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Fabrication of PANI-coated honeycomb-like MnO2 nanospheres with enhanced electrochemical performance for energy storage
Electrochimica Acta 180 (2015) 977–982
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Fabrication of PANI-coated honeycomb-like MnO2 nanospheres with enhanced electrochemical performance for energy storage Xiaowu Sun, Mengyu Gan, Li Ma* , Huihui Wang, Tao Zhou, Shiyong Wang, Wenqin Dai, Huining Wang College of Chemistry & Chemical Engineering, Chongqing University, Chongqing 400044, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 May 2015 Received in revised form 9 September 2015 Accepted 10 September 2015 Available online 12 September 2015
A new binary composite of MnO2/PANI has been synthesized as electrode material for energy storage. In the composite, the MnO2 nanospheres with a honeycomb-like structure are coated with PANI layers, in which MnO2 serve as a scaffold for PANI layer and PANI act as a protective coating layer to restrain MnO2 nanospheres from dissolution in acidic electrolyte, and causing a synergic effect. The composite, when the mass ratio of MnO2 and aniline is 1:1, exhibits a high specific capacitance of 565 F g1 (specific capacity of 126 mAh g1) at a discharge current density of 0.8 A g1 and a high specific capacity of 143 mAh g1 at the scan rate of 20 mV s1 in 0.5 M Na2SO4-0.5 M H2SO4 solution. In addition, the electrode material retains about 77% of the initial capacitance after 1000 cycles of charge-discharge at 8 A g1. These results demonstrate that the special morphology and shorter diffusion pathways (SEM and EIS) make the binary composite a promising candidate for future energy storage applications. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Honeycomb-like MnO2 PANI Protective effect
1. Introduction With the decrease of finite fossil-fuel supplies and worrying environmental issues, energy conversion and storage have undoubtedly arisen worldwide attention. Consequently, it is very important and indispensable to develop rechargeable batteries and supercapacitors materials for energy storage [1–4]. Transition metal oxide (such as MnO2 [5,6], Co3O4 [7,8], NiO [9,10]) have been extensively studied for rechargeable batteries and supercapacitors applications owing to their good structural stability and excellent charge storage characteristics [2,11,12]. Compared to other transition metal oxides, MnO2 is generally regarded as the most promising one for the electrode material of batteries and supercapacitors because of its good cycle stability, cost effectiveness, high surface area, ideal charge storage characteristics and environmental friendliness [6,13–17]. Thus far, the published results have established that the electrochemical performances of MnO2 are greatly concerned with the particle sizes, controlled morphology and the specific surface area [18,19]. Nanostructured MnO2 with various morphologies have been found, for example, nanowires [20], nanosheets [21], nanorods [14] and hierarchical spheres [6,19]. With the purpose of maximizing the
* Corresponding author. Tel.: +86 2365106159; Fax: +86 2365106159 E-mail address: [email protected] (L. Ma). http://dx.doi.org/10.1016/j.electacta.2015.09.056 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
electrochemical activity, honeycomb MnO2 nanospheres have been used in our works because this “opened” structures will further increase the energy storage density of electrode materials [22]. However the poor electrical conductivity and instability even in mildly acidic electrolyte of MnO2 greatly limit its application in energy storage. If a protective material can be incorporated with MnO2 nanospheres, it should be possible to serve as an obstacle to restrain the dissolution process when MnO2-based materials are operated in acidic medium. The conducting polymer polyaniline (PANI) may be a good candidate for this protective effect, and has the additional benefits of its easy synthesis, relatively high conductivity, low cost and reversible redox states [16,23–26]. When the surface of MnO2 nanosphere is coated by PANI layer, MnO2/PANI composites not only can develop their respective advantages, but will produce a synergic effect [27,28]. On the basis of this synergic effect, serving MnO2 as a scaffold for PANI layer might also overcome the weakness of PANI as an individual electrode material (the low cycle life and poor mechanical properties resulting from the swelling and shrinkage during the doping and dedoping processes [29,30]). In this work, MnO2/PANI nanocomposites are synthesized by a simple method to overcome the shortcomings of electrode materials based on MnO2 and to integrate the advantages of MnO2 and PANI. Herein, we have prepared PANI-coated honeycomb-like MnO2 nanospheres by two-step polymerization method
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Fig. 1. Schematic illustration of the growth process of MnO2/PANI nanocomposite.
(the first step is solution polymerization and the second step is insitu polymerization [14,31,32]). In order to further investigate the protective effect of PANI coating layer for MnO2 in acidic mediums, we fabricate three different proportions of the composite material (the mass ratio of MnO2 and aniline 1:2, 1:1, 2:1 are designated as MP1, MP2 and MP3, respectively) and stabilize this three composite materials in the mixed Na2SO4-H2SO4 acidic aqueous solution for the testing of electrochemical characterization.
measured from KBr sample pellets on a FTIR spectrophotometer (NICOLET-5700). The morphology of the composites was characterized by a field-emission scanning electron microscope (FE-SEM, JEOLJSM-6335F). 2.5. Electrochemical measurements
Potassium permanganate (KMnO4), oleic acid, ammonium persulfate (APS, 98%) and aniline monomer were purchased from Chuandong Chemical Reagent Company (Chengdu, China). All chemicals used were of analytical grade and used without further purification.
All electrochemical experiments were carried out using a threeelectrode system at room temperature, and used 0.5 M Na2SO40.5 M H2SO4 acidic solution as an electrolyte. For making a working electrode, as-prepared materials (MP1, MP2, MP3 and MnO2), acetylene black and Teflon (8:1:1 wt.%) in N-methylpyrrolidone and coated on carbon paper to form a homogeneous film. Cyclic voltammetry (CV) was performed between 0.2 and 1.0 V at different scan rates using a CHI604 electrochemical workstation. Charge-discharge processes were carried out galvanostatically at various current densities in 0-0.8 V voltage range, using Autolab (PG302 N).
