Chang-Feng Zhao‡
,
Ke Lu‡ and
Houyi Ma*
Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. E-mail: hyma@sdu.edu.cn; Fax: +86-531-88564464; Tel: +86-531-88364959
First published on 31st October 2016
Facile interfacial growth of MnO2 nanoparticles on carbon nanofibers (MnO2/CNF) and the application of this material as a high-performance positive electrode material in asymmetric supercapacitors are presented. Rational combination of MnO2/CNF positive and CNF negative electrode materials enabled asymmetric devices with extended operating voltage windows of 1.8 V, and much higher power densities (4434 W kg−1) and energy densities (36 W h kg−1) compared with similar devices being reported. The assembled novel hybrid aqueous asymmetric capacitor shows excellent cycling stability with 90.9% capacity retention for 1000 cycles. The asymmetric supercapacitors were based on low cost and environmental friendly materials and have great potential for practical devices.
Among these pseudocapacitive transition metal oxides, MnO2 in particular has been considered as one of the most scalable alternatives due to its natural abundance, high theoretical specific capacitance, relatively low cost, and environmental friendliness.18 A key limitation to the use of MnO2 active materials is its limited electronic conductivity (10−5 to 10−6 S cm−1) and poor rate capability, which impede its practical application in energy storage.23,24 To this end, binary and/or ternary nanostructured electrodes made by integrating metal oxide particles with carbon hosts via simple procedures with high electrochemical performance need to be developed.4,8 In MnO2/carbon nanofiber (MnO2/CNF) binary nanocomposites, the carbon nanofiber (CNF) can not only contribute to the maximum accessible surface area/overall capacity through the EDLC mechanism but can also provide effective conductive networks for electrical transport by bridging MnO2 particles.8 So, surface growth/incorporation of MnO2 particles on CNF surfaces can overcome these limitations and improve electrochemical activity, and can therefore serve as high-performance candidates for SCs. As for practical applications, however, the energy densities, operating voltages, and capacitances of SCs still need to be improved significantly. Hitherto, asymmetric supercapacitors combining MnO2-based positive electrodes with carbon-based negative electrodes have been constructed to maximise specific capacitance and enlarge the operating voltage window.8
Directed interfacial grown of MnO2 nanoparticles on highly conductive CNF surfaces was obtained through a straightforward one-step hydrothermal processing technique.25 The conductivity of the pseudocapacitive MnO2 was effectively enhanced and its electrochemical performance was accordingly maximised. In view of the interconnected CNF’s superior electrical transport properties, direct/continuous surface growth of MnO2 nanoparticles enables excellent supercapacitive performance, which is obviously desirable for high-performance supercapacitors. An advanced asymmetric supercapacitor with MnO2/CNF positive and CNF negative electrodes in a mild aqueous electrolyte (0.5 M Na2SO4) was fabricated. The asymmetric supercapacitor demonstrated excellent cycle stability in a large voltage window of 1.8 V, high energy density (36 W h kg−1), and much higher power density (4434 W kg−1) compared with similar devices being reported. Benefiting from these features, the as-fabricated hybrid supercapacitor can serve as a promising candidate for next-generation energy storage systems.
CNFs were synthesized according to the hydrothermal Te@C nanocable template process.26,27 Te nanowires were prepared using a hydrothermal method reported by Yu’s group.27,28 Typically, the as-obtained Te wires were dispersed in 24 mL water with vigorous magnetic stirring, and then 1.5 g of glucose was added to the mixture to form a clear solution. The final solution was transferred into a Teflon-lined stainless steel autoclave and reacted at 200 °C for 10 h. The products were collected by centrifugation and were then washed with deionized water and absolute ethanol three times. After removal of the core of the product with 30 mL of an aqueous solution containing 2 mL of hydrochloric acid (36.5 wt%), 5 mL of H2O2 (30 wt%), and 20 mL of double-distilled water (HCl:
H2O2
:
H2O = 2
:
5
:
20, v/v) at room temperature for 12 h, the obtained black powder was further calcined at 600 °C for 2 h in an N2 flow, yielding pure CNFs.
