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An efficient α-MnO2 nanorods forests electrode for electrochemical capacitors with neutral aqueous electrolytes
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Research Paper
An efficient a-MnO2 nanorods forests electrode for electrochemical capacitors with neutral aqueous electrolytes Ashwani Kumara , Amit Sangera , Arvind Kumara , Yogesh Kumarb , Ramesh Chandraa,* a b
Nanoscience Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee, 247667, India Department of Physics, ARSD College, University of Delhi, New Delhi, 110021, India
A R T I C L E I N F O
Article history: Received 7 October 2016 Received in revised form 18 October 2016 Accepted 26 October 2016 Available online xxx Keywords: supercapacitor reactive sputtering a-MnO2 nanorods electrochemical energy storage
A B S T R A C T
In this work, a facile and novel approach is presented to maintain stoichiometry, to achieve stable nanostructure and high surface area manganese oxide (a-MnO2). The synthesis of a-MnO2 nanorods forest has been carried out using reactive DC magnetron sputtering technique without use of surfactant/ wet chemical procedure. The structural parameters, vibrational response and surface morphology of the a-MnO2 nanorods are characterized using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform Infrared spectroscopy (FT-IR), X-ray Photoluminescence spectroscopy (XPS), Field emission scanning electron microscopy (FE-SEM), Transmission electron miscopy (TEM) and BET surface area. The a-MnO2 nanorods have smooth surface and uniform diameters. The high surface area with lower lattice energy of poly nano-crystalline structure is expected to enhance the utilization ratio of electrode materials and facile de-intercalation process. The Mn-O-Mn bonded, MnO6 octahedral based and the well-developed tetragonal a-MnO2 with (2 2) tunnel is obtained. The mesopores in a-MnO2 provide wide channels, which is extremely suitable to transport ions into micropores present in a-MnO2. The electrochemical measurements are performed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) techniques. The MnO2 electrode exhibits high specific capacitance (346 F g1 at 0.71 A g1 in 1 M aqueous Na2SO4) and good cycling stability (86.6% retention after 1500 cycles). The present approach opens a new avenue to design an efficient chemical reaction free nanorods. ã 2016 Published by Elsevier Ltd.
1. Introduction The environment friendly energy storage/conversion devices have attracted a great attention of the researchers to reduce the dependency of society on the fossil fuels. It is excellent to generate the energy from sustainable and renewable resources like solar energy, hydro energy and wind energy etc. The intermitted renewable energy sources are required to sustain the efficient energy storage systems [1]. The production of clean energy and electrochemical energy storage are one of the great technology achievements to partially full fill need of the huge energy consumption in the twenty first century [2,3]. In a broad spectrum of different energy storage technologies, supercapacitors emerge as attractive candidates with high power density, high reliability, fast charging-discharging and superior cyclic lifetime than that of batteries [3,4]. Therefore, electrochemical supercapacitors an
* Corresponding author. E-mail address: ramesfi[email protected] (R. Chandra).
energy storage devices have been employed in various applications such as the electronic gadgets, emergency doors, digital cameras, portable power backup in hybrid electric vehicles [4,5]. Generally, supercapacitors can be classified into three categories: electric double layer capacitors (EDLCs), pseudocapacitors and hybrid electrochemical capacitors which are differentiated by their charge-storage mechanisms. The electric double layer capacitors store the electric charge via reversible electrostatic charge at the electrode electrolyte interface. The hybrid electrochemical capacitors, generally consists of two different electrode, one capacitivetype electrode and another battery-type faradaic electrode within the same cell. This type of electrochemical capacitor (EC) expected to achieve capacitance and enhanced the energy density than that of EDLCs [5–8]. However, the experimental result based on hybrid design are not satisfactory, in fact, they are far below than the expected theoretical values. Though, in pseudocapacitors, fast and reversible redox reactions take place near the top surface of the electroactive materials, as well as throughout the volume of electroactive materials, resulted in high specific pseudocapacitance (typically 300–1000 F/g) compared to carbon-based EDLCs
http://dx.doi.org/10.1016/j.electacta.2016.10.168 0013-4686/ã 2016 Published by Elsevier Ltd.
Please cite this article in press as: A. Kumar, et al., An efficient a-MnO2 nanorods forests electrode for electrochemical capacitors with neutral aqueous electrolytes, Electrochim. Acta (2016), http://dx.doi.org/10.1016/j.electacta.2016.10.168
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[9,10]. The conducting polymers, transition metal oxides, are the potential redox materials for pseudocapacitors. In the conducting polymers, the stability is a big issue. On the other hand, the metal oxide is still being explored via functional groups, reactive site and voids and channels in the materials. The transition metal oxides (e.g. MnO2, Co3O4, and RuO2 etc.) are considered as highly appreciated electrode materials for pseudocapacitors because of their several oxidation states for effective and multiple charge transfer [11–15]. Among these, manganese oxides (MnxOy) can be considered as promising electrode material due to its high theoretical specific capacitance (Csp 1370 F g1), environmental benignity and low cost [16–19]. Among various crystallographic structures of manganese dioxide (MnO2), a-MnO2 exhibit the biggest tunnel size of 2 2 tunnels, designed with double chains of MnO6 octahedra. Hence, a-MnO2 can store more electrolyte cations, while enabling the conversion of Mn+4 to Mn+3 ions for charge balance in reversible redox reactions delivering it with the highest specific capacitance. The performance of a supercapacitor highly depends on the crystal structure, surface morphology, electrical contact with current collector, number of ions (concentration of electrolyte) and exposed surface area of active materials. It is well studied that the one-dimensional (1-D) nanostructured morphology such as nanotubes, nanorods, and nanowires have large accessible surface area with short diffusion paths for electron/ion transfer to achieve high capacitance, better stress/strain accommodation and improved electrochemical properties. Therefore, it has a great significance to develop controllable synthesis approaches for 1D MnO2 nanomaterials and to explore their potential applications. The great efforts have been carried out to fabricate the 1-D MnO2 nanostructures with several polymorphs, such as a-MnO2
nanorods/nanowires, b-MnO2 nanorods/nanotubes, g-MnO2 nanowires/nanotubes, and d-MnO2 nanofibers [4,16,20–26]. The previous reports follow a common technique i.e. hydrothermal route for the synthesis of crystalline a-MnO2 nanorods using MnSO4 and KMnO4 [7]. However, such modifications in the morphology of nanostructured materials could not make any significant improvement in redox reactions [27]. Therefore, it is required to synthesize controlled 1-D a-MnO2 nanostructures with favorable phase structure, well defined and compact surface morphology, and reproducibility. Among various advanced fabrication techniques Physical vapour deposition (PVD) technique, particularly sputtering has been considered as an effective way to synthesize the high-quality contamination free 1-D nanostructures for supercapacitor electrodes [28]. In this work, we report a novel approach for the first time to synthesize one dimension (1-D) a-MnO2 nanorods using reactive magnetron sputtering. The sputtering provides some essential advantages such as large specific surface area, high purity, time saving, uniformity and reproducibility over chemical synthetic routes. To the best of our knowledge, we are reporting the highest specific capacitance for sputtered synthesis a-MnO2 nanorods. The electrochemical characteristics of the as-prepared a-MnO2 nanorods as an electrode material for supercapacitors have been investigated and the results are discussed in detail. 2. EXPERIMENTAL SECTION 2.1. Synthesis of sputtered a-MnO2 nanorods forest The a-MnO2 nanorods were deposited directly on liquid nitrogen cooled cold finger made of oxygen-free high thermal
Fig. 1. Schematic illustration of the fabrication process of a-MnO2 nanorods on copper cold finger.
