Optimizing Energy Storage Performance: In situ Synthesized Manganese Oxide (Mn 3 O 4 ) Nanoparticle-Based Symmetric Supercapacitor on a Paper Substrate

In this study, a redox reaction is employed to synthesize manganese oxide (Mn 3 O 4 ) nanoparticles using potassium permanganate as a precursor in the presence of diethyl amine. The structural characterization reveals the formation of the tetragonal phase of Mn 3 O 4 nanoparticles with a space group of I41/amd. A free-standing Mn 3 O 4 -based paper electrode is fabricated and its electrochemical performances are investigated. The electrode exhibits a maximum specific capacitance value of ~353 Fg (cid:0) 1 and an areal capacitance of ~530 mFcm (cid:0) 2 at a current density of 0.2 Ag (cid:0) 1 . A symmetric supercapacitor-based device is also designed using Mn 3 O 4 nanoparticles as an active material in a gel electrolyte configuration. The Mn 3 O 4 device achieves specific and areal capacity values of ~208 mAhg (cid:0) 1 and 260 mAcm (cid:0) 2 , respectively, at a current density of 0.3 Ag (cid:0) 1 . The device delivers maximum energy and power density values of ~104 Whkg (cid:0) 1 and ~220 Wkg (cid:0) 1 , respectively, with ~92% specific capacity retention at 0.3 Ag (cid:0) 1 after 5000 cycles. The above results suggest that the Mn 3 O 4 -based device has the potential for energy storage applications.


Introduction
Electrochemical supercapacitors, with their impressive versatility in a broad range of energy storage applications, have garnered significant attention. [1,2]Supercapacitor based devices are capable to deliver high power density, superior charge-discharge cycles and capacitance retention ability. [3]Supercapacitors offer high value of energy and power densities that reduced the gap between the conventional dielectric capacitors and batteries. [3,4]8][9][10] The paper based electrodes display various applications in self-powered generators, [8] supercapacitors, [11] batteries, [12] memory devices, [13] thermoelectric generator [14] and also received much importance for fundamental studies. [15,16]ased on charge storage mechanism, the supercapacitors are categorized into two different types, such as, electrical double layer capacitor and pseudocapacitor. [3]Different carbon based materials such as, carbon nanotube, graphene and activated carbon are considered as excellent electrode material for electrical double layer capacitor, which produce limited value of specific capacitance. [3,17]Whereas, in pseudocapacitors, redox reaction occurs on the surface or near the surface region of the active electrode material and enhance the value of specific capacitance.
Transition metal oxides are extensively applied for pseudocapacitor application due to their variable oxidation states, large surface area, excellent chemical stability and high electrical conductivity. [18,19]Manganese oxide based electrode material revealed promising candidate for pseudocapacitor in the form of various crystallographic structure such as MnO, MnO 2 , Mn 2 O 3 and Mn 3 O 4 .Among the various manganese oxides, Mn 3 O 4 exist as a mixed valence state of Mn 2 + /Mn 3 + with spinel structure. [20]Beside the supercapacitor study, Mn 3 O 4 nano material exhibits range of applications, such as, photocatalysis, [21] dye degradation, [22] photoluminescence, [23] spintronics [24] and memory devices. [25]Various methods have been applied to optimize the electrochemical performances of the fabricated Mn 3 O 4 electrode. [20][28][29][30] One-step reduction protocol was reported for the synthesis of reduced graphene oxide-Mn 3 O 4 nano composite and exhibited specific capacitance value of 243 F g À 1 at 0.5 A g À 1 with higher charge-discharge stability. [31]Mn 3 O 4 -graphene oxide nanocubes showed the energy and power density value of 168 Wh kg À 1 and 675 W Kg À 1 , respectively. [32]Mn 3 O 4 micro-cube based electrode exhibited enhanced the supercapacitor performance with fast charge-discharge rate. [33]ollow structured cube shaped Mn 3 O 4 with metal organic framework provided more redox active sites for ion exchange that improved the electrochemical performances with better cycling stability. [34]Mn 3 O 4 nanoparticle based asymmetric device achieved the specific capacitance value of 144 F g À 1 at 0.5 A g À 1 with energy and power density of 3.33 Wh kg À 1 and 422.5 W Kg À 1 , respectively. [35]Various morphologies of Mn 3 O 4 such as, nanorods, [36] nanowire, [37] and nanofibers [38] showed high value of specific capacitance, energy density and power density with long lifespan.Polyaniline supported Mn 3 O 4 nanoparticles exhibited the specific capacitance value of 352 F g À 1 at 0.5 A g À 1 with 90 % of capacitance retention after 10 4 cycles. [39]aper based asymmetric device prepared with Mn 3 O 4 delivered specific capacitance value of 471 F g À 1 at the current density 1.0 mA cm À 2 .The above device exhibited energy and power density 47 Wh kg À 1 and 202.5 W Kg À 1 , respectively, with excellent charge-discharge stability. [40]otassium permanganate is a powerful oxidizing agent.The oxidation of organic molecule by KMnO 4 and the corresponding reduction of KMnO 4 , with the formation of various types of manganese oxides based system such as MnO, MnO 2 , Mn 2 O 3 and Mn 3 O 4 , are well documented in the literature. [41]In this work, an in-situ, single pot reaction was performed for the synthesis of organic molecule stabilized Mn 3 O 4 nanoparticles.During the reaction between diethylamine and KMnO 4 yielded tetraethyl hydrazine, oxidative product of diethylamine, and potassium permanganate was reduced with the formation of Mn 3 O 4 nanoparticles.The synthesized Mn 3 O 4 material was characterized by X-ray diffraction, transmission electron microscope, X-ray photoelectron spectroscopy and Raman spectroscopic techniques.The as synthesized Mn 3 O 4 nanoparticle based gold-coated paper electrode was fabricated to investigate the electrochemical properties of the material.A symmetric device based on Mn 3 O 4 nanoparticle demonstrated superior specific capacitance and energy storage performances.

