High-performance porous molybdenum oxynitride based fiber ...

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High-Performance Porous Molybdenum Oxynitride Based Fiber Supercapacitors Dan Ruan,†,# Rui Lin,‡,# Kui Jiang,§ Xiang Yu,∥ Yaofeng Zhu,⊥ Yaqin Fu,⊥ Zilong Wang,*,† He Yan,*,§ and Wenjie Mai*,† †

Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Department of Physics, ‡Department of Chemistry, and ∥Analytical and Testing Center, Jinan University, Guangzhou, Guangdong 510632, People’s Republic of China § Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong ⊥ Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, Zhejiang 310018, People’s Republic of China S Supporting Information *

ABSTRACT: Scalable manufacturing of flexible, fiber-shaped energy-storage devices has enabled great technological advances in wearable and portable technology. Replacing inefficient oxides with inexpensive and high-performance oxynitrides with more favorable three-dimensional (3D) structures is critical if the practical applications of these technologies are to be realized. Here, we developed a facile and controllable approach for the synthesis of 3D porous micropillars of molybdenum oxynitride (MON), which exhibit high conductivity, robust stability, and excellent energy-storage properties. Our fiber electrode, containing a 3D hierarchical MON-based anode, yields remarkable linear and areal specific capacitances of 64.8 mF cm−1 and 736.6 mF cm−2, respectively, at a scan rate of 10 mV s−1. Moreover, a wearable asymmetric supercapacitor based on TiN/MON//TiN/MnO2 demonstrates good cycling stability with a linear capacitance of 12.7 mF cm−1 at a scan rate of 10 mV s−1. These remarkable electrochemical properties are mainly attributed to the synergistic effect between the chemical composition of oxynitride and the robust 3D porous structure composed of interconnected nanocrystalline morphology. The presented strategy for the controllable design and synthesis of novel-oxide-derived functional materials offers prospects in developing portable and wearable electronic devices. We also demonstrate that these fiber supercapacitors can be combined with an organic solar cell to construct a self-powered system for broader applications. KEYWORDS: fiber supercapacitor, molybdenum oxynitride, 3D porous structure, high-performance, self-powered system

1. INTRODUCTION

Molybdenum trioxide (MoO3) is an excellent anode material because of its low cost, diversity of chemical valence states, and high work function (6.9 eV).7 Specifically, a large difference between the work functions of the anode and cathode pair will lend ASCs a wide operating voltage, deemed the dominant factor for capacitive performance.8 However, the application of MoO3 is limited by its disappointing intrinsic conductivity (10−5 S cm−1).9 Therefore, most reported MoO3 anodes suffer from performance degradation, sluggish faradic redox kinetics, and poor rate capability.10 To circumvent this issue, various carbon materials have been utilized as supports to provide larger contact surface areas and enhanced electrical conductivity.11 However, the carbon supports tend to have low capacitance, which lowers the overall performance of wearable ASCs. Therefore, the development of a MoO3-based anode

Recently, modern smart electronics, such as smart watches/ phones and e-skins, have enabled a range of functionalities to be conveniently integrated into on-body equipment.1,2 However, powering these devices remains challenging. In various forms of energy-storage systems, wearable asymmetric supercapacitors (ASCs) have attracted substantial attention because of their fast chargeability, ultralong cycle life, and suitable power density.3 In particular, considering the need to fit the curved surface of the human body and be integrable into everyday textiles, wearable ASC devices with fiber shapes have attracted much attention for their promising mechanical advantages of low weight, minuscule volume, and high flexibility.4−6 However, the electrodes of these wearable ASCs require further development to meet the critical requirement of sufficient energy density. For ASCs, higher energy densities are mainly achieved by increasing the operating voltage, which can be enabled by introducing suitable anode materials. © 2017 American Chemical Society

Received: May 26, 2017 Accepted: August 16, 2017 Published: August 16, 2017 29699

