Hierarchical porous carbon derived from sulfonated pitch for electrical double layer capacitors
Graphical abstract
Introduction
Electrical double layer capacitors (EDLCs) have stimulated extensive interest due to their advantages of high power capability, superior reversibility and long cycle life, which are required for new energy storage devices [1], [2]. Charge storage in EDLCs utilizes electrostatic adsorption of the electrolyte ions at the electrode–electrolyte interface [3]. An ideal electrode material is expected to possess a large surface area, an optimal pore size distribution and excellent conductivity for fast transport of the electrolyte ions and charges [4].
Microporous activated carbons are the most common electrode material because of their high surface area, good conductivity, and chemical inertness [5]. Microporous activated carbon electrodes exhibit high specific capacitances at low current densities. However, the capacitance decreases dramatically with increasing current density. This is mainly caused by many small micropores and irregularly curved pores which slow down the ion transport rate and thus limit power storage. To solve this problem, activated carbons with controllable pore sizes and high surface areas are required.
Currently, hierarchical porous carbons (HPCs) with micro-, meso- and macropore structures have attracted much attention. In these materials the micropores provide abundant adsorption sites which are the primary contributors to the large specific capacitances [6]; the mesopores facilitate the diffusion of the electrolyte ions so they reach the available surface area and the macropores act as ion-buffering reservoirs to ensure adequate penetration of the electrolyte into the electrode materials [7]. In fact, the use of HPCs as electrode materials has been demonstrated to simultaneously achieve high energy and power densities.
The most common method to synthesize HPCs is the template method which has three steps: template replication, carbonization and removal of the template [8]. Various templates such as hierarchical porous silica monoliths [9] and powdery silica [10] have been used. For example, Kim et al. [11] used a beta zeolite as a hard template to synthesize HPC with micropores of 1 nm diameter and mesopores of 10–30 nm. Ma et al. [12] reported the synthesis of micro- and mesoporous carbons spheres by colloidal silica as template. Yamada et al. [13] used colloidal crystals as templates to synthesize 3D ordered porous carbons. Oschatz et al. [14] prepared the carbide-derived carbon materials (CDCs) containing micro-, meso-, and macropores through combining a soft-template approach with subsequent chlorine etching. This new synthesis route avoids the use of hard templates, thus the corresponding template removal process is avoided. Liang and Dai [15] reported the synthesis of highly ordered porous carbon by using self-assembled block copolymers as soft templates. In this method, the extra step of generating a template was unnecessary. Adelhelm et al. [16] used an organic polymer as the template to prepare a meso- and macroporous carbon using spinodal decomposition of the templates and a carbon precursor. Liu and co-workers [17] developed a new way to prepare nanoporous carbon using metal-organic frameworks as the template. The obtained nanoporous carbon with micro-, meso- and macropore structures exhibited remarkable electrochemical performance as an electrode of EDLC. However, all of the above methods contain complicated multistep procedures, which are tedious, cost and low yields, limiting their practical applications. Thus, new template-free methods are needed to synthesize HPCs. Lv et al. [18] reported the synthesis of hierarchical porous carbon foams from the bioresource banana peel through a self-template approach by utilizing its natural pore and zinc ions as the self-template.
In present work, sulfonated pitch (SP) is employed as the raw materials to prepare the HPCs. The SP contains a high concentration of oxygen-containing groups, providing many reactive sites for activation. More importantly, the presence of heteroatoms, e.g. S, O and N enhances the polarity of the carbon surface and increases the affinity of the surface for aqueous electrolytes. It is believed that sulfonated pitch is partly dissolved and partly dispersed in hydrosols [19]. This occurs through nano-scale contact between the reagents and the precursor, which should make it possible to prepare HPCs with large surface areas and well-developed porous structures. Moreover, the oxygen-containing groups in the sulfonated pitch are unstable and can decompose to CO2 and CO during heat treatment, which can assist in creating additional pores [20]. These properties should enable sulfonated pitch a promising candidate for producing HPCs with the characteristics of preparation simplicity and easy scalability. Combing the advantage of commercially availability, sulfonated pitch precursor shows great potential in the energy storage field.
Section snippets
Material preparation
Sulfonated pitch, purchased from Originchem Co., Ltd, was used as the raw material to prepare HPCs. Sulfonated pitch was added to KOH solutions with different KOH to precursor mass ratios and stirred for 1 h. The mixtures were dried at 80 °C for 12 h, and then transferred into a tube furnace and heat-treated at 800 °C for 2 h under a flow of nitrogen. After activation, the samples were washed three times with 1 M HCl solution. Then they were repeatedly rinsed by deionized water for three to
Material characterization
Information about the chemical state of the elements anchored in the sulfonated pitch surface was obtained from XPS. Wide scan spectra of the sulfonated pitch and the HPC samples are shown in Fig. 1a. The oxygen O 1s peak located at 532 eV is remarkably strong in the sulfonated pitch spectrum, indicating a high concentration of oxygen-containing groups. The sulfur S 2p spectra of the samples are displayed in Fig. 1b. There are two distinctive peaks in the sulfonated pitch spectrum. The strong
Conclusions
In conclusion, high conductivity HPCs with a large surface area and with micro-, meso- and macroporous structures have been prepared by the simple KOH activation of sulfonated pitch. The SBET and Vtot of the HPCs increased with the activation agent to precursor ratio. The optimum activation agent to precursor ratio was determined to be 1.5, which resulted in a specific surface area of 2602 m2 g−1 and a porosity volume of 1.28 cm3 g−1. A symmetrical EDLC using the HPC-1.5 sample as the electrode
Acknowledgments
This research was supported by the National High Technology Research and Development Program of China (863) (2011AA11A232, 2013AA050905), the National Nature Science Foundation of China (51172160, 50902102) and NSF of Tianjin City (11JCYBJC07500).
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