# Electrodeposition of α-MnO2/γ-MnO2 on Carbon Nanotube for Yarn Supercapacitor

## Abstract

Yarn supercapacitors have attracted renewed interest as promising energy storage for wearable devices due to their lightweight, long cycling lifetime and excellent weavability. There has been much effort to fabricate high performance yarn supercapacitor by depositing pseudo-capacitive materials on the outer surface of the carbon fibers. However, a key challenge still remains to achieve high capacitance and high mass loading without sacrificing the cycling stability. Herein, we perform a phase-controlled of MnO2 at various deposition temperatures with ultrahigh mass loading of 11 mg/cm2 on a MWNT sheets and fabricate it to yarn structure to achieve high capacitance without decreasing in the electrochemical performance. The structure of optimized sample (MnO2/CNTs-60, deposition at 60 °C) consists of the composite of primary α-MnO2 nanosheets and secondary γ-MnO2 nanoparticles. The heteronanostructures of MnO2 provide facile ionic and electric transport in the yarn electrode, resulting in improvement of electrochemical performance and cycling stability. The MnO2/CNTs-60 yarn electrode with ultrahigh mass loading delivers a high areal capacitance of 3.54 F/cm2 at 1 mA/cm2 and an excellent rate capability. Finally, the MnO2/CNTs-60 device exhibits an outstanding high areal energy density of 93.8 μWh/cm2 at the power density of 193 μW/cm2, which is superior to previously reported symmetric yarn supercapacitors.

## Introduction

With the rapid development of portable devices and wearable electronics, the yarn supercapacitors has been continuously demanded because of their high power density, lightweight, long cycling lifetime and excellent weavability1,2,3. The multiwalled carbon nanotubes (MWNTs) as electrode materials has been utilized in yarn supercapacitors due to its high surface area, good mechanical strength, flexibility and excellent electrical conductivity4,5,6,7. However, the MWNTs yarn supercapacitors have several urgent disadvantages such as low specific capacitance and low energy density, leading to seriously suffering from their practical applications. Recently, the pseudocapacitive-type electrode materials have gained much attention due to getting the high capacitance by the charge stored through ion adsorption and surface redox reactions. Among various materials, manganese oxide (MnO2) is a promising material because of the abundant resources, low fabrication cost, and high theoretical capacitance8,9,10. More importantly, it has a wide potential window in a neutral aqueous electrolyte and therefore can achieve higher energy density than other cathode materials such as NiO, Ni(OH)2, Ni-Co and PANI11,12,13,14,15,16. However, the using a solely single phase MnO2 as electrode for supercapacitors due to some inherent disadvantages such as poor electrical conductivity and slow ion transport rate is poor in low rate capacity and cycle stability17,18. In order to overcome the drawbacks of MnO2, the co-existence of two-phase MnO2 materials exhibiting improved electrochemical performance due to synergy effect is one of the promising solutions18,19.

The fabrication of MnO2 on the MWNTs yarn through the electrodeposition is one of the important strategies to improve the capacitance of the MWNTs fiber-based supercapacitors20,21,22,23. Up to now, however, when an electrode is produced by the electrodeposition method in a yarn supercapacitor, the MnO2 are directly electrodeposited on the yarn electrode, so that the acceptable load of the MnO2 is limited. In several reported papers, the active material was electrodeposited on twisted CNT yarns and CNT coated spiral nylon fibers used as the core structure, wherein the amount of active material was limited to less than 20 wt%20,21,22. Therefore, a small active material loading exhibits low capacitance and energy stored, which restrict their practical application for high energy systems24,25. Generally, to provide a feasible energy for commercial devices, the high active loading of 8–10 mg/cm2 is required26,27. However, the increase in the loading active material significantly reduces the charge storage capacity, including specific capacitances and rate performance because of the low electrical conductivity, slow ion diffusion and poor mechanical stability of the MnO2 active material.

