- Email: [email protected]
cellulose fiber composite for supercapacitor
Journal Pre-proof Construction of flexible electrodes based on ternary polypyrrole@cobalt oxyhydroxide/cellulose fiber composite for supercapacitor Shuaishuai Yang, Lijian Sun, Xianhui An, Xueren Qian
PII:
S0144-8617(19)31122-1
DOI:
https://doi.org/10.1016/j.carbpol.2019.115455
Reference:
CARP 115455
To appear in:
Carbohydrate Polymers
Received Date:
3 August 2019
Revised Date:
6 October 2019
Accepted Date:
6 October 2019
Please cite this article as: Yang S, Sun L, An X, Qian X, Construction of flexible electrodes based on ternary polypyrrole@cobalt oxyhydroxide/cellulose fiber composite for supercapacitor, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115455
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Construction of flexible electrodes based on ternary polypyrrole@cobalt
oxyhydroxide/cellulose
fiber
composite for supercapacitor
Shuaishuai Yang, Lijian Sun, Xianhui An, Xueren Qian* Key Laboratory of Bio-based Material Science and Technology of Ministry of
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*Corresponding author. [email protected]
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Highlights
PPy@cobalt oxyhydroxide/cellulose fiber composite was prepared via a facile
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method.
Cobalt oxyhydroxide was grown on the surface of cellulose fiber at room
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temperature.
The composite electrode had a higher specific capacitance and better cycle
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Education, Northeast Forestry University, Harbin, China
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stability.
Abstract: With the development of flexible electronic devices, flexible energy storage systems have been research hotpot. Conductive polymers is potential pseudocapacitor materials in energy storage field. Meanwhile, cellulose fiber with natural, degradable, renewable and flexible properties is one of tremendous promising alternatives to the
flexible substrates. Hence, a polypyrrole@cobalt oxyhydroxide/cellulose fiber composite electrode is prepared via “liquid phase reduction” strategy in open system at room temperature. The composite electrode exhibits excellent electrochemical properties, which has a high specific capacitance and capacitance retention. The highest specific capacitance of 571.3 F g-1 at 0.2 A g-1 is obtained. Besides, the specific capacitance of the composite electrode has no significant loss, showing high cycle
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stability (93.02% after 1000 cycles). The excellent electrochemical properties can be ascribed to the introduction of cobalt oxyhydroxide, which restrains the volumetric
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change of polypyrrole in the electrochemical redox process, and promotes the rapid
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migration of electrons.
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1. Introduction
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electrochemical properties
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Keywords: cellulose fiber, polypyrrole, cobalt oxyhydroxide, electrode material,
In the past decades, the development of the flexible electronic devices (e.g., smart phone, wearable sensor, implantable medical devices and so on) had greatly stimulated
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the demand for miniaturized flexible energy storage systems (Wang 2010; Wang, 2012; Wang & Wu, 2012). Currently, supercapacitors with fast charge/discharge, long cyclic lifetime and high power density had been intensively studied. However, a relatively low energy density of the supercapacitors limited their expansive application. To date, the strategies of enhancing energy density for supercapacitors mainly included two
inspects, i.e., increasing capacitance (C) and/or increasing working voltage (V) based on the formula of energy density (E=CV2/2)(Yan, Wang, Wei, & Fan, 2014). From these point of views, the capacitance can be enhanced by improving some crucial factors of electrode materials (e.g., pore size, surface area, electrical conductivity, functional groups, etc.). The strategy of improving working voltage is to employ highvoltage electrolytes and various supercapacitor device configurations.
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Based on the charge storage mechanisms, supercapacitors are classified as
electrochemical double layer capacitors (EDLCs) with physical adsorption of ions at
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the interface of the electrode surface and the electrolyte, and pseudocapacitors with a
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fast reversible faradaic charge transfer at the electrode surface. Among them, EDLCs electrode material refers to carbon material (e.g., carbon black (Wang et al., 2014),
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graphene, CNT (Yang et al., 2013), etc.). Pseudocapacitive electrode material mainly includes conductive polymers (e.g., polyaniline (Mondal, Barai, & Munichandraiah, Sk,
&
Yue,
2014),
polypyrrole
(Shi
et
al.,
2014),
poly(3,4-
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2007;
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ethylenedioxythiophene) (Liu, Hu, Xue, Zhang, & Zhu, 2008; Ravit, Abdullah, Ahmad, & Sulaiman, 2019)), transition metal oxides (e.g., RuO2 (Kuratani, Kiyobayashi, & Kuriyama, 2009), MnO2 (Huang, Li, Dong, Zhang, & Zhang, 2015), NiO (Cai et al.,
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2015), Co3O4 (Dong et al., 2012), etc.), transition metal sulfides (MoS2 (Islama, Wang, Warzywodac, & Fan, 2018), NiCo2S4 (Zhu, Ji, Wu, & Liu, 2015), etc.), transition metal hydroxides (Co(OH)2 (Jiang et al., 2011), Ni(OH)2 (Li, Xu, 2015), CoMn-LDHs (Jagadale et al., 2016), NiMn-LDHs (Guo et al., 2016),
etc.), transition metal
carbides (Ti3C2 (Zhu, Huang, 2016; Boota, et al., 2016; Qin et al., 2018; Li et al., 2017;
Yan et al., 2017), V2C (Shan et al., 2018)), and cobalt oxyhydroxide (Zheng et al., 2009; Zheng et al., 2010). Among these electrode materials, CPs are organic polymers that conduct electricity through a conjugated bond system along the polymer chain. In the past two decades, CPs are extensively explored for energy storage application due to their reversible faradaic redox reaction, high charge density, and lower cost as compared with the other transition materials (such as metal oxides, grapheme, etc.).
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(Ryu, Kim, Park, Park, & Chang, 2002; Rudge, Raistrick, Gottesfeld, & Ferraris, 1994; Burke, 2007). At the same time, paper, as one of the most ancient flexible products
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invented A.D. 105 years, is one of a tremendous promising alternatives to the flexible
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substrates because of their wide availability, low cost, light weight, environmental friendliness, recyclability and bendability (Yao et al., 2013; Tobjörk, & Österbacka,
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2011; Zheng et al., 2013; Zhang, 2015; Lin, Gritsenko, Liu, Lu, & Xu, 2016; PerezMadrigal, Edo, & Aleman, 2016). As mentioned above, although CPs have so many
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advantages in energy storage, they exhibit a volumetric expansion in redox process,
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which lead to the collapse of electrode materials. Researchers have tried plenty of strategies to improve the drawbacks (Karaca, Dhawale, Kim, & Lokhande, 2019; Zhang, Li, et al., 2019; Dias et al., 2019). In our previous researches, CPs were
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incorporated into cellulose fibers via in situ oxidation polymerization method to prepare the flexible and conductive material (Ding, Qian, Yu, & An, 2010; Mao, Wu, Qian, & An, 2014; Mao, Liu, Qian, & An, 2015; Mao, Dong, Qian, & An, 2017). Besides, cobalt oxyhydroxide with excellent electrochemical reversibility and semimetallic conductivity is less concerned as electrode material.
