Fiber-Shaped Electrochemical Capacitors Based on Plasma

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Letter

Fiber-Shaped Electrochemical Capacitors based on Plasma-Engraved Graphene Fibers with Oxygen Vacancies for AC Line Filtering Performance Jingxin Zhao, Yan Zhang, Jiaxian Yan, Xiaoxin Zhao, Jixun Xie, Xin Luo, Jianhong Peng, Juanjuan Wang, Leichao Meng, Zhongming Zeng, Conghua Lu, Xinhua Xu, Yafei Dai, and Yagang Yao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02060 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Fiber-Shaped Electrochemical Capacitors based on Plasma-Engraved Graphene Fibers with Oxygen Vacancies for AC Line Filtering Performance Jingxin Zhao+b,d,e, Yan Zhang+b, Jiaxian Yan+c, Xiaoxin Zhaob, Jixun Xieb, Xin Luod, Jianhong Penge, Juanjuan Wangb, Leichao Menge, Zhongming Zengd, Conghua Lub, Xinhua Xub, Yafei Daic*, Yagang Yaoa* a

National Laboratory of Solid State Microstructures, College of Engineering and Applied

Sciences, Jiangsu Key Laboratory of Artificial Functional Materials, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China b

School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, P. R.

China c

School of Physics Science & Technology and Jiangsu Key Laboratory for NSLSCS,

Nanjing Normal University, Nanjing 210023, China d

Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications,

CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P. R. China e

College of Physics and Electronic Information Engineering, Qinghai University for

Nationalities, Xining, 811600, China

E-mail: [*] [email protected]; [email protected] [email protected] [+] These authors contribute equally to this work.

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Rapid advances in flexible and wearable electronics have enhanced demand for electrochemical capacitors (ECs) with fast frequency responses and supernal electrochemical supercapacitive properties. Here, we report the development of a fiber-shaped supercapacitor device based on Ar plasma-treated, highly electrically conductive reduced graphene oxide (rGO) fibers. Typical rGO-based, fiber-shaped ECs exhibit ultrafast frequency responses (phase angle = -81.1º), a short resistor-capacitor (RC) time constant (0.471 ms at 120 Hz), and excellent cycle stability. The superior performance of our device relative to most advanced carbon-based ECs is promising for alternating current line filtering performance. KEYWORDS: fiber-shaped electrochemical capacitors, plasma-engraved graphene fiber, oxygen vacancies, ultrafast frequency responses, AC line-filtering performance

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Electrochemical capacitors (ECs) are characteristic energy storage devices with performance complementary to those of Li-ion batteries. ECs are often taken into account a prospective power source for advanced energy storage due to their large capacity, low cost, supernal power density, excellent reversibility, and long life cycle.1-7 Alternating current (AC) line filtering is a pivotal factor capable of attenuating leftover AC ripples on direct current voltage generatrix at 120 Hz and implementing high frequency operations in the majority of circuit powered electronics.8,

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As this

parameter is an important application of ECs in energy storage fields, many recent research efforts have been dedicated to the design of wearable, miniaturized, thin, and in-plane ECs with AC line filtering performance.10,

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However, the relatively large

planar structures of these supercapacitors have limited their use in the development of AC line filtering.8-11 Accordingly, the construction of fiber-shaped supercapacitors (FSSCs) with high AC line filtering performance remains challenging. Excellent AC line filtering performance requires electrode materials to exhibit excellent electric conductivity, high ion diffusion and electron transfer rates, and splendid electrochemical stability. Previous attempts to fabricate EC electrodes with improved AC line filtering performance have explored various carbonaceous materials.8, 12, 13 However, the performance of these electrodes remains unsatisfactory for AC line filtering. For instance, Holloway and co-workers demonstrated that activated carbon electrode-based ECs function more like resistors than capacitors, which is unsuitable for AC line filtering.8 Additionally, Pan et al. indicated that multiwall CNT-based ECs could not availably filter a 120 Hz ripple when deposited 3

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onto various metal current collectors.14 Recently, graphene material has been thinked over among the most prospective ECs electrode for AC line filtering because of their superior electroconductivity, high theoretical specific surface areas (SSAs), excellent chemical durability, and good mechanical property.15,

