High-Performance Solid-State Supercapacitors Fabricated by Pencil Drawing and Polypyrrole Depositing on Paper Substrate
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A solid-state powerful supercapacitor (SC) is fabricated with a substrate of Xerox paper. Its current collector based on a foldable electronic circuit is developed by simply pencil drawing. Thin graphite sheets on paper provide effective channels for electron transmission with a low resistance of 95 Ω sq−1. The conductive organic material of polypyrrole coated on thin graphite sheets acts as the electrode material of the device. The as-fabricated SC exhibits a high specific capacitance of 52.9 F cm−3 at a scan rate of 1 mV s−1. An energy storage unit fabricated by three full-charged series SCs can drive a commercial light-emitting diode robustly. This work demonstrated a simple, versatile and cost-effective method for paper-based devices.
KeywordsSupercapacitor Paper Pencil drawing Polypyrrole
Supercapacitors (SCs), as the promising energy storage devices, have attracted tremendous attention for a set of features, such as high power density, fast rates of charge–discharge process, long cycling life and improved safety [1, 2, 3]. Particularly, SCs can provide much higher power density than batteries and higher energy density than conventional capacitors, which bridge the gap between those two kinds of typical energy storage devices [4, 5, 6]. According to the underlying energy storage mechanism, SCs can be classified into two categories [4, 5]. One is electrochemical double-layer capacitors (EDLCs) which store electrical energy by electrostatic accumulation of charges between the surfaces of the electrode materials and electrolyte . Although EDLCs exhibit ultrahigh power density and distinguished long-term cycling performance, the stored energy is limited by the finite electrical charge separation at the interface between electrolyte and electrode materials . The other type of SC is the so-called pseudocapacitor, which stores energy due to fast and reversible redox reactions occurring on the surface or near surface of the active electrode materials. Compared to EDLCs, pseudocapacitors have high energy density but low power density and short cycle life .
Paper is inexpensive, foldable, environmentally benign nature and widely used in our daily life. Commonly, paper is composed of cellulose fibres with a typical diameter of about 20 μm. In recent years, paper is becoming a promising flexible substrate for various electronics, such as solar cells , transistors , displays  and energy storage devices [10, 11, 12]. The realization of paper-based devices is highly desired not only for their wide range of applications, but also for their compatibility with printed electronics. Aimed at low cost, environmentally benign nature and wide range of applications, paper was exploited for the substrate of our SCs. Carbon materials, as the most typical electrode materials for EDLCs, have been extensively studied in the past decades due to their good conductivity, robust mechanical character and stable electrochemical behaviour [13, 14, 15, 16, 17]. Among carbon materials, graphene has some fascinating features, such as large surface area, high flexibility, excellent conductivity and good chemical/thermal stability [18, 19]. However, high temperature and vacuum are needed during the synthesis process of graphene [20, 21, 22, 23]. Consequently, some unfavourable issues emerge such as high cost, elaborate fabrication or difficulty in large-scale fabrication. Herein, we got inspiration from ordinary writing manners and successfully drew arbitrary shapes of current collectors for our SCs using a pencil. Multilayered graphene (thin graphite sheets) was transferred onto the paper substrates during this simple process, which provided an effective transmission path for electrons. For the sake of enhancing the electrochemical performance of the devices, polypyrrole (PPy) was deposited on the pencil drawing paper, which was also used as the pseudocapacitive material in this research. Compared to other conductive polymers, PPy has greater density and a great degree of flexibility in electrochemical process [24, 25], which result in a high volumetric capacitance and high mechanical performance. After the deposition of PPy, two PPy thin graphite sheet paper electrodes were assembled with a gel electrolyte of H3PO4/polyvinyl alcohol (PVA). The as-fabricated solid-state SCs exhibited good flexibility and a high specific capacitance of 52.9 F cm−3 at a scan rate of 1 mV s−1, which is much higher of some SCs than in prior literatures [26, 27]. This technique represents a low cost, applied and versatile fabrication method for paper-based energy devices.
To get paper-based SCs, a piece of Xerox paper (1.5 cm2) was drawing by a 4B pencil (86 % graphite and 14 % clay) until its sheet resistance reduced to about 95 Ω sq−1 (~150 times of scratching). After that, a layer of thin graphite sheets was deposited on the paper. Then, the graphite–paper composite (the area is about 1.0 × 1.0 cm) was immersed in a solution that contained 0.2 M NaClO4 and 5 % (V:V) pyrrole monomer, and PPy was grown on the drawn paper via an electrochemical deposition process. Three-electrode configuration was used in this deposition process with Ag/AgCl as the reference electrode, platinum foil as the counter electrode and the drawn paper as the working electrode. A constant voltage of 0.8 V was applied during the process. Then the as-grown sample was washed with deionized water and dried at room temperature. In order to seek for the dependence of SC performance on PPy deposition time, the deposition time of PPy on the drawn paper was different. Finally, two pieces of 1.0 × 1.5 cm functionalized paper were used as electrodes with the opposite area of 1.0 × 1.0 cm. A gel composite H3PO4/PVA was used which acted as the separator and the electrolyte between the two electrodes. After the gel electrolyte dried completely, the quasi-solid-state SC was prepared.
3 Results and Discussions
In order to demonstrate the flexibility of our devices, the bent state is shown in Fig. 4c. We also test the CV performance at original/bent state; as shown in Fig. 4d, the CV curves of the device just have a little influence. An example for the application of the connected SCs is shown in Fig. 4e and the inset, where three arbitrary PPy-G-paper-based SCs are connected in series. They can drive a commercial LED (Fig. 4e) as an energy source when it has been fully charged. We picked two SCs (devices 1 and 2) and measured their capacitances, which are 16.1 and 11.3 mF at the current density of 20 A cm−3, respectively. As shown in Fig. 4f, when they are connected in series, the capacitance of the whole device is calculated to be 7.6 mF, when in parallel, it is 32.8 mF. The results reveal that the capacitance of the connected SCs roughly obeys the basic rule of series and parallel connections of capacitors. So we can take various connections of our SCs to meet a wide variety of demands in practice.
In summary, we fabricated SCs on Xerox paper using pencil drawing and PPy deposition successfully. The thin graphite sheets drawn by pencil acted as a good EDLC material and a good current collector. The SCs based on PPy-G-paper electrodes showed high specific capacitance of 52.9 F cm−3 at a scan rate of 1 mV s−1. In addition, three SCs connected in series can drive a commercial LED. This method of fabricating the energy storage devices is of low cost and environment friendly, and the paper SCs can potentially guide the development of paper electronics for its low cost and high compatibility.
This work was supported by the National Basic Research Program (2011CB933300) of China, the National Natural Science Foundation of China (11204093, 11374110) and ‘the Fundamental Research Funds for the Central Universities’, HUST: 2012QN114, 2013TS033.
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