High Rate Performance Nanocomposite Electrode of Mesoporous Manganese Dioxide/Silver Nanowires in KI Electrolytes

In recent years, manganese dioxide has become a research hotspot as an electrode material because of its low price. However, it has also become an obstacle to industrialization due to its low ratio of capacitance and the low rate performance which is caused by the poor electrical conductivity. In this study, a KI solution with electrochemical activity was innovatively applied to the electrolyte, and we systematically investigated the rate performance of the mesoporous manganese dioxide and the composite electrode with silver nanowires in supercapacitors. The results showed that when mesoporous manganese dioxide and mesoporous manganese dioxide/silver nanowires composite were used as electrodes, the strength of the current was amplified five times (from 0.1 to 0.5 A/g), the remaining rates of specific capacitance were 95% (from 205.5 down to 197.1 F/g) and 92% (from 208.1 down to 191.7 F/g) in the KI electrolyte, and the rate performance was much higher than which in an Na2SO4 electrolyte with a remaining rate of 25% (from 200.3 down to 49.1 F/g) and 60% (from 187.2 down to 113.1 F/g). The morphology and detail structure were investigated by Scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectrometry and Nitrogen adsorption-desorption isotherms. The electrochemical performance was assessed by cyclic voltammograms, galvanostatic charge/discharge and electrochemical impedance spectroscopy.

In this study, a KI solution with electrochemical activity was innovatively applied into an electrolyte. Subsequently, the rate performance of the mesoporous manganese dioxide and the composite electrode with silver nanowires in supercapacitors was systematically investigated. The results showed that when mesoporous MnO2 and mesoporous MnO2/silver nanowires were composited as an electrode, the strength of the current was amplified 5 times (from 0.1 to 0.5 A/g), the remain rates of specific capacitance were 95% and 92% in the KI electrolyte, and the rate performance were much higher than a Na2SO4 electrolyte, where the remain rate were 25% and 60%. Results indicate that KI electrolytes may become a new mode for supercapacitors to change their specific capacitance.

Effect of the Amount of Acid on the Structure of MnO2
Because potassium permanganate is a strong oxidizing agent and ethylene glycol is a strong reducing agent, the exothermic redox reaction between the two of them leads to the formation of MnO2 and other oxidized products, such as aldehydes or a mixture of aldehydes, carboxylic acids and even CO2. The formation process may be explained as follows: Once the ethylene glycol was added into the KMnO4 acid solution, MnO2 nuclei were formed immediately, which acted as the precursor. Concentrated sulfuric acid was then added in the solution, increasing the oxidizability of the potassium permanganate and further increasing the reaction rate. On the other hand, acid can also "etch" the originally produced MnO2, thereby increase the specific surface area. However, adding an excess of acid could not increase of surface area correspondingly, because if the acid concentration is too high, the oxidation of potassium permanganate would become stronger and the reaction rate was further increased, the originally produced MnO2 nuclei was too late to be etched, so the regeneration ability also cannot continue [25].
The samples of MnO2-2, MnO2-4, MnO2-5, MnO2-7 were prepared by adding 2, 4, 5 and 7 mL of concentrated sulfuric acid into the reaction conditions, respectively. The BET surface areas of the samples were 204.41, 232.96, 203.54 and 175.57 m 2 /g, respectively, and the pore diameters of the samples were 5.31, 3.64, 3.90 and 5.71 nm, respectively. As can be seen in Figure 1, all of the isotherms were type IV and displayed hysteresis loops in the relative pressure range of 0.6-0.9. Furthermore, a plateau appeared in the adsorption branch, suggesting that all of the samples were mesoporous. However, the nitrogen sorption isotherm of MnO2-4 exhibited a hysteresis loop in the relative pressure range of 0.4-0.7 and the pore diameter was the smallest among all samples. This conclusion indicates that the mesoporous structure of MnO2-4 was more uniform than the others, so the MnO2-4 was used for the preparation of the composite materials.  Figure 2 shows the scanning electron microscopy (SEM) images of mesoporous MnO2 and mesoporous MnO2/silver nanowires nanocomposite. As shown in Figure 2a, the mesoporous MnO2 has a uniform particle morphology. And in Figure 2b, the mesoporous MnO2 and the silver nanowires have dispersed uniformly, which provide a chance to improve the electrochemical performance of supercapacitors.  Figure 3a depicted the X-ray diffraction (XRD) pattern of the as-prepared mesoporous MnO2, silver nanowires and mesoporous MnO2/silver nanowires. As shown in Figure 3a, the main diffraction peaks at 2θ = 35.6° and 63.8° in the as-prepared mesoporous MnO2, the diffraction angles at 38.2°, 44.5°, 64.5° and 77.2° in the as-prepared silver nanowires, all the reflections can be indexed to the face centered cubic structure according to the previous published papers [26,27]. In contrast with the as-prepared silver nanowires, the diffraction peaks at 38.2°, 44.5°, 64.5° and 77.2° also existed in the mesoporous MnO2/silver nanowires; moreover, the diffraction angles at 35.6° which respected the mesoporous MnO2 crystalline phase caused a slight shift to occur, and set the diffraction angles at 30.1°.

