Electrochemical Energy Storage Properties of Ni-Mn-Oxide Electrodes

Jun 3, 2019 - Electrochemical Energy Storage Properties of Ni-Mn-. Oxide. Electrodes. for. Advance. Asymmetric. Supercapacitor Application. Apurba Ray...
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Article Cite This: Langmuir 2019, 35, 8257−8267

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Electrochemical Energy Storage Properties of Ni-Mn-Oxide Electrodes for Advance Asymmetric Supercapacitor Application Apurba Ray,† Atanu Roy,† Samik Saha,†,‡ Monalisa Ghosh,∥ Sreya Roy Chowdhury,⊥ T. Maiyalagan,⊥ Swapan Kumar Bhattacharya,§ and Sachindranath Das*,† Department of Instrumentation Science, ‡Department of Physics, and §Physical Chemistry Section, Department of Chemistry, Jadavpur University, Kolkata 700032, India ∥ Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India ⊥ Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, Chennai 603203, Tamil Nadu, India Downloaded via GUILFORD COLG on July 18, 2019 at 11:22:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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ABSTRACT: In this work, we report a facile one-spot synthesis process and the influence of compositional variation on the electrochemical performance of Ni-Mn-oxides (Ni:Mn = 1:1, 1:2, 1:3, and 1:4) for high-performance advanced energy storage applications. The crystalline structure and the morphology of these synthesized nanocomposites have been demonstrated using X-ray diffraction, field emission scanning electron microscopy, and transmission electron Microscopy. Among these materials, Ni-Mn-oxide with Ni:Mn = 1:3 possesses a large Brunauer−Emmett−Teller specific surface area (127 m2 g−1) with pore size 8.2 nm and exhibits the highest specific capacitance of 1215.5 F g−1 at a scan rate 2 mV s−1 with an excellent long-term cycling stability (∼87.2% capacitance retention at 10 A g−1 over 5000 cycles). This work also gives a comparison and explains the influence of different compositional ratios on the electrochemical properties of Ni-Mn-oxides. To demonstrate the possibility of commercial application, an asymmetric supercapacitor device has been constructed by using Ni-Mn-oxide (Ni:Mn = 1:3) as a positive electrode and activated carbon (AC) as a negative electrode. This battery-like device achieves a maximum energy density of 132.3 W h kg−1 at a power density of 1651 W kg−1 and excellent coulombic efficiency of 97% over 3000 cycles at 10 A g−1.

1. INTRODUCTION To fulfill the ever-increasing demand for clean, eco-friendly, safe, and high-performance energy sources, the supply of energy is a great concern and has made it urgent to develop alternative energy conversion technologies based on renewable energy resources.1−3 The transportation of such renewable energy from the place of production to the place of applications is yet another perplexing task. Thus, the development of high-performance, high-capacity, and stable energy storage devices has received a considerable interest as one of the best alternatives. Significant developments on energy storage devices such as conventional capacitors and batteries have already been achieved worldwide, but the commercial applications are still lagging behind the global demand.4−6 Li-ion batteries have already been well established with high energy density, but they offer low power density and safety is of great concern.7 On the other hand, conventional capacitors are known for their high power density with very low energy density. Therefore, as emerging energy storage technology, electrochemical capacitors or supercapacitors prove themselves as a promising solution, which can yield similar power density to conventional capacitor with high energy density as batteries.8 © 2019 American Chemical Society

It has several advantages such as long life, low cost, shorter charge/discharge time, and better safety. A supercapacitor can store energy on the basis of two mechanisms, either by nonfaradic charge accumulation, known as electric doublelayer capacitor (EDLC) or by fast reversible faradic reactions on the electrode surface, known as pseudocapacitor.9−11 Generally, carbon-based materials such as activated carbon (AC), carbon nanotube (CNT), graphene, etc., are best known for EDLC electrode materials, which can offer high power density compared to commercial capacitors.12,13 On the other hand, various conducting polymers such as polyaniline, polypyrrole, and transition-metal oxides such as RuO2, IrO2, NiO, Ni(OH)2, CuO, MnO2, V2O5, Mn3O4, Fe2O3, Co3O4, etc. are primarily used as pseudocapacitor electrode materials, which can deliver high energy density.14,15 Among them, RuO2 and IrO2 have already been well established due to their excellent supercapacitive behavior, but toxicity and high cost limit their commercial application. Numerous efforts have already been made to improve the supercapacitor performance, Received: April 1, 2019 Revised: May 20, 2019 Published: June 3, 2019 8257

