Nitrogen-doped bagasse carbon spheres/graphene composite for high-performance supercapacitors
Graphical abstract
Bagasse was transformed to nitrogen-doped carbon spheres and used as supercapacitor electrodes, which is the high-value utilization of renewable resources.Illustration for the preparation of bagasse-based carbon spheres/RGO composite, and application as supercapacitor electrodes.
Introduction
Carbon spheres have attracted much interest as a kind of promising materials for various applications, such as sensors [1], catalysts [2], adsorbents [3,4], and energy storage [5]. Many different methods have been developed for the fabrication of carbon spheres, including chemical vapor deposition [6], template method [7], hydrothermal carbonization [8]. In particular, hydrothermal carbonization is the most effective and convenient technique in the synthesis of carbon spheres, owing to its low cost, simple and environmentally friendly process. The common carbon precursors used in hydrothermal carbonization are saccharides, such as glucose, sucrose, agarose and cellulose [9]. Typically, the final product presents well-dispersed, micrometer-sized carbon spheres, and at the same time, both specific surface area and pore volume of the final product increase [10]. However, saccharides are carbon precursor extracted from raw biomass. Their separation and purification processes are complicated, resulting in high cost and limited applications. So, using raw biomass resource as carbon precursor is far more direct, cost-effective and eco-friendly.
Bagasse, consisting of cellulose, hemicelluloses and lignin, is an important agricultural biomass from sugar industry. For a long time, bagasse has been considered as waste materials. The utilization of bagasse is limited to direct combustion. It is noted that bagasse has plenty of advantages, such as abundant production, wide distribution, renewability and biodegradation. The environmentally-friendly utilization of bagasse has drawn much attention, and many researches in relation to bagasse have sprung up in the past few years [11]. One of the most promising utilizations of bagasse is served as carbon precursor to prepare carbon materials with functional structure. However, converting lignocellulosic biomass into hydrothermal carbon material with spherical morphology remains as a challenging task. The previous research used heterogeneous mixture of solid bagasse and distilled water as substrates, which cannot obtain carbon spheres but hydrolysates and solid pulp instead [12]. According to previous study, soluble and non-structural carbohydrates are easier to convert into the hydrothermal carbons with spherical shape [9]. So it can be inferred that bagasse can be converted into carbon spheres under the circumstance of solubilization. Due to the complex structure, finding the dissolution system for bagasse is still a rising subject. At present, there are some effective solvent systems for bagasse, such as DMSO/LiCl [13,14], ionic liquids [15,16], and NaOH/urea solution [17]. It is noted that NaOH/urea solution is a rising solvent system in these few years. To our knowledge, there has been little report about the dissolution of bagasse in NaOH/urea solution to produce carbon spheres. Particularly, this process is simple and low-cost, and the urea can act as nitrogen source for further nitrogen-doped reaction.
In addition, it is noted that the carbohydrates are easier to convert into the hydrothermal carbons in micrometer size and spherical shape with the presence of chemical agents like graphene oxide (GO) and metal ions [9]. GO is the chemical exfoliation product of graphite powders, which has been extensively studied as a precursor for carbon-based active materials. Recent researches have reported that GO can act as a template for the carbon spheres seeding and promote the carbonization under hydrothermal conditions, leading to higher carbonization degree and conductivity [18]. Therefore, it will be a promising way to induce the production of carbon spheres by using GO as template and accelerant.
In this investigation, for the first time, we used bagasse as a cost-effective carbon precursor for the synthesis of hierarchical structured carbons. As shown in Fig. 1, firstly, we used NaOH/urea solution as the green solvent to prepare homogeneous bagasse/GO mixture solution. Subsequently, the N-doped bagasse-based carbon spheres/RGO composited with three-dimensional structure was obtained by the facile hydrothermal carbonization. During this process, the urea acted as nitrogen source and GO was the template and accelerant. Finally, the composite was fabricated into a supercapacitor electrode to study its electrochemical property.
Section snippets
Materials
The bagasse was collected from Guitang sugar refinery (Guangxi, China). Natural flake graphite was purchased from XFNANO Materials Tech Co., Ltd. (Nanjing, China). All other chemical reagents were of analytical grade.
Dissolution of bagasse in NaOH/urea solution
The bagasse was washed with hot water to remove residual sucrose, and extracted with methylbenzene/ethanol (V/V = 2:1) to remove pigment, starch and wax. Then, bagasse powder was obtained by ball-milling for 8 h. NaOH/urea aqueous solution was prepared by mixing of NaOH, urea and
Characterization of bagasse-based carbon spheres/RGO composite
FT-IR spectra of as-prepared HBG with different reaction temperatures, reaction times and mass ratios of bagasse to GO are shown in Figure S1. The results demonstrate that with the increase of GO content, the intensities of oxygen-containing bands in HBG decreased gradually, indicating the higher carbonization degree. With the increase of reaction temperature and reaction time, the peaks at 1550 and 1197 cm−1 corresponding to CN and C–N stretching vibrations gradually increased, indicating
Conclusions
In summary, a novel bagasse-based carbon spheres/RGO composite was obtained for high-performance supercapacitors. Using NaOH/urea solution as solvent, the connections among cellulose, hemicelluloses and lignin in bagasse were weakened to form the homogeneous bagasse solution. With the assistance of GO which was acted as the growing template and accelerant agent, carbon spheres with high carbonization degree from bagasse were loaded on the graphene sheets and in turn acted as the spacers. As a
SYNOPSIS
Bagasse was transformed to nitrogen-doped carbon spheres and used as supercapacitor electrodes, which is the high-value utilization of renewable resources.
Acknowledgements
This work was supported by State Key Laboratory of Pulp and Paper Engineering (No. 2020ZR05) and the Fundamental Research Funds for the Central Universities (No. 2020ZYGXZR066).
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Authors with equal contributions to this work.