The Electrode Materials of Supercapacitor Based on TiO2 Nanorod/MnO2 Ultrathin Nanosheet Core/Shell Arrays

A hierarchical structure of TiO2 nanorod/MnO2 ultrathin nanosheet core/shell nanocomposite arrays on a conductive substrate has been prepared by two facile steps of hydrothermal reaction and annealing progress, which serving as electrodes present great potential application for high-performance supercapacitors. By adjusting the concentration of precursor aqueous solution, it can be found that the thickness of the MnO2 shell in the as-designed hierarchical electrode material can be facilely controlled. By comparison, the obtained TiO2 nanorod/MnO2 ultrathin nanosheet as an electrode material can achieve the best electrochemical performance in terms of the area-specific capacitance up to 34.79mF/cm from the cyclic voltammetry (CV) test at the scan rate of 5mV/s. Furthermore, the composited electrode has also demonstrated good stability, with the capacitance retention rate of about 91% through the cycle experiment test after 1000 cycles.


Introduction
In recent years, nanomaterials have been extensively studied [1][2][3]. More and more electronic products and electric vehicles are coming into people's lives in the past decade, following the increased demand for electric energy storage systems. The electrochemical supercapacitor, as an excellent electric energy storage device, attracts extensive research interests [4][5][6][7][8], because of the good cycle stability and excellent reversibility, fast recharge capability, cleanliness and environmental friendliness, high power density, and so on.
Among various metal oxides, manganese dioxide (MnO 2 ) is a very important electrode material because of the low cost, abundant reserves, ease of synthesis, and environmental protection and other outstanding characteristics [32][33][34][35]; the most important is that MnO 2 has very high theoretical specific capacitance. However, the poor conductivity of manganese dioxide (<10 -5 S/cm) greatly affects its application in the field of energy storage, especially for supercapacitors. Therefore, many researches have been carried out to improve the pure conductivity and low practical capacitance of MnO 2 . It has been widely accepted that manganese dioxide nanostructures with a high specific surface area can effectively improve the utilization ratio of active materials of MnO 2 . Furthermore, designing hierarchical electrodes by combining carbon materials or transition metal compound is aimed at achieving higher conductivity and electrical performance. The capacitance performance of these manganese dioxide-based composite materials is better than that of single manganese dioxide.
In recent years, titanium dioxide (TiO 2 ) is another potential electrode material in the development of capacitor materials [36,37]. Benefiting from the low cost, low toxicity, and abundant reserves, compared to MnO 2 , TiO 2 possesses higher conductivity and electrochemical stability. In this work, we have prepared a hierarchical structure of the TiO 2 nanorod/MnO 2 ultrathin nanosheet core/shell nanocomposite on the FTO substrate (as a current collector) directly through two facile steps of hydrothermal reaction and annealing progress, and the binder-free of the obtained electrode avoids the morphology damage, the pore blockage, and the reduced conductivity of electrode materials, which is favorable for the electrochemical behavior. Meanwhile, it should be noted that the favorable electrode architecture design of conductive FTO as the current collector decorated by the hierarchical structure of the TiO 2 /MnO 2 core/shell nanocomposite can achieve rapid ion and electron migration reactions simultaneously due to the structure of the vertical array and the porous abundance, which enhanced the activity of the electrode, thereby reducing internal resistance and improving capacitor performance. Finally, we have studied the electrochemical properties of materials, which also presented good capacitive properties in terms of high capacitance, low internal resistance, good rate capability, and long cycling stability, presenting great potential application for supercapacitors.

Experiment
2.1. Materials. The analytical grade reagents, tetrabutyl titanate (C 16 H 36 O 4 Ti), concentrated hydrochloric acid (HCl), potassium permanganate (KMnO 4 ), sodium sulfate (Na 2 SO 4 ), and acetone (C 3 H 6 O) were all purchased from Shanghai Chemical Reagent Company, and all of the water in our experiment is deionized water. The FTO conductive glass was purchased from Wuhan Jingge Solar Technology Co., Ltd., and the pretreatment of the FTO conductive glass was based on our previous report [38].

The First
Step of the Preparation: TiO 2 Nanocore Array. Rutile TiO 2 was prepared with a simple hydrothermal reaction progress based on the literature [39]; in our experiment, the conductive glass of FTO was used as the electrode substrate, on which the electrode materials were grown. Specific experiment is described as follows for deionized water, concentrated hydrochloric acid, and tetrabutyl titanate, with a volume ratio of 30 : 30 : 1, first, mix equal volumes of deionized water and concentrated hydrochloric acid together, under magnetic stirring, and then add tetrabutyl titanate into the above mixture drop by drop with vigorous stirring. 20 minutes later, the mixture was transferred into an autoclave (Teflon-lined), in which the FTO collector was placed vertically, and then, the reactor was sealed and maintained at 150°C for 4 h, cooling down naturally. Finally, take out the FTO substrate which has been covered with the obtained TiO 2 nanorod arrays, wash them with deionized water and alcohol by turns for several times, and dry them naturally.

