Enhanced supercapacitive performance of MnOx through N2/H2 plasma treatment

  • Shenggao Wang
  • Mingchen Zhou
  • Xujie Wang
  • Yangwu Mao
  • Quanrong Deng
  • Geming WangEmail author
Original Paper


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.


MnOx 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.

Experimental section

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

Figure 1 shows the XRD patterns of the as-prepared MnOx and plasma-treated MnOx-NH and MnOx-N. It can be seen that MnOx is poorly crystallized, which may be attributed to the small particle size and amorphous nature property of the sample (Zhang et al. 2015). The three weak diffraction peaks of 2θ at around 12º, 37º, and 66º can be indexed to birnessite-type MnO2 (JCPDS no. 80-1098). It can be deduced from the comparison of XRD patterns that plasma-treated MnOx-N is more poorly crystallized than MnOx, and MnOx-NH is almost amorphous. These results show that the plasma treatment can destruct the crystal structure of manganese oxide and the materials become more amorphous.
Fig. 1

XRD patterns of manganese oxides before and after plasma treatment

Further information on the elemental composition, the valence state of Mn, and the oxygen vacancy in the samples is obtained from XPS measurement. Figure 2a–d shows the Mn valence state in manganese oxide which can be determined by calculating the splitting width of Mn 3 s orbit peak from XPS spectra. From Fig. 2b–d, it is found that the peak splitting width of MnOx, MnOx-N, and MnOx-NH is 4.89 eV, 5.01 eV, and 5.10 eV, respectively. According to the relationship between the peak splitting width and the oxide state of manganese (Toupin et al. 2002; Pang et al. 2015), the average valence state of Mn in MnOx, MnOx-N, and MnOx-NH is about 3.8, 3.6, and 3.4, respectively. MnOx, the as-synthesized manganese oxide, is nearly stoichiometry, and it can react with plasma and tend to form nonstoichiometry oxide. Plasma, especially hydrogen plasma, is beneficial for the reduced reaction of Mn to form ionic defects in manganese oxide. Then, multivalent manganese coexists in plasma-treated samples. Furthermore, the O 1 s peaks in the XPS are believed to be related with the loss of oxygen (oxygen vacancy) in the manganese dioxide samples (Ma et al. 2015; Zhu, et al. 2017). Figure 2e–g shows the asymmetrical O 1 s signals of MnOx, MnOx-N, and MnOx-NH, respectively. All of the three signals can be deconvoluted into two components. The one at binding energy about 529.0 eV is assigned to surface lattice oxygen (OLatt) species, and the other at binding energy about 531.4 eV is related to the surface adsorbed oxygen (OAds) species, which locate at the surface oxygen vacancies of manganese dioxide (Wang, et al. 2012). The estimated OAds:OLatt molar ratios in the MnOx-NH, MnOx-N, and MnOx are 25.8, 16.39, and 13.8% respectively, indicating that the plasma treatment can increase the oxygen vacancy density.
Fig. 2

XPS wide scan survey spectra of MnOx-NH (a) and Mn 3 s core-level spectra of MnOx (b), MnOx-N (c), MnOx-NH (d) and asymmetrical O 1 s signals of MnOx (e), MnOx-N (f), and MnOx-NH (g)

Figure 3 shows the isotherms of MnOx, MnOx-N, and MnOx-NH. The specific surface area of MnOx, MnOx-N, and MnOx-NH are 153 m2/g, 134 m2/g, and 129 m2/g, respectively. Correspondingly, the pores’ volume of MnOx, MnOx-N, and MnOx-NH is 0.582 cm3/g, 0.513 cm3/g, and 0.527 cm3/g, respectively. And the pore size distribution becomes a little broader after plasma treatment.
Fig. 3

a Nitrogen adsorption–desorption isotherms of MnOx, MnOx-N, and MnOx-NH, and b the pore size distribution curves of MnOx, MnOx-N, and MnOx-NH

Figure 4 shows TEM images of MnOx, MnOx-N, and MnOx-NH. It can be seen that most of MnOx fabricated by alcohol reacting with potassium permanganate are flakes. And some manganese oxides are interconnected nanoparticles which create abundant space and, thus, lead to porous nanostructures. After plasma treatment, the manganese oxide flakes break and the interconnected nanoparticles aggregate slightly. The aggregation of manganese oxide particles results in the reduction of specific surface area, which is consistent with the results showed in Fig. 3, but, at the same time, micropores form as flakes break and the interconnected nanoparticles aggregate, especially for MnOx-NH. It is estimated that several factors cause the formation of micropores. During the short-time plasma treatment with the temperature being lower than 100 °C, the interconnection among nanoparticles can resist the aggregation to some extent. Furthermore, because of the reaction between manganese oxide and plasma, gas emits out from manganese oxide and, thus, micropores are formed.
Fig. 4

TEM images of MnOx (a), MnOx-N (b), and MnOx-NH (c)