2.2. Preparation of honeycomb-like MnO2 nanospheres
3. Results and discussion
MnO2 were synthesisized in a typical procedure [6,33], dissolving 1 g potassium permanganate (KMnO4) in 450 ml deionized water, and stirring until the mixture completely dissolved. Then a total of 10 ml of oleic acid was added to KMnO4 solution with simultaneous vigorous stirring. Under the condition of 30 C uniform stirring for 24 hours, the nanospheres were synthesisized, then the product was collected by centrifugation, and washed several times with toluene and alcohol to remove residual reactants. honeycomb MnO2 nanospheres were obtained after dried at 60 C for 12 h.
3.1. Morphology and structure characterizations
2. Experimental 2.1. Materials
2.3. Preparation of MnO2/PANI nanocomposite
The directional assembly process of the composites is illustrated in Fig. 1. Honeycomb-like MnO2 nanospheres are synthesized via microemulsion method. The binary composites have been prepared with a two-step protocol. After the first step of fabrication of polyaniline within 30 min (using APS as oxidant), the polyaniline is allowed to orient assembly on the surface of MnO2 nanospheres because of the interaction between the hydroxyl groups on the surface of MnO2 spheres and the amino groups of PANI. With the second aniline adding into the mixture, the MnO2 spheres are wrapped by PANI through in-situ polymerization.
The prepared MnO2 nanospheres (0.2 g) were dispersed in water (50 ml) containing 0.1 g of aniline to get a tawny suspension with the aid of ultrasonication. 0.1 g aniline was slowly added into 25 ml 2 M HCl solution and stirred violently. Then 0.5 g APS (dissolved in 25 ml of 2 M HCl solution) was given drop-wise within 30 min under constant stirring in the reaction mixture. Then the yellow brown suspension was added into the reaction mixture in 30 min. The molar ratio of aniline and APS is 1:1, the concentration of hydrochloric acid is 1 M. The reaction was stirred for 8 h. Finally, the composite was filtered and rinsed with distilled water, 1 M HCl and ethanol in sequence, and then dried at 60 C for 12 h. 2.4. Material characterization Powder X-ray diffraction (XRD) patterns were performed in a PANalytical Empyrean Diffractometer equipped PIXcel 3D detector, using Cu (Ka) radiation with the 2u-angle recorded from 10 to 70 . Fourier transformation infrared spectra (FTIR) of the samples were
Fig. 2. XRD patterns of different samples: MnO2, MnO2/PANI.
X. Sun et al. / Electrochimica Acta 180 (2015) 977–982
Fig. 3. FTIR spectra of PANI, MnO2 and MnO2/PANI.
XRD pattern of the layered porous structure MnO2 and MnO2/ PANI composite are shown in Fig. 2. For MnO2 nanospheres, significant XRD peaks are recorded at 2u = 12.1, 24.6, 36.5 and 65.5 , corresponding to (001), (002), (100) and (110) crystal planes, which is in good agreement with the reported patterns for birnessite-type MnO2 [6,33]. XRD pattern of MnO2/PANI composite shows two broad peaks at 2u of 20.5 and 25.3 , which represent the periodicities parallel and perpendicular to the PANI chain, respectively [34]. It indicates that the PANI has been prepared. For composite, the characteristic peaks of MnO2 at 2u of 12.1 and 24.6 almost disappeared and the intensity of other diffraction peaks is lower than pure MnO2. This observation might be due to the surface of MnO2 nanospheres have been densely grafted with the PANI layer [16,28]. FT-IR spectra of pure PANI, MnO2 and MnO2/PANI nanocomposite are given in Fig. 3 to obtain more specific information of the PANI layer coating onto the surface of MnO2 nanospheres. For MnO2, the bands at 3422 and 1647 cm1 can be assigned to stretching and bending vibrations of the hydroxyl group of water molecule and OH in the lattice, which implies that hydroxyl groups existed in the as-synthesized MnO2 nanospheres [6,33]. The characteristic band of MnO2 is Mn–O stretching vibration band at 518 cm1.The peaks of C–C, C–H (2800-3000 cm1) and C–H (1300-1500 cm1) are assigned to the characteristic peaks of oleic acid, indicating that oleic acid existed in the nanospheres [6,33]. In the spectra of pure PANI, the characteristic peaks at 1570 and 1482 cm1 correspond to the C=C stretching of quinoid and benzenoid rings, the peaks at 1299 and 1242 cm1 are related to the stretching vibrations of C=N and C–N, the peaks located at 1129 and 826 cm1 can be ascribed to N=Q=N stretching vibration and
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out of plane bending of C–H, respectively [25,28,35]. As for MnO2/ PANI, the characteristic FTIR spectrum is similar to that of pure PANI, the peaks at 1570, 1482, 1299, 1242, 1129 and 826 cm1 that belonging to pure PANI can still be observed, verifying the formation of PANI in the composites. Furthermore, two characteristic absorption peaks at 619 and 524 cm1, which are attributed to the vibrations of Mn–O–Mn and Mn–O [16,36], are also observed in the MnO2/PANI composite. It should be noted that the peaks of composites are shifted slightly to higher wave numbers, resulting from the strong interaction between the PANI and MnO2, which means PANI has been coated on the surface of MnO2 nanospheres. The morphology and structure of honeycomb MnO2 nanospheres and MnO2/PANI nanocomposites are elucidated by SEM observations (Fig. 4). Obviously, Fig. 4a shows a typical SEM image of monodisperse MnO2 nanospheres with the diameter about 90 nm, having a hierarchical porous structure that is formed by the self-assembly of MnO2 nanoplatelets [33]. Compared with the MnO2 nanospheres, the diameter of MnO2/PANI nanocomposites have increased obviously, as presented in Fig. 4b. As can be seen from the inset, PANI grew around the MnO2 nanosphere and covered the hierarchical structure on its surface, which is in good agreement with the results of XRD and FTIR spectra. It indicates that the composites have been fabricated successfully. Such MnO2 nanospheres with the “opened” structure in favor of high specific surface areas for the hybrid material, in association with the protective function PANI shell might result in high capacity and excellent cycling performance when it is used as a supercapacitor material in acidic electrolyte. 