Elsevier

Journal of Power Sources

Volume 242, 15 November 2013, Pages 148-156
Journal of Power Sources

A novel asymmetric supercapacitor with an activated carbon cathode and a reduced graphene oxide–cobalt oxide nanocomposite anode

https://doi.org/10.1016/j.jpowsour.2013.05.081Get rights and content

Highlights

  • Well-dispersed Co3O4–rGO nanocomposite is prepared.

  • An asymmetric supercapacitor is developed using Co3O4–rGO anode and carbon cathode.

  • The asymmetric supercapacitor shows a high level of energy and power.

Abstract

Co3O4–reduced graphene oxide (rGO) nanocomposites are prepared by co-precipitation of Co(OH)2 with graphite oxide (GO) to form a Co(OH)2–GO precursor, followed by thermal treatment. It is found that the ratio between Co3O4 and rGO has a significant effect on their electrochemical activities. The specific capacitance of 636 F g−1 is achieved when the mass ratio of Co3O4 to rGO is equal to 80.3:19.7. A novel asymmetric supercapacitor is further fabricated with the Co3O4–rGO nanocomposite as the anode and activated carbon as the cathode in 6 M aqueous KOH solution as electrolyte. The assembled asymmetric supercapacitor can cycle reversibly in a voltage of 0–1.5 V and exhibits a high energy density of 35.7 Wh kg−1 at power density of 225 W kg−1. Moreover, the asymmetric supercapacitor shows an excellent cycling stability with capacitance retention of 95% after 1000 cycles at a current density of 0.625 A g−1.

Introduction

In recent years, the increasing power demands from electric vehicles and power tools have inspired a surge of exploitation for highly effective electrical energy storage devices. As promising energy storage devices, supercapacitors are exhibiting attractive characteristics such as high power density, low equivalent series resistance and long cycle life [1], [2], [3]. Depend on the charge storage mechanisms, supercapacitors can be classified into two different categories: (1) electrical double-layer capacitors (EDLCs), the electrical charge of which is stored at the interface between the electrode and the electrolyte, and (2) redox electrochemical capacitors, in which capacitance arises from reversible Faradaic reactions taking place at the electrode/electrolyte interface [4]. The electrical double layer capacitors, usually based on porous carbon as electrode material, have been commercially applied due to their stable physicochemical properties, good conductivity, low cost and long cycling life [5], but lower specific capacitance and energy density limit its application fields. On the other side, pseudocapacitors can provide high specific capacitance which is often several times higher than that of EDLCs because of the higher levels of charge storage from redox reactions [1], [6], [7], however, only low energy density can be obtained due to the limited cell operating voltage (<1.0 V) [8], [9].

To solve this problem, asymmetric systems have been extensively explored by combining a battery like Faradic electrode (as energy source) and a capacitive electrode (as power source) to increase the operation voltage, which leads to a notable improvement of the energy density [10], [11], [12]. Currently, activated carbon with high surface area and relatively good electrical conductivity is widely used as the cathode material for asymmetric supercapacitors [13], [14], [15], [16]. As for the anode material, transition metal oxides such as MnO2 [17], [18], NiO [19], SnO2 [20] and Co3O4 [21], [22] are a group of very promising materials for energy storage to replace expensive and toxic RuO2 [7], [23]. In particular, Co3O4 is of special interest due to its high theoretical capacitance (up to 3560 F g−1), and well-defined redox behavior [24], [25], [26]. However, Co3O4 usually delivers poor rate capability and reversibility during the charge/discharge process due to its relatively poor electrical conductivity and large crystallite size [27]. Therefore, to reduce the crystal size of Co3O4 and increase the electrical conductivity of the whole electrode is highly desired. Efforts have been expended to increase the electrical conductivity by dispersing them into carbonaceous materials such as activated carbon, carbon nanotubes and graphene. Among them, graphene, a two-dimensional carbon material, is a palpable choice owing to its excellent electronic conductivity and high theoretical surface area (up to 2630 m2 g−1) [28], [29], which not only improve the conductivity, but also suppress the crystal size. However, due to the irreversible agglomeration phenomena during fabrication of most graphene-based composites, which are prepared by reduction of graphene oxide followed with loading of pseudocapacitive nanomaterials, the excellent electric and surface properties of graphene sheets are not completely revealed. Therefore, seeking the effective strategies to synthesize well-dispersed graphene-based composites remains important.

Graphite oxide is a typical derivative of graphene with quasi-2D structure decorated by oxygen functionalities, which can act as nucleation centers to anchor active materials onto GO sheets. Thus it is feasible to obtain well-dispersed rGO-based nanocomposites [30], [31]. Recently, Co3O4–rGO nanocomposites have already been prepared through a two-step surfactant assisted method [32] and extensively studied for application in Li-ion batteries [33], [34], showing improved charge storage capacity and cycling performance. However, to the best of our knowledge, no reports have been found on the fabrication of asymmetric supercapacitors with Co3O4–rGO nanocomposite materials. In this work, the Co3O4–rGO nanocomposites are prepared by a facile co-precipitation approach in combination with subsequent calcination process, which were further employed as the anode materials for asymmetric supercapacitor with activated carbon as the corresponding cathode. A high cell voltage of 1.5 V is achieved in the aqueous electrolyte due to the synergetic effects, so as to result in high energy and power density as well as excellent cycling stability. The effect of rGO on the structural and electrochemical performance of Co3O4–rGO nanocomposites has also been systematically investigated.

Section snippets

Preparation of Co3O4–rGO nanocomposites and activated carbon

Graphite oxide (GO) was synthesized from graphite powder by modified Hummers method [35], [36]. GO was redispersed in distilled water for further use. Co3O4–rGO nanocomposites were prepared by a co-precipitation approach in combination with subsequent calcination process. Typically, 100 ml of the GO dispersion (0.69 mg ml−1) was first sonicated for 1 h. Then 10 ml of CoCl2 (0.5 M) was dropped into above suspension and stirred for 50 min. Subsequently, the aqueous solution of NH3·H2O (25–28%)

Material characterization

The chemical composition of the GO, rGO and rGO-1050 is presented in Table 1. After thermal expansion of GO under an argon atmosphere at 250 °C, the oxygen content of 49.634 wt.% for GO significantly decreases to 22.164 wt.% for rGO. As the annealing temperature up to 1050 °C under a hydrogen atmosphere, oxygen on the surface of rGO-1050 is progressively removed with a final value of only 0.489 wt.%, indicating a high degree of carbonization.

TG analysis is conducted in air atmosphere to

Conclusions

Co3O4–rGO nanocomposites were successfully prepared by a co-precipitation approach in combination with subsequent thermal treatment. An asymmetrical supercapacitor based on 80.3%Co3O4–19.7%rGO nanocomposite as the anode and activated carbon as the cathode was assembled with 6 M KOH electrolyte as the electrolyte. The as-fabricated asymmetric supercapacitor device possesses a potential window as high as 1.5 V and exhibits a maximum specific capacitance of 114.1 F g−1 at a current density of

Acknowledgment

We gratefully acknowledge the support of this work by International Cooperation Project of the Ministry of Science and Technology (No. 2010DFB90690-4), International Cooperation Project of Shanxi Province (No. 2010081031-2), CAS Innovative Foundation (No. Y2SC831791), National Nature Science Foundation of China (No. 51002166, 51061130536, 51172251) and Shanxi Province Science Foundation for Youths (No. 2010021023-3).

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