Direct Synthesis of MnO 2 Nanorods on Carbon Cloth as Flexible Supercapacitor Electrode

MnO2 nanorod/carbon cloth (MnO2/CC) composites were prepared through in situ redox deposition as freestanding electrodes for flexible supercapacitors. The CC substrates possessing porous and interconnecting structures enable the uniform decoration of MnO2 nanorods on each fiber, thus forming conformal coaxial micro/nanocomposites. Three-dimensional CC can provide considerable specific surface area for highmass loading ofMnO2, and the direct deposition process without using polymeric binders enables reliable electrical connection of MnO2 with CC. The effect of MnO2 decoration on the electrochemical performances was further investigated, indicating that the electrode prepared with 40min deposition time shows high specific capacitance (220 F/g at a scan rate of 5mV/s) and good cycling property (90% of the initial specific capacitance was maintained after 2500 cycles) in 1M Na2SO4 aqueous solution. This enhanced electrochemical performance is ascribed to the synergistic effect of good conductivity of carbon substrates as well as outstanding pseudocapacitance of MnO2 nanorods. The obtained MnO2/CC compositing electrode with the advantages of low cost and easy fabrication is promising in applications of flexible supercapacitors.


Introduction
Manganese oxide, as one of the most promising electrode materials for supercapacitors, has attracted significant attention with its excellent electrochemical properties such as high theoretical pseudocapacitance (∼1370 F/g), low cost, and environmental friendliness [1,2].However, in practice such high theoretical capacitance has never been reached which is mainly caused by its inherent poor electrical conductivity and dense morphology of the oxide [3,4].Particularly, when the mass loading of MnO 2 on the electrode is high (which is essential for obtaining high energy density), densely packed MnO 2 enormously reduce the available surface area participating in the electrochemical process and increase the difficulty for electrolyte penetrating into the bulk MnO 2 and thus raise the contact resistance of the electrode [5][6][7].All these phenomena remarkably limit kinetics of charge transfer reaction and finally hinder the improvement of specific capacitance.Therefore, a reasonable-structured MnO 2 coating with good electrical connection becomes essential in designing high-performance electrodes for MnO 2 -based supercapacitors [8,9].
To handle these issues, one powerful and straightforward approach is to transfer bulk MnO 2 to nanoscale structures, that is, nanometer-thick thin films, nanosheets, nanorods, and nanotubes [10,11], and also grow nano-MnO 2 onto highly conductive cores [12][13][14].The nanostructured MnO 2 possessing large surface area could significantly enhance the efficiency in utilizing the electrode material, while the conductive backbone on which MnO 2 dispersed can provide electrical pathway through the backbone to MnO 2 to accomplish the charge storage reaction.
Various conductive substrates could be supporting backbones for MnO 2 , including the carbon-based materials [15,16] and various high-conductivity metal oxides [17,18].Among them, carbon cloths (CCs) are considered as promising conductive substrates due to their reasonable cost, excellent chemical stability, and 3D porous network texture.The 3D porous architecture not only allows high mass loading of MnO 2 but also facilitates the diffusion of active species and transport of electrons, leading to good electrochemical performance [7,19].Additionally, the flexible nature of carbon cloths is advantageous for the fabrication of flexible supercapacitors from the design and packaging perspectives.In fact, in many subjects of functional materials research the utilization of 3D conductive backbones clearly indicates nano-MnO 2 shell grown on micron-size conductive carbon fibers can be an effective solution for constructing outstanding supercapacitors [12,20].
In this work, we reported the binder-free integration of large-area MnO 2 nanorods on CCs through a simple electrodeposition method.The one-step synthesized MnO 2 /CC electrodes with the advantages of low cost and being easily scalable exhibit considerable electrochemical properties with high specific capacitance and good cycling property, indicating their broad applications in flexible energy storage device.

Synthesis of MnO 2 on Carbon
Cloth.The CC substrates were firstly cleaned by sonication sequentially in acetone, ethanol, and DI water for 30 min each.After being dried, an electrochemical deposition process was then conducted to decorate MnO 2 nanostructures onto pristine CC using Autolab electrochemical workstation (PGSTAT-302 N, Eco Chemie B.V. Company, Utrecht, Netherlands) with a threeelectrode system.The process was carried out under galvanostatic conditions at the constant current density of 1 mA/cm 2 .A precursor solution with equal volume of 0.1 M MnSO 4 ⋅H 2 O and 0.1 M Na 2 SO 4 was used as the electrolyte.The cleaned CC was used as working electrode, a platinum wire was used as counter electrode, and KCl-saturated Ag/AgCl was used as reference electrode.The formation of MnO 2 in the electrochemical deposition process occurs via the following reactions [21]: Briefly, Mn(II) ion is firstly oxidized to the intermediate species of Mn(III) ion, which is thermodynamically unstable, and then disproportionation reaction of Mn(III) ion occurs to generate MnO 2 before Mn(III) ion is reduced to be Mn(II) ion.
By varying the depositing time from 10 min to 50 min, different morphologies of MnO 2 films were obtained.After deposition, these samples were washed with distilled water to eliminate any loosely attached chemical substance and dried naturally, and then the flexible MnO 2 /CC electrodes were fabricated.The overall process is scalable.

