- Nano Express
- Open Access
Ultrafine MnO2 Nanowire Arrays Grown on Carbon Fibers for High-Performance Supercapacitors
© The Author(s). 2016
- Received: 28 September 2016
- Accepted: 15 October 2016
- Published: 20 October 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.
- Ultrafine nanowires
- Large area
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.
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.
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.
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.
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