Elsevier

Nano Energy

Volume 19, January 2016, Pages 165-175
Nano Energy

Rapid communication
Nitrogen and sulfur co-doped porous carbon nanosheets derived from willow catkin for supercapacitors

https://doi.org/10.1016/j.nanoen.2015.10.038Get rights and content

Highlights

  • Willow catkin is effectively converted into cross-linked porous carbon nanosheets.

  • N,S-PCNs1-1 electrode shows excellent capacitive performance.

  • The assembled symmetric supercapacitor exhibits high energy density.

Abstract

A facile one-step pyrolysis and activation synthesis method is utilized to convert a common biomass of willow catkin into interconnected porous carbon nanosheets (PCNs), and then followed by effective nitrogen and sulfur co-doping. Owing to the unique hollow and multilayered structure of willow catkin fiber, the pore structure of obtained carbons can be controlled by adjusting the mass ratio of raw material to alkali. As a result, the nitrogen and sulfur co-doped PCNs demonstrate a high specific capacitance of 298 F g−1 at 0.5 A g−1 and 233 F g−1 at 50 A g−1, revealing excellent rate performance. In addition, the electrode demonstrates superb cycling stability with only 2% capacitance loss after 10,000 cycles. Furthermore, the assembled symmetric cell with a wide voltage range of 1.8 V yields a remarkable specific energy of 21.0 Wh kg−1 at 180 W kg−1. These exciting results exhibit a green and low-cost design of electrode materials for high performance supercapacitors.

Graphical abstract

The willow catkin derived nitrogen and sulfur co-doped porous carbon nanosheets (N,S-PCNs1-1) are prepared by a facile one-step pyrolysis and activation synthesis method, and then followed by effective nitrogen and sulfur co-doping. As a result, the as-obtained carbon processes cross-linked graphene-like structure with high specific surface area and interconnected pore texture, resulting in high specific capacitance, excellent rate performance and cycling stability.

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Introduction

Supercapacitor or electrochemical capacitor is a sort of common energy storage devices which can operate at a much higher power density than battery and possess greater energy density than conventional dielectric capacitor. During the past few years, major progress has been made in improving energy storage for supercapacitors [1], [2], [3], [4]. Electrochemical capacitors can be categorized into electric double-layer capacitors (EDLCs) that are based on electrostatic charge diffusion and accumulation in the double layer formed at the surface of inert electrodes [5], and pseudocapacitors that are primarily dominated by reversible Faradaic reactions on the active material’s surface [6].

Carbon materials, such as activated carbon [7], carbon nanotubes [8], graphene [9], [10], carbon aerogels [11] and carbide-derived carbons [12], have been comprehensively researched for EDLCs electrode materials. Among above carbon materials, activated carbon, due to their high surface areas and low cost, has been regarded as first candidate for commercial supercapacitor devices. Nevertheless, the capacitance and power characteristics of conventional activated carbons (CAC) are limited due to its intrinsically inappropriate and blocked pore texture, which in turn restricts pore accessibility of the electrolyte ions at high charge/discharge rates. As is known, the key to obtain excellent power performance is to reduce the ion transportation time in electrode materials [13]. Therefore, carbon materials with two-dimensional (2D) and open porous structure possess inherent superiorities owing to the reduced ion transportation times. Especially, graphene and graphene hybrid materials, with high in-plane electrical conductivity and 2D flat surface, have superior power–energy combination performance to that of CAC. At present, the extensively used methods for synthesis of graphene are mainly chemical vapor deposition [14], chemical reduction [15], [16], arc-discharge synthesis [17], high-intensity ultrasound exfoliation [18] and microwave radiation [19].

However, even the most economically and facilely prepared graphene materials are nowhere near cost and facility competitive with direct pyrolysis carbon precursors, such as biowaste [20], [21], [22] or biomass [23], [24], [25], [26], [27] with containing cellulose, hemicellulose and lignin biopolymer, into PCNs with graphene-like morphology. Willow catkin, as a kind of common biomass, is inedible to human and has no economic value. The worse is that willow catkin breathed into the nasal cavity may cause intense stimulation, cough and asthma. Especially in spring, the willow catkin can easily cause skin allergic reactions, such as itching, eye conjunctival inflammation. But we are surprised to find that it is an ideal carbon precursor for synthesis of value-added PCNs by one-step pyrolysis and activation method.

