Nanostrucutured MnO2-TiN nanotube arrays for advanced supercapacitor electrode material

The capacitance of MnO2 supercapacitors (SCs) is not high as expected due to its low conductivity of MnO2. The synergistic effects of MnO2 with high theoretical specific capacitance and TiN with high theoretical conductivity can extremely enhance the electrochemical performance of the MnO2-TiN electrode material. In this work, we synthesized different nanostructured and crystalline-structured MnO2 modified TiN nanotube arrays electrode materials by hydrothermal method and explained the formation mechanism of different nanostructured and crystalline-structured MnO2. The influences of MnO2 nanostructures and crystalline-structures on the electrochemical performance has been contrasted and discussed. The specific capacitance of δ-MnO2 nanosheets-TiN nanotube arrays can reach 689.88 F g−1, the highest value among these samples TN-MO-SS, TN-MO-S, TN-MO-SR, TN-MO-RS, and TN-MO-R. The reason is explained based on MnO2 nanostructure and crystalline-structure and electron/ion transport properties. The specific capacitance retention rates are 97.2% and 82.4% of initial capacitance after 100 and 500 cycles, respectively, indicating an excellent charging-discharging cycle stability.

www.nature.com/scientificreports/ as expected due to the low conductivity of MnO 2 (10 −3 ~ 10 −4 S m −1 ) 16 . Therefore, MnO 2 needs to be compounded with other materials with good electrical conductivity to improve the overall electrochemical performance including specific capacitance, charge/discharge performance, and cycle characteristics, researchers have made many attempts to prepare supercapacitor electrodes by mixing MnO 2 with highly conductive materials [17][18][19][20][21][22][23][24] , Since transition metal nitrides have great electrical conductivity, electrochemical characteristics, chemical stability and long service life, TiN, VN, WN, CrN and TiVN are widely used as electrode materials of SCs [25][26][27] . TiN has been used in electric devices such as microelectronics, semiconductor device electrodes, lithium ion batteries, fuel cells and SCs as a low-cost transition metal nitride with good conductivity (4000 ~ 55,500 s cm −1 ) and stability [28][29][30] . Tang et al. used urea and TiCl 4 to prepare TiN as a SCs electrode material with a specific capacitance of 407 F g −131 .
In this work, MnO 2 nanosheet spheres, nanosheets, nanorod spheres, and nanorods are synthesized on TiN nanotube arrays for obtaining an electrode material for SCs, where the nanostructured MnO 2 is more chemically stable than MoS 2 32 and has better electrochemical performance than layered MnO 2 33 . The composition and morphology are measured by using XRD, SEM and EDS. The electrochemical performances of all samples in an electrolyte containing K + are measured and discussed.