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conductivity (OFHC) copper by reactive DC sputtering technique (Fig. 1). The distance between target (Mn) and cold finger was kept at 5 cm. Prior to deposition, the custom designed sputtering chamber was initially evacuated to a base pressure of 5 106 m Torr. The working pressure was kept constant at 30 m Torr by constant flow of Ar (40 sccm) and O2 (10 sccm) gas using mass flow controller. The deposition of a-MnO2 was carried out for a period of 1 h by applying 65 W sputtering power at 194 C. After deposition, a-MnO2 nanorods were scratched carefully from cold finger at room temperature.
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1100) and Raman spectroscopy (Renishaw, United Kingdom). The specific surface area of a-MnO2 nanorods were determined using BET principle and the pore parameters using BJH method by surface area analyzer (Autoabsorb-1-C Quantachrome, USA at 77 K). The electrochemical measurements were carried out using electrochemical workstation (CHI-660D) in three electrode arrangement. The aqueous solution of 1 M Na2SO4 was used as an electrolyte with a pH of 7 and conductivity value of 2 V1 cm2 mol1. 2.3. Electrode synthesis
2.2. Characterization The XRD measurements of as deposited a-MnO2 nanorods were obtained in u–2u geometry using X-ray diffractometer (Bruker AXS, D8 advance) operating at 20 kV and 40 mA with CuKa radiation (l = 1.5418 Å). The surface morphologies, elemental composition, and the crystal structure of the samples were characterized by FE-SEM (Carl Zeiss, Ultra plus), TEM (FEI TECNAI G2), XPS (Perkin Elmer model 1257), FT-IR (Nicolet NEXUS Aligent
To fabricate the working electrode for electrochemical supercapacitors, the a-MnO2 nanorods, conducting carbon and polytetrafluoroethylene (PTFE) as the binder were mixed in the weight ratio of 70:20:10 by mortar and pestle using some drop of ethanol as solvent. The mixture was coated on to the graphite sheet current collector (area 1 cm2) with the active material mass loading 1.4 mg and further dried at 80 C for 10 h. The specific capacitance (Cs) of active material from the cyclic voltammetry (CV) and
Fig. 2. (a) SEM image of a-MnO2 nanorods with diameter of 27 nm and lengths of 1.5–2 mm on average, (b) EDX elemental mapping of a-MnO2 nanorods (c) TEM image of a-MnO2 nanorods, (d). High resolution TEM image and corresponding SAED pattern of a-MnO2 nanorods.
Please cite this article in press as: A. Kumar, et al., An efficient a-MnO2 nanorods forests electrode for electrochemical capacitors with neutral aqueous electrolytes, Electrochim. Acta (2016), http://dx.doi.org/10.1016/j.electacta.2016.10.168
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galvanostatic charge-discharge (GCD) curve was calculated using the Eqs. (1) and (2), respectively. Cs ¼
I mv
3.1. Morphological and Structural properties ð1Þ
Where I is the average current (A), m is the mass of a-MnO2 nanorods and v is scan rate (Vs1) Cs ¼
I Dt mDv
3. Results and discussion
ð2Þ
where, Cs is the specific capacitance (F g1), I is the applied current (A), m is the mass of the active material, Dt is the discharge time and DV is the potential window (V). Specific energy density (Wh kg1) and Specific power density (W kg1) of the active material were calculated using GCD curves as per Eqs. (3) and (4), respectively [29]. E¼
1 C S DV 2 2 3:6
ð3Þ
P¼
E 3600 Dt
ð4Þ
Electrochemical impedance spectroscopic (EIS) response was recorded with a 0 V DC bias using a small AC excitation of 10 mV from 100 kHz to 10 mHz.
The SEM image of as-grown DC magnetron sputtered a-MnO2 is shown in Fig. 2a.The rod like structure is appeared clearly in SEM image. The measurement of the typical length is 1.5–2 mm and average diameter of rods is found to be 27 nm. As shown in Fig. 2b, EDX elemental mapping shows the uniform elemental distribution of manganese, and oxygen in the a-MnO2 nanorods. Fig. 2c reveals that these nanorods have smooth surface and uniform diameters without any catalyst, which was observed in low magnification TEM image. The growth of nanorods is due to the condensation of small clusters formed by collisions among sputtered material during transport at low temperature. Subsequently, the selfassembly on the OFHC copper is endorsed by Coulombic interactions between the arriving clusters and the growing structures [30].In addition, there is no apparent defect or dislocation in a-MnO2 nanorods. Furthermore, the corresponding high resolution (HR)-TEM image (Fig. 2d) depicts a polycrystalline structure, with the d-spacing of 2.35 Å, 1.65 Å and 1.37 Å correspond to (121), (600) and (002) planes respectively in selected area electron diffraction pattern (inset Fig. 2d) [31]. The poly nanocrystalline structure is typically related to high surface area with lower lattice energy. This is expected enhancement in the utilization ratio of electrode materials and facile de-intercalation process [32,33]. X-ray diffraction curve of a-MnO2 nanorods are
Fig. 3. Crystal structure analysis of the a-MnO2 nanorods: (a) XRD pattern, (b) FTIR spectra, and (c) Raman spectra.