Hence, cobalt oxyhydroxide is introduced to CPs and cellulose fibers to prepare the binder-free flexible electrode, in order to restrain the volumetric change of CPs in the redox process, and promote the rapid migration of electrons. The work is of great significance to prepare the polypyrrole@cobalt oxyhydroxide/cellulose fiber composite flexible electrode to solve the flexible and electrochemical problems of supercapacitors. To our knowledge, the metal Co was introduced into cellulose fibers
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based composite through the reduction of NaBH4, which on the one hand would
provide path for electron rapid transmission. On the other hand, the crystallinity of
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materials synthesized at room temperature is relatively low, and thereby the materials
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have a large number of crystal defects, which are conducive to the transmission of electrons and ions. Besides, the Co(OH)2 would be converted to CoOOH in open
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system based on the mechanism of the reaction, and it also has a excellent conductivity. So the strategy was beneficial to overcome the drawback (poor cyclic stability) of
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cellulose/PPy composite electrode (Xu et al., 2017).
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In this study, a conductive and flexible composite electrode constructed with polypyrrole (PPy), cobalt oxyhydroxide and cellulose fibers was successfully prepared via “liquid phase reduction” strategy in open system at room temperature. The
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PPy@cobalt oxyhydroxide/cellulose fiber composite electrode showed the excellent electrochemical properties. The highest specific capacitance of 571.3 F g-1 at 0.2 A g-1 in 0.6 M H2SO4 electrolyte was obtained when the molar ratio of CoCl2 to NaBH4 was 1:1. Besides, the specific capacitance of composite electrode had no significant loss, showing high cycle stability (93.02% after 1000 cycles).
2. Experimental 2.1 Materials Cobalt chloride (CoCl2·6H2O) and pyrrole were purchased from Sinopharm Chemical Reagent Co. Ltd. Sodium borohydride (NaBH4) and ferric chloride (FeCl3·6H2O) were purchased from Shanghai Macklin Biochemical Co. Ltd. and
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Tianjin Guangfu Technology Development Co. Ltd., respectively. Canada market
bleached softwood kraft pulp as cellulose fiber source was provided by Mudanjiang
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Hengfeng Paper Co. Ltd (Heilongjiang, China) and was beaten to 37 oSR before use.
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The diameter of cellulose fibers is about 15 μm, and the component of cellulose fibers
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is almost all cellulose.
2.2 Preparation of PPy@cobalt oxyhydroxide/cellulose fiber composite electrode
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The PPy@cobalt oxyhydroxide/cellulose fiber composite electrode was prepared
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using “liquid phase reduction” strategy in open system at room temperature. Firstly, 0.5 g cellulose fibers and a certain amount of CoCl2·6H2O (1 mmol) were dispersed in deionized water, and then a certain amount of NaBH4 (0.5-2 mmol) was slowly added
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to the above mixture suspension under continuous stirring for 6 h to react completely. The resultant mixture was filtered and washed with deionized water. Afterwards, the PPy@cobalt oxyhydroxide/cellulose fiber composite electrode was prepared via in situ oxidative polymerization method. 0.25 mL of pyrrole was dispersed in the cobalt oxyhydroxide/cellulose fiber suspension under continuous stirring. Subsequently,
0.973 g FeCl3·6H2O was added into the above mixture and stirred in order to initiate the polymerization reaction to proceed. After 6 h, the above mixture was filtered and washed with deionized water, PPy@cobalt oxyhydroxide/cellulose fiber composite was obtained, and noted as PCC. Where, PCC-21, PCC-11 and PCC-12 represent the samples prepared at the molar ratios of CoCl2·6H2O to NaBH4 of 2:1, 1:1 and 1:2, respectively. For comparison, the cobalt oxyhydroxide/cellulose fiber and
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PPy/cellulose fiber composite electrodes were fabricated, and marked as CC and PC,
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respectively.
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2.3 Characterization
Scanning electron microscope (SEM) was carried out to investigate the
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morphology of composite electrodes (SEU 8010). The X-ray photoelectron spectra (XPS) and Fourier transform infrared (FTIR) spectra were recorded on Thermo
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respectively.
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ESCALAB 250XI and Thermo Fisher Scientific Nicolet 6700 spectrometer,
2.4 Electrochemical Tests
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All of the electrochemical tests were carried out in three-electrode system, using
a 0.6 M H2SO4 electrolyte. The prepared PPy@cobalt oxyhydroxide/cellulose fiber composite electrode was served as working electrode, Pt electrode was used as counter electrode, and Ag/AgCl in 1 M KCl was used as reference electrode. Cyclic voltammetry, galvanostatic charge/discharge, and EIS were used to test the
electrochemical performance using an electrochemical workstation (CHI-660E). The scan rates of the cyclic voltammetry ranged from 5 to 100 mV s-1. The galvanostatic charge/discharge tests were conducted with a current density of 0.2, 0.5, 1, 2, and 3 A g-1, with a potential range of -0.1 to 0.5 V (vs. Ag/AgCl). EIS was conducted with a 5 mV amplitude and frequencies ranging from 0.01 Hz to 100 kHz. The cyclic stability of composite electrode was conducted at a current density of 3 A g-1. The specific
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f
capacitance and Coulombic efficiency was calculated according to the following
It1 mΔV
Coulombic effciency(%) =
t1 100 % t2
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Cg =
pr
equations:
(1)
(2)
Pr
where, Cg (F g-1) is specific capacitance, I (A g-1) is discharge current, t1 (s) is discharge time, m (g) is mass of loaded active materials, △V (V) is potential window, and t2 (s)
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is charge time.
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3. Results and discussion
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Fig. 1 Illustration of preparation process of PPy@cobalt oxyhydroxide/cellulose fiber composite
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flexible electrode material. a. CC; b. PC; c. PCC-11; d. SEM image of PCC-11.
The preparation process of PPy@cobalt oxyhydroxide/cellulose fiber composite electrode can be seen in Fig. 1. Firstly, the precursor of cobalt complex was in situ
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obtained in “liquid phase reduction” strategy in open system at room temperature.
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However, the suspension was eventually turned into brown rather than pink. We assume that pink cobalt hydroxide is oxidized to cobalt oxyhydroxide (CoOOH) by
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oxygen in the open system according to Eqs (3) to (7). During the process, cobalt ions are reduced by NaBH4 to form cobalt metal, and the Co metal is introduced into composites to improve the conductivity. Besides, the crystallinity of the composite is relatively lower, which also cause crystal defects to facilitate electron and ion transport (Xu et al., 2017). Then, PPy was in situ synthesized and loaded on the surface of cobalt oxyhydroxide/cellulose fiber composite material.
(3)
4Co2B + 3O2 = 8Co + 2B2O3
(4)
B2O3 + 3H2O = 2B(OH)2
(5)
2Co + 2H2O + O2 = Co(OH)2
(6)
4Co(OH)2 + O2 = CoOOH + 2H2O
(7)
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Pr
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2CoCl2 + 4NaBH4 + 9H2O = Co2B + 4NaCl + 12.5H2 + 3B(OH)3
of ro -p re al P ur n
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Fig. 2 SEM images of cellulose fiber (a1-a3), CC (b1-b3), PC (c1-c3), PCC-21 (d1-d3), PCC-11 (e1-e3), and PCC-12 (f1-f3).
