Tuning the Surface Morphology and Pseudocapacitance of MnO2 by

Subscriber access provided by EDINBURGH UNIVERSITY LIBRARY | @ http://www.lib.ed.ac.uk

Article

Tuning the Surface Morphology and Pseudocapacitance of MnO2 by Facile Green Method Employing Organic Reducing Sugars Ediga Umeshbabu, Ponniah Justin, and Gangavarapu Ranga Rao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00390 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Energy Materials

Tuning the Surface Morphology and Pseudocapacitance of MnO2 by Facile Green Method Employing Organic Reducing Sugars Ediga Umeshbabu,† Ponniah Justin,‡ and G. Ranga Rao*† †

‡

Department of Chemistry, Indian Institute of Technology Madras, Chennai - 600036, India Department of Chemistry, Rajiv Gandhi University of Knowledge Technologies, RK Valley,

Kadapa - 516330, Andhra Pradesh, India

Abstract In the present work, three different MnO2 nanostructures especially nanoneedles, hollow tubes and nanorods of MnO2 have been synthesized by a simple redox reaction between permanganate and organic sugars at room temperature. The MnO2 samples were characterized by the variety of analytical techniques. The results illustrate that the organic reducing sugars of mannose, galactose and glucose effectively tune the morphology, crystallinity and pore structure of the MnO2 material. The nanoneedles and hollow tubes were found to be β-MnO2, while the nanorods were of α-MnO2. The formation of different MnO2 nanostructures appears to be a kinetic driven process, which proceeds in a quite distinctive way in the presence of different organic reducing sugars. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) tests were conducted to evaluate the charge storage behavior of the α- and β-MnO2 nanostructures. Among all three MnO2 samples, β-MnO2 composed of nanoneedles delivered a large specific capacitance, CS (∼365 F g-1 at 0.5 A g-1) with improved rate capability (56% retention at 12 A g-1) and excellent cyclability (82% retention at 2000 cycles). The elegant combination of the high specific surface area (∼146 m2 g-1) and 1D-

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 43

nanoneedles structure of β-MnO2, enhances the electrode-electrolyte contact area and hence provides the number of active sites for fast charge-discharge propagations.

KEYWORDS: Green synthesis, Reducing sugars, Manganese dioxide, Nanoneedles, Hollow tubes, Nanorods, Supercapacitor, Inner/outer surface charge

1. Introduction Electrochemical

supercapacitors

(ESs),

sometimes

denoted

as

supercapacitors

or

ultracapacitors, have attracted increased attention in electrochemical energy storage systems. Advanced functional materials have been extensively investigated to meet the requirements of faster and more efficient energy storage and delivery systems compared to the conventional rechargeable Li-batteries.1-3 The challenge is to develop newer materials to improve the power densities and long cycling stability. Based on the storage mechanisms, the electrode materials can be separated into electrical double-layer capacitor (EDLC) materials4 and pseudocapacitive materials.5-10 EDLC materials are carbons which facilitate physical charge storage at the electrode/electrolyte interface through the formation of a Helmholtz double layer. On the other hand, pseudocapacitive materials are mainly the binary and ternary oxides which undergo faradaic redox activity and generate plenty of charges by faradic redox reactions within the bulk material.5,6,9 Among many binary metal oxides reported, MnO2, NiO, Co3O4, and V2O5 are more encouraging as an alternative to RuO2 for ES applications.11-18 Manganese oxide stands out as one of the most competitive electrode materials for pseudocapacitors because it has the required chemical and thermal stability, and high theoretical CS (1370 F g−1, 0.9 V window), ideal capacitive response and excellent cycle stability.19,20 However, the crystal structure, morphology, and electrical conductivity are some of the factors that can restrict its use as a good 2 ACS Paragon Plus Environment

Page 3 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pseudocapacitive material exhibiting nearly rectangular shape cyclic voltammetry curves.21 It has been proposed that MnO2 materials store electric charge by faradaic reactions transpiring on the surface due to the adsorption/desorption of electrolyte ions (eqn. 1) and in the bulk by the intercalation/deintercalation process of cations or protons (eqn. 2).

However, both these

mechanisms encompass redox reactions between III/IV oxidation states:22

MnO2 ( surface ) + C+ + e− ↔ MnO2− C+ ( surface ) MnO2 ( bulk ) + C+ + e− ↔ MnOO− C+ ( bulk )

(1) (2)

where C is the electrolyte cations such as H+, K+, Na+ and Li+ ions. Since both the electrons and ions (protons and/or cations) are participating in the charge storage mechanism, which is essential to achieve large electronic and ionic conductivity in MnO2 based electrodes. MnO2 can crystallize many polymorphic forms, enabled by its MnO6 octahedral building unit either by sharing edges or corners, to form chain-like tunnel type-MnO2 (α-, β-, and γ-phase), or layered-type MnO2 (δ-phase) and/or the spinel-type MnO2 (λ-phase). 21-27 The nature of reducing agent and the presence of certain inorganic cations in the reaction bath strongly influence the formation of different phases.24-28 Recent breakthroughs reveal that among all MnO2 polymorphs, α-, β- and δ-MnO2 are promising and largely studied for ESs and Li-ion batteries.26-35 For example, Wang et al., reported a conventional reflux method to synthesize α-MnO2 nanoneedles, which showed a large CS of 234 F g-1 and excellent stability with 75% retained capacitance up to 500 cycles in 1M Na2SO4 solution.30 In another study, Zhang et al., have synthesized ultrafine βMnO2/polypyrrole nanorods composite by in-situ coprecipitation method. This composite delivers a high CS of 294 F g-1 and good cyclability with 92% retained capacity up to 1000 cycles at 1 A g-1.35 Munaiah et al., have fabricated δ-MnO2 hollow-spheres by a one-pot butyric acidmediated redox reaction between KMnO4 and Na2S2O4.20 The δ-MnO2 hollow-spheres electrode 3 ACS Paragon Plus Environment


showed a large CS (∼283 F g-1) and better stability with 63% retention up to 1000 cycles at 0.5 mA cm-2 in 0.1 M Ca(NO3)2 electrolyte.20 Lai et al., fabricated MnO2 nanoflakes coated over carbon horns (MnO2/CNH) by facile solution approach and tested as anode material for Li-ion batteries. The hybrid MnO2/CNH nanocomposite exhibited excellent reversible capacity of 565 mAh g-1 at 0.45 A g-1 and remarkable cyclic performance with 100% capacity retention even up to 50 cycles.31 Further, Devaraj et al., reported electrochemical deposition of δ-MnO2 nanoparticles on conducting substrate using neutral surfactant Triton X-100.36 The results confirmed that the CS of MnO2 greatly dependent on the concentration of TrtonX-100 (0 to 100 mM) and the high CS of 355 F g-1 was achieved for 10 mM TrtonX-100 and 0.5 M MnSO4 solution. All these studies show that the CS of MnO2 is influenced by its dimensions, morphology, crystalline structure and porous textures. Besides, the control of the nanoscale morphology of MnO2 offers a window of opportunities to further enhance the capacitance by effectively carrying out the redox reactions both at the surface and in the bulk of the electrode material. Further, this also enhances the electrode/electrolyte contact greatly during the charge storage mechanism. Therefore, the development of well-crystallized MnO2 with nanostructured morphology by facile and cost-effective green method is highly desirable. In the present study, we prepared a large quantity of nanostructured MnO2 samples with αand β-crystal structures by a simple redox reaction between permanganate and organic sugars at room temperature. Permanganate is a versatile oxidizing agent and widely used for the oxidation of several organic acids under different media.37-39 The organic sugars of mannose, galactose and glucose were chosen as reducing agents and favorably reduce the permanganate (MnO4−) to MnO2. The as-obtained hydrated MnOx precursors are amorphous or poorly crystalline in nature. Calcining these precursors at an elevated temperature result in well-crystalline counterparts.24,28


Page 4 of 43

Page 5 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The influence of reducing sugar on tuning the physicochemical and electrochemical properties of MnO2 nanostructures have been studied and compared with the literature to gain insights into the factors that determine the charge storage performance. Remarkably, the synthesized β-MnO2 nanoneedles with high specific surface area exhibited excellent energy storage performance with high capacitance and improved cyclability.