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shown in Fig. 3a. The four characteristic peaks at 37.52 , 54.82 , 56.46 and 65.41 are assigned for (121), (530), (600) and (002) planes of tetragonal phase of MnO2 (JCPDS ICDD no. 72-1982) with lattice parameters of a = b = 9.759 Å, c = 2.84 Å. The broad and low intensity XRD peaks confirmed the low degree crystallinity nature of a-MnO2 nanorods. Fig. 3b shows the FT-IR spectrum peaks appeared at 643, 522 and 405 cm1 are the vibration modes of Mn O bonds, and the peaks appeared at 1405 and 794 cm1 correspond to the vibration modes of the Mn OMn bonding in a-MnO2 [34,35]. The FT-IR observations support the XRD predication. In addition, Fig. 3c depicts the Raman spectrum of as deposited a-MnO2 nanorods which gives one strong band located at 634 cm1 with another four weak bands found at 104, 171, 347 and 421 cm1; these bands demonstrate the characteristic spectral features of typical MnO2. The strong band observed at 634 cm1 occurs due to the symmetrical Mn O vibrations, indicate the well-developed tetragonal structure which consists of (2 2) tunnel. The Raman band observed at 634 cm1 which can be related to Mn O vibrations occurs perpendicular to the direction of MnO6 octahedral that change significantly upon ion types in the tunnels. The low-frequency Raman band at 171 cm1 is observed as an external vibration that develops from the translational motion of the MnO6 octahedra, which is concerned with tunnel ions. Described Raman bands indicates the formation of a-MnO2 nanorods type crystal structure [36]. Further information on the oxidation state of the material was studied using XPS. Fig. 4a of XPS characterization result indicates
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the presence of elements like manganese ‘Mn’ and Oxygen ‘O’. The resulting high resolution scans of Mn-2p3/2 and Mn-2p1/2 were fitted with four Gaussian peaks, p1–p4, respectively, as shown in Fig. 4b, where p1 and p2 are responsible for the observed 2p3/2 peak of Mn4+, and p3 and p4 for the 2p1/2 peak. For a-MnO2 nanorods, binding energy of p1 and p3 are 641.79 and 653.40 eV, respectively, which can be attributed to the loose surface structure around the MnO6 octahedra structure. The binding energies of p2 and p4 are 644.69 and 655.81 eV, respectively, which are attributed to the body MnO6 octahedra. These two different states are present in the ratio of 1.8 in the detected depth of the XPS [37]. In Fig. 4c, the O1 s spectra can be deconvoluted in to three components exhibit three peaks positioned at 529.6 eV, 530.31 eV, and 532.18 eV which are attributed to Mn O Mn bonding, Mn OH bonding, and H OH bonding, respectively [38]. The BET analysis curve between adsorbed quantity and relative pressure P/P0 is shown in Fig. 5a. The adsorption plots of a-MnO2 nanorods are classified in Type II and IV isotherm according to the IUPAC classification of pores, which indicate the interactive nature of adsorptive-adsorbent [39]. The initial jump to the N2-volume adsorbed at low relative pressures is the indication of the presence of micropores in the nanorods structure. The quantity of adsorbed N2 is quite low, it implies the low concentration of micropores corresponds to low surface area of the material. The Inflection point in the adsorption curve occurs near the completion of the first adsorbed monolayer of N2 over the porous surface of a-MnO2 nanorods. The monolayer to unrestricted multilayer adsorption is
Fig. 4. XPS spectra of a-MnO2 nanorods (a) Full spectra, and (b) Mn 2p. Fitted peaks p1 and p2 are responsible for the observed 2p3/2 peak of Mn4+, and fitted peaks p3 and p4 for the 2p1/2 peak (open circles: measured data, solid lines: fitted data) (c) Core level O1s spectra (open stars: measured data, solid lines: fitted data).
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Fig. 5. (a) Nitrogen adsorption-desorption isotherms. The inset shows pore size distribution of the obtained a-MnO2 nanorods.
observed from such curves. The capillary condensation/complete pore filling is indicated from hysteresis at higher relative pressures. At the higher relative pressures, the slope of the adsorption curve shows increased uptake of adsorbate (N2) subsequently pores get completely filled [40]. This hysteresis loop is indicative of presence of mesoporous as shown in the pore size distribution from the pore volume versus pore diameter curve. The pore size is centered at 4 nm. This size of pore is quite suitable to accommodate more and more solvated and de-solvated ions, like Na+ and SO4. Ionics liquids can also be suitable electrolyte for such electrode materials. The Pore volume versus pore diameter curve indicates the presence of micropores and mesoporous structure. There is a very negligible amount of macropore present in the materials. The mesopores provides wide channel to transport ions to micropores. The hysteresis loop from nitrogen adsorption/desorption analysis indicates high specific surface area of 164.2 m2 g1 with pore volume 0.24 cm3 gm1 and average pore diameter of 4.7 nm for the mesoporous a-MnO2 nanorods (inset Fig. 5a). The high BET surface area and porous features of the a-MnO2 nanorods facilitate better electrochemical performances with easy transport of electrons through electrode matrix. High surface area of sputtered
Fig. 6. Electrochemical characterization of a-MnO2 nanorods electrode based pseudocapacitors (a) CV curves at different scan rates ranging from 5 to 50 m Vs1, (b) Specific capacitance calculated from the CV curves as a function of scan rates, (c) GCD curves at different current densities, and (d) Specific capacitance calculated from the GCD curves as a function of current densities.