3.2. Electrochemical behavior of MnO2/PANI composite in 0.5 Na2SO40.5 H2SO4 acidic electrolyte The capacitive performance of MP1, MP2 and MP3 were evaluated by CV and galvanostatic charge/discharge (GCD) techniques in 0.5 M Na2SO4-0.5 M H2SO4 solution. Fig. 5a shows CV curves of MP1, MP2, MP3 and MnO2 at 20 mV s1. For MnO2 the electrochemical response current shown an irreversible CV curve and even cannot form a close cycle (the inset in Fig. 5a). This phenomenon may be based on the non-reversibly reductive dissolution of the initial MnO2, where Mn4+ sites were reduced to soluble Mn2+ species with an accompanying insertion of protons from the acid electrolytes. The shapes of CV curves of the former three samples are almost the same, both composites exhibit redox peaks of PANI, where MP1, MP2 and MP3 exhibit higher capacity than MnO2 electrode. It can be attributed to protective response of PANI coating layer. Fig. 6 reveals the mechanism of the dissolution of MnO2 and the protection of PANI coating layer in mixed electrolyte. Fig. 6a shows the dissolution of MnO2, the reasons for such dissolution phenomenon may be attributed to the following equation [28,37,38]:
Fig. 4. FESEM images of MnO2 nanospheres (a), MnO2/PANI (b).
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Fig. 5. CV curves of MP1, MP2, MP3 and MnO2 at scan rate of 20 mV s1, the inset is enlarged CV curve of MnO2 (a). CV curves of MP2 at different scan rates of 5, 10, 20, 50 mV s1 (b).
MnO2 + H+ + e fi MnOOH
(1)
equation:
3+
Mn sites may be further reduced, as shown in Eq. (2), or undergo disproportionation, as in Eq. (3) +
2+
MnOOH + 3H + e ! Mn
(aq) + 2H2O
2MnOOH + 2H+ ! MnO2 + Mn2+ (aq) + 2H2O
(2)
(3)
The overall reaction may be written as MnO2 + 4H+ + 2e ! Mn2+ (aq) + 2H2O
(4)
Fortunately, PANI coating layer effectively prevents such decomposition of MnO2 and the mechanism of the protection is shown in Fig. 6b. Proton (red particles) will stay at the PANI layer because of redox reactions of PANI and Na+ (yellow particles) will not only be deeply intercalated into the porous channels, but also produced redox reactions on the surface of manganese oxides. According to the reports by Brousse et al. [39] that our composites are battery type materials, and specific capacity in mAh g1 should be used rather than specific capacitance in F g1 when assessing the electrochemical performance of composites. And the specific capacity of MP1, MP2 and MP3 are 94.8, 143 and 60.4 mAh g1 at a scan rate of 20 mV s1, respectively. Obviously, MP2 has the best electrochemical performance. The thickness of the protective layer of PANI has increased with the proportion of aniline increases in polymerization process. However, if the protective layer was too thick (like MP1), the properties would greatly decrease, resulting in the reductive surface area ratio of the composites and the shortcomings of PANI. CV curves of MP2 at different scan rates between 5 and 50 mV s1 are presented in Fig. 5b. From the CV response currents, we can see that MP2 is reversibly reduced and oxidized in mixed electrolyte at various scan rates, reflecting strong protective effect of PANI. To investigate the electrochemical performance of MnO2/PANI composite GCD curves were measured at a current density of 0.8 A g1 within the potential window 0-0.8 V (Fig. 7a). The specific capacitance (Cm) of the composite can be calculated according to
Cm ¼
IDt mDV
ð5Þ
where Cm is the specific capacitance (F g1), I is the chargedischarge current (A), Dt is the discharge time (s), DV is the potential window (V), m is the mass of active material in the working electrode (g) [40]. The calculated Cm is 410, 565 and 340 F g1 (specific capacity: 91, 126 and 76 mAh g1) for MP1, MP2 and MP3, respectively. Obviously, MP2 showed a preferable performance of the capacitor and the utilization of MnO2 and PANI. To assess the potential of MP2 as electrode materials for ECs, GCD measurements at various current densities were carried out. As illustrated in Fig. 7b, all the curves exhibit an equilateral triangle shape, indicating high reversibility of the nanocomposite during the process of charge and discharge. The process of taking a longer time at lower current density is attributed to the sufficient insertion or release of H+ and Na+ during the charging and discharging steps. The specific capacitance or capacity of MP2 is much higher than that of MP1 and MP3 at the same current density, as revealed in Fig. 7c. At the current density of 16 A g1, the Cm of MP1, MP2 and MP3 are 140, 345, 112 F g1 (specific capacity: 31.1, 76.7 and 24.9 mAh g1), and retains around 34.1%, 61.1% and 32.9% of the specific capacitance at 0.8 A g1, respectively. This result reveals that MP2 has a preferable rate capability. The enhancement of capacitive performance in acidic mediums of MP2 can certainly be attributed to the protecting existence of PANI layer and larger specific surface area of this nanocomposite.To further demonstrate the electrochemical characteristics of the as-prepared composites, the relationship of energy density (E) and power density (P) is displayed in The Ragone plots (Fig. 7d). For MP2, an energy density of 100.4 Wh kg1 is obtained at a power density of 640 W kg1 and still maintains 60.4 Wh kg1 at a high power density of 12800 W kg1 in 0.5 M Na2SO4-0.5 M H2SO4 solution. Obviously, the energy density of MP2 is much higher than that of MP1 and MP3 at the same power density, indicating that MP2 is a good candidate for energy storage.
