3D (Three-dimensional) sandwich-structured of ZnO (zinc oxide)/rGO (reduced graphene oxide)/ZnO for high performance supercapacitors

doi:10.1016/j.energy.2014.03.003

Energy

Volume 69, 1 May 2014, Pages 266-271

https://doi.org/10.1016/j.energy.2014.03.003 Get rights and content

Highlights

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Sandwich structure of graphene–ZnO hybrid was synthesized facilely.
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This structure prevents the agglomeration of Gr/Gr, and ZnO/ZnO.
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This material demonstrates improved electrochemical properties.

Abstract

3D (Three-dimensional) ZnO (zinc oxide)/rGO (reduced graphene oxide)/ZnO sandwich-structured were synthesized by adding zinc oxide powder to the reaction of graphitic oxide reduction and heating. In this reaction, ZnO nanorods arrays with the size of 20–40 nm grew on both surfaces of rGO sheets directly. Compared with the rGO, the as-obtained ZnO/rGO/ZnO exhibited higher surface area. The electrochemical performance of ZnO/rGO/ZnO electrodes was investigated for energy storage. The maximum specific capacitance of 275 F g⁻¹ at scan rate of 5 mV s⁻¹ by chronopotentiometry was achieved for ZnO/rGO/ZnO sandwich-structured in 1.0 M Na₂SO₄. The hybrid ZnO/rGO/ZnO sandwich-structured exhibited an excellent rate capability and excellent long-term cycling stability as compared with rGO. Such results demonstrated that this ZnO/rGO/ZnO sandwich-structured was a promising candidate of electrode material for high-performance supercapacitor.

Introduction

As an electrochemical energy storage device, supercapacitor shows great promising application in hybrid electric vehicles, electrical vehicles, portable electronic devices, and backup power due to its high power density, long cycling life, and short charging time [1], [2], [3], [4], [5], [6]. Carbon materials such as active carbon, carbon nanotubes, and mesoporous carbon materials have been extensively investigated as the electrodes for supercapacitor. Graphene has recently attracted enormous attention due to its unique 2D nanostructure, high theoretical surface area, excellent electrical conductivity, superior mechanical properties, and good electrochemical stability [7], [8]. Thus, graphene nanosheets have been considered as promising materials for their potential application in supercapacitors and other electrochemical energy storage devices [9], [10]. However, graphene nanosheets tend to form agglomerates or restack to form graphite through van der Waals interaction during graphene preparation and electrode formation processes [11]. The restacked structure would lead to a dramatic decrease of specific surface area of graphene, leading to loss of their specific capacitors when used in supercapacitor [12], [13], [14], [15], [16]. Therefore, the prevention of aggregation and restacking is of paramount significance to advance the application of graphene in these crucial areas [14], [17], [18], [19], [20], [21], [22]. Many methods have been developed to solve this problem such as using surfactant to stabilize graphene nanosheets, grafting polymer chains to change the polarity of graphene surface, and decorating graphene surface with metal or semiconductor inorganic nanoparticles to decrease the π-stacking interactions between graphene sheets. Such as Wu et al. prepared graphene/RuO₂ composites with surface areas increasing from 108 to 281 m² g⁻¹. Although the surface area of the graphene was increased by anchoring these nanostructures, the resulting specific area was still much lower than the theoretical surface area of a single graphene area (2630 m² g⁻¹) [23], [24]. It is still needs to be improved the specific surface area of graphene considering the large-area application in supercapacitors.

Recently, like other metal oxide, ZnO has been used as a potential candidate for supercapacitor application owing to its low cost, natural abundance, and environmental friendliness. However, the poor electrical conductivity of ZnO, remains a major challenge and limits rate capability for high power performance, thus hindering its wide application in an energy storage system. Hybridization of graphene with ZnO offers a powerful way to obtain high specific capacitance [25]. There are several reports on the synthesis of graphene–ZnO nanocomposites. Various methods such as hydrothermal, solvothermal, ultrasonic spray pyrolysis have been employed to obtain graphene–ZnO composites. However, enhancing the electrochemical properties of graphene–ZnO nanostructures to meet large-scale application requirements in supercapacitors is still a challenge. It is still needs to be improved synthetic method of graphene–ZnO nanostructures for high performance of supercapacitors.

In this work, we synthesized 3-dimensional sandwich structure of ZnO nanorods and rGO (reduced graphene oxide) nanocomposites during GO (graphite oxide) rapid thermal reduction. The resulting ZnO nanorods arrays are grown on between two rGO sheets. This prevents graphene sheets from restacking and providing electrolyte access to the electroactive oxide phase. Electrochemical measurements show that the as-prepared graphene oxide and ZnO (ZnO/rGO/ZnO) sandwich-structured exhibit high specific capacitance, excellent rate capability, and excellent long-term cycle stability and they are promising materials for high-performance supercapacitor.

Section snippets

Materials

Graphite powders (325 mesh, 99.995%) were purchased from Alfa Aesar. All other reagents were commercially available and analytic grade and were used directly without any purification. Double distilled water was used throughout the experiments.

Synthesis of GO, and ZnO/rGO/ZnO sandwich-structured

GO was prepared from natural graphite powder by the modified Hummers method, and GO using rapidly thermal reduction process were previously reported by our group elsewhere. Briefly, Graphite powder (1.0 g), sodium nitrate (1.2 g) and sulfuric acid (45 ml)

Morphological and structural characterizations

As shown in Fig. 1, ZnO/rGO/ZnO sandwich-structured was formed during the graphite oxide rapid thermal reduction. Fig. 2 shows the morphologies of the as-synthesized graphite oxide and ZnO-rGO nanocomposites. Fig. 2(a) is an SEM (scanning electron microscope) image of our synthesized graphite oxide sheets. The sheets are still stacking even the acid is intercalated between the graphite oxide sheets. Fig. 2(b) and (c) is the low high and high magnification SEM images of ZnO–rGO nanocomposites.

Conclusion

In summary, ZnO nanorods grown on graphene sheets were synthesized by a simple chemical vapor deposition reaction during graphite oxide rapid thermal reduction. The FESEM analysis revealed the ZnO–graphene composites were sandwich-like nanostructures. In comparison with the specific capacitance of rGO electrode, the ZnO–graphene nanocomposite show superior electrochemical performance such as high specific capacitance and good cyclic stability. These results suggest that the as-synthesized

Acknowledgments

The authors are grateful for support from the National Natural Science and Henan Province United Foundation of China (No. U1204601), Natural Science Foundation of Henan Province (No. 122300410298), Natural Science Foundation of Education Department of Henan Province (No. 13A480365), and PhD Foundation of Zhengzhou University of Light Industry (No. 2010 BSJJ 029).

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Citation Excerpt :
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3D (Three-dimensional) sandwich-structured of ZnO (zinc oxide)/rGO (reduced graphene oxide)/ZnO for high performance supercapacitors

Highlights

Abstract

Introduction

Section snippets

Materials

Synthesis of GO, and ZnO/rGO/ZnO sandwich-structured

Morphological and structural characterizations

Conclusion

Acknowledgments

Energy

J Power Sources

Energy

Energy

Energy

Electrochem Commun

Mater Chem Phys

Appl Surf Sci

J Power Sources

Carbon

Carbon

Scr Mater

J Alloy Compd

A review of electrode materials for electrochemical supercapacitors

Chem Soc Rev

A nanostructured electrochromic supercapacitor

Nano Lett