- Nano Express
- Open Access
Hierarchical Heterostructures of NiCo2O4@XMoO4 (X = Ni, Co) as an Electrode Material for High-Performance Supercapacitors
© Hu et al. 2016
- Received: 27 March 2016
- Accepted: 9 May 2016
- Published: 18 May 2016
Hierarchical heterostructures of NiCo2O4@XMoO4 (X = Ni, Co) were developed as an electrode material for supercapacitor with improved pseudocapacitive performance. Within these hierarchical heterostructures, the mesoporous NiCo2O4 nanosheet arrays directly grown on the Ni foam can not only act as an excellent pseudocapacitive material but also serve as a hierarchical scaffold for growing NiMoO4 or CoMoO4 electroactive materials (nanosheets). The electrode made of NiCo2O4@NiMoO4 presented a highest areal capacitance of 3.74 F/cm2 at 2 mA/cm2, which was much higher than the electrodes made of NiCo2O4@CoMoO4 (2.452 F/cm2) and NiCo2O4 (0.456 F/cm2), respectively. Meanwhile, the NiCo2O4@NiMoO4 electrode exhibited good rate capability. It suggested the potential of the hierarchical heterostructures of NiCo2O4@CoMoO4 as an electrode material in supercapacitors.
- Nanosheet arrays
To meet the increasing requirement for portable electronics, hybrid electronic vehicles and other micro- and nanodevices, numerous studies have been carried out to develop many kinds of energy storage systems. As an important energy storage device, the widely studied supercapacitors, also known as electrochemical capacitors, have been believed as a promising candidate due to their high specific power, long cycling life, fast charge and discharge rates, and reliable safety [1–9]. Though these supercapacitors demonstrated these distinctive advantages, as compared with the batteries and fuel cells, the relatively lower energy densities seriously block their large-scale practical application [4, 10]. So far, various electrode materials which include carbon materials [11, 12], transition metal oxides [2, 13–15], and conducting polymers [16, 17] have been designed and synthesized to enhance the electrochemical properties for the practical applications in the supercapacitors.
Recently, some bimetallic oxides, such as NiCo2O4 [15, 17–20], ZnCo2O4 [21, 22], NiMoO4 , and CoMoO4 [24, 25], have been developed as a new electrode material used for supercapacitors because of their excellent electrical conductivity and multiple oxidation states (as compared with the binary metal oxides) for reversible Faradaic reactions . For fully utilizing the advantages of active materials and thus optimizing the performance of these materials, plenty of efforts has been devoted, i.e., realizing additive/binder-free electrode architectures, which eliminate the “dead surface” and release complicated process in traditional slurry-coating electrode and meaningfully improve the utilization rate of electrode materials even at high rates [4, 27], constructing 3D hierarchical heterostructures, which can provide efficient and fast pathways for electron and ion transport [20, 28], and exploring smart integrated array architectures with rational multi-component combination, which can achieve the synergistic effect from all individual constituents [29–31]. Taken some successful examples, CoxNi1 − xDHs/NiCo2O4/CFP composite electrodes were prepared by a hydrothermal route and an electrodeposition process, showing high capacitance of ∼1.64 F/cm2 at 2 mA/cm2, good rate capability, and excellent cycling stability ; 3D hierarchical NiCo2O4@NiMoO4 core-shell nanowire/nanosheet arrays delivered a high areal capacitance of 5.80 F/cm2 at 10 mA/cm2, excellent rate capability, and high cycling stability . Despite these notable achievements, it is still a hard task to design and construct 3D hierarchical heterostructures made of the bimetallic oxides with improved electrochemical properties for the supercapacitors.
Herein, we report hydrothermal growth of hierarchical heterostructures of NiCo2O4@XMoO4 (X = Ni, Co) as an electrode material for the supercapacitors with improved performances. Within these hierarchical heterostructures, high electrochemical activity of NiCo2O4 not only shows outstanding pseudocapacity but also can be regarded as a backbone to provide reliable electrical connection to the XMoO4 (X = Ni, Co). Between them, the NiCo2O4@NiMoO4 electrode material showed a highest areal capacitance of 3.74 F/cm2 at 2 mA/cm2, which was much higher than the NiCo2O4@CoMoO4 electrode material (2.452 F/cm2), and good rate capability, implying its prospect as an alternative electrode material in the supercapacitors.
Synthesis of NiCo2O4@XMoO4 (X = Ni, Co) Composite Nanosheet Arrays
All the reactants here were analytically graded and used without further purification. The synthesis of the composite nanosheet arrays was described briefly as follows: Firstly, the NiCo2O4 nanosheet arrays were grown on the Ni foam according to a reference . Secondly, the product of as-grown NiCo2O4 nanosheet arrays was put into a 60-mL Teflon-lined autoclave, which contained 0.5 mmol of NiCl2·6H2O (or CoCl2·6H2O), 0.5 mmol of Na2MoO4·2H2O, and 50 mL of deionized water. The autoclave was sealed and maintained at 120 °C for 2 h (or 1 h) in an electric oven and then cooled down to room temperature. The NiCo2O4@XMoO4 (X = Ni, Co) composites on the Ni foam were carefully washed with deionized water and absolute ethanol, successively, and then dried at 60 °C overnight. Lastly, the samples were annealed at 400 °C for 1 h at a ramping rate of 1 °C/min.
As-synthesized products were characterized by means of a D/max-2550 PC X-ray diffractometer (XRD; Rigaku, Cu-Kα radiation), a scanning electron microscopy (SEM; S-4800), and a transmission electron microscopy (TEM; JEM-2100 F) equipped with an energy-dispersive X-ray spectrometer (EDX).
In conclusion, 3D hierarchical heterostructures of the NiCo2O4@XMoO4 (X = Ni, Co) composite nanosheet arrays have been successfully designed and prepared for the supercapacitors. In such a novel nanostructure, the mesoporous NiCo2O4 nanosheet arrays grown directly on the Ni foam not only acted as a good pseudocapacitive material but also used as a hierarchical framework for loading NiMoO4 or CoMoO4 electroactive material. Notably, the NiCo2O4@NiMoO4 composite electrode showed excellent rate capability as well as a highest areal capacitance of 3.74 F/cm2 at 2 mA/cm2, which was much higher than the values for the NiCo2O4@CoMoO4 electrode (2.452 F/cm2) and NiCo2O4 electrode (0.456 F/cm2). The total capacitance retention of the NiCo2O4@CoMoO4 and NiCo2O4@NiMoO4 electrodes after 2000 cycles is ~95.5 and ~83.1 %, respectively. Based on these electrochemical properties, the NiCo2O4@NiMoO4 composite electrode material may be more appropriate for practical applications.
We gratefully thank the Institute of Functional Nano & Soft Materials (FUNSOM) for supporting our work.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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