MnO2/CNF nanocomposites were prepared using a simple in situ hydrothermal method.25 In a typical synthesis, 0.5 g of SDBS and 0.2 g of the CNFs were dissolved in 40 mL of water with 0.005 M MnSO4 and 0.005 M KMnO4, and the mixture was stirred for 1 h. The mixture was then transferred into a Teflon-lined stainless steel autoclave and reacted at 160 °C for 14 h. The products were collected by centrifugation, washed with deionized water three times, and were dried overnight at 60 °C. For comparison, α-MnO2 nanowires were also synthesized according to a hydrothermal method which has been previously reported.29
As shown in Fig. 2a, the broad (002) peak of the CNF matrix ranged from 20° to 30°, suggesting that the carbon host was amorphous in character. What is more, compared to the typical XRD patterns of the resultant MnO2/CNF and MnO2 samples, in addition to the diffraction peaks of the carbon substrates, all of the other peaks can be indexed to the tetragonal phase of α-MnO2 (JCPDS No. 44-0141).25,29 This indicates that the MnO2 nanoparticles had a relatively high crystallinity after combination/interfacial growth. Highly crystalline MnO2 can be formed, as confirmed by XRD, TEM and HRTEM. The XPS patterns of the MnO2/CNF sample (Fig. 2b) provide further insights into the elements’ oxidation states. The binding energy in the case of Mn 2p peaks are 642.0 and 653.8 eV, and demonstrate that the Mn element in the MnO2/CNF sample was present in the chemical state of Mn(VI).18,22 This corresponds well with the structure information obtained from S/TEM and XRD.
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Fig. 2 (a) Powder X-ray diffraction spectra of the MnO2/CNF, CNF and MnO2 samples. (b) High-resolution Mn 2p and C 1s (inset) of the MnO2/CNF sample. |
To assess the feasibility of using MnO2/CNF and CNFs for construction of asymmetric supercapacitor devices, the electrochemical properties of MnO2/CNF binary composites, α-MnO2, and CNFs were studied separately in 0.5 M Na2SO4 using CV, galvanostatic charge–discharge, and EIS techniques. Cyclic voltammetry (CV) was initially employed to estimate their electrochemical properties in the three-electrode cell in Na2SO4 (−0.2 to 0.8 V vs. SCE). Fig. 3a and b illustrates the CV curves of the as-synthesized MnO2/CNF positive electrode and the CNF negative electrode at different scan rates. Accordingly, all of the current densities have been normalized based on the mass of electroactive material. For the MnO2/CNF composites (Fig. 3a), the shape of the CV curves gradually transforms from ideal rectangular to oval with increasing scan rate, which may be caused by the increased internal resistance of the electrode, the limited conductivity of the aqueous electrolytes, and the slow diffusion kinetics of Na+.31–33 Additionally, the area of the curve is commonly used to calculate the value of the specific capacitance. CV curves of the CNF materials (Fig. 3b, −1.0 to 0.2 V vs. SCE) exhibit a rectangular shape under different scan rates without obvious distortion, suggesting a good electrical double-layer capacitive (EDLC) nature with good ion transport in the carbon electrode.4,34 Although influenced by charge-transfer resistance, the changes of current with the applied potential did not give straight lines that parallel the horizontal axis for both positive and negative electrodes. Typical charge/discharge linear curves of MnO2/CNF materials collected at different current rates are shown in Fig. 3c, indicating the good electrochemical reversibility. Furthermore, the non-obvious iR drop for these discharge curves further proves its high electrical conductivity and/or lower internal resistance.34 The calculated specific capacitance of the composite electrodes is 335, 309, 285, 264, 254 F g−1 at 1, 2, 5, 8, 10 A g−1, respectively, and is summarized in Fig. 3d. According to TG analysis (ESI, Fig. S1†), the amounts of the carbon component in MnO2/CNF composites are estimated to be 18%. So, the specific capacitance contributed by MnO2 for the MnO2/CNF composites were about 89.9% and 90.1% at current densities of 1 A g−1 and 5 A g−1. As can be found, the porous CNF matrix not only contributes to the overall specific capacitance values but also maintains high capacitance values even at high current densities. In contrast, the discharge capacitance obtained by the pure α-MnO2 electrode is much smaller (Fig. 3d). The specific capacitances calculated from CV curves are also taken into account to verify the result from the charge-discharge curves. According to the CV measurements, the capacitances are 438, 329, 290, 210, and 159 F g−1 at 5, 20, 30, 50, and 80 mV s−1, respectively. So, we can conclude that the high capacitive current responses of MnO2-containing composite electrodes mainly result from the efficient redox reaction of MnO2 within the effective interconnected conductive framework provided by the CNF hosts.4 It was found that the pure CNF negative electrode has quite good specific capacitances, which are 188, 169, 155, 140, 132 F g−1 at 1, 2, 5, 8, 10 A g−1, respectively, and in the potential range between −1.0 and 0.2 V vs. SCE. Furthermore, the cyclic stability of electrodes made with MnO2/CNF and CNFs were evaluated by a 2000 cycle galvanostatic charge–discharge test conducted at 8 A g−1. Both positive and negative electrodes retain more than 90% (91.4% retention for the positive electrode and 93.5% retention for the negative electrode) of the initial capacitance after 2000 cycles at 8 A g−1 (Fig. 3e). However, as can be seen, the cycling performance of the pure α-MnO2 electrode is very limited, which further convinces us of the necessity and effectiveness of combining it with carbon scaffolds. EIS, an effective tool to analyze the internal resistance and the electron transfer rate at the electrode–electrolyte interface, was performed to explore related electrodes. Fig. 3f shows typical Nyquist plots for different electrodes. As noted, all of them had a straight line at low frequencies and a semi-circle at high frequencies. The circuit used to fit the impedance spectrum is listed in the inset. The values of the charge-transfer resistance for MnO2/CNF, MnO2 and the CNFs were determined to be 0.62, 1.3, and 0.50 Ω cm2, respectively. The lower charge-transfer resistance values imply the easy penetration of the electrolyte within the working electrode and the maximized utilization of the electrode materials. Overall, the cycling performance and capacity of this binary material is comparable or even superior to that of similar materials (see Table S1†). Taking the above-mentioned electrochemical results into consideration, the MnO2/CNF and CNF materials can be selected as positive and negative electrodes to construct asymmetric supercapacitors.
