Weibing Xuab,
Bin Mua,
Wenbo Zhanga and
Aiqin Wang*a
aState Key Laboratory of Solid Lubrication, Center of Eco-Materials and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China. E-mail: aqwang@licp.cas.cn; Fax: +86 931 8277088; Tel: +86 931 4968118
bUniversity of the Chinese Academy of Sciences, Beijing 100049, P. R. China
First published on 20th July 2015
Kapok fiber/MnO2 (TKF/MnO2) composites with a tubular structure are successfully fabricated via a facile hydrothermal process. Potassium permanganate and kapok fiber served as the manganese source and the template, respectively. The effects of operating parameters including material proportion, reaction temperature, reaction time and the growth mechanism of MnO2 are studied in detail. A maximum specific capacitance of 117 F g−1 has been achieved at 0.25 A g−1 in 1 M Na2SO4 and 95% specific capacitance is maintained after 1000 cycles, which demonstrates the potential application of tubular TKF/MnO2 composites in supercapacitors. The superior electrochemical performances of the obtained composites are attributed to their hollow structure, thin wall thickness, and orderly pore passages, which can facilitate ion diffusion and improve the utilization of the electroactive sites of MnO2.
An emerging solution to overcome the above drawbacks is to utilize the synergistic effect of binary or ternary hybrid systems composed of conducting polymers, carbon materials and metal oxide. For instance, the hybrid systems composed of polyaniline and metal oxide have been widely investigated to improve the cycle capability of polyaniline and conductivity of metal oxide.24,25 The hybrid material composed of Ni(OH)2 and MnO2 also displayed the enhanced electrochemistry activity.26 In addition, hollow micro-/nano-structured materials have been considered as one of promising electrode materials for supercapacitor due to their unique attractive advantages,27 such as the short diffusion lengths of ions, large material/electrolyte contact area, good strain accommodation, and the unusual mechanical and electrical properties endowed by confining the dimensions.28–30 However, the common synthetic techniques based on hard-template for the preparation of hollow structures are costly and complicated including electrochemical deposition,31 chemical oxidative polymerization,32 electro spinning.33,34
As an agricultural product, kapok fiber (KF) shows a significantly homogeneous hollow tube shape.35–37 It carries large specific surface and good surface activity to makes it much easier to attach metal oxide,38,39 and thus it could be applied in designing the novel electrode materials of supercapacitor. In this paper, the tubular kapok fiber/MnO2 (TKF/MnO2) composites are prepared based on the NaClO2-treated natural KF (TKF) by a simple hydrothermal method in the presence of KMnO4. The hollow structure is kept well during the hydrothermal process, and the possible growth mechanism of MnO2 on the surface of TKF is proposed. The structure–property relationship between the morphology and electrochemical performances of the as-prepared TKF/MnO2 composites are investigated in detail, and the results reveal that the TKF/MnO2 composites shown good electrochemical performance as electrode materials for supercapacitors.