DOI: 10.1021/acsami.7b07522 ACS Appl. Mater. Interfaces 2017, 9, 29699−29706

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ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustrating the Synthesis Procedure of the Ti@TiN/MON Electrode and Assembly of the Wearable ASC with Cotton Yarn

Figure 1. Morphology characterization of the electrodes. (a−c) SEM images of Ti@TiO2/MoO3; (d−f) Ti@TiN/MON electrodes at different magnifications.

oxynitride (MON) and to enable the assembly of wearable ASCs, thus promoting the development of energy-storage electronics. Herein, we designed a novel in situ nitridation strategy to produce TiN-nanotube-supported mesoporous three-dimensional (3D) MON micropillars as anodes for wearable ASCs. This strategy cannot only construct high-performance oxynitride but also produce a conductive TiN layer. Compared with a carbon support, the TiN support shows superior capacitance performance that can bring better performance to the anode and cathode.17 In addition, the 3D electrode architecture of MON micropillars maximized the number of active sites accessible to ions during the charge/discharge process and simultaneously provided high energy density without compromising the intrinsic power density.18 Furthermore, in the nitridation process, low-valence-state molybdenum atoms were produced with nitrogen dopant. As the result, the 3D hierarchical TiN/MON anode electrode yielded a remarkable specific capacitance of 64.8 mF cm−1/736.6 mF cm−2 at a scan rate of 10 mV s−1; when it was applied in a

with both high conductivity and capacitance is a key challenge in the creation of a high-performance wearable ASC. Fortunately, great progress has been made in the discovery of metal nitrides that can dramatically boost conductivity while also providing more active sites for electrochemical reactions, thus representing promising systems in energy technology.12,13 Additionally, recent studies have revealed that certain oxides present within metal nitrides very effectively improve the electrochemical capacity and the reaction kinetics, without deterioration of the rich structural morphology and material conductivity.14−16 For example, Yu et al. synthesized “holey” tungsten oxynitride nanowires by nitridation of WO3 nanowires and applied them as an electrochemical electrode. They found that the synergistic effect between the two phases (oxide and nitride) greatly enhanced the energy-storage performance, which suggests that metal oxynitrides with complex compositions may show improved electrochemical activity compared with single-phase oxides or nitrides.14 Drawing on these advances, we anticipate the development of a controllable preparation process to construct an optimized molybdenum 29700