Characterization.
The morphologies of the samples were characterized by field emission scanning electron microscopy (FESEM, Hitachi, S-4800, Japan) equipped with energydispersive X-ray (EDX).The structures of the samples were examined by X-ray diffraction (XRD, Bruker D8 Advance) with Cu-Ka radiation (1.5418 Å) operating at 40 kV, 100 mA.X-ray photoelectron spectroscopy (XPS) was carried out at room temperature in ESCALAB 250 system.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests were conducted on an Autolab work station in 1 M Na 2 SO 4 aqueous solution.Galvanostatic charging/discharging and cycling tests were conducted using a battery measurement system (LAND CT2001A, Wuhan LAND Electronics, Wuhan, China).All the electrochemical experiments were conducted at room temperature.

Results and Discussions
3.1.Structural Characterizations.After electrochemical deposition, MnO 2 was successfully decorated onto the surface of CC.Different depositing durations were preset to investigate the morphology variation of the heterostructures.Typical SEM images of the MnO 2 /CC with different depositing time are shown in Figure 1, which clearly demonstrate that the well-established nano-MnO 2 has been grown on CC.Figures 1(a), 1(c), 1(e), 1(g), and 1(i) show the carbon fiber after electrochemical deposition of MnO 2 for 10, 20, 30, 40, and 50 min, respectively, while the enlarged SEM images of the corresponding local area are presented in Figures 1(b), 1(d), 1(f), 1(h), and 1(j), respectively.From the magnified SEM images, it can be found that MnO 2 film consists of large quantities of nanorods, which appears to have a rough, uniform, and worm-like shape.With the relatively short electrochemical depositing duration such as 10 min, the CC surface is covered mostly by the MnO 2 particle islands with some carbon fiber surface exposed to the environment (Figures 1(a) and 1(b)).When the depositing duration is prolonged, the MnO 2 particle islands on the surface of the carbon fibers evolve into more compact film.With the depositing time more than 40 min, the carbon fiber is fully covered by dense MnO 2 nanorods as shown in Figures 1(g)-1(j).These results indicate that the morphology of the product can be readily controlled by simply varying the depositing time of MnO 2 .
EDX, XRD, and XPS analysis were employed to determine the composition and crystallinity of MnO 2 deposits, as shown in Figure 2, where the typical MnO 2 /CC with depositing time of 40 min was chosen as the testing sample.The EDX analysis is conducted to study the chemical composition of the sample, as shown in Figure 2(a), in which C, Mn, and O elements can be detected.Considering that the C element is coming from the CC substrate, the nanostructure obtained by electrochemical deposition is manganese oxide.To explore the chemical structure and phase purity of the as-prepared deposit, XRD was employed as shown in Figure 2(b).

