Hydrangea-like NiMoO4-Ag/rGO as Battery-type electrode for hybrid supercapacitors with superior stability

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Abstract

It is a great challenge to design electrode materials with good stability and high specific capacitance for supercapacitors. Herein, a three-dimensional (3D) hydrangea-like NiMoO4 micro-architecture with Ag nanoparticles anchored on the surface has been designed by adding EDTA-2Na, which was assembled with reduced graphene oxide (rGO) and named as NiMoO4-Ag/rGO composite. Benefiting from the synergetic contributions of structural and componential properties, NiMoO4-Ag/rGO composite exhibits a high specific capacitance of 566.4 C g−1 at 1 A g−1, and great cycling performance with 90.5% capacitance retention after 1000 cycles at 10 A g−1. The NiMoO4-Ag/rGO electrode shows an enhanced cycling stability due to the two-dimensional towards two-dimensional (2D-2D) interface coupling between rGO and NiMoO4 nanosheets, and the stable 3D hydrangea-like micro-architecture. Moreover, NiMoO4-Ag/rGO with 5–15 nm pore structure and enhanced conductivity exhibits improved charge transfer and ions diffusion. Besides, NiMoO4-Ag/rGO//AC capacitor displays an outstanding energy density of 40.98 Wh kg−1 at 800 kW kg−1, and an excellent cycling performance with 73.3% capacitance retention at 10 A g−1 after 8000 cycles. The synthesis of NiMoO4-Ag/rGO composite can provide an effective strategy to solve the poor electrochemical stability and slow electron/ion transfer of NiMoO4 material as supercapacitors electrode.

Introduction

NiMoO4, a typical low-cost and environmentally friendly spinel, presents a high reversible capacity and electrochemical activity because of its feasible oxidation state[1]. Besides, the inverse spinel structure of NiMoO4 can provide ion channels and achieve effective charge storage as electrode materials for supercapacitors[2], [3], [4]. However, NiMoO4 materials face two difficult problems of low conductivity and poor cycling stability, which are associated with slow charge/ion transfer, as well as an excessive volume change after the process of electrochemical cycles[5], [6]. They lead to that NiMoO4 materials show an essentially low dynamics of electrode reactions in supercapacitors, and their specific capacitance are much lower than the theoretical value (≥3000 F g−1)[7], such as reported NiMoO4 nanorods[8], porous worm-like NiMoO4[9] and NiMoO4 nanosheets[10], which exhibited 594, 1088.5 and 1221.2 F g−1 at 1 A g−1, respectively. Moreover, the breakdown of NiMoO4 structure may result in poor cycle life[11]. For example, the mesoporous NiMoO4 nanorod/reduced graphene oxide composites[12] and mesoporous β-NiMoO4 nanorods[13] maintain 81.1% and 80.2% of its initial specific capacitance.

It is noteworthy that combining with carbon materials can improve conductivity and stability of NiMoO4, such as NiMoO4/active carbon[14], NiMoO4/hollow spheres of carbon[15] and NiMoO4/graphene[16], et al. The reported composite of NiMoO4 and carbon material exhibited good rate performance, high specific capacitance and long cycle life, which were contributed to the enhanced electrical conductivity and stability. Among these carbon materials, reduced graphene oxide (rGO) is a popular and promising nanocarrier because of its large specific surface area, long-range π-π conjugated bonds, and excellent electro-response and mechanical properties [17], [18]. For example, Rastabi[16] et al. prepared the 3D NiMoO4/rGO composite by using a co-precipitation method, in which the oxygen-containing functional groups on graphene oxide have been removed by ammonia solution. The electrochemical results of the 3D NiMoO4/rGO composite showed that its specific capacitance can reach 932 F g−1. However, the improvement in specific capacitance of NiMoO4/rGO composite is limited when compared with that of NiMoO4. According to previous reports, combining NiMoO4 and metal with good conductivity is an effective strategy to improve the performance of NiMoO4 for supercapacitors [19], [20], [21], while Ag is a good candidate. Especially, the Ag nanoparticles have large active surface areas and can enhance charge transfer during electrochemical testing.