Samples | TKF (g) | Time (h) | Temperature (°C) | Capacitancea (F g−1) |
---|---|---|---|---|
a The specific capacitance calculated according to eqn (1). | ||||
TKF/MnO2-1 | 0.2 | 4 | 140 | 3.6 |
TKF/MnO2-2 | 0.4 | 4 | 140 | 7.4 |
TKF/MnO2-3 | 0.6 | 4 | 140 | 3.2 |
TKF/MnO2-4 | 0.8 | 4 | 140 | 2.4 |
TKF/MnO2-5 | 0.4 | 0.5 | 140 | 42.0 |
TKF/MnO2-6 | 0.4 | 1 | 140 | 117.0 |
TKF/MnO2-7 | 0.4 | 2 | 140 | 13.0 |
TKF/MnO2-8 | 0.4 | 6 | 140 | 9.4 |
TKF/MnO2-9 | 0.4 | 1 | 100 | 12.5 |
TKF/MnO2-10 | 0.4 | 1 | 120 | 21.0 |
TKF/MnO2-11 | 0.4 | 1 | 160 | 33.3 |
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The reaction time is also optimized at 140 °C for different time of 0.5 h, 1 h, 2 h, 4 h, and 6 h, and the obtained samples are denoted as TKF/MnO2-2, 5–8, respectively. The FTIR spectra of TKF/MnO2-2, 5–8 are displayed in Fig. S3.† The characteristic peaks appeared at about 1741, 1375 and 1255 cm−1 decreased significantly with the increasing hydrothermal time from 0.5 h to 1 h. It can be found that three absorption bands are very strong in the TKF/MnO2-5 (0.5 h), but they almost disappear (TKF/MnO2-6) as the hydrothermal time increases to 1 h. Furthermore, the absorption bands in the 1000–1450 cm−1 region also significantly decrease, which may be ascribed to the deacetylation and dehydration occurred during the hydrothermal process.48 It also can be found that the Mn–O stretching vibration is very weak in the TKF/MnO2-5, and the relative intensity of the characteristic absorption bands of Mn–O increases when the reaction time increases to be 1 h, indicating that the content of MnO2 increases with the increasing hydrothermal time. Fig. S4† exhibited the typical XRD patterns of TKF/MnO2-2, 5–8. The diffraction peak of TKF/MnO2-5 is low indicating a poor crystalline feature in these composites. With the increase in the reaction time, the diffraction peaks assigned to α-MnO2 become sharp and high. It indicates that the crystallinity and the size of the MnO2 particle become high and large as the hydrothermal time increase. Therefore, the crystalline and morphology of the MnO2 nanostructures can be controlled very well by the reaction time. The electrochemical performance of the synthesized TKF/MnO2-5–8 composites as electrode materials for supercapacitors is evaluated, as shown in Table 1. It is worth noting that the TKF/MnO2-6 composite exhibits the highest specific capacitance with the value of 117 F g−1. Based on these above, the optimum reaction time is 1 hour in this work.
The reaction temperature is also optimized at different hydrothermal temperature for 1 h with the addition of TKF (0.4 g), as shown in Table 1 of samples of TKF/MnO2-6, 9–11, which corresponded to 100 °C, 120 °C, 140 °C and 160 °C, respectively. As a control, 0.4 g of pure TKF is also hydrothermally treated at 100, 120, 140, and 160 °C for 1 h in order to investigate the structure change of TKF with the increasing hydrothermal temperature. The obtain products are donated as TKF-100, TKF-120, TKF-140, and TKF-160, respectively. The FTIR spectra of the TKF-100–160 are presented in Fig. S5.† By comparing the absorption peaks of four samples, obvious change of absorption peak can be observed. The intensity of absorption peaks at 2914 cm−1 (C–H stretching vibration), 1735 cm−1, 1375 cm−1, and 1254 cm−1 (CO ester) decrease with the increasing hydrothermal temperature from 100 °C to 160 °C. Furthermore, the absorption band at 3000–3700 cm−1 becomes broad. This may be ascribed to the deacetylation reaction with an increase of the temperature.48 The digital photos of TKF-100–160 and TKF/MnO2-6, 9–11 are exhibited in Fig. S6.† It clearly indicates that the transformation of the color of the products before and after addition of KMnO4. The FTIR spectra of the TKF/MnO2-6, 9–11 composites are displayed in Fig. S7.† The intensity of the characteristic peaks located at around 1741, 1375 and 1255 cm−1 decreased significantly with the temperature increase from 100 °C to 120 °C. The three peaks almost disappear when the temperature up to 160 °C (TKF/MnO2-11). This result is attributed to the high temperature in favour for the degradation reactions of the acetyl ester group in TKF.49,50 The decreases in the intensity and the broad band at 1000–1460 cm−1 suggest that dehydration occurred with the increasing hydrothermal temperature.44 In addition, the intensity of the characteristic peaks appeared at about 517 cm−1 and 620 cm−1 increase as the temperature elevated from 100 °C to 120 °C. As the temperature further rise, the relative intensity of the two peaks is not significantly change, which may be ascribed to the similar content of the MnO2 in the composite. The XRD patterns of the TKF/MnO2-6, 9–11 composites are displayed in Fig. S8.† The diffraction peak of the cellulous becomes low in the all of composites indicating a poor crystallinity. As for the TKF/MnO2 composites, the diffraction peaks assigned to MnO2 become sharp and high with the increasing hydrothermal temperature, indicating the crystallinity and the size of the MnO2 particle become high and large. However, it should be mentioned that when the reaction temperature up to 160 °C (TKF/MnO2-11), the tubular structure of TKF is destroyed completely (Fig. S9†). This suggests that the TKF is reacted with KMnO4 during hydrothermal process. By contrast, the sample of TKF/MnO2-6 prepared at 140 °C for 1 h exhibits the highest specific capacitance. In summary, the detailed ratio-, time-, and temperature-dependent studies illustrate that the optimum quality of TKF, temperature and reaction time is 0.4 g, 140 °C and 1 h, respectively. Therefore, TKF/MnO2-6 composite is selected as representative for the further study.