Microstructure Characterization
To obtain more specific information about the as-prepared mesoporous MnO2/silver nanowires, Fourier transform infrared spectrometry (FT-IR) spectra of the mesoporous MnO2, silver nanowires and the composite were obtained. As seen from Figure 3b, the spectrum of MnO2 shows a broad absorption band at 3424 cm −1 , which can be assigned to the O-H stretching vibration, and the peak at 532 cm −1 should be ascribed to the contribution of the Mn-O vibrations, while other three pronounced peaks centered at 1081, 1384 and 1620 cm −1 are O-H stretching vibrations on Mn atom. In contrast with the mesoporous MnO2, the typical O-H absorption bands at about 3000, 1620, 1081 and 589 cm −1 for the mesoporous MnO2/silver nanowires almost appear, moreover, the peaks at 2322, 2033 and 1641 cm −1 of the as-prepared silver nanowires belong to bending mode of hydroxyl groups and carbonyl groups, respectively, are still observed, suggesting that the addition of the silver nanowires modified the groups on the surface of the MnO2 [28].

Electrochemical Characterizations
As is well-known, silver is an excellent electronic conductor, and it has been used to dope electrode materials to improve their electrochemical performance in the past. The detail conductivity data of the samples are shown in Table 1. The chart shows that silver can change the electrical resistivity and electrical conductivity of mesoporous MnO2-4, and with an increase to the added amount of the silver, the electrical resistivity of the sample decreases and the electrical conductivity increases. Therefore, in the following tests of the electrical performance, a sample of MnO2:Ag = 1:1 was used.
To evaluate the potential applications as an electrode material for supercapacitors, the electrochemical properties of the mesoporous MnO2-based electrode material were investigated in a three-electrode system in 0.5 M Na2SO4 and firstly studied in 0.1 M, 0.2 M and 0.5 M KI electrolyte ( Table 2). Some of the reported data beat the performance of the present paper, but KI electrolyte was studied in the first, and it may become a new area for supercapacitors to change the specific capacitance.   Figure 4a shows the CV curves of the mesoporous MnO2 electrode at a scan rate of 5 mV/s in different aqueous electrolytes (0.5 M Na2SO4, 0.1 M, 0.2 M and 0.5 M·KI) in the range of −0.3-0.7 V. It is clear that the CV curve of mesoporous MnO2 in 0.5 M Na2SO4 has no redox peaks over the potential range. The shape of the CV shows an approximate rectangular mirror image characteristic of capacitive behavior. This may be explained by the fact that mesoporous MnO2 occurred redox reaction in each potential. Therefore, there are generally no particularly prominent redox peaks in the CV curves. However, when the conductive of the electrode material is good and the scan rate is low, the cation can not only undertake the redox reaction on the surface, it is also possible carry out the redox reaction in the channels. In that case, obvious redox peaks will appear. Additionally, specific redox peaks have appeared in the CV curves of mesoporous manganese dioxide in KI electrolyte. The possible redox reaction of I − at the electrode/electrolyte interface can be expressed as shown in Equation (1)(2)(3)(4) [49][50][51]. These redox reactions may be the reasons that a higher specific capacitance of the electrode materials occurs KI than in Na2SO4 electrolyte. Figure 4b presents the charge/discharge curves of mesoporous MnO2 at the experimental condition of 0.2 A/g in 0.1, 0.2 and 0.5 M KI electrolyte in the range of 0-0.30 V. It can be seen that with the increase of the concentration of I − , the capacitance of the mesoporous MnO2 electrode also increases. All of the specific capacitance of the mesoporous MnO2 in the KI electrolyte are higher than those in the Na2SO4 electrolyte. Figure 4c shows the charge/discharge curves of the mesoporous MnO2 electrode measured at different current densities (0.1, 0.2 and 0.5 A/g) in 0.5 M KI electrolyte. From the detailed data shown in Table 3, it is clear that the mesoporous MnO2 electrode material has a high rate of specific capacitance in 0.5 M KI electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were taken to study the storage kinetics of all of the as-prepared materials. As shown in Figure 4d, the EIS of all samples was composed of a partially overlapped semicircle and a straight sloping line. Additionally, the diameter of the semicircle is related to the resistance of the electrode materials. As the diameter decreased, so did the resistance. It was found that the resistance of the electrode materials was the smallest in the 0.5 M KI electrolyte. This is related to the previous discussion. Figure 5a shows the CV curves of the mesoporous MnO2/silver nanowires electrodes at a scan rate of 5 mV/s in different aqueous electrolytes (0.5 M Na2SO4, 0.1 M, 0.2 M and 0.5 M KI) in the range of −0.3-0.7 V. It can be seen that the CV curve of the mesoporous MnO2/silver nanowires in 0.5 M Na2SO4 has redox peaks appeared over the potential range. However, the area of the CV curve of the mesoporous MnO2/silver nanowires in 