2. Experimental section 2.1 Materials preparation In our experiments, we used analytical grade chemicals as received from SD Fine Chemicals, India. In a typical synthesis, 1.58 g of KMnO4 dissolved in 100 mL of deionized (DI) water and stirred for 30 min to obtain a homogeneous purple color solution. To this solution, 100 mL of 7.2 g of mannose dissolved separately in DI water was added slowly and stirred for another 1 h to form a black colored precipitate. The precipitate was centrifuged, rinsed with ethanol and DI water to remove the unreacted components if any. The centrifuged product was dried in an oven at 80 °C for 12 h. Further, the dried amorphous powder was calcined in an electric furnace from room temperature to 400 °C at a heating rate of 5 °C min-1 and kept for 3 h in flowing air. Subsequently the sample was allowed to self-cooling to room temperature in order to obtain pure crystalline β-MnO2 nanoneedles (MnO2-NNs). Similarly, β-MnO2 hollow tubes (MnO2-HTs) and α-MnO2 nanorods (MnO2-NRs) were prepared by replacing mannose with galactose and glucose, respectively. 2.2 Materials characterization Thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG) of amorphous MnOx powders were performed on TA make TGA Q500V20.10 Build 36 instrument 5 ACS Paragon Plus Environment


under continuous air flow with a linear heating rate of 20 °C/min. The crystallographic phase identification of MnO2 samples was done employing Bruker AXS D8 advanced diffractometer with monochromatic Cu Kα radiation (40 kV, 30 mA) at a scan rate of 0.02° per min. Standard Scherrer’s formula, dXRD = Kλ/(β cos θ) was used to evaluate the mean crystallite size (dXRD) of MnO2 samples, where K is particle shape dependent factor (~0.89 for spherical particles), θ is the angle of the diffraction peak, β is full width at half maximum, and λ is the wavelength of xray (λ = 0.15406 nm). The surface textural properties of MnO2 samples were obtained by BET method, and surface morphologies were studied by FESEM and TEM analysis as detailed in our previous study.40 2.3 Electrochemical measurements The electrochemical characteristics of different MnO2 nanostructures were investigated by a homemade three-electrode setup attached to CHI7081C electrochemical workstation (CH Instruments). The detailed procedure for the fabrication of working electrodes was described in elsewhere.27 The total active material weight on each of the electrode was about ∼1.2 mg cm-2. The reference electrode and counter electrode used were saturated calomel electrode (SCE) and Pt foil (1×1 cm2), respectively. Freshly prepared 1 M Na2SO4 aqueous solution was used as an electrolyte. The cyclic voltammograms were obtained at scan rates of 2, 5, 10, 20 and 50 mV s-1 between 0 and 0.9 V (vs. SCE). Galvanostatic charge-discharge curves were recorded between 0 to 0.9 V (vs. SCE) at current densities from 0.5 to 12 A g-1. Electrochemical impedance spectroscopy was carried between 10 mHz and 100 kHz at 0.6 V bias potential. Before final measurements, the MnO2 coated Ni foil working electrodes were stabilized by dipping them in an electrolyte solution for about 30 min and recording 25 CV cycles.


Page 6 of 43

Page 7 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. Results and discussion 3.1 Morphology and structural characterization The α-MnO2 consists of double chains of edge-shared MO6 octahedra, which are linked at corners to form (2×2) and (1×1) tunnels in the tetragonal unit cell, whereas β-MnO2 is composed of single chains of MO6 octahedra to form (1×1) tunnels.23-25,41 It has been reported in the literature that the MnO2 structural and surface textural properties including crystal structure, surface morphology, surface area and porosity strongly depend on the synthetic conditions, reagents, concentration and pH of the solution.25,27,41-43 Here, we have synthesized α- and βMnO2 nanostructures with different morphologies by a simple redox reaction between permanganate and organic sugars such as mannose, galactose and glucose. The formation of MnO2 and the associated reaction mechanism between permanganate and glucose has been reported by Wang et al.39 However, in our system, it is difficult to elucidate the exact reaction mechanism involving redox reaction between KMnO4 and different reducing sugars because of several possible oxidation states. The reactivity towards the reduction of KMnO4 is expected to be quite different as a result of differences in isomeric structural units of sugars as shown in Figure S1, even though all the three sugars possess identical empirical formula. As a result, the sugars can influence the pH, temperature and the rate of nucleation and precipitation during the course of the reaction. Further, the coordination of permanganate ion with the position of hydroxyl group can have profound influence on the morphology and crystalline nature owing to the van der Waals forces, hydrophobic interactions, electrostatic, steric, ionic, entropic factors, and hydrogen bonds involved concurrently in the reaction medium with diversifying strength, range and selectivity.27,44,45. However, based on literature reports, including theoretical and experimental studies, we have described the plausible formation 7 ACS Paragon Plus Environment


mechanism of different MnO2 nanostructures under facile redox reaction (Scheme 1). Initially due to redox reaction between MnO4− and sugar, a large number of MnOx nuclei are formed in a short-time which aggregate further to nanosized particles.37 Even though all the sugars possess identical empirical formula, their stereochemistry and reactivity towards a particular reaction are different.46-48 These differences primarily lead to the formation of nanoparticles of different phases and morphologies. Moreover, the reaction of permanganate with reducing sugars is highly sensitive to temperature, pH, and stereochemistry of sugar.49 The formed nanoparticles can undergo Ostwald ripening and self-assembling processes during the sintering at an elevated temperature and produce specific MnO2 nanostructures.27,50 It is possible that the ratio of KMnO4 to organic sugars can also influence the formation of different nanostructures of MnO2. Thermal stability of MnOx precursor samples was determined by TGA and DTG measurements and corresponding profiles are presented in Figure 1. A progressive weight loss occurred up to 400 °C in all three TGA curves, corresponds to the removal of adsorbed, intercalated and structural water molecules during the crystallization process of MnO2.27 The small weight loss observed at around 450 °C, which can definitely be ascribed as loss of oxygen molecule resulting from phase transformation of MnO2 to Mn2O3 (4MnO 2 → 2Mn 2O3 + O 2 ) .42 The changes in the phase and morphology of manganese oxide are seen by heating the sample at 500 °C for 3 h (Figure S2). The precursor MnOx is transformed to Mn2O3 at 500 °C with the slight distortion of nanoneedles morphology. In order to transform the amorphous MnOx into crystalline MnO2 our samples were calcined at 400 °C, allowing the minimum temperature to safeguard against the phase transition of MnO2 to Mn2O3. The XRD pattern of the uncalcined MnOx precursors (Figure S3) shows a single broad diffraction peak at about 36.5° indicating poor crystalline nature of the samples.28 After sintering the precursors at 400 °C, the samples


