Two-Dimensional Mn3O4 Nanowalls Grown on Carbon Fibers as Electrodes for Flexible Supercapacitors.

Emerging flexible and wearable electronic devices necessitates the development of fiber-type energy storage devices to power them. Supercapacitors received great attention for applications in flexible and wearable devices due to their scalability, safety, and miniature size. Herein, we report the fabrication of a flexible supercapacitor using manganese(II,III) oxide (Mn3O4) nanowalls (NWs) grown by electrochemical deposition on carbon fiber (CF) as electrode-active material. Here, CF serves as both a substrate for the growth of Mn3O4 NWs and a current collector for making a lightweight supercapacitor. Two-dimensional Mn3O4 NWs were uniformly grown on CF with high surface coverage. A three-dimensional nanostructured electrode is obtained using these individual two-dimensional Mn3O4 NWs. The Mn3O4 NWs grown on CF are characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. A symmetric sandwich-type supercapacitor is fabricated using two-dimensional Mn3O4 NW electrodes in an aqueous 1 M Na2SO4 electrolyte. The Mn3O4 NW supercapacitor electrode exhibits a specific capacitance of 300.7 F g-1 at a scan rate of 5 mV s-1. The assembled symmetric sandwich-type supercapacitor displayed high flexibility even at a bending angle of 180° without altering its performance. The Mn3O4 NW supercapacitor also displayed a long cycle life of 7500 cycles with 100% capacitance retention.

Introduction

As a result of the recent advancements in the field of flexible and wearable electronic devices, flexible energy storage devices are attracting considerable attention.[1−6] The existing energy storage devices like Li-ion batteries are less attractive for these applications due to limitations like safety, bulkiness, nonflexibility, and excessive weight.[7−9] On the other hand, electrochemical capacitors or supercapacitors emerged as an alternative for these applications.[10−12] Supercapacitors utilizing flexible electrodes have received high demand in the scientific community due to their feasible incorporation in the flexible electronic devices.[13,14] Not only the electrodes but also the current collector substrates should be flexible to achieve the total flexibility. A prerequisite for a good supercapacitor for flexible application is that the performance of the supercapacitor should not be altered at severe bending and twisting conditions.[15−17] Nevertheless, the low energy density of supercapacitors is still a roadblock in many applications.

Carbon nanomaterials such as activated carbons, graphene, carbon nanotube (CNT), etc. have been widely used as electrode materials in supercapacitors.[18−21] Carbon nanomaterial-based supercapacitors exhibit high power densities, but the low specific capacitance and energy density remain as significant challenges.[22] Two-dimensional (2D) nanomaterials are explored as supercapacitor electrodes due to their large surface area and excellent electrochemical performance.[23] Examples for 2D nanomaterials used as electrode materials in supercapacitors are graphene, transition-metal dichalcogenides, MXenes, and phosphorene. Graphene, a single sheet of carbon with 2D nanostructure, has attracted attention in the past decade.[24,25] Transition-metal dichalcogenides such as MoS2, MoSe2, VS2, WS2, etc. attracted attention in the supercapacitor research due to their unique properties such as large surface area, variable oxidation states, etc.[23,26,27] MXenes are emerging materials for supercapacitor application due to their intrinsic electronic conductivity, hydrophilic nature, and good mechanical properties.[28,29] Phosphorene is the most recently explored material for supercapacitor applications.[30] But all of these 2D materials are costly due to various reasons such as high production cost, sophisticated machinery required for the processing, availability of precursor materials, etc.