Fig. 6. Schematic illustration of dissolution of MnO2 nanospheres in acidic mediums (a), and the protective function of PANI (b). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
X. Sun et al. / Electrochimica Acta 180 (2015) 977–982
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Fig. 7. Charge-discharge curves of MP1, MP2 and MP3 at current density of 0.8 A g1 (a). Charge–discharge curves of MP2 at different current densities (b). Specific capacitances of MP1, MP2 and MP3 at different discharge currents (0.8-16 A g1) in 0.5 M Na2SO4-0.5 M H2SO4 solution (c). Ragone plots of MP1, MP2 and MP3 (d).
The electrochemical stability of MP2 was further investigated in the range of 0-0.8 V at 0.8 A g1 in 0.5 M Na2SO4-0.5 M H2SO4 aqueous solution. As shown in Fig. 8 (black arrow), the initial average capacitance (first cycle) of MP2 was 466 F g1 (specific capacity: 103.6 mAh g1), and it decreased to 359 F g1 (specific capacity: 79.8 mAh g1) after 1000 cycles. The capacitance retained about 77% of the initial capacitance after 1000 charge-dischange cycles (red arrow), compared to the first cycle, indicating a good cycling stability of the composite. The reasons for the decrease in capacitance with the increase of cycle numbers may be attributed to the inevitable dissolution of MnO2 in acidic medium and the degradation of PANI which caused by the swelling and shrinking of PANI under aqueous environment. EIS (Electrochemical impedance spectroscopy) was employed to characterize the kinetic features of the ion diffusions in the
Fig. 8. Capacitance retention of MP2 in the range of 0-0.8 V at 8 A g1. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
electrode. Fig. 9 shows the Nyquist plot obtained in the frequency range of 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. And the enlarged portion of the high frequency region is shown in the inset. The Nyquist plots of MP1, MP2 and MP3 are all composed of a semicircular in the high frequency region and a straight line in the low frequency region. In the high frequency region, the intersection of the curve on X-axis is related to ESR (equivalent series resistance) of the electrode which contains contact resistance, intrinsic resistance of substrate and electrolyte resistance [41–44]. From the inset, MP3 presents a higher ESR value which can be attributed to the poor conductivity of MnO2. In the low frequency region, the slope of the curve shows the Warburg impedance [14,43,44] which represents the diffusion of H+ and Na+ in the electrode. It can be observed that MP2 displays
Fig. 9. Nyquist impedance plots of MP1, MP2 and MP3.
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a more vertical straight line compared to MP1 and MP3, indicating that MP2 has a faster ion diffusion rate. Compared to MP2, less diffusion channel of MP1 which caused by the thicker PANI layers and the lower content of honeycomb-like MnO2 resulting in a slower diffusion rate. For MP3, the protective layer of PANI is too thin, and the dissolution of MnO2 in acidic medium might lead to collapse of the composite structure. 4. Conclusions In summary, we have first synthesized the novel MnO2/PANI nanocomposites via targeting assembly polyaniline on the surface of honeycomb-like MnO2 nanospheres, where MnO2 is synthesized in microemulsion method. In the nanocomposite, PANI not only can effectively store energy, enhance capacitive performance of the composites, but also act as a good coating layer to restrain MnO2 nanospheres from dissolution in acidic electrolyte. MnO2 nanospheres serve as a support material for PANI and solve the disadvantages of PANI as individual electrode material. At the same time of developing their own strengths, composites successfully avoid the defects of MnO2 and PANI as a single material, and produce an excellent synergic effect. Electrochemical tests demonstrate that capacitive performance of the binary composite is strongly dependent upon the mass ratio of MnO2 and aniline. When the ratio is 1:1, the electrode shows a high specific capacitance of 565 F g1 (specific capacity of 126 mAh g1) at a discharge current density of 0.8 A g1 and a high specific capacity of 143 mAh g1 at the scan rate of 20 mV s1 in 0.5 M Na2SO4-0.5 M H2SO4 solution. A maximum energy density as high as 100.4 Wh kg1 can be obtained at a power density of 640 W kg1 for the composite materials. Moreover, the nanocomposite retains about 77% of the original capacitance after 1000 cycles of chargedischarge at 8 A g1, exhibiting a good cycling stability and rate capability. These results demonstrate the binary composite is a promising candidate in energy storage applications. References [1] B.E. Conway, Journal of the Electrochemical Society 138 (1991) 1539–1548. [2] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X.W.D. Lou, Advanced Materials 24 (2012) 5166–5180. [3] K. Chen, Y. Dong Noh, K. Li, S. Komarneni, D. Xue, The Journal of Physical Chemistry C 117 (2013) 10770–10779. [4] Y. Zhou, B. Wang, C. Liu, N. Han, X. Xu, F. Zhao, J. Fan, Y. Li, Nano Energy 15 (2015) 654–661. [5] A.L.M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, Nano Letters 9 (2009) 1002–1006. [6] J. Zhu, J. He, ACS applied materials & interfaces 4 (2012) 1770–1776. [7] Z.-S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.-M. Cheng, ACS nano 4 (2010) 3187–3194.
