Chitin based heteroatom-doped porous carbon as electrode materials for supercapacitors
Introduction
Currently, developing environmentally friendly, sustainable and affordable energy resources is still a great challenge for global researchers. For achieving this goal, supercapacitors have been regarded as one of the most promising electrochemical energy sources for energy storage and conversion devices, because of their high power density and long cycle life (Simon & Gogotsi, 2008). It is anticipated to develop new cost-effective electrode materials with improved performance. Carbon-based materials are expected to deliver outstanding performance as the electrode material for supercapacitors, because of their advantageous properties such as light weight, fast charging/discharging rates and bipolar operational flexibility (He et al., 2013, Liu et al., 2015; Wahid, Puthusseri, & Ogale, 2014; Zhang & Zhao, 2009). Porous activated carbon materials, especially, are appealing materials for supercapacitors because of their low cost, extraordinary tunable structure and surface chemical properties, as well as chemical, mechanical and thermal stability (Elmouwahidi, Zapata-Benabithe, Carrasco-Marin, & Moreno-Castilla, 2012; Gao et al., 2015; Sudhan, Subramani, Karnan, Ilayaraja, & Sathish, 2017).
Electrical double layer capacitance (EDLC) and pseudocapacitance are two types of capacitance based on different electrochemical energy storage mechanisms. In electrical double-layer capacitors, ions diffuse and accumulate in the electrical double layer formed along the interface between the electrolyte and the electrode, and pseudocapacitance results from the reversible Faradic redox reactions of electroactive species on the electrode surface (Yang, Liu et al., 2016, Zhang et al., 2013). To date, heteroatom-doped porous carbons have attracted increased interest, owing to their potential scale of energy storage and conversion (Ling et al., 2016, Ramasahayam et al., 2015). Nitrogen and oxygen incorporation can not only improve the surface hydrophilicity and conductivity of carbon materials, but also contribute pseudocapacitance to promote electrochemical redox reactions (Milczarek, Ciszewski, & Stepniak, 2011). Diverse methodologies have been developed to introduce nitrogen or oxygen species into the carbon framework by using various substances such as seaweed, melamine, and acetonitrile as precursors, or through chemical vapor deposition (CVD) or quick KOH activation processes (Raymundo-Piñero, Leroux, & Béguin, 2006; Tomohiro, Masashi, Hideya, & Satoru, 2013; Yang, Yu et al., 2016, Zhang et al., 2008). Yet, given the great potential in the field of electrochemical applications, it would be more significant to explore infusing sustainable biomass with a large amount of heteroatoms, as a precursor to synthesizing novel carbons for electrodes.
Chitin is a type of nontoxic, abundant, naturally occurring polysaccharide. Its annual biosynthesis amount is about multibillion tons, which is almost same with the annual production of cellulose. Chitin consists of β-(1, 4)-linked-2-acetamido-2-deoxy-d-glucopyranose units (GlcNAc), and contains about 6.9 wt% nitrogen from N-acetamido groups. Thus chitin has received much attention as an amino-functional biomass precursor for synthesis of nitrogen-doped carbon using simple pyrolysis (Nogi, Kurosaki, Yano, & Takano, 2010; Yan et al., 2015, Yang et al., 2012). To the best of our knowledge, there is few literatures discussing the preparation of nitrogen-containing carbonaceous materials from chitin biomass and their potential application in electrochemical capacitors (Chen, 2015, Qu et al., 2015).
Chitin can be obtained from diverse biomass sources such as crustacean (crab, shrimp, and krill) shells, insects, and fungal mycelia. Insect cuticles may be the best choice, as they contain much less inorganic matter than crustacean shells. A cicada is an insect of the order homoptera, suborder auchenorrhyncha, in the superfamily cicadoidea. Cicada sloughs, also known as periostracum cicadae, are the shells that are shed when cicada larvae become imagoes. This material is considered a new alternative source for chitin (Sajomsang & Gonil, 2010). The recovery rate of chitin from cicada slough is much higher than that from rice-field crab shell. Hence, cicada slough is expected to be an ideal biomass candidate for the preparation of nitrogen-rich porous carbon. In this study, we report a new strategy to fabricate cicada slough-derived carbon (CSC-x) with desirable performance as the electrode material for electrochemical capacitors via combined precursor air carbonization and a KOH chemical activation process. CSCs offer the features of well-developed microporosity, large surface area, and abundant doped oxygen and nitrogen heteroatoms. The synergetic effects of microporous structure and heteroatoms (oxygen and nitrogen) incorporated into the carbon framework, on the capacitive behaviors of CSC-based electrodes, have been investigated comprehensively.
Section snippets
Raw materials
Cicada sloughs of C. atrata Fabricius were collected in the Xiasha campus of Hangzhou Dianzi University. Acetone, ethanol, hydrochloric acid (HCl), potassium hydroxide (KOH), and polytetrafluoroethylene (PTFE) latex (60 wt% dispersion in H2O) were supplied by Sinopharm Chemical Regent Co. Ltd., China. All reagents were of analytical grade and used as received without further purification. Millipore water was used in all experiments.
Nickel foam (99.9%, 1.5 cm in thickness), acetylene black and
Structural characterization
To better understand the porous structure of resultant carbons derived from cicada slough, N2 adsorption/desorption and pore size distribution were performed as shown in Fig. 2. The shape of all the isotherms in Fig. 2(a) can be defined as a combined I/IV type according to IUPAC classification. The steep increase at relatively low pressure (P/P0), and the hysteresis loop observed in the isotherms, reveal the existence of abundant micropores and a small quantity of mesopores, respectively, that
Conclusions
In summary, a facile synthesis approach was demonstrated to prepare novel heteroatom dual-doped carbons from cicada slough via mild air pre-carbonization and KOH activation. The resultant CSC-2 possessed well-developed micro porosity (0.73 cm3 g−1), high specific surface area (1745 m2 g−1), and rich heteroatoms oxygen and nitrogen. Its unique properties of microporous structure and surface chemical composition (especially pyridinic-N, pyrrolic-N and quinone/carbonyl (CO) species) had synergistic
Acknowledgements
The authors are grateful for financial support from the National Natural Science Foundation of China (Project No. 21406044) and the Zhejiang Province Nature Science Foundation of China (Grant No. LQ17B060006).
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