DOI: 10.1021/acs.langmuir.9b00955 Langmuir 2019, 35, 8257−8267

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electrochemical energy storage properties of Ni-Mn-oxide with different molar ratios of Ni:Mn for supercapacitor applications, which is still a least explored area in modern research. Evolving our research interest, in this work, we report a facile ethylene glycol-mediated wet chemical synthesis method to fabricate cauliflower-like morphology consisting spherical particle-like structures of Ni-Mn-oxide with different molar ratios of Ni and Mn (Ni:Mn = 1:1, 1:2, 1:3, and 1:4). The electrochemical study of Ni-Mn-oxide electrodes shows that the compositional ratios strongly affect their electrochemical properties. In addition, this Ni-Mn-oxide-based battery-like asymmetric supercapacitor performs very well at high current density with a columbic efficiency of 97.0% after 3000 cycles, indicating excellent cycling stability. These designed Ni-Mnoxide nanoparticles also show promising battery-like capacitive performances compared to those of existing reported articles, which have been discussed earlier. Thus, the higher electrochemical properties have been ascribed to the synergistic effect of both metal oxides and high electrochemical surface area. The good supercapacitive performances of these cauliflowerlike electrode materials are highlighted to be a promising electrode for future energy storage applications.

but the low energy density and low operating voltage still pose great challenges for researchers.4,16−18 Although many binary metal oxides exhibit excellent supercapacitor performances, ternary or higher-order composite metal oxides have attracted great attention for energy storage application in the last few years.19,20 These higherorder metal oxides provide a large number of additional redox active sites for fast faradic reactions and thus offer the possibilities for improving the pseudocapacitance properties as well as delivering high energy density. In general, the longrange nanostructure can minimize the internal electron transfer resistance compared to noncontinuous nanoparticles-based nanostructure framework. The thin layer of nanomaterials can offer high surface area as well as decrease the diffusion length of electrolyte ion, which are the most challenging issues for high-performance supercapacitors.21 A comparable porosity (∼2−50 nm) also plays an important role in mass transport of electrolyte, as well as the open space among neighboring nanomaterials facilitates fast redox reaction, which improves the charge−discharge rate.22 Thus, high-performance supercapacitors necessitate such electrode materials having different morphologies, high specific surface areas, and low charge transfer resistances that deliver high specific capacitance. Among various binary, ternary pseudocapacitive transitionmetal oxides, Ni-Mn-oxide has extensively attracted research interest as a potential candidate for energy storage electrode materials due to its several advantages such as low cost, earthabundance, easy synthesis procedure, tunable morphology, high theoretical capacitance, and large potential windows.19,23 Conversely, the observed specific capacitance for Ni-Mn-oxide is still lower than the theoretical value (∼1000 F g−1 or 922 mAh g−1).24,25 In contrast, there are few reports describing the application of Ni-Mn-oxide nanocomposites-based electrodes for high-performance energy storage applications. Wu et al. reported the electrochemical capacitor performance of nanosize nickel-manganese-oxide powder synthesized by the hydrothermal route, which showed a 284 F g−1 capacitance value at a scan rate of 5 mV s−1 in 2 M NaCl electrolyte.26 In another study, Zhang et al. synthesized a porous NiMn2O4 by an epoxide-driven sol−gel process that exhibited a specific capacitance of 243 F g−1 at a scan rate of 5 mV s−1 in 1 M Na2SO4 electrolyte.24 Sankar et al. reported the electrochemical intercalation/deintercalation mechanism of NiMn2O4 for high-performance pseudocapacitor application, which possessed a specific capacitance of 202 F g−1 in 1 M Na2SO4 electrolyte at 0.5 mA cm−2 and exhibited excellent cyclic stability over 15 000 cycles.27 Pang et al. also studied the electrochemical energy storage performance of porous NiMn2O4 via calcination of oxalate precursors, which showed a specific capacitance of 180 F g−1 at 250 mA g−1 current density.28 Chavan et al. reported spray-deposited mixed nickelmanganese-oxide thin films for supercapacitor applications, and the maximum capacitance was found to be 460 F g−1 at a scan rate of 5 mV s−1 in 2 M KOH electrolyte.15 It is still challenging and better improvements are yet to be achieved to increase the specific capacitance, energy density, power density, and long-term stability. Synthesis of nanostructure with suitable morphology and size may be one of the best solutions to overcome these problems. However, to the best of our knowledge, there is no such report available in the literature on the influence of compositional molar ratios on electrochemical energy storage properties of Ni-Mn-oxide. This has motivated us to study the