Ag+ + e− → Ag + 0.7996 V | (5) |
MnO4− + 2H2O + 3e− → MnO2(s) + 4OH− + 0.59 V | (6) |
It is worth noting that the solution is weak alkaline after potassium permanganate decomposed at the neutral conditions according to the eqn (6). Owing to the difference in the reactive activity of the hydroxyl groups between cellulose and TKF, potassium permanganate might be reduced into manganese dioxide using TKF under hydrothermal condition at 140 °C compared with that of cellulose/Ag NPs composites obtained under hydrothermal condition at 120 °C. To our knowledge, the natural fiber/MnO2 composites with tubular structure are prepared for the first time using natural fibers and potassium permanganate as raw materials without added other reductant.
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Fig. 2 SEM images of (a and b) TKF/MnO2-5, (c and d) TKF/MnO2-6, (e and f) TKF/MnO2-7, and (g and h) TKF/MnO2-2. |
The energy dispersive X-ray spectroscopy (EDS) analysis of TKF/MnO2-6 demonstrated the distribution of manganese, oxygen, carbon components of TKF/MnO2-6 (Fig. 3a–d). It is obvious that the above three components are uniformly distributed on the tubular TKF/MnO2-6 composites. This result implied that the MnO2 nanoparticles are fully confined within the surface of tubular TKF during the hydrothermal treatment, and the two components (TKF, MnO2) are strongly coupled together in the hybrids. In addition, the samples which prepared at 0.5 h (TKF/MnO2-5) and 2 h (TKF/MnO2-7) are also characterized by EDS (ESI Fig. S10 and S11†), and the manganese content of the two samples is 23.4% and 29.2%, respectively. It is higher than the samples of TKF/MnO2-6 (20.83%). According to the hydrothermal process of the pure carbohydrates.63 This process mainly includes three important steps: (1) dehydration of the carbohydrate to (hydroxymethyl) furfural; (2) polymerization towards polyfurans; (3) carbonization via further intermolecular dehydration. The samples of 5–7 are prepared at the hydrothermal temperature 140 °C for 0.5 h, 1 h, and 2 h, respectively. Only the first step of dehydration might take place during this process accompanied with the release of some gases, such as CO2. The ratio of C/O of the samples of 5–7 is 1.584, 1.384, and 1.523 with the increase in the reaction time. The changes in the manganese content of samples might be related to the ratio of C/O during the hydrothermal process. Furthermore, the value of capacitance of TKF/MnO2-5 and TKF/MnO2-7 is less than that of TKF/MnO2-6 (Table 1), and this result strongly suggests that the size and morphology of the MnO2 anchored on the TKF surface has a great impact on specific capacitance of composite.
