In the development of portable electronics such as digital telecommunication systems and systems using pulse laser techniques, the lack of lightweight, inexpensive, sustainable energy storage devices has been a limiting factor (Deng et al.
2016; Zhao et al.
2014b). Among the currently employed energy storage devices, supercapacitors, which are promising candidates because of their excellent cycling stabilities, higher power density than batteries, and higher energy density than conventional physical capacitors, have attracted significant research interest (Simon et al.
2014; Winter and Brodd
2004; Zong et al.
2016; Li et al.
2014,
2017a,
b; You et al.
2017). Supercapacitors can be divided into two categories according to their charge storage mechanisms: pseudocapacitors and electrical double layer capacitors (EDLCs) (Wang et al.
2012; Winter and Brodd
2004). Pseudocapacitors store electrical energy by fast, reversible faradaic reactions (redox reactions, electrosorption or intercalation) occurring on the electrode materials, typically of metal oxides (Jeong et al.
2016; Wang et al.
2013a) or conducting polymers (Wang et al.
2014,
2015; Zhao et al.
2017). EDLCs are associated with an electrode-potential-dependent accumulation of electrostatic charge at the interface between the electrolyte and the electrode, typically of carbon (Ghosh and Lee
2012). The use of carbon here is merited by the many desirable properties of this material, such as low processing costs, chemical inertness, thermo-stability, non-toxicity, wide availability, high electrical conductivity, and a long and stable cycle life (Jiang et al.
2013; Xu et al.
2016; Zhai et al.
2011). Of the various carbon materials with different morphologies and distinct physical and chemical properties (Fan and Shen
2015; Zhai et al.
2011), carbon microspheres have several advantages, such as their regular spheroidal shape, and their adjustable porosity and particle size (which can decrease the resistance of ion diffusion and thus improve the electrochemical performance) (Duffy et al.
2012; Liu et al.
2013). Moreover, the macroporosity created as the carbon microspheres aggregate can promote the generation of ion-buffer reservoirs, which could decrease the diffusion distance of electrolyte ions to the carbon surface (Jiang et al.
2013; Zhao et al.
2014a). Hence, carbon microspheres have become one of the most promising electrode materials in supercapacitor research and development (Liu et al.
2013; Ma et al.
2014; Zhu et al.
2015). As a useful, simple approach, heteroatom doping (e.g. with nitrogen, sulfur, boron and/or phosphorus) is often used to modify the intrinsic electronic structure (Lee et al.
2012), enhance the wettability of the electrode surface (Zhu et al.
2015) or introduce new pseudocapacitive interactions (Biel et al.
2009) in order to improve the electrochemical performance of carbon electrodes (Chen et al.
2014b; Li et al.
2007). There are two main methods employed in heteroatom doping; one is by direct pyrolysis of heteroatom-containing precursors (Xu et al.
2015; Zhu et al.
2015) and the other is by post-treating carbons with dopants (Chen et al.
2014b; You et al.
2013).
Cellulose, the most abundant renewable biopolymer in nature (Klemm et al.
2005), is a promising pyrolysis precursor for providing carbonous species. Chitosan is a pseudonatural polysaccharide composed of glucosamine and N-acetyl glucosamine (Rinaudo
2006). As chitosan is derived from chitin, the second most abundant and renewable biopolymer after cellulose, it is also almost inexhaustible (Rinaudo
2006). Chitosan is also a commonly used source of carbon species self-doped with nitrogen. Cysteine, a natural amino acid containing both nitrogen and sulfur, is a good candidate for providing nitrogen and sulfur doping; it has been used to functionalize carbohydrates in the production of dual-doped carbon microspheres (Wohlgemuth et al.
2012). Recently, work on the reductive amination and preparation of 2,3-dialdehyde cellulose (DAC) beads (Lindh et al.
2014),
l-cysteine-functionalized DAC (LC-DAC) beads (Ruan et al.
2016) and chitosan-crosslinked DAC (CS-DAC) beads (Ruan et al.
2018) has been reported by our group for use in applications such as palladium adsorption and Congo red dye adsorption. All three of these micrometer-sized biopolymer beads are interesting candidates for a carbon source, with or without heteroatom doping.
The aim of this study was to broaden the application of these cellulose beads to include energy storage by using DAC, CS-DAC and LC-DAC beads as a carbon source to provide microscale porous carbon beads doped with heteroatoms (N and/or S), or un-doped beads, for use as an electrode material for manufacturing supercapacitors.