h-MoO3 Hybrid Material for High Performance Supercapacitor

May 16, 2017 - α-MnO2/h-MoO3 hybrid metal oxide materials with different weight ratios were successfully synthesized. The prepared nanocomposite mate...
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Research Article pubs.acs.org/journal/ascecg

α‑MnO2/h-MoO3 Hybrid Material for High Performance Supercapacitor Electrode and Photocatalyst Parasseri Muhammed Shafi,† Rengasamy Dhanabal,† Angamuthuraj Chithambararaj,† Sivan Velmathi,‡ and Arumugam Chandra Bose*,† †

Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli 620015, India Organic and Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli-620015, India

‡

ABSTRACT: α-MnO2/h-MoO3 hybrid metal oxide materials with different weight ratios were successfully synthesized. The prepared nanocomposite material has been demonstrated to be an electrode material for potential application in electrochemical charge storage devices as well as an efficient photocatalyst for dye degradation. The results showed high ionic conductivity, better charge transfer ability, viability for high current density, and enhanced specific capacitance (Sc) with appreciable cyclic stability. Electrochemical performances of the prepared materials were investigated by cyclic voltammetry (CV) analysis, galvanostatic charge−discharge (CD) techniques, and electrochemical impedance spectroscopy (EIS). We were able to achieve a maximum Sc value of 412 F/g and 358 F/g at a current density of 1 A/g for the composite with 25% h-MoO3 mass loading from a three electrode cell and two electrode symmetric cell configuration, respectively, and it could deliver an outstanding energy density of 50 Wh kg−1 with a power density of 1 KW kg−1, which is a very appreciable result compared to bare metal oxide electrode materials. In addition, 94% of methylene blue (MB) degradation was observed for the prepared composite material under visible light irradiation. The possible photocatalytic mechanism is schematically illustrated. KEYWORDS: Nanocomposite, Electrochemical storage, Symmetric cell, Ionic conductivity, Cyclic stability, Dye degradation

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INTRODUCTION Increased demand of fossil fuels along with the exponential uses brought us to the current energy crisis and to the polluted environment. Owing to the invention of electricity and human civilization, the growth of industries has become countless. Several organic pollutants released from textiles, leather, plastic, and pharmaceutical industries directly have a detrimental effect on the environment and human beings. Hence, sustainable energy and green environmentalism have equal importance in day to day life. Researchers and scientists have been making an enormous effort toward the development of such smart materials to solve current issues in energy and the environment. Metal oxides have been attracted due to their favorable applications in energy storage, biosensing, gas sensing, and wastewater management.1−4 Among the various metal oxides, the nanostructured MnO2 has intensive attraction in charge storage and dye degradation applications due to its favorable electrochemical and charge transfer ability. While coming to the charge storage application, supercapacitors achieved considerable attention due to their ecofriendly nature. Among the various materials studied for supercapacitor electrodes, (i) carbon materials, (ii) metal oxides, and (iii) conducting polymers received great attention. A variable oxidation state, excellent chemical and electrochemical stability, © 2017 American Chemical Society

and ease of preparation and handling made transition metal oxides the best choice for supercapacitor electrode materials. Hydrated RuO2 prepared by a sol−gel process at a low temperature has a specific capacitance as high as 720 F/g due to a solid state pseudo-Faradaic reaction. However, high cost, low porosity, and the toxic nature of RuO2 limit commercialization of supercapacitors using this material. These drawbacks of RuO2 led to the study of low-cost metal oxides as alternatives, such as MnO2, MoO3, VOx, and WO3.5−8 MnO2 has attracted much more attention because of its favorable properties, which meet the particular requirements, and it is widely used as a supercapacitor. However, poor electrical conductivity and stability versus size result in less specific capacitance and further limit practical commercialization. To overcome these most pressing challenges and to get better kinetics of ion/electron transport, several modifying techniques were employed like doping, conductive wrapping, and heterostructured nanocomposites.9−13 Tremendous efforts have been made to demonstrate various transition metal oxide composites (TMOCs) as a supercapacitor electrode application. For Received: January 14, 2017 Revised: April 24, 2017 Published: May 16, 2017 4757