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However, reduced graphene oxide (rGO) (graphene

containing partial oxygen-containing groups, such as ketone, hydroxyl, epoxy and carboxyl groups) is generally reduced using a wet chemical method, which introduces impurities, and the resulting electrical conductivity is inferior to that of pristine graphene. Therefore, highly electrically conductive rGO electrode materials produced using a simple, rapid, and direct treatment method are urgently needed. Plasma technologies are considered among the most promising options for rapid rGO fabrication. These technologies are generated using electrical fields, which lead to electron acceleration and the reintroduction of ions to reduce GO.17,18 Accordingly, we designed a highly electrically conductive rGO fiber using a one-step Ar plasma-engraving strategy to ensure AC line filtering performance. Our new rGO fiber-based FSSC device with AC line filtering can serve as an proper energy storage device for wearable electronic devices. In this work, we designed the first highly electrically conductive rGO fiber exhibiting both AC line filtering and electrochemical energy storage performances. In this configuration, the FSSC device displayed a specific capacitance value of 306 μF cm-2 and splendid cycle life, with no obvious damage in capacitance after 10 000 cyclic test. The as-prepared device could be run at a high rate, which could reach 2000

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V s-1. Additionally, the assembled device embraced excellent AC line filtering property (-82.3º at 120 Hz) with an extremely short RC time constant, τRC (approximately 0.32 ms). To obtain rGO fibers, GO fibers were fabricated by wet-spun technology with a GO dispersing agent and subsequently reduction process using a chemical method. Finally, the as-prepared rGO fibers were placed on a glass support for plasma treatment, after which highly electrically conductive rGO fibers were obtained. The as-fabricated rGO fibers and rGO fibers subjected to plasma treatment for different lengths of time [P-rGOx; x: plasma treatment time (30, 60, 120, or 240 s)] were characterized using FESEM (30, 60, 120, and 240 s). Figure 1(a)–(e) demonstrates that the rGO and P-rGOx fibers exhibited typical rough and wrinkled surfaces, which may have contributed to the SSAs of electrode. The nitrogen adsorption-desorption isotherm of the P-rGO240 fiber is tested, suggesting that the obtained sample possess a large BET surface area of 210 m2 g−1 and a appropriate pore size distribution [Figure S1(a) and (b)]. Raman spectrum was also used to weigh the structures of rGO fiber and P-rGOx samples. The intensity ratio (I(D)/I(G)) is normally used to evaluate defects. Here, high ratio manifests a greater level of graphene defects.19 As displayed in Figure 1(f), calculated I(D)/I(G) values decreased as the plasma-treated time increased, indicating an increase in the electrical conductivity of the sample. The observed electrical conductivities of the samples [Figure 2(supporting information; SI)] are consistent with Figure 1(f). The electrical conductivities of the samples will decrease and tend to stable when the plasma-treated time continue to increase [Figure S2 (SI)]. Figure S3 (SI) revealed 5

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two obvious peaks at approximately 1340 and 1506 cm-1, which corresponded to the D and G bands, respectively.20

Figure 1. Structure characterization of plasma-engraved graphene fiber. a)-e) SEM images of the as-prepared rGO fiber and P-rGOx (x=plasma-treated rGO fibers in different time (30, 60, 120 and 240s)). f) I (D)/I (G) after different plasma-treated time. In order to further investigate chemical composition and structural changes of P-rGOx fibers, X-ray photoelectron spectroscopy (XPS) was executed. As shown in Figure S4 (SI), variations in the oxygen functional groups of all samples were analyzed using XPS, and a higher O/C atomic ratio (55.8%) was obtained for the as-fabricated rGO fibers. Notably, the oxygen-containing groups (-O-, -COOH, and -OH) decreased as the Ar plasma treatment time increased, indicating oxygen replacement. Next, the electrochemical properties of the prepared p-rGO240 fibers were tested to weigh the AC line filtering performance. Figure 2(a) exhibits the cyclic voltammetry (CV) curves of an as-prepared p-rGO240 fiber at different scan rates (10–2000 V s-1). Nearly rectangular CV curves exhibited a typical electrical double-layer capacitive 6

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characteristic even at an ultrahigh scan rate (2000 V s-1), which revealed electron transfer and fast ion diffusion.16 The splenid rate capability of assembled device was confirmed using galvanostatic charge/discharge (GCD) test, which exhibited quasi-triangular shapes and no obvious decreases in voltage. These curves revealed the high conductivity of the electrodes [Figure 2(b)]. Figure 2(c) demonstrates phase angle versus frequency response of assembled device. Here, a phase angle (-84.6º) was measured at 120 Hz. The Nyquist plot of our device exhibited a almost vertical line characteristic without the semi-circular shape characteristic of conventional ECs at high frequencies. This result demonstrates the rapid electron transfer and ion diffusion within the electrode [Figure 2(d)].21

Figure 2. Electrochemical performances of p-rGO240 fiber. (a) CV curves. (b) GCD curves. (c) Plot of phase angle versus frequency. (d) Nyquist plot (inset: the enlarged view in high frequencies).