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resolution TEM (HRTEM) image of MoO3 (inset in Figure 2a) exhibits the lattice-resolved fringe with the spacing of 0.326 nm, corresponding to the (021) plane of MoO3. The selected area electron diffraction image demonstrates the hexahedral monocrystalline structure of the MoO3 (inset in Figure 2a). A closer examination of a MON hexahedral micropillar reveals the highly porous texture throughout the whole pillar (inset in Figure 2b). Apparently, compared with pure-phase MoO3 shown in Figure 2a, an obvious enrichment of the nanocrystal and mesoporous appearance occurs on the MON surface after nitridation. Additionally, the HRTEM image (Figure 2c) of the MON proves the existence of crystalline MoO2 and Mo2N. The observed lattice spacings of 0.243 and 0.19 nm correspond to the (−211) plane of MoO2 and the (202) plane of Mo2N, respectively. From the HRTEM images, we can find the interconnected nanocrystalline structure that can reduce interfacial resistance and accelerate the rate of transport of electrons and ions formed after nitridation.20 The scanning transmission electron microscopy/energy-dispersive spectroscopy elemental mapping images (Figure 2d) of the MON samples show that the Mo, N, and O elements are distributed throughout the whole micropillar, suggesting the successful transformation from MoO3 to the MON micropillar. The X-ray diffraction (XRD) patterns (Figures 3a and S8c) confirm the structure change from oxide to oxynitride. They demonstrate that hexagonal MoO3 (JCPDS no. 05-0508) slowly converted into a MON containing MoO2 (JCPDS no. 65-5787) and Mo2N (JCPDS no. 25-1366). The XRD results reveal the full conversion from oxide to nitride when the thermal treatment time is up to 90 min (Figure S8c). Moreover, the formation of oxynitride can be verified by the X-ray photoelectron spectroscopy (XPS) result. A detailed analysis was conducted around the peaks at 230 and 395 eV standing for the Mo 3d and N 1s-Mo 3p3/2 peaks. Peak deconvolution of the Mo 3d XPS spectrum divulges the contribution from MoO3 (d5/2 at 235.4 eV), MoO2 (d5/2 at 230.0 eV), and Mo2N (d5/2 at 229.3 eV).21,22 The presence of Moδ+ (0 < δ+ < 4) in Mo2N corresponds to a lower Mo phase with the N coordination, which means that the valence state of the molybdenum ion from MoO3 is gradually reduced upon thermal treatment in NH3 (Figure 3b).21 Additionally, the N 1s-Mo 3p3/2 spectrum (Figure 3c) also reveals the strongest peak of N (1s at 396.7 eV) and the other peaks of MoO3 (3P3/2 at 398.5 eV), MoO2 (3P3/2 at 395.9 eV), and Mo2N (3P3/2 at 394.5 eV).23,24 The XPS results of other elements are shown in Figure S7. All the above results confirm the formation of MON. The effect of the nitridation strategy in capacitance behavior was studied as follows. Figure 4a shows the cyclic voltammetry (CV) curves of the molybdenum oxide and MON electrodes (Ti@TiO2/MoO3 and Ti@TiN/MON) at a scan rate of 100 mV s−1 between the potential of −0.3 and −1 V with a saturated calomel electrode as the reference electrode. All the oxynitride samples show higher capacitance performance compared with that of the oxide one. Specially, the sample of Ti@TiN/MON-2 shows the highest capacitance of 64.8 mF cm−1/736.6 mF cm−2 (Figure 4b), which is much higher than that of Ti@TiN/MON-3 (considered to be Ti@TiN/Mo2N, 55.8 mF cm−1/634.3 mF cm−2). The improvement in capacitance performance may come from the higher conductivity of MON brought by the nitridation strategy. Then, the electrochemical impedance spectroscopy (EIS) was applied to evaluate the conductivity of different samples. In Figure 4c, the Nyquist plots reveal that the equivalent series resistance

wearable ASC of TiN/MON//TiN/MnO2, this wearable device exhibited an excellent capacitance performance of 12.7 mF cm−1/75.1 mF cm−2. Notably, the as-prepared ASC device showed a benign mechanical performance of highly flexible and tailorable properties, which would be seen as a promising application in wearable devices. After being charged by a nonfullerene organic solar cell for 30 s,19 our wearable ASC could successfully power a light emitting diode (LED, green). On the basis of these facts, it is believed that our wearable ASC, upon being integrated using simple preparation methods with 3D electrode active materials, will hold great promise as an alternative electrode in energy-storage systems.

2. RESULTS AND DISCUSSION The fabrication process of the wearable supercapacitor as a functional energy textile is shown in Scheme 1. First, an array of TiO2 nanotubes was prepared on the surface of a Ti wire via an anodization method (see Figure S1 for details). Then, the orderly 3D MoO3 micropillars were successfully synthesized on the surface of the Ti@TiO2 wire through a hydrothermal method (donated as Ti@TiO2/MoO3, Figure 1a−c). To produce a high conductive TiN layer and to introduce dopants such as low-valence-state Mo and N atoms, the as-prepared Ti@TiO2/MoO3 materials were thermally treated in NH3 atmosphere for 30, 60, and 90 min (denoted as MON-1, MON-2, and MON-3, respectively). The morphologies of the as-synthesized electrodes were characterized by field-emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As observed from SEM images in Figures 1d−f and S2, the as-fabricated MON on the TiN layer inherited the morphology of 3D MoO3 micropillars and formed a rough surface with a mesoporous structure. It is believed that this structure comes from the anion exchange and phase transformation process.16 This unique structure would provide efficient channels allowing fast and easy access of electrolyte ions to the surface of the electrode and increase the surface area bringing more active sites for ion adsorption. The TEM images confirmed the morphology change from MoO3 (Figure 2a) to MON (Figure 2b) after thermal treatment in an NH3 atmosphere. A high-