Page 8 of 43

Page 9 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exhibit a highly crystalline form of MnO2 (Figure 2). The MnO2-NRs sample shows prominent diffraction peaks at 12.7, 18.0, 28.7, 37.5, 42.0, 49.8, 60.2 and 69.6° which are all well indexed to the (110), (200), (310), (121), (301), (411), (521) and (451) planes of pure tetragonal α-MnO2 with lattice parameters a = b = 0.9815 nm and c = 0.2847 nm (JCPDS no. 72-1982; space group: I4/m (87)). While the other MnO2-HTs and MnO2-NNs samples show diffraction peaks at 28.6, 37.3, 42.7, 56.5, 59.3, 64.7 and 72.3° corresponds to the (110), (101), (111), (211), (220), (002) and (112) planes of tetragonal β-MnO2 with lattice parameters a = b = 0.4404 nm and c = 0.2876 nm (JCPDS no. 81-2261; space group: P42/mnm (136)). The broadened XRD peaks are common for all three samples, which indicate smaller crystalline particle size of the samples. Further, the (110), (310) and (121) diffraction planes for MnO2-NRs, and (110), (101) and (211) diffraction planes for MnO2-HTs and MnO2-NNs were selected to assess the mean crystallite sizes (dXRD) of samples by Scherrer's formula. The estimated dXRD values for MnO2-NNs, MnO2-NRs and MnO2-HTs samples are 19.1, 21.2, and 18.5 nm, respectively. The morphology evolution of α- and β-MnO2 samples was characterized by field emission scanning electron microscope (FESEM) analysis, as depicted in Figure 3. The organic reducing sugars would have a great effect on the formation of the three kinds of MnO2 arrays. On employing mannose as reducing agent, MnO2 is formed with uniform 1D-nanoneedles (MnO2NNs, Figure 3A-C), while replacing the mannose with galactose and glucose reducing agents, MnO2 exhibited hollow tubes (MnO2-HTs, Figure 3D-F) and nanorods (MnO2-NRs, Figure 3GI). Transmission electron micrographs (TEM) in Figure 4 show the differences in morphology and microstructures of MnO2 samples. The representative TEM image of MnO2-NNs sample (Figure 4A) reveals the diameter of nanoneedles is about 10 nm and their lengths are between150 and 200 nm. The MnO2 sample that is synthesized by the galactose reducing agent (Figure 4C)



manifests hollow-interior of the tubes (which is clearly shown in FESEM image, Figure 3F) with diameter in the range of 80 nm. The MnO2-NRs sample TEM image (Figure 4E) clearly depicts nanorods with dimensions of 20-25 nm and hundreds of nanometer lengths. The clear lattice fringes are perceived in high-resolution TEM images of all three MnO2 samples (Figure 4B, D and F). The inter-planer spacings for MnO2-NNs sample are 0.306 and 0.161 nm (Figure 4B), and for MnO2-HTs sample is 0.240 nm (Figure 4D) which correspond to the (110), (211) and (101) planes of β-MnO2 (JCPDS no. 81-2261). The inter planer spacing for MnO2-NRs sample is 0.310 nm (Figure 4F) which is in good agreement with (110) plane of α-MnO2 (JCPDS no. 721982). Nitrogen adsorption-desorption measurements were implemented to evaluate the surface textural characteristics of resulting α- and β-MnO2 nanostructures (Figure 5). For all three MnO2 samples, the nitrogen uptake gradually increases from a relative pressure (P/P0) of 0.45 and extending almost up to 0.99. Further, the slope of adsorption isotherm steeper indicates a good fraction of textural porosity. According to the IUPAC, the isotherms of all the three samples are classified as type IV with H3-type hysteresis loop, which is typical of mesoporous materials. The calculated BET specific surface area values for MnO2-NNs, MnO2-HTs and MnO2-NRs are 146, 105 and 53 m2 g-1, respectively. The BJH method was employed to obtain the pore-size distribution analysis of MnO2 samples (insets of Figure 5). The MnO2-NNs exhibits bimodal pore-size distribution with an average pore diameter of 2.30 nm, while the other MnO2-HTs and MnO2-NNs samples show the unimodal pore-size distribution with pore diameters of 4.68 and 6.06 nm, respectively. It is reported that the increased specific surface area along with proper pore-size distribution (particularly in the range of 2 to 5 nm) of an electroactive material possibly offers enormous number of active sites, their easy accessibility to electrolyte ions with less


Page 10 of 43

Page 11 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hindrance to ion diffusion boosting the electrochemical performance.1,6,27 Therefore, the high specific surface area, indispensable pore diameter and uniform 1D-nanoneedles of the β-MnO2 sample are expected to favour the effective ion transport inside the pore system. These factors contribute to the increase in electrode-electrolyte interface area and electrochemical performance of β-MnO2 nanoneedles compared to β-MnO2 hollow tubes and α-MnO2 nanorods.

3.2 Electrochemical studies In order to explore the MnO2 nanostructures as ES electrodes, all the three MnO2 samples underwent CV tests at different sweep rates (2 to 20 mV s-1) between 0 and 0.9 V (vs. SCE), as shown in Figure 6A-C. The CV curves for all three electrodes show nearly rectangular shape, indicating lower contact resistance and ideal capacitive behavior of the MnO2 nanostructures.34,51 The quasi-rectangular shape of CV profiles transpires on account of several overlapping redox centers on MnO2 based electrodes leading to repetitive intercalation and extraction processes of Na+ cations into the bulk material through a tunnel or an interlayer spacing of structure.52 As the sweep rate rises, the position of the oxidation and reduction peaks in the voltammograms of all three MnO2 electrodes moved towards slightly higher and lower potentials, respectively. At higher sweep rates, the ion transport into the interior of the electrode surface diminishes greatly resulting in unsymmetrical CV profiles.27,53 The comparison CV curves of all the three MnO2 electrodes is presented in Figure 6D. It is obvious that MnO2-NNs exhibits larger CV loop and higher anodic and cathodic currents compared to MnO2-HTs and MnO2-NRs, demonstrating its superior charge storage capability. Further, we employed Trasatti analysis to quantitatively distinguish the contribution of charges in the inner surface regions of the electrodes (capacitive elements) from the outer surface



Page 12 of 43

regions of the electrodes (diffusion-controlled insertion processes) by examining the scan rate dependence of the current.54-57 Normally, at higher scan rates, the electrode exhibits lower CS due to the diffusion resistance to the remote areas in the interior parts of the active material. At higher scan rates only the outer regions of the electrode can be accessed efficiently by the ions. However, at lower scan rates both inner and outer surface pores of the electrode are fully involved As scan rates increase, only outer regions of the electrode can be accessed by the ions, while at lower scan rates both interior and superficial surface pores of the electrode employed copiously for the charge propagation. In general, the mechanism of charge storage is completely scan rate dependent phenomena and the total charge, qt stored in the electrode at a constant potential is expressed as the sum of two distinct mechanisms by equation (3):57

(3)

q =q +q t i o

where qi represents the inner surface charge and qo is the outer surface charge of the electrode. The qi is completely sweep rate dependent and which is difficult to access the electrolyte ions at higher sweep rates owing to diffusion-controlled intercalation processes. In contrast, qo is independent of sweep rate and is fully accessible to electrolyte ions at all the sweep rates. The charge storage dependence on sweep rate can be expressed by eqn. (4) and (5):6,58 q (ν ) = q 1 q (ν )

=

o 1 q t

+

kν +

−1

' kν

2 1

(4) 2

(5)

where q represents the surface charge on the electrode, k and k' are constants, and ν is the sweep rate. By using eqn. (4), the capacitive charge stored at the outer surface, qo can be obtained by extrapolation of q to ν = ∞ from the plot of q vs. ν–1/2 (Figure 7A). From equation (5), the qt 12 ACS Paragon Plus Environment