Electrochemical Evaluations.
In order to examine the electrochemical characteristics of the obtained electrodes, samples with size of 1 cm 2 were tested by CV measurements.The specific CV curves of the electrodes are shown in Figure 3(a) with MnO 2 deposition of 0 min (pure CC), 10 min, 20 min, 30 min, 40 min, and 50 min at the scan rate of 50 mV/s in 1 M Na 2 SO 4 electrolyte.The CV curves show a near symmetric rectangular shape and exhibit near mirrorimage current response on voltage reversal, suggesting a fast, reversible reaction as well as excellent capacitive performance of the hybrid electrode.The specific geometric capacitance of pure CC, due to limited electroactive sites, is as low as 0.0168 mF/cm 2 at the scan rate of 50 mV/s, and the specific geometric capacitance of CC is three magnitudes lower than that of MnO 2 /CC.Moreover, after deposition, carbon material underneath is blocked to participate in the charge storage process, considering the process is mainly conducted at or near the surface of the active materials.Hence, the capacitance contribution from carbon core can be negligible to the MnO 2 /CC composites.
In Figure 3(a), with short deposition time the CV curves are closer to rectangular shape, while with the deposition time extending CV curves deform gradually.This result is due to the fact that more MnO 2 loading on the carbon surface participating in reactions and thus pseudocapacitance become the main electrochemical mechanism replacing double-capacitance.Moreover, for the depositing time from 10 min to 40 min, the geometric capacitance of MnO 2 /CC increases with the depositing time prolonged, while when the depositing duration continues to extend to 50 min, the geometric capacitance decreases (as shown in Figure 3(b)).
As the corresponding mass uptake of MnO 2 increases from 0.146 mg/cm 2 to 0.715 mg/cm 2 , the specific gravimetric capacitance depending on the depositing time is plotted in Figure 3(b), in which the same tendency could also be found.That is to say, the specific capacitance of MnO 2 /CC rises from 91 F/g (10 min) to 177 F/g (40 min) and then drops to 122 F/g (50 min).This phenomenon can be explained as follows: when the depositing time is less than 40 min, the MnO 2 film could not fully cover the carbon fiber and as the depositing time increases, the coverage ratio increases apparently (as shown in Figures 1(a)-1(f)); MnO 2 film totally overlays carbon fiber at the depositing time of 40 min (as shown in Figures 1(g)-1(h)), and in this condition, its utilization is most sufficient; when the duration extends to 50 min, MnO 2 loading continues to increase leading the film to be more compacting, which reduces the active site exposing to the electrolyte and thus limits the electrochemical performance of the hybrid electrode.These phenomena reveal that too thick MnO 2 film may limit its optimal electrochemical performance as mentioned earlier.Thus, the employ of MnO 2 film with appropriate thickness and surface morphology is critical to improving the performance of the hybrid electrode, which can be easily controlled by the fabrication process.
The CV curves of the electrodes with 40 min deposition at different scan rates are shown in Figure 3(c).It can be found that the CV shape remains nearly rectangular at a high scan rate of 100 mV/s, indicating a small polarization resistance.As the mass loading of MnO 2 is 0.6428 mg/cm 2 for 40 min depositing process, the calculated specific capacitance is 220 F/g at 5 mV/s and keeps 144 F/g at 100 mV/s scan rate, showing a good rate capability.The dependence of gravimetric capacitance on different scan rate is plotted in Figure 3(d).It is obvious that with the scan rate increasing the specific capacitance decreases substantially.The decline of specific capacitance can be explained as follows: at a low scan rate, the ions in the electrolyte can diffuse sufficiently into the MnO 2 coatings for more charge storage, while at a higher scan rate, the diffusion time is reduced for insertion/extraction of protons or Na + , leading to a lower capacitance.
The electrochemical performance of MnO 2 /CC-40 min electrode was further investigated by galvanostatic charging/discharging technique.Figure 4(a) shows the charging/ discharging curves at various current densities from 0.5 to 15 mA/cm 2 .It could be observed that the discharge curves are symmetric with their corresponding charge counterparts, indicating superior reversibility and good Coulombic efficiency.These discharge curves are approximately straight line after a small voltage drop (IR drop), which reflects the equivalent series resistance of electrodes, that is, ionic resistance of electrolyte, intrinsic resistance of the hybrid electrode, and interfacial contact resistance between electrode and electrolyte.The specific capacitance obtained from the discharging curves is calculated according to the following equation [9]: where  (mA) is the applied current, Δ (s) is the discharge time, Δ (V) is the sweep potential range, and  (g) is the mass of active material, respectively.As shown in Figure 4(b), the specific capacitance of MnO 2 /CC-40 min decreases distinctly as the current density increases for low current density from 0.5 mA/cm 2 (∼193 F/g) to 4 mA/cm 2 (∼149 F/g).As the current density continues to increase to 15 mA/cm 2 , the areal capacitance drops very little and finally keeps stable (∼149 F/g).It is noteworthy that the electrode maintained 77.2% retention of its initial specific capacitance measured at a high rate of 15 mA/cm 2 (23.3 A/g), which is superior to the previously reported values of other MnO 2 /carbon-based electrodes including MnO 2 /CNT@carbon microelectrode (gravimetric capacitance remains less than 50% with the increase of current density from 0.5 mA/cm 2 to 3 mA/cm 2 [21]), MnO 2 /CFP (75% retention from 1 A/g to 10 A/g [23]), and PPy/MnO 2 @carbon cloth (capacitance retention of 70% from 0.2 A/g to 5 A/g [24]).Moreover, the specific capacitance of MnO 2 /CC-40 min electrode is substantially higher than the value reported for MnO 2 nanosheets/carbon nanofibers (151.1 F/g at the current density of 1 A/g [15]).This suggests that our electrode with its prior feature for fast charge/discharge under a large current density might have huge potential in supercapacitor applications.Besides high specific capacitance, good cycling performance is also an important property for high-performance supercapacitors.The long-term stability of the MnO 2 / CC-40 min electrode was examined through galvanostatic charge/discharge cycling at a current density of 2 mA/cm 2 in 1 M Na 2 SO 4 electrolyte.Figure 5 displays the specific capacitance retention and Coulombic efficiency of the hybrid electrode as a function of charge/discharge cycling number.From this figure, the Coulombic efficiency keeps >95% from the 40th cycle and finally reaches to 99%.The capacitance retention decreases about 10% of the initial value over the first 300 cycles, while, then, remaining quite stable for the last 2200 cycles.The inset of Figure 5 shows the last several cycles of charging/discharging curves between −0.1 V and 0.7 V, which exhibit almost the same shape, revealing excellent long-term cyclability of the hybrid array electrode.The excellent cycling performance for MnO 2 /CC reveals the good structural stability as well as close contact between MnO 2 nanorods and carbon fibers.
To further understand the fundamental behavior of the prepared supercapacitor electrodes, EIS measurements for MnO 2 /CC electrodes with depositing time from 10 min to 50 min have been carried out and the corresponding Nyquist plots are shown in Figure 6.The EIS data can be fitted by an equivalent circuit consisting of internal resistance (  ), charge transfer resistance ( ct ), double-layer capacitance ( dl ), pseudocapacitance ( ps ), and Warburg impedance (  ), as presented in the inset of Figure 6.At high-frequency region, the Nyquist plots of MnO  low frequency region, the plots of MnO 2 /CC electrodes with deposition time from 10 min to 40 min demonstrate steeper Warburg tail, indicating their lower diffusion-resistance.Moreover,  ps for the five electrodes is calculated to be 0.00989 F, 0.01198 F, 0.01239 F, 0.01531 F, and 0.01221 F, respectively, representing the fact that the MnO 2 /CC-40 min electrode has the best capacitive property, which could be due to the optimized loading mass of electrochemical active material (MnO 2 ), and thus providing reasonable active area for electrochemical reactions.All the results suggest that the MnO 2 /CC-40 min electrode has very small resistance with good ion response, indicating that the obtained structure could indeed act as a good supercapacitor electrode.

