E ﬀ ect of Concentration on the Charge Storage Kinetics of Nanostructured MnO 2 Thin-Film Supercapacitors Synthesized by the Hydrothermal Method

: In this study, amorphous manganese oxide (MnO 2 ) nanostructured thin ﬁlms were synthesized by a simple hydrothermal method. It is well known that the nanostructure plays a crucial role in energy storage applications. Herein, MnO 2 nanostructures ranging from plates to ﬂakes were synthesized without the use of any hard or soft templates. The 4 + oxidation state of Mn was conﬁrmed by X-ray photoelectron spectroscopy. The MnO 2 nanoﬂake structure has a speciﬁc surface area of 46 m 2 g − 1 , which provides it with an excellent rate capability and an exactly rectangular cyclic voltammogram (CV) curve. The MnO 2 nanoﬂake electrode has a high speciﬁc capacitance of about 433 Fg − 1 , an energy density of 60 Whkg − 1 at 0.5 mAcm − 2 , and an excellent cyclic stability of 95% over 1000 CV cycles in 1 M aq. Na 2 SO 4 . Kinetics analysis of the charge storage in the nanoﬂake MnO 2 sample shows a 55.6% di ﬀ usion-controlled contribution and 44.4% capacitive-controlled contribution to the total current calculated at a scan rate of 100 mVs − 1 from the CV curve.


Introduction
The use of fossil fuels has posed significant problems to modern society, such as increasing fuel costs, pollution, and global warming [1]. These issues can be resolved by developing renewable energy sources and storage technologies [2]. The automobile industry (automobile vehicles) has the largest contribution to environmental pollution. A new variety of vehicles based on electric energy can reduce these problems. The currently available energy storage devices are batteries and capacitors, that exhibit low power density and energy density, respectively. However, modern electronic applications require both the power and the energy density simultaneously, which can be fulfilled by supercapacitors, that also fill the gap between capacitors and batteries [3].
Supercapacitors have high power density, long cycle life, fast charging rate, wide operating temperature range, and enhanced safety and efficiency [4,5]. Supercapacitors are used as auxiliary power sources in electric vehicles, which have a significant potential development in the vehicle

Characterization of the Prepared Samples
The photoluminescence (PL) spectrofluorometer (Flouromax4, Horiba Ltd., Japan) was used to record PL emission spectra of the material. The structural study of materials was conducted on X-ray diffraction (XRD) spectroscopy, D2 phaser, Bruker AXS Analytical Instruments Pvt. Ltd., Karlsruhe, Germany. X-ray photoelectron spectroscopy (XPS) measurements were carried out on K-alpha, and Thermo Scientific, UK. Surface morphology of the materials were observed at 10 kV potential by field emission scanning electron microscopy (FE-SEM) S-4800 HITACHI, Ltd., Tokyo, Japan. Surface area of the nanostructured material were studied by the Brunauer-Emmett-Teller (BET) method on Quantachrome NOVA 1000e, Austria. The electrochemical measurements, were carried out in a three-electrode system on potentiostat PGSTAT302N Metrohm autolab, Netherlands. The deposited weight of the active material was determined by the weight difference method, i.e., the difference in the weight of substrate before and after deposition. Figure 1a shows the XRD patterns of the MnO 2 thin films synthesized by the hydrothermal method with different concentrations of Mn precursor for diffracting angles 2θ varying between 20 • to 80 • . The observed XRD peaks at 43.5, 44.48, 50.7 and 74.6 • marked with ∆ were attributed to the SS substrate. No extra peaks besides the SS peaks were observed, indicating that the prepared thin film materials were X-ray amorphous. The amorphous nature may be due to the low synthesis temperature, which resulted in the presence of water molecules in the material. The material was further characterized by XPS to obtain information about its chemical state.