To solve the low specific capacitance and high-cost issues associated with carbon supercapacitors, transition-metal oxides (TMOs) have been evolved. TMO-based supercapacitors exhibit high specific capacitance and comparatively high energy density compared to carbon supercapacitors. TMOs such as manganese oxide, ruthenium oxide, and vanadium oxide have exhibited high specific capacitance and provided high energy density.[31−34] Among the TMOs, manganese oxide has achieved much attention due to its natural abundance, high specific capacitance, and low cost. Manganese dioxide (MnO2) and manganese(II,III) oxide (Mn3O4) are two manganese oxides with different oxidation states used as electrodes in supercapacitor applications. MnO2 is a well-studied electrode material, whereas Mn3O4 is scarcely studied for supercapacitor electrode applications.[35−39] Various Mn3O4 morphologies such as nanoparticles, nanofibers, and nanocrystals are invariably used for obtaining high specific capacitance in supercapacitors.[40−42] Recently developed chromium-doped Mn3O4 nanocrystal-based supercapacitor electrodes exhibited a capacitance of 272 F g–1 in a three-electrode cell configuration at a current density of 0.5 A g–1 in aqueous 1 M Na2SO4 electrolyte.[42] Hausmannite Mn3O4 thin film-based supercapacitor electrodes made by electrochemical deposition procedure exhibited an electrode specific capacitance of 210 F g–1 in aqueous 3 M Na2SO4 electrolyte at a current density of 0.5 A g–1 and ∼70% capacitance retention after 4000 charge–discharge cycles.[43] Mn3O4 nanorods/nickel foam composite electrodes were synthesized via a hydrothermal method and used as cathode material in a supercapacitor, which displayed an electrode specific capacitance of 263 F g–1 at a current density of 1 A g–1 in 4 M NaOH electrolyte.[44] In another report, single-crystalline Mn3O4 nano-octahedrons prepared via hydrothermal method were used as supercapacitor electrodes, and they achieved electrode specific capacitances of 195 and 244 F g–1 at scan rates of 100 and 50 mV s–1, respectively, in 1 M Na2SO4 electrolyte.[45]

Carbon fiber (CF)-based electrodes have attracted much interest in the fabrication of flexible supercapacitors, as they can easily be incorporated with the wearable fabrics due to their bendable and flexible nature.[46−48] In addition to their flexible nature, they exhibit good electronic conductivity, high mechanical strength, excellent chemical and electrochemical stabilities, and lightweight.[49−52] They are used as either a substrate for depositing electrode-active material to synthesize supercapacitor electrodes or current collectors for supercapacitors. The unique features of CFs mentioned above help in obtaining high-flexibility and lightweight supercapacitors. Herein, we report the fabrication of flexible supercapacitor electrodes consisting of Mn3O4 nanowalls (NWs). We grew Mn3O4 NWs on CF by the electrochemical deposition procedure and used as electrode-cum-current collectors for fabricating a flexible supercapacitor. Two-dimensional individual Mn3O4 NWs grown on CF enabled the electrodes to obtain a hierarchical three-dimensional porous structure. Since no additional current collectors and polymeric binders are used in this study, the as-fabricated supercapacitor electrodes, as well as the supercapacitor cell, are lightweight.

Results and Discussion

Figure a,b shows the scanning electron microscope (SEM) images of the Mn3O4 NWs grown on CF substrate. The Mn3O4 NWs are grown with high density on the CF substrate. Figure c,d shows the high-resolution SEM images of Mn3O4 NWs, and it is clear that the NWs comprise a porous architecture. This porous structure of the Mn3O4 NWs enhances the faradic reactions between the electrode-active material and the electrolyte, thereby enhancing the specific capacitance of the supercapacitor. It can be seen that the surface coverage of Mn3O4 NWs is very high and the second layer of NWs is grown on top of the first layer of NWs while maintaining the porous architecture. Energy-dispersive X-ray spectroscopy analysis (EDAX) was used to determine the distribution of Mn3O4 NWs on the CF substrate. Figure S1 shows the EDAX mapping images as well as the EDAX spectra of Mn3O4 NWs grown on the CF substrate. The EDAX mapping is carried out for the elements carbon, nitrogen, manganese, and oxygen. The presence of carbon and nitrogen arises from the CF substrate, which is highly nitrogen-rich. The EDAX mapping images clearly show that the Mn3O4 NWs are distributed uniformly in the CF substrate over the entire region. The uniform growth of two-dimensional NWs on CF can also be viewed from the atomic force microscopy (AFM) image, as shown in Figure a. The two-dimensional porous architectures of supercapacitor electrodes are much preferred for obtaining high specific capacitance and energy density. Due to the porous architecture of the electrodes, more electrolyte ions can penetrate through it, which enhances the redox performance of the Mn3O4 NW electrodes. The three-dimensional topographical AFM image shown in Figure b represents the vertical growth of Mn3O4 NWs on the CF substrate. The electrochemical deposition procedure is a versatile method to prepare nanostructured electrodes, where a uniform growth can be achieved.