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was as follows: 0.3 M Mn(CH3COO)2 and 0.2 M KMnO4 were dissolved in 20 mL of deionized water separately under vigorous stirring. Once it completely dissolved, a KMnO4 solution was added quickly to the Mn(CH3COO)2 solution, and stirring was continued for 4 h. A dark brown precipitate was collected and washed several times with deionized water and ethanol. Finally, the resultant material was dried in a hot air oven at 80 °C for 12 h. The as-prepared samples were calcinated at 450 °C for 3 h to obtain the α-MnO2 nanoparticles. Preparation of h-MoO3/α-MnO2 Nanocomposite. The nanocomposites were prepared using a chemical precipitation method. The reaction procedure was as follows: 1.23 g of ammonia heptamolybdate tetrahydrate (AHM) was dissolved in 10 mL of deionized water, and 2.5 mL of concentrated nitric acid (HNO3) was added dropwise to the solution. Once it reached 0.5 mL, the solution precipitated, and it started to deprecipitate on further addition of HNO3, over which the solution became colorless. After 15 min of stirring, an appropriate amount of asprepared α-MnO2 was mixed to the above solution, and the stirring was continued for 10 min and then kept at 120 °C for 1 h in an oil bath. After cooling to room temperature, the mixture was collected by centrifugation and washed subsequently with water and ethanol. Finally, the product was dried at 70 °C for 9 h in a hot air oven. The composites were prepared in different weight ratios of α-MnO2/h-MoO3 (1:1, 2:1, 3:1, and 4:1), and the prepared materials are labeled as MnO2−MoO3 (1:1), MnO2−MoO3 (2:1), MnO2−MoO3 (3:1), and MnO2−MoO3 (4:1), respectively. Material Characterization. The crystallanity and multiphase composition of samples were analyzed by the X-ray diffraction technique using an Ultima III Rigaku X-ray diffractometer (1.5406 Å) for a 2θ range of 5 to 80°. A PerkinElmer FTIR (Fourier Transform Infrared) spectrometer was used to identify functional groups present in the samples. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed with an EXSTAR6200 thermal analyzer under a nitrogen gas atmosphere from 30 to 1000 °C at a heating rate of 10 °C/min. Morphological analyses of the samples were carried out by transmission electron microscopy (TEM, JEOL JEM-2010, 200 kV), TEM imaging with a selected area electron diffraction (SAED) pattern, and elemental mapping and were employed to emphasize the presence and proportions of each element (Mn, Mo, and O) in the composite material. Surface area and pore size distribution studies were carried out by Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) analysis. Both BET and BJH studies were derived from the same N2 adsorption desorption isotherm (Micromeritics, Gemini V 2380, surface area and pore size analyzer, U.S.A). The N2 adsorption desorption isotherm was performed under liquid nitrogen conditions (77 K). The electrochemical behavior of the samples was studied using a three electrode cell system as well as a two electrode cell system (SP-150, Bio Logic Corporation, France). The working electrode was prepared as follows: the active material, acetylene black, and PVDF were taken in a weight ratio of 8:1:1 and grinded together using a pestle and mortar. A known weight of this mixture was dispersed in dimethylformamide (DMF) solution in a ratio of 20 mg/mL by an ultrasound sonicator for a few minutes in order to get a homogeneous slurry. Twenty microlitres of this slurry was then drop casted on a 1 cm2 area of a stainless steel foil (working electrode), yielding an active material loading of 0.32 mg cm−2. Then, the working electrode was allowed to dry at room temperature. The electrochemical behavior of the materials was characterized using a KCl saturated Ag/AgCl reference electrode and a platinum wire counterelectrode along with a prepared working electrode in a three electrode system. In a two electrode system, two similar working electrodes were used as positive as well as negative electrodes (mass of active material on individual electrode = 0.32 mg cm−2) to form a symmetric system, and 1 M Na2SO4 was used as an electrolyte. The cyclic voltammetry is recorded for different scan rates, such as, 2 mV/s, 5 mV/s, 10 mV/s, 20 mV/s, 50 mV/s, and 100 mV/s in a potential range of 0−1 V. Galvanostatic charge−discharge techniques were employed at different current densities in a potential window of 0−0.8 V. Then, the electrochemical impedance spectroscopy was recorded for a frequency range of 100 mHz to 100 kHz. The photocatalytic dye degradation of the MoO3/MnO2 nanocomposite photocatalyst was investigated using

example, Jiang et al. fabricated Ni(OH)2/MnO2 core−shell nanowires as an electrode and reported a maximum Sc value of 355 F/g in a neutral electrolyte (1 M Na2SO4, 70.4 wt % MnO2) with high cycling stability due to the synergistic effect between the core and shell.14 Huang et al. investigated the electrochemical behavior of Co3O4@MnO2 core−shell arrays on Ni foam which have been fabricated via a hydrothermal approach. The studies showed a high Sc value (560 F/g at a current density of 0.2 A/g) with good rate capability and excellent cycling stability (95% capacitive retention after 5000 cycles).15 Lu et al. investigated the electrochemical behavior of the WO3−x@Au@MnO2 hybrid structure and achieved a maximum Sc value of 588 F/g at a scan rate of 10 mV/s and 1195 F/g at a current density of 0.75 A/g with even an increased capacitive retention over 5000 cycles.16 Yang et al. fabricated 1D NiCo2S4@MnO2 heterostructures and could achieve a remarkable specific capacitance (1337.8 F/g at a current density of 2 A/g) and excellent cycling stability (82%) after 2000 cycles.17 Among the various nanostructured materials, 1D materials (nanowires, nanofiberes, nanorods, and nanotubes) are more preferable because of its better electrical conductivity and enhanced ionic insertion/deinsertion capability.17−20 Hence, in the present work, we opted for the h-MoO3 rods to fabricate the α-MnO2/h-MoO3 composite. Several metal oxides have been developed toward the degradation of toxic organic pollutants which are hazardous to the environment and human beings. TiO2, ZnO, SnO2, WO3, NiO, CeO2, and MoO3 are well established for dye degradation under UV and visible light irradiation.4,21−24 The MnO2 nanoparticles with a bandgap (Eg = 1.68 eV) are not active in the visible region; however, the absorption of MnO2 can be tuned in the visible light region by modifying it with suitable metal oxides.24−27 There are several modifications that have been made on MnO2 nanoparticles in order to tune the absorption range and use of dye degradation in visible light irradiation.24−27 Saravanakumar et al. decorated the MnO2 nanowire with embedding silver nanoparticles on it, and it exhibited efficient dye degradation under visible light irradiation.25 Ye et al. have prepared MnOx-BiOI heterogeneous nanostructure photocatalysts using the photodeposition method, and the photocatalytic activity is well investigated.26 Liu et al. have fabricated flower-like MnO2/BiOI composites using a simple and costeffective approach and could achieve 97.8% degradation of methylene orange (MO) and 91.7% degradation of MB.24 Xu et al. constructed MnO2/TiO2 nanotube array nanocomposite photocatalysts through an electrodeposition method and could obtain remarkable activity in the photocatalytic degradation of acid Orange II under visible light radiation.27 Hence, the present work demonstrates the fabrication of the αMnO2/h-MoO3 composite as a supercapacitor electrode and photocatalyst in visible light irradiation. Though there are few reports on the MnO2−MoO3 nanocomposite for electrochemical energy storage applications, to the best of our knowledge this work is the first to report on α-MnO2/h-MoO3 composite material for supercapacitor electrodes and visible light photocatalysts.28,29 Moreover, the investigations on the charge transport mechanism and ionic conductivity on α-MoO3 and hMoO3 nanocrystals encouraged us to choose h-MoO3 rods as compositing material. Detailed charge transport studies on αMoO3 and h-MoO3 were reported in our previous paper.30

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EXPERIMENTAL SECTION

Synthesis of α-MnO2. The MnO2 nanoparticles were prepared by using a simple chemical precipitation method. The synthesis procedure 4758

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pure MnO2 and MnO2−MoO3 composites has been recorded from 1100 to 500 cm−1 as shown in Figure 2. The bands observed

organic dye MB under visible light irradiation. A total of 0.025 g of each photocatalyst was added into 1 L of double distilled water containing 10 mg of MB dye. The photocatalytic chamber consists of a tungsten halogen lamp, and reactions are carried out under visible light irradiation with an E = 350 W and λ ≥ 420 nm at ambient temperature. Prior to irradiation, the suspension was stirred for 30 min under dark conditions to establish the adsorption−desorption equilibrium on the surface of catalysts. Finally, at the given time interval, 4 mL of catalyst containing suspension was centrifuged and used to measure the change in concentration using a UV−vis spectrophotometer (UV-2600, Shimadzu, Japan) ranging from 250 to 800 nm.