The SEM images of cellulose fibers (a1-a3), CC (b1-b3), PC (c1-c3), PCC-21 (d1-d3), PCC-11 (e1-e3) and PCC-12 (f1-f3) at low and high magnifications are shown in Fig. 2. The surface of the cellulose fibers is smooth without any substance (Fig. 2a1-a3). It can be seen that the flaky particles were deposited on the surface of cellulose fibers (Fig. 2b1-b3), but which are not uniform. PPy was loaded on the surface of CC to obtain PCC (PCC-21 (d1-d3), PCC-11 (e1-e3) and PCC-12 (f1-f3). Compared with PCC-11, the
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f
distribution of PPy in PC is irregular on the surface of cellulose fibers. Whereas, the
distribution of PPy in PCC-11 electrode material with spherical is directional, which
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may be attributed to the role of Co complex. Besides, SEM-mapping was investigated
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in Fig. S1 to reveal the distribution of different substances in the composite electrode. The result showed four elements (i.e., C, N, O and Co) were uniformly distributed.
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Furthermore, XPS was used as an analytical tool to reveal the composition of the sheet
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material.
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Fig. 3 XPS survey spectra of (a) cellulose fiber, (b) CC, and (c) PCC-11, and (d) Co 2p XPS
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spectrum of CC.
XPS is a proven reliable method for intensive investigation for the oxidation state
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of atoms in the top few layers of material surfaces with partially filled valence bonds (Yang, Liu, Martens, & Frost, 2010). In this study, we utilize the tool to explore the
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chemical state of the composite flexible electrodes. The X-ray photoelectron spectra (XPS) test of CC and PCC-11 are shown in Fig. 3. Fig. 3a, b, and c show the full survey spectra of cellulose fibers, CC and PCC-11, respectively. In addition, d exhibits the fitting spectrum of Co 2p of CC. From the spectra of Fig. 3b and c, the main peaks (Co 2p and N 1s) can be observed clearly, demonstrating synthesized Co compounds and PPy on the surface of cellulose fibers. However, there is no characteristic peak of cobalt
in the composite flexible electrode (PCC-11). We suppose that the cobalt peak is masked by PPy. To further identify the chemical composition of Co compounds, we analyze the fitting peaks of Co 2p of CC according to Principal Component Analysis (Artyushkova, Levendosky, Atanassov, & Fulghum, 2007). Fig. 3d presents fitting peaks of Co 2p of CC. The Co 2p peaks include Co 2p3/2 and Co 2p1/2 with the binding energy of 780.30 and 796.06 eV, respectively. The Co 2p3/2 peak is mainly
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deconvoluted into three peaks with binding energies of 780.21, 781.88, and 785.96 eV, as shown in Fig. 3d. It is difficult to distinguish the chemical state of Co for the similar
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binding energies of Co oxides, hydroxides and oxyhydroxide (McIntyre, & Cook,
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1975). Nevertheless, a detailed analysis of the Co 2p shake-up structure allows the assignment of the oxidation state of cobalt (Dhawale et al., 2015). From the fitting
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peaks of Co 2p3/2, we can see that the main peak is sharp and centers at 780.21 eV with a highest intensity, which can be ascribed to CoⅢ(i. e. CoOOH), (Cataldi, Casella,
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Desimoni, & Rotunno, 1992; Mitton, Walton, & Thompson, 1993; Barr, 1978; Stoch,
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& Gablankowska-Kukucz, 1991), indicating the presence of CoOOH in composite
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electrode material.
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Fig. 4 FTIR spectra of CC (a) and cellulose fibers (b).
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The FTIR analysis was carried out to investigate the chemical structure of electrode materials. The FTIR spectrum of CC showed four characteristic peaks, i.e., 3335, 2906,
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1636, and 671 cm-1. The broad band at 3335 cm-1 is attributed to the stretching bond of hydrogen bonded hydroxyl group (–OH) in cellulose. The peak at 2906 cm-1 ascribes
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to the stretching absorption of C-H in cellulose, and/or the interaction of O-H bond with the near cobalt atom (Antony, Peulon, Legrand, & Chausse´, 2004). The characteristic peak at 1636 cm-1 corresponds to the Co-O bond in the crystal structure of cobalt oxyhydroxide (Cudennec, & Lecerf, 2001). The peak at 671 cm-1 is attributed to Co-O2- complex in the oxide (Kandalkar et al., 2010).
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Fig. 5 (a) Comparative CV profiles of PC, PCC-21, PCC-11, and PCC-12 electrodes recorded using a scan rate of 50 mV s−1; (b, c) CV profiles of (b) PC and (c) PCC-11 electrodes recorded at different
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scan rates (5, 10, 20, 30, 40, and 50 mV s−1); (d) Comparative GCD profiles of PC, PCC-21, PCC-
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11, and PCC-12 electrodes recorded using a current density of 0.5 A g−1; (e, f) GCD profiles of (e) PC and (f) PCC-11 electrodes recorded at different current densities (0.2, 0.5, 1, 2, and 3 A g−1); (g) Comparative specific capacitance profiles of PC and PCC-11 electrodes recorded at various current
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densities (0.2, 0.5, 1, 2, and 3 A g−1); (h) Coulombic efficiency of PCC-11 electrode over 1000 cycles at a current density of 3 A g−1, and (i) Cyclic stability of PCC-11 electrode over 1000 cycles at a current density of 3 A g−1.
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Fig. 6 Nyquist plots of PC and PCC-11 electrodes (a), Nyquist plots of PCC-11 electrodes before
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and after 1000 cycles (b), and EIS circuit of PCC-11 electrodes (c).
As we all known, electrolyte is extremely important to improve potential window in
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the study of supercapacitors. Generally, PPy, as one of pseudo-capacitive materials, is
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stable in acid electrolytes such as sulphuric acid (Ding et al., 2014; Mo, Chen, Gao, Chen, & Li, 2018; Xu et al., 2015). As mentioned above, the electrochemical performance of PPy/cellulose fibers and PPy@cobalt oxyhydroxide/cellulose fiber
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electrodes were investigated in 0.6 M H2SO4 electrolyte using three-electrode system over the potential window of -0.1~0.5 V. Fig. 5a shows comparative CV profiles of PC, PCC-21, PCC-11, and PCC-12 electrodes recorded using a scan rate of 50 mV s−1. The PCC-11 electrode has a highest area owing to high conductivity, which is useful for the fast transition of electrons. Fig. 5b and c exhibit the CV profiles of (b) PC and (c) PCC-
11 electrode at different scan rates (5, 10, 20, 30, 40, and 50 mV s−1). The curves change from rectangle-like to ellipse owing to polarization effect of pseudo-capacitive materials with the improvement of scan rate (Huang, Li, Dong, Zhang, & Zhang, 2015; Li, Wang, et al., 2015). Fig. 5d shows comparative GCD profiles of PC, PCC-21, PCC11, and PCC-12 electrodes using a current density of 0.5 A g−1. The PCC-11 electrode emerges the longest discharge time, indicating it possesses the largest specific
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capacitance. Fig. 5e and f further explore the charge/discharge performance of PC and PCC-11. The GCD curves exhibit a good symmetrical triangle shape, revealing the
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excellent charge transport within the electrode (Sheng et al., 2019). For comparison,
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the specific capacitance of PC and PCC-11 were evaluated at different current densities (Fig. 5g). The specific capacitance of the PC electrode is 340.3, 257.5, 210.0, 166.7,
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and 140.0 F g-1 at the current densities of 0.2, 0.5, 1, 2, and 3 A g-1, respectively. Besides, the specific capacitance of the PCC-11 electrode is 571.3, 461.7, 373.3, 310.0, and
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235.7 F g-1 at the current densities of 0.2, 0.5, 1, 2, and 3 A g-1, respectively. The result
1
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shows that specific capacitance of PCC-11 is 231.0, 204.2, 163.3, 143.3, and 95.7 F ghigher than that of PC. The improvement of the specific capacitance might be due to
cobalt oxyhydroxide also acts as a bridge between PPy and cellulose fiber, which also
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has an active effect on the structure of composite electrodes. To further explore the flexibility of composite electrode, a series of experiments
were carried out and the results are exhibited in Figs. S2 and S3. As shown in Fig. S2, the composite electrode could be folded into different shapes. Besides, the CV curves of the composite electrode almost unchanged after bending into different angles (Fig.
