Elsevier

Carbohydrate Polymers

Volume 229, 1 February 2020, 115455
Carbohydrate Polymers

Construction of flexible electrodes based on ternary polypyrrole@cobalt oxyhydroxide/cellulose fiber composite for supercapacitor

https://doi.org/10.1016/j.carbpol.2019.115455Get rights and content

Highlights

  • PPy@cobalt oxyhydroxide/cellulose fiber composite was prepared via a facile method.

  • Cobalt oxyhydroxide was grown on the surface of cellulose fiber at room temperature.

  • The composite electrode had a higher specific capacitance and better cycle 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 stability (93.02% after 1000 cycles). The excellent electrochemical properties can be ascribed to the introduction of cobalt oxyhydroxide, which restrains the volumetric change of polypyrrole in the electrochemical redox process, and promotes the rapid migration of electrons.

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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