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RESULTS AND DISCUSSION An α-MnO2/h-MoO3 nanocomposite matrix was achieved through intermingled growth of h-MoO3 rods in between αMnO2 nanoneedles by employing a simple chemical precipitation method. Figure 1 shows the typical powder XRD patterns

Figure 2. FTIR spectra recorded for samples α-MnO2 and α-MnO2/hMoO3 composite materials at different proportions.

at 525, 571, and 703 cm−1 are corresponding to Mn−O stretching and bending vibrations in MnO6 octahedral units of the α-MnO2 phase.31,32 For the MnO2−MoO3 composites of all proportions, characteristic peaks of α-MnO2 and h-MoO3 are present. The sharp peaks at 974 cm−1 and 919 cm−1 are ascribed to the oxygen symmetry stretching mode (νs) of MoO.30,33 The absorption peaks at 603 and 527 cm−1 are attributed to different stretching and bending vibrations of O−Mo−O with varying Mo−O bond lengths and are assigned to bridge oxygen asymmetric and symmetric modes (νas and νs). Figure 3a shows the TGA curve recorded for the sample bare MnO2, MnO2−MoO3 (2:1), and MnO2−MoO3 (3:1). The weight loss observed around 100 °C is attributed to the desorption of physically adsorbed water molecules and solvents during the reaction. For pure MnO2, a constant weight loss of 20% is observed from 500 to 1100 °C; it is due to the continuous removal of oxygen by self-decomposition of MnO2 to Mn2O3 and from Mn2O3 to Mn3O4.34,35 An abrupt weight loss of 6% and 7% is observed between 490 and 530 °C for MnO2−MoO3 (2:1) and MnO2−MoO3 (3:1), respectively. This is due to the combination of decomposition of MnO2 to Mn2O3 and phase transformation of h-MoO3 to α-MoO3.30 Moreover, this weight loss is less (6%) for MnO2−MoO3 (2:1) compared to MnO2−MoO3 (3:1; 7%). There is no further mass reduction observed after 630 °C for these composites. Also, the total weight loss for the MnO2− MoO3 composite is 13%, which is much less compared to pure MnO2 since it has a 25% weight loss after 1050 °C. Hence, the results clearly point out that the incorporation of h-MoO3 to the MnO2 matrix enhanced its thermal stability. The UV−visible diffuse reflectance spectra of pure MnO2, MoO3, and MnO2− MoO3 composites are shown in Figure 3b. The sharp absorption edge around 400 nm corresponding to pure MoO3 indicates that the visible light absorption can be attributed to the electronic band gap transition.23 Whereas there is no sharp absorption edge observed for pure MnO2, MnO2−MoO3 composites exhibit a broad absorption in the visible light region from 300 to 600 nm. This implies that MoO3 is influencing the optical absorption of MnO2.36,37

Figure 1. XRD patterns of α-MnO2 and α-MnO2/h-MoO3 composite materials at different proportions.

of prepared α-MnO2 and α-MnO2/h-MoO3 composites in different weight ratios. All the peaks corresponding to the pure MnO2 nanoparticle can be assigned to tetragonal structured αMnO2 (JCPDS card no: 44-0141). The broadened peaks observed corresponding to pure α-MnO2 indicate its smaller crystallite size. The XRD profile for the MnO 2 −MoO 3 composites clearly reflects the mixed double phase nature of the material. In addition to the peaks corresponding to α-MnO2, the new strong peaks observed at 2θ = 9.7°, 19.6°, 25.99°, and 29.5° are indexed as (100), (200), (210), and (300) crystal planes of the h-MoO3 phase, respectively (JCPDS card no: 210569). There is no further impurity peak from a third phase, ensuring the purity of this binary composite. The phase formation was further confirmed by FTIR analysis and is well supported by the XRD result. The FTIR spectrum for 4759

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Figure 3. (a) TGA curve for pure α-MnO2, α-MnO2/h-MoO3 (2:1) and α-MnO2/h-MoO3 (3:1). (b) UV−Visible diffuse refluctance spectra (DRS) of pure α-MnO2, pure h-MoO3 and α-MnO2/h-MoO3 composites.

Figure 4. TEM and HRTEM images of bare MnO2 (a, b, and c) and MnO2−MoO3 (3:1) composites (d, e, and f). Insets c and f show SAED patterns of the corresponding sample. EDAX and corresponding elemental mapping of MnO2−MoO3 (3:1; g−j).

got diffracted. The d spacings of 0.46, 0.22, 0.30, and 0.24 nm are corresponding to the planes (200), (301), (110), and (211), respectively. The inset to Figure 4c shows the selected area electron diffraction (SAED) patterns of bare MnO2, composed of multiple concentric rings with different diameters, clearly demonstrating the polycrystalline nature of the sample. These results are well matched with the JCPDS card number 44-0141. Figure 4d and e show the MnO2−MoO3 composite at a 3:1 ratio. It is observed from the figure that MoO3 nanocrystals are grown like long rods surrounded by small MnO2 needles to form a fibrous root structure. The MoO3 stands for lateral root and MnO2 stands for the root hair in this fibrous root system. Here, MoO3 rods acts like a backbone in this heterostructure, and MnO2 provides a large surface area to adsorb and intercalate charged ions. Furthermore, the d spacing corresponding to the

