Construction of zeolitic imidazolate frameworks-derived NixCo3−xO4/reduced graphene oxides/Ni foam for enhanced energy storage performance

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Abstract

Direct assembling 2D zeolitic imidazolate frameworks-derived (ZIFs)-derived Ni-Co oxide on Ni foam (NixCo3−xO4/Ni foam) is a very attractive way to obtain high performance for electrochemical energy storage device. In this work, the highly optimized NixCo3−xO4/Ni foam was prepared via facile co-precipitation, ion-exchange method and subsequently composited with rGO, acting as binder-less electrode for high properties hybrid supercapacitors. As electrode materials, the pure Co3O4/Ni foam shows 94.40 C g−1 (209.78 F g−1) at 1 A g−1, while the highly optimized NixCo3−xO4/Ni foam exhibits a high capacity of 870.3 C g−1 (1934F g−1) at 1 A g−1. Compared with NixCo3−xO4/Ni foam, the reduced graphene oxides (rGO) modified NixCo3−xO4/Ni foam electrodes possess a porous conductive structure, displaying more superior capacity of 1440.99 C g−1 (3202.22F g−1) at 1 A g−1 and a higher cycling stability of 76.10% retention value after 1000 cycles. Subsequently, the NixCo3−xO4/rGO/Ni foam electrodes were assembled as a hybrid supercapacitor, exhibiting a maximum energy density of 36.31 Wh kg−1, a maximum power density of 8000 W kg−1 and a good cycling stability of 76.29% retention value after 3000 cycles. The NixCo3−xO4/rGO/Ni foam also could light the blue LED device. Thereby, it is proved that NixCo3−xO4/rGO/Ni foam is a promising materials for hybrid supercapacitors.

Introduction

Recently, Metal-organic frameworks (MOFs) derived nanomaterials have attracted extensive attention for its application as an electrode for energy storage [1], sensing [2], catalysis [3] and so forth. As a type of porous crystalline materials, MOFs formed with metal ions and organic ligands has been extensively explored as an ideal template material to direct prepare carbon or metal oxides by virtue of their tunable pore size and large specific surface area [4], [5]. According to previous report, as sacrificial template, some MOFs with novel structures have been synthesized to porous transition metal oxides, expecting to improve performance in their applications fileds [6], [7]. For instance, Zeolitic imidazolate frameworks (ZIFs), a species of novel MOFs, are well suitable to be used as the precursor to obtain ZIFs-derived metal oxides owing to high chemical and thermal stability, as well as, mild reaction conditions [8]. Specifically, ZIFs are composed of tetrahedrally coordinated metal ions (Co ions or Zn ions) and imidazolate ligands, having countless active sites on their surface [9]. Thanks to the large specific surface area and countless active sites, the electron transfer efficiency in electrochemical reaction can be enhanced, which is beneficial in improving performance for electrochemical capacitors [10].