S3), showing the excellent flexibility. In addition, the specific capacitance of PCC-11 is higher than most of other CPsbased composites (in Table 1). We also tested the specific capacitance of PCC-11 electrode based two-electrode system (Fig. S4). To investigate the charge transfer and ion diffusion performance of composite electrode, we simulated the equivalent circuit according to electrochemical impedance spectroscopy (EIS) in Fig.6c, which mainly
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include three parts, i.e., equivalent series resistance (ESR), charge transfer resistance
(Rct) and Warburg diffusion (W). ESR is related to the electrolyte, electrode material,
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and the interfacial between the electrode and current collector. Rct represents the electron transfer at the interface of between electrode and electrolyte. W, the straight
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line at the low frequency with 45˚, is related to the ion diffusion of the electrolyte to
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the electrode. From Fig. 6a, we prove that the resistance value of PC (~5.9 Ω) is higher than that of PCC-11 (~3.5 Ω). Compared with PC, PCC-11 exihibits more excellent ion
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diffusion. Hence, the introduction of cobalt oxyhydroxide can improve the
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electrochemical performance of composite electrode. To explore the stability of electrode material, the cyclic stability of PCC-11 is performed using GCD test at a current density of 3 A g-1. From Fig. 5h, the Coulombic
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efficiency of PCC-11 electrode keeps ~100% or even higher, which indicates the composite electrode possesses a better stability. Fig. 5i demonstrates the capacitance retained 93.02% after 1000 cycles. The capacitance still retained 80.76% even after 5000 cycles (Fig. S5). The slight rise of specific capacitance might be due to the activation of the electrode materials and slowly decreases thereafter as cycling number
increases due to the degradation of the active materials in the long cycling process (Wei et al., 2010). On the other hand, the decrease of specific capacitance might be ascribed to the rise of resistance after 1000 cycles from Fig. 6b. In a word, the excellent electrochemical performance of PPy@cobalt oxyhydroxide/cellulose fiber composite electrode is due to the following reasons: (1) cobalt oxyhydroxide with semi-metallic property (5~12.8 S cm-1) (Fukunaga, Kishimi,
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Ozaki, & Sakai, 2005; Liu et al., 2017) improves the electric conductivity of the electrode and boosts the electron transfer rate and the electrochemical performance of
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the electrode; (2) cobalt oxyhydroxide might act as a bridge between PPy and cellulose
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the stability of the composite electrode.
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fibers, which restrains the expansion of PPy in charge/discharge process to promote
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of
4. Conclusions PPy@cobalt oxyhydroxide/cellulose composite electrode was successfully prepared via “liquid phase reduction” strategy in open system at room temperature. The results showed the introduction of cobalt oxyhydroxide not only promoted the conductivity of electrode but also improved its electrochemical performance. The electrochemical test demonstrated that the composite electrode had a high specific
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capacitance of 571.3 F g-1 at a current density of 0.2 A g-1. Meanwhile, it also had a
robust cyclic stability, which can retain 93.02% capacitance at a current density of 3 A g-1 after 1000 cycles. The study provides a simple and cost-effective method to
re
-p
manufacture the flexible electrodes for application in energy storage.
Acknowledgement
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The financial support from the National Natural Science Foundation of China
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na
(grant no. 31770620) is gratefully acknowledged.
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Jo
ur
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lP
re
-p
Electrochimica Acta, 186, 562-571.
Specific Current Electrode material
Power
Energy
capacita
Cyclic stability
Ref.
density
density
density
391 F g-
0.5 A g-
429.3 m
4.1 mWh
1
1
PPy/PEDOT:PSS@M
13.44 F
2 mA
12 mW
1.87 mWh
94.3% after
(Sun, Li, Zhao, Cai,
WCNT/SF
cm-1
cm-1
cm−3
cm−3
5000 cycles
& Ge, 2019)
nce
PPy@TOBC/rGO
Ti3C2/PPy
(Sheng et al., 2019) −3
W cm
−3
cm
184.36 F 2 mV s-
83.33% after (Wu et al., 2019)
g
1
4000 cycles
ro of
nanocomposite
-1
268.5
0.5 A g-
450.4 W
23.8 Wh
88% after 10000
(Zhang, Li, et al.,
F g-1
1
kg−1
kg−1
cycles
2019)
PPy@UIO-66@cotton
565 F
0.8 mA
90% after 500
(Zhang, Tian, et al.,
fabric electrode
g−1
cm-2
cycles
2019)
PANI
500
1Ag
-1
−1
(Song et al., 2019)
−1
kg
0.1A g-
﹥70% after
(Zhang, Zhang,
g−1
1
1000 cycles
Zhao, & Wu, 2010)
na 575 F
ur
−1
g
Jo
59.2 Wh
480 F PANI/GN
PANI/MnO2
W kg
−1
lP
g
re
426 F
-p
PPy@CA
0.4 A g-
720
90.2% after (Jia et al., 2019)
W kg
1
52.7 Wh
−1
−1
kg
2000 cycles
255.7 F
0.2 A g-
﹥93% after
(Zhu, Xu, et al.,
g−1
1
1000 cycles
2015)
PPy/GN
(Mohd Abdah, 409.88
0.5 A g
-
297.32
42.53 Wh
86.3% after
CNFs/PPy/MnO2
Abdul Rahman, & −1
Fg
1
W kg
−1
kg
−1
3000 cycles Sulaiman, 2019)
780.4 F
0.5 A g-
533.02
58.04 Wh
90.54% after
PTh/Al2O3
(Vijeth et al., 2019) −1
g
529.3 F
W kg
1
0.1 A g-
−1
100 W
−1
kg
5000 cycles
38.42 Wh
98.5% after
PPy/CNT/MnO2
(Zhou et al., 2015) g
−1
1
kg
571.3 F
0.2 A g-
g−1
1
461.7 F
0.5 A g-
g−1
1
−1
kg
PPy@cobalt 373.3 F 1 A g-1
oxyhydroxide/cellulos
This work
−1
-p
g e fiber
235.7 F
lP
g−1
3Ag
−1
80.76% after
-1
na
g
re
310.0 F 2 A g-1
1000 cycles
ro of
−1
1000 cycles
Table 1. Comparison of electrochemical performance of the PPy@cobalt
ur
oxyhydroxide/cellulose fiber electrode and previously reported conductive polymers
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electrodes.