Page 13 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stored in the electrode can be obtained by extrapolation of q to scan rate ν = 0 from the plot of 1/q vs. ν1/2 (Figure 7B). In addition, the capacitance related to insertion process ( qi ) is obtained from the difference between q0 and qt . The calculated qi and qo values for MnO2-NNs, MnO2HTs and MnO2-NRs electrodes are presented in Figure 7C. Among the three MnO2 nanostructured electrodes, the MnO2-NNs based electrode exhibits higher total charge as well as outer surface charges. This demonstrates that large amount of charge is located at sites that are easily accessible with lowest ohmic drops along with fewer diffusion limitations of irreversible redox transitions. The galvanostatic charge-discharge (GCD) measurements of different MnO2 nanostructures were recorded at a fixed current density of 1 A g-1, as shown in Figure 8A. The GCD curves of all three MnO2 electrodes are highly linear and symmetrical, revealing good capacitive behavior and superior irreversible faradaic reactions between Na+ and MnO2. Moreover, no obvious voltage drop (IR drop) observed in all three MnO2 electrodes, indicating the fast I-V response of the electrodes in a voltage window between 0 and 0.9 V (vs. SCE).27,59 It is imperative that the MnO2-NNs exhibits longer charge-discharge time compared to the MnO2-HTs and MnO2-NRs, demonstrating its highest surface redox activity. Based on the active material weight, the CS values can be obtained for all the three MnO2 electrodes from the galvanostatic discharging curves according to the equation (6):58,59

C

s

=

i m( ∆V ∆t )

(6)

where i, m and ∆V ∆t , respectively, are the constant discharge current, electrode active material weight, and slope of discharge curve. The plot of CS with the change of current densities (0.5 to 12 A g-1) for all MnO2 electrodes is presented in Figure 8B. All the three MnO2 electrodes 13 ACS Paragon Plus Environment


displayed maximum CS values at a smaller current density of 0.5 A g-1 and it gradually reduces with current density. The reason is that at lower current densities, the diffusion of electrolyte ions can effortlessly access to both inner and outer surface pores of MnO2 electrode. As a result complete insertion and deinsertion reactions occur in both inner and outer surface pores of the MnO2 electrode. But, when the current density is increased, the utilization of redox reaction sites has been inadequate and only the superficial surface pores are available for electrolyte ions leading to sluggish transport of electrolyte ions into the pores of active MnO2 electrode.40,60 However, the MnO2-NNs electrode exhibits the high CS of 365 F g-1 at 0.5 A g-1, while MnO2HTs and MnO2-NRs exhibit only 245 and 152 F g-1 at the same current density. The rate capability of MnO2-NNs is superior as it could retain the capacitance of 205 F g-1 even when the current density increases up to 24 times, while the other MnO2-HTs and MnO2-NRs retain only 124 and 61 F g-1. It is worth noting here that the CS and rate capability of our MnO2-NNs are much higher than the reported MnO2 nanostructures (nanoplates, spheres, particles and hybrid composites).36,39,43,52,55,59,61,62 The performance of MnO2-NNs, MnO2-HTs and MnO2-NRs demonstrates that the effective electrochemical active surface area is apparently different in all the three MnO2 electrodes. The number of available redox reaction active sites in MnO2-NNs is larger than the MnO2-HTs and MnO2-NRs due to its high specific surface area, smaller pore size and 1D-nanoneedles-like morphology. The number of available active sites (Z) for the redox reaction per mole can be calculated from the CS values using equation (7),63 Z =

Cs ×M ×∆V F

(7)

where ∆V, M and F are the voltage window of charge-discharge curves (∼0.9 V), molecular weight of MnO2 (∼86.93 g mol-1), and Faraday constant (∼96485 C mol-1), respectively. The calculated Z values for the three MnO2 electrodes at different current densities are shown in 14 ACS Paragon Plus Environment

Page 14 of 43

Page 15 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8C. The MnO2-NNs shows the highest number of redox active sites at all the current densities compared to MnO2-HTs and MnO2-NRs. The good cyclic stability of an electroactive material is the foremost requirement in the development of supercapacitor devices for practical applications. To evaluate the cyclic stability of resulting MnO2 electrodes, the GCD measurement was performed at 1 A g-1 over 2000 cycles and corresponding results are shown in Figure 9A. The repeated GCD curves of all three MnO2 electrodes (Figure 9B-D) show an identical triangle curve shapes (symmetrical charge-discharge profiles) indicating all three electrodes have ideal capacitive behavior. As shown in Figure 9A, the CS of all MnO2 electrodes slowly reduces with cycle number. However, MnO2-NNs sample exhibits excellent stability with 82% retained capacitance by the end of 2000 cycles compared to MnO2-HTs (75%) and MnO2-NRs (71%). In Table 1, we have summarized the results of our MnO2 materials along with the reported values for various MnO2 nanostructures. We observe that our MnO2-NNs electrode showed a very high CS and excellent cyclability. The high specific surface area and porous 1D-nanoneedles-like morphology are the essential factors in improving electrochemical performance of MnO2-NNs electrode, which can provide an efficient path for complete electrolyte penetration into the MnO2 electrode during the electrochemical chargedischarge processes. Therefore, the majority of the electroactive sites of the porous 1Dnanoneedles of MnO2 effectively utilized, resulting in higher capacitance and better cyclability. Electrochemical impedance spectroscopy (EIS) is an essential tool to gain insight into the capacitive behavior and study the kinetic features of our nanostructured MnO2 electrodes during the electrochemical charge-discharge processes. Nyquist plots (Z″ vs. Z′) of three different MnO2 electrodes are depicted in Figure 10A. The plots of Nyquist data is used to analyze by a complex nonlinear least squares fitting method to the Randle equivalent circuit, as shown in the inset of



Figure 10A. The Nyquist plots of the three MnO2 electrodes are similar with two depressed semicircles at high-frequency region and a spike at low-frequency region, representing the electrode process largely controlled by electrochemical reaction at higher frequencies and by mass transfer at low frequencies.55,57 The transition point at higher and lower frequency regions is known as the ‘Knee frequency’. Figure 10B depicts enlarged view of the high frequency region Nyquist plots. The first intercept point on the real axis, Z′ in the high-frequency region represents equivalent series resistance, Rs (which is known as the solution resistance).13,34 The Rs is almost same for all three MnO2 electrodes (1.02, 1.80 and 1.42 Ω for MnO2-NNs, MnO2-HTs and MnO2-NRs, respectively) due to similar electrolyte testing system. The semi-circle at highfrequency region indicates double layer capacitance, Cdl (rate of redox reactions at the electrodeelectrolyte interface) and ionic charge transfer of the electrode, Rict (which typically occurs at the interfaces between solids and ionic solutions).64,65 The Rict and Cdl of MnO2-NNs are 1.05 Ω and 320 µF, which are lower than those of MnO2-HTs (2.87 Ω and 353 µF) and MnO2-NRs (2.04 Ω and 462 µF). The resistance at the medium frequency region is ascribed to the diffusion controlled process, and the entire intrinsic capacitance as well as resistance arise as a result of ion insertion/de-insertion processes.27 Such kind of phenomena can be correlated with film capacitor, Cf (4.76, 5.31 and 3.17 mF for MnO2-NNs, MnO2-HTs and MnO2-NRs respectively) which is in parallel with the Rect (electron transfer resistance).27 In low-frequency region, the close vertical lines (along with Z″) with its slope gradually changing from 45° to 90° represents the Warburg finite-length diffusion (W), which is owing to the diffusion/transport of ions into the pores of an electrodes during the electrochemical charging-discharge.66 Here a nonhomogeneous diffusion in the less-accessible sites controls the impedance and close vertical line along the Z″ corresponds to the pseudocapacitive behavior of the corresponding MnO2 16 ACS Paragon Plus Environment