2. EXPERIMENTAL SECTION 2.1. Synthesis Procedure. In this distinct synthesis approach, to synthesize Ni-Mn-oxide nanoparticles with Ni:Mn = 1:1, stoichiometric amounts of 1 mmol Ni(CH3COO)2·4H2O and 1 mmol Mn(CH3COO)2·4H2O were used as cation precursors and dissolved in 70 mL of ethylene glycol under continuous magnetic stirring. Next, 0.15 g of poly(vinylpyrrolidone) (PVP; average mol. wt., 40 000) was added into the solution and the reaction was kept under continuous magnetic stirring for 20 min to make it a homogeneous transparent solution. The resulting mixture was then transferred to a hot-bath magnetic stirrer, heated at 175 °C for 4 h in an oil bath arrangement, and then cooled to room temperature. The obtained precipitate was washed with ethanol and distilled water several times and finally collected by centrifugation at 8000 rpm. Then, the sample was dried at 90 °C for 48 h. Finally, the as-prepared sample was annealed at 400 °C for 4 h in a hot-air furnace. In this same synthesis process, we have synthesized the other samples of different molar ratios of Ni:Mn (1:2, 1:3, and 1:4). 2.2. Materials Characterization. The phase structures of all of the Ni-Mn-oxide nanocomposites were recorded by powder X-ray diffraction using the Rigaku Miniflex-600 benchtop diffractometer with a Cu Kα radiation source (λ = 1.542 Å) in the range 10−80°. The size and surface morphology of the samples were further examined by field emission scanning electron microscopy (FESEM), JEOL JSM 6700F (Japan). The morphological analysis of the sample was performed using a high-resolution transmission electron microscope (JEOL, JEM 2100 PLUS) operating at 200 kV. Fourier transform infrared (FTIR) spectra were recorded with a PerkinElmer FT-IR spectrum RX1 spectrometer, using KBr pellets at room temperature. X-ray photoelectron spectroscopy (XPS) analysis was carried out with SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) using Mg Kα radiation (1253.6 eV). The specific surface areas were calculated by N2 adsorption−desorption characteristics at 77 K, using a Belsorp Mini II (BEL, Japan) spectrometer via the Brunauer−Emmett−Teller (BET) method. The pore size distributions of the samples have derived the isotherms using the Barrett−Joyner−Halenda (BJH) model. 2.3. Electrochemical Measurement. All of the electrochemical studies of all Ni-Mn-oxide nanocomposites have been carried out in a multichannel electrochemical analyzer (CS313, CorrTest, China) at room temperature in 1 M Na2SO4 aqueous electrolyte solution. In this three-electrode system, the potential was measured with respect to Ag/AgCl (sat. KCl) as a reference electrode and platinum (Pt) foil (1 cm × 1 cm) was used as the counter electrode. For working electrode 8258

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Figure 1. (a) XRD and (b) FTIR spectra of Ni-Mn-oxides and (c−f) FESEM images with particles distribution curve (inset) of Ni:Mn = 1:1, 1:2, 1:3, and 1:4, respectively. fabrication, a Teflon-coated graphite rod has been used as the current collector. The working electrodes were prepared by mixing Ni-Mnoxide composite (80 wt %), activated powder (AC) (15 wt %), and poly(vinylidene fluoride) (PVDF, 5 wt %) in 200 μL of N-methyl pyrrolidine (NMP) solution and sonicated for 30 min to form a gel. Next, the prepared gel was dropped on the one flat end of the Tefloncoated graphite rod (4 mm diameter). The electrodes were dried at 60 °C for 6 h, and the deposited mass of the electrode material was approximately 1.0 mg. The electrochemical measurement, including cyclic voltammetry (CV), galvanostatic charge−discharge (GCD), and electrochemical impedance spectroscopy (EIS), has been carried out at room temperature, and ECLab software has been used to fit the EIS data. The specific capacitance (Csp) in F g−1 for all Ni-Mn-oxide electrodes in three-electrode system has been calculated from CV plots using eq 1

NMP solvent. Both the electrodes were prepared by uniformly coating on one side of a 2 cm × 3 cm steel plate. The electrodes were dried at 60 °C temperature for 4 h. Two electrodes were then sandwiched by a nonconducting separator (Whatman filter paper of pore diameter 25 μm) with 1 M Na2SO4 aqueous electrolyte. To solidify the electrolyte, the ASC device was kept at room temperature overnight. The charge balance theory (Q+ = Q−) has been adopted to calculate the mass ratio of the ASC electrodes (eq 3). Q+ = Q− m+ C × ΔV − = − m− C+ × ΔV+

Csp(F g ) =

where C refers to the specific capacitance (F g ), ΔV represents the potential window (V), and m indicates the mass of the electrode material on both the positive and negative electrodes (g).16,31 All electrochemical studies of ASC such as CV, GCD, and EIS have been carried out under ambient conditions, and the specific capacitance (Csp) (F g−1), energy density (Ecell) (W h kg−1), and power density (Pcell) (W kg−1) of this ASC device have been calculated using eqs 4−6

∫V c i(V ) dV a

2 × m × v × (Vc − Va)

(3) −1

V

−1

(2)

(1)

where m refers to the mass of the electrode (g), ν is the scan rate (V s−1), and (Vc − Va) represents the applied potential window.2,22,29,30 2.4. Electrochemical Performance of Asymmetric Supercapacitor (ASC) Device. To fabricate the solid-state asymmetric supercapacitor device (ASC), activated carbon (AC) and the synthesized Ni-Mn-oxide (Ni:Mn = 1:3) composite have been used as the negative and positive electrode materials, respectively. The negative electrode was prepared by mixing 95 wt % activated carbon and 5 wt % PVDF in NMP solvent. The positive electrode was also prepared in a similar way by mixing 85 wt % Ni-Mn-oxide nanoparticles, 10 wt % activated carbon, and 5 wt % PVDF in

Csp =

8259

I × Δt m × ΔV

(4)

Ecell =

1 × Csp × ΔV 2 2 × 3.6

(5)