Figure 2. TEM images of (a) the MoO3 micropillar and (b) the porous MON micropillar; (c) HRTEM image showing the lattice spacing; and (d) elemental mapping images of MON. 29701


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Figure 3. (a) XRD patterns of MoO3 and MON-2; high-resolution XPS spectra of (b) Mo 3d and (c) N 1s-Mo 3p3/2.

Figure 4. (a) CV curves at a scan rate of 100 mV s−1, (b) linear capacitance, and (c) Nyquist plots of the Ti@TiO2 and Ti@TiO2/MoO3 electrodes under different nitridation times; (d) CV curves, (e) calculated linear/areal capacitance, and (f) GCD curves of the Ti@TiN/MON-2 electrode.

the current density increases from 0.5 to 5 mA cm−1. The hydrophilicity of electrode materials is of great significance because it can provide an efficient access of the ions to the electrode/electrolyte interface when the electrode is used. The hydrophilicities of MoO3 and MON are measured and shown in Figure S5. The contact angles of the water droplets on MoO3 and MON samples gradually decrease along with the increasing annealing duration. These results prove that, after thermal treatment in NH3, the samples show superior wettability compared with pure MoO3. On the basis of the above results, the improved capacitance performance of 3D MON may stem

(Rs) of the electrodes decreased after nitridation. The charge transfer resistance (Rct) value of Ti@TiN/MON-2 is 0.8 Ω, which is much smaller than that of Ti@TiO2/MoO3 (9.4 Ω). This result proves the introduced metal nitride can effectively promote conductivity and simultaneously facilitate electron transfer, resulting in superior capacitance performance.7 Moreover, as shown in Figure 4e, the Ti@TiN/MON-2 electrode also exhibits a satisfactory rate capability, which maintains 71% capacitance when the scan rate increases from 10 to 100 mV s−1. The galvanostatic test shows a similar result (Figure 4f): the decrease in capacitance is just about 25% when 29702


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Figure 5. (a) CV curves of the optimized FSC recorded over different potential windows; (b) linear and areal capacitances with an increase in the potential window; (c) CV curves and (d) GCD curves of the FSC device; (e) Ragone plot of the FSC device (red) compared with other FSC devices (black dots); (f) long cycle time of the FSC device.

retains high linear and areal capacitances of 5.9 mF cm−1/34.9 mF cm−2 even at a high current density of 2 mA cm−1. These results indicate that our device obtains promising application as a wearable power source. Herein, the power and energy densities are two important parameters to characterize the electrochemical performance of the FSC device (Figure 5e). The maximum power density of our device reaches 8.84 mW cm−2 when it was operated at a short discharge time of 4.4 s. Simultaneously, our FSC device get the highest energy density of 23.7 μWh cm−2. These two results are superior to many previous reports such as Ti@TiN/C-FSC (2.69 μWh cm−2 and 809 μW cm−2),17 PANI/stainless steel-FSC (0.95 μWh cm−2 and 100 μW cm−2),26 MnO2-coated CNT-FSC (1.14 μWh cm−2 and 210 μW cm−2),27 and CNT/ordered mesoporous carbon-FSC (1.77 μWh cm−2 and 43 μW cm−2).28 As the long cycling life at high scan rate is an important requirement for potential applications, the cycling-life test was carried out on the FSC device at a scan rate of 50 mV s−1 for 8000 cycles. Our FSC device shows good stability with 84.5% of its initial capacitance retained (Figure 5f). With a view to the application of Ti@TiN/MON//Ti@TiN/MnO2-FSC as a wearable electronic, the CV curves of our device with the bending angles of 0°, 120°, and 180° are shown in Figure 6b. There is no variation significantly observed from these CV curves. As is shown in the stress−strain curves (Figure S11, measured at the rate of 10 mm/min), the ultimate tensile strength of the Ti@ TiO2/MoO3 electrode (the whole diameter of 280 μm) is 350

