Hollow Carbon Microspheres/MnO2 Nanosheets Composites: Hydrothermal Synthesis and Electrochemical Behaviors
This article reported the electrochemical behaviors of a novel hollow carbon microspheres/manganese dioxide nanosheets (micro-HC/nano-MnO2) composite prepared by an in situ self-limiting deposition method under hydrothermal condition. The results of scanning electron microscopy reveal that MnO2 nanosheets homogeneously grow onto the surface of micro-HC to form a loose-packed microstructure. The quantity of MnO2 required in the electrode layer has thereby been reduced significantly, and higher specific capacitances have been achieved. The micro-HC/nano-MnO2 electrode presents a high capacitance of 239.0 F g−1 at a current density of 5 mA cm−2, which is a strong promise for high-rate electrochemical capacitive energy storage applications.
KeywordsManganese oxide Hollow carbon microspheres Composite electrode Supercapacitor
With the rapid depletion of fossil fuels and increasingly worsened environmental pollution caused by vast fossil fuel consumption, there is currently a strong demand to make efficient use of energy and to seek renewable and clean energy sources . Supercapacitor is an attractive power source, which has properties intermediate to those of batteries and electrostatic capacitors [2, 3, 4, 5]. Supercapacitors are capable of storing electrical charge but are distinguished from electrochemical cells as they are essentially maintenance-free, possess a longer life cycle, require a very simple charging circuit, experience no memory effect, and are generally much safer [6, 7]. Unfortunately, supercapacitors deliver an unsatisfactory energy density, which is lower than that of batteries . Thus, most current research works have focused on energy density enhancement of supercapacitors. It is well-known that their performance depends intimately on the properties of their active electrode materials . In general, metal oxides/hydroxides and conducting polymers store charge in a faradic or redox-type process similar to batteries, which enables high energy density but is, in general, kinetically unfavorable.
Among all these materials, manganese oxide (MnO2) has been conceived as a promising supercapacitive material because of its low cost, low toxicity, and environmental safety, as well as high theoretical capacities [10, 11, 12]. However, the utilization efficiency of active materials is usually very low due to the fact that the pseudo-capacitive reaction of MnO2 is a surface reaction and that only the surface or a very thin surface layer of the oxide can participate in the pseudo-capacitive reaction . So, the highly ordered metallic nanostructures should be one of interests for the development of supercapacitors’ electrode materials . Thus, a composite electrode material architecture which incorporates nanoscopic MnO2 film on materials that have outstanding electrical conductivity, excellent mechanical flexibility, large specific surface area, and high thermal and chemical stability support [e.g., carbon nanofoams, templated mesoporous carbon, carbon nanotubes (CNTs)] is thought as ideal to optimize both the electrochemical performance and mass-loading of the ultrathin MnO2 [15, 16, 17]. So, the supports should have high high-surface-area, good electrical conductivity, and most importantly lighter. Thus, the utilization efficiency of active materials MnO2 can been improved and higher specific capacitances have been achieved.
We present an alternative route here to construct a novel composite of hollow core/shell electrode material for supercapacitors by incorporating MnO2 nanosheets (nano-MnO2) onto hollow carbon microspheres (micro-HC). Maintaining such hollow carbon, the utilization efficiency of active materials MnO2 will be improved. The core material is micro-HC, which has been prepared by simple, efficient, and economical synthetic technique. The results showed that the as-prepared micro-HC/nano-MnO2 composite material had a considerably high specific capacitance of 239.0 F g−1 in neutral electrolytes at a current density of 5 mA cm−2. The effects of reaction temperature on structure and electrochemical performance of micro-HC/nano-MnO2 composite were also investigated.
2 Experimental Sections
Potassium persulfate (APS), potassium permanganate, methylacrylic acid (MAA), dimethylformamide (DMF), and absolute alcohol were purchased from Sinopharm Chemical Reagent Co. Ltd., and used as received without any further purification. Styrene (St) from Sinopharm Chemical Reagent Co. Ltd. was purified via distillation prior. Divinylbenzene (DVB) was purchased from J&K Scientific Ltd.