Result and Discussion
Phase composition of Mn 3 O 4 was investigated using X-ray diffraction technique within the range (2θ) from 15°to 80°, Figure 1A.The majority of the diffracted peaks are matched with the tetragonal phase of Mn 3 O 4 structure, space group I41/ amd (no.141), with lattice constant values of a = b = 5.76 Å, c = 9.45 Å, where α = β = γ = 90°(ICDD: 01-089-4837, blue colour bar) and arranged in [Mn(II)Mn(III) 2 O 4 ] spinal like structure. [42,43]t was reported that an intermediated phase of MnOOH was formed during the growth of tetragonal Mn 3 O 4 nanoparticles. [20,34]The diffracted peak position at 25.8°(green colour symbol) belongs to the orthorhombic phase of MnOOH structure (ICDD: 00-024-0713).The formation of sharp and well defined diffraction peaks revealed the crystalline nature of the synthesized nanoparticles.Figure 1B shows the unit cell structure of tetragonal Mn 3 O 4 nanoparticles aligned along the b-axis.In this spinal structure, two different oxidation states of manganese cation, Mn(II) and Mn(III), occupy in tetrahedral and octahedral sites, respectively. [42]igure 2A shows deconvoluted Raman spectrum of Mn 3 O 4 , recorded within the frequency range from 20 to 1450 cm À 1 .Nine Raman active modes (ν 1 -ν 9 ) were extracted from the spectrum, (a) below 500 cm À 1 (ν 1 -ν 4 ), blue colour region, (b) between 500-700 cm À 1 (ν 5 -ν 8 ), green colour region and (c) above 1000 cm À 1 (ν 9 ), purple colour region.The high intensity Raman active mode positioned at ~658.8 cm À 1 (ν 7 ) corresponds to the symmetric MnÀ O vibration of Mn(II) cation in the tetrahedral coordination (MnO 4 ), the characteristic feature of Mn 3 O 4 spinal structure. [44,45]The distortion in the tetrahedral unit is responsible for the origin of low intensity peaks positioned at ν 3 ~289.5 cm À 1 , ν 4 ~374.2cm À 1 and ν 5 ~485.2cm À 1 , due to the asymmetric vibration of MnÀ O bond. [46]The Mn 3 O 4 structure consists of mixed valence state with the combination of Mn(II)O 4 (tetrahedra) and Mn(III)O 6 (octahedra).Large variation of the MnÀ O bond length is due the presence of Mn 3 + cation, resulted distorted octahedra unit in the orthorhombic structure of MnOOH.[48] The vibrational bands at ~568.2 cm À 1 (ν 6 ) and ~723.7 cm À 1 (ν 8 ) are attributed to the stretching modes of MnO 6 octahedra.Broad diffused peak at ~1188.1 cm À 1 (ν 9 ) corresponds to the Mn-OÀ H structural vibration from the intermediate phase of MnOOH. [48]he oxidation state and elemental composition of the synthesized Mn 3 O 4 nanoparticles was investigated by X-ray photoelectron spectroscopy (XPS) technique.The survey spectrum, Figure 2B, exhibited the elemental peaks of manganese, carbon, nitrogen and oxygen.The C1s spectrum, Figure 2B (inset), deconvoluted into two peaks positioned at ~284.50 and ~285.43 eV, credited to the binding energy of CÀ C and CÀ N bond, respectively, originated from tetraethyl hydrazine stabilizer.For Mn 3 s spectrum, two peaks positioned at ~84.0 eV and ~89.5 eV, splitting width ~5.5 eV, Figure 2C, is consistent with Mn 3 O 4 system. [43]In Mn 2p spectrum, Figure 2D, two prominent peaks at ~641.8 eV and 653.2 eV was noticed, correspond to the Mn 2p 1/2 and Mn 2p 3/2 , respectively, with the spin orbit splitting of ~11.4 eV, consistent with Mn 3 O 4 system. [43]Two peaks positioned at 529.6 eV and 530.7 eV in the deconvoluted O 1s spectrum, Figure 2E, associated with MnÀ OÀ Mn and MnÀ OH bond interaction.The N 1s spectrum, Figure 2F, analysis revealed two peaks, positioned at ~399.3 eV and ~401.1 eV, generated from CÀ N and NÀ N bonds, respectively, originated from organic molecule.Figure 3A show the transmission electron microscopy images of the tetraethyl hydrazine stabilized Mn 3 O 4 particles and the dark spots revealed the formation of Mn 3 O 4 nanoparticles.The selected area electron diffraction (SAED) pattern (inset), indicate the crystalline nature of the Mn 3 O 4 nanoparticles.The particle size distribution is within the range of 8-15 nm, calculated from the histogram profile, Figure 3B.