0.5 M Na2SO4 was obviously smaller than that of the other CV curves, which reflects the fact that the specific capacitance of the mesoporous MnO2/silver nanowires in 0.5 M Na2SO4 electrolyte was smaller than the specific capacitance of the mesoporous MnO2/silver nanowires in KI electrolyte. Figure 5b presents the charge/discharge curves of the mesoporous MnO2/silver nanowires composite electrode at the experimental condition of 0.5 A/g in 0.1, 0.2 and 0.5 M KI electrolyte in the range of 0-0.35 V. For the pure MnO2, the large Warburg region indicates great variation in the ion diffusion path lengths and an increase of ion movement obstruction in the process of ion diffusion into the interior of the electrode. This implies that silver nanowire dopant in MnO2 can promote the ion diffusion rate, which explains the fast current response in the CV curve and the ideal capacitive behavior of the mesoporous MnO2/silver nanowires composite electrode. Obviously, the charge/discharge curve results confirm that the silver nanowires dopant in MnO2 not only enhances the electron transfer efficiency but also facilitates ion diffusion [52][53][54]. Additionally, due to the synergistic effect of the electrical conductivity of the electrode and the electrolyte, the specific capacitance of the mesoporous MnO2/silver nanowires reaches its maximum in 0.2 M KI electrolyte. Figure 5c presents the charge/discharge curves of the mesoporous MnO2/silver nanowires composite electrode with different current densities in 0.2 M KI in the range of 0-0.35 V. It is clear that with an increase of the current density, the charge/discharge voltage also increases, and a larger voltage drop appeared. These results imply that the changing test conditions also affected the concentration of iodine ions in the electrode surface and the electrolyte. As shown in Figure 5d, the EIS of all of the samples was composed of a partially overlapped semicircle and a straight sloping line. It is noted that for all samples, at very high frequency, the intercepts at real part Z' were almost the same, indicating the same combination resistance of ionic resistance of electrolyte, intrinsic resistance of active materials and contact resistance at the active material/current collector interface [55,56]. All of the relevant specific capacitances (Cs) were calculated from the galvanostatic charge/discharge according to Equation (5) [57][58][59], and all of the details of the data are shown in Tables 3 and 4.
where C s is the specific capacitance (F/g), i is the discharge current (A), ∆t is discharge time (s), m is the active mass of electrode (g), and ∆V is the total potential deviation (V). It can be observed that the manganese dioxide-based nanocomposite materials exhibited the highest specific capacitance, and the specific capacitance of the mesoporous MnO2 that was tested in KI electrolyte was higher than that which was in Na2SO4 electrolyte. Besides, with the increase of the concentration of I − , the specific capacitance of the mesoporous MnO2 electrode material also increased accordingly. In addition, the mesoporous MnO2 showed a good ratio discharge property in 0.5 M KI electrolyte. In fact, the specific capacitance reached 205.5, 200.6 and 197.1 F/g at 0.1, 0.2 and 0.5 A/g, respectively. This may be because of the effect of the electrical conductivity of the electrolyte. On the other hand, the specific capacitance of mesoporous MnO2 also changed when the blended with silver nanowires. However, according to the detailed data as shown in Table 4, the specific capacitance of mesoporous MnO2/silver nanowires reached its maximum in 0.2 M KI electrolyte. This may be due to the synergistic effect of the electrical conductivity of the electrode and the electrolyte, the conductivity of the composite electrode and the ion activity of the electrolyte were worked together to the electrode materials, make sure that it has a good ratio discharge property under different test conditions [60,61]. Finally, the mesoporous MnO2/silver nanowires show a good ratio discharge property in 0.2 M KI electrolyte, with the specific capacitance reaching to 208.1, 200.5 and 191.7 F/g at 0.1, 0.2 and 0.5 A/g, respectively.

Preparation of Mesoporous MnO2
First, 1.730 g KMnO4 was dissolved in 100 mL of deionized water and stirred with a magnetic stirrer for 30 min to form a homogeneous solution at room temperature. Then, 2 mL concentrated sulfuric acid were added in 2.5 mL of ethylene glycol and aged for 20 min with stirring at ambient conditions. Then the mixture was poured into the above KMnO4 solution. The mixture was aged for 20 min with stirring at ambient conditions. The deposits were collected by centrifugation, washed several times with deionized water and absolute ethanol until the pH of the wash was neutral, and then dried at 60 °C overnight under air atmosphere. This sample was denoted as MnO2-2 for the following tests [25]. In the remaining three experiments, 4, 5 and 7 mL concentrated sulfuric acid were added in the ethylene glycol solution, respectively, and the collected samples were correspondingly denoted as MnO2-4, MnO2-5 and MnO2-7 for the following tests.

