One-pot hydrothermal synthesis of Mn3O4 nanorods grown on Ni foam for high performance supercapacitor applications
© Li et al.; licensee Springer. 2013
Received: 12 November 2013
Accepted: 6 December 2013
Published: 19 December 2013
Mn3O4/Ni foam composites were synthesized by a one-step hydrothermal method in an aqueous solution containing only Mn(NO3)2 and C6H12N4. It was found that Mn3O4 nanorods with lengths of 2 to 3 μm and diameters of 100 nm distributed on Ni foam homogeneously. Detailed reaction time-dependent morphological and component evolution was studied to understand the growth process of Mn3O4 nanorods. As cathode material for supercapacitors, Mn3O4 nanorods/composite exhibited superior supercapacitor performances with high specific capacitance (263 F · g-1 at 1A · g-1), which was more than 10 times higher than that of the Mn3O4/Ni plate. The enhanced supercapacitor performance was due to the porous architecture of the Ni foam which provides fast ion and electron transfer, large reaction surface area, and good conductivity.
KeywordsManganese oxide Ni foam Nanorod Hydrothermal synthesis Supercapacitor
Pseudocapacitors, based on reversible redox reactions at/near the surface of the electrode, represent one type of supercapacitors having the potential for high energy densities [1–3]. As is known, the excellent electrode should primarily meet the following key requirements: (1) a large number of electroactive sites, (2) high transport rates of both electrolyte ions and electrons, and (3) high electronic conductivity . Among various pseudocapacitor electrode materials, RuO2 has been extensively studied because of its ultrahigh theoretical capacitance (2,000 F · g-1 in a wide potential window of 1.4 V), a nearly metallic electrical conductivity and excellent chemical stability . However, RuO2 has the drawbacks of high cost and toxicity. Therefore, extensive efforts have been made to search for alternative materials, such as Ni, Co, or Mn-based oxides/hydroxides [6–9]. Because the energy density of a supercapacitor is proportional to the square of the cell voltage, the energy density of Ni- and Co-related materials is limited by the narrow potential window .
Mn3O4 is a potentially interesting electrode material for electrolytic supercapacitors due to its low cost, non-toxicity, environmental compatibility, and intrinsically high capacity [11, 12]. However, the capacitance property of Mn3O4 has been rarely investigated because of its poor electronic conductivity. A common strategy with poor electronic conductors is to combine them into composites with conducting substrates such as nanoporous gold, various carbon materials, and Ni foam [13, 14]. Ni foam, as a commercial material with high electronic conductivity and a desirable three-dimensional (3D) structure is widely used as the electrode substrate material [15, 16]. It would not only reduce the diffusion resistance of electrolytes but also provide a large surface area for loading active material. There have been some reports on the synthesis of Ni- and Co-based oxides/hydroxides on Ni foam [17–20]. However, there are very few reports on the fabrication of Mn-based oxides/hydroxides on Ni foam, except for the MnO2/CNT/Ni foam electrode [21, 22]. To the best of our knowledge, one-pot hydrothermal synthesis of Mn3O4 nanorods structures on Ni foam has not been reported.
Here, we report facile direct synthesis of Mn3O4 nanorods on Ni foam with diameters of about 100 nm and lengths of 2 to 3 μm via one-pot hydrothermal process, without any additional surfactant. The extraordinary redox activity of the Mn3O4/Ni foam composite is demonstrated in terms of pseudocapacitive performance. The effect of reaction time on the crystal growth mechanism and supercapacitor performance of the Mn3O4/Ni foam is well discussed.
Hexamethylene tetramine (C6H12N4) and Mn(NO3)2 (50%) solution were purchased from Shanghai Chemical Reagent Company (Shanghai, China), while Ni foam (5 g/100 cm2) was purchased from Changsha Liyuan New Material Co., Ltd. (Changsha, China). All reagents used in this experiment were of analytical grade without further purification. The Ni foam was immersed in concentrated hydrochloric acid for 10 min and then washed with acetone, ethanol, and distilled water several times before use.
Synthesis of samples
In a typical procedure, 3 mL Mn(NO3)2 (50%) solution and 2 g C6H12N4 were dissolved in 17 mL distilled water. After vigorously stirring, the resulting solution and the pre-cleaned Ni foam were transferred into a Teflon-lined stainless autoclave. The autoclave was sealed at 120°C for 10 h and then cooled to room temperature naturally. The products were washed with distilled water several times, and finally dried in a vacuum desiccator at 50°C. The deposit weight of Mn3O4 was accurately determined by calculating the weight difference between the Ni foam coated with Mn3O4 after the hydrothermal process and the Ni foam before the hydrothermal process.