Morphological and elemental analysis for the pure MnO2 metal oxide and its composites with h-MoO3 (MnO2−MoO3 (3:1)) are shown in Figure 4. It is very clear that the MoO3 rods of several nanometers were uniformly grown interstitially through the MnO2 needle-like structures. Since the incorporation of additives will affect the physical and chemical properties of the composite material, the optimization of compostional ratio is very crucial. Also, in order to improve the conductivity and its resultant capacitance value, the proportion of h-MoO3 nanocrystals to MnO2 nanoparticles has to be examined properly. Figure 4a and b show the bare MnO2 nanoparticles with a needlelike structure of 10−20 nm diameter and 30−40 nm length. Figure 4c shows the HRTEM image of bare MnO2 nanoparticles. The fringes which are observed clearly in the HRTEM image provide the d spacing of the crystal plane from which the X-rays 4760

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Figure 5. (a) Nitrogen adsorption/desorption isotherm. (b) Pore size distribution of each sample. (c and d) Cyclic voltammogram and EIS recorded for the MnO2−MoO3 (3:1) sample on different substrates (SS foil and Ni foil), respectively.

composite MnO2−MoO3 (3:1) is also calculated (Figure 4f). Apart from the d spacing of α-MnO2 planes ((211) and (301)), the lattice fringe distances of 0.29 and 0.25 nm correspond to (300) and (310) planes of h-MoO3, respectively (JCPDS: 210569). Here also, SAED patterns corresponding to MnO2− MoO3 composites (inset Figure 4f) exhibit a polycrystalline nature. Elemental analysis by energy-dispersive X-ray (EDX) spectroscopy of the composite MnO2−MoO3 (3:1) and elemental mappings are shown in Figure 4g−j. The elemental mapping clearly confirms the presence of individual components manganese (Mn), molybdenum (Mo), and oxygen (O) in the composite material. These results are very consistent with the XRD and FTIR analysis. The N2 adsorption/desorption isotherms and pore size distribution curves of all prepared samples were studied, and the results are expressed in Figure 5a,b. All samples possess a small hysteresis from p/p0 = 0.4 to 0.95, revealing the coexistence of both micropores and mesopores.1 More precisely, samples MnO2 and MnO2−MoO3 (3:1) exhibited relatively large adsorption at low relative pressure with better hysteresis. This clearly points out the mesoporous nature of the synthesized material. The bare MnO2 sample exhibited a maximum BET surface area of 83.8 m2/g. However, the BET surface area for all composites showed a considerable reduction from that of bare MnO2. The specific surface area evaluated for composites MnO2−MoO3 (4:1), MnO2−MoO3 (3:1), MnO2−MoO3 (2:1), and MnO2−MoO3 (1:1) are 35, 52.1, 39, and 23.1, respectively. Here, the composite MnO2−MoO3 (3:1) has better specific surface area than the rest of the composites. The pore size distribution of samples bare MnO2, MnO2−MoO3 (4:1), MnO2−MoO3 (2:1), and MnO2−MoO3 (1:1) reflect broad

distributions around 15, 13, 13.2, and 11.5 nm, respectively. However, the composite MnO2−MoO3 (3:1) exhibited a narrow pore size distribution at 15 nm. Electrochemical capacitive behavior of the as-prepared composites material has been evaluated by fabricating the composite material as a working electrode in a three electrode electrochemical analyzer. The prepared working electrodes were subjected to this three electrode system and characterized with a cyclic voltammetry experiment, galvanostatic charge−discharge techniques, and electrochemical impedance spectroscopy with 1 M Na2SO4 electrolyte. In addition to the SS foil working electrode, nickel foil has also been used initially to compare the effect of substrate material on the electrochemical behavior of the prepared composite materials. The typical CV curves and EIS are shown in Figure 5c and d, respectively. It is noted that the CV curve recorder at 10 mV/s for both the SS foil electrode and nickel foil electrode almost overlap each other. This clearly points out the independence of the pseudocapacitance property on substrate material; beyond that, it confirms the property exclusively coming from the prepared composite material. From the EIS plot, the charge transfer resistance (Rct) value is found to be the same for both electrodes (10 Ω), which is obtained from the diameter of the semicircle in the high frequency region.38 This again proved the substrate-free nature of the composite material. However, the series resistance, Rs, coming from the electrode, electrolyte, current collector, and electrode/current collector contact resistance (the real part intercept at the beginning of the semicircle) is more for the nickel foil than the SS foil. Hence, we opted for the SS foil as the substrate for the working electrode to proceed with further electrochemical studies. 4761

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Figure 6. (a) CV curve recorded at a scan rate of 10 mV/s for all samples. (b) CV curve of MnO2-MoO3(3:1) at different scan rates, (c) Chargedischarge curve recorded for all samples at a current density of 1 A/g. (d) CD curve of MnO2-MoO3(3:1) at various current densities.

desorption of electrolyte active species on the MnO2/MoO3 surface (non-Faradaic storage or electric double layer capacitance) and intercalation/deintercalation of charged ions in between the tunnels/pores of the MnO2/MoO3 electrode through the redox reaction (Faradic storage or pseudocapacitance).40−42 All composite material showed a similar rectangular shaped CV curve together with high symmetricity between an anodic and cathodic regime, implying a positive synergetic effect and improved pseudocapacitive nature of the electrode with MoO3 mass loading to the composite material. It is quite general that the integral area under the CV curve is directly proportional to the specific capacitance (Sc) value of the respective material. The area bounded by pure h-MoO3 is much less compared to all other samples, indicating its limited specific capacitance. However, it is well proven from Figure 6a that as MoO3 mass loading increases in the composite material, the area under the curve increases. It can be attributed to the incremental pseudocapacitive contribution to the total capacitance due to the improved ionic conductivity of the composite electrode as a synergetic effect. Here, the composite with 25% MoO3 mass loading (MnO2−MoO3 (3:1)) gave a maximum integral area. Further mass loading of MoO3 rods to the composite minimize the Sc value. It may be due to the inability to access active tunnels on MnO2 with a higher mass loading of MoO3. The composite with 50% MoO3 loading showed an even lower Sc value than the pure MnO2 electrode. Hence, the optimum proportion of MoO3 rods on MnO2 material will provide better accessibility to avail the active sites for ionic storage, and it will accelerate ionic transport throughout the electrode material, which results in a high Sc value. In order to examine the excellence in pseudocapacitive nature of the composite material, sample