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To reveal the theoretical structures of the as-fabricated p-rGO240 fiber, periodic density-functional theory (DFT) computations were performed using the generalized gradient approximation (GGA) of the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional as implemented in the Vienna ab initio simulation package (vasp).22, 23 Through the projector augmented wave (PAW) potential,24 interactions between ionic cores and valence electrons were described. The energy cutoff of the plane wave basis set was set at 450 eV, and convergence criterions for atomic structure relaxation were that total energy converged to 10-4 eV and the force on each atom fell to below 0.02 eV·Å-1. Using a periodically repeated hexagon, a two-dimensional sheet was modeled with 4 × 4 and 5 × 5 supercells. The distance between two adjacent sheets was 20 Å. A (3 × 3 × 1) Monkhorst–Pack k-grid was used for numerical integrations in the Brillouin zone. The adsorption energies (Ead) of oxygen-containing functional groups (-COOH, -OH, and -O-) on the surface of p-rGO240 were defined as Ead = Egra + Eoxi – Etot, where Egra, Eoxi and Etot represent the energies of clean graphene, isolated adsorbed oxygen-containing groups, and the slab after adsorption, respectively. Generally, the surface of GO includes three typical oxygen-containing groups, they are hydroxyls (-OH), carboxyls (-COOH), and epoxides (-O-). The adsorption energies of these oxygen-containing groups have been calculated as approximately 0.35, 1.30, and 3.91 eV, respectively, indicating that different energy levels are required to dissociate these groups experimentally and that the concentrations of oxygen-containing groups will continue to decrease. Based on the above analysis, GOs with various concentrations of oxygen-containing groups were considered, and the optimized geometry structures are shown in Figure 3. To

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analyze the relationship between the concentrations of oxygen-containing groups and the conducting properties of graphene, the band gap corresponding to graphene was calculated using different concentrations of oxygen-containing groups. On the one hand, the band gap does not directly indicate the conductivity, but the band gap indirectly reflects the conductivity. The smaller the band gap, the easier the electronic transition. On the other hand, previous studies have found that with the decrease of oxygen concentration of graphene oxide, the band gap decreases and the conductivity increases. However, there is a lack of relevant theoretical study. Therefore, we believe that the change of conductivity can be explained by our calculation of the band gap, and our calculation can be mutually confirmed with the previous research results. The results are listed in Table S1. Here, we used the O/C atomic ratio to represent the concentration of oxygen-containing groups. Therefore, it was found that with the decrease of oxygen concentration, the band gap decreases and the conductivity increases.25

Figure 3. The optimized structure of p-rGO240 with partial oxygen-containing functional groups. Red, blue and grey balls represent oxygen, hydrogen and carbon atoms, respectively. As shown in Table S1 (SI), the band gap of p-rGO240 continued to decrease as the

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concentration of oxygen-containing groups decreased. This finding indicates better electrical conductivity, consistent with our experimental results demonstrating that the electrical conductivity of the samples increased with a longer Ar plasma treatment time. Of course, electrical conductivity did not alter with the increasing of Ar plasma treatment time once oxygen-containing groups with lower adsorption energies had been almost completely removed. Electrochemical properties of as-fabricated symmetrical EC device (Figure 4(a)) constructed from two twisted p-rGO240 fibers and the practical applications for AC line-filtering performance were explored. Figure 4(b) presents CV curves of an as-prepared symmetrical FSSC device weighed with scan rate of 10–2000 V s-1 between the operation voltage of 0 and 0.8 V. These curves exhibited quasi-rectangular form, indicating the desired capacitive behavior. Notably, the discharge current density exhibited an perfect linear increase as the scan rates of the CV curves increased [Figure 4(c)].

