Ultra-thin bacterial cellulose/poly(ethylenedioxythiophene) nanofibers paper electrodes for all-solid-state flexible supercapacitors
Introduction
With the concepts of flexible displays, artificial skins, and intelligent glasses proposed in recent years, portable and wearable electronics such as electronic papers and roll-up displays have drawn widespread concerns [[1], [2], [3]]. Compared with conventional energy-storage devices, flexible energy-storage devices not only must be satisfied with the prime requirements of high power density, steady energy density, long cycle life, and environmental friendliness, but also should have good mechanical properties including high flexibility, stretchability, and lightweight [4,5]. Among diverse energy-storage devices, supercapacitor is a promising candidate owing to the rapid charge and discharge rate, steady energy density, high power density, excellent durability, and wide working temperature range [6,7]. Moreover, a supercapacitor, which is flexible, will further meet the requirements of stretchability and lightweight [8,9]. Currently, most of the available commercial supercapacitors are produced by deposition method and slurry-coating technology [10,11], which lead to the supercapacitors provided are too heavy, thick, bulky and rigid to meet the demands of flexible energy-storage devices [12] due to the existences of heavy metal substrates, binders, and conductive additives.
Thin film all-solid-state flexible supercapacitor would be a favorite in solving the above mentioned problems, which needs a kind of appropriate material with good electrochemical performance and deformable mechanical properties to work both as current collector and electrode. Conducting polymers could be promising to develop a lightweight and flexible supercapacitor electrode with high performance because of their intrinsic light-weight, flexibility, and good conductivity [[13], [14], [15]]. Recently, many research efforts have focused on depositing conducting polymers onto free-standing or porous substrates such as textile [3], cellulose paper [[16], [17], [18]], biochar [19,20] and graphene paper [12], aiming to get desirable electrode material. Among these substrates, cellulose, which is kind of abundant renewable polymer [21], possess the inherent characters of excellent mechanical properties and flexibility, causing that the cellulose-based composites are attractive in applications of flexible polymer displays [16,17,22,23]. For example, Anothumakkool et al. fabricated highly conducting PEDOT on the flexible cellulose paper by inducing surfactant-free interfacial polymerization at the interface of two immiscible liquids [16]; Zhang et al. developed an flexible electrode by simply loading PEDOT:PSS on cellulose paper and conductive graphite foils via spin-coating of its commercially available solution [17]. Nevertheless, thinking about the big diameter size of cellulose fibers (about 20 μm), the mass loading of deposition is limited by chemical polymerization or physical spin-coating, because of the small thickness of conducting polymers (approximately 50 nm) [3,24]. In addition, the PEDOT was simply coated on the surface of the substrates in the above mentioned works, which gave very weak interaction between substrates and polymers. In order to solve these issues, we choose bacterial cellulose (BC) as substrate instead of common cellulose. Bacterial cellulose, which mainly comprises ultrafine nanofibers (less than 100 nm in width) [25], has absorbing characteristics of high aspect ratios, high porosity, high surface area, good flexibility, and superior hydrophilicity [25,26]. Membranes made up of BC nanofibers possess a robust three-dimensional (3D) porous structure with high specific surface area, which is beneficial for increasing the mass loading of active materials such as conducting polymers [27]. Meanwhile, the inherent hydrophilic functional groups such as OH and COOH [28,29] make it easily integrate with conductive polymers [30,31], which strenghthen the interaction between substrates and polymers. As a matter of fact, the BC do has huge advantages as the substrates for flexible supercapacitor, which was demonstrated in our previous works [26,29].
On the basis of our previous studies, our group has developed a simpler method for preparation of 3D, flexible, freestanding, lightweight BC-PEDOT electrodes, which no longer needed conductive additives such as carbon nanotubes, graphene and carbon black and owned high capacitance and low electrical resistance at the same time. This has been achieved by absorbing the oxidant on the surface of the BC nanofibers and then manipulating the slow polymerization of PEDOT only on the BC nanofibers' surface rather than in the vacant spaces. Thus, the PEDOT fabricated in this method holds highly ordered polymer chains and displays strong adhesion to BC nanofibers. The highly ordered PEDOT brings about high capacitance and low electrical resistance. The strong physical and chemical interactions between PEDOT and BC give rise to great mechanical properties, excellent flexibility and stability of the composite. The 3D nanoporous networks of the flexible BC-PEDOT electrode and the hydrophilicity of BC nanofibers offer diffusion channels to electrolyte ions, which are beneficial to the permeation of aqueous electrolytes and enhance supercapacitors performance. So the resulted flexible BC-PEDOT electrodes presented a high areal capacitance of 442.2 mF cm-2 at a current density of 1 mA cm−2. And the area capacitance measured by the symmetric supercapacitors fabricated with BC-PEDOT10 paper electrodes was about 127.6 mF cm-2 at the current density of 0.5 mA cm−2 which is also superior to those solid current collectors made from other thin-film-electrode materials reported recently [16,[45], [46], [47], [48], [49], [50], [51], [52]].
Section snippets
Preparation of the BC-PEDOT paper
All the chemicals were analytical grade and used without purification except ethylenedioxythiophene (EDOT). EDOT which was purchased from Aldrich Chemicals was distilled before using. BC membranes were produced and purified by the previously reported method [32,33]. At the initial stage, BC membranes were cut up and then pulped by a mechanical homogenizator at the speed of 12,000 r min−1 [29], which aimed at acquiring the suspension of BC nanofibers. Subsequently, the nanofibres suspension was
Results and discussion
We fabricated the BC-PEDOT paper by absorbing the oxidant on the surface of the BC nanofibers and then manipulating the polymerization of the EDOT on the BC nanofibers' surface. As illustrated in Fig. 1, ferric chloride and hydrochloric acid solution was firstly introduced drop-by-drop (about one drop per 10 s) into BC nanofibers and EDOT suspension. And then the small amount of Fe3+ ions could be easily adsorbed on the surface of BC because of the hydrophilic interaction between Fe3+ ions and
Conclusion
In summary, we skillfully make use of the intrinsic hydrophilicity of the BC nanofibers to absorb oxidant and then control that the EDOT is only polymerized on the surface of BC nanofibers in order. In this way, a kind of 3D, flexible, freestanding BC-PEDOT paper electrode, which no longer needed any other additives, was fabricated successfully. Benefitting from the hydrophilicity and three-dimensional porous network of BC, the capacity of the conductive polymer PEDOT was exploited sufficiently
Acknowledgements
This work was financially supported by NSFC International (Regional) Joine Research Project NSFC-SNSF (51661135023), NSFC (21103057),the 973 Program of China (2014CB643506), the Fundamental Research Funds for the Central Universities (HUST: 2016YXMS031), and the Director Fund of the WNLO and the Open Funds of the State Key Laboratory of Electroanalytical Chemistry (SKLEAC201607). The authors thank the Analytical and Testing Center of HUST and the Center for Nanoscale Characterization & Devices
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