The cyclic voltammogram is recorded for each sample at different scan rates (2 mV/s, 5 mV/s, 10 mV/s, 20 mV/s, 30 mV/ s, 50 mV/s, 100 mV/s, and 150 mV/s) in a potential range of 0 to 1 V. Typical CV curves for α-MnO 2 , α-MnO 2 /h-MoO 3 composites, and h-MoO3 at a scan rate of 10 mV/s are shown in Figure 6a. The symmetric CV curve between the anodic and cathodic regime with a rectangular shape is the signature of an ideal pseudocapacitive electrode material.39 Here, from Figure 6a, it is very clear that the recorded voltammogram corresponding to all composite samples showed a symmetric rectangular shape, indicating that a fast redox reaction takes part over the electrode material. Whereas the CV curve corresponding to MnO2 has a small variation from its ideal rectangular behavior, a small slanted discharging from 0.8 to 0.6 V was observed, which indicates poor capacitive behavior rather than a slight resistive nature of the pure MnO2 electrode material. The poor ionic conductivity and weak electrode electrolyte interface results in such a limited potential application of the MnO2 electrode material.40 However, the MnO2/MoO3 composite electrode exhibits good rectangularity with a small redox peak and pointed out the enhancement in the pseudocapacitive behavior of the composite material. This improved result is attributed to the enhancement in the ionic and electronic conductivity of the composite electrode through the intimate bonding between MnO2 and MoO3 nanocrystals. This will make a far easier Faradaic process of electrochemically active species on the electrode. Further, the variable oxidation states of MnO2 such as Mn3+ and Mn4+ provide an easier ionic transport pathway to intercalate/deintercalate the active species of electrolyte on the electrode materials through the exchange of electron from Mn4+ ↔ Mn3+.41 Hence, there is a combination of surface adsorption/ 4762

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Figure 7. (a) EIS recorded for MnO2, MoO3 and MnO2-MoO3 composites, Inset figure (i): EIS for all composites, Inset figure (ii): the original and fitted EIS curve for MnO2-MoO3(1:1) sample along with equivalent circuit. (b) capacitive retention for 1000 cycles. Inset figure: capacitive retention at higher current densities.

MnO2−MoO3 (3:1) is further subjected to the CV technique at a higher voltage scan rate (Figure 6b). A linear dependence is observed between the scan rate and current density, indicating good rate capability and Faradaic behavior of the electrode. Also, the rectangularity is maintained with inconsiderable deviation at a higher scan rate (2−150 mV/s). This again demonstrates excellent rate capability and static pseudocapacitive nature due to the easy accessibility of ions over the electrode interface.43 To further investigate the potential application in electrochemical capacitors, a constant current galvanostatic charge− discharge (CD) technique was employed for all prepared electrodes at a current density of 1 Ag−1. The typical CD curve recorded in 0.1 M Na2SO4 electrolyte for a potential window 0− 0.8 V is shown in Figure 6c. Triangular symmetry and a linear slope exhibited from the charging and discharging curve again ensured appreciable electrochemical performance for the prepared electrodes. A good linear variation of charging and discharging is observed for all samples, which is another typical characteristic of an ideal capacitor, reflecting the constant charging and discharging rate. Again the symmetry between the charging and discharging curves implies high reversibility and high Columbic efficiency.44 The specific capacitance (Sc) value corresponding to each material is calculated from eq 1.45 Sc =

I Δt mΔV

composite matrix, the Sc value has been reduced. Hence, the composite material with 25% MoO3 rods is optimized as the best pseudocapacitor electrode material. Thus, it can be concluded that the MnO2 material is responsible for the Faradaic storage in the composite material, whereas MoO3 rods have made the ionic transportation and accessibility of active sites easier. So the proper proportion of MnO2 and MoO3 in the composite material contributed equally to get a maximum capacitance as a positive synergetic effect. Here also, we further analyzed the rate capability of the prepared composite electrode by recording the CD curve at different current densities. The resultant CD curve for MnO2−MoO3 (3:1) is shown in Figure 6d. It is obvious to see the symmetric triangular curve across various scan rates with minimum IR drop (at the beginning of discharge curve). It reveals the excellence in rate capability.43 The charge storage mechanisms behind the capacitive property of MnO2 and MnO2/MoO3 composite electrode materials was further confirmed by electrochemical impedance spectroscopy (EIS). A typical EIS plot measured on the complex plane at a bias voltage of 10 mV with a frequency range between 100 mHz and 100 kHz is shown in Figure 7a. It is obvious to see that the EIS spectra of h-MoO3 possess a straight line with small bending at the higher frequency region, indicating its remarkable conductive nature. However, all other EIS curves are almost similar, composed of three parts. The first one is the x intercept with the starting curve in the higher frequency region and one semicircle part in the medium frequency region followed by a straight line part in the low frequency region. The first part is effective series resistance/combinational resistance (Rc), which includes the ionic resistance in the electrolyte, contact resistance of the active material to the current collector, and intrinsic resistance of the active material. The semicircle part at medium frequency gives details about the charge transfer resistance at the electrode electrolyte interface (diameter of semicircle = charge transfer resistance, Rct), and the linear part at higher frequency implies the ideal capacitive behavior.46 This linear part with a slope of 45° is known as the Warburg resistance (Zw) owing to the frequency dependent ion diffusion at the electrode electrolyte interface. Figure 7a (inset figure i) shows the zoomed EIS diagram for all composites. All EIS curves have been fitted with an equivalent circuit by using the Z-Fit method (nonlinear fitting by EC-Lab@ Software, Version 11.02 − August 2014). The equivalent circuit and fitted curve corresponding to MnO2− MoO3 (1:1) are shown in Figure 7a (inset figure ii) for reference.

