- Nano Express
- Open Access
Ultrafine MnO2 Nanowire Arrays Grown on Carbon Fibers for High-Performance Supercapacitors
Nanoscale Research Letters volume 11, Article number: 469 (2016)
Large-area ultrafine MnO2 nanowire arrays (NWA) directly grew on a carbon fiber (CF, used as a substrate) by a simple electrochemical method, forming three-dimensional (3D) hierarchical heterostructures of a CF@MnO2 NWA composite. As an electrode for supercapacitors, the CF@MnO2 NWA composite exhibits excellent electrochemical performances including high specific capacitance (321.3 F g−1 at 1000 mA g−1) and good rate capability. Further, the overall capacitance retention is ~99.7 % capacitance after 3000 cycles. These outstanding electrochemical performances attribute to a large number of transport channels for the penetration of electrolyte and the transportation of ions and electrons of electrodes. The as-prepared CF@MnO2 NWA composite may be a promising electrode material for high-performance supercapacitors.
For electrochemical energy storage applications, a nanoscale solution to supercapacitors has attracted considerable attention due to their unique advantages such as faster charging/discharging rate, higher power density, much longer lifetimes, and safer operation [1–4]. Up to now, the design and synthesis of nanomaterials have promoted significant advancements in supercapacitors. One-dimensional (1D) nanostructures are believed to facilitate the electrical transport along the axial direction . So far, a wide variety of nanowires, including carbonaceous materials [6, 7], transition metal oxides [8–14], conducting polymers [15, 16], and hybrid composites [17–19], have been synthesized to acquire enhanced electrochemical properties as an electrode in supercapacitors. Among them, 1D nanostructured transition metal oxides with high capacity and low cost have been a popular topic. In particular, manganese oxide (α-MnO2) with high theoretical pseudocapacitance (~1370 F g−1) has attracted intense attention due to a one-electron transfer and the complete reduction of MnIV to MnIII as well as its environmental compatibility and earth abundance . However, its poor intrinsically electrical conductivity (10−5 to 10−6 S cm−1) and large volume expansion during repeated cycling processes will limit its practical applications in supercapacitors [21, 22]. Thus, the design and synthesis of an electrode material based on MnO2 nanowires that provides a high electrical conductivity and a reduced volume expansion are needed.
Recently, three-dimensional (3D) hierarchical heterostructures by assembling 1D MnO2 nanostructures and conductive backbones, e.g., carbon materials [22, 23], nickel foam , and Co3O4 , have been demonstrated to show improved electrochemical properties in supercapacitors. In particular, a constitution within 3D heterostructures made of dense ultrafine nanowire arrays will result in high specific surface area and plentiful porosities, forming a great number of electrochemically active sites with shorter diffusion pathways for ions and electrons [26, 27]. Thus, the active materials of the electrode will improve their utilization, i.e., easily participate in reversible redox reactions with the electrolyte solution, enhancing electrochemical kinetics during the charging and discharging process [12–27]. For samples, 3D Co3O4@MnO2 hierarchical nanoneedle arrays by a hydrothermal approach showed excellent electrochemical performances such as high specific capacitances of 932.8 F g−1 at a scan rate of 10 mV s−1 as well as long-term cycling stability ; hierarchical CNTs@NCS@MnO2 core-shell composites via a chemical polymerization coating followed by a hydrothermal process exhibited a high specific capacitance of 312.5 F g−1 at a current density of 1 A g−1 and a good rate capability (76.8 % retention with the charge-discharge rate increasing from 1 to 10 A g−1) . However, in the two cases, using a special template (Co3O4 nanoneedles or CNTs) with unavailable sizes can make large-scale preparation and manipulations more difficult.
Herein, using commercial carbon fibers (CF) as a substrate, large-area ultrafine MnO2 nanowire arrays (NWA) directly grew on a CF, forming 3D hierarchical heterostructures of a CF@MnO2 NWA composite by a simple electrochemical method. The as-fabricated electrodes by the CF@MnO2 NWA composite exhibited an improved specific capacitance of 321.3 F g−1 at 1000 mA g−1 and an excellent cycling stability in 0.5 M Na2SO4 aqueous solution, i.e., the specific capacitance of the electrodes showing 99.7 % retention after 3000 cycles.