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Fig. 3 (a–d) Element mapping images of TKF/MnO2-6 (e) EDX curve of TKF/MnO2-6, the inset shows the atomic ratio of Mn, O and C. |
In order to further comparison the structure change of the KF, TKF, TKF-140 and TKF/MnO2 composites, the FTIR spectra of the pristine KF, TKF, TKF-140 and the TKF/MnO2-6 composite are showed in Fig. 4. It can be seen the broad absorption band at about 3393 cm−1 characteristic of the stretching vibration of –OH in cellulose of KF. The absorption band at 2914 cm−1 is assigned to asymmetric and symmetric stretching vibration in –CH2 and –CH3 group in the acetyl ester groups of cellulose in the KF. The three important ester bands at 1741, 1375 and 1255 cm−1 are associated with the stretching and bending vibrations of carbonyl bonds (CO) of the acetyl ester group in the KF. The stretching vibration of C–C in different substituted aromatic rings in lignin at 1592 and 1463 cm−1 in KF. The bands in the 1000–1450 cm−1 region correspond to C–O stretching (ester, or ether of carbohydrate and polysaccharide) and O–H bending vibrations. When the KF is treated with NaClO2, the absorption band of the hydroxyl groups becomes broad. Furthermore, the relative intensity of the absorption peaks at 2914 cm−1, 1735 cm−1, 1375 cm−1, 1254 cm−1, and 1000–1450 cm−1 increase. It is reported that the NaClO2 would produce chlorine dioxide under an acidic condition, which rendered the oxidation of lignin.64 Due to part of hydrogen bonds and lignin is broken, thus leading to increase in the amorphous part in cellulose and release of more hydroxyl groups. The absorption bands at 1592 and 1463 cm−1 almost disappear in TKF, which indicates the broken of the lignin in the TKF. The TKF exhibited good hydrophilic property and it is fully wet after stirred with distilled water for 30 min. When the TKF is processed at 140 °C for 1 hour without addition KMnO4, the relative intensity of the absorption peaks at 2914 cm−1, 1735 cm−1, 1375 cm−1, 1254 cm−1, and 1000–1450 cm−1 further increase, which may be ascribed to dehydration occurred during the hydrothermal process.44 However, the characteristic peaks appeared at about 1741, 1375 and 1255 cm−1 almost disappears, as shown in the FTIR spectrum of TKF/MnO2-6 composite. This result may be due to the deacetylation by the cleavage of acetyl groups linked as an ester group to the celluloses in TKF.48–50 Furthermore, the weak alkaline condition will be help to the deacetylation reaction, which result from the reduction reaction of KMnO4 under neutral conditions as depicted in eqn (6). This result also agrees well with that of Fig. 1d. The stretching vibration of –OH in cellulose undergoes a shift from 3393 cm−1 in the TKF to 3430 cm−1 in the TKF/MnO2-6 composite. In addition, the relative intensity of absorption band decrease significantly accompanied with a red shift in the 1000–1450 cm−1 region. This phenomenon may be associated to the anchor of MnO2 particle on the TKF surface. The characteristic peaks of MnO2 appear at about 628 and 508 cm−1, which could belonged to the Mn–O vibrations of MnO6 octahedral in α-MnO2 (Fig. 4). The peak located at 1648 cm−1 could be related to the bending vibration mode of the absorbed water.65
The phase purity and crystallographic structure of the TKF/MnO2-6 have been identified by X-ray power diffraction. The XRD patterns of the TKF/MnO2-6 composites and the TKF are shown in Fig. 5. It can be found that the characteristic diffraction peaks of cellulose appear in the position of 2θ close to 15.59° and 22.60° for TKF, corresponding to the (110) and (200) crystallographic planes. In case of TKF/MnO2-6 composite, the characteristic diffraction peaks of cellulose almost disappear. This may be due to the crystallinity has been destroyed during the hydrothermal process. All the diffraction peaks can be indexed to α-MnO2, which are consistent with the reported values (JCPDS Card 44-0141). No characteristic impurity peaks are observed, indicating the high purity of the sample.
The TGA curves of TKF and the TKF/MnO2-6 composite are showed in Fig. 6. The TKF showed a dramatic mass loss (about 78% of weight) from around 250 °C to 350 °C, due to the decomposition of oxygen-containing groups.51 The 45% weight loss accompanied by an endothermic reaction is observed at 140–450 °C at the curve of the TKF/MnO2-6 composite assigned to the loss of water produced by the decomposition and dehydroxylation of the TKF and the high valence MnOx decomposed to a lower valence state along with the removal of more stable oxygen.52,53 As calculated from the TGA curves, the MnO2 content of TKF/MnO2-6 composite is about 23 wt%.