Conclusion
In summary, MnO 2 nanorods were successfully synthesized on flexible CC substrates through a facile electrodepositing method.The effect of depositing time on the morphology and electrochemical properties of the composites has been investigated, verifying that the MnO 2 /CC electrode with 40 min depositing time demonstrates optimal specific capacitance, good rate capability, and good cyclic stability.Such excellent electrochemical properties are attributed to the synergetic effect of nanostructured MnO 2 and interconnected porous CC acting as a conductive backbone.The prepared electrode may have promising potential as the high-performance flexible supercapacitor.

Figure 2 (
b) shows the XRD patterns of the pristine CC and MnO 2 /CC composites.Compared with CC, two additional characteristic peaks at 36.9 ∘ and 66.3 ∘ (marked by black arrows) appear in the pattern of MnO 2 /CC composites, which endorse the presence of amorphous birnessite-type MnO 2 (JCPDS, Card number 18-0802).It can be seen that these MnO 2 peaks are broad and unclear, indicating the

Figure 1 :
Figure 1: SEM images of MnO 2 /CC integrated structures with the left side are low-magnitude views and with the right side are corresponding enlarged views of the left side.

Figure 3 :
Figure 3: (a) CV of the pure CC and MnO 2 /CC composites at a scan rate of 50 mV/s in 1 M Na 2 SO 4 electrolyte.(b) Gravimetric and geometric capacitance as a function of depositing time.(c) CV of the MnO 2 /CC-40 min at various scan rates.(d) Gravimetric capacitance of the MnO 2 /CC-40 min as a function of scan rate.

Figure 4 :
Figure 4: (a) Galvanostatic charging/discharging curves of MnO 2 /CC-40 min electrode at different current density.(b) Changing tendency of the specific capacitance depending on current densities.

Figure 5 :
Figure 5: Cycling performance and Coulombic efficiency of the electrode up to 2500 cycles at the current density of 2 mA/cm 2 .The inset showing the charging/discharging curves after 2000 cycles.

Figure 6 :
Figure 6: Nyquist plots of the MnO 2 /CC electrode.The insets are the equivalent circuit and the magnification of Nyquist plots at high frequency.