Herein, to overcome the aforementioned drawback and achieve both high capacitance and loading, the MnO2 was directly deposited on the MWNTs sheets through the electrodeposition technique, and then it was fabricated to yarm structure using biscrolling method. By depositing MnO2 onto MWNTs sheets, it dramatically expands the loading of active materials in yarn to as high as 11 mg/cm2. The MnO2 material composed of primary α-MnO2 nanosheets and secondary γ-MnO2 nanoparticles was grown on the surface of MWNTs sheets using an electrodeposition method at the different deposition temperature. Among them, the MnO2/CNTs-60 yarn electrode exhibits excellent areal capacitance of 3.54 F/cm2 at 1 mA/cm2. It is one of the highest values reported for MnO2-based yarn supercapacitors in gel electrolytes. In addition, it avoids the problem of general mechanical separation of composite materials during long-term cycling, and can improve the cycling stability. The MnO2/CNTs-60 device shows high areal energy density of 93.8 μWh/cm2 at the power density of 193 μW/cm2. This performance is the highest value in the most of the symmetric yarn supercapacitors.

## Results and Discussion

A schematic illustration of the fabrication process for the yarn supercapacitor is presented in Fig. 1a. The five layers of MWNT sheets were stacked on a glass slide. Subsequently, the stacked MWNT sheets were immersed into a 0.1 M Mn(CH3CO2)2.(H2O)n aqueous solution for 40 mins. After deposition, the MnO2/MWNT hybrid sheets were washed with ethanol/water (volume ratio of 1:1). The MnO2/MWNT hybrid sheets were peeled off from the glass slide and then twisted to form yarn supercapacitor through an electric motor.

The MnO2 was directly deposited on the MWNTs sheets through the electrodeposition at different temperatures and its morphologies of the all yarn samples, as presented in Figs 1b–e and S1, were observed through the SEM. At 25 °C of deposition temperature, interconnected MnO2 nanosheets grown on the surface of the MWNTs sheets are shown in Fig. 1b (MnO2/CNTs-25). When the deposition temperature increases at 40, 60 and 80 °C, respectively, it can be seen that not only similar sheets are observed but also small particles are on the nanosheets (MnO2/CNTs-40, 60 and 80, respectively, Fig. 1c–e). The nanosheets are preferred as primary structure to grow on the MWNTs sheets at the early stages of electrodeposition, but the morphologies of secondary particles in the MnO2/CNTs yarn depend on the deposition temperature. Conversely, at 25 °C, the growth of the primary nanosheets is predominant and secondary morphology is not observed. This is because more nucleation sites are allowed to occur on the surface of the nanosheets at the increase in the temperature.

The crystal structure of the electrodeposition MnO2 is investigated by X-ray diffraction (XRD) and shown in Fig. 2(a). The two characteristic peaks of MnO2/CNT-25 yarn electrode at the diffraction angle 2θ = 37.5°, 65.5° are indexed to the (211) and (002) of the α-MnO2 phase (JCPDS 44-0141). The intensity of diffraction peaks is broaden, indicating the poor crystallinity of α-MnO2 in the composite. When the deposition temperature increases from 40 °C to 80 °C, there is not only the α-MnO2 phase, but also two diffraction peaks corresponding to the γ-MnO2 at 2θ = 42.1° and 55.5° (JCPDS 14-0644), which are assigned to the (300) and (160) crystal plane. This indicates that the α-MnO2 phase nanosheets was initially grown on the MWNTs sheets, while the nanoparticles with γ-MnO2 phase were secondarily grown from the deposition temperature of 40 °C, which is consistent with the SEM results. With the increase in the electrodeposition temperature up to 80 °C, the intensity of diffraction peaks of α-MnO2 phase is sharper, indicating high crystallinity of α-MnO2 phase compared to the other samples. Moreover, the diffraction peaks of γ-MnO2 phase for MnO2/CNT-80 yarn electrode are clearly observed, indicating that the large amount of γ-MnO2 phase is formed compared to the other samples. Overall, as the deposition temperature increases, the main crystalline phase of samples has changed from a pure α-MnO2 into a mixture of α-MnO2 and γ-MnO2.

