Full Length ArticleConverting biomass waste into microporous carbon with simultaneously high surface area and carbon purity as advanced electrochemical energy storage materials
Graphical abstract
Introduction
Porous carbons with well-developed porosity, good electrical conductivity and durable physicochemical properties have attracted great research attentions due to their wide availability and environmentally benign nature. These properties enable them to be superior candidates as electrode materials for electrochemical energy storage [1], [2], [3], [4], [5]. It has been widely acknowledged that pore structure and stability of carbon materials are two critical factors that decide the capacity, the rate capability and the cycling stability of the constructed electrodes [6], [7], [8].
With electrical double layer capacitor (EDLC) as an example, EDLCs store charges by the electrostatic adsorption or accumulation of electrolyte ions onto the charged surface of electrodes. Generally, the energy density (E) of electrochemical double-layer capacitors (EDLCs) can be described by E = ½ CV2, where V is the voltage window and C is the capacitance [9], [10]. To increase the energy density, usually there are two strategies, i.e., increasing C or/and V [2], [10], [11], [12]. For carbon materials, large specific surface area (SSA) is beneficial to the accumulation or adsorption of electrolyte ions in carbon pores and thus to the improvement of C value while high carbon purity could enhance the electron transfer efficiency and the corrosion resistance of the carbon framework and thus help to widen the voltage window, especially in the organic electrolyte [6], [7], [8], [13], [14], [15]. Hence, developing carbon materials featuring both abundant pore structure and high graphitization are highly desirable to boost the performances of carbon-based EDLCs.
Thus far, various synthesis strategies including hard template methods [16], [17], self-assembly methods [18], [19], [20], chemical or physical activation methods [10], [15], [21], [22], [23] and bio-template methods [24], [25] have been explored to design and engineering the carbon materials. As a result, various carbon types including mesoporous carbons, hierarchically porous carbons, doped carbons and graphene-like carbons have been reported and employed as electrode materials [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Nevertheless, the expensive raw materials and complex synthesis process often render many of above-mentioned methodologies unsuitable for industrial application. To this end, many research focuses have shifted to seek for renewable natural resources as precursors in the preparation of carbon materials. In this regard, biomass has recently aroused great interests of researchers due to its abundance, low-cost, environmental friendly and natural 2D or 3D carbon-based microstructures [26], [27]. In fact, substantial progresses have been made along this direction where high surface area carbons, heteroatom-doped carbons and hard carbons were successfully synthesized based on controllable physical or chemical activation methods [5], [26], [27], [28], [29], [30], [31]. These biomass-derived carbons show incremental improvements in boosting the energy and power densities of constructed electrodes [5], [15], [27], [28], [29], [30], [31], [32]. Moreover, those progresses also highlight the great potential of unexploited biomass waste in the development of carbon materials.
In the present work, we demonstrate a new type of microporous carbon (MPC) material derived from biomass waste pomelo peels by a simple high-temperature activation procedure. The as-obtained porous carbon features a high surface area, a dominantly microporous structure and a high carbon purity enabling the construction of supercapacitors with improved capacitance and enlarged voltage window (up to 3 V) in commercial organic electrolyte. In addition, the as-obtained MPC with high porosity and carbon purity can also be excellent substrate for the growth of electrochemically active materials. Enclosing the SnO2 nanocrystals into the framework of MPC by a melt infiltration method, the obtained SnO2-MPC composite, as expected, exhibits outstanding lithiation capability in term of high capacity, excellent rate capability and high cycling stability. At a high current density of 5 A g−1, the SnO2-MPC electrode can still deliver a reversible capacity of approximately 400 mAh g-1 even after 2000 cycles, which is among the highest levels of currently reported SnO2-based lithium ion anode materials.
Section snippets
Synthesis of microporous carbons (MPCs) and SnO2-MPC composite
The microporous carbons were prepared by a pre-carbonization process followed with a simple high-temperature activation procedure. In a typical procedure, biomass waste pomelo peels were firstly washed, dried at 80 °C for 12 h and then cut into small pieces. The dried pieces were than grinded into yellowish-brown solid powder. For the pre-carbonization procedure, 5 g of pomelo peel powder were put into a crucible in a horizontal furnace which was then heat-treated at 700 °C for 1 h under N2 the
Synthesis and structural characterization of MPC
The formation process of microporous carbon (MPC) is illustrated in Fig. 1 from which dried pomelo peel powders experience a pre-carbonization process and subsequent a high-temperature chemical activation process to yield MPCs. Pomelo peels are rich in organic acid and protein which are easy to transform to carbon framework upon pyrolysis or carbonization. As shown in Fig. 1, suffering from a carbonization process, yellow-wish brown pomelo peels convert into black powers during which process
Conclusion
In summary, microporous carbon (MPC) with simultaneously high surface area and high carbon purity can be readily prepared from biomass waste through a chemical activation methodology. Activation temperature poses a significant impact on the porosity and impurity content. High-temperature activated MPC-900 possesses critical features of high surface area, low impurity content and partially graphitic structure, cooperatively endowing the constructed organic supercapacitor with remarkable
Acknowledgement
This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 51376054 and 51406131).
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