The detailed pore textural characteristics of the samples are analyzed by nitrogen adsorption–desorption technique. The BET specific surface areas and pore volumes of TKF and TKF/MnO2 composites are summarized in Table S1.† It can be found that the TKF exhibits the smallest surface area and pore volume with 0.8875 m2 g−1 and 0.0012 cm3 g−1, respectively. This result is attributed to the TKF presents a regular hollow tubular structure with outer and inner diameters of 20–25 and 16–23 μm, and the BET technique cannot detect the surface areas and pore volumes.66 When the MnO2 is anchored on the surface of TKF, the specific surface areas and pore volumes of all TKF/MnO2 composites can be detected due to the introduction of MnO2 nanoparticles. It should be noting that the surface areas and pore volumes of the TKF/MnO2 composites increase significantly with the increase in the hydrothermal time from 0.5 h to 1 h. At the hydrothermal time 1 h, the TKF/MnO2-6 composite exhibits the maximum the surface areas and pore volumes up to 222.41 m2 g−1 and 0.496 cm3 g−1. As the hydrothermal time further increase (TKF/MnO2-2, 7–8), the surface areas and pore volumes greatly decreased. It might be assigned to the change of the MnO2 morphology with increase in the hydrothermal time, as depicted in Fig. 2. The diameters of the manganese dioxide particles anchored on the surface of TKF become larger after being hydrothermally treated for 1 h, which might be result in the decrease in the surface areas and pore volumes. It also can be observed that the surface area and pore volume also are affected by the hydrothermal temperature. The BET specific surface areas and pore volumes increased with the increasing hydrothermal temperature and the pore structure parameters reach the maximum value at 140 °C (TKF/MnO2-6). This may be ascribed to the fact that the high temperature is favor for the formation of manganese dioxide. However, the pore structure parameters of TKF/MnO2-11 decrease while the hydrothermal temperature is up to 160 °C. This decrease might be related to the collapse of the hollow structure of composites as shown in the SEM images (Fig. S9†).
The Fig. 7 shows the nitrogen adsorption–desorption isotherms and pore size distributions of TKF and TKF/MnO2-6 composite. For TKF/MnO2-6 sample, the nitrogen adsorption isotherms show an evident type IV isotherm, and a type H4 hysteresis loop also can be observed in the adsorption–desorption isotherms (Fig. 7a), indicating the existence of mesopore size in the composite.67,68 The slope shape increase at the medium relative pressure of 0.35–0.65 P/P0 associated with capillary condensation taking place in mesopores, illustrating the presence of well developed mesoporosity.69,70 Fig. 7b shows the pore size distribution of TKF/MnO2-6. It can be seen that the porosity of the composite is essentially consisted of mesopores, in which the average pore size is around 8.92 nm. In the case of the TKF, there is no significant adsorption–desorption occurs during the increase pressure process, this result is ascribed to the fact that the diameter of the hollow tubular structure of TKF is up to micron level, and the pore size distribution of the TKF is not detected. In addition, the nitrogen adsorption–desorption isotherms and pore size distributions of TKF/MnO2-2, 5–11 composites are tested as shown in Fig. S12.† The adsorption–desorption isotherms of all these samples are exhibit a typical IV isotherm with a H3 hysteresis loop. It worth noting that mesopores are dominated in the all samples and the pore size is around range from 5.12 nm to 30 nm. For comparison, it is clearly found that surface areas and the pore volumes of TKF/MnO2-6 are the largest among all of the samples.
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Fig. 7 (a) N2 adsorption–desorption isotherms of TKF and TKF/MnO2-6 composite and (b) BJH pore size distribution of TKF/MnO2-6 composites. |
To obtain more information about the ability of the TKF/MnO2-6 composite to function as electrodes in supercapacitors, EIS measurements are carried out in 1.0 M Na2SO4 solution at open circuit potential (vs. SCE), which could provide useful information about the redox reaction resistance and equivalent series resistance. Fig. 8c presents the typical Nyquist plots of the composite. At the high-frequency region, the real axis provides quantitative information on the effective internal resistance (Rs), which is mainly attributed to the uncompensated solution resistance. It can be seen that the Rs values of TKF/MnO2-6 is about 2.0 Ω. The semicircle corresponded to the charge-transfer resistance (Rct) at the electrode–electrolyte interface. It could be seen that the diameter of the capacitive reactance arc of the TKF/MnO2-6 electrode is about 5.0 Ω. It could be inferred that the charge transfer resistance of electrode materials prepared with TKF/MnO2-6 composites is small, and it facilitates charge-transfer and the migration of the electrolyte ions in the interior of the pores of electrode materials. Furthermore, the line at low frequencies is closer to a parallel line along the imaginary axis for an ideal capacitor.