(1)

where I (A) is the applied constant current in amperes, Δt (s) is the discharge time in seconds, m (g) is the mass of active material in grams, and ΔV (V) is the applied potential window of charge− discharge curve in volts. Estimated Sc values corresponding to pure MnO2, pure MoO3, MnO2−MoO3 (4:1), MnO2−MoO3 (3:1), MnO2−MoO3 (2:1), and MnO2−MoO3 (1:1) are 228, 62.5, 323, 412, 397, and 168 F/g, respectively. As Δt is directly proportional to the Sc value, the electrode which possesses a higher Δt value will have a higher pseudocapacitance. From Figure 6c, it is clearly seen that the Δt value has increased with MoO3 mass loading into the composite material. Interestingly, improved discharge time was observed as the MoO3 feeding ratio increased. The sample with a MnO2/MoO3 compositional ratio of 3:1 (MnO2−MoO3 (3:1)) exhibited a maximum Sc of 412 F/ g. This may be due to the easier accessibility of active sites for Faradaic storage and enhanced ionic transportation, which are achieved by the proper intermingled growth of MoO3 with MnO2 needles. Further, on addition of MoO3 rods to the 4763

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ACS Sustainable Chemistry & Engineering The components Cdl and CL are the double layer capacitance at the grains and limit capacitance, respectively.12 The fitted values of all components corresponding to each material are tabulated in Table 1. Here, the MnO2 electrode exhibited a larger Rct (42.5

10 times (10 A/g). The reduction in Sc value toward higher current density is attributed to the time deficit of the hybrid electrode to accommodate the radicals in their respective sites. However, this appreciable capacitive retention at higher current density again proved the excellent electrochemical behavior of the hybrid electrode material. To further investigate the real life application of the prepared composite electrode, a symmetric two electrode system is employed. In typical symmetric cell fabrication, the prepared composite material is used as a positive and negative electrode, and a filter paper soaked with 1 M Na2SO4 is sandwiched in between the positive and negative electrodes. The typical CV curves for pure MnO2, MoO3, and MnO2−MoO3 composites at a scan rate of 5 mV/s are shown in Figure 8a. The CV curve of the symmetric capacitor proves that the operating potential of the symmetric cell could be extended up to 2 V. Similar to three electrode characterization, here also a symmetric rectangular shaped cyclic voltammogram is observed for all samples. But in spite of all other samples, the pure MoO3 electrode exhibits a large difference from rectangularity and symmetricity with less hysteresis behavior. However, all other samples (pure MnO2 and MnO2−MoO3 composites) manifest good symmetric rectangular shape with large hysteresis, clearly illustrating the excellent electrochemical behavior of the prepared materials. Again, the MnO2−MoO3 (3:1) composite exhibited a large area of hysteresis, clearly revealing the enhanced specific capacitance of the material. Also from Figure 8a, compared to pure α-MnO2, the MnO2−MoO3(3:1) composite cell exhibited a reduced peak to peak potential difference (the difference in potential between anodic and cathodic peaks), elucidating the fast ionic transport through the electrode−electrolyte interface. The peak potential separation (ΔE) is reduced from 520 mV to 260 mV for the MnO2−MoO3 (3:1) composite from pure α-MnO2. This clearly explains the faster electron transfer kinetics through the electrode−electrolyte interface. Further, to analyze the rate capability of the MnO2−MoO3 (3:1) electrode material, cyclic voltammetry with different scan rates (2−150 mV/s) was recorded and is presented in Figure 8b. Interestingly, at higher scan rates, the shape of the CV curve is almost maintained, which demonstrates the high rate capability of the material in a symmetric capacitor system. A similar result is observed from the galvanostatic charge− discharge technique (Figure 8c and d). All samples possess good triangularity, elucidating the pseudoconstant charging and discharging behavior. The specific capacitance (Sc) of the total cell and individual electrode is calculated using the following equations:1,43

Table 1. Calculated Values of Rc, Rct, Zw, Cdl, and CL by Fitting the Experimental Data with Z Fit Method (EC Lab11.02) Based on the Equivalent Circuit

pure MnO2 MnO2−MoO3(4:1) MnO2−MoO3(3:1) MnO2−MoO3 (2:1) MnO2−MoO3 (1:1)

Rc (Ω)

Rct (Ω)

Zw (Ω S−1/2)

Cdl (μF)

CL (F)

11.03 10.82 4.3 3.08 9.15

42.53 17.25 5.37 10.25 18.7

196.5 49.7 11.13 43 34.7

15.8 23.3 30 24 12.4

0.058 0.036 0.019 0.275 0.011

Ω). Whereas almost all composites showed a low Rct value, the MnO2−MoO3 (3:1) exhibited a smaller semicircle with a lower Rct (5.3 Ω). It can be attributed to the reduced electrode electrolyte interface resistance or the enhanced ionic/electronic conductivity through the electrode−electrolyte interface for the composite material. Also, the finite slope of the straight line in the low frequency region represents the diffusive resistance of electrolyte ions at electrode pores or tunnels.1,38 It has been clearly observed in Figure 7a that the composite MnO2−MoO3 (3:1) has a higher slope compared to the rest of the composites and the MnO2 electrode. These results suggest that the MnO2/ MoO3 composite with 25% MoO3 loading has a lower diffusive resistance of the electrolyte and better ionic conductivity (faster charge transport) through the electrode−electrolyte interface. Further, the electronic conductivity of the prepared electrodes was measured using a four-probe technique (Ecopia Hall Effect Measurement System HMS-5000). The conductivity values are observed to be 0.90, 0.98, 2.29, 1.17, and 0.92 S cm −1 corresponding to pure MnO2, MnO2−MoO3 (4:1), MnO2− MoO3 (3:1), MnO2−MoO3 (2:1), and MnO2−MoO3 (1:1), respectively. The capacitive retention of electrode material across cycle number and current density was studied for 1000 charge− discharge cycles (Figure 7b) and 1−10 A/g current densities (Figure 7b inset), respectively. Here, the main highlight of this work is the cyclic stability of prepared α-MnO2/h-MoO3 hybrid materials, as it is an another critical requirement of the supercapacitor. Here, all hybrid material possesses more than 97.5% of capacitive retention of its maximum value after 1000 cycles (more precisely, the material with 20% and 50% of MoO3 mass loading exhibited 99% capacitive retention). Except MnO2−MoO3 (2:1), all hybrid electrodes attained a maximum value of Sc after a certain number of cycles and are maintained almost at a constant value. Hence, this attenuation of specific capacitance value corresponding to hybrid material is far better than the MnO2 electrode material (≈11%), as clearly shown in Figure 7b. Hence, the composition of α-MnO2 with h-MoO3 rods not only enhanced its mechanical stability and electronic/ ionic conductivity to get a high capacitance value but also improved its structural stability toward thousands of cycles. This appreciable long-term cycle stability was attributed to the synergetic effect between α-MnO2 nanoneedles and h-MoO3 rods. The investigations of specific capacitance with varying current densities point out a moderate stability. The hybrid materials maintained almost 50% of their initial value (at a current density of 1 A/g) when the current density increased to

