Elsevier

Composites Part B: Engineering

Volume 121, 15 July 2017, Pages 68-74
Composites Part B: Engineering

Bioinspired Co3O4/graphene layered composite films as self-supported electrodes for supercapacitors

https://doi.org/10.1016/j.compositesb.2017.03.025Get rights and content

Abstract

A bioinspired Co3O4/graphene composite film was fabricated by an electrostatic self-assembly of poly(diallyldimethylammonium chloride)-stabilized porous Co3O4 flakes and graphene oxide nanosheets under vacuum filtration-induced directional flow in combination with subsequent thermal annealing. The resulting composite film was then empolyed as a self-supported electrode for supercapacitors. The narce-like layered architecture allows for an intimate interface contact and strong interactions between the two. This ensures a large ion-accessible surface and high structural integrality of the electrode during charging-discharging cycles. The high porosity of Co3O4 is capable of affording short diffusion paths of charges and high electrochemical utilization of the electrode. Graphene sheets also construct a highly-conductive platform for fast charge transport and electrochemical reactions. Such unique bioinspired morphology and synergistic effects between porous Co3O4 flakes and graphene sheets promise excellent electrochemical performance. As expected, the free-standing electrode exhibits a specific capacitance of 623.8 F/g at a scan rate of 5 mV/s, and retains 83% of initial capacitance in the current density increasing from 1.0 to 8.0 A/g, suggesting large energy storage and high rate capabilities. The retention of initial capacitance remains 87% after 1000 cycles at 20 mV/s, indicating excellent cycling stability and reversibility.

Introduction

Supercapacitors have been considered as a promising candidate for energy storage system due to their high power density, long cyclic life, fast charging process, and friendly-environmental features [1], [2]. Based on the charge storage mechanisms, supercapacitors can be classified into electric double layer capacitors (EDLCs) and faradaic pseudo-capacitors. Over the past years, considerable efforts have mainly been made to develop novel nanostructured electrode materials for endowing such supercapacitors with large capacitance and hence high energy density, and high rate performance [3]. Generally, carbon-based nanomaterials such as graphene and nanotubes allow EDLCs with fast charge transport and thus high power density due to their high electronic conductivity. In contrast, transition metal oxides and conducting polymers are capable of providing pseudo-capacitors with high energy density due to their large intrinsic capacitance and rich redox reactions on/near the electrode surface [4].

Among transition metal oxides, Co3O4 is one of the most investigated electrode materials due to its ultrahigh theoretical specific capacitance (3560 F/g), natural abundance, environmental safety, and controllable nanostructures [5], [6], [7]. Various Co3O4 nanostructures such as Co3O4 nanoparticles [8] Co3O4 nanorods [9], Co3O4 nanoboxes [10], and Co3O4 nanowires [11], have been fabricated through particular techniques. However, it suffers from poor electronic conductivity and the limited diffusion depth of electrolytes into the interior electrode, thus resulting in the relatively low power capability and a modest specific capacitance [12]. Moreover, the breakage and even the pulverization of Co3O4-based electrodes caused by the drastic volume change during the cycling processes usually leads to the capacity decay [13]. Therefore, much attention has recently been paid to combine Co3O4 with graphene which has large specific surface area, excellent electronic conduction, and high mechanical flexibility as well as its greater specific capacitance compared to conventional carbon nanomaterials [14]. Till now, the electrodes based on Co3O4/graphene nanocomposites have been frequently reported to possess greatly-improved supercapacitor performance with respect to the Co3O4-only counterparts [15], [16].

In this work, we report on the fabrication of the bioinspired Co3O4/graphene nanocomposite films as self-supported supercapacitor electrodes. Typically, Co3O4 flakes were first mixed with a water-soluble cationic surfactant of poly(diallyldimethylammonium chloride) (PDDA) to produce the positively-charged Co3O4 flakes. Highly-ordered Co3O4/GO layered nanocomposites were then formed by an electrostatic self-assembly of PDDA-stabilized Co3O4 flakes and the negatively-charged graphene oxide (GO) sheets under vacuum filtration-induced directional flow. Bioinspired Co3O4/reduced GO (RGO) composite films were finally obtained by thermal annealing. Of note, the electrode active materials with layered morphologies have been reported to be electrochemically preferable due to their high structural tolerance to ion transportations and redox reactions [17], [18], [19]. Such a bioinspired nanostructure was herein found to intensify the synergistic effect between pseudo-capacitive Co3O4 flakes and conductive RGO sheets. More important, the as-fabricated Co3O4/RGO nanocomposite film could be served as binder-/conductive additive-free self-supported electrodes [20], which exhibit excellent electrochemical performance, for instance, high specific capacitance (623.8 F/g at a scan rate of 5 mV/s) and excellent cycling stability (retaining 87% of initial capacitance after 1000 cycles).

Section snippets

Materials

GO was prepared through the modified Hummers method [21], [22]. Urea and Co(NO3)2·6H2O were purchased from Shanghai Sinopharm Chemical Reagent Co, Ltd, PDDA and Triton X-100 were purchased from Aladdin. All chemicals were used as received without further purification.

Synthesis of Co3O4 flakes

According to the literature method [19], Triton X-100 (2 mL, 2.5 mmol) was first added into deionized water (25 mL) to form a clear aqueous solution after stirring for a while. Subsequently, Co(NO3)2·6H2O (1.45 g, 5 mmol) was added

Fabrication of bioinspired Co3O4/RGO nanocomposite films

Fig. 1 shows TEM images of GO, and Co3O4. The exfoliated GO sheets (Fig. 1a) contain a few stacking folds due to the distortions arising from high fraction of sp3 hybridized bonds, structural defects, and extremely large aspect ratio. Co3O4 was produced by thermal decomposition of the layered Co2(OH)2CO3 precursor (Fig. 1b) as systematically described by Rao and coworkers [19]. The layered 2D morphology can be well retained in the Co3O4 flake with added porosity, as shown in Fig. 1c. The

Conclusions

This work reports a bioinspired Co3O4/RGO layered composite film which was fabricated by a PDDA-assisted electrostatic self-assembly of porous Co3O4 flakes and GO nanosheets followed by thermal annealing under inert gas atmosphere. The resulting composite film was then employed as a free-standing electrode. The specific capacitance is as high as 623.8 F/g at a scan rate of 5 mV/s. The retentions of initial specific capacitance are up to 83% from 1.0 to 8.0 A/g, and 87% over 1000 cycles at

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51273057, and 51673061), the Program for New Century Excellent Talents in University (NCET-12-0709), and the Funds for Distinguished Young Scientists of Hubei Province (2015CFA048).

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