Asymmetric supercapacitors using the MnO2/CNF as positive and CNF as negative materials were assembled. The mass ratio of the positive to the negative was optimized at 0.66 on the basis of charge balance theory. Due to the facile kinetics on both the positive (battery-type material) and negative (capacitor-type material) electrodes, this class of device ensures high power and high energy density.34 CV measurements were first performed to explore the stable potential window of the as-prepared asymmetric supercapacitor. The operating potential was investigated as shown in Fig. 4a. When the cell voltage reaches 1.8 V, there is no bump which may caused by some irreversible side-reactions. Consequently, the cell voltage of the asymmetric supercapacitors was selected as 0–1.8 V. As shown in Fig. 4a, each CV curve keeps a similar shape without apparent distortion under a series of scan rates (5 to 80 mV s−1), indicating that the as-fabricated asymmetric supercapacitors show good capacitive behavior with quasi-rectangular shapes. This large potential window, along with the fast charge/discharge properties, can meet the demand of practical applications. Representative galvanostatic charge/discharge curves for the asymmetric prototypes at different current densities are shown in Fig. 4b. Both the MnO2/CNF composite and the CNFs contribute to the whole capacitance, and no noticeable change was perceived for the shapes of the CV and discharge curves, which illustrates the fast charge–discharge performance of the asymmetric devices. Specific capacitance values (based on the total mass of positive and negative active materials), calculated based on the discharge curves, are summarized in Fig. 4c. As shown in Fig. 4c, the calculated specific capacitance of the asymmetric devices was 80 F g−1 at a current density of 0.5 A g−1. The specific capacitance decreases slightly due to the voltage drop with increased current density but is maintained at 52 F g−1 even at 5 A g−1, demonstrating the excellent rate capability.Both the pseudocapacitance of the MnO2/CNF nanocomposite and the EDLC of the CNFs are beneficial for high-performance energy storage devices. Additionally, the long-term cycling stability also should be taken into consideration. The asymmetric supercapacitors were further examined using a long-term charge–discharge test at a scan rate of 2 A g−1 (Fig. 4d). The capacitance retention of the device is maintained at 90.2% of its original specific capacitance after 1000 cycles (88% after 5000 cycles at a high current rate of 5 A g−1, Fig. S2†). The EIS of the asymmetric supercapacitor device was also tested, and is shown in Fig. 4e. The small charge-transfer resistance (Rct, 2.3 Ω cm2) is also beneficial for high performance devices. The Ragone plot (energy density vs. power density) was further employed to show the electrochemical performance of MnO2/CNF//CNF asymmetric supercapacitors. The plot reveals a significant increase in the supercapacitor performance when pairing different electrodes to construct a hybrid supercapacitor. The calculated energy density (normalized to the whole active weight) of our asymmetric supercapacitor is 36 W h kg−1 at a power density of 450 W kg−1, which is higher than or comparable to previously reported similar hybrid supercapacitors. The maximum power density is 4433 W kg−1, comparable to commercially available supercapacitors. This tentative test suggests that these devices possess better energy and power densities, and are very promising candidates for high performance energy storage systems or portable electronic devices.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra23195k |
‡ C. F. Zhao and K. Lu contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2016 |