Electrochemical Performance of Free Standing Mn 3 O 4 Electrode
The electrochemical performance of tetraethyl hydrazine stabilized Mn 3 O 4 nanoparticles was investigated on paper electrode by cyclic voltammetry (CV) method within the potential window from À 0.2 to 0.8 V, under different scan rates (20-200 mV s À 1 ), in presence of 6 M KOH electrolyte solution, Figure 4A.At the scan rate of 20 mV s À 1 , the redox active peaks appeared at 0.46 V (oxidation) and 0.08 V (reduction), in accordance with the oxidation state of Mn 2 + /Mn 3 + , which involved the intercalation and de-intercalation of electrolyte ion (K + ) on the surface and interior (Equation ( 1)) of the bulk Mn 3 O 4 .During the charging process, the K + ion captured and stored inside the Mn 3 O 4 spinal structure.These stored charges (K + ) returned to the KOH solution during discharge process and released the electron via the external circuit. [20,30,49]The other possible mechanism where Mn 3 O 4 electrode could react with KOH electrolyte through a pseudo-capacitive process via an intermediate step, Equation (2). [50] The redox peaks in CV the profile were shifted towards higher potential region with increasing the scan rate that revealed the irreversible redox process, Figure 4A.The electrode showed maximum current density value ~2.3A g À 1 under the scan rate of 200 Mv s À 1 .
Galvanostatic charge-discharge (GCD) profiles of Mn 3 O 4 based electrode, Figure 4B, was recorded under different current densities, varying from 3.0 to 0.2 A g À 1 , within the potential window from À 0.2 to 0.8 V.A sharp potential (IR) drop and followed by sluggish decay was observed towards the end GCD profile.Figure 5A display the graphical representation of specific capacitance (C p ) and areal capacitance (C a ) versus current density of the fabricated Mn 3 O 4 electrode.The electrode delivered maximum specific capacitance value of ~353 F g À 1 and areal capacitance ~530 mF cm À 2 , at current density of 0.2 A g À 1 , calculated using the relations, where I is current (mA), a is active surface area (cm 2 ), ΔV is voltage window (0.8 V) and Δt is the discharge time (s), extracted from GCD curves.At higher current density, ion interaction capability of the Mn 3 O 4 electrode was decreased due to fast kinetics resulted from lowering the specific capacitance value.At low current density, both inner and outer surface sites of the Mn 3 O 4 material are available for charge storage, hence improved the capacitance performance. [51]The material showed ~93 % retention of the initial capacitance value under the current density of 0.8 A g À 1 after 5000 cycles, Figure 5A, inset.][53][54][55] The current I ð Þ variation in the Mn 3 O 4 electrode is directly proportional to the applied scan rate (ν) and expressed in terms of I~aν b , [1][2][3]56,57] where both the parameters a and b are the adjustable. A liner fitting of log (I) versus log (ν) at 0.6 V produced the b-value ~0.64, Figure 6A, suggested the diffusion controlled process is predominant in the active material.The diffusion (inner) and capacitive (outer) current contributions of the active material at 0.6 V was extracted using the Equation, (5) (k 1 v and k 2 v 1=2 correspond to the surface capacitive and diffusion control term, respectively, k 1 and k 2 are constant).The values of k 1 (slope) and k 2 (intercept) were extracted by plotting I=v 1=2 as a function of v 1=2 , Figure 6B.The diffusive and capacitive contributions, in terms of percentage, of the Mn 3 O 4 based electrode is represented in Figure 6C, implied that diffusive control mechanism is dominated in the charge storage process.