The morphology of samples was characterized by scanning electron microscopy (SEM, JEOL JSM-6700 F, Akishima-shi, Japan) at an accelerating voltage of 10 kV. The obtained samples were characterized by X-ray powder diffraction (XRD) on a Bruker D8 advanced X-ray diffractometer (Madison, WI, USA) with Cu Ka radiation (λ = 1.5418 Å) at a scan rate of 0.02° · s-1. Raman spectra were obtained using LabRAM HR UV/vis/near-IR spectrometer (Kyoto, Japan) with an argon-ion continuous-wave laser (514.5 nm) as the excitation source.
The electrochemical measurements were performed in a standard three-electrode cell on a CHI 760D potentiostat at room temperature, where 1 cm2 (1 × 1 cm) of the obtained composite was used as the working electrode, a Pt plate was chosen as the counter electrode and a saturated calomel electrode (SCE) was selected as the reference electrode. A 4-M NaOH solution was used as the electrolyte.
Results and discussions
Electrochemical capacitance of Mn3O4/Ni foam electrode
where C (F · g-1) is the specific capacitance; i (A · g-1) is the discharge current density, Δt (s) is the discharge time, and ΔV (V) is the discharge potential range. The specific capacitance values of the Mn3O4/Ni foam composite evaluated from the discharge curves are 293, 263, 234, 214, and 186 F · g-1 at the current density of 0.5, 1, 2, 3, and 5 A · g-1, respectively (Figure 4c). The significant capacitance decrease with increasing discharge current density is likely to be caused by the increase of potential drop due to electrode resistance and the relatively insufficient Faradic redox reaction of the Mn3O4/Ni foam composite under higher discharge current densities. It is noteworthy that the specific capacitance of the as-prepared Mn3O4/Ni foam composite is higher than of the previously reported Mn3O4 in other forms, i.e., Ma et al. reported a specific capacitance of 130 F · g-1 (in 1 M Na2SO4 electrolyte at a current density of 1 A · g-1) for Mn3O4/graphene nanocomposites prepared by a one-step solvothermal process , and Wang et al. reported a specific capacitance of 159 F · g-1 (in 6 M KOH electrolyte at a scan rate of 5 mV · s-1) for Mn3O4/graphene synthesized by mixing graphene suspension in ethylene glycol with MnO2 organosol . The high capacitance of the as-prepared Mn3O4/Ni foam composite can be attributed to the positive synergistic effects between Mn3O4 and Ni foam. The skeleton of Ni foam could reduce the aggregation of the Mn3O4 nanorods, making the Mn3O4 nanorod accessible for electronic and ionic transport pathways and enhancing the utilization of the active materials. Furthermore, Ni foam also provides a highly conductive network for electron transport during the charge and discharge processes.
The endurance test was conducted using galvanostatic charging-discharging cycles at 1 A · g-1 (insert of Figure 4d). The discharge capacitance loss after 2,000 consecutive cycles is about 20%. The specific capacitance degradation is estimated to be from 263 to 205 F · g-1 (Figure 4d). Although the Ni foam serves as a conductive matrix to promote fast Faradaic charging and discharging of the Mn3O4 nanorods, its loose structure leads to the flaking off of the nanorods from the Ni foam substrate.
Time-dependent Mn3O4/Ni foam composite properties
Electrochemical capacitance of Mn3O4/Ni plate electrode- comparison with Mn3O4/Ni foam
A facile one-step hydrothermal method was successfully developed to synthesize Mn3O4 nanorods on Ni foam. The complete absence of any surfactant enabled the product to have high purity. The formation process was proposed to include the dissolution of nanosheets, followed by the formation of uniform nanorods. The obtained Mn3O4 nanorods have diameters of about 100 nm and lengths of 2 to 3 μm. A high specific capacitance of 263 F · g-1 has been achieved for the Mn3O4/Ni foam at 1 A · g-1, which is higher than that of the Mn3O4 composite on other substrates. Porosity may enhance the electrolyte/Mn3O4 contact area and shorten the electrolyte diffusion length in the nanostructures. The cost-effective fabrication and remarkably high specific capacitance provide great potential for this type of hybrid nanostructure to be used as an active electrode for supercapacitor application.
This work was sponsored by the National Science Foundation of China (51171092), the Research Fund for the Doctoral Program of Higher Education of China (20090131110019) and the Independent Innovation Foundation of Shandong University (2012HW004).
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