Elsevier

Chemical Engineering Journal

Volume 330, 15 December 2017, Pages 1166-1173
Chemical Engineering Journal

Nitrogen and oxygen-codoped carbon nanospheres for excellent specific capacitance and cyclic stability supercapacitor electrodes

https://doi.org/10.1016/j.cej.2017.08.070Get rights and content

Highlights

  • N, O codoped carbon nanospheres are prepared using sodium alginate and polyaniline.

  • N, O codoping improves electrochemical performance via pseudocapacitive effect.

  • The electrode displays high specific capacitance and exceptional cycling stability.

Abstract

Nitrogen and oxygen-codoped carbon nanospheres were prepared by adopting sodium alginate (SA) as the carbon precursor in the presence of polyaniline (PANI) through direct carbonization process. A typical sample (PANI/SA-800) obtained at the carbonization of 800 °C shows regular a spherical shape (∼130 nm in average diameter) and a relatively rich oxygen (11.71 at.%) and nitrogen (3.48 at.%) doping. When PANI/SA-800 is used as an electrode material for electrochemical capacitor, a high specific capacitance of 627 F g−1 at a current density of 1 A g−1 in 3 M H2SO4 aqueous electrolyte is achieved. Furthermore, the electrode also displays exceptional cycling stability for 10,000 charge/discharge cycles with retention ratio up to 224.9% in acidic electrolyte at a high current density of 7 A g−1. The renewable, cost-effective, and high electrochemical performance nitrogen and oxygen-codoped carbon nanospheres provide a promising candidate for advanced supercapacitor electrodes.

Introduction

Nowadays, it becomes rather urgent to develop green, sustainable, high efficiency and low cost energy storage materials owing to the shortage of traditional fossil fuels and the rapid increased greenhouse gas [1], [2], [3], [4], [5], [6]. Supercapacitors, possessing a high power density, ultra-long cycle life, subsecond charging and excellent safety properties, have stimulated tremendous interest in hybrid electric vehicles, digital electronic products, and renewable energy applications [7]. In general, supercapacitors have two type charge-storage mechanism, namely electrical double layer capacitance (EDL-capacitance) and faradaic pseudo-capacitance [8]. Electrical double layer capacitors (EDLCs) store electrical energy via electrostatic charge accumulated in the double layer at the electrode/solution interface, commonly use carbon materials [9]. On the other hand, pseudocapacitors utilize reversible faradaic redox reaction occurring at the electrode surface formed by active materials, such as metal oxides [10], [11] or conducting polymers [12].

It is well-known that electrode material is a pivotal factor for the properties of supercapacitors. Among the electrode materials, carbon materials such as activated carbon [13], [14], hierarchical porous carbon [15], [16], and carbon aerogels [17], [18] have been widely used as supercapacitor electrode materials due to their high surface area, long cycle life and low cost [19]. However, the specific capacitance of carbon materials is lower than that of transition metal oxides and conductive polymers. Recently, carbon spheres are emerged as one of the most outstanding candidate for advanced supercapacitor electrode material due to their merits of regular morphology, particle size, and high packing density, which can decrease the resistance of ion diffusion and thereby resulting in enhanced electrochemical performance [20], [21], [22]. For instance, Lei et al. [23] synthesized mesoporous carbon nanospheres by a chemical vapor deposition method, which showed a relatively low specific capacitance of 225 F g−1 at a current density of 0.25 A g−1. Ma et al. [9] reported the preparation of size controllable mesoporous carbon microspheres (MCMs) by template method and further sodium hydroxide (NaOH) etching process. The as-synthesized MCMs delivered the high specific capacitance of 289 F g−1 at 1.0 A g−1. Suslick groups [24] fabricated carbon microspheres by ultrasonic spray pyrolysis aqueous precursors, and achieved the maximum specific capacitance of 360 F g−1 at the scan rate of 5 mV s−1. Unfortunately, the reported methods for preparing carbon spheres are time consuming, energy-intensive, high-cost or tedious because of the additional synthesis of special templates and extra activation process.

