Elsevier

Nano Energy

Volume 11, January 2015, Pages 211-218
Nano Energy

Rapid communication
Large-scale synthesis of coaxial carbon nanotube/Ni(OH)2 composites for asymmetric supercapacitor application

https://doi.org/10.1016/j.nanoen.2014.09.030Get rights and content

Abstract

Coaxial carbon nanotube-nickel hydroxide (CNT/Ni(OH)2) composites are prepared by a simple, one step and inexpensive chemical coprecipitation method. The coaxial coating of nickel hydroxide provides a three dimensional (3D) structure for easy access of electrolyte. Asymmetric supercapacitors (ASCs) are fabricated using coaxial CNT/Ni(OH)2 composites as positive electrode and reduced graphene oxide (rGO) as negative electrode. The operation voltage is expanded to 1.8 V in spite of the use of aqueous electrolyte, revealing a high energy density of 35 W·h·kg−1 at a power density of 1.8 kW·kg−1. This strategy for choice of coaxial metal hydroxide CNT composites provides a promising route for next generation supercapacitors with high energy as well as power densities.

Introduction

Electrochemical capacitors, also called supercapacitors, which store energy in two closely spaced layers with opposing charges, are used to power up electrical vehicles, consumer electronics, memory backup, and military devices [1], [2], [3], [4], [5]. Electrochemical supercapacitors bridge a gap between batteries which offer high energy densities but are slow, and conventional capacitors which are fast but have low energy density [1], [2], [3], [4], [5]. However, for some power applications, a major limitation of supercapacitor is that they have unsatisfactory energy density (typically 5–10 W·h·kg−1) compared to lead acid batteries (30–40 W·h·kg−1), and lithium ion cells (160 W·h·kg−1) [6]. There are two ways to improve device performance: increasing energy density of supercapacitors or increasing the power density of batteries. Current research on the development of supercapacitors has mainly focused on improving their energy density, making them a comparable technology to that of batteries [7], [8], [9], [10]. A major effort has been directed toward increasing energy density of supercapacitor either by increasing capacitance ‘C’ of the material or the operation voltage window ‘V’, or both, since the energy stored is proportional to CV2 [11], [12], [13]. Some investigations have utilized organic electrolytes which have wide potential window in cell. However, most of organic electrolytes have poor ionic conductivity, high cost, raise environmental concerns, and can be used only in cells that can be fabricated in oxygen free environment [14], [15].

On the other hand, fabrication of asymmetric supercapacitors (ASCs) using aqueous electrolytes is an efficient way to improve energy density. Transition metal oxides (e.g., NiO, MnO2, etc.) and activated carbon have been used to fabricate ASCs with cell voltages ranging from 1.2 to 2.0 V in aqueous electrolytes [16], [17], [18]. However, the poor electrical conductivity of metal oxide electrodes resulted in compromises of power density. Recently, ASCs fabricated using metal oxide/CNT composites exhibited good energy and power density [19]. A composite material containing cost-effective pseudocapacitive material such as nickel hydroxide (Ni(OH)2) is of interest because of its good conductivity and dual energy storage mechanism. An ASC fabricated with Ni(OH)2/carbon composites shows high energy and power densities [20], [21], [22] However, these methods are expensive, multi-step and some of them used nickel or graphene foam as current collector that brings limitation in achieving higher volumetric energy density, which is crucial for practical device applications.

Nanoporous carbon can also be considered as one of potential carbon-based electrode materials for supercapacitor application [23], [24], [25]. However, if one can consider composites of carbon matrix with metal oxide, one-dimensionally shaped carbon nanotubes (CNTs) with hollow interior space are ideal candidate as they can possibly provide more reaction sites with conducting channels. To date, many reports are available in literatures for development of metal oxide/CNT composites for supercapacitor application based on synergy of excellent properties [26], [27]. For the effective utilization of CNTs, the homogeneous dispersion of individual nanotubes throughout the matrix is critical. In addition, strong interfacial adhesion between the CNTs and the matrix is also required for improving the properties of composites. However, it is well known that the CNTs have a strong tendency to agglomerate due to their nanosize and high surface energy [28]. The grafting of chemical functionalities on the CNT surface creates the electrostatic stability required for a colloidal dispersion. There have been some reports available for the addition of dispersing aids to result in better dispersion and improved electrical and mechanical properties, however, it has been often observed that undesired impurities are inevitably introduced. The surface oxidation of CNTs is a versatile route [28], [29], which can realize various surface modifications with different functional groups and improve the dispersion in matrix without causing undesired impurities (Fig. 1b-c).

In this work, we have adopted a simple, one-step, and cost-effective chemical synthesis method for coaxial composites consisting of nickel hydroxide and oxidized CNT (o-CNT). In the present study, we aim to grow coaxial thin layer of nickel hydroxide (Ni(OH)2) onto oxidized CNT surface (Fig. 1a) and a related ASC study for high energy densities. In our chemical reaction system, the precursor of Ni(OH)2 gets adsorbed on o-CNT surface due to interfacial electrostatic interaction between the Ni species and the oxygen functional groups present on o-CNT surface [30], [31], [32]. By optimizing temperature and time, we can coat coaxially nickel hydroxide onto oxidized CNT surface. This design for composites has the advantage of synergy for nickel hydroxide (pseudocapacitive capability due to nickel hydroxide) and CNT (EDLC due to CNTs). An ASC fabricated with o-CNT-Ni(OH)2 and reduced graphene oxide (rGO) exhibit a significantly elevated cell voltage of 1.8 V and a specific capacitance of 78.3 F·g−1 at charge/discharge current density of 2 A·g−1. The corresponding specific energy and specific power are 35 W·h·kg−1 and 1807 W·kg−1, respectively.