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Fabrication of PANI-coated honeycomb-like MnO2 nanospheres with enhanced electrochemical performance for energy storage Xiaowu Sun, Mengyu Gan, Li Ma* , Huihui Wang, Tao Zhou, Shiyong Wang, Wenqin Dai, Huining Wang College of Chemistry & Chemical Engineering, Chongqing University, Chongqing 400044, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 3 May 2015 Received in revised form 9 September 2015 Accepted 10 September 2015 Available online 12 September 2015
A new binary composite of MnO2/PANI has been synthesized as electrode material for energy storage. In the composite, the MnO2 nanospheres with a honeycomb-like structure are coated with PANI layers, in which MnO2 serve as a scaffold for PANI layer and PANI act as a protective coating layer to restrain MnO2 nanospheres from dissolution in acidic electrolyte, and causing a synergic effect. The composite, when the mass ratio of MnO2 and aniline is 1:1, exhibits a high specific capacitance of 565 F g1 (specific capacity of 126 mAh g1) at a discharge current density of 0.8 A g1 and a high specific capacity of 143 mAh g1 at the scan rate of 20 mV s1 in 0.5 M Na2SO4-0.5 M H2SO4 solution. In addition, the electrode material retains about 77% of the initial capacitance after 1000 cycles of charge-discharge at 8 A g1. These results demonstrate that the special morphology and shorter diffusion pathways (SEM and EIS) make the binary composite a promising candidate for future energy storage applications. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Honeycomb-like MnO2 PANI Protective effect
1. Introduction With the decrease of finite fossil-fuel supplies and worrying environmental issues, energy conversion and storage have undoubtedly arisen worldwide attention. Consequently, it is very important and indispensable to develop rechargeable batteries and supercapacitors materials for energy storage [1–4]. Transition metal oxide (such as MnO2 [5,6], Co3O4 [7,8], NiO [9,10]) have been extensively studied for rechargeable batteries and supercapacitors applications owing to their good structural stability and excellent charge storage characteristics [2,11,12]. Compared to other transition metal oxides, MnO2 is generally regarded as the most promising one for the electrode material of batteries and supercapacitors because of its good cycle stability, cost effectiveness, high surface area, ideal charge storage characteristics and environmental friendliness [6,13–17]. Thus far, the published results have established that the electrochemical performances of MnO2 are greatly concerned with the particle sizes, controlled morphology and the specific surface area [18,19]. Nanostructured MnO2 with various morphologies have been found, for example, nanowires [20], nanosheets [21], nanorods [14] and hierarchical spheres [6,19]. With the purpose of maximizing the
* Corresponding author. Tel.: +86 2365106159; Fax: +86 2365106159 E-mail address: [email protected] (L. Ma). http://dx.doi.org/10.1016/j.electacta.2015.09.056 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
electrochemical activity, honeycomb MnO2 nanospheres have been used in our works because this “opened” structures will further increase the energy storage density of electrode materials [22]. However the poor electrical conductivity and instability even in mildly acidic electrolyte of MnO2 greatly limit its application in energy storage. If a protective material can be incorporated with MnO2 nanospheres, it should be possible to serve as an obstacle to restrain the dissolution process when MnO2-based materials are operated in acidic medium. The conducting polymer polyaniline (PANI) may be a good candidate for this protective effect, and has the additional benefits of its easy synthesis, relatively high conductivity, low cost and reversible redox states [16,23–26]. When the surface of MnO2 nanosphere is coated by PANI layer, MnO2/PANI composites not only can develop their respective advantages, but will produce a synergic effect [27,28]. On the basis of this synergic effect, serving MnO2 as a scaffold for PANI layer might also overcome the weakness of PANI as an individual electrode material (the low cycle life and poor mechanical properties resulting from the swelling and shrinkage during the doping and dedoping processes [29,30]). In this work, MnO2/PANI nanocomposites are synthesized by a simple method to overcome the shortcomings of electrode materials based on MnO2 and to integrate the advantages of MnO2 and PANI. Herein, we have prepared PANI-coated honeycomb-like MnO2 nanospheres by two-step polymerization method
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Fig. 1. Schematic illustration of the growth process of MnO2/PANI nanocomposite.
(the first step is solution polymerization and the second step is insitu polymerization [14,31,32]). In order to further investigate the protective effect of PANI coating layer for MnO2 in acidic mediums, we fabricate three different proportions of the composite material (the mass ratio of MnO2 and aniline 1:2, 1:1, 2:1 are designated as MP1, MP2 and MP3, respectively) and stabilize this three composite materials in the mixed Na2SO4-H2SO4 acidic aqueous solution for the testing of electrochemical characterization.
measured from KBr sample pellets on a FTIR spectrophotometer (NICOLET-5700). The morphology of the composites was characterized by a field-emission scanning electron microscope (FE-SEM, JEOLJSM-6335F). 2.5. Electrochemical measurements
Potassium permanganate (KMnO4), oleic acid, ammonium persulfate (APS, 98%) and aniline monomer were purchased from Chuandong Chemical Reagent Company (Chengdu, China). All chemicals used were of analytical grade and used without further purification.
All electrochemical experiments were carried out using a threeelectrode system at room temperature, and used 0.5 M Na2SO40.5 M H2SO4 acidic solution as an electrolyte. For making a working electrode, as-prepared materials (MP1, MP2, MP3 and MnO2), acetylene black and Teflon (8:1:1 wt.%) in N-methylpyrrolidone and coated on carbon paper to form a homogeneous film. Cyclic voltammetry (CV) was performed between 0.2 and 1.0 V at different scan rates using a CHI604 electrochemical workstation. Charge-discharge processes were carried out galvanostatically at various current densities in 0-0.8 V voltage range, using Autolab (PG302 N).
