Construction of flexible electrodes based on ternary polypyrrole@cobalt oxyhydroxide/cellulose fiber composite for supercapacitor
Introduction
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 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 high-voltage electrolytes and various supercapacitor device configurations.
Based on the charge storage mechanisms, supercapacitors are classified as electrochemical double layer capacitors (EDLCs) with physical adsorption of ions at the interface of the electrode surface and the electrolyte, and pseudocapacitors with a 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), graphene, CNT (Yang et al., 2013), etc.). Pseudocapacitive electrode material mainly includes conductive polymers (e.g., polyaniline (Mondal, Barai, & Munichandraiah, 2007; Sk & Yue, 2014), polypyrrole (Shi et al., 2014), poly(3,4-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., 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 et al., 2015), CoMn-LDHs (Jagadale et al., 2016), NiMn-LDHs (Guo et al., 2016), etc.), transition metal carbides (Ti3C2 (Boota et al., 2016; Li et al., 2017; Qin et al., 2018; Yan et al., 2017; Zhu et al., 2016), V2C (Shan et al., 2018)), and cobalt oxyhydroxide (Zheng et al., 2009, 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.). (Burke, 2007; Rudge, Raistrick, Gottesfeld, & Ferraris, 1994; Ryu, Kim, Park, Park, & Chang, 2002). At the same time, paper, as one of the most ancient flexible products invented A.D. 105 years, is one of a tremendous promising alternatives to the flexible substrates because of their wide availability, low cost, light weight, environmental friendliness, recyclability and bendability (Lin, Gritsenko, Liu, Lu, & Xu, 2016; Perez-Madrigal, Edo, & Aleman, 2016; Tobjörk & Österbacka, 2011; Yao et al., 2013; Zheng et al., 2013; Zhang et al., 2015). As mentioned above, although CPs have so many advantages in energy storage, they exhibit a volumetric expansion in redox process, which lead to the collapse of electrode materials. Researchers have tried plenty of strategies to improve the drawbacks (Dias et al., 2019; Karaca, Gökcen, Pekmez, & Pekmez, 2019; Zhang, Li, et al., 2019). In our previous researches, CPs were 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 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 materials synthesized at room temperature is relatively low, and thereby the materials 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 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 cellulose/PPy composite electrode (Xu et al., 2017).
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 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).
Section snippets
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 Tianjin Guangfu Technology Development Co. Ltd., respectively. Canada market bleached softwood kraft pulp as cellulose fiber source was provided by Mudanjiang Hengfeng Paper Co. Ltd (Heilongjiang, China) and was beaten to 37 °SR before use. The diameter of cellulose fibers is
Results and discussion
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 obtained in “liquid phase reduction” strategy in open system at room temperature. However, the suspension was eventually turned into brown rather than pink. We assume that pink cobalt hydroxide is oxidized to cobalt oxyhydroxide (CoOOH) by oxygen in the open system according to Eqs. (3), (4), (5), (6), (7). During the process, cobalt
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 capacitance of 571.3 F g−1 at a current density of 0.2 A g−1. Meanwhile, it also had a robust cyclic
Acknowledgement
The financial support from the National Natural Science Foundation of China (grant no. 31770620) is gratefully acknowledged.
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