Figure 1 (a and b) suggest the composition and the nanotube structure of TNTM, indicating that it consists of Ti (JCPDS card No. 65-3362) of Ti mesh and TiN (JCPDS card No. 65-5759) of vertically aligned TiN nanotubes. As shown in the XRD patterns of TN-MO-SS, TN-MO-S, TN-MO-SR, TN-MO-RS, and TN-MO-R (Figure 1c), TN-MO-SS and TN-MO-S mainly contain δ-MnO2 crystals (JCPDS card No. 80-1098), while the MnO2 in TN-MO-SR, TN-MO-RS, and TN-MO-R is α-MnO2 (JCPDS card No. 44-0141). In addition, TN-MO-S also contains a little α-MnO2. According to the SEM images in Figure 1(d-h), the MnO2 nanostructures in TN-MO-SS, TN-MO-S, TN-MO-SR, TN-MO-RS and TN-MO-R are nanosheet spheres, nanosheets, nanorods, nanorod spheres and dispersed nanorods, respectively. The EDS spectra in Figure 1(d-h) insets further demonstrate the compositions of all samples. Equation (1) and (2) depict the chemical reactions for generating MnO2 in M-1 (Equation (1)), M-2 and M-3 (Equation (2))solutions[26,27]. Figure 2 shows the crystal growth process under hydrothermal reaction condition. At first, a number of crystal nuclei rapidly form in the solution, which aggregate into nanoparticles. Afterwards, nanosheets grow through the Ostwald ripening mechanism around the nanoparticles due to the particular lamellar crystal structure of δ-MnO2 and the intercalation of K+. As the hydrothermal reaction continues, the nanosheet spheres gradually disintegrate and form the intercalated nanosheets. Meanwhile, since α-MnO2 is more stable than δ-MnO2 thermodynamically, the δ-MnO2 phase begins to transform into the α-MnO2 phase with the α-MnO2 nuclei generating in the δ-MnO2 nanosheets. Then, the δ-MnO2 crystal domains diffuse to the α-MnO2 nucleus and convert into α-MnO2, while α-MnO2 nanorods grow through the Ostwald ripening mechanism[26,28]. In M-2 and M-3 solutions, the strong reducibility of Cl- and the presence of H+ greatly accelerate the chemical reaction and phase transition speed.
Figure 3(a and b) show the cyclic voltammetry curves with the sweep speed of 5 mV•s-1 and corresponding specific capacitances of samples TN-MO-SS, TN-MO-S, TN-MO-SR, TN-MO-RS, and TN-MO-R. TN-MO-S has the largest specific capacitance of 689.88 F•g-1. The specific capacitances of TN-MO-SS, TN-MO-SR, TN-MO-RS and TN-MO-R are 577.45 F•g-1, 407.23 F•g-1, 143.65 F•g-1 and 152.03 F•g-1, respectively. Figure 3(c and d) show the cyclic voltammetry curves with the sweep speed of 5 mV•s-1 and corresponding specific capacitances of TNTM, TO-MO-S and TN-MO-S. Obviously, the specific capacitance of TN-MO-S is about 6.1 times and 2.5 times of TNTM and TO-MO-S, respectively. The results demonstrate that the synergistic effects of MnO2 nanosheets and TiN nanotube arrays significantly increase the specific capacitance. The specific capacitance mainly depends on the surface area of MnO2 and the capacity of K+ . TN-MO-S and TN-MO-SS have large specific surface area and great capacity for K+ due to layered crystal structure (Figure 4). The 3D structures formed by the intercalation of nanosheets benefit to energy storage with electrolyte ions intercalation/deintercalation and provide numerous chemical reaction sites. In addition, the contact between MnO2 nanosheets and TiN nanotubes is more sufficient and tighter than that of MnO2 nanorods, which facilitates the transport of electrons between the substrate and the active substance (Figure 3e). Since the hydrothermal reaction time during the preparation of TN-MO-S is longer than TN-MO-SS, TN-MO-S contains more hydrates to adsorb more K+ than TN-MO-SS, which further improves the pseudo-capacitance. TiN nanotube arrays can not only provide high-speed channels for electron transport, but also expands the specific surface area as a support for active substances providing more space for the ion intercalation/deintercalation during the electrochemical process. Besides, TiN nanotube arrays directly contact with the substrate without the requirement of adhesion agent, which efficiently promotes the charge transfer between the interface.
Figure 5(a and b) show the curves and corresponding specific capacitrances of TN-MO-S at different scan rates. The cyclic voltammetry curves maintain symmetrical shapes from 0.005 V•s-1 to 0.1 V•s-1, indicating the magnification capacity of the electrode material. The specific capacitance decreases with the increase of scan rate because of the insufficient Faraday reaction time at high scanning rate. Figure 5c shows the charging-discharging curves of TN-MO-S at different current densities. The nearly symmetrical triangular outlines manifest the capacitive and reversible characters of the electrode. The Nyquist plot, corresponding fitted curve and the equivalent circuit of TN-MO-S are shown in Figure 5d. The internal resistance (R1) and the charge transfer resistance (R2) of the electrode are low as 1.183 Ω and 52.23 Ω, respectively, indicating the excellent electronic conductivity and electron diffusion. Figure 5e depicts the cycle stability of TN-MO-S by charging-discharging measurements at a current density of 2 A•g-1 for consecutive 500 cycles. The specific capacitance of the electrode maintains 97.2% and 82.4% of initial capacitance after 100 and 500 cycles, respectively. Figure 6 shows the composition and morphology of TN-MO-S after 500 charging-discharging measurement cycles. Generally, the composition and morphology hardly change as shown in Figure 6(a-c). Meanwhile, as shown in Figure 6(d and e), the amount of MnO2 nanosheets deposited in some areas of the sample is reduced, indicating that the lost of active substance is the main reason for the specific capacitance attenuation. However, it can be obviously observed in Figure 6(d and e) that MnO2 nanosheets firmly and uniformly grow on not only the nanotube array surface but also the walls of nanotubes. The close integration of MnO2 nanosheets and TiN nanotubes improves the transportation of electrons and ions so that TN-MO-S has great electrochemical performance as a SCs electrode.