Enhanced supercapacitive performance of MnOx through N2/H2 plasma treatment
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This work relates to the research on the effect of plasma on the performance of manganese oxide. Manganese oxide nanoflakes were prepared through the reaction of KMnO4 and alcohol, and then treated by N2 and N2/H2 plasma. The crystal structure of manganese oxide can be destructed by plasma treatment and manganese oxide become more amorphous and aggregated than those of as-synthesized MnOx, both of which have been proven by XRD and TEM techniques. Results of XPS confirm that the ionic defects and oxygen vacancy are also formed in manganese oxide by plasma. The electrochemical behavior was studied using CV, GCD, and EIS method in 0.5 M Na2SO4 solution. The results show that N2/H2 plasma treatment can fascinate the coexistence of mixed valence of Mn and the formation of oxygen vacancies, reduce the charge-transfer resistance, and then enhance the capacitive performance efficiently.
KeywordsMnOx Plasma Supercapacitors
In recent years, supercapacitors attract considerable attention in the area of energy storage due to its high power density, high energy density, superior cycling stabling, and wide operating temperature range (Zhu et al. 2011; Simon et al. 2014). In terms of the fundamental charge storage mechanisms, supercapacitors can be classified into two types, which are electrical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store charge by physical ion adsorption, while pseudocapacitors store charge through fast reversible Faradaic redox reactions on/near the surface of active materials (Zhang et al. 2013a, b; Kang et al. 2015; Ho et al. 2014). Compared with EDLCs, pseudocapacitors have higher specific capacitance and more widespread application. Among all kinds of pseudocapacitors electrode materials, manganese oxides are the most promising materials, because of their low cost, rich resources, high theoretical specific capacitance, and environmental friendliness (Ho et al. 2014; Lee et al. 2015; Jiang et al. 2015). However, the low conductivity and accessible active surface area limit their practical application (Lee et al. 2010; Huang et al. 2015; Shimamoto et al. 2013). To enhance the capacitive performance of MnO2, the good electron conduction and the ion diffusion paths are needed to be obtained (Teng et al. 2010; Li et al. 2015). Carbon nanotubes, graphene and conducting polymer have been used to enhance conductivity and accessible active surface of MnO2 (Pi et al. 2016; Xiao and Xu 2013; zhang et al. 2013b). However, owing to the interface resistance, the MnO2 conductivity resulting from the conductive material can only be enhanced to a limited degree (Wu et al. 2012). To avoid the interface resistance, Kang et al. prepared Au-doped MnO2, which remarkably improved conductivity and capacitive performance of MnO2 (Kang et al. 2013). Liu et al. reported that Co-doped MnO2 exhibited better rate capability and electron transport ability than pure MnO2 (Liu and Yue 2014). Li et al. synthesized Cu-doped hollow-structured MnO2 via the hydrothermal process, which demonstrated 642 mAh/g at the current density of 100 mA/g (Li et al. 2013). On the other hand, oxygen vacancies also played a very important role in enhancing the intrinsic conductivity of MnO2. Zhai et al. prepared hydrogenated MnO2 nanorods by annealing the as-synthesized MnO2 nanorods in H2 at 250 °C for 3 h, and on account of the oxygen vacancies, the hydrogenated MnO2 exhibits high capacitance of 449 F/g at 0.75 m A/cm2 (Zhai et al. 2014). Song et al. claimed that the mixed valence state facilitated the formation of more ionic defects and electronic defects, thus improving the capacitive performance of MnOx (Song et al. 2012).
Plasma modification technology is regarded as an economical and efficient technology applied to various areas, which only works on/near the surface of material (Dorraki et al. 2015; Sahin et al. 2015). Through the plasma modification, the surface structure and valence state of materials can be changed efficiently. It is also expected that plasma treatment can efficiently change the chemical states of Mn in manganese oxide and, thus, significantly enhance its capacitive performance. Therefore, in this paper, we treated MnOx through N2 and N2/H2 plasma, and compared their capacitive performance. Electrochemical measurements showed that, after the N2/H2 plasma treatment, the conductivity and rate capability of MnOx were greatly enhanced, and the relaxation time constant of MnOx was reduced greatly. It shows that N2/H2 plasma treatment is a promising way to enhance the conductivity and capacitive performance of MnOx.
Synthesis and plasma treatment of MnOx
All the chemicals used were of analytical grade and were used without further purification. In a typical experiment, 3 g KMnO4 was dissolved in 200 mL deionized water and stirred with a magnetic stirrer for 30 min to form a homogeneous solution at room temperature. 200 mL ethanol was then added to the solution with constant stirring for 5 h. Later, the precipitate was collected by filtration, and sequentially washed for several times with deionized water and absolute ethanol, and then dried at 60 °C for 12 h. The final product is denoted as MnOx. Afterwards, a given fraction of MnOx (500 mg) was modified by N2/H2 plasma in the microwave plasma generator (2.45 GHz). During the 5 min plasma modification process, the input microwave power was 300 W, and chamber pressure was kept at 0.5 kPa, the N2 and H2 flow rates were kept at 50sccm and 5sccm, respectively. The sample modified by N2/H2 plasma was denoted as MnOx-NH. For comparison, MnOx was also modified by N2 plasma with the same process, which was denoted as MnOx-N.
Characterization and electrochemical measurements
The morphologies and chemical composition of the products were characterized by transmission electron microscopy (TEM, JEM-2010EX, 200 kV), X-ray diffraction (XRD, D8 Brucker), and X-ray photoelectron spectroscopy (XPS, Thermal Scientific INC). Electrochemical studies were performed through a workstation (AMETEK, Princeton perstat4000). The surface and the pore size distribution were evaluated based on the Brunauer–Emmett–Teller (BET) method and the Barret–Joyner–Halenda (BJH) method, respectively. Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out by a standard three-electrode cell configuration. The platinum electrode and Ag/AgCl electrode served as counter and reference electrode, respectively. All the electrochemical tests were performed in 0.5 M Na2SO4 solution. The working electrodes were prepared as follows. First, 5 mg material was dissolved into the mixture solution of deionized water (800 μL), alcohol (800 μL), and 5% Nafion (80 μL). Then, it was poured into the ultrasonic vibration for 0.5 h to achieve homogeneous catalyst ink. At last, 5 μL of the ink was dropped on the glass carbon electrode to act as working electrode, and thus, the mass of the active electromaterials was about 0.0149 mg.
Results and discussion
In summary, plasma treatment can efficiently change the structure of MnOx and valence state of Mn. Compared with as-synthesized MnOx, plasma-treated manganese oxides show better conductivity, rate capability and cycling stability. The better capacitive performance of MnOx-NH can be attributed to the existence of mixed valence of Mn and oxygen vacancies. It shows that plasma treatment is a promising way to enhance the conductivity and capacitive performance of MnOx.
Financial support from National Natural Science Foundation of China (Grants No.: 51272187, 11704288), the Science and Technology Supporting Program of Hubei Province (Grants No.: 2015BAA093, 2013CFA012), and the Scientific Project provided by Wuhan Government (Grants No.: 2016010101010026) was greatly acknowledged.
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