On the other hand, the doping of heteroatoms, such as nitrogen and oxygen, in a carbon matrix can further enhance the energy density and specific capacitance of the carbon material electrodes, so the relevant researches have attracted much attention in recent years [25], [26], [27], [28], [29]. Particularly, the heteroatoms dual-doped carbon materials originated from renewable biomass resources have elicited tremendous interests owing to their cost-effective and potential large-scale production for supercapacitor applications. The preparation of nitrogen and boron dual-doped porous carbon by adopting chitosan as the raw material with the aid of boric acid was reported by Ling [30]. The as-obtained porous carbon electrode possessed a high specific capacitance of about 227 F g−1 at 2 mV s−1. Tian and co-workers reported the synthesis of nitrogen and oxygen-containing hierarchical porous carbon through the carbonization of Enteromorpha prolifera, and revealed a specific capacitance of 234 F g−1 at 0. 5 A g−1 [31]. Yu et al. [32] synthesized nitrogen and oxygen dual-doped hierarchical porous carbon via adopting Enteromorpha as the biomass precursor, and the resultant sample showed a specific capacitance of 201 F g−1 at a current density of 1 A g−1. More recently, Qiu groups reported a scalable production of B/N co-doped 2D carbon nanosheets by using gelatin (an animal derivative consisted of various proteins) as carbon and nitrogen precursor, exhibiting a specific capacitance of 358 F g−1 at a current density 0.1 A g−1 [33]. To date, however, some biomass derived carbon materials utilized as carbon precursors haven’t been fully researched. Furthermore, it remains a significant challenge to develop cost-effective, sustainable and high efficiency precursors for advanced supercapacitor electrodes.

Sodium alginate (SA), a natural sea biopolymer, is composed of (1,4)-β-d-mannuronic acid (M) and α-l-guluronic acid (G) residues [34]. Sodium alginate has been widely used in food, biological and pharmaceutical industry due to its non-toxicity and good biocompatibility. However, its application in electrochemical capacitors has not attracted enough attention. To the best of our knowledge, there are few reports on the preparation of nitrogen and oxygen-codoped carbon nanospheres through direct carbonization of the nanostructured polyaniline (PANI)/SA. In this paper, we present a facile yet sustainable approach for production of nitrogen and oxygen-codoped carbon nanospheres. The PANI/SA nanospheres are synthesized by in situ oxidation polymerization of aniline monomer using sodium alginate as a green template. Then, the nitrogen and oxygen-codoped carbon nanospheres are produced via adopting sodium alginate as oxygen source and polyaniline as nitrogen source through direct carbonization of the PANI/SA nanospheres. When evaluated as the electrode in supercapacitors, the nitrogen and oxygen-codoped carbon nanospheres exhibit a high specific capacitance and exceptional long-term electrochemical stability. Therefore, these carbon nanospheres provide a promising electrode material for the production of electrochemical capacitors.

Section snippets

Materials

Aniline (analytical reagent) was purchased from Tianjin Fuchen Chemical Reagent Factory. Sodium alginate (SA, chemically pure, viscosity: 200 ± 20 MPa·s) was provided by Aladdin Chemistry Co., Ltd. Ammonium persulfate (APS, analytical reagent) was supplied by Tianjin Guangfu Fine Chemical Research Institute. The other reagents are analytical grade and as they are without further purification. Deionized water was utilized throughout the experiments.

Synthesis of PANI/SA nanospheres

Typical PANI/SA nanospheres were synthesized by in

Results and discussion

The morphology and microstructure of the original PANI/SA and carbonized PANI/SA samples are examined by SEM and TEM. As can be seen in Fig. 1a, the pristine PANI/SA composites have regular spherical shape and their surface is very rough. Compared with the pristine PANI/SA nanospheres, the PANI/SA-600, PANI/SA-700, PANI/SA-800, and PANI/SA-900 (Fig. 1b–e) still maintain the initial morphology of PANI/SA. However, the surface of carbonized PANI/SA nanospheres becomes smoother than original PANI/SA

Conclusions

In summary, a simple and sustainable method is provided to produce nitrogen and oxygen-codoped carbon nanospheres from PANI/SA precursor for use in advanced supercapacitor electrodes. The heteroatoms codoped process plays an important role on the properties of these carbonized PANI/SA electrodes. Under ideal carbonization temperature, the PANI/SA-800 sample exhibits significant O, N content, pseudocapacitive species and proper degree of graphitization, which are conducive to the improvement of

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