Full Length ArticleWalnut shell derived porous carbon for a symmetric all-solid-state supercapacitor
Introduction
Ever-worsening environmental concerns coupled with diminishing fossil fuels have encouraged the development of efficient energy storage devices. The most promising energy storage devices are lithium ion batteries with high energy densities and supercapacitors with good power densities [1], [2]. In the search for new energy storage devices, supercapacitors have attracted a great deal of attention because they have high power densities, good stabilities and are light weight [3].
Two typical supercapacitors are the electric double layer capacitor (EDLC) and the pseudocapacitor which can store energy by accumulating charges in the electrical double layer through electrostatic interactions or by Faradic redox reactions, respectively [4], [5]. Due to their high capacitances, metal oxides and conducting polymers have been widely used as pseudocapacitor electrode materials. However the large-scale application of these materials has been hindered by limited resources, low conductivities and poor stabilities. In contrast, the electrode materials for EDLCs include various carbon materials such as carbon nanotubes, carbon aerogels and graphene which all exhibit good stabilities and are naturally abundant [6].
Biomass waste products have attracted tremendous attention as carbon sources since they are abundant, sustainable and low cost [7], [8]. For example, bean dregs [9], dead leaves [10], waste coffee beans [11], banana peels [12], and pomelo peels [13] have all been used to prepare carbon materials with good electrochemical performances in supercapacitors. It can be concluded that utilizing these biomass waste products in this way not only reduces environmental waste, but also provides new energy storage materials. In addition, supercapacitive performance has been shown to be strongly affected by the size, morphology, porosity and composition of the electrode material [14], [15], [16]. The large specific surface areas of carbon materials coupled with their small pore sizes and porous structures can improve the contact between the electrode material and the electrolyte, which improves the capacitive performance.
In this work, low-cost easily available walnut shells were used as a carbon source to prepare a porous carbon material. The carbon material was prepared using a simple hydrothermal method followed by the activation routes with K2CO3. The structure of the obtained material was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The obtained porous carbon material was then used as the electrode material for a symmetric all-solid-state supercapacitor. The properties of the supercapacitor were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) cycles and electrochemical impedance spectroscopy (EIS).
Section snippets
Preparation of materials
Walnut shells were obtained from Hebei province, China and were washed, dried, ground with an electric grinder and then screened through a 50 mesh screen to obtain a powder. The powder (5 g) was dispersed into 45 mL distilled water and stirred for 0.5 h. The dispersion was then transferred into a Teflon-lined stainless steel autoclave and maintain at 180 °C for 12 h. After cooling, the precipitate was collected by centrifugation, washed and finally dried at 60 °C. The obtained product is denoted as
Results and discussion
A schematic illustration of the preparation route for sample-K2CO3-700 is shown in Fig. 1a. The morphologies of the samples at different stages of the preparation were observed by SEM and the results are shown in Fig. 1b–d. The powder (Fig. 1b) consisted of large blocks of material. However, in sample-H (Fig. 1c), these blocks were changed to small particles after the hydrothermal treatment. After activation of the sample with K2CO3 and annealing, the morphology of the sample changed to a
Conclusions
Low-cost readily available walnut shells were used as a carbon source to prepare the new porous carbon material. The porous carbon material was prepared by a simple hydrothermal method followed by the activation routes with K2CO3. The specific capacitance of the obtained porous carbon material was 255 F/g at 0.5 A/g in a three-electrode setup. The walnut shell derived carbon material was also used as the electrode material in a symmetric all-solid-state supercapacitor. The properties of the
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