Pcell =

3600 × Ecell Δt

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Figure 2. (a) TEM image, (b) HRTEM image, (c) SAED pattern, and (c) EDS elemental profile of Ni-Mn-oxide nanoparticle (Ni:Mn = 1:3). where I/m is the current density (A g−1), Δt is the discharge time (s), and ΔV is the applied potential window (V).1,2,17,32

D=

0.9λ β cos θ

(7)

where λ, β, and θ are the wavelength of the X-ray, full width at half-maximum, and Bragg diffraction angle, respectively.15 The average grain sizes are obtained to be 7, 10, 14, and 12 nm for Ni:Mn = 1:1, 1:2, 1:3, and 1:4, respectively, and these average grain sizes are also confirmed using the XPowder software. The FTIR spectra (Figure 1b) in the range 4000−400 cm−1 of all Ni-Mn-oxide with different molar ratios of Ni:Mn (1:1, 1:2, 1:3, and 1:4) show the appearance of strong band between 3000 and 3500 cm−1 corresponding to the stretching vibration of O−H group due to the physically adsorbed water molecule on the surface of the material and due to the presence of residual ethylene glycol.4,16,37 These physioadsorbed H2O molecules improve the electrochemical performance of the material due to an increase of wettability of the electrode material.2,5,38 The absorption bands at 2300 and 1760 cm−1 are observed due to the presence of C−O and CO stretching vibration in residual ethylene glycol, respectively. The band centered at 1402 cm−1 reveals the presence of stretching vibration of CO2 adsorbed from the atmosphere. Two main characteristic peaks at 474 cm−1 and around 535 cm−1 correspond to the presence of Ni−O and Mn−O stretching bonds in Ni-Mn-oxide nanomaterials.39−43 The FESEM images (Figure 1c−f) of all Ni-Mn-oxide mesoporous nanostructures show that they are irregularly selfassembled into a spherical nanostructure with diameters varying from 20 to 30 nm, as shown in the particles distribution curve (Figure 1c−f, inset). Each material is composed of numerous tiny Ni-Mn-oxide nanoparticles, and these interlinked nanoparticles form ample and porous structure. Moreover, these porous structures along with high surface area also increase the effective contact area for electrolyte ions at the electrode−electrolyte interface. This shortens the ion-diffusion path as well as facilitates the fast redox reaction.4,44 On the other hand, the interlinked nanoparticles enhance the efficient electron transport, which impressively improved the supercapacitive performance. To investigate the surface properties of Ni-Mn-oxide nanostructure, the N2 adsorption−desorption isotherms and

3. RESULTS AND DISCUSSION 3.1. Structural Studies. The structure, phase, and purity of the prepared powder samples have been analyzed from Xray diffraction patterns (Figure 1a) of Ni-Mn-oxide with different molar ratios of Ni:Mn = 1:1, 1:2, 1:3, and 1:4. The diffraction peaks due to the presence of different phases have also been assigned with different symbols. The Bragg diffraction peak positions and broadening of these observed XRD patterns have been identified and compared to the standard inverse cubic spinel structure of NiMn2O4 pattern (JCPDS card no. 71-0652).33 The diffraction peaks at 2θ = 18.38, 30.32, 35.98, 43.52, 57.56, 63.01, and 76.01° are observed, which correspond to the diffraction planes (111), (200), (311), (400), (511), (440), and (533) due to the presence of NiMn2O4 phase. There is one peak of NiO (JCPDS card no. 47-1049) observed at 2θ = 37.23° corresponding to the (111) plane for Ni:Mn = 1:1.34 The XRD peaks appeared to be sharper as the molar ratio of Mn is increased. Almost same XRD pattern has been followed by Ni:Mn = 1:2 and 1:4 materials, but a different XRD pattern is observed for Ni:Mn = 1:3. The presence of NiMnO3 (JCPDS card no. 75-2089) and Mn2O3 (JCPDS card no. 2-896) phases with NiMn2O4 is observed for this nanomaterial with molar ratio of Ni:Mn = 1:3, which may be due to the synergistic effect of both oxide materials at a particular molar ratio of Ni:Mn.14,35 The formation of distinct crystallite phase significantly depends on the annealing temperature of the nanomaterials. To keep the desired nanosized (∼ below 30 nm) electrode material, in this work, the synthesized samples have been annealed at 400 °C. The existence of different phases with low crystallinity may be due to relatively low annealing temperature of the materials.36 The XRD patterns suggest that the formation of Ni-Mn-oxide nanomaterials occurs at a suitable annealing temperature. The average grain size of the nanomaterials has been determined from the most intense peak using the well-known Debye−Scherrer formula, eq 7 8260