from the two following aspects. First, the metal oxide and nitride in the nanocrystals may play an important role in the enhanced active-site accessibility, conductivity, and chargetransfer efficiency due to a synergistic combination.7,14 Second, the 3D MON micropillars with the mesoporous structure cannot only decrease the diffusion length of the ions but also increase the contact area with the electrolyte and improve the active material utilization.18 On the basis of the high-performance MON-2 materials, we designed a wearable fiber-shaped solid-state ASC device (marked as Ti@TiN/MON//Ti@TiN/MnO2-FSC), in which Ti@TiN/MnO2 worked as the cathode. Here, TiN/MnO2 is a mature cathode material with high performance for a fiber supercapacitors (FSC; the synthesis and morphology are shown in the Supporting Information).25 The wearable FSC device was fabricated by a novel facile design, which can avoid mixingadditive assembly and allow membrane-free preparation (see the Supporting Information for details).17 The CV curves and the corresponding capacitances shown in Figure 5a, b imply a suitable operating potential of 1.5 V for the Ti@TiN/MON// Ti@TiN/MnO2-FSC device. The nearly rectangular CV curves (Figure 5c) and triangular galvanostatic charge−discharge (GCD) curves (Figure 5d) reveal its ideal capacitive property. According to the CV results (Figure S10), the maximum linear and areal capacitances of the Ti@TiN/MON//Ti@TiN/ MnO2-FSC device are calculated to be 12.7 mF cm−1/75.1 mF cm−2 when the scan rate is 10 mV s−1. Our device still 29703


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Figure 6. (a) CV curves of the original ASC, the two separated parts, and the serial and parallel connections of the two separated parts; (b) CV curves of the device in different bending angles; (c) model of the self-powered smart system; (d) magnified images of the solar cell and fabric weaved by the FSC; (e, f) integrated system of simultaneous solar-energy harvesting and storage.

energy to the FSC device. Then, in consideration of the stitchability of the FSC device, Figure 6d shows that our devices could be woven well by introducing commercial cotton yarns, which demonstrates that our FSC devices are well-utilized and combined in wearable electronics. As a result, the performance of an integrated system of simultaneous solar-energy harvesting (by nonfullerene organic solar cells) and storage (by Ti@TiN/ MON//Ti@TiN/MnO2-FSC) is also demonstrated (Figure 6e, f), where the FSC devices (in series) woven in the textile could power a green LED in the meantime after the energy conversion from polymer solar cells (in series). In general, with excellent flexibility, tailorability, and stitchability, this Ti@ TiN/MON//Ti@TiN/MnO2-FSC device presents great application potential in wearable energy-storage systems.

MPa, while the ultimate tensile strength of the Ti@TiN/MON electrode is 178.4 MPa. Here, the total strength of the electrodes mainly depends on the intermetal wire. These above results demonstrate that the annealing process reduces the flexibility of the electrode, which could be explained by the intrinsic property of the metal. The FSC device shows the ultimate tensile strength of 103 MPa when the strain is 5.3%. Here, after the breakage of the two electrodes, the solid-state electrolyte is not completely destroyed in tests (Figure S11b). These results exhibit that the tensile strength of the fiber is robust enough for use in wearable electronics. These results indicate that our device has superior flexibility and is suitable for a flexible and wearable device, which exhibits a wide difference in deformation and strength. According to practical applications, a single FSC device was cut to test the tailorable ability (Figure 6a), from which the parted device showed 49.8% and 50.1% capacitance retentions compared to the original CV result. Then, we reconnected the two damaged parts of the FSCs in series or parallel to explore their mechanical and capacitive stabilities. From the result, it is only a 5.1% loss that the capacitance of the paralleled devices showed compared with that of the untouched FSC device, revealing that the FSC device that we fabricated can be resumed via a simple connection. In the future development of a self-powered smart system of simultaneous solar-energy harvesting and storage (Figure 6c), nonfullerene organic solar cells designed by Yan at HKUST19 were used as the photovoltaic cell energy harvesting module because of its portability and mobility for offering enough