2.2 Preparation of Crosslinked Poly (styrene-co-methylacrylic acid) Hollow Spheres Aggregations
Firstly, MAA (0.431 g) was dispersed in 100 mL deionized water, and followed by the addition of St dropwise in a single-necked round-bottom flask at room temperature. In this process, the mixture was stirred under a nitrogen atmosphere all the time. Sequentially, the temperature of the mixture was increased to 80 °C; APS (0.135 g) was added into the reaction system. The polymerization continued for 24 h at 80 °C. After the reaction, the non-crosslinked P(St-co-MAA) template spheres were centrifuged and washed with both ethanol and deionized water, and then dispersed in 150 mL deionized water. Secondly, the obtained non-crosslinked P(St-co-MAA) template spheres solution (100 mL), deionized water (55 mL), and APS (0.135 g) were mixed in a single-necked round-bottom flask at room temperature. The mixture was stirred under a nitrogen atmosphere for 30 min. The mixture of St (2.083 g), MAA (0.431 g), and DVB (0.078 g) was added into the reaction system drop by drop. The polymerization continued for 24 h at 80 °C. After the reaction, the prepared product was centrifuged and washed with both ethanol and deionized water, then dispersed in 150 mL DMF and stirred for 12 h. The crosslinked P(St-co-MAA) was centrifuged and washed with both ethanol and deionized water. In this process, P(St-co-MAA) template spheres were removed and finally, the crosslinked poly (styrene-methylacrylic acid) hollow spheres aggregations were obtained in a powder form.
2.3 Preparation of Hollow Carbon Microspheres
Hollow carbon microspheres (micro-HC) were prepared through a simple polymer carbonization method involving two steps. The prepared crosslinked P(St-co-MAA) hollow spheres aggregations were firstly pre-oxidized at 320 °C for 5 h and subsequently pyrolyzed at 700 °C under nitrogen atmosphere for 2 h. After the above procedure, the obtained powder, called hollow carbon microspheres (micro-HC), was cooled down to room temperature . The micro-HC was further modified using the nitric acid oxidation method. A typical process for modification of hollow carbon microspheres (micro-HC) can be summarized as follows: 0.1 g carbon powder was dispersed in 10 mL of nitric acid solution (65 wt%) and the reaction was kept at 80 °C under refluxing process for 2 h; after oxidation, samples were recovered and washed thoroughly with deionized water until the pH was close to 7; the resultant product was further dried at 60 °C for 16 h.
2.4 Preparation of Micro-HC/nano-MnO2
In a typical synthesis of micro-HC/nano-MnO2 composites procedure, 100 mg HC was added into 300 mL deionized water and the solution was ultrasonically stirred for 1 h. 250 mg of KMnO4 was subsequently added to the solution, which was ultrasonically dispersed for 30 min. Then, the mixture was moved into a water bath with vigorous magnetic stirring. The morphology and nanostructure of HC/MnO2 composites can be tuned by adjusting solution temperature (30, 60, and 90 °C). The resulting samples were washed with deionized water several times then dried at 60 °C for 12 h.
2.5 Structure Characterization
The microstructure and morphology of the as-prepared samples were characterized by transmission electron microscope (TEM, JEOL, JEM-2010, Japan) and field emission scanning electron microscope (SEM, JEOL, JSM-6701F, Japan). Crystallite structure was determined by X-ray diffraction (XRD) using a Rigaku D/MAX 2400 diffractometer (Japan) with CuKα radiation (λ = 1.5418 Å) operating at 40 kV and 60 mA. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out in air at a heating rate of 10 °C min−1 on a NETZSCH STA 449F3.
2.6 Electrode Preparation and Electrochemical Measurements
The working electrodes were prepared as follows. 80 wt% of micro-HC/nano-MnO2 was mixed with 7.5 wt% of acetylene black and 7.5 wt% of conducting graphite in an agate mortar until a homogeneous black powder was obtained. To this mixture, 5 wt% of poly (tetrafluoroethylene) was added together with a few drops of ethanol. The resulting paste was pressed at 10 MPa into nickel foam (ChangSha Lyrun New Material Co. Ltd., 90 PPI, 2 mm) then dried at 80 °C for 12 h. Each carbon electrode contained approximately 8 mg of electroactive material and had a geometric surface area ≈1 cm2.
