Elsevier

Journal of Power Sources

Volume 310, 1 April 2016, Pages 145-153
Journal of Power Sources

Nitrogen-doped porous carbon with an ultrahigh specific surface area for superior performance supercapacitors

https://doi.org/10.1016/j.jpowsour.2016.01.052Get rights and content

Highlights

  • A special staged KOH activation method was applied to synthesized Nitrogen-doped porous carbon materials via silk cocoon.

  • We obtained an ideal Nitrogen-doped porous carbon with an ultrahigh specific surface area (3841 cm2 g−1).

  • The prepared sample displays superior electrochemical performance than many other Nitrogen-doped porous carbon materials.

Abstract

Owing to its abundant nitrogen content, silk cocoon is a promising precursor for the synthesis of Nitrogen-doped porous carbon (N-PC). Using a simple staged KOH activation, the prepared sample displays particular nanostructure with ultrahigh specific surface area (3841 m2 g−1) and appropriate pore size, providing favorable pathways for transportation and penetration of electrolyte ions. Additionally, the doped nitrogen atoms ensure the samples with pseudocapacitive behavior. Those special characteristics endow N-PCs with high capacity, low resistance, and long-term stability, indicating a wonderful potential for application in energy-storage devices.

Introduction

Supercapacitors, also known as electrochemical capacitors (EC), have been extensively developed due to their various advantages, such as high power energy, long service life and good stability [1], [2], [3], [4], [5]. There are two types of supercapacitors based on the different charge-storage mechanism, electric double-layer capacitors (EDLCs) and pseudocapacitors. The energy storage principle of EDLCs is based on the separation of charged species in an electrical double layer across the electrode/solution interfaces [6]. Therefore, high surface area and suitable pores for electrolyte ions are significant to the EDLCs. On the other hand, pseudocapacitors can attain higher capacitance than EDLCs because they store and transport electrolyte ions by electrosorption, reduction-oxidation reactions, and intercalation processes. However, the inferior electrical conductivity and poor cycle stability result in a limitation for their practical applications.

As we all know, the surface area, pore size distribution, and surface microstructure of carbon materials are essential to the performance of supercapacitors. Therefore, many nanostructured carbon materials, including porous carbons (PCs) [7], [8], carbon fibers [9], carbon aerogels [10] and carbon nanotubes [11], have been widely used in the EDLCs as electrode materials. Among all these carbon materials, PCs present numerous advantages than others, such as, low-cost, high surface area, adjustable pore structure, varieties of forms and ease of process ability. Those desirable merits ensure that many kinds of PC-based materials can be applied in the high performance supercapacitors [12], [13], [14], [15], [16], [17], [18], [19].

Recently, for the purpose of further improving their applied performance, much effort has been devoted to synthesize and tailor the microstructures of PC materials. Among them, the nitrogen functionalized PCs is especially popular and important in the applications of adsorption and electrochemistry due to its specific structures and unique properties which can offer both high surface area for large amounts of potential active sites and channels for reactant/product transfer, respectively. Moreover, the doped nitrogen atoms endow the porous carbon with remarkable pseudocapacitive behavior. Most pathways for the production of nitrogen-containing PCs (N-PCs) rely on post-treatment of ammonia [20], [21], [22] or templating route combined with activation of nitrogen-rich carbon precursors [23], [24], [25]. Nevertheless, ammoxidation of PCs leads to a decrease in their surface area and pore volume, the nitrogen mainly distributed on the surface and often unstable, which limit their wide applications. In addition, templating route often suffers from these drawbacks such as time-consuming and severe synthetic conditions. Furthermore, most of the templates are based on surfactants and block-copolymers, which are rather expensive and non-renewable. One of the alternatives is to carbonize biomaterials, especially biomasses, to have nitrogen firmly incorporated in the carbon frame. For example, Senoz et al. reported the synthesis of microporous nitrogen-doped carbon materials from chicken feather fibers (composed of keratin, a fibrous structural protein also found in wool, hair, and some animal shells) [26]. Fan et al. reported on employing chitosan to produce nitrogen-containing microporous carbon materials [27]. Li et al. synthesized nitrogen-doped PCs by direct carbonization and activation of eggshell membranes [28]. In general, these were feasible methods to fabricate N-PCs, however, most of nitrogen-doped porous carbons obtained from above methods have comparatively low specific surface area (<2000 m2 g−1), which impeded their applications to a considerable extent in supercapacitors electrode materials [29], [30], [31], [32]. Therefore, it is still a challenge to develop a facile and benign method to synthesize N-PCs with ultrahigh specific surface area from simple available precursors.