Electrochemical Energy Storage Performance of Mn 3 O 4 Device
Symmetrical supercapacitor device was constructed using gel electrolyte, sandwiched between two Mn 3 O 4 based electrodes and tested using cyclic voltammetry technique within the voltage range from 0.0 to 1.0 V under different scan rates (20-200 mV s À 1 ), Figure 7A.GCD profile of the Mn 3 O 4 device was recorded under different current densities from 0.3 A g À 1 to 1.0 A g À 1 , Figure 7B, and a non-linear pattern revealed the pseudo-capacitive behaviour of the device.The specific capacity (Q s ) and areal capacity (Q a ) values of the device were calculated from CV curves under the condition of different current densities by using the following relations, where I is current, 'm' is the mass and 'a' is the active surface area of the Mn 3 O 4 electrode.The calculated specific capacity and areal capacity values are plotted in the Figure 8A. Figure 8A, inset, shows the variation of charge storage (q) as a function of current density in the device.Maximum charge storage value ~750 C g À 1 was achieved by the device at 0.3 A/g.Maximum values of Q s ~208 mAh g À 1 and Q a ~260 mA cm À 2 were achieved in the device at the current density of 0.3 A/g.The stability (chargedischarge) of the device was tested for 5000 cycles at 0.3 A g À 1 and the device retained ~92 % of the specific capacity at the end the cycle, Figure 8B.
The capacitive behaviour was originated from the surface controlled (outer, q 0 ) and diffusion-controlled (inner, q i ) charge components that contribute the total voltammetry charge of the device, q t v ð Þ ¼ q o þ q i .The total charge, q t v ð Þ, of the device was calculated from the following relation, when ffi ffi ffi v p ! 0, Figure 9A.The outer charge ðq o Þ was obtained from the relation, is the charge at a specific scan rate, Figure 9B.
The linear fitting and the y-axis intercept estimated the value of total charge ðq t Þ~1052 C g À 1 , outer charge q 0 ð Þ~221 C g À 1 and inner charge ðq i Þ~831 C g À 1 of the device.The results suggested the dominance of diffusion ðq i Þ control process (78.9 %), Figure 9C.The variation of device charge contributions (q i and q 0 ) versus scan rate is exhibited in Figure 9D.With rising the scan rate, the contribution from inner charge (q i ) was diminished because the electrolyte ions are not able to get sufficient time to reach the inner sites of the Mn 3 O 4 material.

