One-dimension MnCo2O4 nanowire arrays for electrochemical energy storage
Graphical abstract
Introduction
Under the circumstance of energy and resources crisis, one of the most serious problems need to be solved is to search for high-power, low-cost and environmentally friendly energy storage devices. Among varies energy storage devices, supercapacitors and Li-ion batteries (LIBs) are most widely used [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Metal oxide nanostructures are promising electrode materials for lithium ion batteries and supercapacitors because of their high specific capacity/capacitance, typically 2–3 times higher than that of the carbon/graphite based materials [17].
Nowadays, many binary transition-metal oxides with spinel structure, such as NiFe2O4 [18], CuCo2O4 [19], MgCo2O4 [20], ZnCo2O4 [21], [22], NiCo2O4 [23], [24], [25], [26], [27], ZnMn2O4 [28], [29], and CoMn2O4 [30], etc, have been investigated as electrodes for supercapacitors and LIBs because of their low cost, environmentally friendly property and excellent electrochemical performances. Among them, there are few works about MnCo2O4 [31]. P. Lavela prepared the MnxCo3-xO4 spinel oxide series (x= 1, 1.5, 2, 3) used as anode materials for LIBs, capacities of higher than 400 mAh g−1 was sustained after 50 cycles [32]. The MnCo2O4 quasi-hollow microsphere anode materials for LIBs possessed reversible capacity of 755 mAh g−1 at a current density of 200 mA g−1 after 25 cycles [33]. However, most of the researches of MnCo2O4 focus on powder materials or compact films; there still exist several drawbacks, such as relatively low conductivity, low utilization of active materials and inferior structure stability.
One-dimension nanowire arrays have attracted intense attention for their excellent physical and chemical properties and are promising electrode materials for both supercapacitors and LIBs. The nanowire array architecture, just as the sphere-like particle, is one of the architectures which can provide large specific area, resulting in high utilization of active materials. And the nanowire array architecture can also alleviative the stress caused by volume changes during electrochemical processes. In a previous work [34], single-crystalline Co3O4 nanowire arrays grown on nickel foam were prepared by hydrothermal synthesis method for supercapacitor application, which exhibited noticeable pseudocapacitive performance with a high capacitance of 754 F g−1 at 2 A g−1 and 610 F g−1 at 40 A g−1, as well as excellent cycling performance. Besides, Co(OH)2 [35], MnO2 [36], LiMn2O4 [37], CoO [38], Na2V6O16 [39], ZnV2O6 [40] and polyanilineand [41], with nanowire structure were all widely investigated.
The hydrothermal method is one of the most widely used methods to fabricate nanostructured materials [42], [43], [44], [45], [46], [47], because the size, morphology, and crystal structure of the products can be easily controlled [48], and the reaction temperatures are also lower than other preparation methods. In this present work, we investigate the electrochemical properties of MnCo2O4 nanowire array prepared by a facile hydrothermal method coupled with an annealing treatment. The MnCo2O4 nanowire array electrode grown directly on nickel foam avoids the use of polymer binders and conducting additives so that ensures good mechanical adhesion and electrical connection. Through electrochemical measurements, the MnCo2O4 nanowire array is found to be a promising pseudocapacitive material with high specific capacitance and great rate capability. What's more, as an anode material for LIBs, the MnCo2O4 nanowire array also shows excellent electrochemical properties.
Section snippets
Preparation of materials
All the reagents used in the experiment were of analytical grade. The cobalt nitrate, manganese nitrate, urea and ammonium fluoride were obtained from Shanghai Chemical Reagent Co. All aqueous solutions were freshly prepared with high purity water (18 MΩ resistance).
MnCo2O4 nanowire arrays were prepared by a facile hydrothermal synthesis method according to our previous work [49], [50]. The reaction solution was obtained by mixing 1 mmol of Mn (NO3)2, 2 mmol of Co (NO3)2, 6 mmol of NH4F and 15 mmol
Characterization
Fig. 1a shows the XRD pattern of the product after annealing treatment. The standard XRD patterns of Ni and MnCo2O4 are included in the graph too. Expect for the peaks of Ni foam substrate (PDF No. 65-2865), all the diffraction peaks can be indexed as a spinel structure phase of space group Fd3 m MnCo2O4 (PDF No. 23-1237). No additional peaks for other phases are observed. The BET surface area of the MnCo2O4 product is 41.22 m2 g−1 (Fig. 1b).
Fig. 2 shows the SEM images of the as-prepared MnCo2O4.
Conclusions
We have illustrated a fast and facile hydrothermal method for the growth of MnCo2O4 nanowire array. The MnCo2O4 nanowire array exhibits noticeable pseudocapacitive performance with a high capacitance of 349.8 F g−1 at 1 A g−1 and 328.9 F g−1 at 20 A g−1 as well as excellent cycling stability. As an anode material for LIBs, the MnCo2O4 nanowire array delivers an initial specific discharge capacity of 1288.6 mAh g−1 at 100 mA g−1, with capacity retention of 92.7% after 50 cycles. The excellent
Acknowledgement
This work was supported by the Key Science and Technology Innovation Team of Zhejiang Province (2010R50013).
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