Page 16 of 43

Page 17 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrodes.9 The W values for MnO2-NNs, MnO2-HTs and MnO2-NRs are 227, 283 and 196 mMho, respectively. Among three electrodes MnO2-NRs has very less inclination of the Nyquist plot to imaginary axis and also possess a minimum W value, which indicates the MnO2-NRs electrode has faster electrolyte diffusion kinetics than the other two electrodes. Further, the experimentally obtained impedance data of our nanostructured MnO2 electrodes is converted to the specific capacitance (CS) by using the equation (8),27,67

Cs =

1

(8)

-2π fZ "

where ƒ and Z" represent the measured frequency and imaginary part of the impedance of the electrodes. Figure 10C represents Bode plot of the frequency dependence of capacitance (normalized by active material weight) for three different MnO2 electrodes. The CS values of MnO2 electrodes decrease with frequency up to 10 Hz and then remain constant. All three MnO2 electrodes display maximum CS values at lower frequencies. This is due to complete diffusion of the electrolyte ions into the pores of the electroactive material which means that both inner and outer surfaces of MnO2 electrodes are fully involved. However, at large frequency region, the penetration of the ions in the electrolyte solution is firmly limited by the outer surface of electrodes, since the inner surface is not accessed giving rise to low capacitance. The maximum CS values for MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes at 10 mHz are 330, 240 and 153 F g-1, respectively, which are in good agreement with the GCD results in Figure 8B. To evaluate the quantitative frequency response, plot of imaginary capacitance, C″ vs. frequency, f was derived from the resulting EIS data (Figure 10D). At any frequency region, the MnO2-NNs electrode exhibited superior frequency response than the MnO2-HTs and MnO2-NRs electrodes. The maxima of the resultant convex curves represent the frequency f0, at which



Page 18 of 43

purely resistive behavior is transformed into purely capacitive behavior. The reciprocal of f0 provides relaxation time constant (τ0), which represents a quantitative measure of how fast a device can reversibly be charged and discharged. It can be obtained from the eqn. τ 0 = 1 2π f 0 .68 The calculated τ0 values for MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes are 1.4, 1.7 and 3.1 s, respectively. Generally, the smaller τ0 favored for the electrochemical capacitors in order to ensure the fast charge-discharge characteristics. Therefore, the low τ0 for MnO2-NNs indicates faster accessibility of electrolyte ions during the electrochemical process. In the view of high CS and superior rate capability, we further determine the energy and power density of our nanostructured MnO2 electrodes from the discharge curves according to equations (9) and (10),6,20

1 Cs V 2 2 E Power density (Pd ) = d t

Energy density (E d ) =

(9) (10)

where Cs, V and t are the specific capacitance, potential window (0.9 V), and discharge time. Figure 11 depicts Ragone plot of the energy density vs. power density of MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes. As seen in Figure 11, the Ed of MnO2-NNs gradually decreases from 42.1 to 23.2 Wh kg-1, as Pd increases from 225 to 5200 W kg-1. On the contrary, the other two electrodes show drastic decrease in Ed (27.6-13.9 Wh kg-1 for MnO2-HTs and 17.1-6.9 Wh kg-1 for MnO2-NRs, respectively) as the current density progressively ramped from 0.5 to 12 A g-1. Therefore, the MnO2-NNs electrode can provide an ultra-high energy density while retaining its intrinsically high power density, indicating great potential as an electrode material for ESs. This study clearly demonstrates that the high specific surface area along with 1D-nanoneedles of βMnO2 (MnO2-NNs) effectively increase the ion transport at the electrode-electrolyte interface. 18 ACS Paragon Plus Environment

Page 19 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


This is significant for improving the electrochemical activity of MnO2-NNs than MnO2-HTs and MnO2-NRs.

4. Conclusion In summary, we have synthesized α- and β-MnO2 sample with nanostructured morphologies especially nanoneedles, hollow tubes and nanorods by a simple redox reaction between KMnO4 and organic sugars of mannose, galactose and glucose. It is found that the redox reaction between permanganate and mannose leads to 1D nanoneedles-like β-MnO2. Owing to their high specific surface area and unique nanoneedles-like morphology of the MnO2 outperforms the counterpart β-MnO2 hollow tubes and α-MnO2 nanorods, when applied as a supercapacitor electrode. The CS values of three different MnO2 electrodes decreased as β-MnO2 nanoneedles (365 F g-1) > β-MnO2 hollow tubes (245 F g-1) > α-MnO2 nanorods (152 F g-1). The exceptional supercapacitive performance with high CS, stable cycling performance and great rate capability combine with low-cost and environmentally friendly nature of the β-MnO2 nanoneedles open up an avenue to fabricate a promising electrode for electrochemical energy storage-conversion devices.

AUTHOR INFORMATION Corresponding Author *

G. Ranga Rao, Email: [email protected], Tel. 91 44 22574226, Fax.91 44 2257 4202

Acknowledgement The authors acknowledge Department of Science and Technology, New Delhi, for providing experimental facilities under DST-FIST Schemes.



Supporting information The isomeric structural differences of mannose, galactose and glucose sugars, powder XRD pattern of uncalcined MnO2 precursors, and XRD and FESEM image of Mn2O3 sample obtained after calcination of MnOx precursor at 500 °C are presented in the supporting information.

References (1) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nature Mater. 2008, 7, 845–854. (2) Lang, X.; Hirata, A.; Fujita, T.; Chen, M. Nanoporous Metal/Oxide Hybrid Electrodes for Electrochemical Supercapacitors. Nature Nanotech. 2011, 6, 232–236. (3) Huang, M.; Li, F.; Dong, F.; Zhang, Y. X.; Zhang, L. L. MnO2-Based Nanostructures for High-Performance Supercapacitors. J. Mater. Chem. A 2015, 3, 21380–21423. (4) Gu, W.; Yushin, G. Review of Nanostructured Carbon Materials for Electrochemical Capacitor Applications: Advantages and Limitations of Activated Carbon, Carbide-Derived Carbon, Zeolite-Templated Carbon, Carbon Aerogels, Carbon Nanotubes, Onion-like Carbon, and Graphene, WIREs Energy Environ. 2014, 3, 424–473. (5) Augustyn, V.; Simon, P.; Dunn, B. Pseudocapacitive Oxide Materials for High-Rate Electrochemical Energy Storage. Energy Environ. Sci. 2014, 7, 1597–1614. (6) Umeshbabu, E.; Ranga Rao, G. Vanadium Pentoxide Nanochains for High-Performance Electrochemical Supercapacitors, J. Colloid Interface Sci. 2016, 472, 210−219. (7) Rajeshkhanna, G.; Umeshbabu, E.; Ranga Rao, G. In Situ Grown Nano-architectures of Co3O4 on Ni-foam for Charge Storage Application, J. Chem. Sci. 2017, 127, 157−166.