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any other impurity. The high-resolution Ni 2p core-level XPS images (Figure 3b) reveal two major peaks at binding energies 854.9 and 872.2 eV of Ni 2p3/2 and Ni 2p1/2, respectively.16,46 Similarly, the deconvoluted Mn 2p spectra (Figure 3c), located at 642.6 and 653.4 eV, correspond to Mn 2p3/2 and Mn 2p1/2. The lower binding energy of O 1s band (Figure 3d) consists of three components around 529.8, 531.2, and 532.8 eV.1,16 The components existing at 529.8 and 531.2 eV correspond to lattice oxygen (Oβ, i.e., O2− ions), and the other peak at 532.8 eV can be assigned to the physisorbed and chemisorbed surface-adsorbed oxygens (Oα, i.e., O22−, O−, OH− species). The presence of Ni and Mn in atomic ratio Ni:Mn = 27.16:72.84, or approximately 1:3 has, also been confirmed by CASA XPS software. 3.2. Electrochemical Studies. The CV curves (Figure 4a−d) for all electrodes in the potential range of −0.2−1.3 V display typical pseudocapacitive behaviors with distinct redox peaks due to Mn3+ ↔ Mn4+ and Ni2+ ↔ Ni3+.47−49 Figure 4a− d represents the CV curves of Ni-Mn-oxide nanomaterials at different molar ratios of Ni:Mn (1:1, 1:2, 1:3, and 1:4) electrode at various scan rates varied from 2 to 100 mV s−1. It has been observed that the current enhanced with increasing scan rate and the shape of the CV curves follows almost same nature. The same patterns with scan rates for all samples signified the existence of surface redox reactions and pseudocapacitive behaviors due to the faradic redox reaction processes (Ni2+ ↔ Ni3+, Mn3+ ↔ Mn4+) related to eq 827,50

the corresponding pore size distribution curves (see Figure S1 for BET images) show typical type IV isotherm, which corresponds to the mesoporous nature.45 The Brunauer− Emmett−Teller (BET) surface area of all samples have been calculated to be 43.63, 50.96, 127.09, and 61.78 m2 g−1, respectively for Ni:Mn = 1:1, 1:2, 1:3, and 1:4. These results also exhibit that the BET surface area of the material increases with increase of Mn molar ratio and reach maximum (127.09 m2 g−1) for Ni:Mn = 1:3, and then decrease with further increase in the Mn molar ratio. The corresponding BJH pore size distribution (see Figure S1 inset) shows average pore sizes of 5.1, 13.6, 8.2, and 17.2 nm, which confirm the mesoporous nature. This high surface area (127.09 m2 g−1) with average pore size 8.2 nm for Ni:Mn = 1:3 could effectively enhance the electrochemical performance providing a large number of electrochemically active sites as well as allow better penetration of electrolyte ions for supercapacitor applications.1,2,17 This is the main factor for which the Ni:Mn = 1:3 nanomaterial offers the highest specific capacitance value. The morphology of Ni-Mn-oxide (Ni:Mn = 1:3) powder has further been confirmed using TEM images (Figure 2a). This TEM image shows nearly dispersed Ni-Mn-oxide nanoparticles with the mean size of 16 nm. The high-resolution TEM (HRTEM) image (Figure 2b) shows that the lattice fringes spaced by 0.25 nm correspond to the (400) plane of face-centered phase of Ni-Mn-oxide (Ni:Mn = 1:3) due to the presence of NiMn2O4 phase, as shown in the XRD pattern. The selected area diffraction pattern (SAED) (Figure 2c) confirms the possible presence of different planes in Ni-Mn-oxide powder and which is also consistent with XRD analysis. The EDS profile (Figure 2d) also clearly reveals the presence of Ni, Mn, and O in the as-prepared Ni-Mn-oxide powder with almost Ni:Mn = 1:3. The XPS analysis has been further carried out to examine the chemical composition and chemical state of Ni-Mn-oxide (Ni:Mn = 1:3). The survey spectra (Figure 3a) of Ni:Mn = 1:3 mainly show the presence of Ni, Mn, and O species without

NiMn2O4 + Na + + e− ↔ NaNiMn2O4

(8) +

Equation 8 clearly shows that during charging time, Na ions from electrolyte diffuse into the electrode material (∼0.41 V vs Ag/AgCl) and absorb one electron, and subsequently during discharge time, Na+ ions are freed from electrode material (∼0.29 V vs Ag/AgCl) to the electrolyte.16,51 The oxidation/ reduction peaks for Ni2+/Ni3+ and Mn3+/Mn4+ for these NiMn-oxide nanoparticles are also located at ∼0.64/0.47 and 0.94/0.87 V vs Ag/AgCl, respectively. On the other hand, with increasing scan rates, the anodic peaks shifted toward higher positive potential and cathodic peaks shifted toward lower negative potential due to the polarization of electrode and an interfacial resistance effect of the electrode materials at higher scan rates.4,5 The comparative CV curves (see Figure S2) at 40 mV s−1 scan rate of all Ni-Mn-oxide electrodes reveal that all samples demonstrate pseudocapacitive performance. Remarkably, the capacitive current and capacitive behavior of the Ni:Mn = 1:3 electrode material is higher than that of other NiMn-oxide electrodes, which suggests that the electrode (Ni:Mn = 1:3) possesses the highest capacitance value and charge storage capacity due to synergistic effects between nickel oxide and manganese oxide and high specific surface area. The specific capacitances (Csp) for all Ni-Mn-oxide electrodes have been calculated using eq 1, and the maximum Csp values obtained were ∼488.5, 767.4, 1215.5, and 547.7 F g−1 at 2 mV s−1 for Ni:Mn = 1:1, 1:2, 1:3, and 1:4 nanostructures, respectively. The variation of Csp vs scan rates (ν) plots (Figure 5a) demonstrated that the Csp value increases first with increasing Mn molar ratios to the highest value 1215.5 F g−1 for Ni:Mn = 1:3, and then decreases. Csp also decreases with increasing scan rates for all electrodes. The increase of Csp with Mn molar ratio is achieved due to the increase of specific surface area with the molar ratio, with the highest being for the Ni:Mn = 1:3 nanostructure (127.09 m g−1). The increased surface area meritoriously enhances the charge storage

