Carbon nanotubes and manganese oxide hybrid nanostructures as high performance fiber supercapacitors

Manganese oxide (MnO2) has long been investigated as a pseudo-capacitive material for fabricating fiber-shaped supercapacitors but its poor electrical conductivity and its brittleness are clear drawbacks. Here we electrochemically insert nanostructured MnO2 domains into continuously interconnected carbon nanotube (CNT) networks, thus imparting both electrical conductivity and mechanical durability to MnO2. In particular, we synthesize a fiber-shaped coaxial electrode with a nickel fiber as the current collector (Ni/CNT/MnO2); the thickness of the CNT/MnO2 hybrid nanostructured shell is approximately 150 μm and the electrode displays specific capacitances of 231 mF cm−1. When assembling symmetric devices featuring Ni/CNT/MnO2 coaxial electrodes as cathode and anode together with a 1.0 M Na2SO4 aqueous solution as electrolyte, we find energy densities of 10.97 μWh cm−1. These values indicate that our hybrid systems have clear potential as wearable energy storage and harvesting devices. Manganese dioxide is a promising material for energy storage applications, but is limited by its brittleness and poor conductivity. Here, manganese dioxide domains are electrochemically deposited onto carbon nanotube networks to produce flexible and conductive hybrid fiber-shaped supercapacitors.

Of the many outstanding achievements, an effort made by Liu et al. 26 is noteworthy: they built up a typical supercapacitor exhibiting 110 mF cm −1 energy density, which appears to be the highest value reported to date for purely EDL-based FSCs. The rGO being electrochemically deposited on nickel-coated multifilament cotton yarns was also used as an active material. Its high energy density was attributed to the large surface area of rGO. As current collectors, nickel-coated cotton yarns were synthesized via electroless deposition 26 , thus preserving high flexibility and the best electrical resistance has been optimized at ca. 1.3 Ω cm −1 , a value which is much higher than that of normal nickel fibers (ca. 5.1 × 10 −2 Ω cm −1 ).
For pseudo-capacitive FSCs, energy density values based on reversible redox interactions can be as high as many times that of purely EDL-based capacitors 1 . Therefore, the loading of the redox-active materials has been a challenge in the fabrication of  the pseudo-capacitive FSCs. Redox-active materials, especially inorganic systems such as MnO 2 , RuO 2 , Ni(OH) 2 , and Co(OH) 2 , are highly brittle and exhibit poor electrical conductivity values 27 . In this context, Kim and co-workers 11,28] have recently reported an effective approach that is able to overcome both the brittleness and low electrical conductivity. In particular, manganese oxide (MnO 2 ) nanoparticles were uniformly deposited on a piece of a CNT sheet via drop casting a dispersion containing MnO 2 nanoparticles which was then twisted into bi-scrolled yarns. MnO 2 loading rate could be maximized up to 93 wt% while the CNT sheet based yarns retained excellent flexibility and mechanical durability. In this case, the maximum energy density reported was 60.6 mF cm −1 . Therefore, wrapping MnO 2 with CNT sheet constitutes an alternative to maximize the loading ratio of the redox-active materials. However, the full activation of MnO 2 is not possible via physically wrapping the redox-active materials with CNT sheets. Hybrid structures resembling tissue cells and capillaries in living organisms inspired our studies; one could visualize MnO 2 domains as tissue cells and CNT networks as capillaries. In this study we describe the synthesis of CNT/MnO 2 hybrid nanostructures by electrochemically inserting nanostructured MnO 2 domains into continuously interconnected CNT networks. These CNT/MnO 2 hybrid nanostructures are self-conducting, and the transfer of electrons during charge and discharge can be achieved rapidly; this occurs almost independently of the thickness of the CNT/MnO 2 system. We also fabricate different coaxial fibershaped electrodes using CNT/MnO 2 hybrid nanostructures as the pseudo-capacitive materials. The fiber core, which functions as current collectors can be either metal wires/fibers or carbon fibers. As the pseudo-capacitive materials of FSCs, our CNT/MnO 2 hybrid nanostructures can be loaded up to a maximum thicknesses of ca. 150 μm, while both the Faradic efficiency and the mechanical properties are retained with excellent performance. In particular, we obtain specific capacitances of 231 mF cm −1 , which is about four times the value reported for the CNT-sheet-wrapped MnO 2 electrodes 11 . We believe these results pave the way to establish new assemblies with a high impact in energy-related applications.