Page 20 of 43

Page 21 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Rajeshkhanna, G.; Umeshbabu, E.; Justin, P.; Ranga Rao, G. Spinel ZnCo2O4 Nanosheets as Carbon and Binder Free Electrode Material for Energy Storage and Electroreduction of H2O2. J. Alloys Compd. 2017, 696, 947−955. (9) Venkateswarlu, P.; Umeshbabu, E.; Naveen Kumar, U.; Nagaraja, P.; Tirupathi, P.; Ranga Rao, G.; Justin, P. Facile Hydrothermal Synthesis of Urchin-like Cobalt Manganese Spinel for High-Performance Supercapacitor Applications, J. Colloid Interface Sci. 2016, 472, 210−219. (10) Bryan, A. M.; Santino, L. M.; Lu, Y.; Acharya, S.; D’Arcy, J. M. Conducting Polymers for Pseudocapacitive Energy Storage. Chem. Mater. 2016, 28, 5989−5998. (11) Hu, C. -C.; Chang, K. -H.; Lin, M. -C.; Wu, Y. -T. Design and Tailoring of the Nanotubular Arrayed Achitecture of Hydrous RuO2 for Next Generation Supercapacitors. Nano Lett. 2006, 6, 2690−2695. (12) Ragupathy, P.; Vasan, H. N.; Munichandraiah, N. Synthesis and Characterization of NanoMnO2 for Electrochemical Supercapacitor Studies. J. Electrochem. Soc. 2008, 155, A34−A40. (13) Justin, P.; Meher, S. K.; Ranga Rao, G. Tuning of Capacitance Behavior of NiO Using Anionic, Cationic, and Nonionic Surfactants by Hydrothermal Synthesis. J. Phys. Chem. C 2010, 114, 5203–5210. (14) Rajeshkhanna, G.; Umeshbabu, E.; Ranga Rao, G. Charge Storage, Electrocatalytic and Sensing activities of Nest-like Nanostructures Co3O4. J. Colloid Interface Sci. 2017, 487, 20–30. (15) Selvan, R. K.; Perelshtein, I.; Perkas, N.; Gedanken, A. Synthesis of Hexagonal-Shaped SnO2 Nanocrystals and SnO2@C Nanocomposites for Electrochemical Redox Supercapacitors. J. Phys. Chem. C 2008, 112, 1825−1830.



(16) Aravindan, V.; Cheah, Y. L.; Mak, W. F.; Wee, G.; Chowdari, B. V. R.; Madhavi, S. Fabrication of High Energy-Density Hybrid Supercapacitors Using Electrospun V2O5 Nanofibers with a Self-supported Carbon Nanotube Network. ChemPlusChem 2012, 77, 570–575. (17) Lu, X.; Wang, G.; Zhai, T.; Yu, M.; Gan, J.; Tong, Y.; Li, Y. Hydrogenated TiO2 Nanotube Arrays for Supercapacitors. Nano Lett. 2012, 12, 1690−1696. (18) Cui, Z.; Yuan, W.; Li, C. M. Template-Mediated Growth of Microsphere, Microbelt and Nanorod α-MoO3 Structures and their High Pseudocapacitances. J. Mater. Chem. A 2013, 1, 12926–12931. (19) Wei, W.; Cui, X.; Chen, W.; Ivey, D. G. Manganese Oxide-Based Materials as Electrochemical Supercapacitor Electrodes. Chem. Soc. Rev. 2011, 40, 1697–1721. (20) Munaiah, Y.; Sundara Raj, B. G.; Kumar, T. P.; Ragupathy, P. Facile Synthesis of Hollow Sphere Amorphous MnO2: The Formation Mechanism, Morphology and Effect of a Bivalent Cation-Containing Electrolyte on Its Supercapacitive Behavior. J. Mater. Chem. A 2013, 1, 4300−4306. (21) Dubal, D.P.; Dhawale, D.S.; Salunkhe, R.R.; Lokhande, C.D.; A novel chemical synthesis of Mn3O4 thin film and its stepwise conversion into birnessite MnO2 during super capacitive studies. J. Electroanal. Chem. 2010, 647, 60-67. (22) Toupin, M.; Brousse, T.; Bélanger, D. Charge Storage Mechanism of MnO2 Electrode Used in Aqueous Electrochemical Capacitor, Chem. Mater. 2004, 16, 3184−3190. (23) Sun, C.; Zhang, Y.; Song, S.; Xue, D. Tunnel-Dependent Supercapacitance of MnO2: Effects of Crystal Structure, J. Appl. Cryst. 2013, 46, 1128–1135.


Page 22 of 43

Page 23 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Brousse, T.; Toupin, M.; Dugas, R.; Athouël, L.; Crosnier, O.; Bélanger, D. Crystalline MnO2 as Possible Alternatives to Amorphous Compounds in Electrochemical Supercapacitors. J. Electrochem. Soc. 2006, 153, A2171−A2180. (25) Devaraj, S.; Munichandraiah, N. Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties. J. Phys. Chem. C 2008, 11, 4406−4417. (26) Ghodbane, O.; Pascal, J. -L.; Fraisse, B.; Favier, F. Microstructural Effects on ChargeStorage Properties in MnO2-Based Electrochemical Supercapacitors. ACS Appl. Mater. Interfaces 2010, 2, 3493−3505. (27) Meher, S. K.; Ranga Rao, G. Enhanced Activity of Microwave Synthesized Hierarchical MnO2 for High Performance Supercapacitor Applications. J. Power Sources 2012, 215, 317−328. (28) Aswathy, R.; Munaiah, Y.; Ragupathy, P. Unveiling the Charge Storage Mechanism of Layered and Tunnel Structures of Manganese Oxides as Electrodes for Supercapacitors. J. Electrochem. Soc. 2016, 163, A1460−A1468. (29) Yu, L. L.; Zhu, J. -J.; Zhao, J. -T. Beta-Manganese Dioxide Nanoflowers Self-Assembled by Ultrathin Nanoplates with Enhanced Supercapacitive Performance. J. Mater. Chem. A 2014, 2, 9353–9360. (30) Chen, S.; Zhu, J.; Han, Q.; Zheng, Z.; Yang, Y.; Wang, X. Shape-Controlled Synthesis of One-Dimensional MnO2 via a Facile Quick-Precipitation Procedure and Its Electrochemical Properties. Crystal Growth & Design 2009, 9, 4356−4361. (31) Lai, H.; Li, J.; Chen, Z.; Huang, Z. Carbon Nanohorns as a High-Performance Carrier for MnO2 Anode in Lithium-ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 2325−2328.



(32) Ma, Z.; Zhao, T.; Reduced Graphene Oxide Anchored with MnO2 Nanorods as Anode for High Rate and Long cycle Lithium ion Batteries. Electrochim. Acta 2016, 201, 165−171. (33) Batchelor-McAuley, C.; Shao, L.; Wildgoose, G. G.; Green, M. L. H.; Compton, R. G. An Electrochemical Comparison of Manganese Dioxide Microparticles Versus α and β Manganese Dioxide Nanorods: Mechanistic and Electrocatalytic Behavior. New J. Chem. 2008, 32, 1195−1203. (34) Zhu, J. -J.; Yu, L. -L.; Zhao, J. -T. 3D Network Mesoporous Beta-Manganese Dioxide: Template-free Synthesis and Supercapacitive Performance. J. Power Sources 2014, 270, 411−417. (35) Zang, J. F.; Li, X. In Situ Synthesis of Ultrafine β-MnO2/Polypyrrole Nanorod Composites for High-Performance Supercapacitors. J. Mater. Chem. 2011, 21, 10965−10969. (36) Devaraj, S.; Munichandraiah, N. The Effect of Nonionic Surfactant Triton X-100 During Electrochemical Deposition of MnO2 on Its Capacitance Properties. J. Electrochem. Soc. 2007, 154, A901−A909. (37) Kim, H.; Watthanaphanit, A.; Saito, N. Simple Solution Plasma Synthesis of Hierarchical Nanoporous MnO2 for Organic Dye Removal. ACS Sustainable Chem. Eng. 2017, 5, 5842−5851. (38) Umashankar, V.; Radhamani, R.; Ramadoss, K.; Murty, D. S. R. Simultaneous Separation and Preconcentration of Trace Elements in Water Samples by Coprecipitation on Manganese Dioxide Using D-glucose as Reductant for KMnO4. Talanta 2002, 57, 1029−1038. (39) Wang, H. -Q.; Yang, G. -F.; Li, Q.-Y.; Zhong, X. -X.; Wang, F.-P.; Li, Z. -S.; Li, Y. -H. Porous nano-MnO2: Large Scale Synthesis via a Facile Quick-Redox Procedure and Application in a Supercapacitor. New J. Chem., 2011, 35, 469−475.