Synthesis of CF@MnO2 NWA Composite
Firstly, a piece of carbon fibers (~4 × 1 cm2) was carefully cleaned with deionized water and absolute ethanol in sequence for several times. Secondly, the electrochemical deposition was carried out in a standard three-electrode glass cell consisting of a clean carbon fiber working electrode, a platinum plate (~1.5 × 1.5 cm2) counter electrode, and a saturated calomel reference electrode (SCE). MnO2 nanowire arrays were electrodeposited on the carbon fibers using an Autolab electrochemical workstation (PGSTAT302N potentiostat), in which deposition conditions included a current density of 0.75 mA cm−2, a solution at 70 ± 2 °C containing 0.1 M manganese acetate (Mn(CH3COO)2) and 0.02 M ammonium acetate (CH3CO2NH4) with 10 % dimethyl sulfoxide (DMSO), and an electrodeposit surface area of 1 × 1 cm2. The electrodeposition process was carried out in a water bath, in which the temperature was carefully set at 70 °C. After deposition for 20 min, the CF@MnO2 NWA composite was ultrasonically washed with deionized water and absolute ethanol several times and then placed in a vacuum oven at 60 °C for 2 h. Finally, the as-prepared CF@MnO2 NWA composite was annealed at 200 °C for 3 h in air. The mass of the CF@MnO2 NWA composite was obtained by a weight difference before and after deposition, and the mass of active material MnO2 per unit area (1 × 1 cm2) of the electrode is about ~1.27 mg.
Powder X-ray diffraction (XRD; Rigaku with Cu-Kα radiation and a normal θ–2θ scan) was used to characterize the phases in the collected products. Morphological observation and structural analysis of the products were carried out with a scanning electron microscope (SEM; S-4800) and a transmission electron microscopy (TEM; JEM-2100F operated at 200 kV) equipped with an energy-dispersive X-ray spectrometer (EDX). The mass of the electrode materials was weighed on an XS analytical balance (Mettler Toledo; δ = 0.01 mg).
Electrochemical performances were performed on the Autolab electrochemical workstation using a three-electrode mode in a 0.5 M Na2SO4 solution. The reference electrode was a SCE, and the counter electrode was a platinum plate. Standard current-voltage curves were recorded in a potential range of −0.1 to 0.9 V.
Results and Discussion
The phase of the as-grown products was characterized by XRD, as shown in Fig. 1a. In this pattern, two strong peaks marked with “#” are originated from the carbon fibers (substrate) and the other peaks marked with “*” can be indexed to those of the tetragonal phase of α-MnO2 (JCPDS: 44-0141). No peaks associated with other crystalline forms of manganese oxide were detected in the pattern. The results indicate a high purity of MnO2 material on the carbon fibers. Figure 1b, c shows a low-magnification SEM image revealing the general morphology of the product. It can be seen that the MnO2 nanowires grew densely and uniformly on the surface of each carbon fiber, forming a CF@MnO2 NWA composite. In fact, these dense MnO2 nanowires were intersected with each other to form numerous voids or space clearances, as suggested by a high-magnification SEM image in Fig. 1d. Here, the commercial carbon fibers can also be used as a roll-to-roll media to realize mass production of the CF@MnO2 NWA composite, which is expected to continuously fabricate supercapacitors in the future .
The TEM image and the EDX pattern of the CF@MnO2 NWA composite are shown in Fig. 2. Each MnO2 nanowire has a length of ~1.5 μm (Fig. 2a). Notably, individual MnO2 nanowires have a diameter as fine as ~9 nm. Combining the SEM observation, the as-grown composite made of ultrafine MnO2 nanowire arrays and carbon fibers has remarkable advantages for its application as an electrode in the supercapacitors. Clearly, those numerous voids or space clearances among the ultrafine nanowires can provide a large number of transport channels for the penetration of electrolyte and the transportation of ions and electrons of electrodes. Furthermore, the ultrafine MnO2 nanowires could greatly increase the specific surface area of the composite. These characteristics will promote the electrochemical reactions to occur on the surface and interface of the composite, finally improving the performance of the supercapacitors. The high-resolution TEM image in Fig. 2b reveals the crystalline nature of the MnO2 nanowire, and the observed d-spacing of 0.30 nm matches well with the (110) facets of the tetragonal phase of α-MnO2. The corresponding fast Fourier transform (FFT) pattern in Fig. 2c agrees with the  zone axis of the tetragonal phase of α-MnO2. Figure 2d presents the EDX of the MnO2 nanowires, which clearly indicates the presence of Mn and O elements (C signals originate from the TEM grid).