Electrochemical Impedance Study of Free Standing Mn 3 O 4 Electrode and Device
The impedance property of the device was recorded within the frequency range from 200 kHz to 100 mHz. Figure 10A shows the Nyquist plot (Z' vs. Z") of the device.A significant difference was observed in the Nyquist plot for the Mn 3 O 4 device and freestanding Mn 3 O 4 electrode towards low frequency region.The vertical line is the characteristic of a capacitive behaviour of the device, indicates the fast ion diffusion. [1]For the freestanding electrode, the deviation in the low frequency capacitive behaviour (arc shape) was observed along the Z' axis that implies more resistive value of the electrode.The equivalent series resistance (R s ) values ~15.5 Ω and ~0.45 Ω were obtained for the free-standing electrode and device, respectively.The charge transfer resistance (R t ) of the device is ~25.0Ω, extracted from the diameter of the semicircle, Figure 10A, inset.The device exhibited high electrical conductivity and low charge transfer resistance as compared with the freestanding electrode, due to the longer ion diffusion path through the gel electrolyte.Figure 10B display the Bode plot, phase angle (ϕ) as a function of frequency, of the freestanding electrode and device.Bode plot for the device exhibited more    (A) Trasatti method was used to extract the total charge (q À 1 vs. υ 1/2 ) and (B) outer charge (q vs. υ À 1/2 )) contributions of the THMO device.(C) Outer (q o ) and inner (q i ) charge contributions (%) and (D) variation of device charges (q i and q o ) with respect to different scan rate.
angle value increased with increasing frequency.Capacitance (C) and frequency (f) data were extracted from the Nyquist plot using the equation, where, where, the Z" parameter corresponds to the imaginary part of the device impedance (Ω).The graphical representation of capacitance as a function of frequency for the Mn 3 O 4 based device is illustrated in Figure 10C and the device showed a maximum value of capacitance ~600 μF at 100 mHz. Figure 10D shows the variation of imaginary (C") and real (C') capacitance as a function of frequency.The imaginary and real capacitances are calculated from the equations, where, ω is angular frequency and Z is the electrical impedance of the device. [58]t lower frequency, electrolyte ions can easily access inside the active material that caused the high value of C'.At higher frequency, ions are not able to follow the time response hence decrease in C' value.The C" curve formed a peak like feature at a particular frequency (f 0 )~170 Hz, associated with the relaxation process of the device.The relaxation time (τ 0 = 1/f 0 ) of the device is 5.8 ms, the minimum time required for charge transfer of electrolyte ions at the interface (electrode/electrolyte) region of the Mn 3 O 4 material, which demonstrates that the device can be fully charged within 5.8 ms.
The energy density (E) and power density (P) performances of the Mn 3 O 4 device was calculated from GCD profile by using the following Equations, respectively, where R I2 I1 I � dt is area under the curve, m is the mass of Mn 3 O 4 material, V is potential window and Dt is the discharge time (s). Figure 11A display the variation of energy (E) and power (P) density as a function of current density.The device delivered maximum energy density value of ~104 Wh kg À 1 and a power density of ~84 W kg À 1 at 0.3 A g À 1 .
The maximum power density value of ~220 W kg À 1 was reached at 1.0 A g À 1 .The energy and power performances of the previously reported manganese oxide (Mn 3 O 4 ) based supercapacitor devices are summarized using a Ragone plot, Figure 11B.