All samples were investigated by X-ray photoelectron spectroscopy (XPS). The Mn and O elemental spectra of the MnO2/CNTs-60 sample are shown in Fig. 2b,c and the other samples are present in Figs S2S5. On the basis of the analysis of the Mn 2p spectrum, the characteristic peaks at 641.7 and 653.3 eV correspond to the Mn 2p1/2 and Mn 2p3/2 spin-orbit peaks. The spin-energy separation of two peaks is 11.6 eV, which is in good accordance with previously reported values for the MnO2 materials28,29,30. In the Mn 3 s spectrum, the binding energy separation of the two peaks for Mn 3 s means an average oxidation state of Mn of MnO228,29. According to previous reports, the separation value of 4.7 eV and 5.4 eV corresponds to Mn4+ and Mn3+29,30. The binding energy separation is 5.2 for MnO2/CNTs-25, 5.2 for MnO2/CNTs-40, 5.1 for MnO2/CNTs-60, and 4.9 for MnO2/CNTs-80, respectively, which suggests an intermediate oxidation state peak between Mn4+ and Mn3+. This means that the deviation from Mn4+ is a result of the formation of defects during the electrodeposition process. Finally, the oxidation states of Mn in MnO2 were estimated by the O 1s peak. The O 1s peaks are deconvoluted with three components, representing the Mn-O-Mn component at 530.2 eV, Mn-O-H component at 531.5 eV, and the H-O-H at 532.6 eV (Figs 2c and S8). The valence of Mn can be also calculated to be 3.42 through the intensities ratio of the Mn-O-Mn and Mn-OH according to a previous study. This result is in good agreement with the XPS analysis of the Mn 3s spectrum31.

In order to confirm the two phases in the MnO2/CNTs yarn electrodes, transmission electron microscopy (TEM) characterization was conducted. Figure 3a displays the α-MnO2 nanosheets with amorphous structure in the MnO2/CNTs-25 sample. In the case of MnO2/CNTs-40 electrode, similar large particles corresponding to the amorphous of α-MnO2 are observed at low magnification TEM image (Fig. 3b), as well, the small particles with orderly lattice planes can be clearly observed in the inset of Fig. 3b. The orderly lattice planes are assigned to the (300) plane (d = 0.21 nm) of γ-MnO2 crystal structure, confirming the existence of two types phases in the MnO2/CNTs-40 yarn electrode. Moreover, at higher temperatures, the amorphous nanosheets are basically present for the samples and it can be seen that the size of the particles with an orderly lattice plane increase. In the HRTEM images of the MnO2/CNTs-60 and 80 samples (Fig. 3c,d), γ-MnO2 present as well as there is other orderly lattice plane, which is indexed to the (211) plane (d = 0.24 nm) of α-MnO2 crystal structure. As mentioned in the XRD result, it is confirmed that the α-MnO2 crystal structure with high crystallinity appears. Meanwhile, the TEM element mapping shows the uniform distributions of Mn and O elements in the MnO2/CNTs-60 profile (Fig. S6). Hence, it is verified that the co-existence of two MnO2 phases is showed in the MnO2/CNTs-40, 60 and 80 samples.