3 Results and Discussion
3.1 Preparation and Characterization of Materials
The micro-HC/nano-MnO2 composite was also studied by TGA and DSC in the temperature range of 25–1,000 °C, as shown in Fig. 1b. TGA data showed the presence of three distinct stages: the sample weight decreasing gradually up to ~200 °C, a sharp reduction in sample weight below ~400 °C, and additional step in weight loss observed at ~900 °C. DSC data showed a broad exothermic peak at 340 °C and a narrow endothermic peak at 900 °C. The significant weight loss stage below ~200 °C was due to the loss of crystal water. In the temperature range of 200–400 °C, the weight loss can be attributed to the combustion of carbon. At temperatures exceeding ~400 °C the weight loss can be attributed to reduction of Mn4+ species and the formation of Mn2O3 [27, 28]. The weight loss at ~900 °C was related to the formation of Mn3O4 . The TGA data showed that micro-HC/nano-MnO2 composite contained about 30 wt% of carbon and 60 wt% of MnO2. Hence, the hollow carbon microspheres may be totally coated by MnO2.
The morphology of MnO2 with different dimensionalities and morphologies attract much attention due to their novel and unexpected properties , the reason of which is that the morphology of MnO2 closely relates to the specific surface area and therefore, the specific capacitance. In this work, the effects of reaction temperatures on the structure and properties of micro-HC/nano-MnO2 were studied. Figure 2f–h shows the different surface structures of nano-MnO2 fabricated at different temperatures. It can been seen from the pictures that the size of nanosheet is obtained, which is about 30 nm at 30 °C and 20 nm at 60 °C. With the reaction temperature increasing, the nanosheet became thinner. Even the thickness of each paper-shaped “petal” was about 10 nm and more nanosheets were curled into nano-rod at 90 °C. Compared with the petal-shaped MnO2, the specific surface area is larger, which is a good choice for electrochemical capacitors’ material . As we well know that the unique microstructure played a basic role in electrochemical accessibility of electrolyte to MnO2 active material and the fast diffusion rate within the redox phase.
3.2 Electrochemical Performance
Figure 3b shows the CVs of the micro-HC/nano-MnO2 composite prepared at different reaction temperatures. These curves show no peaks in the range between 0 and 1 V, making clear that the electrode capacitor was charged and discharged at a pseudoconstant rate over the complete voltammetric cycle . At higher reaction temperature, the area of rectangular characteristic became larger, indicating almost ideal capacitive behavior for the obtained materials. The reason is that the petal-shaped MnO2, which may not be a good choice for electrochemical capacitors, could reduce the specific surface area, leading to capacitance fading.
A novel method was used to synthesize micro-HC/nano-MnO2 composites through self-limiting deposition of nanoscale MnO2 on the surface of carbon under hydrothermal method. The composite with high-rate transportation of both electrolyte ions and electrons throughout the electrode matrix has superior electrochemical utilization of MnO2, resulting in the excellent electrochemical performance. The electrochemical studies indicated that the prepared micro-HC/nano-MnO2 composite presented high capacitance of 239.0 F g−1 in 1 M Na2SO4 solution. Through tuning the reaction temperature, the specific capacitance of micro-HC/nano-MnO2 composite prepared at 90 °C were higher than that of micro-HC/nano-MnO2 composite prepared at 30 and 60 °C. The facile synthesis approach may pave the way for successfully employing carbon-based composites for microelectronics, photovoltaic, chemical sensors and energy storage, and conversion applications.
This work was supported by the National Natural Science Foundation of China (51203071, 51363014 and 51362018), China Postdoctoral Science Foundation (2014M552509), the Key Project of Chinese Ministry of Education (212183) and the Natural Science Funds for Distinguished Young Scholars of Gansu Province (1111RJDA012).
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