PII:
S0144-8617(19)31122-1
DOI:
https://doi.org/10.1016/j.carbpol.2019.115455
Reference:
CARP 115455
To appear in:
Carbohydrate Polymers
Received Date:
3 August 2019
Revised Date:
6 October 2019
Accepted Date:
6 October 2019
Please cite this article as: Yang S, Sun L, An X, Qian X, Construction of flexible electrodes based on ternary polypyrrole@cobalt oxyhydroxide/cellulose fiber composite for supercapacitor, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115455
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Construction of flexible electrodes based on ternary polypyrrole@cobalt
oxyhydroxide/cellulose
fiber
composite for supercapacitor
Shuaishuai Yang, Lijian Sun, Xianhui An, Xueren Qian* Key Laboratory of Bio-based Material Science and Technology of Ministry of
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*Corresponding author. [email protected]
e-
Highlights
PPy@cobalt oxyhydroxide/cellulose fiber composite was prepared via a facile
Pr
method.
Cobalt oxyhydroxide was grown on the surface of cellulose fiber at room
al
temperature.
The composite electrode had a higher specific capacitance and better cycle
ur n
f
Education, Northeast Forestry University, Harbin, China
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stability.
Abstract: With the development of flexible electronic devices, flexible energy storage systems have been research hotpot. Conductive polymers is potential pseudocapacitor materials in energy storage field. Meanwhile, cellulose fiber with natural, degradable, renewable and flexible properties is one of tremendous promising alternatives to the
flexible substrates. Hence, a polypyrrole@cobalt oxyhydroxide/cellulose fiber composite electrode is prepared via “liquid phase reduction” strategy in open system at room temperature. The composite electrode exhibits excellent electrochemical properties, which has a high specific capacitance and capacitance retention. The highest specific capacitance of 571.3 F g-1 at 0.2 A g-1 is obtained. Besides, the specific capacitance of the composite electrode has no significant loss, showing high cycle
oo
f
stability (93.02% after 1000 cycles). The excellent electrochemical properties can be ascribed to the introduction of cobalt oxyhydroxide, which restrains the volumetric
pr
change of polypyrrole in the electrochemical redox process, and promotes the rapid
e-
migration of electrons.
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1. Introduction
al
electrochemical properties
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Keywords: cellulose fiber, polypyrrole, cobalt oxyhydroxide, electrode material,
In the past decades, the development of the flexible electronic devices (e.g., smart phone, wearable sensor, implantable medical devices and so on) had greatly stimulated
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the demand for miniaturized flexible energy storage systems (Wang 2010; Wang, 2012; Wang & Wu, 2012). Currently, supercapacitors with fast charge/discharge, long cyclic lifetime and high power density had been intensively studied. However, a relatively low energy density of the supercapacitors limited their expansive application. To date, the strategies of enhancing energy density for supercapacitors mainly included two
inspects, i.e., increasing capacitance (C) and/or increasing working voltage (V) based on the formula of energy density (E=CV2/2)(Yan, Wang, Wei, & Fan, 2014). From these point of views, the capacitance can be enhanced by improving some crucial factors of electrode materials (e.g., pore size, surface area, electrical conductivity, functional groups, etc.). The strategy of improving working voltage is to employ highvoltage electrolytes and various supercapacitor device configurations.
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f
Based on the charge storage mechanisms, supercapacitors are classified as
electrochemical double layer capacitors (EDLCs) with physical adsorption of ions at
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the interface of the electrode surface and the electrolyte, and pseudocapacitors with a
e-
fast reversible faradaic charge transfer at the electrode surface. Among them, EDLCs electrode material refers to carbon material (e.g., carbon black (Wang et al., 2014),
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graphene, CNT (Yang et al., 2013), etc.). Pseudocapacitive electrode material mainly includes conductive polymers (e.g., polyaniline (Mondal, Barai, & Munichandraiah, Sk,
&
Yue,
2014),
polypyrrole
(Shi
et
al.,
2014),
poly(3,4-
al
2007;
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ethylenedioxythiophene) (Liu, Hu, Xue, Zhang, & Zhu, 2008; Ravit, Abdullah, Ahmad, & Sulaiman, 2019)), transition metal oxides (e.g., RuO2 (Kuratani, Kiyobayashi, & Kuriyama, 2009), MnO2 (Huang, Li, Dong, Zhang, & Zhang, 2015), NiO (Cai et al.,
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2015), Co3O4 (Dong et al., 2012), etc.), transition metal sulfides (MoS2 (Islama, Wang, Warzywodac, & Fan, 2018), NiCo2S4 (Zhu, Ji, Wu, & Liu, 2015), etc.), transition metal hydroxides (Co(OH)2 (Jiang et al., 2011), Ni(OH)2 (Li, Xu, 2015), CoMn-LDHs (Jagadale et al., 2016), NiMn-LDHs (Guo et al., 2016),
etc.), transition metal
carbides (Ti3C2 (Zhu, Huang, 2016; Boota, et al., 2016; Qin et al., 2018; Li et al., 2017;
Yan et al., 2017), V2C (Shan et al., 2018)), and cobalt oxyhydroxide (Zheng et al., 2009; Zheng et al., 2010). Among these electrode materials, CPs are organic polymers that conduct electricity through a conjugated bond system along the polymer chain. In the past two decades, CPs are extensively explored for energy storage application due to their reversible faradaic redox reaction, high charge density, and lower cost as compared with the other transition materials (such as metal oxides, grapheme, etc.).
oo
f
(Ryu, Kim, Park, Park, & Chang, 2002; Rudge, Raistrick, Gottesfeld, & Ferraris, 1994; Burke, 2007). At the same time, paper, as one of the most ancient flexible products
pr
invented A.D. 105 years, is one of a tremendous promising alternatives to the flexible
e-
substrates because of their wide availability, low cost, light weight, environmental friendliness, recyclability and bendability (Yao et al., 2013; Tobjörk, & Österbacka,
Pr
2011; Zheng et al., 2013; Zhang, 2015; Lin, Gritsenko, Liu, Lu, & Xu, 2016; PerezMadrigal, Edo, & Aleman, 2016). As mentioned above, although CPs have so many
al
advantages in energy storage, they exhibit a volumetric expansion in redox process,
ur n
which lead to the collapse of electrode materials. Researchers have tried plenty of strategies to improve the drawbacks (Karaca, Dhawale, Kim, & Lokhande, 2019; Zhang, Li, et al., 2019; Dias et al., 2019). In our previous researches, CPs were
Jo
incorporated into cellulose fibers via in situ oxidation polymerization method to prepare the flexible and conductive material (Ding, Qian, Yu, & An, 2010; Mao, Wu, Qian, & An, 2014; Mao, Liu, Qian, & An, 2015; Mao, Dong, Qian, & An, 2017). Besides, cobalt oxyhydroxide with excellent electrochemical reversibility and semimetallic conductivity is less concerned as electrode material.