Figure 1

(a–d) SEM images of Mn3O4 NWs grown on CF at different magnifications (scale bars: (a) 20 μm, (b) 2 μm, (c) 200 nm, and (d) 200 nm).

Figure 2

AFM (a) phase image and (b) three-dimensional topographical image of the Mn3O4 NWs grown on CF.

To understand the structure of two-dimensional Mn3O4 NWs grown on CF, X-ray diffraction (XRD) analysis is carried out. Figure represents the XRD spectrum of the Mn3O4 NWs grown on CF. A broad peak positioned at 20–30° represents the amorphous nature of the CF. The CF is used as received, and no surface treatment is performed prior to use. The other peaks belong to the amorphous Mn3O4 phase (JCPDF no. 3-1041).[53] From the XRD analysis, it is clear that the as-synthesized Mn3O4 NWs are amorphous. Raman spectroscopy is a versatile tool used in determining the structure of nanomaterials. Figure shows the signature Raman spectrum of the as-synthesized Mn3O4 NWs on CF. Two major peaks associated with CF can be observed, one positioned at ∼1384 cm–1 and the other at ∼1602 cm–1. The peak at ∼1384 cm–1 is associated with the disordered carbon, and the one at ∼1602 cm–1 represents the graphitic carbon.[54] Two minor peaks are located at ∼309 and ∼385 cm–1 and a dominant peak at ∼669 cm–1. They are attributed to the specific vibrations of pure Mn3O4.[55−57] From the XRD and Raman spectroscopic analyses, the formation of Mn3O4 phase is confirmed.

Figure 3

XRD spectrum of Mn3O4 NWs grown on CF.

Figure 4

Raman spectrum of Mn3O4 NWs grown on CF.

To understand the electrochemical properties of the Mn3O4 NWs grown on CF, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD) measurement have been carried out in a three-electrode cell configuration. Figure a shows the Nyquist plots obtained for the Mn3O4 NW electrode within a frequency range of 106–0.1 Hz, and the inset shows the high-frequency region. It shows that the Mn3O4 NW supercapacitor electrode displays an electrochemical series resistance (ESR) of 300 mΩ. Figure b represents the CV curves obtained at different scan rates of 100, 50, 25, 10, and 5 mV s–1. The near-rectangular nature of the CV curves with a large area under the curve represents that the Mn3O4 NW electrodes exhibited good charge storage capabilities. Due to the two-dimensional NW architecture, the effective surface area of the supercapacitor electrodes has increased, which in turn stimulated the pseudocapacitive charge storage by the Mn3O4-active material. Figure c shows the GCD curves of the Mn3O4 NW supercapacitor electrodes, which indicate the typical nature of the charge–discharge process of a supercapacitor with fast charging and discharging capabilities.

Figure 5

(a) Nyquist plots obtained in the frequency range of 106–0.1 Hz (inset: Nyquist plot in the high-frequency region), (b) CV curves at different scan rates, and (c) GCD curves at different current densities for Mn3O4 NW supercapacitor electrodes in aqueous 1 M Na2SO4 electrolyte.