Results
Synthesis and characterization of fiber-shaped Ni/CNT/MnO 2 electrodes. Figure 1a schematically illustrates the process for fabricating a coaxial fiber-shaped CNT/MnO 2 hybrid nanostructured electrode, in which a nickel fiber (O.D. 200 μm) was used as the current collector (denoted as Ni/CNT/MnO 2 electrode). Two key features need to be following during preparation: (i) the interconnected CNT networks with thicknesses of ca. 300 nm were prepared by "dipping and drying", and (ii) nanostructured MnO 2 domains were electrochemically deposited within the CNT networks by cyclic voltammetry (CV). The thickness of the MnO 2 domains was precisely controlled by controlling CV scan rate. These two steps (step (i) and step (ii)) were repeated for several cycles until the CNT/MnO 2 hybrid nanostructures with a desirable thickness were achieved. For example, a Ni/CNT/MnO 2 electrode carrying CNT/MnO 2 with a total thickness of 9.1 μm was prepared after 9 cycles of MnO 2 deposition at a CV scan rate of 50 mV s −1 and 8 cycles for depositing the CNT networks. Energy dispersive spectroscopy (EDS) analysis of carbon (contained in CNTs), manganese, and oxygen (present in MnO 2 ) are shown in Supplementary Figure 1a-c. The data indicated that the CNT/MnO 2 hybrid nanostructures are indeed constructed by MnO 2 and CNTs. Elemental EDS line scans (Fig. 1b) are overlapped with element mappings of carbon, manganese, and oxygen (Fig. 1c), and scanning electron microscopy (SEM) images (Fig. 1d). These provided a clear composition of the CNT/MnO 2 hybrid nanostructures. It is clear that MnO 2 domains have penetrated into the CNT networks (Fig. 1e, f). The initial thickness of CNT networks was 300-400 nm but it expanded to 1.1-1.3 μm after the electrochemical deposition of the nanostructured MnO 2 domains. In our experiments, manganese acetate dissolved in 0.1 M sodium sulfate aqueous solution was used as the bath for electrodepositing the nanostructured MnO 2 domains. In particular, manganese ions (Mn 2+ ) penetrated into the whole CNT networks via selfdiffusion. The expansion of the CNT networks from 300-400 nm to 1.1-1.3 μm could be due to the formation of the nanostructured MnO 2 nanoparticles within the CNT networks, similar to previous observations 5,29 . A schematic illustration is shown in Fig. 1e, depicting the distribution of CNT/MnO 2 hybrid nanostructures. While MnO 2 domains were densely packed, the CNT networks exhibited plenty of entangled cavities (Fig. 1d, f). EDS mapping for sodium (cation of electrolyte, Na 2 SO 4 ) was also recorded (Supplementary Figure 1d). In this case, sodium ions were uniformly distributed through the CNT/MnO 2 hybrid nanostructures, thus indicating a high accessibility of the electrolyte. Supplementary Figure 1e shows an SEM image of a crosssectional area at low resolution. Neither the physically overwrapping method 11 nor the "layer-by-layer" method 30 are applicable for depositing MnO 2 nanostructures into the heavily interconnected CNT networks we produced with high electrical conductivity values.
As mentioned above, the thickness of the MnO 2 domains deposited electrochemically depends on the CV scan rate. As mentioned above, the ratio of MnO 2 within the CNT/MnO 2 hybrid nanostructures varies as a function of the CV scan rate, estimated to be 49.39 wt%, 66.13 wt%, 82.99 wt%, 90.71 wt%, 95.13 wt%, and 97.99 wt% for scan rates of 100, 50, 20, 10, 5, and 2 mV s −1 , respectively. These values were calculated from EDS analysis for each CNT/MnO 2 sample. Thus, for the 2 mV s -1 deposited shell, the weight and capacitance of active material are both dominated by MnO 2 , even though CNT plays important role for mechanical support and electron percolation. Nanostructured MnO 2 domains embedded within CNT/MnO 2 hybrids produced via electrochemical deposition exhibit the δcrystalline phase, confirmed by X-ray diffraction (XRD) patterns (Supplementary Figure 5). As redox-active materials, δand αcrystalline nanostructured MnO 2 always resulted in the best capacitance values (γ, λ, and then β) 32,33 . The MnO 2 domains involved in the CNT/MnO 2 hybrid nanostructures were built up by aggregating MnO 2 nanoparticles. The average grain size of MnO 2 nanoparticles were in the range of 5-30 nm, and strongly dependent on the CV scan rate.