Hence, cobalt oxyhydroxide is introduced to CPs and cellulose fibers to prepare the binder-free flexible electrode, in order to restrain the volumetric change of CPs in the redox process, and promote the rapid migration of electrons. The work is of great significance to prepare the polypyrrole@cobalt oxyhydroxide/cellulose fiber composite flexible electrode to solve the flexible and electrochemical problems of supercapacitors. To our knowledge, the metal Co was introduced into cellulose fibers
oo
f
based composite through the reduction of NaBH4, which on the one hand would
provide path for electron rapid transmission. On the other hand, the crystallinity of
pr
materials synthesized at room temperature is relatively low, and thereby the materials
e-
have a large number of crystal defects, which are conducive to the transmission of electrons and ions. Besides, the Co(OH)2 would be converted to CoOOH in open
Pr
system based on the mechanism of the reaction, and it also has a excellent conductivity. So the strategy was beneficial to overcome the drawback (poor cyclic stability) of
al
cellulose/PPy composite electrode (Xu et al., 2017).
ur n
In this study, a conductive and flexible composite electrode constructed with polypyrrole (PPy), cobalt oxyhydroxide and cellulose fibers was successfully prepared via “liquid phase reduction” strategy in open system at room temperature. The
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PPy@cobalt oxyhydroxide/cellulose fiber composite electrode showed the excellent electrochemical properties. The highest specific capacitance of 571.3 F g-1 at 0.2 A g-1 in 0.6 M H2SO4 electrolyte was obtained when the molar ratio of CoCl2 to NaBH4 was 1:1. Besides, the specific capacitance of composite electrode had no significant loss, showing high cycle stability (93.02% after 1000 cycles).
2. Experimental 2.1 Materials Cobalt chloride (CoCl2·6H2O) and pyrrole were purchased from Sinopharm Chemical Reagent Co. Ltd. Sodium borohydride (NaBH4) and ferric chloride (FeCl3·6H2O) were purchased from Shanghai Macklin Biochemical Co. Ltd. and
oo
f
Tianjin Guangfu Technology Development Co. Ltd., respectively. Canada market
bleached softwood kraft pulp as cellulose fiber source was provided by Mudanjiang
pr
Hengfeng Paper Co. Ltd (Heilongjiang, China) and was beaten to 37 oSR before use.
e-
The diameter of cellulose fibers is about 15 μm, and the component of cellulose fibers
Pr
is almost all cellulose.
2.2 Preparation of PPy@cobalt oxyhydroxide/cellulose fiber composite electrode
al
The PPy@cobalt oxyhydroxide/cellulose fiber composite electrode was prepared
ur n
using “liquid phase reduction” strategy in open system at room temperature. Firstly, 0.5 g cellulose fibers and a certain amount of CoCl2·6H2O (1 mmol) were dispersed in deionized water, and then a certain amount of NaBH4 (0.5-2 mmol) was slowly added
Jo
to the above mixture suspension under continuous stirring for 6 h to react completely. The resultant mixture was filtered and washed with deionized water. Afterwards, the PPy@cobalt oxyhydroxide/cellulose fiber composite electrode was prepared via in situ oxidative polymerization method. 0.25 mL of pyrrole was dispersed in the cobalt oxyhydroxide/cellulose fiber suspension under continuous stirring. Subsequently,
0.973 g FeCl3·6H2O was added into the above mixture and stirred in order to initiate the polymerization reaction to proceed. After 6 h, the above mixture was filtered and washed with deionized water, PPy@cobalt oxyhydroxide/cellulose fiber composite was obtained, and noted as PCC. Where, PCC-21, PCC-11 and PCC-12 represent the samples prepared at the molar ratios of CoCl2·6H2O to NaBH4 of 2:1, 1:1 and 1:2, respectively. For comparison, the cobalt oxyhydroxide/cellulose fiber and
oo
f
PPy/cellulose fiber composite electrodes were fabricated, and marked as CC and PC,
pr
respectively.
e-
2.3 Characterization
Scanning electron microscope (SEM) was carried out to investigate the
Pr
morphology of composite electrodes (SEU 8010). The X-ray photoelectron spectra (XPS) and Fourier transform infrared (FTIR) spectra were recorded on Thermo
ur n
respectively.
al
ESCALAB 250XI and Thermo Fisher Scientific Nicolet 6700 spectrometer,
2.4 Electrochemical Tests
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All of the electrochemical tests were carried out in three-electrode system, using
a 0.6 M H2SO4 electrolyte. The prepared PPy@cobalt oxyhydroxide/cellulose fiber composite electrode was served as working electrode, Pt electrode was used as counter electrode, and Ag/AgCl in 1 M KCl was used as reference electrode. Cyclic voltammetry, galvanostatic charge/discharge, and EIS were used to test the
electrochemical performance using an electrochemical workstation (CHI-660E). The scan rates of the cyclic voltammetry ranged from 5 to 100 mV s-1. The galvanostatic charge/discharge tests were conducted with a current density of 0.2, 0.5, 1, 2, and 3 A g-1, with a potential range of -0.1 to 0.5 V (vs. Ag/AgCl). EIS was conducted with a 5 mV amplitude and frequencies ranging from 0.01 Hz to 100 kHz. The cyclic stability of composite electrode was conducted at a current density of 3 A g-1. The specific
oo
f
capacitance and Coulombic efficiency was calculated according to the following
It1 mΔV
Coulombic effciency(%) =
t1 100 % t2
e-
Cg =
pr
equations:
(1)
(2)
Pr
where, Cg (F g-1) is specific capacitance, I (A g-1) is discharge current, t1 (s) is discharge time, m (g) is mass of loaded active materials, △V (V) is potential window, and t2 (s)
ur n
al
is charge time.
Jo
3. Results and discussion
f oo pr
Fig. 1 Illustration of preparation process of PPy@cobalt oxyhydroxide/cellulose fiber composite
Pr
e-
flexible electrode material. a. CC; b. PC; c. PCC-11; d. SEM image of PCC-11.
The preparation process of PPy@cobalt oxyhydroxide/cellulose fiber composite electrode can be seen in Fig. 1. Firstly, the precursor of cobalt complex was in situ
al
obtained in “liquid phase reduction” strategy in open system at room temperature.
ur n
However, the suspension was eventually turned into brown rather than pink. We assume that pink cobalt hydroxide is oxidized to cobalt oxyhydroxide (CoOOH) by
Jo
oxygen in the open system according to Eqs (3) to (7). During the process, cobalt ions are reduced by NaBH4 to form cobalt metal, and the Co metal is introduced into composites to improve the conductivity. Besides, the crystallinity of the composite is relatively lower, which also cause crystal defects to facilitate electron and ion transport (Xu et al., 2017). Then, PPy was in situ synthesized and loaded on the surface of cobalt oxyhydroxide/cellulose fiber composite material.
(3)
4Co2B + 3O2 = 8Co + 2B2O3
(4)
B2O3 + 3H2O = 2B(OH)2
(5)
2Co + 2H2O + O2 = Co(OH)2
(6)
4Co(OH)2 + O2 = CoOOH + 2H2O
(7)
Jo
ur n
al
Pr
e-
pr
oo
f
2CoCl2 + 4NaBH4 + 9H2O = Co2B + 4NaCl + 12.5H2 + 3B(OH)3
of ro -p re al P ur n
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Fig. 2 SEM images of cellulose fiber (a1-a3), CC (b1-b3), PC (c1-c3), PCC-21 (d1-d3), PCC-11 (e1-e3), and PCC-12 (f1-f3).