Sc(cell) =

I Δt M ΔV

Sc(single) =

4I Δt = 4Sc(cell) M ΔV

(2)

(3)

And the corresponding energy density, E (Wh/kg), and power density, P (W/kg), were estimated from eqs 4 and 5:47 E=

0.5Cs(ΔV )2 3.6

(4)

P=

E Δt

(5)

where M represents the total weight of active material in both positive and negative electrodes. All remaining letters represent its usual meaning. MnO2−MoO3 (3:1) has a long discharge time, 4764

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Figure 8. Electrochemical characterizations by symmetric cell system. (a) Cyclic voltammetry for all prepared samples at a scan rate of 5 mV/s, (b) CV recorded for MnO2-MoO3 (3:1) at different scan rates, (c) Comparative CD curve of all samples at 1 A/g, (d) CD measured for MnO2-MoO3 (3:1) at different current densities, (e) Sc versus current density plot of MnO2-MoO3 (3:1) electrode and (f) Ragone plot corresponding to MnO2-MoO3 (3:1) symmetric cell.

efficiency. The specific capacitances at various current densities (1 to 10 A g−1) are calculated and plotted in Figure 8e. More than 50% of its specific capacitance is retained (358 to 180 A g−1) when the current density is increased from 1 to 10 A g−1. The Ragone plot derived from the CD techniques corresponding to the MnO2−MoO3 (3:1) composite symmetric cell is shown in Figure 8f. Since the cell operates on an extended potential window of 2 V, the energy density and power density could be drastically enhanced. The cell could deliver a maximum energy density of 50 Wh kg−1 at a power density of 1000 W kg−1. Moreover, even at a power density as high as 10 kW kg−1, the device still delivers an energy density of 25 Wh kg−1. Figure 9a shows the complex impedance plot corresponding to pure MnO2 and the MnO2−MoO3 (3:1) composite symmetric

indicating its enhanced specific capacitance. The specific capacitance of pure MnO2, pure MoO3, MnO2−MoO3 (4:1), MnO2−MoO3 (3:1), MnO2−MoO3 (2:1), and the MnO2− MoO3 (1:1) symmetric cell are calculated to be 49.5, 9.5, 78, 89.5, 81.5, and 40 F g−1, respectively, and those of individual electrodes are calculated to be 198, 38, 312, 358, 326, and 160 F g−1, respectively. These capacitance values clearly pointed out the practical application of the prepared composite materials. In order to examine the cell performance at high current density, charge−discharge techniques at various current densities were performed for the MnO2−MoO3 (3:1) composite (Figure 8d). Even at high current density, the linearity and symmetricity of the CD curve are maintained, reflecting the appreciable cell performance at high current density and high Coulombic 4765

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Figure 9. EIS plots (a) and Bode plots (b) for pure MnO2 and MnO2−MoO3 (3:1) composite symmetric cells. Frequency versus real and imaginary part of impedance Z for the pure MnO2 (c) and MnO2−MoO3 (3:1) composite (d) symmetric cells.

Hz) and 1 s (f 0 = 1 Hz), respectively. The reduction of τ0 from 10 to 1 s for the MnO2−MoO3 (3:1) composite is direct evidence of fast pulse power delivery of the composite cell due to fast and easier diffusion of the electrolyte ion through the electrode electrolyte interface.51 The long cycling life of the MnO2−MoO3 (3:1) composite symmetric cell is studied for 10 000 cycles by performing CD techniques at 4 Ag−1 (Figure 10). After 10 000 cycles of constructive charging and discharging, the cell possesses 95% of its maximum capacitance. This excellent capacitive retention clearly manifests the superior electrochemical reversibility of the electrode material in a symmetric cell configuration. Interest-

cell. The EIS plot is shown the same as the three electrode measurement with different parameter values. The charge transfer resistance (Rct) has been calculated corresponding to pure MnO2 and the MnO2−MoO3 (3:1) composite symmetric cell as 30 Ω and 6.65 Ω, respectively, from the equivalent circuit fitting (circuit diagram is shown in Figure 7a,ii). The reduced charge transfer resistance observed for the MnO2−MoO3 (3:1) cell elucidates its better ionic transport through the electrode electrolyte interface. The total impedance of pure MnO2 and the MnO2−MoO3 (3:1) composite symmetric cells was observed to be increasing with decreasing frequency (Figure 9b). However, at low frequency, the phase shift approaches the maximum angle, indicative of ideal capacitive behavior.48 The phase angle, −45°, is found in the lowest frequency region, reflectjng the fast pulse power delivery of the symmetric cell. The phase angle is at a maximum for MnO2−MoO3 (3:1) composite cells (≈80°), which indicate the excellent capacitive behavior. The response time or relaxation time (τ0) is another important factor (referred to as the figure of merit), which explains the rate performance of a capacitor.49,50 The response time can be extracted from the frequency (f 0) at which the capacitive and resistive contribution to the total impedance get equal with each other. Hence, the phase difference between the real and imaginary part of Z will be 45°.50 Consequently, the frequency versus real and imaginary part of Z cross each other at this particular frequency (f 0). Typical plots of Z′ versus Z″ as a function of frequency corresponding to pure MnO2 and MnO2 −MoO3 (3:1) composite symmetric cells are shown in Figure 9c and d, respectively. Response time is considered the measure of pulse power performance (P0 = E0/τ0), where E0 is the available energy. The response time is also calculated from the plot (τ0 = 1/f 0) for respective cells. The relaxation time corresponding to pure MnO2 and MnO2−MoO3 (3:1) composite cells are 10 s (f 0 = 0.1