Page 24 of 43

Page 25 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Umeshbabu, E.; Rajeshkhanna, G.; Justin, P.; Ranga Rao, G. Synthesis of Mesoporous NiCo2O4-rGO by a Solvothermal Method for Charge Storage Applications. RSC Adv. 2015, 5, 66657−66666. (41) Wang, X.; Li, Y. Selected-Control Hydrothermal Synthesis of α- and β-MnO2 Single Crystal Nanowires. J. Am. Chem. Soc. 2002, 124, 2880−2881. (42) Ghodbane, O.; Pascal, J. -L.; Fraisse, B.; Favier, F. Structural in Situ Study of the Thermal Behavior of Manganese Dioxide Materials: Toward Selected Electrode Materials for Supercapacitors. ACS Appl. Mater. Interfaces. 2010, 2, 3493−3505. (43) Zhang, Y.; Sun, C.; Lu, P.; Li, K.; Song, S.; Xue, D. Crystallization Design of MnO2 Towards Better Supercapacitance. CrystEngComm 2012, 14, 5892−5897. (44) Bishop, K. J. M.; Wilmer, C. E.; Soh, S.; Grzybowski, B. A. Nanoscale Forces and Their Uses in Self-Assembly. Small 2009, 5 1600−1630. (45) Jun, Y.-W.; Choi, J. -S.; Cheon, J. Shape Control of Semiconductor and Metal Oxide Nanocrystals through Nonhydrolytic Colloidal Routes. Angew. Chem. Int. Ed. 2006, 45, 3414−3439. (46) Azmat, R.; Naz, R.; Qamar, N.; Malik, I. Kinetics and Mechanisms of Oxidation of dFructose and d-Lactose by Permanganate ion in an Acidic Medium. Natural Science, 2012, 4, 466−478. (47) Azmat, R.; Naz, R.; Khalil, A.; Uddin, F. Kinetics and Mechanism of Oxidation of DGalactose

and

D-Maltose

with

Potassium

Permanganate

in

Acidic

Medium

by

Spectrophotometry. Asian J. Chem. 2008, 20, 829−837. (48) Okoro, H. K. and Odebunmi, E. O., Kinetics and Mechanism of Oxidation of Sugar and Sugar Alcohols by Vanadium Pentoxide. Sci. Res. Essays 2010, 5, 2588−2598. 25 ACS Paragon Plus Environment


(49) Tripathi, R.; Upadhyay, S. K. Alkaline KMnO4 Oxidation of Reducing Sugars in Microemulsions: Inhibition Effect of Surfactants. Kinet Catal. 2015, 56, 261−266. (50) Bag, S.; Raj C. R. Facile Shape-Controlled Growth of Hierarchical Mesoporous δ-MnO2 for the Development of Asymmetric Supercapacitors. J. Mater. Chem. A 2016, 4, 8384–8394. (51) Li, Y.; Zhao, N.; Shi, C.; Liu, E.; He, C. Improve the Supercapacity Performance of MnO2‑ Decorated Graphene by Controlling the Oxidization Extent of Graphene. J. Phys. Chem. C 2012, 116, 25226−25232. (52) Ko, W. -Y.; Chen, L. -J.; Chen, Y. -H.; Chen, W. -H.; Lu, K. -M.; Yang, J. -R.; Yen, Y. -C.; Lin, K. -J. One-Step Solvothermal-Processed 3D Spinel-Type Manganese Oxide Microspheres and Their Improved Supercapacitive Properties. J. Phys. Chem. C 2013, 117, 16290−16296. (53) Syedvali, P.; Rajeshkhanna, G.; Umeshbabu, E.; Kiran, G. U.; Ranga Rao, G.; Justin, P. In Situ Fabrication of Graphene Decorated Microstructured Globe Artichokes of Partial Molar Nickel Cobaltite Anchored on a Ni Foam as a High Performance Supercapacitor Electrode. RSC Adv. 2015, 5, 38407–38416. (54) Ardizzone, S.; Fregonara, G.; Trasatti, S. ‘‘Inner” and ‘‘Outer” Active Surface of RuO2 Electrodes. Electrochim. Acta 1990, 35, 263−267. (55) Zhang, X.; Sun, X.; Zhang, H.; Li, C.; Ma, Y. Comparative Performance of Birnessite-type MnO2 Nanoplates and Octahedral Molecular Sieve (OMS-5) Nanobelts of Manganese Dioxide as Electrode Materials for Supercapacitor Application. Electrochim. Acta 2014, 132, 315−322. (56) Duay, J.; Sherrill, S. A.; Gui, Z.; Gillette, E.; Lee, S. B. Self-Limiting Electrodeposition of Hierarchical MnO2 and M(OH)2/MnO2 Nanofibril/Nanowires: Mechanism and Supercapacitor Properties. ACS Nano 2013, 7, 1200−1214.


Page 26 of 43

Page 27 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(57) Sen, P.; De, A.; Chowdhury, A. D.; Bandyopadhyay, S. K.; Agnihotri, N.; Mukherjee, M. Conducting Polymer Based Manganese Dioxide Nanocomposite as Supercapacitor. Electrochim. Acta 2013, 108, 265−273. (58) Sathiya, M.; Prakash, A. S.; Ramesha, K.; Tarascon, J. M.; Shukla, A. K. V2O5-Anchored Carbon Nanotubes for Enhanced Electrochemical Energy Storage. J. Am. Chem. Soc. 2011, 133, 16291−16299. (59) Chang, J.; Jin, M.; Yao, F.; Kim, T. H.; Le, V. T.; Yue, H.; Gunes, F.; Li, B.; Ghosh, A.; Xie, S.; Lee, Y. H. Asymmetric Supercapacitors Based on Graphene/MnO2 Nanospheres and Graphene/MoO3 Nanosheets with High Energy Density. Adv. Funct. Mater. 2013, 23, 5074−5083. (60) Subramanian, V.; Zhu, H.; Vajtai, R.; Ajayan, P. M.; Wei, B. Hydrothermal Synthesis and Pseudocapacitance Properties of MnO2 Nanostructures. J. Phys. Chem. B 2005, 109, 20207−20214. (61) Zhang, J.; Zhao, X. S. A Comparative Study of Electrocapacitive Properties of Manganese Dioxide Clusters Dispersed on Different Carbons. Carbon 2013, 5, 1−9. (62) Chen, S.; Zhu, J.; Wu, X.; Han, Q.; Wang, X. Graphene Oxide-MnO2 Nanocomposites for Supercapacitors. ACS Nano 2014, 5, 2822−2830. (63) Wan, C.; Yuan, L.; Shen, H. Effects of Electrode Mass-Loading on the Electrochemical Properties of Porous MnO2 for Electrochemical Supercapacitor. Int. J. Electrochem. Sci. 2014, 9, 4024−4038. (64) Conway, B. E.; Birss, V.; Wojtowicz, J. The Role and Utilization of Pseudocapacitance for Energy Storage by Supercapacitors. J. Power Sources 1997, 66, 1−14.