Figure a represents the gravimetric capacitance obtained for the Mn3O4 NW supercapacitor electrodes at different scan rates. The supercapacitor electrode exhibited a high gravimetric capacitance of 300.7 F g–1 at 5 mV s–1. The enhanced performance is attributed to the hierarchical porous electrode nanostructures associated with the Mn3O4 NWs, in which more number of electrolyte ions can be moved inside the porous architecture, thereby resulting in an enhanced faradic reaction. The supercapacitor electrode displayed a high area specific capacitance of 571 mF cm–2 at 5 mV s–1, and the variation in the areal capacitance with respect to the scan rate is shown in Figure b. Supercapacitors with high areal capacitance are greatly preferred for integrating them with flexible and thin wearable electronic devices. Hence, the present Mn3O4 NW supercapacitor electrodes are potential candidates for flexible and wearable applications. As shown in Figure c, a gravimetric energy density of 41.7 Wh kg–1 is obtained for the Mn3O4 NW electrodes, which is higher than those of the other Mn3O4 supercapacitors in the literature.[41,58,59] But the present Mn3O4 supercapacitor electrodes exhibit attractive features such as safety, environment friendliness, and flexibility. An areal specific energy density of 79.4 mWh cm–2 at 5 mV s–1 is obtained for Mn3O4 NW supercapacitor electrodes, and the variation in the areal specific energy density of the supercapacitor electrode is depicted in Figure d. A comparison of the Mn3O4 NW supercapacitor electrodes with the existing flexible/wearable manganese oxide-based supercapacitors in the literature is given in Table S1. To examine the electrochemical cycling stability of the Mn3O4 NW supercapacitor electrode, CV measurement has been carried out in a three-electrode cell configuration for 3000 cycles. The capacitance of the electrode is found to be increased during the initial cycles due to the activation process and found to maintain a 100% retention until 3000 cycles, as shown in Figure S2. A capacitance retention of 100% after 3000 cycles clearly shows the good electrochemical stability of the Mn3O4 NW supercapacitor electrode. It is of great interest to examine the microstructure of the supercapacitor electrode after the cycling test, and Figure S3 shows the field emission SEM image of the electrode after completion of 3000 continuous cycles. From the SEM image, it is clear that the porous electrode nanostructure is unaltered after the cycling test, which is the reason behind excellent capacitance retention even after completion of 3000 continuous cycles. Some of the pores are looking hazy, mainly due to the solidification of electrolyte after the cycling just before the SEM analysis.

Figure 6

(a) Gravimetric capacitance (in F), (b) areal capacitance (in mF cm–2), (c) gravimetric energy density (Wh kg–1), and (d) areal energy density (in mWh cm–2) for Mn3O4 NW supercapacitor electrode in aqueous 1 M Na2SO4 electrolyte.

The Nyquist plot of the Mn3O4 NW electrode-based symmetric supercapacitor cell is shown in Figure a. The inset of the figure shows the Nyquist plot in high-frequency regions, and the ESR of the supercapacitor cell is calculated to be 1.19 Ω. This ESR value is high, as it is obvious that in an assembled supercapacitor cell, the ESR increases. Figure b shows the CV curves obtained at various scan rates. The near-rectangular nature of the curves shows that the supercapacitor exhibits good charge storage capability. Figure c represents the GCD curves of the Mn3O4 NW supercapacitor at different current densities of 0.5, 1, 2, and 3 mA cm–2. The symmetric charge–discharge curves indicate that the present supercapacitor exhibits good Coulombic efficiency. The flexibility of the Mn3O4 NW supercapacitor is examined by performing the CV studies at different bending angles of 0 (straight position), 30, 60, 90, and 180° (Figure d). It can be seen from the figure that even at a severe bending angle of 180°, the capacitance of the supercapacitor is retained almost at 90% of its initial value. The inset of Figure d shows the digital images of the supercapacitor bend at an angle of 180°, twisted on a finger, and weaved on a textile fabric glove, which shows that the Mn3O4 NW supercapacitor is highly bendable, twistable, and capable of incorporating with the daily wearable textiles. The cycle life of the Mn3O4 NW supercapacitor is estimated by performing CV study for continuous 7500 cycles at a constant scan rate of 50 mV s–1. Figure e represents the capacitance retention at different cycles of the Mn3O4 NW supercapacitor. A repetitive increase and decrease in the capacitance can be observed from the plot, which is obvious for supercapacitors and associated with the ion diffusion mechanisms through the supercapacitor electrode nanostructures.[15] Initially, an increase in the capacitance (an increase of 33.4% capacitance retention) was observed after 2700 cycles followed by a slow decrease to about 105%. The repetitive increase and decrease are associated with the ion diffusion mechanisms through the hierarchical three-dimensional porous network of Mn3O4 NWs. It is proposed that during the initial cycles, additional pores might have opened as a result of electrolyte diffusion toward the inner parts of the porous three-dimensional network of Mn3O4 NW electrodes. This enhances the faradic reactions, and hence an increase in capacitance is expected. As the cycling proceeds, a degradation of the electrode nanostructure may happen, which in turn deteriorates its performance, and hence a decrease in the capacitance is observed. Even after 7500 cycles, the Mn3O4 NW supercapacitor displayed a capacitance retention of 105%, a 5% enhancement from its first cycle, indicating its potential application for prolonged usage in various applications. Room-temperature ionic liquids are also potential electrolyte candidates to obtain high specific capacitance and energy density, but their high cost limits their use in commercial applications.[60−63] A Ragone plot comparing the energy density and power density of Mn3O4 supercapacitor electrodes with other supercapacitor electrodes in the literature is depicted in Figure S4. The energy density of the Mn3O4 supercapacitor electrodes is higher than that of many of the supercapacitor electrodes such as Mn3O4 nanofibers,[64] ZnO/activated carbon nanofiber composite,[65] Mn3O4 nanosheets,[59] ZnO/CNT nanocomposite,[66] Mn3O4/graphene composite,[67] etc.