Figure 3. XPS images of Ni-Mn-oxide nanoparticle with Ni:Mn = 1:3 (a) survey spectrum, (b) Ni 2p, (c) Mn 2p, and (d) O 1s. 8261

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Figure 4. CV curves at different scan rates Ni-Mn-oxide nanoparticles for (a) Ni:Mn = 1:1, (b) Ni:Mn = 1:2, (c) Ni:Mn = 1:3, and (d) Ni:Mn = 1:4.

Figure 5. (a) Specific capacitance vs scan rate, (b) log(ip) vs log(ν), (c) CTotal vs ν1/2, and (d) Cout vs ν−1/2 of Ni-Mn-oxide nanoparticles.

diffusion-controlled.1,4 The obtained b values of Ni-Mn-oxide nanostructures from Figure 5b are 0.64, 0.72, 0.84, and 0.80 for Ni:Mn = 1:1, 1:2, 1:3, and 1:4, respectively, which indicates that the charge store is due to a combination of surfacecontrolled and diffusion-controlled processes. Usually, for an electrode, the total Csp is the sum of both contributions such as due to surface-controlled capacitance (Cout) and diffusioncontrolled capacitance (Cin), which can be expressed as eq 10 according to the Trasatti relation2,52

mechanism, providing a large number of redox active sites as well as allows better penetration of electrolyte ions. To investigate the charge storage mechanism of Ni-Mnoxide with different molar ratios of Ni:Mn = 1:1, 1:2, 1:3, and 1:4, the relationship between scan rate (ν) and anodic peak current (Ip) has been analyzed using the well-known power law expressed as eq 9 Ip = a × v b

(9)

where a and b are adjustable parameters; b = 1 indicates that the charge storage mechanism is mainly surface-controlled, while b = 1/2 represents that the charge storage mechanism is

C Total = Cin + Cout(F g −1) 8262

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Figure 6. GCD plots at different current densities of Ni-Mn-oxide nanoparticles for (a) Ni:Mn = 1:1, (b) Ni:Mn = 1:2, (c) Ni:Mn = 1:3, and (d) Ni:Mn = 1:4.

Figure 7. (a) Specific capacitance vs current density, (b) retention for Ni:Mn = 1:3, (c) EIS plots, and (d) Bode plots with equivalent circuit (inset) of Ni-Mn-oxide nanoparticles.

respectively. Hence, the diffusion-controlled process contributes significantly over the surface-controlled process to the total charge storage.1,17 The galvanostatic charge/discharge (GCD) cycles (Figure 6) at different current densities (4.0, 6.0, 8.0, and 10.0 A g−1) for four electrodes show typical faradic behavior of charge storage process. The discharge curves of all electrodes are almost symmetric in nature with a small IR drop at the beginning of discharge, implying good redox reversibility. The initial IR drop is very low for Ni:Mn = 1:3 observed from the GCD curve (Figure 6c), which confirms the good contact between active material and the current collector. As clearly

At higher scan rates, due to time restrictions, the diffusion of electrolyte ions inside the electrode pores is limited, and as a result, charge storage only depends on the outer surface of the electrode. Consequently, at lower scan rates, the specific capacitance due to inner contribution (Cin) involves the effect of diffusion and redox reactions of electrolyte ions. Thus, the contribution of both Ctotal and Cin can be determined by extrapolating the Csp vs ν1/2 at ν → 0 (Figure 5c), while Cout is obtained at a high scan rate ν → ∞ (Figure 5d) from the Cout vs ν−1/2 plot. The calculated CTotal values are 492.7, 724.3, 1248.6, and 509.1 F g−1, and Cout are 76.3, 173.6, 620.1, and 146.2 F g−1 for Ni:Mn = 1:1, 1:2, 1:3, and 1:4 electrodes, 8263

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Langmuir Table 1. Values of Different Charge Transfer Parameters sl. no.

sample name Ni:Mn

Rs (Ω)

Rct (Ω)

CPE (mF cm−2)

n (0 < n < 1)

Wo (Ω s−1)

Cp (F cm−2)

1. 2. 3. 4.