Based on this, a study is designed to construct NiMoO4-Ag/rGO composite by a facile two-step hydrothermal process. It can be seen from the morphological characterization that rGO and NiMoO4 form a hydrangea-like structure with lots of stacking layers, and the Ag nanoclusters are evenly attached on the surface of rGO and NiMoO4. The NiMoO4-Ag/rGO composite shows an excellent electrochemical performance and its specific capacitance (566.4 C g−1) is more than 2 times than that of NiMoO4 (212.9 C g−1) or NiMoO4-Ag (378.6 C g−1) electrode material at 1 A g−1. Moreover, the NiMoO4-Ag/rGO composite displays an excellent cycle stability, which can retain 90.5% of initial capacitance after 1000 cycles at 10 A g−1. Furthermore, the assembled NiMoO4-Ag/rGO//AC capacitor exhibits a high energy density of 40.98 Wh kg−1 at 800 W kg−1. Importantly, the hybrid capacitor still shows 73.3% of capacitance retention after 8000 cycles at 10 A g−1, which exhibits an excellent cyclic stability. When compared with NiMoO4[22], CoMoO4-NiMoO4[23] and CoMoO4-NiMoO4·xH2O[24] electrode materials for the application of supercapacitors, our NiMoO4-Ag/rGO composite shows better electrochemical performance. It can be contributed to the combination of NiMoO4 flakes and rGO sheets that greatly improve active sites for electrochemical redox reaction. Moreover, the introduction of Ag nanoparticles can effectively accelerate the charge transfer and enhance the specific capacitance of composite. Moreover, due to the 2D-2D interface coupling formed by rGO and NiMoO4 nanosheets, as well as the stable 3D hydrangea-like micro-architecture, the composite shows an enhanced cycling stability. Therefore, this work provides an effective strategy to design and construct stable electrode materials with high electrochemical activity for supercapacitors.

Section snippets

Materials

Ni(NO3)2·6H2O, Na2MoO4·H2O, AgNO3, Disodium ethylenediaminetetraacetic acid (EDTA-2Na), active carbon (AC), KOH and polytetrafluoroethylene (PTFE) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). All reagents and solvents were of analytical grade and used as received.

Preparation

Graphite oxide (GO) was made of scaly graphite as raw material, and was prepared by modified Hummers method[25].

Microstructure characterizations

The preparation process of 3D hydrangea-like NiMoO4-Ag/rGO composite is illustrated in Fig. 1. Firstly, Ni2+ and MoO42- were controlled by EDTA2- to generate flake-like NiMoO4. NiMoO4 and GO sheets were assembled into a stable 3D hydrangea-like NiMoO4/rGO. Then, the Ag nanoparticles were anchored to the surface of NiMoO4/rGO and formed NiMoO4-Ag/rGO. The morphology and microstructure of NiMoO4-Ag/rGO composite were observed by FESEM. As shown in Fig. 2a and Fig. S1, NiMoO4 presents a 3D

Conclusion

In summary, a facile two-step hydrothermal method was utilized to prepare the unique 3D hydrangea-like NiMoO4-Ag/rGO composite which is applied as the high electrochemical activity and stability electrode materials for supercapacitors. 3D hydrangea-like micro-architecture and 2D-2D interface coupling formed by rGO and NiMoO4 nanosheets enhances the cycling stability of composite. Moreover, NiMoO4 flakes, rGO sheets and Ag nanoparticles can greatly improve the electrochemical activity of the

CRediT authorship contribution statement

Bingji Huang: Conceptualization, Methodology, Visualization, Data curation, Writing–original draft, Writing–review & editing, Investigation. Dachuan Yao: Conceptualization, Methodology, Visualization, Data curation. Jingjing Yuan: Data curation, Writing–original draft, Writing–review & editing. Yingrui Tao: Visualization, Data curation, Investigation. Yixuan Yin: Resources, Validation. Guangyu He: Conceptualization, Resources, Supervision. Haiqun Chen: Conceptualization, Resources, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful for the financial support from National Natural Science Foundation of China (grant number 22078028, 21978026), Changzhou Key Laboratory of Graphene-Based Materials for Environment and Safety (grant number CM20153006, CE20185043), PAPD of Jiangsu Higher Education Institution, and Postgraduate Research & Practice Innovation Program of Jiangsu Province (grant number SJCX20-0954, KYCX20-2564).

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