Methods
Preparation of TiO 2 NTAs on mesh (TONM). All reagents are analytical grade and used without further purification. A large piece of raw Ti mesh (50 meshes, 99.5% purity) with a thickness of 0.12 mm was cut into square pieces of 2.5 × 2.5 cm 2 , which were ultrasonically degreased in acetone, isopropanol, and methanol for 15 min, respectively, then chemically etched in a mixture of HF and HNO 3 aqueous solution (HF:HNO 3 :H 2 O = 1:4:10 in volume, total 20 mL) for 10 s, afterward rinsed with deionized water and finally dried in air. Electrochemical anodic oxidation was performed at 60 V direct current voltages for 24 h in DEG solution containing 1.5 vol.% HF, using Ti mesh as the working electrode and Pt plate as a counter electrode. The as-prepared samples were ultrasonically rinsed with deionized water and dried in the air 33 .
Preparation of TiN NTAs on mesh (TNNM). TONM samples in a quartz boat were placed in the heating center of a horizontal quartz tube vacuum furnace. Prior to heating, the system was evacuated and flushed with high pure N 2 to eliminate oxygen. Afterward, the furnace was heated in N 2 to 750 °C, and then changed to NH 3 flow keeping a flow rate of 100 mL/min for 5 h while the temperature was maintained. Finally, the furnace cooled down to room temperature in N 2 .
Preparation of MnO 2 modified TNNM. Different precursor solutions were employed for synthesizing MnO 2 nanostructures by hydrothermal synthesis method. A TNNM sample was placed at the bottom of the reaction solution in a sealed 150 mL Teflon-lined autoclave, which was put into a muffle furnace for hydrothermal reaction. The solution compositions and reaction solutions are summarized in Table 1.
Characterization. The crystalline phase compositions of the samples were measured by a Rigaku D/ Max 2550VB3 + /PC X-ray diffractometer (XRD) equipped with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm). Nanostructures and elemental distributions of the samples were characterized by a Schottky field emission scanning electron microscopy (FESEM, FEI Nova NanoSEM 450) equipped with energy dispersive spectroscope (EDS, EDAX).
Electrochemical performance measurement. Electrochemical measurement was measured by CHI 660E electrochemical system using a three-electrode system where the samples as a working electrode, Pt foil as a counter electrode, and Ag/AgCl electrode as a reference electrode in 2 mol/L KCl solution. Cyclic voltammetry (CV) curves were obtained in a voltage range from − 0.2 V to 0.8 V at different scan rates of 5, 10, 20, 40, 60, 80 and 100 mV s −1 , respectively. Galvanostatic charge/discharge curves were recorded in a potential window from − 0.2 V to 0.8 V at a series of current densities. The electrochemical impedance spectroscopy (EIS) was conducted in the frequency from 100 kHz to 10 mHz at an open-circuit potential vibration of 5 mV 33 .    (2)) solutions 34,35 . Figure 2 shows the crystal growth process under hydrothermal reaction conditions. At first, a number of crystal nuclei rapidly form in the solution, which aggregates into nanoparticles. Afterward, nanosheets grow through the Ostwald ripening mechanism around the nanoparticles due to the particular lamellar crystal structure of δ-MnO 2 and the intercalation of K + . As the hydrothermal reaction continues, the nanosheet spheres gradually disintegrate and form the intercalated nanosheets. Meanwhile, since α-MnO 2 is more stable than δ-MnO 2 thermodynamically, the δ-MnO 2 phase begins to transform into the α-MnO 2 phase with the α-MnO 2 nuclei generating in the δ-MnO 2 nanosheets. Then, the δ-MnO 2 crystal domains diffuse to the α-MnO 2 nucleus and convert into α-MnO 2 , while α-MnO 2 nanorods grow through the Ostwald ripening mechanism 34,36 . 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 34 .  (Fig. 4) 11 . The 3D structures formed by the intercalation of nanosheets benefit energy storage with electrolyte ions intercalation/de-intercalation and provide numerous chemical reaction sites. In addition, the contact between MnO 2 nanosheets and TiN nanotubes is more sufficient and tighter than that of MnO 2 nanorods, which facilitates the transport of electrons between the substrate and the active substance (Fig. 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/de-intercalation 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 5a,b show the curves and corresponding specific capacitance 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 a 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 is shown in Fig. 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 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 Fig. 6a-c. Meanwhile, as shown in Fig. 6d,e, the amount of MnO 2 nanosheets deposited in some areas of the sample is reduced, indicating that the loss of the active substance is the main reason for the specific capacitance attenuation. However, it can be observed in Fig. 6d,e that MnO 2 nanosheets firmly and uniformly grow on not only the nanotube array surface but also the walls of nanotubes. The close integration of MnO 2

Conclusions
In summary, various MnO 2 nanostructures are synthesized on TiN nanotube arrays by hydrothermal method including nanosheet spheres, nanosheets, nanorod spheres, and nanorods for developing an advanced electrode material of SCs. The TiN nanotubes with excellent conductivity and great specific surface area provide highly efficient paths for charge transport and more electrochemical reaction sites. The nanostructured MnO 2 with high theoretical specific capacitance of 1370 F g −1 improves the pseudocapacitance reaction and specific capacitance of the electrode material. The specific capacitance of δ-MnO 2 nanosheets-TiN nanotube arrays can reach 689.88 F g −1 because of its good magnification capacity and its excellent electronic conductivity and electron/ ion transport properties. Its specific capacitance retention rate is 97.2% and 82.4% of initial capacitance after 100 and 500 cycles, respectively, indicating a good charging-discharging cycle stability. Hence, the synergistic effect of TiN and MnO 2 can extremely enhance the electrochemical performance of the electrode material for SCs.  www.nature.com/scientificreports/