Silkworm cocoon produced by the bombyx mori silkworm is one of the most abundant nontoxic protein-based biopolymer in nature, which is cultivated more than 480 000 ton per year all over the world [33]. Detailed investigations indicate that the silk consists of two main proteins, sericin and fibroin, which is rich in nitrogen and carbon elements. In recently years, however, compared with their widely used in fabrics, their potential applications in material science have attracted some attention, such as electrode materials. Liang and his group synthesized carbon microfibers with hierarchical porous structure from silk cocoon which could be used as electrodes of supercapacitors [34]. In addition, Yun et al. reported microporous carbon nanoplates from regenerated silk proteins for supercapacitors [35]. Recently, Hou and his co-workers have prepared hierarchical porous nitrogen-doped carbon nanosheets derived from silk for ultrahigh-capacity battery anodes and supercapacitors [36]. Those researches clearly demonstrate that silk cocoon is a promising carbonaceous raw material for supercapacitor electrodes.

Herein, we reported a facile synthesis (staged KOH activation) of N-PCs with abundant micropores and mesopores as well as ultrahigh specific surface area (>3800 m2 g−1) by silkworm cocoon. Using the as-prepared N-PCs as a charge storage material in supercapacitors, we have achieved a high specific capacitance of 408 F g−1 at a current density of 0.5 A g−1 and an excellent capacitance retention of 96% after 20 000 cycles at a high current density of 25 A g−1. Those results clearly demonstrate that the electrochemical performance of N-PCs derived from silk cocoon is superior to that of most activated carbons used for commercial purposes.

Section snippets

Synthesis of N-PCs

Cocoons of Bombyx mori obtained from the South China Agriculture University in China were directly used as the starting material without any pretreatment. The schematic diagram for the synthesis of N-PCs derived from silk cocoon is shown in Scheme 1. In a typical process, the silk cocoon was first pre-carbonized in a tube furnace at 400 °C under argon atmosphere for 2 h, and the heating rate was 2 °C min−1. Then, the carbonized sample was ground into powder and thoroughly mixed with KOH, the

Results and discussion

The element analysis results reveal that the employed silk cocoon is mainly composed of C (27.30 at.%), O (16.24 at.%), and N (8.70 at.%), as shown in Table S1. Fig 1a presents the TGA curve of raw silk cocoon under argon atmosphere protection. The weight sharply decreases when the temperature is over 250 °C, suggesting the rapid pyrolysis of proteins and fibers within silk cocoon, which is similar to the decomposition of silk fibroins [37]. The weight loss of silk cocoon is about 50.5%,

Conclusions

In summary, nitrogen doped porous carbon materials were synthesized through carbonization and further staged activation of silk cocoon. This kind of material exhibits ultra-large specific surface area of 3841 m2 g−1 and suitable pore size, which endows high charge storage capacity with a specific capacitance of 408 F g−1 in 6 M KOH at a current density of 0.5 A g−1 and excellent cyclic stability (96% of capacity retention) over 20 000 cycles. Remarkably, the N-PC-700 based symmetric capacitor

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. U1501242, 21201065, 21371061, and 21571066), the Key Program of Science Technology Innovation Foundation of Universities of Guangdong Province (cxzd1113), and the key Laboratory of Functional Inorganic Materials Chemistry (Heilongjiang University), Ministry of Education.

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