1:1 1:2 1:3 1:4

1.29 0.99 0.33 0.52

5.52 3.01 2.39 2.62

0.586 0.136 0.815 0.291

0.62 0.42 0.68 0.48

6.835 1.785 0.979 2.759

0.331 0.276 0.679 0.2118

Figure 8. (a) CV curves at different scan rates, (b) GCD curves at different current densities, (c) Csp vs current density, (d) Ragone plot with comparative study, (e) EIS plots before and after cycling with equivalent circuit (inset), and (f) Coulombic efficiency with practical application (inset) of ASC device of Ni-Mn-oxide nanoparticles of Ni:Mn = 1:3.

To discuss the conductive behavior of the as-prepared NiMn-oxide electrodes, the EIS study has been carried out in the frequency range of 0.01 Hz to 100 k Hz in 1 M Na2SO4 electrolyte with an AC perturbation amplitude of 10 mV at a fixed dc potential of 0.55 V. The EIS plots for electrodes (Figure 7c) show a depressed semicircle at high frequency and a straight line in the low-frequency region. The diameter of the semicircle usually represents the charge transfer resistance (Rct), while the straight-line portion in the low frequency is attributed to the frequency-dependent diffusion control Warburg impedance (Wo).53,54 The intersection with the real Z-axis (Z′- axis) corresponds to the electrochemical series resistance (Rs), including the intrinsic resistance of the active material, electrolyte resistance, and contact resistance between the electrode and the current collector. The equivalent circuit model corresponds to the experimental data (Figure 7d, inset), which show that a constant phase element (CPE) is introduced in parallel with the series combination of Rct and W0 to fit impedance data by taking into account the porous nature of the electrode material, inhomogeneity of the electrode surface, and charge distribution allied with electrode geometry. The porous nature of the electrode has an important contribution to the electrochemical charge storage process, which deviates from the performance of the pure capacitive behavior. However, for practical purpose, the capacitive performance of an electrode could be sensibly represented as CPE in parallel with Rct.55,56 The fitted charge transfer parameters obtained from the equivalent circuit model are listed in Table 1. The Rs values of Ni-Mn-oxides are 1.29, 0.99, 0.33, and 0.52 Ω, respectively, for Ni:Mn = 1:1, 1:2, 1:3, and 1:4, signifying their

observed in CV results, the Ni-Mn-oxide (Ni:Mn = 1:3) shows a longer discharge time than other three samples. The Csp (F g−1) values for all electrodes have been calculated from GCD curve using eq 111,4,5 Csp =

i

(

dV

m × − dt

)

(F g −1) (11)

where i is the applied current density and dV/dt is the average slope of the discharge curve. The Ni-Mn-oxide electrodes with different molar ratios (Ni:Mn = 1:1, 1:2, 1:3, and 1:4) achieve maximum capacitances of 269.5, 342.7, 1009.2, and 531.3 F g−1 at 4 A g−1 current density. It has also been observed from Figure 7a that the Csp first increases with increase of Mn molar ratio, becomes maximum (1009.2 F g−1) for Ni:Mn = 1:3, and then decrease, which is well consistent with CV results. This highest specific capacitance is attributed to the nearly full redox reaction provided by the high specific surface area of the Ni:Mn = 1:3 electrode. This GCD result is also consistent with CV results. The cycling stability of Ni-Mn-oxide electrode (Ni:Mn = 1:3) has been studied at a fixed current density of 10 A g−1 for 5000 cycles, as shown in Figure 7b. This electrode shows 87.2% capacity retention after 5000 cycles, indicating excellent cycling stability with good electrical conductivity. In this case, the increase of specific capacitance at the first few cycles is observed due to the activation effect of the electrode material and an increase of mobility of the surface charge and electrolyte ions.16 Next, capacitance gradually degrades to a stabilized level up to 87.2%. 8264

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NiMn2O4/rGOH and [email protected] The EIS images of this device before and after 3000 cycles (Figure 8e) show that the impedance increases sharply and becomes almost vertical parallel to imaginary y-axis in the low-frequency region, which demonstrates the pure capacitive behavior of the ASC device. The small semicircular portion in the high-frequency region represents the charge transfer resistance (Rct) at electrode− electrolyte interface combined with intrinsic resistance (Rs) due to ionic resistance of electrolyte and intrinsic resistance of the current collector. The equivalent circuit corresponding to the EIS data (Figure 8e, inset) shows that a slight increase of Rct from 4.2 to 5.6 Ω is observed after 3000 cycles. Interestingly, this Ni-Mn-oxide electrode-based ASC device exhibits excellent cycling stability and columbic efficiency of 97% over 3000 cycles (Figure 8f), which is one of the key factors to be a promising supercapacitor electrode for practical applications.