Mechanism of energy storage for the CNT/MnO 2 hybrid nanostructures. Similar to MnO 2 -based pseudo-capacitive materials, our CNT/MnO 2 hybrid nanostructures store energy via both redox interactions and ion intercalation/deintercalation. Equation (1) describes the chemical description for the energy storing mechanism 34,35 : where E + denotes the cations of electrolyte. Mn(IV) in MnO 2 displays a high oxidation state, having a higher potential than that of E + during charging the supercapacitor. In other words, Eq. (1) can be rearranged in to Eq. (2), where E + remained largely as cations but being electrically balanced by electrons, e − :  The best reported data for the specific capacitance in MnO 2 using CNT sheets was 60.6 mF cm −1 , which is about one-fourth the value of the performance of our CNT/MnO 2 multi-layered structure. Figure 2 shows CV curves (Fig. 2a), galvanostatic charge/discharge curves (Fig. 2b), cycling performance (Fig. 2c), and electrochemical impedance spectroscopy (EIS) spectra (Fig. 2d) for the coaxial Ni/CNT/MnO 2 electrodes. All experiments were carried out using a three-electrode system in a 1.0 M Na 2 SO 4 solution. The Ni/CNT/MnO 2 hybrid nanostructured electrode used in Fig. 2 was electrochemically prepared using CV scan rates of 10 mV s −1 , and the overall thickness of the CNT/MnO 2 hybrid nanostructure was about 48 μm. Quasirectangular-shaped CV curves were observed when the CV scan rates ranged from 2 mV s −1 to 100 mV s −1 (Fig. 2a), as well as symmetrically shaped galvanostatic charge/discharge curves (Fig. 2b), thus indicating the presence of an ideal pseudocapacitive material 29,36 . The specific capacitance vs. current densities for this Ni/CNT/MnO 2 electrode was calculated based on the galvanostatic charge/discharge curves. It was found to be 51.3, 49.1, 44.1, 37.5, and 36.2 mF cm −1 for current densities of 0.1, 0.2, 0.5, 0.7, and 1.0 mA cm −1 , respectively. The specific capacitance in weight of a 10-layered (CV rate for electrochemical deposition, 10 mV s −1 ) CNT/MnO 2 hybrid electrode was calculated to be 236.5 F g −1 at a current density of 0.1 mA cm −1 by the mass of CNT/MnO 2 hybrid nanostructuresd shell. Long charge/ discharge cycling tests were performed at a current density of 2 mA cm −1 , and the electrochemical stability of the CNT/MnO 2 hybrid nanostructures remained nearly constant even after 3000 cycles (Fig. 2c). We also measured the diameter of the electrode after certain cycles of charging and discharging processes; changes were not observed, indicating the excellent durability of the CNT/MnO 2 hybrid electrode. The CNT/MnO 2 hybrid electrode remained stable while the working potential for the supercapacitor ranged from 0 V to 0.8 V (vs. Ag/AgCl). In the EIS measurements, the Nyquist plots (Fig. 2d) showed a vertical line in a low-frequency region and the low equivalent series resistance was calculated to be 6.8 Ω cm −1 .