Such a composite of the ultrafine MnO2 nanowire arrays directly grown on carbon fibers is used as a bind-free electrode for the supercapacitor which was examined in a 0.5 M Na2SO4 solution. Figure 3a gives cyclic voltammetry (CV) curves of the as-fabricated CF@MnO2 NWA electrode at different scan rates. All the curves display a nearly rectangular and symmetric characteristic, which suggests an ideal pseudocapacitive nature with fast charge and discharge processes. This could be thanks to the electrolyte coming in great numbers to the active surface of the electrode materials. Galvanostatic charge-discharge curves of the CF@MnO2 NWA electrode were recorded between a potential window range of −0.1 to 0.9 V at various current densities, as shown in Fig. 3b. They appear to be an almost symmetric shape, which reflects an ideal electrochemical capacitive nature and good reversible redox reaction in the whole potential range.
Based on the discharge curves, the discharge-specific capacitance can be obtained referring to the following formula :
where C is the specific capacitance (F g−1) and I, Δt, ΔV, and m are the discharge current (A), the discharge time (s) taken in the potential range, the potential windows (V), and the mass of the active materials (or the mass of the total electrode materials) (g), respectively. In line with the formula, the discharge-specific capacitances of the electrode material (CF@MnO2 NWA) were calculated from the discharge curves to be 321.3, 277, 212.5, and 175 F g−1 at the current densities of 1000, 2000, 5000, and 10,000 mA g−1, respectively, as shown in Fig. 3c. These specific capacitances here are much higher than that in the reported works on this material with a similar composite structure [31–34].
As we know, for the supercapacitors, the rate capability is also an important aspect in their high-power applications. Figure 3c reflects the specific capacitance of the CF@MnO2 NWA electrode measured at different current densities. As can be seen, the CF@MnO2 NWA electrode kept 54.5 % of its specific capacitance (from 321.3 to 175 F g−1) as the current density increased from 1000 to 10,000 mA g−1, i.e., 10 times increase. The excellent rate capability also ascribed to the dense ultrafine space clearances inside the CF@MnO2 NWA composite. Obviously, the space clearances can support numerous electrolytic accessible passageways and thus greatly help the electrolyte to penetrate into the active materials, effectively promoting the redox reactions that occurred on the surfaces and interfaces. So, even at a high rate, a considerably high specific capacitance can be obtained. In addition, these numerous passageways within the CF@MnO2 NWA composite are expected to slow down the volume expansion upon a long-term cycle of repeating CV test . Noticeably, the CF substrate could also affect the final capacitance of the composite electrode material. By comparing the specific capacitances of the blank CF and the CF@MnO2 NWA electrode (Fig. 3d), the influence from the bare CF can be nearly negligible.
A high stability should also be required by high-performance supercapacitors. Here, a long-term cycle stability of an as-fabricated CF@MnO2 NWA electrode was evaluated by repeating CV test for 3000 cycles at a scan rate of 50 mV s−1. As shown in Fig. 4, after 3000 cycles, the capacitance retention is as high as ~99.7 %, that is, the overall loss of the capacitance is as low as ~0.3 %. In other words, this electrode represented a capacity decay rate per cycle of only 0.0001 %, which is considerably lower than those of the reported MnO2-based cathodes [31–33, 36, 37]. The credit for such high stability could be given to those numerous ultrafine spaces inside the MnO2 nanowire as well as the good electrical conductivity from the carbon fibers, of which the electron collection became facile and much more electrons became involved in the electrochemical reactions. It is believed that the electrode material of the CF@MnO2 NWA composite can be highly competitive for a long cycle life.
In summary, large-area ultrafine MnO2 nanowire arrays were directly grown on carbon fiber substrates resulting in a 3D CF@MnO2 NWA composite by an easy electrochemical deposition. The CF@MnO2 NWA electrode demonstrates excellent electrochemical performances, i.e., ultrahigh specific capacitances of 321.3 and 175 F g−1 at current densities of 1000 and 10,000 mA g−1, respectively, a good rate capability, and a long cycling stability with a capacitance loss of 0.3 % after 3000 cycles. These overall fine electrochemical performances should owe to the effective electron and ion-transport pathways that originated from the distinctively microstructural characteristics of the CF@MnO2 NWA composite. So, it makes the CF@MnO2 NWA composite a promising electrode material for the high-performance supercapacitors.