To evaluate the durability of the TKF/MnO2-6 composite, the long-term cycling stability of the TKF/MnO2-6 electrodes is also tested by continuous cyclic voltammetric scans at the scan rates of 100 mV s−1 for 1000 cycles in 1 M Na2SO4 solution and the result is shown in Fig. 8d. The capacitance retention rate can maintain 95% of its original specific capacitance after 1000 cycles, which could be attributed to the introduction of TKF, and hollow tubular structure of the as-prepared composites. The hollow channels can not only promote accessibility of electrolyte ions to active sites and fast diffusion, but also shorten the transport length for ions and storage charge during charging/discharging processes effectively.72
An asymmetric supercapacitor is fabricated based on the TKF/MnO2-6 composites to further value its application. More specifically, the TKF/MnO2-6 composite is used as the positive electrode and the commercial activated carbon (AC) is treated as the negative electrode, and the mass ratio of positive and negative is about 1.4 according to the eqn (2). The electrochemistry test results of the asymmetric supercapacitor are shown in Fig. 8. Based on the three-electrode electrochemical measurements of both TKF/MnO2-6 composite and AC electrode, a certain mass of AC supported on Ni foam is chosen to balance the capacitance of two electrodes.73 The CV curves of the asymmetric capacitor shows that the cell voltage could be extended to 2.0 V (Fig. 9a). The typical CV result at various scan rates indicates ideal capacitor behaviour, with a nearly rectangular CV shape. The GCD curves (Fig. 9b) exhibited great charge–discharge potentials that are nearly proportional to the charge or discharge time. The specific capacitances calculated from the GCD curves at current density 0.25 A g−1 is 50 F g−1. The energy density is 27.8 W h kg−1 and the power density is 330 W kg−1 according to the eqn (3) and (4). The results indicates that the supercapacitor made by our method have a great superiority in the shortage of energy density. Moreover, EIS is applied in this study in order to understand the interfacial process of the asymmetric supercapacitor. The data is presented in Fig. 9c in the Nyquist plot form. The semicircle and the oblique line indicate that the electrochemical reaction is controlled by charge transfer at high frequency range and electrolyte diffusion at low frequency range.
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Fig. 9 (a) CV curves at different scan rates, (b) GCD curves at different current density, and (c) EIS plots in 1 M Na2SO4 electrolyte for the TKF/MnO2-6//AC supercapacitor. |
Table S2† demonstrates the comparison in the capacitive performance of supercapacitors based on various natural fiber composites presented in literature and this work. It can be seen that the capacitive performance of the present TKF/MnO2 composites surpasses many other natural fiber composites. In addition, the TKF/MnO2 composites facilely fabricated through a green hydrothermal process compared with the reported complicated preparation process of other natural fiber composites. Furthermore, in comparison with the high price of other supports, such as carbon nanotube, graphene gel, the present TKF is quite cost-effective and it can be produced on large-scale.
Footnote |
† Electronic supplementary information (ESI) available: FTIR spectra and XRD patterns of TKF/MnO2-1–11, SEM of TKF/MnO2-11, FTIR spectra and digital photos of TKF-100–160 and TKF/MnO2-6, 9–11, EDS of TKF/MnO2-5 and TKF/MnO2-7, the BET specific surface areas and volumes of TKF and TKF/MnO2-2, 5–11 composites are summarized in Table S1, the nitrogen adsorption–desorption isotherms and pore size distributions of TKF and TKF/MnO2-2, 5–11 composites, CV, GCD and ESI analysis of TKF/MnO2-2, 5–11, the comparison in the capacitive performance of supercapacitors based on various natural fiber composites. See DOI: 10.1039/c5ra13602d |
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