Preparation of Silver Nanowires
First, 5 mL of ethylene glycol was added to a disposable glass vial to which a stir bar was added; the vial was then suspended in an oil bath at 151.5 °C and heated for 1 h under magnetic stirring. At the same time, reagent solutions were prepared. At 1 h, a Cu-additive solution was injected into the heated ethylene glycol. The solution was then heated for an additional 15 min. 1.5 mL of a 0.147 M polyvinylpyrrolidone solution in ethylene glycol, followed by 1.5 mL of a 0.094 M AgNO3 solution in ethylene glycol [26,62]. Then, the reaction was quenched by cooling the reaction vial in a room temperature water bath. Products were then washed with acetone and water.

Preparation of Mesoporous Manganese Dioxide-Based Composite Materials
Silver nanowires and mesoporous MnO2 were mixed directly with different molar ratios, and to reach the uniform by grinding.

Material Characterization
The Nitrogen adsorption-desorption isotherms were tested with a JW-BK Static Physisorption Analyzer. The BET surface area was calculated from the desorption branches in the relative pressure range of 0.05-0.35, and the total pore volume and average pore diameter were evaluated at a relative pressure of about 0.99. The materials were pretreated at 300 °C for 6 h. Pore-size distributions (PSD) were calculated by Barrett-Joyner-Halenda (BJH) plots using the nitrogen desorption isotherm. The pore width was obtained at the peak of the PSD curve. The morphologies of the prepared products were examined by scanning electron microscopy (SEM), using a FEI Quanta 400 F at 20 kV. X-ray diffraction (XRD) patterns were obtained with a Bruker D8 Diffractometer in reflection mode using Cu Kα = 0.154 nm with a voltage of 40 kV. In addition, Fourier transform infrared spectrometry (FT-IR) analyses were performed on a Thermal Nicolet Infrared Spectrometer.

Preparation of Electrodes and Electrochemical Characterization
The working electrodes were fabricated by the as-prepared materials, carbon black and poly tetrafluoroethylene (PTFE) at a weight ratio of 70:20:10. Typically, the mixture was formed to slurry by adding a few drops of 1-methyl-2-pyrrolidinone, then coated onto a stainless steel grid with an apparent area of 1 cm × 1 cm, and finally dried under air atmosphere at 70 °C for 2 h. All electrochemical measurements were done in a three-electrode system. The prepared electrodes were impregnated with different aqueous electrolytes before the electrochemical tests for 0.5 h at room temperature. A Pt wire and an Ag/AgCl cell were used as counter and reference electrodes, respectively. The CV and galvanostatic charge/discharge (GC) were measured by a CHI 660C electrochemical workstation. CV tests were performed between −0.3 and 0.7 V. Galvanostatic charge/discharge curves were measured in the potential range of −0.3-0.7 V at a current density of 0.5 A/g.
The conductivity tests were carried out according to Four Probes Tech type RTS-9. The ambient temperature of the experiments was in the range 23 ± 5 °C and the operating humidity was in the range 73 ± 5 °C. The samples were prepared with a typical diameter of around 13 mm and thick layer of around 0.5 mm by a tablet machine.

Conclusions
In this study, uniform mesoporous MnO2 was obtained through a simple one-pot synthesis procedure at ambient temperature from KMnO4 and ethylene glycol under acidic conditions. Subsequently, the electrochemical performance of the resulting mesoporous MnO2-based electrode materials was tested in KI and Na2SO4 electrolytes. The mesoporous MnO2 showed a good ratio discharge properties in 0.5 M KI electrolyte, the strength of the current was amplified 5 times (from 0.1 to 0.5 A/g), and the remain rate of the specific capacitance was 95% (from 205.5 down to 197.1 F/g). Due to the synergistic effect of the electrical conductivity of the electrode and the electrolyte, the mesoporous MnO2/silver nanowires showed a good ratio discharge property in 0.2 M KI electrolyte, with the strength of the current being amplified five times (from 0.1 to 0.5 A/g), and the remain rate of specific capacitance at 92% (from 208.1 down to 191.7 F/g). The rate performance was also much higher than that in Na2SO4 electrolyte, where the remain rate were 25% (from 200.3 down to 49.1 F/g) and 60% (from 187.2 down to 113.1 F/g), respectively. Results indicate that KI electrolyte may become a new way for supercapacitors to change the specific capacitance.