Figure 7

(a) Nyquist plots obtained at a frequency range of 106–0.1 Hz, (b) CV curves at different scan rates, (c) GCD curves at different current densities, (d) capacitance retention at different bending angles of the supercapacitor [inset: digital images of supercapacitor (i) bend at 180° (scale bar = 2 cm), (ii) twisted on a finger (scale bar = 2 cm), and (iii) weaved on a textile fabric glove (scale bar = 2 cm)], (e) capacitance retention at different cycle numbers of the Mn3O4 supercapacitor, and (f) lighting an light-emitting diode (LED) using two Mn3O4 supercapacitor cells connected in series (scale bar = 2 cm).

While using the liquid electrolytes, electrolyte leakage may happen if appropriate sealing procedures are not adopted, which is not acceptable for wearable applications. In our present study, we used sophisticated sealing procedures during the fabrication of the supercapacitor to prevent the possible leaking. Moreover, we use water-based electrolytes to be safe for wearable applications. Leak-proof supercapacitors can also be developed by using polymer gel electrolytes.[68−70] But the electrochemical performance will be lower than the liquid electrolyte-based supercapacitors due to higher ESR of the polymer electrolyte system. To demonstrate the practical application of the Mn3O4 NW supercapacitor, we have fabricated a supercapacitor stack consisting of two symmetric supercapacitors and lighted up a green light-emitting diode (LED). Two identical Mn3O4 NW symmetric supercapacitor cells were fabricated as discussed before using aqueous 1 M Na2SO4 electrolyte and connected in series. The supercapacitor stack is charged to 2.2 V for a minute and then discharged through an LED (color: Super L. Green, Vf@20 mA = 2 V, radiant power = 3 mW@20 mA), as shown in Figure f. The supercapacitor stack could light up the LED for more than a minute, which indicates its potential for powering various wearable microelectronic devices. The present study enables the development of lightweight flexible supercapacitors for next-generation flexible and wearable devices.

Conclusions

The synthesis of Mn3O4 NWs on CF for the fabrication of a flexible supercapacitor is reported. Electrochemical deposition enables the formation of NW architecture of Mn3O4 on CF substrate with high surface coverage along with uniform growth. The Mn3O4 NWs grown on CF substrate are highly flexible and bendable and hence used as the electrode-cum-current collector for fabricating a flexible symmetric supercapacitor. Here, CF functions as both the substrate for the growth of Mn3O4 NWs and a current collector for the supercapacitor. The porous architecture of the Mn3O4 NWs helps in boosting the pseudocapacitive charge storage, hence enabling to achieve a maximum gravimetric capacitance of 300.7 F g–1 for the supercapacitor electrode at a scan rate of 5 mV s–1. Since the CF is highly flexible and bendable with good mechanical strength, these features are imparted to the as-fabricated supercapacitor, where it could bend even at a severe bending angle of 180° without sacrificing its capacitance. The symmetric supercapacitor also displayed a long cycle life of 7500 cycles, which is very important for its applications in wearable devices.