Section snippets

Synthesis of carbon nanotube/Ni(OH)2 composites

Multiwall carbon nanotubes (MWCNTs) were treated by modified hummers method to convert it into o-CNT. A chemical precipitation method was carried out to obtain the o-CNT-Ni(OH)2 composites. The MWCNTs (40–60 nm) were converted into CNT oxide (o-CNT) to make them more dispersible in deionized water. A specific amount of o-CNT (30 mg) was first dissolved in 50 ml water by ultrasonication for 1 h and was maintained at pH=12 with addition of ammonia. Afterwards, this suspension was mixed with an

Results and discussions

The structures of the pristine nickel hydroxide (prepared without o-CNT), o-CNT, and, o-CNT/Ni(OH)2 powders were characterized by wide-angle XRD, as shown in Fig. 2. The peaks indexed by asterisks correspond to o-CNT. From XRD profile, we can see that nickel hydroxide contains both α- and β-phases. The peaks related to β-phase are (0 0 1), (1 0 0), (1 0 1), (1 0 2), (1 1 0), and (1 1 1). However, only one peak related to α-phase is (0 0 1). The observed peak positions are in good agreement with previous

Conclusion

We have successfully prepared a coaxial nanocomposite material by simple, cost-effective, and facile chemical method and studied synergy of these nanocomposites supercapacitor application. The resultant nanocomposite exhibited significant capacitance, as compared with bare Ni(OH)2 and o-CNT. This significantly improved electrochemical performance was attributed to the fact that Ni(OH)2 was coated coaxially to CNT surface, which could greatly improve conductivity of matrix and dual storage

Acknowledgment

This work was partially supported by The Canon Foundation and The Japan Society for the Promotion of Science (JSPS KAKENHI Grant Number 26708028).

R. R. Salunkhe received his M.S. (2005) and PhD (2009) in Physics from Shivaji University, Kolhapur, India. He is currently working as a research associate at Prof. Yamauchi׳s group of National Institute for Materials Science (NIMS) in Japan. His present research interests include mainly synthesis of novel nanocrystalline metal oxides, conducting polymers, mesoporous carbons, and their electrochemical applications in supercapacitor and Li-ion battery.

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    R. R. Salunkhe received his M.S. (2005) and PhD (2009) in Physics from Shivaji University, Kolhapur, India. He is currently working as a research associate at Prof. Yamauchi׳s group of National Institute for Materials Science (NIMS) in Japan. His present research interests include mainly synthesis of novel nanocrystalline metal oxides, conducting polymers, mesoporous carbons, and their electrochemical applications in supercapacitor and Li-ion battery.

    Jianjian Lin received her master degree from Shandong Polytechnic University in 2011. Currently, she is pursuing her PhD under the supervision of Prof. Jung Ho Kim and Prof. Shi Xue Dou at Institute for Superconducting and Electronic Materials (ISEM) in University of Wollongong, Australia. Her research mainly focuses on materials engineering for energy harvesting.

    Victor Malgras received his bachelor degree in physics engineering at Polytechnic School of Montreal (Canada), his master degree in nanoscience at the University Toulouse III (France), and his PhD in photovoltaics and nanomaterial engineering at the University of Wollongong (Australia). He specialized himself in various fields such as carbon nanotube materials, hybrid surface characterization, and chemical synthesis of semiconducting nanostructures for photovoltaic applications. He is now undergoing a postdoctoral fellow in Prof. Yamauchi׳s group in the National Institute for Materials Science (Japan).

    Shi Xue Dou is Distingiushed Professor and Director of the Institute for Superconducting and Electronic Materials (ISEM) at Australian Institute of Innovative Materials, University of Wollongong. He received his PhD in chemistry in 1984 at Dalhousie University, Canada. He was elected as a Fellow of the Australian Academy of Technological Science and Engineering in 1994. He was awarded a DSc by the University of New South Wales in 1998 and three Australian Professorial Fellowships by Australian Research Council in 1993, 2002 and 2007. He received the Australian Government׳s Centenary Medal in 2003, Vice-Chancellors Senior Excellence Award in 2008, Vice-Chancellor Outstanding Partnership Award in 2012.

    Jung Ho Kim is currently an associate professor at University of Wollongong, Australia. He received his bachelor (1998), master (2000), and PhD (2005) from Sungkyunkwan University, Korea. He has published in more than 120 papers in international refereed journals. He is currently acting as an editorial board member for Scientific Reports (Nature Publishing Group). His major research interest is rationally designed materials with one-, two-, and three-dimension toward energy storage and harvest applications.

    Yusuke Yamauchi received his PhD in 2007 from Waseda University in Japan. After receiving his PhD, he joined the National Institute for Materials Science (NIMS) and started his own research group on ‘Inorganic materials chemistry’. He has published more than 300 papers in international refereed journals. He concurrently serves as a visiting professor at several universities (Tianjin University, Waseda University, and University of Wollongong), and an associate editor of APL Materials published by the American Institute of Physics (AIP). His major research interest is tailored design of novel nanoporous materials with various shapes and compositions toward practical applications.

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