low resistance. Compared to the other three electrodes, the Ni:Mn = 1:3 electrode offers a smaller charge transfer resistant (Rct) value (2.39 Ω), suggesting faster charge transfer properties. The lower value of Wo of Ni:Mn = 1:3 also determines better diffusion of electrolyte ions within the electrode pores, and the higher slope compared to the other three electrodes at low frequency specifies better capacitive performance. The capacitance due to redox reaction is represented by Cp. The Bode phase plot of all electrodes (Figure 7d) also shows that the phase angles approach the maximum values of 66.72, 71.52, 73.24, and 75.29° for Ni:Mn = 1:1,1:2, 1:3, and 1:4, respectively, at 0.01 Hz. The phase angle generally approaches 90° for ideal capacitor at low frequency, whereas it becomes less than 90° for redox material. The obtained phase angle at low frequency concludes that the specific capacitance arises mainly due to the redox nature with a small contribution of surface-controlled double-layer process.4,37,57 The relaxation time constant (τ0) values calculated (using τ0 = 1/2πf) at phase angle 45° where the capacitive and resistive impedances are equal are 0.85, 0.60, 0.82, and 0.46 s for Ni:Mn = 1:1, 1:2, 1:3, and 1:4, respectively. These τ0 values also indicate how fast the electrochemical energy is stored and efficiently distributed. These relatively low values of τ0 signify high energy storage with high rate capability.58,59 3.3. Asymmetric Supercapacitor (ASC). Cyclic voltammetry (CV) has been first carried out at suitable applied potential windows (see Figure S3a) to estimate the stable potential windows of positive and negative electrodes for asymmetric supercapacitor (ASC) application using threeelectrode systems in 1 M Na2SO4 electrolyte. The ASC device potential can be achieved as the sum of potential windows of negative electrode (AC) and positive electrode (Ni-Mn-oxide). Consequently, the maximum stable potential window has been extended up to 2.2 V in 1 M Na2SO4 electrolyte (see Figure S3b). It is also essential to balance the charge stored at two electrodes. The mass ratio between negative and positive electrodes, which has been calculated using eq 3, is 1.4. A wide potential window (0−2.2 V) has been selected for this ASC device to investigate the electrochemical performance of this device and remarkably enhance the energy density proportional to the square of the applied potential. The CV curves (Figure 8a) of this ASC device show a relatively rectangular shape at different scan rates from 2 to 100 mV s−1, indicating a promising supercapacitive nature. The GCD curves (Figure 8b) at different applied current densities also demonstrate a good supercapacitive behavior. The maximum specific capacitance (Csp) of the ASC device has been calculated from GCD plots of 196.5 F g−1 at a current density of 1.5 A g−1 based on the total mass of the active material in the two electrodes. The variation of Csp with different current densities plot (Figure 8c) represents that the capacitance value decreases with increase of current density due to time restriction of diffusion of electrolyte ions. The Ragone plots (Figure 8d) of this ASC device shows that this device achieves maximum specific energy density (Ecell) and power density (Pcell) of 132.33 W h kg−1 and 6054.8 W kg−1, respectively, and comparison with other supercapacitor electrodes performance. More significantly, this ASC can deliver a higher energy density and power density than previously reported supercapacitors, such as (A) reduced graphene oxide, NiMn2O4, and polyaniline (GNMOP);60 (B) NiMnO3-rGO;61 (C) Hierarchical NiMn2O4@CNT;62 (D) graphene oxide/Mn3O4;63 and (E)

4. CONCLUSIONS In conclusion, we have successfully synthesized porous Ni-Mnoxide nanoparticles with different molar ratios of Ni:Mn = 1:1, 1:2, 1:3, and 1:4 utilizing glycol-mediated facile sol−gel technique. The overall electrochemical performance of these electrodes shows that the molar ratio of Ni:Mn influences significantly the charge storage performance of Ni-Mn-oxide. The specific capacitance first increases with increase of Mn molar ratio and attained maximum (1215.5 F g−1) for Ni:Mn = 1:3 due to increase of the BET surface area, providing large numbers of active sites and efficient pathways for fast electron transport at the electrode surface. More importantly, an asymmetric supercapacitor (ASC) assembled from Ni-Mnoxide/steel plate and AC/ steel plate offers a high specific energy density (Ecell) of 132.33 W h kg−1 and power density (Pcell) of 6054.8 W kg−1. Furthermore, this ASC device shows excellent cycle lifespan over 3000 cycles with 97% columbic efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b00955. Nitrogen adsorption/desorption isotherms (Figure S1); CV curve of Ni-Mn-oxide electrodes (Figure S2); and CV curves of negative electrode and positive electrodes, and at different applied potential windows of ASC device (Figure S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], sachindas15@ gmail.com. Tel.: +91 3324572965. ORCID

Monalisa Ghosh: 0000-0002-9417-2940 T. Maiyalagan: 0000-0003-3528-3824 Swapan Kumar Bhattacharya: 0000-0002-1218-1860 Sachindranath Das: 0000-0002-6938-6701 Notes

The authors declare no competing financial interest. 8265

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ACKNOWLEDGMENTS A. Ray (File no. 09/096(0927)/2018-EMR-I) and S. Saha (File no. 09/096(0898)/2017−EMR-I) are grateful to CSIR, Government of India, for financial support. S. Das acknowledges the Department of Science and Technology (DST), Government of India, for providing research support through the “INSPIRE Faculty Award” (IFA13-PH-71) and research grant from RUSA 2.0, Jadavpur University (Ref no. R-11/281/ 19). A. Roy (IF140920) acknowledges the Department of Science and Technology (DST), INSPIRE, Government of India, for providing research support through the “INSPIRE Fellowship”.

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