With the development of the ZIFs derived metal oxides, the developing prospects in application of ZIFs derived metal oxides in the field of electrochemical capacitors has been in the spotlight. Benefiting from the high theoretical capacitance value of 3560F g−1, Co3O4 shows more superior in energy storage compared with ZnO [11]. In addition, ZnO is very liable to corrosion in the alkaline electrolyte, which also limits its application in alkaline solution [12]. Therefore, a lot of effort was put into developing facile and efficient preparation strategies for Co3O4 based on cobalt-rich ZIF-67 with high performance in electrochemical capacitors. For example, Zhang et al. synthesized hollow Co3O4 using ZIF-67 as template for supercapacitor with the high capacitance of 1100F g−1 under the condition of 1.25 A g−1 [13]. Although excellent electrochemical performance was achieved, the similar powdery Co3O4 usually suffers from aggregation due to the surface energy, impacting the performance. Yang et al. provided a facile binder-free strategy to fabricate Ni foam/Co3O4 electrode through ZIFs [14]. The obtained Co3O4 possesses 15–25 nm average size, exhibiting a good electrochemical performance of 1680F g−1 at 0.5 A g−1. The aggregation of power can be significantly decreased by this method, but the bad electrical conductivity of Co3O4 still affects the performance. To improve electrical conductivity of Co3O4, Guan et al. prepared carbon cloth supported NiCo2O4 nanowall arrays with hollow and porous structure for flexible supercapacitor by two-step reactions [15]. In their work, Ni ion was brought into Co3O4 to form of NiCo2O4 via ion-exchange and etching process, achieving a good specific capacitance (1055.3 F g−1) at a small current (2.5 mA cm−2) and expressing good power density and energy density. Despite good properties have been acquired, there remains a lack of details on the influence of different proportion of Ni ion and Co ion in the mixed metal oxides. Properly Ni-Co proportion optimization could increase the electrochemical performance. Besides, the introduced carbon nanomaterials, such as carbon nanotubes [16]and graphene [17], [18], [19], [20] can further enhance the conductivity of NiCo2O4, and thereby increasing their energy storage properties. Especially, Graphene has been widely used in optoelectronic and supercapacitor fields due to its excellent electronic properties [21], [22], [23], [24]. In our previous report, NixCo3−xO4/CNTs nanocomposites were fabricated via directly growth of ZIF-67 on CNTs and this electrode presents a capacitance of 668F g−1 at 1 A g−1 [25]. The unsatisfactory properties may result from improper proportion of Co ion and Ni ion.

Herein, we fabricated a hybrid supercapacitor of optimal NixCo3−xO4 and reduced graphene grow on Ni foam (denoted as NixCo3−xO4/rGO/Ni foam) through facile and effective solution route. It is achieved by direct growing Co-ZIF on Ni foam in the water solution, introducing Ni ion into Co-ZIF using ion-exchange and etching method, annealing Ni-Co precursor process, and then fabricating rGO on NixCo3−xO4/Ni foam by hydrothermal reaction. Benefiting from the well-designed structure, the NixCo3−xO4/rGO/Ni foam displays excellent electrochemical energy storage properties compared with the NixCo3−xO4/Ni foam electrode.

Section snippets

Reagent and materials

The porous Ni foams with thickness of 1.6 mm, hole number of 80–110 pores per inch and average hole diameters of 25 nm were received from Shenzhen Kejing Star Technology Co., LTD. The GO were obtained from Chengdu Organic Chemicals Co. Ltd. Co(NO3)2·6H2O, Ni(NO3)2·6H2O used in this work supplied by Sinopharm Chemical Reagent Co. Ltd. 2-Methylimidazole were acquired from Aladdin Industrial Co. Ltd. In this work, all of the chemical reagent applied in the experiment only analytical grade and

Characterization of NixCo3−xO4/rGO/Ni foam

The XRD analysis of the NixCo3−xO4/rGO/Ni foam is displayed in Fig. 1. The sample exhibits three intense diffraction peaks at around 45.29°, 52.49° and 77.13°, which can be attributed to the standard of spinel Ni (JCPDS 00-001-1266). Except for the three peaks obtained from Ni foam, two other weaker diffraction peaks around 38.55° and 82.3° can be indexed to (2 2 2) and (4 4 4) planes of NiCo2O4 (JCPDS 01-073-1702), respectively. Besides, the other two diffraction peaks of NiCo2O4 coincide with

Conclusions

In summary, the highly optimized NixCo3−xO4 are successfully sandwiched between the rGO and Ni foam with 2D Co-ZIF as template. The preparation procedure could be divided into four steps of co-precipitation, ion exchange, hydrothermal and thermal treatment method. When test as binder-free hybrid supercapacitor, the as-prepared sample exhibits an excellent specific capacity, satisfying cycling stability and rate capability. Besides, an ASC was assembled on the basis of NixCo3−xO4/rGO/Ni foam,

Declaration of Competing Interest

The authors declared that there is no conflict of interest.

Acknowledgement

This work has been supported by the National Natural Science Foundation of China (Grant No. 51521061).

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