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a-MnO2 nanorods clearly shows the advantage of sputtering technique over other synthesis routes [41–46]. 3.2. Electrochemical characterization Fig. 6a shows the CV curves of a-MnO2 nanorods electrode based electrochemical cell at the scan rate of 2–50 m Vs1 which exhibit the capacitive nature of CV curves. The capacitive current significantly enhanced with increase in the scan rate. The CV curves look like the resistive electric double layer capacitor (EDLCs) type response. Subsequently, these is no well-defined peaks in the CV curve. Basically this CV response is an envelope of the locus of reversible redox peaks. The reversible redox peaks do shift a little by few mV each cycle. The measurements were carried out by optimized mass loading and length of nanorods of active materials for better electrochemical performance [31,33]. Fig. 6b depicts the variation of specific capacitance (Cs) of working electrode with respect to different scan rates. The specific capacitance of pseudocapacitors cell was found to be 322 F g1 at the scan rates of 5 m Vs1. Such high specific capacitance is attributed to the unique 1-D nanostructure of a-MnO2 nanorods. As reported in previous investigations, the charge storage mechanism of nanocrystalline MnO2 is basically a surface phenomenon which involves the intercalation/de-intercalation of alkali cations (as shown in Eq. (5)) [31,47]. ðMnO2 Þsurf ace þ Naþ þ e $ðMnOONaÞsurf ace
ð5Þ
It can be found that the integral area of CV curves increases, while specific capacitance decreases with high scan rate. The Na+
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present in the electrolyte solution reacts with the surface of the working electrode at higher scan rate, the rapid movements of ions electrode-electrolyte interface reduce the probability of faradic reactions. However, at low scan rate the ions in aqueous electrolyte use all the available sites of working electrode which leads to high specific capacitance [42,43,48]. Fig. 6c shows the GCD curves of the capacitor cell at different current density, exhibiting the significantly linear electrochemical capacitive characteristics. The GCD characteristics reveal the symmetric nature of all the charging discharging curves even up to 1500 cycles which ascribed to the good stability of electrode and stable electrochemical capacitive performance [33]. As shown in Fig. 6d, the highest Cs value was found to be 346 F g1 from the discharge curve at a current density of 0.71 A g1, indicating comparable capacitive performance with the previous available reports [40,42–46]. From this curve we have found that as specific current enhances the specific capacitance starts to decline, caused by a large IR drop. The retention test was carried out at current density of 7.14 A g1 to explore its cycling stability for long time usage which indicates remarkable capacitance retention of 86.6% after 1500 cycles (Fig. 7a). The GCD curves of few initial and final cycles at current density of 7.14 A g1 are depicted in inset of Fig. 7a. It is believed that Mn4+ ions converting into Mn2+ ions due to oxidation/reduction process occurs in MnO2 electrode. So this may be attributed for reducing the specific capacitance of MnO2 [48]. The excellent Nyquist plot of electrochemical capacitor cell with its fitting circuit is shown in Fig. 7b. An equivalent circuit (inset in Fig. 7b) was used to obtain the Rs (equivalent series resistance), Rct (charge-transfer resistance), W (Warburg
Fig. 7. (a) Specific capacity with cyclic stability vs. cycle number at the current densities of 7.14 A g1, (b) Nyquist plots with corresponding equivalent circuit and expanded high frequency region and (c) Ragone plot of a-MnO2 nanorods electrode based pseudocapacitor.
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impedance), Cp (pseudocapacitance) and Cdl (double layer capacitance), respectively [48,49]. The specific capacitance of MnO2 based cells is measured from the Eq. (9) Cs ¼
1 2pf Z 00
ð9Þ
where, Cs is the specific capacitance in F g1, z00 is the imaginary axis and f is the frequency in Hz. The specific capacitance value was found to be 303 F g1. Fig. 5 depicts that the intercept on the real axis demonstrated the equivalent series resistance (Rs) and charge transfer resistance Rct values of a-MnO2 electrodes based supercapacitors equal to 0.62 and 1.52 V, respectively. These values indicate the high conductivity with low ion diffusion resistance. The vertical lines parallel to the imaginary axis reveal the best behavior of electrode material in capacitor cell at lower frequencies which demonstrated the facile ion diffusion in the structure of the a-MnO2 electrode. For the a-MnO2 electrode based cell the energy and power densities were measured from GCD curves and plotted on the Ragone diagram as shown in Fig. 7c. The maximum energy density of 38.92 Wh kg1 is achieved at a power density of 0.318 kW kg1, while the highest power density 3.59 kW kg1 is obtained at the energy density of 7.9 Wh kg1. 4. Conclusion The high specific surface area a-MnO2 nanorods structures were synthesized by reactive DC sputtering technique. The surface and morphological studies revealed that the nanorods have smooth and compact surface, and uniform diameters. The high surface area with lower lattice energy of poly nano-crystalline structure is expected to enhance the utilization ratio of electrode materials and facile de-intercalation process. The XRD studies further confirms low degree of crystallinity nature of a-MnO2. FTIR, Raman and XPS studies indicate Mn-O-Mn bonding, MnO6 octahedra structure and the well-developed a-MnO2 with tetragonal structure, which consists of (2 2) tunnel. The BET analysis curves indicate the presence of micropores and mesopores. The mesopores provide wide channels extremely suitable to transport ions into micropores. A high specific capacitance of 346 F g1 was measured for 1 M Na2SO4 aqueous neutral electrolytes at the current density of 0.71 A g1. The energy density of 38.92 Wh kg1 and power density of 3.59 kW kg1 with excellent capacitance retention (86.6% after 1500 cycles) demonstrated that the a-MnO2 electrode can be used as a promising electrode material for supercapacitors. Our study has demonstrated that sputtering technique is a very promising method for supercapacitor applications due to its capability of maintaining the stoichiometry and providing microstructures, phases and high specific surface area active materials with reproducibility on a large scale. Therefore, the present work provides a novel and facile strategy for the development of high-performance energy storage devices. Acknowledgments The authors would like to acknowledge the financial support from the University Grant Commission, India (Grant No. 7412-32044). We are grateful to Dr. T. Shripathi at UGC-DAE Consortium for Scientific Research, Indore, India for XPS characterizations. References [1] C.J. Barnhart, M. Dale, A.R. Brandt, S.M. Benson, The energetic implications of curtailing versus storing solar- and wind-generated electricity, Energy & Environmental Science 6 (2013) 2804–2810.