2.2. Preparation of honeycomb-like MnO2 nanospheres
3. Results and discussion
MnO2 were synthesisized in a typical procedure [6,33], dissolving 1 g potassium permanganate (KMnO4) in 450 ml deionized water, and stirring until the mixture completely dissolved. Then a total of 10 ml of oleic acid was added to KMnO4 solution with simultaneous vigorous stirring. Under the condition of 30 C uniform stirring for 24 hours, the nanospheres were synthesisized, then the product was collected by centrifugation, and washed several times with toluene and alcohol to remove residual reactants. honeycomb MnO2 nanospheres were obtained after dried at 60 C for 12 h.
3.1. Morphology and structure characterizations
2. Experimental 2.1. Materials
2.3. Preparation of MnO2/PANI nanocomposite
The directional assembly process of the composites is illustrated in Fig. 1. Honeycomb-like MnO2 nanospheres are synthesized via microemulsion method. The binary composites have been prepared with a two-step protocol. After the first step of fabrication of polyaniline within 30 min (using APS as oxidant), the polyaniline is allowed to orient assembly on the surface of MnO2 nanospheres because of the interaction between the hydroxyl groups on the surface of MnO2 spheres and the amino groups of PANI. With the second aniline adding into the mixture, the MnO2 spheres are wrapped by PANI through in-situ polymerization.
The prepared MnO2 nanospheres (0.2 g) were dispersed in water (50 ml) containing 0.1 g of aniline to get a tawny suspension with the aid of ultrasonication. 0.1 g aniline was slowly added into 25 ml 2 M HCl solution and stirred violently. Then 0.5 g APS (dissolved in 25 ml of 2 M HCl solution) was given drop-wise within 30 min under constant stirring in the reaction mixture. Then the yellow brown suspension was added into the reaction mixture in 30 min. The molar ratio of aniline and APS is 1:1, the concentration of hydrochloric acid is 1 M. The reaction was stirred for 8 h. Finally, the composite was filtered and rinsed with distilled water, 1 M HCl and ethanol in sequence, and then dried at 60 C for 12 h. 2.4. Material characterization Powder X-ray diffraction (XRD) patterns were performed in a PANalytical Empyrean Diffractometer equipped PIXcel 3D detector, using Cu (Ka) radiation with the 2u-angle recorded from 10 to 70 . Fourier transformation infrared spectra (FTIR) of the samples were
Fig. 2. XRD patterns of different samples: MnO2, MnO2/PANI.
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Fig. 3. FTIR spectra of PANI, MnO2 and MnO2/PANI.
XRD pattern of the layered porous structure MnO2 and MnO2/ PANI composite are shown in Fig. 2. For MnO2 nanospheres, significant XRD peaks are recorded at 2u = 12.1, 24.6, 36.5 and 65.5 , corresponding to (001), (002), (100) and (110) crystal planes, which is in good agreement with the reported patterns for birnessite-type MnO2 [6,33]. XRD pattern of MnO2/PANI composite shows two broad peaks at 2u of 20.5 and 25.3 , which represent the periodicities parallel and perpendicular to the PANI chain, respectively [34]. It indicates that the PANI has been prepared. For composite, the characteristic peaks of MnO2 at 2u of 12.1 and 24.6 almost disappeared and the intensity of other diffraction peaks is lower than pure MnO2. This observation might be due to the surface of MnO2 nanospheres have been densely grafted with the PANI layer [16,28]. FT-IR spectra of pure PANI, MnO2 and MnO2/PANI nanocomposite are given in Fig. 3 to obtain more specific information of the PANI layer coating onto the surface of MnO2 nanospheres. For MnO2, the bands at 3422 and 1647 cm1 can be assigned to stretching and bending vibrations of the hydroxyl group of water molecule and OH in the lattice, which implies that hydroxyl groups existed in the as-synthesized MnO2 nanospheres [6,33]. The characteristic band of MnO2 is Mn–O stretching vibration band at 518 cm1.The peaks of C–C, C–H (2800-3000 cm1) and C–H (1300-1500 cm1) are assigned to the characteristic peaks of oleic acid, indicating that oleic acid existed in the nanospheres [6,33]. In the spectra of pure PANI, the characteristic peaks at 1570 and 1482 cm1 correspond to the C=C stretching of quinoid and benzenoid rings, the peaks at 1299 and 1242 cm1 are related to the stretching vibrations of C=N and C–N, the peaks located at 1129 and 826 cm1 can be ascribed to N=Q=N stretching vibration and
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out of plane bending of C–H, respectively [25,28,35]. As for MnO2/ PANI, the characteristic FTIR spectrum is similar to that of pure PANI, the peaks at 1570, 1482, 1299, 1242, 1129 and 826 cm1 that belonging to pure PANI can still be observed, verifying the formation of PANI in the composites. Furthermore, two characteristic absorption peaks at 619 and 524 cm1, which are attributed to the vibrations of Mn–O–Mn and Mn–O [16,36], are also observed in the MnO2/PANI composite. It should be noted that the peaks of composites are shifted slightly to higher wave numbers, resulting from the strong interaction between the PANI and MnO2, which means PANI has been coated on the surface of MnO2 nanospheres. The morphology and structure of honeycomb MnO2 nanospheres and MnO2/PANI nanocomposites are elucidated by SEM observations (Fig. 4). Obviously, Fig. 4a shows a typical SEM image of monodisperse MnO2 nanospheres with the diameter about 90 nm, having a hierarchical porous structure that is formed by the self-assembly of MnO2 nanoplatelets [33]. Compared with the MnO2 nanospheres, the diameter of MnO2/PANI nanocomposites have increased obviously, as presented in Fig. 4b. As can be seen from the inset, PANI grew around the MnO2 nanosphere and covered the hierarchical structure on its surface, which is in good agreement with the results of XRD and FTIR spectra. It indicates that the composites have been fabricated successfully. Such MnO2 nanospheres with the “opened” structure in favor of high specific surface areas for the hybrid material, in association with the protective function PANI shell might result in high capacity and excellent cycling performance when it is used as a supercapacitor material in acidic electrolyte. 