3. CONCLUSION In summary, we designed a scalable synthesis of a 3D Ti@TiN/ MON anode electrode, through one-step nitridation of the precursor oxides from anodization and seed-assisted hydrothermal methods; we produced low-value-states Mo; and we modified the smooth surface of the micropillars, which works as high-performance negative materials. After assembling the whole fiber-shaped device, the solid-state wearable FSC exhibits the high capacitance of 12.7 mF cm−1 /75.1 mF cm−2 at a scan rate of 10 mV s−1. The exceptional electrochemical activity might be mainly attributed to the chemical composition and optimized structure of the mesoporous, crystalline form as well as their robust overall 3D structure. Additionally, the asfabricated FSC devices achieve the maximum power density of 29704


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ACS Applied Materials & Interfaces 8.84 mW cm−2 and the highest energy density of 23.7 μWh cm−2. On the other hand, the FSC device that exhibits excellent flexibility and tailorable properties as desired at the device level is significant for wearable electronics and paves the way for developing a highly on-body device, which is appropriate to apply to the integration of a self-powered system and shows satisfactory application in the near future.

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Stability and Tailorable Performance. J. Mater. Chem. A 2017, 5 (2), 814−821. (6) Chai, Z.; Zhang, N.; Sun, P.; Huang, Y.; Zhao, C.; Fan, H. J.; Fan, X.; Mai, W. Tailorable and Wearable Textile Devices for Solar Energy Harvesting and Simultaneous Storage. ACS Nano 2016, 10, 9201− 9207. (7) Yu, M.; Cheng, X.; Zeng, Y.; Wang, Z.; Tong, Y.; Lu, X.; Yang, S. Dual-Doped Molybdenum Trioxide Nanowires: A Bifunctional Anode for Fiber-Shaped Asymmetric Supercapacitors and Microbial Fuel Cells. Angew. Chem., Int. Ed. 2016, 55, 6762−6766. (8) Chang, J.; Jin, M.; Yao, F.; Kim, T. H.; Le, V. T.; Yue, H.; Gunes, F.; Li, B.; Ghosh, A.; Xie, S.; Lee, Y. H. Asymmetric Supercapacitors Based on Graphene/MnO2 Nanospheres and Graphene/MoO3 Nanosheets with High Energy Density. Adv. Funct. Mater. 2013, 23 (40), 5074−5083. (9) Yao, B.; Huang, L.; Zhang, J.; Gao, X.; Wu, J.; Cheng, Y.; Xiao, X.; Wang, B.; Li, Y.; Zhou, J. Flexible Transparent Molybdenum Trioxide Nanopaper for Energy Storage. Adv. Mater. 2016, 28 (30), 6353−6358. (10) Lu, X.-F.; Huang, Z.-X.; Tong, Y.-X.; Li, G.-R. Asymmetric Supercapacitors with High Energy Density based on Helical Hierarchical Porous NaxMnO2and MoO2. Chem. Sci. 2016, 7 (1), 510−517. (11) Cao, X.; Zheng, B.; Shi, W.; Yang, J.; Fan, Z.; Luo, Z.; Rui, X.; Chen, B.; Yan, Q.; Zhang, H. Reduced Graphene Oxide-wrapped MoO3 Composites Prepared by Using Metal-organic Mrameworks as Precursor for All-solid-state Flexible Supercapacitors. Adv. Mater. 