Photoluminescence (PL)
The PL spectra of all the MnO2 samples are in the range of 375 to 525 nm were recorded at room temperature at 374 nm excitation wavelength using a Xe lamp source. The results are shown in Figure  1b. The PL emission spectra are located within the blue-violet spectral region. The hydrothermally

Photoluminescence (PL)
The PL spectra of all the MnO 2 samples are in the range of 375 to 525 nm were recorded at room temperature at 374 nm excitation wavelength using a Xe lamp source. The results are shown in Figure 1b. The PL emission spectra are located within the blue-violet spectral region. The hydrothermally deposited MnO 2 samples exhibit three peaks at 412, 435, and 462 nm, indicating strong blue emission. The bandgaps of the samples can be calculated from the PL wavelengths using where, h is the Planck constant; c is the velocity of light, and λ is the wavelength of the absorption peak. The bandgap of MnO 2 was calculated to be 3.0 eV for the wavelength of 412 nm from the PL spectra. Along with the violet band-edge transition, the violet and blue emissions observed at 435 and 462 nm may be ascribed to oxygen vacancies and dangling bonds on the surface (or surface defects) of the material respectively [32].

X-ray Photoelectron Spectroscopy (XPS)
XPS studies were performed to investigate the chemical states of the MnO 2 thin film (HM3 sample). The survey scan spectrum in Figure 2a shows the presence of Mn, O, and C elements. The Mn 2p core-level spectra shown in Figure 2b exhibit two major peaks at the binding energies of 653.7 and 642.1 eV corresponding to Mn 2p 1/2 and Mn 2p 3/2 , respectively. The spin energy separation of 11.6 eV in Mn2p confirms the presence of Mn in the +4 oxidation state [33][34][35][36]. The oxidation state of Mn in MnO 2 can be confirmed from the Mn 3s core level spectra (89.1 and 84.0 eV). The Mn 3s doublet spectra has spin energy separation of 4.9 eV for the MnO 2 structures (Figure 2c), indicating that Mn is present in the +4 charge state. Figure 2d shows the O1s core level spectra of MnO 2 . The peaks at 532.5 eV and 529.7 eV were attributed to the presence of H-O-H (water molecule) and Mn-O-Mn bonds, respectively [37][38][39]. These results indicate that Mn in the hydrothermally synthesized MnO 2 has an oxidation state of Mn (IV). Along with the Mn and O peaks, the high-resolution C1s peak in the survey scan spectrum (Figure 2a) was further studied in detail. The results in Figure 2e show that the peak centered at the binding energy of 284.7 eV results from the sp 2 (C-C) bond. The other two observed peaks at the binding energies of 286.2 and 287.9 eV are derivatives of carbon in the C-O group and carbonyl or carboxylic groups (C=O), respectively [40].
Moreover, based on XPS spectra, the peak area of Mn 4+ and Mn 3+ for Mn2p core level spectra mentioned in Table 1, and the ratio of Mn 4+ /Mn 3+ for HM3 sample is 0.77 which ascribed presence of high content of Mn 4+ [41][42][43].  Moreover, based on XPS spectra, the peak area of Mn 4+ and Mn 3+ for Mn2p core level spectra mentioned in Table 1, and the ratio of Mn 4+ /Mn 3+ for HM3 sample is 0.77 which ascribed presence of high content of Mn 4+ [41][42][43].

Field Emission Scanning Electron Microscopy (FESEM)
The surface morphologies of the hydrothermally grown MnO 2 thin films were observed by FESEM. Figure 3 shows the FESEM images recorded at 100 k magnification and an applied potential of 15 kV. At a low concentration of the Mn precursor (HM1), nanoplates of size 200-300 nm were observed. At higher Mn precursor concentrations (HM1 to HM3), the nanostructures converted from nanoplates to nanoflakes. Well-developed and uniformly distributed nanoflakes of size 10-20 nm were observed in the HM3 sample (3 mM concentration of Mn precursor). Such nanoflakes with highly porous structures provide a high SSA. When the concentration of the Mn precursor was increased to 4 mM, the agglomeration of nanoflakes formed, reduces the SSA.  Figure 3 shows the FESEM images recorded at 100 k magnification and an applied potential of 15 kV. At a low concentration of the Mn precursor (HM1), nanoplates of size 200-300 nm were observed. At higher Mn precursor concentrations (HM1 to HM3), the nanostructures converted from nanoplates to nanoflakes. Well-developed and uniformly distributed nanoflakes of size 10-20 nm were observed in the HM3 sample (3 mM concentration of Mn precursor). Such nanoflakes with highly porous structures provide a high SSA. When the concentration of the Mn precursor was increased to 4 mM, the agglomeration of nanoflakes formed, reduces the SSA.