Arico AS, Bruce P, Scrosati B, Tarascon JM, Van Schalkwijk WV (2005) Nanostructured materials for advanced energy conversion and storage devices. Nat Mater 4:366
Chen YJ, Qu BH, Hu LL, Xu Z, Li QH, Wang TH (2013) High-performance supercapacitor and lithium-ion battery based on 3D hierarchical NH4F-induced nickel cobaltate nanosheet-nanowire cluster arrays as self-supported electrodes. Nanoscale 5:9812
Simon P, Gogotsi Y (2008) Materials for electrochemical capacitors. Nat Mater 7:845
Zhu YG, Wang Y, Shi YM, Wong JI, Yang HY (2014) CoO nanoflowers woven by CNT network for high energy density flexible micro-supercapacitor. Nano Energy 3:46
Ma ZP, Shao GG, Fan YQ, Wang GL, Song JJ, Shen DJ (2016) 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
Yu ZN, Tetard L, Zhai L, Thomas J (2015) Supercapacitor electrode materials: nanostructures from 0 to 3 dimensions. Energy Environ Sci 8:702
Lee S, Ha J, Cheng H, Lee JW, Jang TS, Jung YG et al (2014) Surface-coverage-dependent cycle stability of core-shell nanostructured electrodes for use in lithium ion batteries. Adv Energy Mater 4:1300472
Liu W, Li X, Zhu M, He X (2015) High-performance all solid state asymmetric supercapacitor based on Co3O4 nanowires and carbon aerogel. J Power Sources 282:179
Cheng K, Yang F, Ye K, Zhang Y, Jiang X, Yin JL et al (2014) Highly porous Fe3O4-Fe nanowires grown on C/TiC nanofiber arrays as the high performance anode of lithium-ion batteries. J Power Sources 258:260
Chen R, Wang HY, Miao J, Yang H, Liu B (2015) A flexible high-performance oxygen evolution electrode with three-dimensional NiCo2O4 core-shell nanowires. Nano Energy 11:333
Duay J, Sherrill SA, Gui Z, Gillette E, Lee SB (2013) Self-limiting electrodeposition of hierarchical MnO2 and Mn(OH)2/MnO2 nanofibril/nanowires: mechanism and supercapacitor properties. ACS Nano 7:1200
Jiao Y, Liu Y, Yin BS, Zhang SW, Qu FY, Wu X (2014) Hybrid α-Fe2O3@NiO heterostructures for flexible and high performance supercapacitor electrodes and visible light driven photocatalysts. Nano Energy 10:90
Yin BS, Zhang SW, Jiang H, Qu FY, Wu X (2015) Phase-controlled synthesis of polymorphic MnO2 structures for electrochemical energy storage. J Mater Chem A 3:5722
Chen PC, Shen GZ, Shi Y, Chen HT, Zhou CW (2010) Preparation and characterization of flexible asymmetric supercapacitors based on transition-metal-oxide nanowire/single-walled carbon nanotube hybrid thin-film electrodes. ACS Nano 4:4403
Tran HD, Wang Y, D’Arcy JM, Kaner RB (2008) Toward an understanding of the formation of conducting polymer nanofibers. ACS Nano 2:1841
Dubal DP, Lee SH, Kim JG, Kim WB, Lokhande CD (2012) Porous polypyrrole clusters prepared by electropolymerization for a high performance supercapacitor. J Mater Chem 22:3044
Ghosh D, Giri S, Moniruzzaman M, Basu T, Mandala M, Das CK (2014) α MnMoO4/graphene hybrid composite: high energy density supercapacitor electrode material. Dalton Trans 43:11067
Tian W, Wang X, Zhi CY, Zhai TY, Liu DQ, Zhang C et al (2013) Ni(OH)2 nanosheet@Fe2O3 nanowire hybrid composite arrays for high-performance supercapacitor electrodes. Nano Energy 2:754
Huang L, Chen DC, Ding Y, Wang ZL, Zeng ZZ, Liu ML (2013) Hybrid composite Ni(OH)2@NiCo2O4 grown on carbon fiber paper for high-performance supercapacitors. ACS Appl Mater Interfaces 5:11159
Toupin M, Brousse T, Belanger D (2004) Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor. Chem Mater 16:3184
Lee SW, Kim J, Chen S, Hammond PT, Shao-Horn Y (2010) Carbon nanotube/manganese oxide ultrathin film electrodes for electrochemical capacitors. ACS Nano 4:3889