Experimental Section

Materials

Carbon fibers were purchased from Fibre Glast. Manganese acetate tetrahydrate [(CH3COO)2Mn·4H2O] and sodium sulfate (Na2SO4) were purchased from Sigma-Aldrich.

Synthesis of Mn3O4 NWs

Mn3O4 NWs were synthesized on CF by an electrochemical deposition method. Deposition bath consisting of 0.16 M manganese acetate tetrahydrate and 0.16 M Na2SO4 in deionized water was mixed well under continuous vigorous magnetic stirring. A three-electrode cell configuration was used for depositing Mn3O4 NWs on CF substrate, in which the CF substrate, platinum foil, and Ag/AgCl were used as the working electrode, counter electrode, and reference electrode, respectively. The CF substrate was immersed in 0.2 M HNO3, 0.2 M HCl, and deionized water, and each bath was ultrasonicated for 30 min. It was then dried in an oven at 100 °C for 2 h prior to use as a substrate for electrochemically depositing Mn3O4 NWs. Chronoamperometry technique was used, and the Mn3O4 NWs were deposited at a constant voltage of −1.8 V for a period of 20 min. The Mn3O4 NWs grown on CF were washed with deionized water and dried in an oven thereafter.

Characterization of Mn3O4 NWs

The surface morphology of the Mn3O4 NWs grown on CF was characterized by SEM (Zeiss ULTRA-55 FEG SEM). The SEM (Zeiss ULTRA-55 FEG SEM) equipped with the Noran System 7 energy-dispersive spectrometry system with silicon drift detector was used to perform energy-dispersive X-ray spectroscopy. The structure of Mn3O4 NWs grown on CF was determined using XRD analysis (PANalytical Empyrean with 1.8 kW copper X-ray tube). Raman spectroscopy was performed using Renishaw RM 1000B Micro-Raman Spectrometer with Ar, 514 nm excitation unit. The electrochemical studies of Mn3O4 NW electrodes and supercapacitor cell were examined using an electrochemical workstation (Bio-Logic Science Instruments, model SP-150). The mass of CF substrate before and after the deposition of Mn3O4 NWs was measured using a microbalance (Mettler Toledo NewClassic MF, model MS 104S/03) to estimate the mass of active materials used in the supercapacitor electrodes.

Supercapacitor Cell Assembly

A sandwich-type supercapacitor cell was assembled in a symmetric configuration using two identical Mn3O4 NWs grown on CF as electrodes and NWs uncoated portion as the current collectors. The mass loading of Mn3O4 NWs grown on the CF substrate is found to be 9.5 mg (a weight percentage of 2.5%). The Mn3O4 NWs uncoated portion of CF was used as the current-collecting leads, and no separate current collector was used in this study. Whatman paper was used as the separator, and aqueous 1 M Na2SO4 solution was used as the electrolyte.

Electrochemical Characterizations

The Mn3O4 NW electrodes were characterized by CV and GCD measurement in a three-electrode cell configuration. Here, Mn3O4 NWs grown on CF as the working electrode, platinum foil as the counter electrode, and Ag/AgCl (in 1 M KCl) as the reference electrode were used, and aqueous 1 M Na2SO4 solution was used as the electrolyte. A symmetric supercapacitor was fabricated using Mn3O4 NWs grown on CF as electrodes, Whatman paper as the electrolyte separator membrane, and aqueous 1 M Na2SO4 solution as the electrolyte. EIS measurement was performed in the frequency range of 106–0.1 Hz at 0.2 V. CV study was performed in a voltage window of 0–1 V for both individual electrodes and the assembled supercapacitor at different scan rates. GCD was also carried out in the voltage window of 0–1 V in both the cases.

Bending Test

The flexibility of the symmetric supercapacitor fabricated using Mn3O4 NW electrodes was tested by bending the supercapacitor at various angles of 0 (straight position), 30, 60, 90, and 180°. This is performed by carrying out CV study of the supercapacitor cell while bending the supercapacitor at different bending angles at a constant scan rate of 100 mV s–1.