Growth Mechanism of MnO2 Thin Films
The growth mechanism of the MnO2 thin films is shown schematically in Figure 4. After dissolving KMnO4 of 1 mM concentration in DDW, the K + (aq.), and MnO . ions were separated with pink color solution. As the concentration of KMnO4 increased from 1 mM to 4 mM, the color of the solution changed from pink to dark bluish-pink due to presence of more K + and MnO ions. The reaction mechanism is given below [44], During the reaction in hydrothermal, the MnO2 nanoparticles started to self-assemble to minimize the energy of the system. At the lower KMnO4 concentration (1 mM), the Van-der Waals forces and hydrogen bonds between the molecules resulted the formation of MnO2 nanoplates. As the KMnO4 concentration increases, more Mn particles participated in the reaction to minimize the energy of the system, and the synthesized nanostructures were altered by the Ostwald ripening mechanism during the hydrothermal reaction [45]. The agglomeration of MnO2 nanoparticles observed after increasing concentration of the KMnO4 because more MnO ions are in contact with

Growth Mechanism of MnO 2 Thin Films
The growth mechanism of the MnO 2 thin films is shown schematically in Figure 4. After dissolving KMnO 4 of 1 mM concentration in DDW, the K + (aq.) , and MnO − 4 (aq.) ions were separated with pink color solution. As the concentration of KMnO 4 increased from 1 mM to 4 mM, the color of the solution changed from pink to dark bluish-pink due to presence of more K + and MnO − 4 ions. The reaction mechanism is given below [44], Energies 2020, 13, x FOR PEER REVIEW 7 of 18 each other. Hence, the optimization of the exact concentration of the metal precursor (availability of metal ions) plays a vital role in the nanostructure formation. The availability of the exact number of metal ions with respective energies of the system can assist to develop uniform nanostructures.

Surface Adsorption De-Adsorption
To study the influence of the Mn precursor concentration on the surface area and porosity of the MnO2 nanostructured materials, the BET gas adsorption method was employed. The N2 adsorptionde-adsorption isotherms and corresponding Barrett-Joyner-Halenda pore size distribution (inset graph) of the prepared MnO2 nanostructured samples are shown in Figure 5a-d. The isotherms of all the MnO2 samples observed in Figure 5a-d usually correspond to the Type-III category, and the H3 During the reaction in hydrothermal, the MnO 2 nanoparticles started to self-assemble to minimize the energy of the system. At the lower KMnO 4 concentration (1 mM), the Van-der Waals forces and hydrogen bonds between the molecules resulted the formation of MnO 2 nanoplates. As the KMnO 4 concentration increases, more Mn particles participated in the reaction to minimize the energy of the system, and the synthesized nanostructures were altered by the Ostwald ripening mechanism during the hydrothermal reaction [45]. The agglomeration of MnO 2 nanoparticles observed after increasing concentration of the KMnO 4 because more MnO − 4 ions are in contact with each other. Hence, the optimization of the exact concentration of the metal precursor (availability of metal ions) plays a vital role in the nanostructure formation. The availability of the exact number of metal ions with respective energies of the system can assist to develop uniform nanostructures.