The electrochemical performances were conducted for the MnO2/CNTs-25, MnO2/CNTs -40, MnO2/CNTs-60 and MnO2/CNTs-80 electrodes. Two electrodes cell was fabricated in parallel containing an aqueous poly(vinyl alcohol) (PVA)/LiCl gel electrolyte and then assembled to a solid-state yarn supercapacitor. Figure 4a shows the cyclic voltammetry (CV) curves of all samples at scan rate of 10 mV/s and CV curves of all samples at various scan rates are presented in Fig. S7. The quasi-rectangular shaped CV can be seen in all samples, indicating the energy storage by electrochemical double-layer charging capacitance of the CNTs and the pseudocapacitance of MnO2. As the deposition temperature increases up to 60 °C, the capacitance also increases. However, as the deposition temperature is further increased to 80 °C, the capacitance in the MnO2/CNTs-80 yarn electrode decreases. This phenomenon is also observed when the galvanostatic charge-discharge (GCD) curves of all samples were measured. Figure 4b represents the GCD profile of each electrode at the current density of 1 mA/cm2 and the results of measurement at different current densities (1,2,5,10 and 15 mA/cm2) are shown in Fig. S8. The weight, areal and volume capacitances of all samples with MnO2 loadings of 11 mg/cm2 are summarized in Table S1. The MnO2/CNTs-60 yarn electrode delivers the high areal capacitance of 3.56 F/cm2 at 1 mA/cm2, which is higher than the others yarn electrodes (for MnO2/CNTs-25, for MnO2/CNTs-40, for MnO2/CNTs-80). As previously aforementioned, the heterostructures would cause lattice defects between the intersection of two phases, leading to create electrochemical active sites and increase for fast electron transportation. In the case the MnO2/CNTs-80 yarn electrode, however, it has two phases, but the large particle with high crystallinity is the major drawback for its ionic and electronic conductivity in comparison to the MnO2/CNTs-40 and 60, resulting to slightly decrease in the electrochemical performance. The MnO2/CNTs-40, 60 and 80 yarn electrodes also exhibit excellent rate capability performance with capacitance retention of 55.6, 59.6 and 54.1%, respectively, when the current densities increase from 1 mA/cm2 to 15 mA/cm2, demonstrating the advantage of existence of two phases. In addition, it is hard to come off the MnO2 powder from MWNTs sheets because it is wrapped by the MWNTs sheets (Fig. S1). Therefore, the excellent rate capability is obtained due to the intrinsic nature of the heterophases and MWNTs of the MnO2/CNTs-40, 60 and 80 yarn electrodes. Moreover, in the Nyquist and electrical conductivity plots (Figs S9 and S10 in Supporting information), the MnO2/CNTs-60 yarn electrode shows the lowest equivalent series resistance (Rs) value and high electrical conductivity (50.5 S cm−1) compared with the others samples. This is because the MnO2/CNTs-60 yarn electrode has the high surface area and large reactive active sites compared with the others samples. As a result, the MnO2/CNTs-60 yarn electrode exhibits the excellent capacitance characteristic with fast electrolyte ion response. In the contrast, the areal capacitance of MnO2/CNTs-25 yarn electrode retained only 28.2% with the increase of current density. It is indicated that single phase MnO2 as electrodes suffers from low rate capacity due to high resistance and low electrical conductivity.

In our case, the two phases of MnO2 in the MnO2/CNTs composites provides the improvement of the electron transportation between electrode and electrolyte, leading to higher capacitive current than the one phase MnO2. To demonstrate this, the detailed charge storage mechanisms and electrode kinetics capacitances were calculated by Dunn’s method based on the CV curves at various scan rates32,33. The capacitance of all samples obtained from CV curves can be separated as the capacitive charge storage and the diffusion controlled insertion processes. The capacitive-controlled capacitances are 45.7% for MnO2/CNTs-25, 61.3% for MnO2/CNTs-40, 65.7% for MnO2/CNTs-60 and 53.8% for MnO2/CNTs-80, respectively. The high value of capacitive-controlled capacitance means that the charge storage process can be easily facilitated in the electrode and leads to its excellent rate capability. On the other hands, the low capacitive-controlled capacitances values in the other three electrodes indicate slower kinetics, resulting in the poor rate capability. Consequently, the low charge transfer resistance, small electrical resistance and high capacitive-controlled capacitances of MnO2/CNTs-60 yarn electrode establish inherently excellent electrochemical performance.

