Elsevier

Electrochimica Acta

Volume 283, 1 September 2018, Pages 190-196
Electrochimica Acta

General self-template synthesis of transition-metal oxide microspheres and their excellent charge storage properties

https://doi.org/10.1016/j.electacta.2018.06.157Get rights and content

Abstract

Transition metal oxides are promising electrode materials for electrochemical energy storage and conversion to replace the precious metal-based electrode materials. Herein, we demonstrate a universal method for making flower-like transition metal oxides (NiO, Co3O4, NiCo2O4) microspheres as excellent electrode materials for electrochemical capacitors. These as-obtained flower-like microspheres organized from a large number of ultrathin nanosheets, which can provide high surface area and abundant mesoporous structures. Furthermore, the as-fabricated NiO, Co3O4 and NiCo2O4 microspheres exhibit high specific capacities of 1210.6, 802.5 and 842.5 F g−1 at a current density of 0.5 A g−1. Those microspheres also show good cyclability with capacities retention of 94.7%, 87% and 88.6% after 3000 cycles. The general self-template method holds great potential for the fabrication of high-performance charge storage devices.

Introduction

Many efforts have been devoted to studying the energy storage devices due to the increasing future energy use and consequent environmental impacts. Supercapacitors, one of the energy storage devices, have attracted worldwide attention because of their unique characteristics of high power density, fast charge/discharge process, long cycle life and excellent reversibility [1,2], as well as their potential applications in many fields such as mobile electronics, power resources, transportation and aerospace systems [3,4]. To achieve the high-performance charge storage properties, considerable interests have been taken in electrode materials for electrochemical capacitors using transition metal oxides including NiO, Co3O4 and NiCo2O4 [[5], [6], [7]]. These metal oxides can be considered as the most promising electrode materials for charge storage and power delivery owing to their low cost, environmental friendly and high theoretical capacitance [1,8]. However, the preparation of high specific and good stability electrode materials is still a challenge. To solve these problems, many research have been committed to design the nanostructure and morphology of the materials, such as nanoparticles (Co/CNTs) [9], nanotubes (NiCo2O4) [10], nanofibers (carbon-doped Co3O4) [11] and nanosheets (CoS1.097/N-doped C, CoOx) [12,13].

Nanosheets materials are considered as the new popular materials with admirable electronic properties for their high specific surface area and energy storage [14], which plays great important on electrochemical performance by facilitating the high transport rates for both electrons and electrolyte ions. For example, Du et al. suggested that the NiO nanosheet in microspheres show high specific capacitance (762 F g−1 at 1 A g−1) due to their high surface area and porosity [15]. Liu et al. have synthesized NiCo2O4@NiCo2O4 core/shell nanoflake arrays, which exhibit high areal specific capacitances of 1.55 F cm−2 (787 F g−1) at 2 mA cm−2 (1 A g−1), mainly attributed to their unique core/shell and nanosheet structure [16]. The porosities can also shorten ion/electron diffusion distance and accelerate the kinetic process [[17], [18], [19]]. Liao et al. have prepared hierarchical Co3O4 microspheres, which display a specific capacitance of 483.8 F g−1 at 1 A g−1 and good cycling stability (10.5% lose after 2000 cycles) because of their large surface area and unique hierarchical porous structures [20]. Therefore, it is necessary to develop ultrathin high specific and good stability ultrathin metal oxide electrode materials for the purpose of satisfying the advanced energy storage devices.

In the past few years, extensive research has focused on hollow and porous nanostructures for charge storage, such as porous NiO nanoflake arrays [21], starfish-shaped porous Co3O4/ZnFe2O4 hollow nanocomposite [22] and hollow NiCo2O4 submicrospheres [23]. Unfortunately, a large number of the preparation processes are complex and poisonous. Most of the research only consider the method for individual metal oxide which is not suitable for others. Therefore, universality of preparation becomes increasingly vital to the basic research and application of materials. For example, the research of combination of cellulose nanocrystal aerogels for supercapacitor with enhanced performance is made by Yang et al. [24]. Moreover, Wang et al. demonstrates general synthesis of transition metal oxide (NiO, Co3O4, NiCo2O4) nanotubes with enhanced oxygen evaluation reaction (OER) performances [25]. Thus, it is necessary yet very challenging to seek efficient, clean and sustainable preparation methods for fabricating a variety of materials with enhanced energy storage properties using a universal method.

Herein, we report a universal and general self-template method to synthesize transition metal oxides (NiO, Co3O4 and NiCo2O4) microspheres organized from a large number of ultrathin nanosheets via a facile and universal hydrothermal method. This kind of microspheres exhibit excellent charge storage performances. The universality of the approach is a great promotion to the development of advanced electrode materials for high-power battery-type devices [26,27].

Section snippets

Synthesis of NiO microspheres

Firstly, 0.3993 g L-aspartic was added into 15 mL deionized water. Next, 3 mL NaOH (2 M) aqueous solution was slowly added to the previous solution under stirring. Finally, 18 mL glycol (EG) and 0.872 g nickel nitrate hexahydrate (Ni(NO3)2·6H2O) were mixed to the solution with vigorous stirring until the solution turn blue. Subsequently, the transparent blue solution was transferred to 50 mL Teflon-lined autoclave and maintained at 180 °C for 16 h. The obtained precipitates were collected and

Synthetic process

The formation process of NiO, Co3O4 and NiCo2O4 microspheres is more similar to a self-template mechanism [25,28,29], as illustrated in Fig. 1. Take NiO as an example, Ni2+ ions will initially connect with the L-aspartic molecules (Ni(II)-Asp) under the coordination characteristics of transition metal ions [30] to form Ni(II)-Asp complexes which will be fabricated as precursors aggregating to form a core center [25,31,32]. These nucleation centres that allow the subsequent adsorption of Ni(II)

Conclusions

In summary, a universal and general self-template method has been developed to fabricate flower-like transition metal oxides (NiO, Co3O4, NiCo2O4) microspheres. Those transition metal oxides exhibit excellent charge storage properties, which can be ascribed to the large number of ultrathin nanosheets around the surface of microspheres. This unique structures can provide high surface area and abundant mesoporous. This work provides a universal and general method for fabricating metal oxides

Acknowledgements

This work was supported by the Excellence Foundation of BUAA for PhD Students under Grant no. 2017008 and the National Natural Science Foundation of China under Grant nos. 51671010 and 51101007.

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