Obviously,

the

symmetrical

FSSC

device

possesses

an

ultrafast

charging/discharging capability, as demonstrated by the ideal linear relation between logarithmic discharge current and scan rates of 10–2000 V s-1. The highest scan rate at which the symmetrical FSSC device maintained a perfect linear dependence of the discharge current density versus scan rate is the highest value reported to date based on graphene materials [Table S2]. The dependenc of the phase angle versus frequency of our device is disclosed in Figure 4(d). Here, the phase angle at 120 Hz was -81.1º, which was similar to commercial aluminum electrolytic capacitors and comparable to that of most advanced 10

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graphene-based ultrafast ECs.8, 26 Nyquist plot of the as-fabricated device is nearly a vertical line [Figure 4(e)], indicating a high conductivity and rate capability. Based on Nyquist plot characteristic, even at low frequencies and the above-described short relaxation time constant, a series-RC circuit model was constructed to calculate the RC time constant of our device. Figure S5(a) (SI) depicts the plot of CA versus frequency, which exhibited a CA of 264 μF cm-2 and a estimated resistance (1.78 Ω) at 120 Hz. These values yielded a RC time constant of 470 μs, which was comparable to the most previously reported graphene-based ECs, such as UPSCs-25 (G/PH1000, 1 ms), G/CNTCs-MCs (0.82 ms), ErGO-DLC (1.35 ms) and PRG-MSCs (16.4 ms) [Figure S6].15, 27-29 Therefore, our device appears to be a promising replacement for AECs with AC line filtering performance. Additionally, the real and imaginary parts of capacitance (C’ and C”, respectively) obtained from the electrochemical impedance spectroscopy were plotted as a function of frequency [Figure S5(b)] (SI). A full-wave bridge rectifier (smoothing symmetrical FSSC device) and load resistor constituted the AC line filtering electrical circuit [Figure S6] (SI). An input AC signal (1.0 Vpeak|peak, 60 Hz) was generated using a function generator, and the full-bridge rectifier converted the AC signal to a pulsating DC signal [Figure 4(f)]. This was eventually smoothed successfully to a DC output by our device, thus exhibiting a promising for practical AC line filtering.

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Figure 4. Electrochemical performances of the assembled symmetrical FSSCs. (a) Schematic of assembled device. (b) CV curves. (c) Plot of discharge current density versus scan rates. (d) Plot of phase angle versus frequency. (e) Nyquist plot (inset: the enlarged view in high frequencies). (f) AC input signal and rectified pulsating DC signal. The GCD curves of as-assembled device exhibited ideal triangular shape under different current densities [Figure 5(a)], indicating high electrical conductivity. Additionally, our device exhibited excellent cycle stability with a inappreciable damage of capacitance after 10 000 cycles at 1 mA cm-2 [Figure 5(b)], indicating outstanding electrochemical stability, which was mainly attributable to the inherently stable physicochemical properties of p-rGO240. 12

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Portable and wearable electronics rely strongly on the mechanical flexibility of the electrochemical device. To evaluate the flexible performance of our device, the real performance of the prepared device was examined under different bending states. As shown in Figure 5(c), resulting CV curves of our device indicated a stable electrochemical performance with a negligible loss of capacitance, indicating excellent mechanical flexibility. Furthermore, our device exhibited a capacitance retention of up to 97.2% after 4, 000 repeated bending cycles at a bending angle of 90º [Figure 5(d)].

Figure 5. Electrochemical properties of assembled device. (a) GCD curves at different discharge current densities.(b) Cycle stability. (c) The CV curves of assembled device under different bending angles. (d) The capacitance retention after 4,000 bending cycles. Herein, we triumphantly prepared a FSSC device based on highly electrically conductive Ar plasma-treated rGO fibers. This FSSC device exhibited fast frequency with a phase angle (-81.1º), a short RC time constant (0.471 ms at 120 Hz), a high rate (2000 V s-1), and long-term cycle stability for AC line filtering. Moreover, the FSSC 13