Figure 5a shows a Ragone plot of areal energy density versus power density compared with the previously reported supercapacitors. Based on the total surface area of the supercapacitor, including gel electrolyte, the areal energy density and power density of symmetric MnO2/CNTs-60 device was calculated. The maximum areal energy density was 93.8 μWh/cm2 at 193 μW/cm2, which is higher than previously published studies such as (a) PPy/MnO2/rGO, (b) rGO/CNT, (c) PANI/CNT, (d) MnO2/MPNW, (e) pen ink Au/plastic wire, (f) MnO2/ZnO, (g) ZnO nanowire, (h) PEDOT-S:PSS fiber (i) biscolled MnO2/CNT34,35,36,37,38,39,40,41,42. Figure 5b shows the capacitance retention of the symmetric MnO2/CNTs-60 device at a scan rate of 50 mV/s during 1000th cycles. The symmetric MnO2/CNTs-60 device exhibits excellent cycling stability with 98.9% under 1000th cycles because it has a good flexibility by hetero morphologies of MnO2 and MWNT sheets. More importantly, this structure helps to buffer the internal deformation during cycling. In addition, these α-phase components stably maintain long-term cycling due to the large ion tunnels, and multiple junctions between the α- and γ-phases help to further buffer internal crystal deformation. These phenomenons ensure excellent mechanical stability which effectively inhibits electrode degradation and improves cycling stability. In order to demonstrate the practical application of the device and to meet the voltage or power requirements for practical applications, the MnO2/CNTs-60 devices are required to be connected in series or in parallel. As shown in Fig. 5c, the voltage window and current density increase when devices are connected in series and in parallel, respectively. The MnO2/CNTs-60 devices can operate a red light emitting diode (LED, 1.8–2.2 V) even bending. (Fig. S11 in the Supporting information) Moreover, to briefly demonstrate the ability to withstand harsh banding, the MnO2/CNTs-60 sample was measured under different bending angles from 0° to 135° at a scan rate of 50 mV/s. As illustrated in Fig. 5d, the changes in CV curves are negligible, indicating the outstanding flexibility of our devices. In addition, as shown in Fig. 5f, negligible change was observed even knotted. To investigate the stability after bending 1000 cycles, the capacitance retention was maintained after 1000 cycles of bending from 0° to 135°, demonstrating the robust mechanical property of our device. (Fig. 5e).

## Conclusion

A high mass loading of 11 mg/cm2 and the heterophases of MnO2 were deposited on MWNTs sheets through a facile electrodeposition technique, which was made of yarn electrode. When the deposition temperature increases, the α- and γ-phases of MnO2 in MnO2/CNTs can be obtained. The MnO2/CNTs-60 in optimized material is composed of α- and γ-phases of MnO2, which create electrochemical active sites and improve the fast electron transportation. The MnO2/CNTs-60 yarn electrode shows an extremely areal capacitance of 3.54 F/cm2 at 1 mA/cm2 in a gel electrolyte, which is superior to previously reported MnO2 yarn electrodes. Also, the MnO2/CNTs-60 yarn electrode has the good mechanical stability as well as high ionic and electric conductivities of the material due to the heterophases of MnO2 and wrapping of MnO2 particles by MWNT sheet, resulting that it shows excellent cycle retention capacitance with >98% during 1000 charge/discharge cycles. Significantly, the MnO2/CNTs-60 device delivers an extremely high areal energy density of 93.8 μWh/cm2 at the power density of 193 μW/cm2. Our results suggest that the heterostructures with high mass loading enhance the electrochemical performance. It will be the possibility to be applied in practical applications.

## Method

### Materials

Lithium chloride (LiCl, >99%), poly(vinyl alcohol) (PVA, Mw 146,000~186,000) and manganese acetate (Mn(CH3CO2)2.(H2O)n) were purchased from Sigma-Aldrich.

### Electrodeposition of Manganese Oxide (MnO2) on aligned carbon nanotube sheets

As shown in Fig. 1a, the five layers of highly aligned carbon nanotube sheets with the width of ~2 cm and length of 7.5 cm which were drawn from the multiwalled nanotube (MWNT) forest (U053HANYANG-SH158-06, LINTEC Inc.) were stacked on the glass side20,22,42. Subsequently, the stacked MWNT sheets was immersed in a 0.1 M manganese acetate aqueous solution to do the electrodeposition of MnO2 on the MWNT sheets using a potentiostatic method. The electrodeposition of MnO2 on the stacked MWNT sheets was conducted at about 1.3 V for 40 mins using Ag/AgCl as a reference electrode and Pt mesh as a counter electrode in a three electrode system through an electrochemical analyzer (CHI 627b system, CH Instruments, Austin, TX). In order to investigate the effect of temperature on MnO2 growth on the stacked MWNT sheets, the electrodeposition of MnO2 was carried out at various temperature of 25, 40, 60, 80 °C. These samples were named as MnO2/CNTs-25, MnO2/CNTs-40, MnO2/CNTs-60, and MnO2/CNTs-80, respectively. After electrodeposition, all of samples were washed thoroughly by deionized water and then it was peeled off from glass slide and twisted to ~100 turns per meter using an electric motor to form a yarn electrode.