The Next
Step of the Preparation: The Shell of the MnO 2 Ultrathin Nanosheet. The final product of the TiO 2 nanorod/MnO 2 ultrathin nanosheet core/shell composite material was synthesized by the following second step of the hydrothermal process, the TiO 2 nanorod-modified electrode from the above step was placed into an autoclave (the specification is 15 mL), and then 10 mL of potassium permanganate aqueous solution with a given concentration was added. Then, transfer the reactor into an oven and keep it for 6 hours at 160°C, turn off the oven and let it cool down, rinse the electrode with ethanol and deionized water, and dry them in an oven for 8 h at 60°C.
For this step, three parallel experiments were carried out; with other conditions unchanged, the concentration of potassium permanganate aqueous solution was adjusted in the range of 0.03-0.09 M (0.03 M, 0.06 M, and 0.09 M, respectively). Finally, the dried products were annealed in a furnace at 350°C for 2 h, and the final product TiO 2 /MnO 2 nanomaterial was obtained. The differences in the morphology and the electrochemical performance of the products were studied.
The schematic diagram of the preparation of TiO 2 /MnO 2 core-shell structure composite materials is shown in Figure 1. In order to easily distinguish the three kinds of products, in the following, the products obtained at different KMnO 4 concentrations were named as TMnO-1, TMnO-2, and TMnO-3, corresponding to 0.03 M, 0.06 M, and 0.09 M of KMnO 4 aqueous solution, respectively.

Characterization.
The morphology of the products was characterized by field emission scanning electron microscopy (Philip XL-30 ESEM FEG) with an X-ray energy-dispersive spectrometry (EDX) analyses, transmission electron microscopy (TEM) (Philips JEM-2010 at an acceleration voltage of 200 kV), and X-ray diffraction (XRD) (D/max 2200 PC spectrometer with a Cu Kα source).

Electrochemical
Testing. Electrochemical Workstation CHI660d (Chenhua) was used to carry out the electrochemical experiments, with a standard three-electrode cell testing system, in which the obtained composite nanomaterials is the working electrode, Ag/AgCl is the reference electrode, and the platinum wire is the counter electrode. Electrochemical tests of cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out. The electrolyte was 1.0 M Na 2 SO 4 aqueous solution. The CV tests were performed from -0.2 to 0.8 V. The frequency range of the EIS test was from 0.01 Hz to 100 kHz at an AC voltage of 5 mV. The test area of all of the electrode materials is 1 × 1 cm 2 . The following formulation is used to calculate the area-specific capacitance of the product.
In formula (1), C a represents the area-specific capacitance, I represents the discharge current, t is the discharge time, and S i represents the geometric area of the electrode, and in formula (2), Ð I · dv is the integral of the cyclic voltammetry curve area, v is the scanning speed of cyclic voltammetry, S is the geometric area of the electrode, and △V is the voltage window during scanning.

Results and Discussions
3.1. XRD Spectra of the Products. Figure 2 shows the three curves of XRD spectrum, respectively, representing the TMnO-1 (red curve), TMnO-2 (blue curve), and TMnO-3 (purple curve) obtained by our experiments. These materials were all directly tested on the FTO glass without any treatment. The symbol of star in the spectra represents the diffraction peak of the FTO glass base, which corresponds to No. 46-1088 in the standard JPCDS card. Among the three spectral curves, the diffraction peaks TiO 2 TiO 2 /MnO 2 Step 2 Step 1 Step 3

Hydrothermal
The   . 88-1175). Therefore, the XRD results exhibit that both MnO 2 and TiO 2 are presented in the three products.

EDX Spectra of the Products.
The EDX spectra of TMnO-1, TMnO-2, and TMnO-3 are displayed in Figure 3. Except   5 Journal of Nanomaterials for the elements Sn and Si contained in the FTO glass, as well as the Au element contained in the sprayed gold, the existence of Ti, Mn, and O elements can further illustrate the coexistence of the two oxides of titanium dioxide and manganese dioxide in our as-synthesized products of TiO 2 /MnO 2 core-shell structure. At the same time, we can also observe that the content of the Mn element in the product increases obviously by increasing the concentration of potassium permanganate during the reaction, namely, high concentration of the precursor, achieving high content of MnO 2 in the product.   Journal of Nanomaterials