(65) Wang, W.; Guo, S.; Lee, I.; Ahmed, K.; Zhong, J.; Favors, Z.; Zaera, F.; Ozkan, M.; Ozkan, C. S. Hydrous Ruthenium Oxide Nanoparticles Anchored to Graphene and Carbon Nanotube Hybrid Foam for Supercapacitors. Sci. Rep. 2014, 4, 4452. (66) Li, X.; Rong, J.; Wei, B. Electrochemical Behavior of Single-Walled Carbon Nanotube Supercapacitors under Compressive Stress, ACS Nano 2010, 4, 6039–6049. (67) Umeshbabu, E.; Rajeshkhanna, G.; Ranga Rao, G. Urchin and Sheaf-like NiCo2O4 Nanostructures: Synthesis and Electrochemical Energy Storage Application. Int. J. Hydrogen Energy 2014, 39, 15627−15638. (68) Umeshbabu, E.; Rajeshkhanna, G.; Ranga Rao, G. Effect of Solvents on the Morphology of NiCo2O4/Graphene Nanostructures for Electrochemical Pseudocapacitor Application. J. Solid State Electrochem. 2016, 20, 1837−1844. (69) Xu, M.; Kong, L.; Zhou, W.; Li, H. Hydrothermal Synthesis and Pseudocapacitance Properties of α-MnO2 Hollow Spheres and Hollow Urchins. J. Phys. Chem. C 2007, 111, 19141−19147. (70) Zhu, J.; He, J. Facile Synthesis of Graphene-Wrapped Honeycomb MnO2 Nanospheres and their Application in Supercapacitors. ACS Appl. Mater. Interfaces 2012, 4, 1770−1776. (71) Tang, N.; Tian, X.; Yang, C.; Pi, Z. Facile Synthesis of α-MnO2 Nanostructures for Supercapacitors, Mater. Res. Bull. 2009, 44, 2062–2067.


Page 28 of 43

Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




Figure 1. TGA and DTG curves of uncalcined precursors of MnO2-NNs (A), MnO2-HTs (B) and MnO2-NRs (C) samples.


Page 30 of 43

Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Powder XRD patterns of MnO2-NNs, MnO2-HTs and MnO2-NRs samples; MnO2-NNs and MnO2-HTs samples show β-phase and MnO2-NRs sample shows α-phase.



Figure 3. FESEM images of (A-C) MnO2-NNs, (D-F) MnO2-HTs and (G-I) MnO2-NRs samples at different magnifications.


Page 32 of 43

Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. TEM and HRTEM images of (A, B) MnO2-NNs, (C, D) MnO2-HTs and (E, F) MnO2NRs samples at different magnifications.



Figure 5. N2 adsorption-desorption isotherms of (A) MnO2-NNs, (B) MnO2-HTs and (C) MnO2NRs samples; Inset shows corresponding samples BJH pore size distribution curves.


Page 34 of 43

Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Cyclic voltammetry (CV) curves of (A) MnO2-NNs, (B) MnO2-HTs and (C) MnO2NRs electrodes at different scan rates (from 2 to 20 mV s-1); (D) Comparison of the CV curves of MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes at a scan rate of 5 mV s-1.



Figure 7. Variation of voltammetric charge (q) against scan rates: (A) q versus ν −1/2 and (B) 1/q versus ν1/2 for MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes; (C) Inner and outer surface charge storage profiles of MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes, obtained from cyclic voltammograms.


Page 36 of 43

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. (A) Galvanostatic charge-discharge curves of MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes at a current density of 1 A g-1. (B) Specific capacitance and (C) reaction active sites of MnO2-NNs, MnO2-HTs and MnO2-NRs electrode at different current densities.



Figure 9. (A) Cyclic performance of MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes at a current density of 1 A g-1 for 2000 cycles. The initial charge-discharge curves of (B) MnO2-NNs, (C) MnO2-HTs and (D) MnO2-NRs electrodes.


Page 38 of 43

Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. (A) Nyquist plots of MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes at a bias potential of 0.6 V, (B) enlarged view of the high frequency region of the corresponding MnO2 electrodes; (C) Variation of specific capacitance, CS and (D) imaginary specific capacitance, C″ with frequency for MnO2-NNs, MnO2-HTs and MnO2-NRs electrodes.


ACS Applied Energy Materials 1 2 3 4 5 6 7 Figure 11. Ragone plot 8 9 10 electrodes. 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Synthesis method Crystal 27 Structure 28 29Redox reaction β-MnO2 30 β-MnO2 31Redox reaction 32 Redox reaction α-MnO2 33 34Redox reaction α-MnO2 35 36Redox reaction δ-MnO2 37 38Template method MnO2/HPCs 39 α-MnO2 40Microemulsion 41 Hydrothermal Spinel MnO2 42 43Precipitation δ-MnO2 44 45Precipitation α-MnO2 46 β-MnO2/poly47Polymerization 48 pyrrole 49 50Electrodeposited δ-MnO2 51 52Redox reaction α-MnO2 53 54Reflux method MnO2/rGO 55 56 57 58 59 60

Page 40 of 43

(energy vs. power density) of MnO2-NNs, MnO2-HTs and MnO2-NRs

(a)

(b)

(c)

Morphology

SBET (m2 g-1)

CS (F g-1)

Nanoneedles

146

365

82% after 2000 1.0 M Na2SO4

In

Hollow tubes

105

245

75% after 2000 1.0 M Na2SO4

this

Nanorods

53

152

72% after 2000 1.0 M Na2SO4

work

Nanorods

−

229

44% after1000

0.1 M Na2SO4

12

Hollow spheres

−

283

63% after1000

0.1 M Ca(NO3)2

20

Nanoparticles

437

196

Not mentioned

1.0 M Na2SO4

21

Nanoparticles

123

300

70% after 500

0.1 M Na2SO4

26

Nanofibers

156

241

100% after 500 0.1 M K2SO4

27

Nanoparticles

−

250

67% after 1000 1.0 M Ca(NO3)2

29

Nanoneedles

−

233

75% after 500

1.0 M Na2SO4

31

Nanorods

−

294

92% after 1000 1.0 M Na2SO4

35

Nanoparticles

84

355

79% after 500

36

Sphericales

−

200

92% after 1000 0.25 M Na2SO4

43

Nanoneedles

−

200

78% after 1000 1.0 M Na2SO4

51


Cyclic performance

Electrolyte

0.1 M Na2SO4

Ref.

Page 41 of 43


1 2 3 Solvothermal 190 95% after 1000 1.0 M Na2SO4 Spinel MnO2 Microspheres − 4 5 Hydrothermal MnO2/OMS Plate-like 44 308 92% after 1000 1.0 M Na2SO4 6 α-MnO2Nanorods 42 315 Not mentioned 1.0 M LiClO4 7 Polymerization 8 PEDOT 9 10 Hydrothermal α-MnO2 Nanorods 132 168 83% after 100 1.0 M Na2SO4 11 12 Hydrothermal α-MnO2 Hollow sphere 93 167 89% after 350 1.0 M Na2SO4 13 β-MnO2/rGO Nanospheres 210 82% after 1000 1.0 M Na2SO4 − 14 Co-precipitation 15 α-MnO2 Nanorods 120 152 87% after 100 1.0 M Na2SO4 16 Hydrothermal 17 Table 1. Comparison of the electrochemical performance of our MnO2 electrodes with the 18 19 literature reports. 20 21 22 (a) 23 SBET is the BET specific surface area obtained from N2 adsorption-desorption isotherms; (b)CS 24 is the maximum specific capacitance; (c)Cyclic stability (%) after nth number of cycles 25 26 27 28 29 30 31 32 Scheme 1. A schematic mechanism for the formation of MnO2 nanostructures under facile redox 33 34 reaction between permanganate and organic sugars. 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 41 59 ACS Paragon Plus Environment 60

52 55 57

60 69 70 71



Page 42 of 43

Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical abstract

ACS Paragon Plus Environment

Tuning the Surface Morphology and Pseudocapacitance of MnO2 by

Recommend Documents