3.2. Electrochemical behavior of MnO2/PANI composite in 0.5 Na2SO40.5 H2SO4 acidic electrolyte The capacitive performance of MP1, MP2 and MP3 were evaluated by CV and galvanostatic charge/discharge (GCD) techniques in 0.5 M Na2SO4-0.5 M H2SO4 solution. Fig. 5a shows CV curves of MP1, MP2, MP3 and MnO2 at 20 mV s1. For MnO2 the electrochemical response current shown an irreversible CV curve and even cannot form a close cycle (the inset in Fig. 5a). This phenomenon may be based on the non-reversibly reductive dissolution of the initial MnO2, where Mn4+ sites were reduced to soluble Mn2+ species with an accompanying insertion of protons from the acid electrolytes. The shapes of CV curves of the former three samples are almost the same, both composites exhibit redox peaks of PANI, where MP1, MP2 and MP3 exhibit higher capacity than MnO2 electrode. It can be attributed to protective response of PANI coating layer. Fig. 6 reveals the mechanism of the dissolution of MnO2 and the protection of PANI coating layer in mixed electrolyte. Fig. 6a shows the dissolution of MnO2, the reasons for such dissolution phenomenon may be attributed to the following equation [28,37,38]:
Fig. 4. FESEM images of MnO2 nanospheres (a), MnO2/PANI (b).
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Fig. 5. CV curves of MP1, MP2, MP3 and MnO2 at scan rate of 20 mV s1, the inset is enlarged CV curve of MnO2 (a). CV curves of MP2 at different scan rates of 5, 10, 20, 50 mV s1 (b).
MnO2 + H+ + e fi MnOOH
(1)
equation:
3+
Mn sites may be further reduced, as shown in Eq. (2), or undergo disproportionation, as in Eq. (3) +
2+
MnOOH + 3H + e ! Mn
(aq) + 2H2O
2MnOOH + 2H+ ! MnO2 + Mn2+ (aq) + 2H2O
(2)
(3)
The overall reaction may be written as MnO2 + 4H+ + 2e ! Mn2+ (aq) + 2H2O
(4)
Fortunately, PANI coating layer effectively prevents such decomposition of MnO2 and the mechanism of the protection is shown in Fig. 6b. Proton (red particles) will stay at the PANI layer because of redox reactions of PANI and Na+ (yellow particles) will not only be deeply intercalated into the porous channels, but also produced redox reactions on the surface of manganese oxides. According to the reports by Brousse et al. [39] that our composites are battery type materials, and specific capacity in mAh g1 should be used rather than specific capacitance in F g1 when assessing the electrochemical performance of composites. And the specific capacity of MP1, MP2 and MP3 are 94.8, 143 and 60.4 mAh g1 at a scan rate of 20 mV s1, respectively. Obviously, MP2 has the best electrochemical performance. The thickness of the protective layer of PANI has increased with the proportion of aniline increases in polymerization process. However, if the protective layer was too thick (like MP1), the properties would greatly decrease, resulting in the reductive surface area ratio of the composites and the shortcomings of PANI. CV curves of MP2 at different scan rates between 5 and 50 mV s1 are presented in Fig. 5b. From the CV response currents, we can see that MP2 is reversibly reduced and oxidized in mixed electrolyte at various scan rates, reflecting strong protective effect of PANI. To investigate the electrochemical performance of MnO2/PANI composite GCD curves were measured at a current density of 0.8 A g1 within the potential window 0-0.8 V (Fig. 7a). The specific capacitance (Cm) of the composite can be calculated according to
Cm ¼
IDt mDV
ð5Þ
where Cm is the specific capacitance (F g1), I is the chargedischarge current (A), Dt is the discharge time (s), DV is the potential window (V), m is the mass of active material in the working electrode (g) [40]. The calculated Cm is 410, 565 and 340 F g1 (specific capacity: 91, 126 and 76 mAh g1) for MP1, MP2 and MP3, respectively. Obviously, MP2 showed a preferable performance of the capacitor and the utilization of MnO2 and PANI. To assess the potential of MP2 as electrode materials for ECs, GCD measurements at various current densities were carried out. As illustrated in Fig. 7b, all the curves exhibit an equilateral triangle shape, indicating high reversibility of the nanocomposite during the process of charge and discharge. The process of taking a longer time at lower current density is attributed to the sufficient insertion or release of H+ and Na+ during the charging and discharging steps. The specific capacitance or capacity of MP2 is much higher than that of MP1 and MP3 at the same current density, as revealed in Fig. 7c. At the current density of 16 A g1, the Cm of MP1, MP2 and MP3 are 140, 345, 112 F g1 (specific capacity: 31.1, 76.7 and 24.9 mAh g1), and retains around 34.1%, 61.1% and 32.9% of the specific capacitance at 0.8 A g1, respectively. This result reveals that MP2 has a preferable rate capability. The enhancement of capacitive performance in acidic mediums of MP2 can certainly be attributed to the protecting existence of PANI layer and larger specific surface area of this nanocomposite.To further demonstrate the electrochemical characteristics of the as-prepared composites, the relationship of energy density (E) and power density (P) is displayed in The Ragone plots (Fig. 7d). For MP2, an energy density of 100.4 Wh kg1 is obtained at a power density of 640 W kg1 and still maintains 60.4 Wh kg1 at a high power density of 12800 W kg1 in 0.5 M Na2SO4-0.5 M H2SO4 solution. Obviously, the energy density of MP2 is much higher than that of MP1 and MP3 at the same power density, indicating that MP2 is a good candidate for energy storage.