Surface Adsorption De-Adsorption
To study the influence of the Mn precursor concentration on the surface area and porosity of the MnO 2 nanostructured materials, the BET gas adsorption method was employed. The N 2 adsorption-de-adsorption isotherms and corresponding Barrett-Joyner-Halenda pore size distribution (inset graph) of the prepared MnO 2 nanostructured samples are shown in Figure 5a-d. The isotherms of all the MnO 2 samples observed in Figure 5a-d usually correspond to the Type-III category, and the H3 hysteresis loop is mesoporous, according the International Union of Pure and Applied Chemistry (IUPAC) classification criteria [46,47].

Surface Adsorption De-Adsorption
To study the influence of the Mn precursor concentration on the surface area and porosity of the MnO2 nanostructured materials, the BET gas adsorption method was employed. The N2 adsorptionde-adsorption isotherms and corresponding Barrett-Joyner-Halenda pore size distribution (inset graph) of the prepared MnO2 nanostructured samples are shown in Figure 5a-d. The isotherms of all the MnO2 samples observed in Figure 5a-d usually correspond to the Type-III category, and the H3 hysteresis loop is mesoporous, according the International Union of Pure and Applied Chemistry (IUPAC) classification criteria [46,47].  The measured SSA and average pore radius (APR) are summarized in Table 2. HM1, which has a nanoplate morphology, has the minimum SSA with a large average pore radius. As the concentration of the Mn precursor increases from 1 to 3 mM, the SSA increases and APR decreases. As the concentration of the Mn precursor is further increased to 4 mM, the SSA decreases because of the agglomeration of the nanoflakes, which was confirmed by FESEM. The largest SSA of 46 m 2 g −1 with 6 nm APR is observed in the well-developed and uniformly distributed flake-like nanostructures of the HM3 sample. The variations of the SSA and APR are required for intercalation-deintercalation of electrolyte ions in the porous material, which in turn affects the suitability of the material for supercapacitor applications.

Electrochemical Measurements
The supercapacitive behavior of the MnO 2 nanostructured thin films was characterized by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), and cyclic stability measurements. A three-electrode system with platinum as the counter electrode, MnO 2 as the working electrode (1 × 1 cm 2 deposited area), and saturated calomel (SC) as the reference electrode was used for the electrochemical measurements.  [48]. The CV curves shift towards higher current densities as the concentration of Mn precursor increases from HM1 to HM3 and decreases as the concentration is further increased in the HM4 sample. To obtain more information on the electrochemical behavior, the HM3 electrode was characterized at different sweep rates from 20 to 100 mVs −1 . This information will lead to a better understanding of the actual participation of the MnO2 electrode in the redox process [53]. The results are shown in Figure 6b. It can be seen that the current density curves shift towards higher current There are two types of capacitive charge storage mechanisms in MnO 2 electrodes [28]. The first type is the intercalation of alkali metal ions such as Na + ions during reduction in the MnO 2 electrode and de-intercalation during oxidation [49]. A possible reaction mechanism is: The second type of capacitive charge storage mechanism involves a surface process in which adsorption/de-adsorption of alkali metal Na + ions occurs on the surface of the MnO 2 nanostructures [50,51]. The reaction mechanism is given by: The intercalation/de-intercalation process is predominant in crystalline MnO 2 and the surface adsorption/desorption process predominant in amorphous MnO 2 [52]. In the latter case, the capacitive charge storage is due to the surface adsorption/de-adsorption process; therefore, the specific capacitance mainly depends on the SSA of the material. The redox mechanism is mainly governed by the adsorption and de-adsorption of Na + (or H + ) from the electrolyte onto the surface of the porous nanostructured MnO 2 matrix.
To obtain more information on the electrochemical behavior, the HM3 electrode was characterized at different sweep rates from 20 to 100 mVs −1 . This information will lead to a better understanding of the actual participation of the MnO 2 electrode in the redox process [53]. The results are shown in Figure 6b. It can be seen that the current density curves shift towards higher current densities in the HM3 electrode as the scan rate increases. Zhang et al. [54] reported the occurrence of distortions in the CV curves when the scan rate increased. However, in our study, rectangular-like CV curves were consistently observed as the scanning rate increased from 20 to 100 mVs −1 . The absence of redox peaks in all the CV curves suggests that the electrode was charged/discharged at a pseudo-constant rate over the complete CV cycle [55].

