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عنوان فارسی مقاله:
الکترود انعطاف پذیر و ساده بر اساس پلی پیرول / گرافن / کاغذ سلولز باکتریایی برای ابرخازن
عنوان انگلیسی مقاله:
Flexible and freestanding electrode based on polypyrrole/graphene/bacterial cellulose paper for supercapacitor
سال انتشار : 2016
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مقدمه انگلیسی مقاله:
1. Introduction
Flexible energy storage devices have attracted tremendous attention in recent years, the potential applications of which ranging from wearable electronics, stretchable electronics, collapsible displays to on-body sensors [1e5]. Among various types of energy storage devices, supercapacitors are widely recognized as an important class due to high power density, fast dynamic response, moderate energy density, good operational safety, and long life cycle [6e9]. However, restrained by the lack of flexibility and lightweight of conventional supercapacitors, one of the key challenges is to fabricate flexible and freestanding electrodes with high capacitance and good mechanical properties [10]. To date, considerable efforts have focused mainly on improving the gravimetric capacitance of the flexible electrodes, but for wearable and portable electronics, areal and volumetric performance are at a premium [11]. In general, supercapacitors can be classified into two categories based on charge-storage mechanism, namely the electrochemical double-layer capacitors (EDLCs) and pseudo-capacitors (PDCs). Carbon based freestanding film including RGO paper and carbon nanotubes paper are very popular to be employed for flexible electrodes in EDLCs due to the high conductive property and large surface area, and the charges are electro-statically stored by reversible ion adsorption at the electrode/electrolyte interface [12e14]. Despite these carbon based flexible paper has shown long cycle life and good gravimetric capacitance, the areal capacitance (<100 mF cm2 ) is rather low due to the small mass loading (<1 mg cm2 ) [15]. PDCs, based on fast reversible faradic reactions at the surface of electroactive materials, using conducting polymers or metal oxides as active materials provide them with several times higher specific capacitance than the non-faradic EDLCs. PPY, as a kind of traditional conducting polymer, is particularly promising pseudocapacitive electrode material for supercapacitors owing to its high energy storage capacity, high electrical conductivity in doped states, ease of low cost synthesis and low environmental impact [16e18]. Nevertheless, poor cycling stability induced by large volumetric swelling and shrinking during charge/discharge and weak flexibility of PPY limit its use in flexible supercapacitors. A number of methods have been demonstrated to overcome these drawbacks by coupling PPY onto flexible supporting substrates such as office paper, RGO paper, and carbon nanotube film. Therefore, directly electro-polymerization or * Corresponding author. chemical polymerization PPY onto carbon-based electricalconductivity substrates is the main approach. However, the mass loading of these electrodes is generally still very low, resulting in the low areal capacitance, despite that they have obtained high electrical conductivity, good gravimetric performance and rate capability [19]. As has been described recently, porous textiles are also used as alternative substrate. In comparison to printing paper and carbon conductive paper, they can provide three-dimensional (3D) porous network structure, and therefore enable high mass loading. As an interesting ecofriendly biomaterial cellulose, BC is produced by fermentation of bacteria (Acetobacter xylinum, E. coli, etc.). With so many unique properties such as specific ultrafine network structure, good mechanical strength and high water holding capability, BC could be designed and applied in supporting substrates. Furthermore, while great efforts have been devoted to develop high capacitive devices, relatively fewer attentions have been paid to its mechanical properties. Designing efficient, low-cost flexible electrode that can achieve both good electrochemical properties and high mechanical integrity upon bending or folding, tensile strength and lightweight property is highly required [15]. In this point of view, BC membrane can strongly bind with PPY and carbon materials due to the many hydroxyl groups, meanwhile, the stretchable and compressible properties of these structures can be helpful for mechanical properties of flexible electrode. Herein, we demonstrate a simple and low-cost “polymerization and vacuum-filtering” method to fabricate the PPY/RGO/BC flexible electrode. The as-constructed flexible electrode exhibits good electrochemical performance and mechanical properties. The asfabricated paper with high mass-loading (~8.93 mg cm2 ) shows a high areal capacitance of 2100 mF cm2 , good rate performance (75% retention at 50 mA cm2 ) and good cycling stability with ~64.7% of its initial capacitance after 5000 cycles. By direct coupling of two paper electrodes, symmetric supercapacitor can offer large areal capacitance (790 mF cm2 ), high energy density of 0.11 mWh cm2 and power density of 15 mW cm2 . Therefore, this strategy opens up new opportunities for development of high performance flexible energy-storage devices.
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