Incorporation of heteroatoms into carbon materials is crucial to tailor their electron-donor properties and consequently tune the electrical and chemical performance of their surface. The introduction of heteroatoms can decrease charge transfer resistance and improve wettability, thereby enhancing capacitive performance. Representative heteroatoms such as nitrogen (N) [28], phosphorus (P) [29], sulfur (S) [30] and boron (B) [31] can be incorporated in either a single- or dual-doped manner to modify the carbon materials. Compared with single heteroatom doping that improves merely one aspect of properties, co-doping can enhance overall performance of the materials due to the synergetic effect [32], [33], [34]. Therefore, more and more efforts have been focused on studying the multi-heteroatom doping in recent years. N, S co-doped carbon materials have been applied into oxygen reduction reaction (ORR) [35], but have yet to be researched in the fields of energy storage materials.

In this work, we report a one-step pyrolysis and activation synthesis method to convert willow catkin into interconnected PCNs, then followed by N, S co-doping. The obtained carbon materials show unique cross-linked laminar structures and display outstanding electrochemical performances in aqueous electrolyte.

Section snippets

Experimental section

All the chemical reagents in this study are of analytical grade and are used without further purification. All aqueous solutions are prepared with deionized (DI) water.

Material characterization

Fig. 1A and B presents photographs and SEM images of the willow catkin. It is easily observed that the morphology of willow catkin is hollow and thin-walled, contributing to the molten KOH permeation for activation. The heating procedure under argon atmosphere was determined according to the thermogravimetric analysis. As shown in Fig. S1, with the temperature increasing to 400 °C, the weight loss is almost up to its maximum, indicating the preliminary forming of carbon skeleton. Interestingly,

Conclusions

In summary, carbon-based materials derived from biomass/biowaste are attracting extensive attention in the supercapacitor and secondary battery fields due to their low-cost and facile preparation. We employed a one-step pyrolysis and activation followed by N, S co-doping synthesis process to convert willow catkin into heteroatom-doped interconnected porous carbon nanosheets. The N,S-PCNs1-1 specimen exhibits a high specific capacitance of 298 F g−1 at 0.5 A g−1 and excellent rate performance (78.2%

Acknowledgment

We gratefully acknowledge the financial support of this research by the National Natural Science Foundation of China (No. 51572052).

Yiju Li is a Ph.D. candidate in Materials Science and Technology at the Harbin Engineering University. He received his B.S. degree from the Harbin Engineering University in 2013. His current research includes the synthesis of advanced carbon materials and their composites with metal oxides/sulfide, as well as their applications in electrochemical energy storage devices.

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  • Cited by (0)

    Yiju Li is a Ph.D. candidate in Materials Science and Technology at the Harbin Engineering University. He received his B.S. degree from the Harbin Engineering University in 2013. His current research includes the synthesis of advanced carbon materials and their composites with metal oxides/sulfide, as well as their applications in electrochemical energy storage devices.

    Guiling Wang received his Ph.D. in 2006 from the Institute of Chemical Engineering, Harbin Engineering University. He became full professor at the College of Material Science and Chemical Engineering at Harbin Engineering University in 2007. His research interests focus on the design and controlled synthesis of electrode materials of NaBH4 fuel cells, H2O2 fuel cell, supercapacitors and lithium ion batteries.

    Tong Wei received her Ph.D. in 2003 at the Institute of Coal Chemistry, Chinese Academy of Sciences. She became full professor at the College of Material Science and Chemical Engineering at Harbin Engineering University in 2009. Her research interests focus on the design and synthesis of functional carbonaceous nanomaterials, and their application in for energy conversion and storage devices.

    Zhuangjun Fan received his Ph.D. in 2003 at the Institute of Coal Chemistry, Chinese Academy of Sciences. He became full professor at the College of Material Science and Chemical Engineering in 2006, and now he is the Director of the Institute of Advanced Carbon Based Materials at Harbin Engineering University. His research interests focus on the design and controlled synthesis of carbon nanomaterials such as carbon nanotubes and graphene, and their application in energy-related areas such as supercapacitors, Li ion batteries, and fuel cells.

    Peng Yan is a Ph.D. candidate in Materials Science and Technology at Harbin Engineering University. She received her M.S. degree from School of Chemistry and Materials Science, Heilongjiang University in 2007. Her research interests focus on the design and synthesis of electrode materials of direct methanol fuel cell, H2O2 fuel cell and supercapacitors.

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