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Electrochimica Acta xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
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Research Paper
An efficient a-MnO2 nanorods forests electrode for electrochemical capacitors with neutral aqueous electrolytes Ashwani Kumara , Amit Sangera , Arvind Kumara , Yogesh Kumarb , Ramesh Chandraa,* a b
Nanoscience Laboratory, Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee, 247667, India Department of Physics, ARSD College, University of Delhi, New Delhi, 110021, India
A R T I C L E I N F O
Article history: Received 7 October 2016 Received in revised form 18 October 2016 Accepted 26 October 2016 Available online xxx Keywords: supercapacitor reactive sputtering a-MnO2 nanorods electrochemical energy storage
A B S T R A C T
In this work, a facile and novel approach is presented to maintain stoichiometry, to achieve stable nanostructure and high surface area manganese oxide (a-MnO2). The synthesis of a-MnO2 nanorods forest has been carried out using reactive DC magnetron sputtering technique without use of surfactant/ wet chemical procedure. The structural parameters, vibrational response and surface morphology of the a-MnO2 nanorods are characterized using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform Infrared spectroscopy (FT-IR), X-ray Photoluminescence spectroscopy (XPS), Field emission scanning electron microscopy (FE-SEM), Transmission electron miscopy (TEM) and BET surface area. The a-MnO2 nanorods have smooth surface and uniform diameters. The high surface area with lower lattice energy of poly nano-crystalline structure is expected to enhance the utilization ratio of electrode materials and facile de-intercalation process. The Mn-O-Mn bonded, MnO6 octahedral based and the well-developed tetragonal a-MnO2 with (2 2) tunnel is obtained. The mesopores in a-MnO2 provide wide channels, which is extremely suitable to transport ions into micropores present in a-MnO2. The electrochemical measurements are performed using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) techniques. The MnO2 electrode exhibits high specific capacitance (346 F g1 at 0.71 A g1 in 1 M aqueous Na2SO4) and good cycling stability (86.6% retention after 1500 cycles). The present approach opens a new avenue to design an efficient chemical reaction free nanorods. ã 2016 Published by Elsevier Ltd.
1. Introduction The environment friendly energy storage/conversion devices have attracted a great attention of the researchers to reduce the dependency of society on the fossil fuels. It is excellent to generate the energy from sustainable and renewable resources like solar energy, hydro energy and wind energy etc. The intermitted renewable energy sources are required to sustain the efficient energy storage systems [1]. The production of clean energy and electrochemical energy storage are one of the great technology achievements to partially full fill need of the huge energy consumption in the twenty first century [2,3]. In a broad spectrum of different energy storage technologies, supercapacitors emerge as attractive candidates with high power density, high reliability, fast charging-discharging and superior cyclic lifetime than that of batteries [3,4]. Therefore, electrochemical supercapacitors an
* Corresponding author. E-mail address: ramesfi[email protected] (R. Chandra).
energy storage devices have been employed in various applications such as the electronic gadgets, emergency doors, digital cameras, portable power backup in hybrid electric vehicles [4,5]. Generally, supercapacitors can be classified into three categories: electric double layer capacitors (EDLCs), pseudocapacitors and hybrid electrochemical capacitors which are differentiated by their charge-storage mechanisms. The electric double layer capacitors store the electric charge via reversible electrostatic charge at the electrode electrolyte interface. The hybrid electrochemical capacitors, generally consists of two different electrode, one capacitivetype electrode and another battery-type faradaic electrode within the same cell. This type of electrochemical capacitor (EC) expected to achieve capacitance and enhanced the energy density than that of EDLCs [5–8]. However, the experimental result based on hybrid design are not satisfactory, in fact, they are far below than the expected theoretical values. Though, in pseudocapacitors, fast and reversible redox reactions take place near the top surface of the electroactive materials, as well as throughout the volume of electroactive materials, resulted in high specific pseudocapacitance (typically 300–1000 F/g) compared to carbon-based EDLCs
http://dx.doi.org/10.1016/j.electacta.2016.10.168 0013-4686/ã 2016 Published by Elsevier Ltd.
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[9,10]. The conducting polymers, transition metal oxides, are the potential redox materials for pseudocapacitors. In the conducting polymers, the stability is a big issue. On the other hand, the metal oxide is still being explored via functional groups, reactive site and voids and channels in the materials. The transition metal oxides (e.g. MnO2, Co3O4, and RuO2 etc.) are considered as highly appreciated electrode materials for pseudocapacitors because of their several oxidation states for effective and multiple charge transfer [11–15]. Among these, manganese oxides (MnxOy) can be considered as promising electrode material due to its high theoretical specific capacitance (Csp 1370 F g1), environmental benignity and low cost [16–19]. Among various crystallographic structures of manganese dioxide (MnO2), a-MnO2 exhibit the biggest tunnel size of 2 2 tunnels, designed with double chains of MnO6 octahedra. Hence, a-MnO2 can store more electrolyte cations, while enabling the conversion of Mn+4 to Mn+3 ions for charge balance in reversible redox reactions delivering it with the highest specific capacitance. The performance of a supercapacitor highly depends on the crystal structure, surface morphology, electrical contact with current collector, number of ions (concentration of electrolyte) and exposed surface area of active materials. It is well studied that the one-dimensional (1-D) nanostructured morphology such as nanotubes, nanorods, and nanowires have large accessible surface area with short diffusion paths for electron/ion transfer to achieve high capacitance, better stress/strain accommodation and improved electrochemical properties. Therefore, it has a great significance to develop controllable synthesis approaches for 1D MnO2 nanomaterials and to explore their potential applications. The great efforts have been carried out to fabricate the 1-D MnO2 nanostructures with several polymorphs, such as a-MnO2
nanorods/nanowires, b-MnO2 nanorods/nanotubes, g-MnO2 nanowires/nanotubes, and d-MnO2 nanofibers [4,16,20–26]. The previous reports follow a common technique i.e. hydrothermal route for the synthesis of crystalline a-MnO2 nanorods using MnSO4 and KMnO4 [7]. However, such modifications in the morphology of nanostructured materials could not make any significant improvement in redox reactions [27]. Therefore, it is required to synthesize controlled 1-D a-MnO2 nanostructures with favorable phase structure, well defined and compact surface morphology, and reproducibility. Among various advanced fabrication techniques Physical vapour deposition (PVD) technique, particularly sputtering has been considered as an effective way to synthesize the high-quality contamination free 1-D nanostructures for supercapacitor electrodes [28]. In this work, we report a novel approach for the first time to synthesize one dimension (1-D) a-MnO2 nanorods using reactive magnetron sputtering. The sputtering provides some essential advantages such as large specific surface area, high purity, time saving, uniformity and reproducibility over chemical synthetic routes. To the best of our knowledge, we are reporting the highest specific capacitance for sputtered synthesis a-MnO2 nanorods. The electrochemical characteristics of the as-prepared a-MnO2 nanorods as an electrode material for supercapacitors have been investigated and the results are discussed in detail. 2. EXPERIMENTAL SECTION 2.1. Synthesis of sputtered a-MnO2 nanorods forest The a-MnO2 nanorods were deposited directly on liquid nitrogen cooled cold finger made of oxygen-free high thermal
Fig. 1. Schematic illustration of the fabrication process of a-MnO2 nanorods on copper cold finger.