### Supercapacitor assembly

The capacitive performance of solid-state yarn supercapacitor was measured through a two-electrode system. The device was fabricated by placing two MnO2/CNTs yarns in parallel, and then coating the PVA-LiCl (6 M) gel electrolyte. The 3 g of PVA and 6 g LiCl was dissolved in 30 ml deionized water at 90 °C for several hours to prepare the PVA/LiCl gel electrolyte. The Cu wires were attached at the end of two yarns using Ag paste for electrochemical performance measurement.

### Calculation of electrochemical performance

The capacitances of two electrode configuration were calculated from galvanostatic charge-discharge curve by following equation, C = I/(dV/dt) where, I and dV/dt are the discharge current and the slope of the discharge curve, respectively. The specific capacitance of the electrode was calculated by Cs = C/S, where S is area (a), volume (v) and mass (g) of the yarn. The length of the yarn electrodes was fixed to 1 cm. In case of the two electrode systems, area and volume contain both electrodes and the PVA/LiCl gel electrolyte. The specific energy density and power density were calculated from the equation $${E}_{s}=\frac{1}{3600}\frac{1}{2}{C}_{s}{\rm{\Delta }}{V}^{2}$$ and $${P}_{s}=\frac{{E}_{s}}{{\rm{\Delta }}t}$$, where Δt is the discharging time.

### Characterization

The surface morphologies of the materials were observed using a scanning electron microscope (SEM, Hitachi S-4800, Japan). Transmission electron spectroscopy (TEM) images were taken with JEOL-2100F at an acceleration voltage of 200 kV. To determine the mass loading of MnO2 in the MnO2/CNTs yarns, the weight difference of the electrode was measured before and after electrodeposition using a Meter Toledo XP2U semi-microbalance with a readability of 1 μg. The crystal structures of the samples were investigated by X-ray diffraction (XRD, SmartLab, Rigaku). X-ray photoelectron spectroscopy analyses were carried out with Al Kα radiation (XPS, K-alpha plus, Thermo Scientific, USA). All XPS spectra were calibrated using C 1s photoelectron peak at 284.6 eV as the reference. The electrochemical performances of the MnO2/CNTs yarns were obtained by a CHI 660E electrochemical workstation. Electrochemical impedance spectra (EIS) were conducted by applying a sinusoidal voltage of 5 mV in a frequency range from 0.01 to 100 kHz.

## References

1. 1.

Yetisen, A. K. et al. Nanotechnology in Textiles. ACS Nano 10, 3042 (2016).

2. 2.

Zeng, W. et al. Fiber-based wearable electronics: a review of materials, fabrication, devices, and applications. Adv. Mater. 26, 5310 (2014).

3. 3.

Weng, W., Chen, P. N., He, S. S., Sun, X. M. & Peng, H. S. Smart Electronic Textiles. Angew. Chem. Int. Ed. 55, 6140–6169 (2016).

4. 4.

Sun, G. Z. et al. Electrochemical capacitive properties of CNT fibers spun from vertically aligned CNT Arrays. J. Solid State Electrochem. 16, 1775 (2012).

5. 5.

Yang, Z. B., Deng, J., Chen, X. L., Ren, J. & Peng, H. S. A highly stretchable, fiber-shaped supercapacitor. Angew. Chem. Int. Ed. 52, 13453 (2013).

6. 6.

Chen, X. L. et al. Novel Electric Double-Layer Capacitor with a Coaxial Fiber Structure. Adv. Mater. 25, 6436 (2013).

7. 7.

Xu, P. et al. Carbon Nanotube Fiber Based Stretchable Wire-Shaped Supercapacitors. Adv. Energy Mater. 4, 1300759 (2014).

8. 8.