To evaluate the electrochemical properties of MnOx, MnOx-N, and MnOx-NH, CV, GCD, and EIS are conducted in 0.5 M Na2SO4. Figure 5a, b shows the CV comparison of the three electrodes recorded at a scan rate of 30 mV s−1 and 100 mV s−1, respectively. There are distinctive differences among the three curves, not only in the CV area, but also in the curve shapes. In comparison to MnOx, plasma-treated manganese oxide, especially MnOx-NH, exhibits an obvious pseudocapacitive characteristic, which can be attributed to the presence of low valence Mn ions (Zhai et al. 2014). And the pseudocapacitance of MnOx-NH is evidently bigger than those of the other two. Figure 5c shows the variation of specific capacitance versus scan rate. It can be seen that the specific capacitance decreases gradually with the increasing of scan rate, which can be ascribed to the ion diffusion limitation at high scan rate. The decreases of the specific capacitance of the three samples show that the resistance on the ion diffusion in MnOx-NH is smallest. Furthermore, it is well known that reductions of specific surface area and pores volume are not conducive to the improvement of capacitive performance, including capacitance and rate capability. The specific surface areas of MnOx-NH and MnOx-N are smaller than that of MnOx. However, compared with MnOx, the rate capacitances of MnOx-NH and MnOx-N have been improved dramatically and the specific capacitance is enhanced, especially for MnOx-NH. Although, in comparison with the previous reports (Liu and Yue 2014; Li et al. 2013; Zhai et al. 2014; Song et al. 2012), the specific capacitance of MnOx is a little lower, our results show that the capacitance of MnOx treated by N2 and N2/H2 plasma has been enhanced dramatically. Especially for MnOx-NH, at scan rates of 50 mV/s, 100 mV/s and 200 mV/s, the specific capacitances have increased by 184, 200, and 223%, respectively. Figure 5d shows the galvanostatic charge–discharge curves at the current density of 10 A/g. The discharge time of MnOx-NH is far longer than that of MnOx, which also shows that plasma treatment can efficiently enhance the capacitance of manganese oxide. In addition, the IR drops of the three electrodes are 0.21 V (MnOx), 0.14 V (MnOx-N), and 0.11 V (MnOx-NH), respectively (see Fig. 5e). Plasma-treated manganese oxides have smaller IR drops than MnOx, which shows that the electrical conductivity has been improved by plasma.
Fig. 5

CV curves of three electrodes at scan rates of 30 mV s−1 (a) and 100 mV s−1 (b), variation of specific capacitance of three electrodes at different scan rate (c), comparison of galvanostatic discharge–charge curves of three electrodes collected at a current density of 10 Ag−1 (d), and the comparison of IR drop (e)

To further study the effect of plasma treatment on the electrochemical properties of manganese oxide, electrochemical impedance spectroscopy (EIS) measurement is carried out. According to Fig. 6, all these plots show a semicircle in the high-frequency region arising from the chemical reaction process and straight line in the low-frequency region which is related to the ion diffusion in the electrode materials (Pang et al. 2015). The high-frequency intercepting the X-axis represents the equivalent series resistance (ESR) of electrolyte, intrinsic resistance of active materials, and contact interface resistance between the electrode and electrolyte. And the diameter of the semicircle corresponds to the charge-transfer resistance (Kundu and Liu 2013). Figure 6 reveals that, after N2/H2 plasma treatment, both the equivalent series resistance and the charge-transfer resistance drop dramatically. It confirms that plasma treatment, especially N2/H2 plasma, can efficiently enhance the conductivity of MnOx, which is consistent with the results showed in Fig. 5e. Furthermore, the straight line of MnOx-NH in the low frequency tends to more vertical than that of MnOx, which suggests that the N2/H2 plasma treatment can enhance the capacitive behavior of MnO2 (Miller et al. 2010). Clearly, the largest of straight slope suggests the fastest ions diffuse in MnOx-NH. The fastest ion diffusion rate and the smallest charge-transfer resistance, which can be attributed to coexistence of multivalent Mn and large amounts of oxygen vacancies, lead to the best capacitive performance of MnOx-N. This result is in consistence with Song’s work, in which Song et al. claim that the coexistence of multivalent Mn can facilitate the formation of ionic defects and electronic defects. These defects may accelerate the rate of surface redox reaction, and enhance the transportation of charged species and extend the reaction sites from the surface to the subsurface (Song et al. 2012). Figure 6c represents the variation of the imaginary part of the capacitance with the frequency. The peak of plot represents character frequency (f) and the relaxation time constant (τ = 1/f). The small relaxation time constant means that material can deliver high power (Gu and Wei 2015). It shows that, through N2/H2 plasma treatment, the relaxation time constant is enhanced, which is consistent with the rate capability improvement showed in Fig. 5c.
Fig. 6

Nyquist plots (a), the zoom-in Nyquist plots at high-frequency region (b), and evolution of the imaginary part of the capacitance vs. the frequency (c) of MnOx, MnOx-N, and MnOx-NH

Figure 7 shows the long-term cycling test of MnOx, MnOx-N, and MnOx-NH. The cycling life test is conducted by repeating the CV test between 0 and 1 V at a scan rate of 50 mV/s for 1000 cycles. It can be seen that the capacitance retention value of MnOx-NH maintains better than that of MnOx and MnOx-N, which even increases slightly during the first several hundred cycles. The mechanism of the capacitance increment may be ascribed to the structure reorganization and element redistribution of MnOx-NH during electrochemical oxidation and reduction process (Peng et al. 2013). Moreover, it may also explain the activation process of MnOx-NH that allows the trapped cations to diffuse out gradually and confirms that there are more defects in MnOx-NH (Li et al. 2013).
Fig. 7

The cycle performance of MnOx, MnOx-N and MnOx-NH at a scan rate of 50 mV/s


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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Copyright information

© Institute of Chemistry, Slovak Academy of Sciences 2019

Authors and Affiliations

  • Shenggao Wang
    • 1
  • Mingchen Zhou
    • 1
  • Xujie Wang
    • 1
  • Yangwu Mao
    • 1
  • Quanrong Deng
    • 1
  • Geming Wang
    • 1
    Email author
  1. 1.Provincial Key Laboratory of Plasma Chemistry and Advanced MaterialsWuhan Institute of TechnologyWuhanChina

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