Symmetric supercapacitor devices. Two-electrode-type supercapacitors were assembled and the typical electrochemical responses are shown in Fig. 3. Cathode and anode consisted of a single piece of a Ni/CNT/MnO 2 fiber having an identical shell thickness at around 48 μm and lengths of 1.15 cm; a 1.0 M Na 2 SO 4 aqueous solution was used as the electrolyte. Interestingly, this device gives quasi-rectangular-shaped CV curves (Fig. 3a) and symmetrically shaped galvanostatic charge/discharge curves (Fig. 3b), indicating excellent capacitive performances. Cell linear capacitance remained at 76.3% (from 27.0 to 20.6 mF cm −1 ), even as the current density increased from 0.02 to 0.5 mA cm −1 (Fig. 3c). Energy density and power density of this two-electrode device were calculated to be 2.09 μWh cm −1 and  (Fig. 3d). Typical data on cell energy density and power density reported by other groups 7,9,16 , are also given in Fig. 3d for comparison. Our CNT/MnO 2 hybrid nanostructures clearly show the best performance. Finally, two-electrode devices using Ni/ CNT/MnO 2 electrodes having the maximum CNT/MnO 2 shell thickness (149 μm) resulted in energy densities of 10.97 μWh cm −1 , which, to the best of our knowledge, are the best data on the MnO 2 -based FSCs reported hitherto. Energy density has been further enhanced by using bundled Ni/CNT/MnO 2 fibers as electrodes (cathode and anode). In this context, a symmetrical supercapacitor with a triple-bundled Ni/CNT/MnO 2 fiber (three strands of the 1.15-cm long Ni/CNT/MnO 2 fibers were bundled in parallel and were then used as both cathode and anode) as electrodes was assembled. Surprisingly, this was capable of powering a light-emitting diode (LED; Fig. 4a). Note here that three micro-supercapacitor devices connected in parallel were also capable of powering the LED (Supplementary Figure 7). From the CV (Fig. 4b) and galvanostatic charge/discharge curves (Fig. 4c), the capacitance was enhanced nearly twice and three times when the dual and the triple-bundled Ni/CNT/MnO 2 electrodes were used. The measured resistance (Fig. 4d) was 62.5 Ω, 37.5 Ω, and 12.5 Ω, for the single, dual-bundled, and the triplebundled electrodes, respectively. In other words, bundled Ni/ CNT/MnO 2 electrodes result in high energy densities and high power densities, and also lower the internal resistance values of the pseudo-capacitive materials.

Discussion
It is clear that our fiber supercapacitors are potentially capable of driving wearable electronic devices. We established a novel CNT/ MnO 2 hybrid nanostructured material that resembles tissue cells and capillaries. In our case, CNT networks entangle within nanostructured MnO 2 domains, thus allowing electrons to be injected effectively via these conducting CNT networks. Pseudocapacitive domains have been activated via Faradic and the intercalation/deintercalation interactions, and the capability for storing energy therefore has been enhanced. Moreover, the selfconducting properties of our CNT/MnO 2 hybrid nanostructures enabled us to increase the absolute energy density via the maximized loading rate of MnO 2 . The bending stability of the electrode depends entirely on the properties of the current collectors. CNT/MnO 2 hybrid electrodes with carbon fibers as current correctors showed superior bending stabilities; they were able to bend 360 degrees while the electrochemical performance remained unchanged. Other combinatorial nanostructures, such as CNT/TiO 2 , CNT/RuO 2 , CNT/PbO 2 , CNT/Ni(OH) 2 , and CNT/ Co(OH) 2 , are also possible via the electrochemical-deposition/ CNT network formation methodology. We believe that our results pave the way to the design and development of novel wearable energy storage devices.

Methods
Preparation of aqueous dispersions containing mono-dispersed CNTs. Asgrown, entangled multi-walled CNT powders (NC7000 TM ), were purchased from Nanocyl S.A. (Belgium). They were further dispersed into aqueous suspensions via the following steps: (i) 40 g of the as-purchased CNT powders were pre-dispersed into 1000 ml deionized water using a   Fig. 4 Assembly of bundled Ni/CNT/MnO 2 electrodes for a supercapacitor device. a A digital photograph of a light-emitting diode (LED) powered by a triple-bundled two-electrode type of a symmetric supercapacitor device. b CV curves, c galvanostatic charge/discharge curves and d EIS spectra of a single, dual-, and triple-bundled Ni/CNT/MnO 2 electrode (9-layered MnO 2 and 9-layered CNTs, the total thickness is 48 μm) Preparation of the Ni/CNT/MnO 2 electrodes. Nanostructured MnO 2 domains were anodically electrodeposited onto CNT networks via a cyclic voltammetry. A Ni mono-fiber (diameter, 200 μm) was used as the working electrode, a platinum foil as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The Ni fiber was immersed into an aqueous solution containing 0.1 M Mn(Ac) 2 and 0.1 M Na 2 SO 4 . Cyclic voltammograms were recorded between 0.4 and 1.4 V (vs. Ag/AgCl) at a certain scan rate (from 2 to 100 mV s −1 ) for 1 cycle. After electrodepositing the nanostructured MnO 2 domains, the Ni/MnO 2 electrode was rinsed in deionized water and then dried at room temperature. The Ni/MnO 2 electrode was dipped into the mono-dispersed CNT suspension for 10 s and then rapidly removed. Subsequently, the Ni/CNT/MnO 2 electrode was dried using a heat gun. After drying, the Ni/CNT/MnO 2 electrode was subjected to surfactant/ stabilizer removal by washing with abundant ethanol and deionized water, and dried at room temperature. The two steps were repeated several times until desirable Ni/CNT/MnO 2 hybrid nanostructured electrodes were obtained. Four 10layered CNT/MnO 2 hybrid electrodes (CV scan rate for electrochemical deposition, 10 mV s −1 ) were prepared under identical experimental conditions and the energy density of each electrode was measured. The standard deviation (n = 4) was found to be 5.7%, indicating the CNT/MnO 2 hybrid electrodes are reproducible.