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conductivity (OFHC) copper by reactive DC sputtering technique (Fig. 1). The distance between target (Mn) and cold finger was kept at 5 cm. Prior to deposition, the custom designed sputtering chamber was initially evacuated to a base pressure of 5 106 m Torr. The working pressure was kept constant at 30 m Torr by constant flow of Ar (40 sccm) and O2 (10 sccm) gas using mass flow controller. The deposition of a-MnO2 was carried out for a period of 1 h by applying 65 W sputtering power at 194 C. After deposition, a-MnO2 nanorods were scratched carefully from cold finger at room temperature.
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1100) and Raman spectroscopy (Renishaw, United Kingdom). The specific surface area of a-MnO2 nanorods were determined using BET principle and the pore parameters using BJH method by surface area analyzer (Autoabsorb-1-C Quantachrome, USA at 77 K). The electrochemical measurements were carried out using electrochemical workstation (CHI-660D) in three electrode arrangement. The aqueous solution of 1 M Na2SO4 was used as an electrolyte with a pH of 7 and conductivity value of 2 V1 cm2 mol1. 2.3. Electrode synthesis
2.2. Characterization The XRD measurements of as deposited a-MnO2 nanorods were obtained in u–2u geometry using X-ray diffractometer (Bruker AXS, D8 advance) operating at 20 kV and 40 mA with CuKa radiation (l = 1.5418 Å). The surface morphologies, elemental composition, and the crystal structure of the samples were characterized by FE-SEM (Carl Zeiss, Ultra plus), TEM (FEI TECNAI G2), XPS (Perkin Elmer model 1257), FT-IR (Nicolet NEXUS Aligent
To fabricate the working electrode for electrochemical supercapacitors, the a-MnO2 nanorods, conducting carbon and polytetrafluoroethylene (PTFE) as the binder were mixed in the weight ratio of 70:20:10 by mortar and pestle using some drop of ethanol as solvent. The mixture was coated on to the graphite sheet current collector (area 1 cm2) with the active material mass loading 1.4 mg and further dried at 80 C for 10 h. The specific capacitance (Cs) of active material from the cyclic voltammetry (CV) and
Fig. 2. (a) SEM image of a-MnO2 nanorods with diameter of 27 nm and lengths of 1.5–2 mm on average, (b) EDX elemental mapping of a-MnO2 nanorods (c) TEM image of a-MnO2 nanorods, (d). High resolution TEM image and corresponding SAED pattern of a-MnO2 nanorods.
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galvanostatic charge-discharge (GCD) curve was calculated using the Eqs. (1) and (2), respectively. Cs ¼
I mv
3.1. Morphological and Structural properties ð1Þ
Where I is the average current (A), m is the mass of a-MnO2 nanorods and v is scan rate (Vs1) Cs ¼
I Dt mDv
3. Results and discussion
ð2Þ
where, Cs is the specific capacitance (F g1), I is the applied current (A), m is the mass of the active material, Dt is the discharge time and DV is the potential window (V). Specific energy density (Wh kg1) and Specific power density (W kg1) of the active material were calculated using GCD curves as per Eqs. (3) and (4), respectively [29]. E¼
1 C S DV 2 2 3:6
ð3Þ
P¼
E 3600 Dt
ð4Þ
Electrochemical impedance spectroscopic (EIS) response was recorded with a 0 V DC bias using a small AC excitation of 10 mV from 100 kHz to 10 mHz.
The SEM image of as-grown DC magnetron sputtered a-MnO2 is shown in Fig. 2a.The rod like structure is appeared clearly in SEM image. The measurement of the typical length is 1.5–2 mm and average diameter of rods is found to be 27 nm. As shown in Fig. 2b, EDX elemental mapping shows the uniform elemental distribution of manganese, and oxygen in the a-MnO2 nanorods. Fig. 2c reveals that these nanorods have smooth surface and uniform diameters without any catalyst, which was observed in low magnification TEM image. The growth of nanorods is due to the condensation of small clusters formed by collisions among sputtered material during transport at low temperature. Subsequently, the selfassembly on the OFHC copper is endorsed by Coulombic interactions between the arriving clusters and the growing structures [30].In addition, there is no apparent defect or dislocation in a-MnO2 nanorods. Furthermore, the corresponding high resolution (HR)-TEM image (Fig. 2d) depicts a polycrystalline structure, with the d-spacing of 2.35 Å, 1.65 Å and 1.37 Å correspond to (121), (600) and (002) planes respectively in selected area electron diffraction pattern (inset Fig. 2d) [31]. The poly nanocrystalline structure is typically related to high surface area with lower lattice energy. This is expected enhancement in the utilization ratio of electrode materials and facile de-intercalation process [32,33]. X-ray diffraction curve of a-MnO2 nanorods are
Fig. 3. Crystal structure analysis of the a-MnO2 nanorods: (a) XRD pattern, (b) FTIR spectra, and (c) Raman spectra.