Conclusions
This is a first kind of report on the synthesis of organic molecule stabilized Mn 3 O 4 nanoparticles through an in-situ redox reaction route under the ambient condition using KMnO and diethylamine as precursors, where KMnO 4 performed as an oxidizing agent.The oxidation of diethyl amine in presence of potassium permanganate produced tetraethyl hydrazine stabilized Mn 3 O 4 nanoparticles.The structural characterization confirmed the existence of tetragonal phase of Mn 3 O 4 with the space group of I4 1 /amd.Transmission electron microscope image revealed the nanoparticles dispersed within the organic matrix.X-ray photoelectron spectroscopy study confirmed the elemental composition and oxidation state of the synthesized nanoparticles.The electrochemical performance of the synthesized Mn 3 O 4 nanoparticle was investigated on paper based electrode under aqueous and solid state electrolyte environments.The fabricated free standing Mn 3 O 4 based paper electrode exhibited the maximum value of specific capacitance ~353 F g À 1 and areal capacitance ~530 mF cm À 2 , at current density of 0.2 A g À 1 , with an excellent retention of specific capacitance ~93 % after 5000 cycles at 0.8 A g À 1 .A symmetric supercapacitor device was constructed via two-electrode system by using Mn 3 O 4 nanoparticles as an active material in solid-state gel electrolyte arrangement.The device achieved specific capacity and areal capacity value of ~208 mAh g À 1 and 260 mA cm À 2 , respectively, at the current density of 0.3 A g À 1 .The device delivered maximum energy density value of ~104 Wh kg À 1 and power density values of ~220 W kg À 1 with specific capacity retention of ~92 % at 0.3 A g À 1 for 5000 cycles.The results suggested that the organic molecule stabilized Mn 3 O 4 nanoparticle based system enables it to be incorporated in energy storage systems for renewable energy application.

Experimental Material Synthesis
Synthesis process was performed using commercially available potassium permanganate, diethylamine and methanol without any additional purification.
During the synthesis process, 1.5 mL of diethylamine was diluted with 10 mL of methanol in a beaker.Subsequently, an aqueous solution of KMnO 4 (0.2 M) was added under the stirring condition to the diluted diethylamine.In presence of KMnO 4 , the diethylamine was oxidized and form tetraethyl hydrazine (confirmed by

Material Characterization
The X-ray diffraction study was performed using Philips PANalytical X'pert Pro diffractometer with Cu-Kα target.The microscopic analysis was performed using a JEOL (JEM-2100) Transmission Electron Microscope (TEM) analytical instrument.Raman spectrum was recorded using Jobin-Yvon T64000 spectrometer under 514.5 nm excitation of an argon ion laser source.X-ray photoelectron spectroscopy (VG Multilab 2000) technique was applied to investigate the oxidation state of the product.The electrochemical properties were investigated by using Bio-Logic, SP-300.Electrochemical impedance spectroscopy was recorded under varying frequency from 200 kHz to 100 mHz with an amplitude of 5 mV in open circuit voltage.

Fabrication of a Free Standing Mn 3 O 4 Based Electrode
A homogeneous slurry was prepared using Mn 3 O 4 (active material, 90 wt %) and polyvinylidene fluoride (binder, 10 wt %) in presence of N-methyl pyrrolidinone as a solvent.Initially, a printing paper (dimension: 5 cm×1 cm) was treated with polyvinyl alcohol (4 wt %) for 30 minute and dried for an hour in an oven.A conducting layer of gold with the thickness of ~100 nm was coated on both side of the polyvinyl alcohol treated paper by using physical vapour deposition technique (EMSCOPE SC 500).The electrical resistivity of the gold coated paper was achieved ~30 mΩ/cm.The as prepared Mn 3 O 4 based slurry (~3.0 mg) was deposited on both side of the gold coated paper (dimension: 1 cm×1 cm), dried at 80 °C for 6 hours, and used for electrochemical analysis.

Preparation of Solid-State Gel Electrolyte
Polyvinyl alcohol of 2.4 g was initially dissolved in 80 mL of deionized water at 85 °C under stirring condition.The viscous liquid of polyvinyl alcohol (5 mL) was added to 5 mL of KOH (6 M) solution (1 : 1 ratio) and after dried in air, a thick layer of gel electrolyte was formed.

