Growth of zinc cobaltate nanoparticles and nanorods on reduced graphene oxide porous networks toward high-performance supercapacitor electrodes

https://doi.org/10.1016/j.jallcom.2016.01.212Get rights and content

Highlights

  • Porous ZnCo2O4/rGO composite networks can be prepared by a hydrothermal method.

  • These networks are mainly constructed from ZnCo2O4 nanorods and rGO nanosheets.

  • The rGO nanosheets are uniformly covered by ZnCo2O4 nanoparticles.

  • The composite networks can promote capacitive performances as electrode materials.

Abstract

A type of composite network constructed from zinc cobaltate (ZnCo2O4) nanoparticles and nanorods on reduced graphene oxide (rGO) nanosheets has been prepared by a facile hydrothermal method. Transmission electron microscope results reveal that the rGO nanosheets are covered by ZnCo2O4 nanoparticles evenly due to the abundant surface functional groups on surface of original GO, and supported by some cross-linked ZnCo2O4 nanorods in the entire structures. With a rational combination, the composite networks present a meso-/macroporous architecture with a larger specific surface area than those of pristine ZnCo2O4 nanorods. As expected, the prepared ZnCo2O4/rGO electrode exhibits improved electrochemical performances, which shows a high specific capacitance (626 F g−1 at 1 A g−1), excellent rate capability (81% retention of the initial capacitance at 30 A g−1), and long-term cycling stability (99.7% retention after 3000 cycles at 10 A g−1). Such remarkable electrochemical performances of ZnCo2O4/rGO electrode can be due to the effective pathways for both electronic and ionic transport in these porous networks.

Introduction

Supercapacitors, also named electrochemical capacitors or supercapacitors, have attracted increasing attention due to their high power density, fast charge–discharge process and long cycle life [1]. Generally, supercapacitors rely on reversible Faradaic redox reactions at the electrode surface can possess higher specific capacitance than those storing charge by reversible adsorption of ions at the electrical double-layer. Thus, considerable efforts have been dedicated to develop redox-type active materials including metal oxides, such as RuO2, MnO2, NiO, Co3O4, and CuO, and related hydroxides or sulfides [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Recent studies have shown that binary cobalt-based metal oxides (MCo2O4 where M is the metal ion, such as NiCo2O4 and ZnCo2O4) have higher electrical conductivity and better electroactivity and usually show much improved electrochemical performances than the single oxides [12], [13], [14]. MCo2O4 with a spinel structure has been investigated for application in the areas of Li-ion batteries, electrocatalysts, gas sensors, etc. [15], [16], [17], [18].

Zinc cobaltate (ZnCo2O4), has sparked great interest in lithium–ion batteries and supercapacitors [18], [19], [20], [21]. Generally, ZnCo2O4 possesses a spinel structure with the bivalent Zn-ions occupying the tetrahedral sites and the trivalent Co-ions occupying the octahedral sites, usually resulting in their higher electrical conductivity and electrochemical activity. In view of these advantages, various ZnCo2O4 nanostructures as electrode materials for supercapacitors and lithium–ion batteries have been explored. For example, Huang et al. [22] reported that ZnCo2O4 rodlike nanostructure was prepared, exhibiting excellent porous structure and the highest specific capacitance of 604.52 F g−1 a current density of at 1 A g−1. Chen et al. [23] synthesized porous ZnCo2O4 nanoparticles, the ZnCo2O4 electrode exhibited a high specific capacitance of 451 F g−1 was obtained at the scan rate of 5 mV s−1. However, the reported specific capacitances of ZnCo2O4are usually far smaller than its theoretical value [24], [25], [26], [27], [28]. Then, it is highly expectable to develop a rational combination of ZnCo2O4 and carbon material with facilitated charge transport to constructed fast electron and ion transport channels and further improve their electrochemical performances.

Since its discovery, graphene oxide (GO), an analog of graphene, has opened new possibilities owing to its abundant functional groups, large specific surface area, and superior mechanical strength [29], [30]. Reduced graphene oxide (rGO) modified active materials have been extensively exploited in supercapacitors with remarkable electrochemical performance. In this work, we present the electrochemical performances of porous ZnCo2O4/rGO composite networks prepared by a hydrothermal method. Results show that the porous networks are mainly constructed from ZnCo2O4 nanorods and rGO nanosheets, and some ZnCo2O4 nanoparticles are uniformly grown on rGO. The rGO nanosheets show a high electrical conductivity and form a porous and interconnected scaffold, which serve as cross-linked ZnCo2O4 nanorods connecters and, meanwhile, enhance the electrical conductivity of ZnCo2O4. Due to the large specific surface area with meso-/macropores and high electrical conductivity, the prepared ZnCo2O4/rGO electrode shows improved electrochemical performances compared with ZnCo2O4 electrode, including high specific capacitance, excellent rate capability and cycling stability.

Section snippets

Materials

Graphene oxide (GO) was prepared from graphite powder (Aldrich, powder, <20 mm, synthetic) by a modified Hummers method, which has been reported in previous work [31]. The GO powder was dispersed in deionized water to form GO suspension (∼0.75, 1.25, 1.75 and 2.25 mg mL−1). All reagents were of analytical grade and used as received without further purification.

Porous ZnCo2O4/rGO composite networks were prepared by a hydrothermal method and post-annealing process. Typically, Zn(NO3)2·6H2O

Structures and morphologies

Fig. 1a shows the XRD pattern of as-prepared ZnCo2O4/rGO composites. It reveals that several diffraction peaks can be observed at 2θ value of 31.2°, 36.8°, 44.7°, 55.6°, 59.3°, 65.1°, and 77.2°, which could correspond to (220), (311), (400), (422), (511), (440), and (533) planes of cubic ZnCo2O4 (a = 8.0946 Å, space group: Fd−3 m, JCPDS no. 23–1390) [32], respectively. In addition, a broad and weak peak appears at 2θ value of around 25° could be indexed to the (002) plane of disorderedly

Conclusions

In summary, porous ZnCo2O4/rGO composite networks have been prepared by a hydrothermal route followed by annealing at 250 °C in air. The rGO nanosheets play a dual role in the composite networks, in which serve as ZnCo2O4 nanoparticles attached templates and cross-linked ZnCo2O4 nanorods connecters. Interestingly such composite networks can provide high surface area for electrolyte ions diffusion and many fast pathways for electron transport. Benefiting from the rational combination of two

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant. Nos. 11474135, 51202100, and U1232121), and the Fundamental Research Funds for the Central Universities (No. lzujbky-2015-110).

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