Figure 10. Cycling performance of the MnO2−MoO3 (3:1) symmetric cell at a current density of 4 A/g. 4766

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Figure 11. Photodegradation of MB under visible light irradiation. (a) Pure α-MnO2. (b) MnO2−MoO3 (4:1). (c) MnO2−MoO3 (3:1). (d) MnO2− MoO3 (2:1). (e) MnO2−MoO3 (1:1). (f) Pure h-MoO3.

performance. This presumable behavior is due to the synergetic effect of α-MnO2 and h-MoO3 in the hybrid composite material. Photocatalytic Degradation of Pure MnO2, MoO3, and MnO2/MoO3 Nanocomposite Catalyst. The photocatalytic activity of h-MoO3 and the MnO2/MoO3 nanocomposite photocatalyst was evaluated through the photocatalytic degradation of MB dye under visible light irradiation. Figure 11 shows the photodegradation of MB using MnO2, MoO3, and the MnO2/MoO3 nanocomposite photocatalyst. There was no degradation observed for pure MnO2 as shown in Figure 11a. Whereas for MoO3 and MnO2−MoO3 nanocomposites, the aqueous solution of MB was degraded under visible light irradiation, which is confirmed from the gradual decrement in characteristic absorption peak intensity (λmax) at 665, 605, and 250 nm (Figure 11b−f). The percentage of photocatalytic degradation of MB was calculated using eq 6

ingly, the specific capacitance was observed to increase drastically up to 3500 cycles. It is attributed to the electro-activation of the electrode electrolyte interface having a smooth ionic intercalation/deintercalation. The electro-activation is achieved through cycling-induced improvement of the active surface area to have easy access to electrolyte species on the electrode surface. Similar behavior is observed in many previous reports.52−57 After 3500− 8000 cycles, it was stabilized, and a gradual decay of capacitance is observed after 8000 cycles. However, after 10 000 cycles, the MnO2−MoO3 (3:1) composite symmetric cell retained 95% of its maximum capacitance, indicating its practical exploration of the fabricated composite cell. The color of the electrolyte remains transparent after 10 000 cycles, indicating minimal dissolution of the active material to the electrolyte solution, which is the major reason for capacitance loss in MnO2 based electrodes.20 Hence, from the above discussions, it is obvious that the hybrid material MnO2−MoO3 (3:1) possesses better electrochemical 4767

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ACS Sustainable Chemistry & Engineering Co − C t × 100 Co

chemical composition, morphology, and thermal stability were well understood by different characterization techniques such as XRD, FTIR, TEM, SAED, EDX and TG, and DTA. Electrochemical behavior of the prepared α-MnO2 and α-MnO2/hMoO3 composite materials is investigated by cyclic voltammetry analysis, galvanostatic charge−discharge techniques, and electrochemical impedance spectroscopy with three electrode cell and symmetric two electrode cell configurations. For all the composite materials, we observed a better electrochemical performance due to its enhanced ionic conductivity that resulted in a fast ionic transport through the electrode−electrolyte interface, which has been confirmed by EIS analysis. Again, the investigation of electrode stability across high current density and long cycling displayed a better improvement from the pristine MnO2. Hence, an optimum mass loading of h-MoO3 to an αMnO2 matrix has enhanced its electrochemical property to a favorable extent. Further, from the electrochemical investigations of these composite electrodes, we suggest the material with 25% MoO3 mass loading as the best material for supercapacitor electrode application. Also, the higher photocatalytic dye degradation efficiency was observed and is due to the effective charge transfer process of MoO3 and MnO2 in the α-MnO2/hMoO3 nanocomposite.

(6)

where Co and Ct are the initial and residual concentrations of MB at different interval times, respectively. The MnO2−MoO3 (4:1), MnO2−MoO3 (3:1), MnO2−MoO3 (2:1), and MnO2−MoO3 (1:1) nanocomposite takes 40, 60, 75, and 120 min to degrade 94, 93, 94, and 93% of MB as shown in Figure 12. Obviously, the pure MoO3 exhibited relatively less degradation (19% of MB degradation) even after 105 min of irradiation time, which is shown in Figure 12.

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AUTHOR INFORMATION

Corresponding Author

Figure 12. Photocatalytic efficiency of MB using the MnO2−MoO3 nanocomposites under visible light irradiation.

*E-mail: [email protected]. ORCID

Arumugam Chandra Bose: 0000-0001-7590-5961

Plausible Mechanism for Photodegradation of MnO2− MoO3 Nanocomposites. The possible charge transfer process between MoO3 and MnO2 nanocomposite photocatalysts is illustrated in Figure 13. Under visible light irradiation, electrons

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the Director, National Institute of Technology, Tiruchirappalli for availing the UV−visible DRS facility and four probe Hall measurement system. P.M.S. acknowledges MHRD, Government of India for funding the SRF grant to undertake research work.

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REFERENCES

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Figure 13. Charge transfer process in MnO2/MoO3 nanocomposites under visible light irradiation.

are excited from the valence band to the conduction band of MoO3 (Eg = 2.87 eV); at the same time, no electrons can be excited from MnO2 (1.68 eV). The photoexcited electrons at the conduction band and holes at the valence band of MoO3 can be transferred to the conduction and valence band of MnO2 as shown in Figure 13. Finally, holes at the valence band can oxidize the dye molecules, and the electrons at the conduction band could reduce O2 into O2− radicals, which effectively degrades MB dye.

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CONCLUSIONS α-MnO2/h-MoO3 hybrid metal oxide with varying MoO3 mass loadings has been successfully synthesized. The microstructure, 4768

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DOI: 10.1021/acssuschemeng.7b00143 ACS Sustainable Chem. Eng. 2017, 5, 4757−4770