Wei, W., Cui, X., Chen, W. & Ivey, D. G. Manganese oxide-based materials as electrochemical supercapacitor electrodes. Chem. Soc. Rev. 40, 1697 (2011).

9. 9.

Zong, Q. et al. Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon Nanotube Fibers for Ultrahigh-Energy-Density All-Solid-State Wearable Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 10, 37233 (2018).

10. 10.

Xu, P. et al. Stretchable Wire-Shaped Asymmetric Supercapacitors Based on Pristine and MnO2 Coated Carbon Nanotube Fibers. ACS Nano 9, 6088 (2015).

11. 11.

Su, F. H., Lv, X. M. & Miao, M. H. High-performance two-ply yarn supercapacitors based on carbon nanotube yarns dotted with Co3O4 and NiO nanoparticles. Small 11, 854 (2015).

12. 12.

Shi, P. et al. Holey nickel hydroxide nanosheets for wearable solid-state fiber-supercapacitors. Nanoscale 10, 5442 (2018).

13. 13.

Meng, F. C., Zhao, J. N., Ye, Y. T., Zhang, X. H. & Li, Q. W. Carbon nanotube fibers for electrochemical applications: effect of enhanced interfaces by an acid treatment. Nanoscale 4, 7464 (2012).

14. 14.

Wang, K., Meng, Q. H., Zhang, Y. J., Wei, Z. X. & Miao, M. H. High-performance two-ply yarn supercapacitors based on carbon nanotubes and polyaniline nanowire arrays. Adv. Mater. 25, 1494 (2013).

15. 15.

Chen, Y. et al. Design of Novel Wearable, Stretchable, and Waterproof Cable-Type Supercapacitors Based on High-Performance Nickel Cobalt Sulfide-Coated Etching-Annealed Yarn Electrodes. Small 14, 1704373 (2018).

16. 16.

Wen, J., Xu, B., Zhou, J. & Chen, Y. Novel high-performance asymmetric supercapacitors based on nickel-cobalt composite and PPy for flexible and wearable energy storage. J. Power Sources 402, 91 (2018).

17. 17.

Sung, D. Y., Kim, I. Y., Kim, T. W., Song, M. S. & Hwang, S. J. Porously Assembled 2D Nanosheets of Alkali Metal Manganese Oxides with Highly Reversible Pseudocapacitance Behaviors. J. Phys. Chem. C 115, 13171 (2011).

18. 18.

Ma, Z. P. et al. Construction of Hierarchical α-MnO2 Nanowires@Ultrathin δ-MnO2 Nanosheets Core–Shell Nanostructure with Excellent Cycling Stability for High-Power Asymmetric Supercapacitor Electrodes. ACS Appl. Mater. Interfaces 8, 9050 (2016).

19. 19.

Zhu, C. et al. Self-branched α-MnO2/δ-MnO2 heterojunction nanowires with enhanced pseudocapacitance. Mater. Horiz. 4, 415 (2017).

20. 20.

Choi, C. et al. Flexible Supercapacitor Made of Carbon Nanotube Yarn with Internal Pores. Adv. Mater. 26, 2059 (2014).

21. 21.

Choi, C. et al. Stretchable, weavable coiled carbon nanotube/MnO2/polymer fiber solid-state supercapacitors. Sci. Rep. 5, 9387 (2015).

22. 22.

Choi, C. et al. Elastomeric and Dynamic MnO2/CNT Core–Shell Structure Coiled Yarn Supercapacitor. Adv. Energy Mater. 6, 1502119 (2016).

23. 23.

Ren et al. Carbon Nanotube Fibers for Both Wire-Shaped Micro-Supercapacitor and Micro-Battery. Adv. Mater. 25, 1155 (2013).

24. 24.

Belanger, D., Brousse, T. & Long, J. W. Manganese Oxides: Battery Materials Make the Leap to Electrochemical Capacitors. Interface 17, 49 (2008).

25. 25.

Tian, W., Mao, X., Brown, P., Rutledge, G. C. & Hatton, T. A. Electrochemically Nanostructured Polyvinylferrocene/Polypyrrole Hybrids with Synergy for Energy Storage. Adv. Funct. Mater. 25, 4803 (2015).

26. 26.