The SEM images of cellulose fibers (a1-a3), CC (b1-b3), PC (c1-c3), PCC-21 (d1-d3), PCC-11 (e1-e3) and PCC-12 (f1-f3) at low and high magnifications are shown in Fig. 2. The surface of the cellulose fibers is smooth without any substance (Fig. 2a1-a3). It can be seen that the flaky particles were deposited on the surface of cellulose fibers (Fig. 2b1-b3), but which are not uniform. PPy was loaded on the surface of CC to obtain PCC (PCC-21 (d1-d3), PCC-11 (e1-e3) and PCC-12 (f1-f3). Compared with PCC-11, the
oo
f
distribution of PPy in PC is irregular on the surface of cellulose fibers. Whereas, the
distribution of PPy in PCC-11 electrode material with spherical is directional, which
pr
may be attributed to the role of Co complex. Besides, SEM-mapping was investigated
e-
in Fig. S1 to reveal the distribution of different substances in the composite electrode. The result showed four elements (i.e., C, N, O and Co) were uniformly distributed.
Pr
Furthermore, XPS was used as an analytical tool to reveal the composition of the sheet
Jo
ur n
al
material.
f oo pr ePr
Fig. 3 XPS survey spectra of (a) cellulose fiber, (b) CC, and (c) PCC-11, and (d) Co 2p XPS
al
spectrum of CC.
XPS is a proven reliable method for intensive investigation for the oxidation state
ur n
of atoms in the top few layers of material surfaces with partially filled valence bonds (Yang, Liu, Martens, & Frost, 2010). In this study, we utilize the tool to explore the
Jo
chemical state of the composite flexible electrodes. The X-ray photoelectron spectra (XPS) test of CC and PCC-11 are shown in Fig. 3. Fig. 3a, b, and c show the full survey spectra of cellulose fibers, CC and PCC-11, respectively. In addition, d exhibits the fitting spectrum of Co 2p of CC. From the spectra of Fig. 3b and c, the main peaks (Co 2p and N 1s) can be observed clearly, demonstrating synthesized Co compounds and PPy on the surface of cellulose fibers. However, there is no characteristic peak of cobalt
in the composite flexible electrode (PCC-11). We suppose that the cobalt peak is masked by PPy. To further identify the chemical composition of Co compounds, we analyze the fitting peaks of Co 2p of CC according to Principal Component Analysis (Artyushkova, Levendosky, Atanassov, & Fulghum, 2007). Fig. 3d presents fitting peaks of Co 2p of CC. The Co 2p peaks include Co 2p3/2 and Co 2p1/2 with the binding energy of 780.30 and 796.06 eV, respectively. The Co 2p3/2 peak is mainly
oo
f
deconvoluted into three peaks with binding energies of 780.21, 781.88, and 785.96 eV, as shown in Fig. 3d. It is difficult to distinguish the chemical state of Co for the similar
pr
binding energies of Co oxides, hydroxides and oxyhydroxide (McIntyre, & Cook,
e-
1975). Nevertheless, a detailed analysis of the Co 2p shake-up structure allows the assignment of the oxidation state of cobalt (Dhawale et al., 2015). From the fitting
Pr
peaks of Co 2p3/2, we can see that the main peak is sharp and centers at 780.21 eV with a highest intensity, which can be ascribed to CoⅢ(i. e. CoOOH), (Cataldi, Casella,
al
Desimoni, & Rotunno, 1992; Mitton, Walton, & Thompson, 1993; Barr, 1978; Stoch,
ur n
& Gablankowska-Kukucz, 1991), indicating the presence of CoOOH in composite
Jo
electrode material.
f oo pr e-
Pr
Fig. 4 FTIR spectra of CC (a) and cellulose fibers (b).
al
The FTIR analysis was carried out to investigate the chemical structure of electrode materials. The FTIR spectrum of CC showed four characteristic peaks, i.e., 3335, 2906,
ur n
1636, and 671 cm-1. The broad band at 3335 cm-1 is attributed to the stretching bond of hydrogen bonded hydroxyl group (–OH) in cellulose. The peak at 2906 cm-1 ascribes
Jo
to the stretching absorption of C-H in cellulose, and/or the interaction of O-H bond with the near cobalt atom (Antony, Peulon, Legrand, & Chausse´, 2004). The characteristic peak at 1636 cm-1 corresponds to the Co-O bond in the crystal structure of cobalt oxyhydroxide (Cudennec, & Lecerf, 2001). The peak at 671 cm-1 is attributed to Co-O2- complex in the oxide (Kandalkar et al., 2010).
f oo pr e-
Pr
Fig. 5 (a) Comparative CV profiles of PC, PCC-21, PCC-11, and PCC-12 electrodes recorded using a scan rate of 50 mV s−1; (b, c) CV profiles of (b) PC and (c) PCC-11 electrodes recorded at different
al
scan rates (5, 10, 20, 30, 40, and 50 mV s−1); (d) Comparative GCD profiles of PC, PCC-21, PCC-
ur n
11, and PCC-12 electrodes recorded using a current density of 0.5 A g−1; (e, f) GCD profiles of (e) PC and (f) PCC-11 electrodes recorded at different current densities (0.2, 0.5, 1, 2, and 3 A g−1); (g) Comparative specific capacitance profiles of PC and PCC-11 electrodes recorded at various current
Jo
densities (0.2, 0.5, 1, 2, and 3 A g−1); (h) Coulombic efficiency of PCC-11 electrode over 1000 cycles at a current density of 3 A g−1, and (i) Cyclic stability of PCC-11 electrode over 1000 cycles at a current density of 3 A g−1.
f oo pr
Fig. 6 Nyquist plots of PC and PCC-11 electrodes (a), Nyquist plots of PCC-11 electrodes before
Pr
e-
and after 1000 cycles (b), and EIS circuit of PCC-11 electrodes (c).
As we all known, electrolyte is extremely important to improve potential window in
al
the study of supercapacitors. Generally, PPy, as one of pseudo-capacitive materials, is
ur n
stable in acid electrolytes such as sulphuric acid (Ding et al., 2014; Mo, Chen, Gao, Chen, & Li, 2018; Xu et al., 2015). As mentioned above, the electrochemical performance of PPy/cellulose fibers and PPy@cobalt oxyhydroxide/cellulose fiber
Jo
electrodes were investigated in 0.6 M H2SO4 electrolyte using three-electrode system over the potential window of -0.1~0.5 V. Fig. 5a shows comparative CV profiles of PC, PCC-21, PCC-11, and PCC-12 electrodes recorded using a scan rate of 50 mV s−1. The PCC-11 electrode has a highest area owing to high conductivity, which is useful for the fast transition of electrons. Fig. 5b and c exhibit the CV profiles of (b) PC and (c) PCC-
11 electrode at different scan rates (5, 10, 20, 30, 40, and 50 mV s−1). The curves change from rectangle-like to ellipse owing to polarization effect of pseudo-capacitive materials with the improvement of scan rate (Huang, Li, Dong, Zhang, & Zhang, 2015; Li, Wang, et al., 2015). Fig. 5d shows comparative GCD profiles of PC, PCC-21, PCC11, and PCC-12 electrodes using a current density of 0.5 A g−1. The PCC-11 electrode emerges the longest discharge time, indicating it possesses the largest specific
oo
f
capacitance. Fig. 5e and f further explore the charge/discharge performance of PC and PCC-11. The GCD curves exhibit a good symmetrical triangle shape, revealing the
pr
excellent charge transport within the electrode (Sheng et al., 2019). For comparison,
e-
the specific capacitance of PC and PCC-11 were evaluated at different current densities (Fig. 5g). The specific capacitance of the PC electrode is 340.3, 257.5, 210.0, 166.7,
Pr
and 140.0 F g-1 at the current densities of 0.2, 0.5, 1, 2, and 3 A g-1, respectively. Besides, the specific capacitance of the PCC-11 electrode is 571.3, 461.7, 373.3, 310.0, and
al
235.7 F g-1 at the current densities of 0.2, 0.5, 1, 2, and 3 A g-1, respectively. The result
1
ur n
shows that specific capacitance of PCC-11 is 231.0, 204.2, 163.3, 143.3, and 95.7 F ghigher than that of PC. The improvement of the specific capacitance might be due to
cobalt oxyhydroxide also acts as a bridge between PPy and cellulose fiber, which also
Jo
has an active effect on the structure of composite electrodes. To further explore the flexibility of composite electrode, a series of experiments
were carried out and the results are exhibited in Figs. S2 and S3. As shown in Fig. S2, the composite electrode could be folded into different shapes. Besides, the CV curves of the composite electrode almost unchanged after bending into different angles (Fig.
