Research articlePreparation of porous carbons from waste sugar residue for high performance electric double-layer capacitor
Graphical abstract
Introduction
In modern society, limited fossil fuel energy and serious greenhouse effect have prompted researchers to develop a kind of efficient, economical and green energy storage device [1], [2], [3]. Electric double-layer capacitor (EDLC) is considered to be one of the most potential energy storage devices. Taking advantage of the charge separation in a Helmholtz double layer at the interface between the electrolyte and electrode, EDLC possesses the advantages of superior electrochemical performance, environmentally-friendly products and long cycle life [4], [5], [6]. It is well known that the electrochemical performances of EDLC are basically determined by the kind of electrode active materials which include activated carbon (AC) [7], carbon nanotube [8], carbon aerogel [9] and graphene [10]. AC is a relatively inexpensive and high-performance electrode material and also has porous structure, large specific surface area (SSA), variable characteristics of surface chemistry and excellent surface reactivity [11]. Meanwhile, it can be prepared by various raw materials, such as coal, coconut shell, fruit shell and biomass [12], [13], [14], [15]. AC as the most successful electrode material in the commercialization, many electrochemical workers have focused on the exploration of new precursors via adopting different activation agents (KOH, H2SO4 and ZnCl2) to prepare AC with extraordinary performance for EDLC [16], [17], [18]. Zhai et al. [19], [20] used mesophase pitch to prepare ACs with KOH as activation agent and found the AC with SSA of 2258 m2 g− 1. When AC was applied in the supercapacitors, it had the largest specific capacitance of 145 F g− 1 in organic electrolytes as well as high energy density of 31 Wh kg− 1 and power density of 12 kW kg− 1. Wu et al. [21] used enteromorpha overrunning in China Sea as precursor to prepare porous carbons with the activation agent of ZnCl2. The specific capacitance reached 206 F g− 1, and 93% of the initial specific capacitance was retained even after 5000 cycles. Furthermore, the carbon precursor was activated by KOH, which is more efficient in activating precursor with higher SSA and more controllable pore size distribution, compared with H2SO4, ZnCl2 and NaOH [22].
Recycling waste material not only can ease the energy crisis, can also adjust the energy resource structure and environment protection. Moreover, the process of manufacturing vitamin C resulted in the production of high concentrations of waste sugar solution (WSS), which contains various waste materials, such as waste acids and organics, and it will pollute the environment and waste the recyclable resource if discharged directly. In addition, WSS is generally used for the production of low value-added products, such as oxalate and detergent. Waste sugar residue (WSR) obtained by drying WSS can be used to prepare porous carbon, which has not been reported until now. Hence, it is significant and necessary to prepare high value-added item like porous AC based electrodes by recycling WSR. It is feasible for WSR to prepare ACs because of its low ash content and abundant elemental contents of C and O which are conducive to increase the electrochemical performance of ACs [23]. Besides, relatively high alkali metal content in WSR is beneficial to activation by the reduce of the application amount of activation agent to some extent [24].
In this study, WSR was favorable to be the precursor for the preparation of ACs with KOH as activation agent applied in EDLC. The prepared ACs were characterized by Fourier transform infrared (FTIR) spectrometer, scanning electron microscopy (SEM), X-ray diffraction (XRD) and N2 adsorption-desorption. Subsequently, the electrochemical performances of AC based electrodes were investigated by galvanostatic charge-discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. This paper focuses on the effects of carbonization temperature, activation temperature, activation ratio and activation time on the electrochemical performance of ACs prepared by WSR.
Section snippets
Material
The WSS, collected from vitamin C pharmaceutical company in Shandong, China, was dried at 105 °C in the oven for 12 h and the dried WSR was pulverized to pass through a 200-mesh sieve followed by drying at 105 °C for 12 h. Ultimate analysis was carried out by an Elementar vario MACRO cube CHNS elemental determinator [25]. The main characteristics of the WSR are shown in Table 1. Thermodynamic property of WSR was investigated and the thermogravimetry (TG) and derivative thermogravimetry (DTG) are
Characterization of ACs
The proximate analysis of WSR and the yields of carbonization and activation are listed in the Table 1, Table 2. WSR has relatively low ash content, and its carbonization and activation yields are comparable to the carbonization and activation yields of rice husk and peanut shell [28], [29]. Besides, the computational equations of the yields of carbonization and activation are listed in the Supplementary material.
In order to figure out the changes of the surface functional groups, we
Conclusions
In summary, porous carbons with a relatively high SSA of 1953 m2 g− 1 was obtained by two-step carbonization/KOH-activation method with WSR as raw material. The prepared ACs possessed the highest specific capacitance of 273.31 F g− 1 at a current density of 40 mA g− 1. The capacity retention rate of AC-600-700-3-2.5 remained 90.1% after 5000 cycles, and its energy density reduced from 5.09 to 4.59 Wh kg− 1 after 5000 cycles. Meanwhile, the excessive temperature, activation ratio and activation time were
Acknowledgements
This work was subsidized by the Fundamental Research Funds for the Central Universities (China University of Mining & Technology, 2015XKMS043), National Natural Science Foundation of China (Grant 21676292), Natural Science Foundation of Jiangsu Province (BK20161180), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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2021, International Journal of Hydrogen EnergyCitation Excerpt :In addition, the pore size distribution analysis also showed the presence of pores with sizes of less than 4 nm (Fig. 2b and d). Notably, the SSA and pore structure varied when the preparation conditions were changed: 1) Compared with the porous carbon [12] (SBET = 1326 m2/g, Vtotal = 0.741 cm3/g) prepared by the same method without the nitrogen dopant, AM-700 shows a larger SSA and total pore volume (Table 1). Nitrogen dopants such as melamine, urea, and thiourea have been reported as an in-built activating agent, which is beneficial to generate the pore structure [3,26].
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2020, International Journal of Hydrogen EnergyCitation Excerpt :However, in the vitamin C production process, large volumes of waste sugar solution (WSS) are generated as by-product. In addition, there are various wastes in the WSS, such as sewage, gulonic acid, formic acid, and nucleic acid [30,31]. WSS, if dumped directly, can cause environmental pollution.