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shown in Fig. 3a. The four characteristic peaks at 37.52 , 54.82 , 56.46 and 65.41 are assigned for (121), (530), (600) and (002) planes of tetragonal phase of MnO2 (JCPDS ICDD no. 72-1982) with lattice parameters of a = b = 9.759 Å, c = 2.84 Å. The broad and low intensity XRD peaks confirmed the low degree crystallinity nature of a-MnO2 nanorods. Fig. 3b shows the FT-IR spectrum peaks appeared at 643, 522 and 405 cm1 are the vibration modes of Mn O bonds, and the peaks appeared at 1405 and 794 cm1 correspond to the vibration modes of the Mn OMn bonding in a-MnO2 [34,35]. The FT-IR observations support the XRD predication. In addition, Fig. 3c depicts the Raman spectrum of as deposited a-MnO2 nanorods which gives one strong band located at 634 cm1 with another four weak bands found at 104, 171, 347 and 421 cm1; these bands demonstrate the characteristic spectral features of typical MnO2. The strong band observed at 634 cm1 occurs due to the symmetrical Mn O vibrations, indicate the well-developed tetragonal structure which consists of (2 2) tunnel. The Raman band observed at 634 cm1 which can be related to Mn O vibrations occurs perpendicular to the direction of MnO6 octahedral that change significantly upon ion types in the tunnels. The low-frequency Raman band at 171 cm1 is observed as an external vibration that develops from the translational motion of the MnO6 octahedra, which is concerned with tunnel ions. Described Raman bands indicates the formation of a-MnO2 nanorods type crystal structure [36]. Further information on the oxidation state of the material was studied using XPS. Fig. 4a of XPS characterization result indicates
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the presence of elements like manganese ‘Mn’ and Oxygen ‘O’. The resulting high resolution scans of Mn-2p3/2 and Mn-2p1/2 were fitted with four Gaussian peaks, p1–p4, respectively, as shown in Fig. 4b, where p1 and p2 are responsible for the observed 2p3/2 peak of Mn4+, and p3 and p4 for the 2p1/2 peak. For a-MnO2 nanorods, binding energy of p1 and p3 are 641.79 and 653.40 eV, respectively, which can be attributed to the loose surface structure around the MnO6 octahedra structure. The binding energies of p2 and p4 are 644.69 and 655.81 eV, respectively, which are attributed to the body MnO6 octahedra. These two different states are present in the ratio of 1.8 in the detected depth of the XPS [37]. In Fig. 4c, the O1 s spectra can be deconvoluted in to three components exhibit three peaks positioned at 529.6 eV, 530.31 eV, and 532.18 eV which are attributed to Mn O Mn bonding, Mn OH bonding, and H OH bonding, respectively [38]. The BET analysis curve between adsorbed quantity and relative pressure P/P0 is shown in Fig. 5a. The adsorption plots of a-MnO2 nanorods are classified in Type II and IV isotherm according to the IUPAC classification of pores, which indicate the interactive nature of adsorptive-adsorbent [39]. The initial jump to the N2-volume adsorbed at low relative pressures is the indication of the presence of micropores in the nanorods structure. The quantity of adsorbed N2 is quite low, it implies the low concentration of micropores corresponds to low surface area of the material. The Inflection point in the adsorption curve occurs near the completion of the first adsorbed monolayer of N2 over the porous surface of a-MnO2 nanorods. The monolayer to unrestricted multilayer adsorption is
Fig. 4. XPS spectra of a-MnO2 nanorods (a) Full spectra, and (b) Mn 2p. Fitted peaks p1 and p2 are responsible for the observed 2p3/2 peak of Mn4+, and fitted peaks p3 and p4 for the 2p1/2 peak (open circles: measured data, solid lines: fitted data) (c) Core level O1s spectra (open stars: measured data, solid lines: fitted data).
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Fig. 5. (a) Nitrogen adsorption-desorption isotherms. The inset shows pore size distribution of the obtained a-MnO2 nanorods.
observed from such curves. The capillary condensation/complete pore filling is indicated from hysteresis at higher relative pressures. At the higher relative pressures, the slope of the adsorption curve shows increased uptake of adsorbate (N2) subsequently pores get completely filled [40]. This hysteresis loop is indicative of presence of mesoporous as shown in the pore size distribution from the pore volume versus pore diameter curve. The pore size is centered at 4 nm. This size of pore is quite suitable to accommodate more and more solvated and de-solvated ions, like Na+ and SO4. Ionics liquids can also be suitable electrolyte for such electrode materials. The Pore volume versus pore diameter curve indicates the presence of micropores and mesoporous structure. There is a very negligible amount of macropore present in the materials. The mesopores provides wide channel to transport ions to micropores. The hysteresis loop from nitrogen adsorption/desorption analysis indicates high specific surface area of 164.2 m2 g1 with pore volume 0.24 cm3 gm1 and average pore diameter of 4.7 nm for the mesoporous a-MnO2 nanorods (inset Fig. 5a). The high BET surface area and porous features of the a-MnO2 nanorods facilitate better electrochemical performances with easy transport of electrons through electrode matrix. High surface area of sputtered
Fig. 6. Electrochemical characterization of a-MnO2 nanorods electrode based pseudocapacitors (a) CV curves at different scan rates ranging from 5 to 50 m Vs1, (b) Specific capacitance calculated from the CV curves as a function of scan rates, (c) GCD curves at different current densities, and (d) Specific capacitance calculated from the GCD curves as a function of current densities.