SEM and TEM
TMnO-3 at different magnifications, respectively. As the concentration of KMnO 4 in the precursor solution increases, the thickness of the MnO 2 shell wrapped around the TiO 2 increases. Particularly, the net-like porous structure formed by the MnO 2 sheet becomes more and more thick, and the original structure of the TiO 2 array is still preserved well. Therefore, the resulting complex TMnO-x (x = 1, 2, and 3) is the core-shell structure porous array. Figure 5 shows the TiO 2 nanorods prepared by the hightemperature hydrothermal method (Figure 5(a)) and the TiO 2 /MnO 2 core-shell structure composite materials TMnO-1, TMnO-2, and TMnO-3. Figure 5(a) is a pure TiO 2 nanorod with a smooth surface and uniform diameter of 100 nm. Figures 5(b)-5(d) are TEM images of TMnO-1, TMnO-2, and TMnO-3, respectively. The core-shell structure can be seen obviously, and TiO 2 as the nanocore is covered with MnO 2 homogeneously. The thickness of the MnO 2 shell is obviously increased from TMnO-1 to TMnO-3. The large amount deposition of the MnO 2 shell will improve the energy storage ability for the as-designed hierarchical electrode of TiO 2 /MnO 2 due to the increase in active sites. 7 Journal of Nanomaterials have been also conducted directly by using our devices without any treatment to investigate the electrochemical property of the products.

Electrochemical
At a scan rate of 100 mV/s, a comparison diagram of CV results of the three kinds of products is shown in Figure 6(a). The black curve is TMnO-3, the red curve is TMnO-2, and the blue curve is TMnO-1. As can be seen from these curves, they all present a similar shape, in which a tiny redox peak appeared, indicating the pseudocapacitive characteristics of these materials. And they also have the characteristics of a double-layer capacitor. Meanwhile, it can be observed that TMnO-3 has the largest integrated area, indicating that this material presents the best capacitive performance with the highest area capacitance among them. In the TMnO-x core-shell structure electrodes, the redox reaction (pseudocapacitance reaction) occurs between the valence states of Mn (IV) and Mn (III). Due to the insertion and extraction of Na + in the outer layer of MnO 2 [40,41], the mechanism of the charge storage can be expressed by the following: This mechanism indicates that the capacitance performance of the material mainly comes from MnO 2 through  Journal of Nanomaterials the surface Faraday reaction (pseudocapacitive). In addition, the intercomponent synergy of TiO 2 and MnO 2 can further improve the performance of the composite materials. In the SEM and TEM pictures, the TMnO-3 possesses the most advantageous morphology, and hence, the surface MnO 2 of TMnO-3 with more active sites can absorb more electrolyte ions. This result is consistent with the CV measurement result. At the same time, due to this special structure of core-shell arrays, the specific surface area increased and the diffusion path of the ions has been effectively reduced, which greatly increases the diffusion speed rate of the ions and accelerates the kinetics of the Faraday reaction. Moreover, in addition to comparing the CV test results, the GCD tests were also performed for the series of TiO 2 /MnO 2 nanocomposites. The three curves in Figure 6(b) are the GCD curves of TMnO-x at the current density of 0.03 mA/cm 2 .
From the GCD test results, the compound of TMnO-3 exhibits the best electrochemical performance, which is consistent with the cyclic voltammetry tested above.
For the TMnO-3 serving as the supercapacitor electrode with the best capacitance effect, we also conducted a detailed electrochemical performance test on its capacitance. Figure 7(a) gives the CV curves of TMnO-3 at different scanning speeds, and Figure 7(b) shows the GCD curve of TMnO-3 under different current densities. By calculating from formula (2), the area-specific capacitance of TMnO-3 is up to 34.79 mF/cm 2 at the scan rate of 5 mV/s. And from the GCD test, according to formula (1), the area-specific capacitance is 28.87 mF/cm 2 at the current density of 0.015 mA/cm 2 .
We conducted an electrochemical impedance test on TMnO-3, the purpose of which is to further study the property of our electrode material. The Nyquist plot of TMnO-3 is shown in Figure 8(a). In the high-frequency region, there is a semicircle with a smaller diameter, indicating that the charge transfer resistance is small in the product. At the same time, it can be observed that the slope of the curve in the low-frequency region exceeds 45°, indicating a decrease in the diffusion resistance of the electrode. The impedance results once again demonstrate that TMnO-3 has good electrochemical performance.
Another important factor for judging the performance of a material whether with practical application value or not is its cycle stability. Figure 8(b) shows the test result of 1000 cycles of charge and discharge at a current density of 0.2 mA/cm 2 . After the test, the capacitance of the TMnO-3 nanocomposite electrode material still remained 91%, indicating that the nanocomposite has good long cycle stability.

Conclusion
We successfully prepared hierarchical core-shell arrays between TiO 2 nanorods and MnO 2 nanosheets, through two steps of simple low-cost hydrothermal progress and one step of the high-temperature annealing process. By adjusting the concentration of KMnO 4 aqueous solution in the hydrothermal reaction progress, we obtained a series of hierarchical electrodes of TiO 2 /MnO 2 core-shell nanocomposites. And we discussed the different morphologies and electrochemical properties of these electrode materials, achieving the best product that has good properties. These results show that rationally designing a special structure of electrode material can result in better performance. Meanwhile, by combining different electrode materials together, benefiting from the synergistic interaction of the component, this contributes to increasing the electrochemical properties of energetic materials.

Data Availability
All of the data can be found in our manuscript.

Conflicts of Interest
There are no conflicts to declare.