2015, 27 (32), 4695−4701. (12) Xu, J.; Jia, G.; Mai, W.; Fan, H. J. Energy Storage Performance Enhancement by Surface Engineering of Electrode Materials. Adv. Mater. Interfaces 2016, 3 (20), 1600430. (13) Xiao, X.; Yu, H.; Jin, H.; Wu, M.; Fang, Y.; Sun, J.; Hu, Z.; Li, T.; Wu, J.; Huang, L.; Gogotsi, Y.; Zhou, J. Salt-Templated Synthesis of 2D Metallic MoN and Other Nitrides. ACS Nano 2017, 11 (2), 2180−2186. (14) Yu, M.; Han, Y.; Cheng, X.; Hu, L.; Zeng, Y.; Chen, M.; Cheng, F.; Lu, X.; Tong, Y. Holey Tungsten Oxynitride Nanowires: Novel Anodes Efficiently Integrate Microbial Chemical Energy Conversion and Electrochemical Energy Storage. Adv. Mater. 2015, 27 (19), 3085−3091. (15) Wang, S.; Zhang, L.; Sun, C.; Shao, Y.; Wu, Y.; Lv, J.; Hao, X. Gallium Nitride Crystals: Novel Supercapacitor Electrode Materials. Adv. Mater. 2016, 28 (19), 3768−3776. (16) Chen, T. T.; Liu, H. P.; Wei, Y. J.; Chang, I. C.; Yang, M. H.; Lin, Y. S.; Chan, K. L.; Chiu, H. T.; Lee, C. Y. Porous Titanium Oxynitride Sheets as Electrochemical Electrodes for Energy Storage. Nanoscale 2014, 6 (10), 5106−5109. (17) Sun, P.; Lin, R.; Wang, Z.; Qiu, M.; Chai, Z.; Zhang, B.; Meng, H.; Tan, S.; Zhao, C.; Mai, W. Rational Design of Carbon Shell Endows TiN@C Nanotube Based Fiber Supercapacitors with Significantly Enhanced Mechanical Stability and Electrochemical Performance. Nano Energy 2017, 31, 432−440. (18) Lukatskaya, M. R.; Dunn, B.; Gogotsi, Y. Multidimensional Materials and Device Architectures for Future Hybrid Energy Storage. Nat. Commun. 2016, 7, 12647. (19) Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; Inganäs, O.; Gundogdu, K.; Gao, F.; Yan, H. Fast Charge Separation in a Non-fullerene Organic Solar Cell with a Small Driving Force. Nature Energy 2016, 1 (7), 16089. (20) Leng, K.; Chen, Z.; Zhao, X.; Tang, W.; Tian, B.; Nai, C. T.; Zhou, W.; Loh, K. P. Phase Restructuring in Transition Metal Dichalcogenides for Highly Stable Energy Storage. ACS Nano 2016, 10, 9208−9215. (21) Shi, C.; Zhu, A. M.; Yang, X. F.; Au, C. T. On the Catalytic Nature of VN, Mo2N, and W2N Nitrides for NO Reduction with Hydrogen. Appl. Catal., A 2004, 276 (1−2), 223−230. (22) Choi, J. G.; Thompson, L. T. XPS Study of As-prepared and Reduced Molybdenum Oxides. Appl. Surf. Sci. 1996, 93 (2), 143−149.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07522. Details regarding the experimental procedure, characterization, and calculation methods; XPS spectra; SEM and TEM images; XRD patterns; N2 adsorption and desorption isotherms; pore size distribution curve of the TiN/MON-2 electrode; contact angles showing hydrophilicity; schematic illustrating the synthesis procedure of the Ti@TiN/MnO2 electrode and its morphology; typical tensile strength curves; and CV and GCD curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zilong Wang: 0000-0002-3642-9111 He Yan: 0000-0003-1780-8308 Author Contributions #

D.R. and R.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Junpeng Xie and Ms. Ying Zhong for experimental assistance and helpful discussions. We acknowledge financial support from the National Natural Science Foundation of China (grant no. 21376104), the Natural Science Foundation of Guangdong Province, China (grant no. 2014A030306010), and the Science and Technology Planning Project of Guangdong Province, China (grant no. 2016B020244002).

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