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a-MnO2 nanorods clearly shows the advantage of sputtering technique over other synthesis routes [41–46]. 3.2. Electrochemical characterization Fig. 6a shows the CV curves of a-MnO2 nanorods electrode based electrochemical cell at the scan rate of 2–50 m Vs1 which exhibit the capacitive nature of CV curves. The capacitive current significantly enhanced with increase in the scan rate. The CV curves look like the resistive electric double layer capacitor (EDLCs) type response. Subsequently, these is no well-defined peaks in the CV curve. Basically this CV response is an envelope of the locus of reversible redox peaks. The reversible redox peaks do shift a little by few mV each cycle. The measurements were carried out by optimized mass loading and length of nanorods of active materials for better electrochemical performance [31,33]. Fig. 6b depicts the variation of specific capacitance (Cs) of working electrode with respect to different scan rates. The specific capacitance of pseudocapacitors cell was found to be 322 F g1 at the scan rates of 5 m Vs1. Such high specific capacitance is attributed to the unique 1-D nanostructure of a-MnO2 nanorods. As reported in previous investigations, the charge storage mechanism of nanocrystalline MnO2 is basically a surface phenomenon which involves the intercalation/de-intercalation of alkali cations (as shown in Eq. (5)) [31,47]. ðMnO2 Þsurf ace þ Naþ þ e $ðMnOONaÞsurf ace
ð5Þ
It can be found that the integral area of CV curves increases, while specific capacitance decreases with high scan rate. The Na+
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present in the electrolyte solution reacts with the surface of the working electrode at higher scan rate, the rapid movements of ions electrode-electrolyte interface reduce the probability of faradic reactions. However, at low scan rate the ions in aqueous electrolyte use all the available sites of working electrode which leads to high specific capacitance [42,43,48]. Fig. 6c shows the GCD curves of the capacitor cell at different current density, exhibiting the significantly linear electrochemical capacitive characteristics. The GCD characteristics reveal the symmetric nature of all the charging discharging curves even up to 1500 cycles which ascribed to the good stability of electrode and stable electrochemical capacitive performance [33]. As shown in Fig. 6d, the highest Cs value was found to be 346 F g1 from the discharge curve at a current density of 0.71 A g1, indicating comparable capacitive performance with the previous available reports [40,42–46]. From this curve we have found that as specific current enhances the specific capacitance starts to decline, caused by a large IR drop. The retention test was carried out at current density of 7.14 A g1 to explore its cycling stability for long time usage which indicates remarkable capacitance retention of 86.6% after 1500 cycles (Fig. 7a). The GCD curves of few initial and final cycles at current density of 7.14 A g1 are depicted in inset of Fig. 7a. It is believed that Mn4+ ions converting into Mn2+ ions due to oxidation/reduction process occurs in MnO2 electrode. So this may be attributed for reducing the specific capacitance of MnO2 [48]. The excellent Nyquist plot of electrochemical capacitor cell with its fitting circuit is shown in Fig. 7b. An equivalent circuit (inset in Fig. 7b) was used to obtain the Rs (equivalent series resistance), Rct (charge-transfer resistance), W (Warburg
Fig. 7. (a) Specific capacity with cyclic stability vs. cycle number at the current densities of 7.14 A g1, (b) Nyquist plots with corresponding equivalent circuit and expanded high frequency region and (c) Ragone plot of a-MnO2 nanorods electrode based pseudocapacitor.
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impedance), Cp (pseudocapacitance) and Cdl (double layer capacitance), respectively [48,49]. The specific capacitance of MnO2 based cells is measured from the Eq. (9) Cs ¼
1 2pf Z 00
ð9Þ
where, Cs is the specific capacitance in F g1, z00 is the imaginary axis and f is the frequency in Hz. The specific capacitance value was found to be 303 F g1. Fig. 5 depicts that the intercept on the real axis demonstrated the equivalent series resistance (Rs) and charge transfer resistance Rct values of a-MnO2 electrodes based supercapacitors equal to 0.62 and 1.52 V, respectively. These values indicate the high conductivity with low ion diffusion resistance. The vertical lines parallel to the imaginary axis reveal the best behavior of electrode material in capacitor cell at lower frequencies which demonstrated the facile ion diffusion in the structure of the a-MnO2 electrode. For the a-MnO2 electrode based cell the energy and power densities were measured from GCD curves and plotted on the Ragone diagram as shown in Fig. 7c. The maximum energy density of 38.92 Wh kg1 is achieved at a power density of 0.318 kW kg1, while the highest power density 3.59 kW kg1 is obtained at the energy density of 7.9 Wh kg1. 4. Conclusion The high specific surface area a-MnO2 nanorods structures were synthesized by reactive DC sputtering technique. The surface and morphological studies revealed that the nanorods have smooth and compact surface, and uniform diameters. The high surface area with lower lattice energy of poly nano-crystalline structure is expected to enhance the utilization ratio of electrode materials and facile de-intercalation process. The XRD studies further confirms low degree of crystallinity nature of a-MnO2. FTIR, Raman and XPS studies indicate Mn-O-Mn bonding, MnO6 octahedra structure and the well-developed a-MnO2 with tetragonal structure, which consists of (2 2) tunnel. The BET analysis curves indicate the presence of micropores and mesopores. The mesopores provide wide channels extremely suitable to transport ions into micropores. A high specific capacitance of 346 F g1 was measured for 1 M Na2SO4 aqueous neutral electrolytes at the current density of 0.71 A g1. The energy density of 38.92 Wh kg1 and power density of 3.59 kW kg1 with excellent capacitance retention (86.6% after 1500 cycles) demonstrated that the a-MnO2 electrode can be used as a promising electrode material for supercapacitors. Our study has demonstrated that sputtering technique is a very promising method for supercapacitor applications due to its capability of maintaining the stoichiometry and providing microstructures, phases and high specific surface area active materials with reproducibility on a large scale. Therefore, the present work provides a novel and facile strategy for the development of high-performance energy storage devices. Acknowledgments The authors would like to acknowledge the financial support from the University Grant Commission, India (Grant No. 7412-32044). We are grateful to Dr. T. Shripathi at UGC-DAE Consortium for Scientific Research, Indore, India for XPS characterizations. References [1] C.J. Barnhart, M. Dale, A.R. Brandt, S.M. Benson, The energetic implications of curtailing versus storing solar- and wind-generated electricity, Energy & Environmental Science 6 (2013) 2804–2810.
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