Assembly of the symmetric supercapacitors. Two Ni/CNT/MnO 2 hybrid nanostructured electrodes were placed parallel and ca. 500 μm apart, then fixed with tape and inserted into a flexible plastic tube filled with a 1.0 M Na 2 SO 4 aqueous solution. The tube was finally sealed with epoxy resin to prepare the symmetric devices. One end of each electrode was connected to a 180 μm diameter Cu wire using silver paste for electrochemical performance measurements.
Characterization. The materials were characterized by XRD pattern (R-AXIS RAPID-SH, Rigaku) using a Mo radiation (Kα, λ = 0.071069 nm), field emission SEM (Hitachi SU-8020) equipped with an energy-dispersive X-ray spectroscopy (HORIBA X-Max) and SEM (JEOL JSM-6390, Japan). The cross-sectioned electrodes were prepared using a cross-section polisher (CP, JEOL SM-09010). All electrochemical measurements including CV curves, galvanostatic charge/discharge curves, EIS (that was obtained with a sinusoidal potential excitation of 5 mV in the frequency range from 100 kHz to 0.01 Hz), and cycling performance measurements were carried out using an electrochemical workstation (CHI 760E, CH Instruments). The electrochemical performance of the electrode materials was measured in a three-electrode system with 1.0 M Na 2 SO 4 electrolyte solution. Electrode materials, a platinum foil, and an Ag/AgCl electrodes were used as working electrodes, counter electrodes, and reference electrodes, respectively. The electrochemical performance of the symmetric supercapacitors was evaluated in a twoelectrode system. Two symmetry fiber electrodes of the same length (1.15 cm) and from the same synthesis batch were immersed into a beaker containing 1.0 M Na 2 SO 4 solution, with a distance of about 10 mm.
Calculation of the electrochemical performances. The specific linear capacitance of the single electrode-based pseudo-capacitive material (C l ) in a three-electrode cell was calculated from galvanostatic charge/discharge curves, according to the following equation 7 : where I denotes the constant discharge current; Δt denotes the time for a full discharge; l is the length of pseudo-capacitive electrode; and ΔV is the voltage drop on discharge (excluding the V drop ). The capacitance of the supercapacitor device (C cell ) in a two-electrode cell was calculated from their galvanostatic charge/discharge curves at different current densities based on the following equation 14 : where i is the discharging current and dV/dt is the slope of the discharge curve. The device linear capacitances of the FSCs (C cell,l ) was calculated according to the following equation: where l refers to the device length of the FSCs. The linear energy density of the FSCs (E cell,l ) was obtained from the following equation: where ΔE is the operating voltage window in volts. The linear power density of the FSCs (P cell,l ) was calculated from the galvanostatic curves at different charge/discharge current densities by using the following equation: where t discharge is the discharge time.
The overall capacitance of the CNT/MnO 2 hybrid electrode increased proportionally as the length of the electrode increased. Due to the limitation of the surface area of the counter electrode, the maximum length of the CNT/MnO 2 hybrid electrode prepared in this study was restricted to 5.2 cm.
Data availability. The data that support the findings of this study are available from the corresponding author upon request.