOH) enhances the capacitance, however, the accumulation of excessive functional species on the surface may detrimentally affect the charge transport process, inducing slight decrease of the capacitance.26 Figure 4b depicts the areal capacitances of the AECC versus current density, which were derived from GCD curves. Similar to the results of CV, the AECC-12 sample possessed the highest areal capacitance value. For example, at a current density of 5 mA cm-2, the areal capacitance increased from 2020 to 3650 mF cm-2 as the electrode was treated from 1h to 6h, and that of AECC-12 approached the highest value of 5310 mF cm-2, 60 times higher than that of previous chemically activated CC.27 More importantly, this impressive areal capacitance is also significantly higher than some of lately reported carbon-based and metal oxide electrodes including Fe2O3/PPy (382.4 mF cm-2 at 0.5 mA cm-2),25 activated carbon fiber paper (1560 mA cm-2 at 5 mA cm-2),38 electrochemically modified carbon cloth (756 mA cm-2 at 6 mA cm-2),26 and carbon nanofibers (1120 mA cm-2 at 5 mA cm-2),39 WO3-x/MoO3-x (303 mF cm-2 at 5 mA cm-2).40

Figure 5 (a) CVs curves of the flexible all-solid-state symmetric SCs device obtained in different potential windows at a scan speed of 50 mV s-1. (b) CV curves of the flexible all-solid-state symmetric SCs device record at different scan speed. (c) GCD curves of the flexible all-solid-state symmetric SCs

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device record at different current densities. (d) Ragone plots of energy density versus power density for the fabricated flexible all-solid-state symmetric SCs in comparison with some reported devices.

In order to assess the feasibility of AECC electrodes for practical application, an allsolid-state symmetric supercapacitor was fabricated using H2SO4/PVA as a solid electrolyte. CV curves of the symmetric SC recorded within different potential windows are presented in Figure 5a, indicated that the operating potential window can reach a maximum value of 1.5 V. Figure 5b presented the CV curves of the SC collected with different scan speed from 10 to 100 mV s-1. The symmetric and quasi-rectangle CV curves suggest the good capacitive properties, similar to the results tested in threeelectrode test. Figure 5c shows the charge/discharge curves of the solid-state SC device with reasonably symmetric shape. The good linear relationship of voltage over time implied the ideal capacitive feature as well as the desirable charge/discharge characteristic of the SC device. Ragone plots of energy density (E) versus power density (P) of the fabricated device along with some values reported for other symmetric supercapacitor (SSC) and asymmetric supercapacitor (ASC) devices are presented in Figure 5d. Promisingly, the fabricated device approached an energy density of 4.27 mWh cm-3 along with a power density of 1.32 W cm−3 running at a current density of 5 mA cm-2, which are much better than some of recently reported SSC and ASC devices, including MnO2//Fe2O3/PPy ASC,25 MnO2@TiN//EACC-10 ASC,26 OCFP-SSC,38 PANI//WO3-x/MoO3-x ASC,40 MnO2//RGO ASC,41 CoO@PPy//AC ASC.42 In order to prove their practical application of the fabricated SCs, a couple of devices were connected in tandem to power a LED indicator (3 V) successfully (Figure S7). Furthermore, the all solid-state symmetric supercapacitor displayed remarkable mechanical flexibility. Bending and crimping the device induced negligible change of the capacitive performance. After 1,000 bending cycles, the capacitance kept nearly unchanged, indicating the excellent flexibility as a flexible power source for foldable and wearable electronic devices. CONCLUSION A facile and cost-effective chemical acid-etched method has been developed to activate the carbon cloth electrode for supercapacitors. The functional species introduced on the surface of the modified carbon cloth can be rationally engineered simply by the treatment time. The fabricated electrode delivered an ultrahigh areal capacitance of 5310 mF cm-2 at a current density of 5 mA cm-2. Importantly, the capacitance of the AECC-12 electrode exhibited remarkable stability with capacitance loss of less than 7% after 5,000 cycles. A high-performance all-solid-state symmetric SCs has been constructed, which can operate properly within a maximum window of 1.5 V, and the symmetric supercapacitor can achieve a high energy density of 4.27 mWh cm-3 at a

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power density of 1.32 W cm-3. Furthermore, the as-fabricated all-solid-state symmetric SCs displayed outstanding mechanical flexibility. 1,000 bending cycles lead to negligible change of the capacitive performance. The high performance with remarkable stability and mechanical flexibility makes the acid-etched carbon cloth as an intriguing and promising supercapacitor electrode for flexible power sources to various foldable and wearable electronic devices. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.0x00000. Calculation, XRD, XPS, CV curves, GCD curves and a digital picture of devices AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (H. Liu), *E-mail: [email protected] (J. Wang). ORCID Hong Liu: 0000-0003-1640-9620 Jian-Jun Wang: 0000-0002-1876-5103 Notes The authors declare no competing financial interest ACKNOWLEDGMENTS The authors are grateful for the funding from the National Natural Science Foundation of China (Grant No. 51732007, 21802086), the Fundamental Research Funds of Shandong University (2018WLJH64), the Beijing National Laboratory for Molecular Sciences Program (BNLMS201802), and the support from Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong. REFERENCES 1. Larcher, D.; Tarascon, J. M., Towards Greener and More Sustainable Batteries for Electrical Energy Storage. Nature Chem. 2015, 7, 19-29. 2. Simon, P.; Gogotsi, Y., Materials for Electrochemical Capacitors. Nature Mater. 2008, 7, 845-854. 3. Simon, P.; Gogotsi, Y.; Dunn, B., Where Do Batteries End and Supercapacitors Begin? Science 2014,

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343, 1210-1211. 4. Wang, Y.-Y.; Hou, B.-H.; Ning, Q.-L.; Pang, W.-L.; Rui, X.-H.; Liu, M.; Wu, X.-L., Hierarchically Porous Nanosheets-Constructed 3D Carbon Network for Ultrahigh-Capacity Supercapacitor and Battery Anode. Nanotechnology 2019, 30, 214002. 5. Guo, J.-Z.; Yang, A.-B.; Gu, Z.-Y.; Wu, X.-L.; Pang, W.-L.; Ning, Q.-L.; Li, W.-H.; Zhang, J.-P.; Su, Z.-M., Quasi-Solid-State Sodium-Ion Full Battery with High-Power/Energy Densities. ACS Appl. Mater. Interfaces 2018, 10, 17903-17910. 6. Cao, F.; Zhao, M.; Yu, Y.; Chen, B.; Huang, Y.; Yang, J.; Cao, X.; Lu, Q.; Zhang, X.; Zhang, Z.; Tan,

C.;

Zhang,

H.,

Synthesis

of

Two-Dimensional

CoS1.097/Nitrogen-Doped

Carbon

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