S3), showing the excellent flexibility. In addition, the specific capacitance of PCC-11 is higher than most of other CPsbased composites (in Table 1). We also tested the specific capacitance of PCC-11 electrode based two-electrode system (Fig. S4). To investigate the charge transfer and ion diffusion performance of composite electrode, we simulated the equivalent circuit according to electrochemical impedance spectroscopy (EIS) in Fig.6c, which mainly
oo
f
include three parts, i.e., equivalent series resistance (ESR), charge transfer resistance
(Rct) and Warburg diffusion (W). ESR is related to the electrolyte, electrode material,
pr
and the interfacial between the electrode and current collector. Rct represents the electron transfer at the interface of between electrode and electrolyte. W, the straight
e-
line at the low frequency with 45˚, is related to the ion diffusion of the electrolyte to
Pr
the electrode. From Fig. 6a, we prove that the resistance value of PC (~5.9 Ω) is higher than that of PCC-11 (~3.5 Ω). Compared with PC, PCC-11 exihibits more excellent ion
al
diffusion. Hence, the introduction of cobalt oxyhydroxide can improve the
ur n
electrochemical performance of composite electrode. To explore the stability of electrode material, the cyclic stability of PCC-11 is performed using GCD test at a current density of 3 A g-1. From Fig. 5h, the Coulombic
Jo
efficiency of PCC-11 electrode keeps ~100% or even higher, which indicates the composite electrode possesses a better stability. Fig. 5i demonstrates the capacitance retained 93.02% after 1000 cycles. The capacitance still retained 80.76% even after 5000 cycles (Fig. S5). The slight rise of specific capacitance might be due to the activation of the electrode materials and slowly decreases thereafter as cycling number
increases due to the degradation of the active materials in the long cycling process (Wei et al., 2010). On the other hand, the decrease of specific capacitance might be ascribed to the rise of resistance after 1000 cycles from Fig. 6b. In a word, the excellent electrochemical performance of PPy@cobalt oxyhydroxide/cellulose fiber composite electrode is due to the following reasons: (1) cobalt oxyhydroxide with semi-metallic property (5~12.8 S cm-1) (Fukunaga, Kishimi,
oo
f
Ozaki, & Sakai, 2005; Liu et al., 2017) improves the electric conductivity of the electrode and boosts the electron transfer rate and the electrochemical performance of
pr
the electrode; (2) cobalt oxyhydroxide might act as a bridge between PPy and cellulose
Jo
ur n
al
Pr
the stability of the composite electrode.
e-
fibers, which restrains the expansion of PPy in charge/discharge process to promote
re
al P
ur n
Jo -p
ro
of
4. Conclusions PPy@cobalt oxyhydroxide/cellulose composite electrode was successfully prepared via “liquid phase reduction” strategy in open system at room temperature. The results showed the introduction of cobalt oxyhydroxide not only promoted the conductivity of electrode but also improved its electrochemical performance. The electrochemical test demonstrated that the composite electrode had a high specific
ro of
capacitance of 571.3 F g-1 at a current density of 0.2 A g-1. Meanwhile, it also had a
robust cyclic stability, which can retain 93.02% capacitance at a current density of 3 A g-1 after 1000 cycles. The study provides a simple and cost-effective method to
re
-p
manufacture the flexible electrodes for application in energy storage.
Acknowledgement
lP
The financial support from the National Natural Science Foundation of China
Jo
ur
na
(grant no. 31770620) is gratefully acknowledged.
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Electrochimica Acta, 186, 562-571.
Specific Current Electrode material
Power
Energy
capacita
Cyclic stability
Ref.
density
density
density
391 F g-
0.5 A g-
429.3 m
4.1 mWh
1
1
PPy/PEDOT:PSS@M
13.44 F
2 mA
12 mW
1.87 mWh
94.3% after
(Sun, Li, Zhao, Cai,
WCNT/SF
cm-1
cm-1
cm−3
cm−3
5000 cycles
& Ge, 2019)
nce
PPy@TOBC/rGO
Ti3C2/PPy
(Sheng et al., 2019) −3
W cm
−3
cm
184.36 F 2 mV s-
83.33% after (Wu et al., 2019)
g
1
4000 cycles
ro of
nanocomposite
-1
268.5
0.5 A g-
450.4 W
23.8 Wh
88% after 10000
(Zhang, Li, et al.,
F g-1
1
kg−1
kg−1
cycles
2019)
PPy@UIO-66@cotton
565 F
0.8 mA
90% after 500
(Zhang, Tian, et al.,
fabric electrode
g−1
cm-2
cycles
2019)
PANI
500
1Ag
-1
−1
(Song et al., 2019)
−1
kg
0.1A g-
﹥70% after
(Zhang, Zhang,
g−1
1
1000 cycles
Zhao, & Wu, 2010)
na 575 F
ur
−1
g
Jo
59.2 Wh
480 F PANI/GN
PANI/MnO2
W kg
−1
lP
g
re
426 F
-p
PPy@CA
0.4 A g-
720
90.2% after (Jia et al., 2019)
W kg
1
52.7 Wh
−1
−1
kg
2000 cycles
255.7 F
0.2 A g-
﹥93% after
(Zhu, Xu, et al.,
g−1
1
1000 cycles
2015)
PPy/GN
(Mohd Abdah, 409.88
0.5 A g
-
297.32
42.53 Wh
86.3% after
CNFs/PPy/MnO2
Abdul Rahman, & −1
Fg
1
W kg
−1
kg
−1
3000 cycles Sulaiman, 2019)
780.4 F
0.5 A g-
533.02
58.04 Wh
90.54% after
PTh/Al2O3
(Vijeth et al., 2019) −1
g
529.3 F
W kg
1
0.1 A g-
−1
100 W
−1
kg
5000 cycles
38.42 Wh
98.5% after
PPy/CNT/MnO2
(Zhou et al., 2015) g
−1
1
kg
571.3 F
0.2 A g-
g−1
1
461.7 F
0.5 A g-
g−1
1
−1
kg
PPy@cobalt 373.3 F 1 A g-1
oxyhydroxide/cellulos
This work
−1
-p
g e fiber
235.7 F
lP
g−1
3Ag
−1
80.76% after
-1
na
g
re
310.0 F 2 A g-1
1000 cycles
ro of
−1
1000 cycles
Table 1. Comparison of electrochemical performance of the PPy@cobalt
ur
oxyhydroxide/cellulose fiber electrode and previously reported conductive polymers
Jo
electrodes.