Stoller, M. D. & Ruoff, R. S. Best practice methods for determining an electrode material’s performance for ultracapacitors. Energy Environ. Sci. 3, 1294 (2010).

27. 27.

Gogotsi, Y. & Simon, P. True Performance Metrics in Electrochemical Energy Storage. Science 334, 917 (2011).

28. 28.

Toupin, M., Brousse, T. & Bélanger, D. Influence of Microstucture on the Charge Storage Properties of Chemically Synthesized Manganese Dioxide. Chem. Mater. 14, 3946 (2002).

29. 29.

Audi, A. & Sherwood, P. Valence-band x-ray photoelectron spectroscopic studies of manganese and its oxides interpreted by cluster and band structure calculations. Surf. Interface Anal. 33, 274 (2002).

30. 30.

Huang, M., Li, F., Dong, F., Zhang, Y. X. & Zhang, L. L. MnO2-based nanostructures for high-performance supercapacitors. J. Mater. Chem. A 3, 21380 (2015).

31. 31.

Toupin, M., Brousse, T. & Belanger, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor. Chem. Mater. 16, 3184 (2004).

32. 32.

Augustyn, V., Simon, P. & Dunn, B. Pseudocapacitive oxide materials for high-rate electrochemical energy storage. Energy Environ. Sci. 7, 1597 (2014).

33. 33.

Yan, W. et al. Mesoporous Manganese Oxide Nanowires for High-Capacity, High-Rate, Hybrid Electrical Energy Storage. ACS Nano 5, 8275 (2011).

34. 34.

Huang, Y. et al. From Industrially Weavable and Knittable Highly Conductive Yarns to Large Wearable Energy Storage Textiles. ACS Nano 9, 4766 (2015).

35. 35.

Kou, L. et al. Coaxial wet-spun yarn supercapacitors for high-energy density and safe wearable electronics. Nat. Commun. 5, 3754 (2014).

36. 36.

Meng, Q. H. et al. Thread-like Supercapacitors Based on One-Step Spun Nanocomposite Yarns. Small 10, 3187 (2014).

37. 37.

Wang, Z. et al. Cellular Structure Fabricated on Ni Wire by a Simple and Cost-Effective Direct-Flame Approach and Its Application in Fiber-Shaped Supercapacitors. ChemSusChem 11, 985 (2018).

38. 38.

Fu, Y. et al. Fiber supercapacitors utilizing pen ink for flexible/wearable energy storage. Adv. Mater. 24, 5713 (2012).

39. 39.

Chen, T. et al. An Integrated “energy wire” for both photoelectric conversion and energy storage. Angew. Chem. Int. Ed. 51, 11977 (2012).

40. 40.

Bae, J. et al. Fiber Supercapacitors Made of Nanowire-Fiber Hybrid Structures for Wearable/Flexible Energy Storage. Angew. Chem. Int. Ed. 50, 1683 (2011).

41. 41.

Wang, Z. et al. All-in-one fiber for stretchable fiber-shaped tandem supercapacitors. Nano Energy 45, 210 (2018).

42. 42.

Choi, C. et al. Improvement of system capacitance via weavable superelastic biscrolled yarn supercapacitors. Nat. Commun. 7, 13811 (2016).

## Acknowledgements

This work was supported by the Creative Research Initiative Center for Self-powered Actuation in National Research Foundation of Korea. Support at the University of Texas at Dallas was provided by Air Force Office of Scientific Research grants FA9550-15-1-0089, and the Robert A. Welch Foundation grant AT-0029.

## Author information

J.H.J. conceived the idea and designed the experiments; J.H.J., J.W.P. and D.W.L. contributed mechanical/electrochemical characterization; J.H.J., S.J.K. and R.H.B. wrote the paper. All authors discussed the results and commented on the manuscript.

Correspondence to Seon Jeong Kim.

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Jeong, J., Park, J.W., Lee, D.W. et al. Electrodeposition of α-MnO2/γ-MnO2 on Carbon Nanotube for Yarn Supercapacitor. Sci Rep 9, 11271 (2019) doi:10.1038/s41598-019-47744-x