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device exhibited excellent mechanical flexibility and prospective AC line filtering performance. Hence, FSSCs are prospective for serving as power sources in various flexible electronics, as they are flexible, lightweight and wearable, and feature a high energy density, power density, and frequency operation. Notes The authors declare no competing financial interest. Acknowledgements J. X. Zhao, Y. Zhang, J. X. Yan, X. X. Zhao, J. X. Xie, X. Luo, J J. Wang, Z. M. Zeng, C. H. Lu, X. H. Xu and Y. G. Yao received funding from National Natural Science Foundation of China (Nos. 51522211, 51602339, 51703241, 21374076, 21574099 and 51273145), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No.QYZDB-SSW-SLH031), the Thousand Youth Talents Plan, the Postdoctoral Foundation of China (No. 2016M601905 and 2017M621855), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399), the Postdoctoral Foundation of Jiangsu Province (No. 1601065B) and the Science and Technology Project of Nanchang (2017-SJSYS-008). J. X. Zhao, J. H. Peng and L. C. Meng received funding from Qinghai university for nationalities 2017 "chunhui plan" cooperative scientific research project of the ministry of education (Z2017043, Z2016110). Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: Materials and Characterizations; Electrochemical Performance Calculations; N2 adsorption/desorption isotherms and the pore size distribution

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of the P-rGO240 fiber; The relationship between electrical conductivity of the samples and plasma-treated time; Raman spectra of the plasma-treated rGO fibers in different time (0, 30, 60, 120 and 240s); XPS of the plasma-treated rGO fibers in different time (0, 30, 60, 120 and 240s) and the O/C atomic ratio under different plasma-treated time; Plot of specific capacitance versus frequency using a series-RC circuit model and plot of the real or imaginary part (C’ or C’’) of specific capacitance versus frequency; The RC time constant of different graphene-based ECs; An AC line filtering electrical circuit composed of a full wave bridge rectifer, smoothing symmetrical FSSCs device and a load resistor; The band gap of rGOs corresponding to different concentration of oxygen-containing functional groups; The plot of scan rates vs graphene-based ECs. These materials are available free of charge via the internet at http://pubs.acs.org.

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Xie, L. Y.; Xie, J. X.; He, B.; Zhou, Z. Y.; Lu, C. H.; Lu, W. B.; Zhu, G.; Yao, Y. G. MOF for template-directed growth of well-oriented nanowire hybrid arrays on carbon nanotube fibers for wearable electronics integrated with triboelectric nanogenerators. Nano Energy 2018, 45, 420-431. [21] Ye, J. L.; Tan, H. B.; Wu, S. L.; Ni, K.; Pan, F.; Liu, J.; Tao, Z. C.; Qu, Y.; Ji, H. X.; Simon, P.; Zhu, Y. W. Direct laser writing of graphene made from chemical vapor deposition for flexible, integratable micro-supercapacitors with ultrahigh power output. Adv. Mater. 2018, 30, 1801384-1801391. [22] Kresse, G.; Furthmller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169-11187. [23] Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. [24] Blochl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953-17979. [25] Yeh, T. -F.; Syu, J. -M.; Cheng, C.; Chang, T. -H.; Teng, H. Graphite oxide as a photocatalyst for hydrogen production from water. Adv. Funct. Mater. 2010, 20, 2255-2262. [26] Rangom,Y.; Tang, X. W.; Nazar, L. F. Carbon nanotube-based supercapacitor with

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excellent ac line filtering and rate capability via improved interfacial impedance. ACS Nano 2015, 9, 7248-7255. [27] Wu, Z. S.; Liu, Z. Y.; Parvez, K.; Feng, X. L.; Mullen, K. Ultrathin printable graphene supercapacitors with AC line-filtering performance. Adv. Mater. 2015, 27 3669-3675. [28] Lin, J.; Zhang, C. G.; Yan, Z.; Zhu, Y.; Peng, Z. W.; Hauge, R. H.; Natelson, D.; Tour, J. M. 3-dimensional graphene carbon nanotube carpet-based microsupercapacitors with high electrochemical performance. Nano Lett. 2012, 13, 72-78. [29] Wang, S.; Wu, Z. -S.; Zheng, S. H.;Zhou, F.; Sun, C. L.; Cheng, H. -M.; Bao, X. H. Scalable

Fabrication

of

Photochemically

Reduced

Graphene-Based

Monolithic

Micro-Supercapacitors with Superior Energy and Power Densities. ACS Nano 2017, 11, 4283-4291.

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A fiber-shaped supercapacitors based on highly electrical conductive rGO fiber by Ar plasma treating that possesses the excellent AC line-filtering performance.

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