He YM, Chen WJ, Li XD, Zhang ZX, Fu JC, Zhao CH et al (2013) Freestanding three-dimensional graphene/MnO2 composite networks as ultralight and flexible supercapacitor electrodes. ACS Nano 7:174
Reddy ALM, Shaijumon MM, Gowda SR, Ajayan PM (2009) Coaxial MnO2/carbon nanotube array electrodes for high-performance lithium batteries. Nano Lett 9:1002
Sun YY, Zhang WH, Li DS, Gao L, Hou CL, Zhang YH et al (2015) Direct formation of porous MnO2/Ni composite foam applied for high-performance supercapacitors at mild conditions. Electrochim Acta 178:823
Kong DZ, Luo JS, Wang YL, Ren WN, Yu T, Luo YS et al (2014) Three-dimensional Co3O4@MnO2 hierarchical nanoneedle arrays: morphology control and electrochemical energy storage. Adv Funct Mater 24:3815
Zhu J, Xu Z, Lu BG (2014) Ultrafine Au nanoparticles decorated NiCo2O4 nanotubes as anode material for high-performance supercapacitor and lithium-ion battery applications. Nano Energy 7:114
Li YH, Wang ZY, Zhang YF (2015) Ultrafine Ag/MnOx nanowire-constructed hair-like nanoarchitecture: in situ synthesis, formation mechanism and its supercapacitive property. J Alloys Compd 644:47
Li L, Li RM, Gai SL, Gao P, He F, Zhang ML et al (2015) Hierarchical porous CNTs@NCS@MnO2 composites: rational design and high asymmetric supercapacitor performance. J Mater Chem A 3:15642
Gao Z, Bumgardner C, Song NN, Zhang YY, Li JJ, Li XD (2016) Cotton-textile-enabled flexible self-sustaining power packs via roll-to-roll fabrication. Nat Commun 7:11586
Ding SJ, Zhu T, Chen JS, Wang ZY, Yuan CL, Lou XW (2011) Controlled synthesis of hierarchical NiO nanosheet hollow spheres with enhanced supercapacitive. J Mater Chem 21:6602
Xia H, Hong C, Shi X, Li B, Yuan G, Yao Q et al (2015) Hierarchical heterostructures of Ag nanoparticles decorated MnO2 nanowires as promising electrodes for supercapacitors. J Mater Chem A 3:1216
Phattharasupakun N, Wutthiprom J, Chiochan P, Suktha P, Suksomboon M, Kalasinaa S et al (2016) Turning conductive carbon nanospheres into nanosheets for high-performance supercapacitors of MnO2 nanorods. Chem Commun 52:2585
Jiang H, Zhao T, Ma J, Yan C, Li CZ (2011) Ultrafine manganese dioxide nanowire network for high-performance supercapacitors. Chem Commun 47:1264
Bao LH, Zang JF, Li XD (2011) Flexible Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber hybrid composites for high-performance supercapacitor electrodes. Nano Lett 11:1215
Tao XY, Du J, Sun Y, Zhou SL, Xia Y, Huang H et al (2013) Exploring the energy storage mechanism of high performance MnO2 electrochemical capacitor electrodes: an in situ atomic force microscopy study in aqueous electrolyte. Adv Funct Mater 23:4745
Lu XF, Wang AL, Xu H, He XJ, Tong YX, Li GR (2015) High-performance supercapacitors based on MnO2 tube-in-tube arrays. J Mater Chem A 3:16560
Bag S, Raj CR (2016) Hierarchical three-dimensional mesoporous MnO2 nanostructures for high performance aqueous asymmetric supercapacitors. J Mater Chem A 4:587
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51602193), and we gratefully thank the Institute of Functional Nano & Soft Materials (FUNSOM) for supporting our work.
JH designed and performed the experiments. JH, GS, and FQ prepared the samples and analyzed the data. JH, FQ, GS, WL, and LW participated in interpreting and analyzing the data. All authors read and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
About this article
Cite this article
Hu, J., Qian, F., Song, G. et al. Ultrafine MnO2 Nanowire Arrays Grown on Carbon Fibers for High-Performance Supercapacitors. Nanoscale Res Lett 11, 469 (2016). https://doi.org/10.1186/s11671-016-1693-1
- Ultrafine nanowires
- Large area