Fabrication of a Device
A symmetric device of Mn 3 O 4 was constructed using a solid state gel electrolyte with an active mass of 1.5 mg in each electrode.The device was protected with polyethylene terephthalate film to prevent leakage.The formation of Mn 3 O 4 nanoparticles (step-I), electrode preparation (step -II) and symmetric device fabrication (step-III) is illustrated in the Scheme 1.

Figure 1 .
Figure 1.(A) X-ray diffraction patter of tetraethyl hydrazine stabilized Mn 3 O 4 system recorded in the 2θ range from 15°to 80°.The diffracted peaks are indexed according to the tetragonal phase of Mn 3 O 4 (blue colour bar) and that associated with an intermediate phase of MnOOH (green colour symbol).(B) Unit cell representation of Mn 3 O 4 structure interconnected via MnO 4 and MnO 6 polyhedra unit.

Figure 3 .
Figure 3. (A) Transmission electron microscopy images of tetraethyl hydrazine stabilized Mn 3 O 4 particles.Inset shows the SAED pattern of Mn 3 O 4 particles.(B) Histogram profile of particle size distribution.

Figure 4 .
Figure 4. (A) Cyclic voltammetric performance of Mn 3 O 4 based working electrode, for three electrode system, at different scan rate (20-200 mV/s).(B) Galvanostatic charge-discharge profiles of Mn 3 O 4 based electrode under the current density from 2.0 to 0.2 A/g.

Figure 5 .
Figure 5. (A) Specific capacitance (F/g) and areal capacitance (mF/cm 2 ) as a function of current density of the Mn 3 O 4 based electrode.Inset: Cycling stability of the electrode (retention of specific capacitance after 5000 cycles; extracted from galvanostatic charge-discharge profile).(B) Comparative histogram profile of the specific capacitance value from the current work and other reported Mn 3 O 4 based system.

Figure 7 .
Figure 7. (A) Cyclic voltammetry performance of the Mn 3 O 4 based device under different scan rate.(B) Galvanostatic charge-discharge profiles of the device under different current density.

Figure 8 .
Figure 8. (A) Bar diagram represents the specific capacity (mAh/g) and areal capacity (mA/cm 2 ) values of the Mn 3 O 4 based symmetric device as a function of current density.Inset figure shows the amount of charge (C/g) stored in device under different current densities.(B) The device retained ~92 % of the specific capacity value under the current density of 0.3 A/g at the end of 5000 cycles, evaluated from the charge-discharge profile.

Figure 9 .
Figure 9. (A)Trasatti method was used to extract the total charge (q À 1 vs. υ 1/2 ) and (B) outer charge (q vs. υ À 1/2 )) contributions of the THMO device.(C) Outer (q o ) and inner (q i ) charge contributions (%) and (D) variation of device charges (q i and q o ) with respect to different scan rate.

Figure 10 .
Figure 10.(A) Nyquist plot (Z' vs. Z") of the freestanding electrode and device.Inset: the magnified view of the Nyquist plot toward higher frequency.(B) Bode plot (phase angle as a function of frequency) for the electrode and device.(C) Capacitance versus frequency curve.(D) Real (C') and imaginary (C") part of the capacitance for the device.

1 H
NMR spectra, FigureS1, A and B, supporting information).Disappearance of NH proton peak of diethylamine in the NMR spectra suggested the chemical transformation into tetraethyl hydrazine.During the reaction, KMnO 4 was reduced and form Mn 3 O 4 nanoparticles.A solid precipitation of tetraethyl hydrazine stabilized Mn 3 O 4 (brown colour) was deposited at the bottom of the beaker.The entire reaction process was continued for the period of 12 h.The solid material was filtered and dried at 80 °C under vacuum.The synthesized material were characterized using different analytical techniques.

Figure 11 .
Figure 11.(A) Variation of energy density and power density of the device with respect to current density.(B) Comparative Ragone plot of the current device and some reported Mn 3 O 4 based devices.