Fig. 6. Schematic illustration of dissolution of MnO2 nanospheres in acidic mediums (a), and the protective function of PANI (b). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
X. Sun et al. / Electrochimica Acta 180 (2015) 977–982
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Fig. 7. Charge-discharge curves of MP1, MP2 and MP3 at current density of 0.8 A g1 (a). Charge–discharge curves of MP2 at different current densities (b). Specific capacitances of MP1, MP2 and MP3 at different discharge currents (0.8-16 A g1) in 0.5 M Na2SO4-0.5 M H2SO4 solution (c). Ragone plots of MP1, MP2 and MP3 (d).
The electrochemical stability of MP2 was further investigated in the range of 0-0.8 V at 0.8 A g1 in 0.5 M Na2SO4-0.5 M H2SO4 aqueous solution. As shown in Fig. 8 (black arrow), the initial average capacitance (first cycle) of MP2 was 466 F g1 (specific capacity: 103.6 mAh g1), and it decreased to 359 F g1 (specific capacity: 79.8 mAh g1) after 1000 cycles. The capacitance retained about 77% of the initial capacitance after 1000 charge-dischange cycles (red arrow), compared to the first cycle, indicating a good cycling stability of the composite. The reasons for the decrease in capacitance with the increase of cycle numbers may be attributed to the inevitable dissolution of MnO2 in acidic medium and the degradation of PANI which caused by the swelling and shrinking of PANI under aqueous environment. EIS (Electrochemical impedance spectroscopy) was employed to characterize the kinetic features of the ion diffusions in the
Fig. 8. Capacitance retention of MP2 in the range of 0-0.8 V at 8 A g1. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
electrode. Fig. 9 shows the Nyquist plot obtained in the frequency range of 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. And the enlarged portion of the high frequency region is shown in the inset. The Nyquist plots of MP1, MP2 and MP3 are all composed of a semicircular in the high frequency region and a straight line in the low frequency region. In the high frequency region, the intersection of the curve on X-axis is related to ESR (equivalent series resistance) of the electrode which contains contact resistance, intrinsic resistance of substrate and electrolyte resistance [41–44]. From the inset, MP3 presents a higher ESR value which can be attributed to the poor conductivity of MnO2. In the low frequency region, the slope of the curve shows the Warburg impedance [14,43,44] which represents the diffusion of H+ and Na+ in the electrode. It can be observed that MP2 displays
Fig. 9. Nyquist impedance plots of MP1, MP2 and MP3.
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a more vertical straight line compared to MP1 and MP3, indicating that MP2 has a faster ion diffusion rate. Compared to MP2, less diffusion channel of MP1 which caused by the thicker PANI layers and the lower content of honeycomb-like MnO2 resulting in a slower diffusion rate. For MP3, the protective layer of PANI is too thin, and the dissolution of MnO2 in acidic medium might lead to collapse of the composite structure. 4. Conclusions In summary, we have first synthesized the novel MnO2/PANI nanocomposites via targeting assembly polyaniline on the surface of honeycomb-like MnO2 nanospheres, where MnO2 is synthesized in microemulsion method. In the nanocomposite, PANI not only can effectively store energy, enhance capacitive performance of the composites, but also act as a good coating layer to restrain MnO2 nanospheres from dissolution in acidic electrolyte. MnO2 nanospheres serve as a support material for PANI and solve the disadvantages of PANI as individual electrode material. At the same time of developing their own strengths, composites successfully avoid the defects of MnO2 and PANI as a single material, and produce an excellent synergic effect. Electrochemical tests demonstrate that capacitive performance of the binary composite is strongly dependent upon the mass ratio of MnO2 and aniline. When the ratio is 1:1, the electrode shows a high specific capacitance of 565 F g1 (specific capacity of 126 mAh g1) at a discharge current density of 0.8 A g1 and a high specific capacity of 143 mAh g1 at the scan rate of 20 mV s1 in 0.5 M Na2SO4-0.5 M H2SO4 solution. A maximum energy density as high as 100.4 Wh kg1 can be obtained at a power density of 640 W kg1 for the composite materials. Moreover, the nanocomposite retains about 77% of the original capacitance after 1000 cycles of chargedischarge at 8 A g1, exhibiting a good cycling stability and rate capability. These results demonstrate the binary composite is a promising candidate in energy storage applications. References [1] B.E. Conway, Journal of the Electrochemical Society 138 (1991) 1539–1548. [2] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X.W.D. Lou, Advanced Materials 24 (2012) 5166–5180. [3] K. Chen, Y. Dong Noh, K. Li, S. Komarneni, D. Xue, The Journal of Physical Chemistry C 117 (2013) 10770–10779. [4] Y. Zhou, B. Wang, C. Liu, N. Han, X. Xu, F. Zhao, J. Fan, Y. Li, Nano Energy 15 (2015) 654–661. [5] A.L.M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, Nano Letters 9 (2009) 1002–1006. [6] J. Zhu, J. He, ACS applied materials & interfaces 4 (2012) 1770–1776. [7] Z.-S. Wu, W. Ren, L. Wen, L. Gao, J. Zhao, Z. Chen, G. Zhou, F. Li, H.-M. Cheng, ACS nano 4 (2010) 3187–3194.
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