Elsevier

Carbon

Volume 71, May 2014, Pages 276-283
Carbon

Synthesis and electrochemical properties of nickel oxide/carbon nanofiber composites

https://doi.org/10.1016/j.carbon.2014.01.052Get rights and content

Abstract

The electrospinning of polyacrylonitrile (PAN) with a polyaniline and graphene sol–gel mixture produced uniform, smooth fibers with an average diameter of 0.3 μm. These electrospun fibers were stabilized for 2 h at 200 °C and then carbonized at 800 °C for 5 h. Composites were prepared by depositing Ni(OH)2 on the carbon nanofibers (CNFs) and calcining them at different temperatures. The composites were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The effect of the calcination temperatures on the electrochemical properties was studied using cyclic voltammetry and electrochemical impedance spectroscopy. The specific capacitance (SC) was found to be highest (738 F g−1) at a calcination temperature of 400 °C. The charge transfer resistance (Rp) decreased as the calcination temperature was increased. However, the electrical double layer capacitance (EDLC) increased with an increase in the calcination temperature. The EDLC increased from 0.144 F g−1 at a calcination temperature of 100 °C to 485 F g−1 at a calcination temperature of 500 °C.

Introduction

Supercapacitors have attracted much interest due to their potential applications in electronic devices, medical devices and portable batteries [1], [2]. Scientists have focused much effort on developing flexible supercapacitor materials that are characterized by high electrochemical capacitance and mechanical strength [2], [3]. Carbon materials, such as graphene [2], [3], carbon nanotubes [2], [4] mesoporous carbon [5], [6], [7] and carbon nanofibers (CNFs) [2], [8], [9], are among the most interesting and promising materials for supercapacitors. Electrospinning is an efficient, inexpensive and versatile technique for producing fine fibers with diameters of a few nanometers up to submicrons [1], [2], [9], [10], [11]. Non-woven fibers produced by electrospinning techniques typically have large surface areas and high porosity [9], [12]. These characteristics have been exploited in a wide range of applications [13]; including biomedical applications such as drug delivery, wound dressing, enzyme immobilization and tissue engineering [14]. Electrospinning has also been used for energy-related applications, e.g., Li-ion batteries, fuel cells, supercapacitors and dye-sensitized solar cells [15]. CNFs have been prepared from different polymeric materials, such as polyacrylonitrile (PAN) [12], polybenzimidazole [15], polyvinyl alcohol [10], etc., using electrospinning followed by heat treatment in a vacuum furnace [12]. The pyrolysis conditions used to produce nanoscale carbon fibers from electrospun PAN nanofibers with different structural orientations control the properties of the produced CNFs [12], [16], [17]. It is worthy to mention that, the limited morphology of carbonaceous materials leads to a low specific capacitance (SC) [18], [19]. However, activated CNFs prepared by electrospinning have exhibited good performance compared to carbon fibers prepared using the melt-blown method because of their high electrical double layer capacitance (EDLC) [18]. The pseudocapacitance (PC) and EDLC can be improved by preparing a composite from a conducting polymer (high PC) and carbon-based material (high EDLC) [20], [21], [22], [23]. Furthermore, the electrochemical stability and excellent SC of transition metal oxides have attracted much attention [24]. Nickel oxide is particularly attractive, because it is cheap, readily available and capable of PC [24], [25], [26], [27].

The goals of this work are to (i) prepare a new composite with a nanosize component which has high PC and EDLC, using Ni oxide/hydroxide nanoparticles supported by CNFs (CNFs were prepared by electrospinning a mixture of PAN, polyaniline (PANI) and graphene and heat treating the resulting material) for supercapacitor applications and (ii) study the effect of the calcination temperature of the CNF-supported Ni(OH)2 nanoparticles on their measured capacitance using cyclic voltammetry and electrochemical impedance spectroscopy. The innovative point here is the combination of PAN with graphene and PANI to produce CNF using the electrospinning technique. Adding graphene to CNF is expected to increase the capacitance of CNF [2] as a result of its exfoliation and high surface area. In addition graphene is expected to reduce the electrical resistance by dissipating the heat generated during capacitor charging/discharging (because of its high exfoliation) and enhancing the contact between NiO and the CNF.

Section snippets

Composite-based CNF preparation

PAN with an average molecular weight of approximately 150,000 g mol−1, polyaniline (emeraldine base) with an average molecular weight of approximately 10,000 g mol−1, nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O) and dimethyl formamide (DMF) were obtained from Sigma–Aldrich, Saudi. Graphene was obtained by reducing graphene oxide, which was synthesized using a modified Hummers method [28], [29], with 0.1 M sodium borohydride.

PAN (10 g) was dissolved in DMF (100 mL) at room temperature under

Results and discussion

Fig. 1 shows SEM micrographs of the produced PAN/PANI/graphene composite electrospun fibers. As observed in the figure, they are randomly oriented and have diameter between 0.3 and 500 μm, smooth surface and straight shape (i.e. no beads). It has been reported that smooth, straight electrospun fibers are usually produced by increasing the PAN solution concentration up to 10.0 wt.% [30]. The length of the produced electrospun fibers is on the order of several centimeters.

Fig. 2 presents the SEM

Summary and conclusions

This work reported (i) the synthesis of Ni oxide/hydroxide nanoparticles supported on CNFs for supercapacitor applications and (ii) the effect of the calcination temperature on their electrochemical properties. CNFs were prepared by electrospinning PAN with PANI and graphene sheets, and these materials were then loaded with Ni(OH)2 nanoparticles by pyrolyzing a Ni nitrate salt. The samples were subsequently calcined at different temperatures. The composites were characterized using SEM, XRD and

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through the research group project No. RGP-VPP-227.

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