Galvanostatic Charge-Discharge (GCD)
To calculate the specific capacitance (C sp ), energy density (ED), and power density (PD) of the hydrothermally grown MnO 2 nanostructured electrode, galvanostatic charge-discharge versus SCE measurements were carried out in 1 M aqueous Na 2 SO 4 electrolyte for 0-1.0 V potential at different current densities. Figure 6c shows the GCD graphs of MnO 2 in HM1 to HM4 samples at 0.5 mAcm −2 current density. The GCD graphs of all the MnO 2 samples (HM1 to HM4) show a linear variation in the potential and symmetric charge/discharge curves, which indicates ideal capacitive performance [56]. C sp , ED, and PD were calculated from the GCD curves using the following equations: where I d is the constant discharge current; T d the discharge time, m the deposited weight of active material; dV the cycling potential window; V the potential during the discharge cycle; C sp the specific capacitance; E the energy density, and P the power density of the materials. The specific capacitances calculated from Equation (5) for HM1 to HM4 at the constant current density of 0.5 mAcm −2 are listed in Table 2 Subramanian et al. [31] reported similar results by varying the hydrothermal dwelling time from 1 to 18 h and achieved the maximum specific capacitance of about 160 Fg −1 . Toupin et al. [57] examined the effect of the annealing temperature on the specific capacitance and obtained the maximum specific capacitance of 180 Fg −1 at 2 mVs −1 for as-prepared MnO 2 samples. Increasing the annealing temperature increased the decomposition of MnO 2 into Mn 2 O 3 and decreased the specific capacitance. Subramanian et al. [53] studied the effect of the Na 2 SO 4 electrolyte concentration on the MnO 2 thin film and observed that there was only a small variation in the capacitance. This small difference resulted from the presence of carbon, which has a finite contribution to the double-layer capacitance. Xiao et al. [58] reported the specific capacitance of approximately 220 Fg −1 in α-MnO 2 nanotubes prepared by a chemical precipitation method. α-MnO 2 nanorods synthesized via the chemical precipitation method were reported to have a specific capacitance 166.2 Fg −1 by Li et al. [59].
The increase in the capacitance with the Mn precursor concentration is due to the change in the nanostructures from nanoplates to nanoflakes. The pores between the nanoflakes can facilitate the penetration of electrolyte ions into the inner region of the nanoflake electrode while the large surface area provides more active sites for electrochemical reactions.
To further investigate the rate capability of the MnO 2 nanostructured electrodes, the GCD curves were measured at various current densities. Figure 6d shows the GCD curves of the HM3 electrode at the current densities of 0.5, 1, and 2 mAcm −2 in the 0-1.0 V potential window. The GCD profiles are highly linear and symmetrical, demonstrating that the nanoflake-structure MnO 3 electrode has excellent reversibility and charge-discharge properties. The calculated specific capacitance for the HM3 electrode, which has nanostructured flakes, at different current densities are provided in Table 3. HM3 shows a high specific capacitance as well as a better rate capability even at the high current densities of 1 and 2 mAcm −2 (5 and 10 Ag −1 ). The HM3 electrode shows specific capacitances of 396 and 366 Fg −1 with 91% and 84% capacitance retention as the current density increases from 1 to 2 mAcm −2 . It is well known that the capacitance decreases with increasing current density because the surface of the electrode is inaccessible at higher current densities [60,61]. The above results for the MnO 2 electrode at high current density are much better than the specific capacitances of about 150 and 122 Fg −1 at 5 and 10 Ag −1 current densities, respectively, reported previously for nanosheet MnO 2 by Kundu et al. [62]. Zhao et al. [63] reported specific capacitances of about 221 and 176 Fg −1 at 0.5 and 1 mAg −1 , respectively. A mesoporous MnO 2 nanowire array exhibited specific capacitances of 493 and 84 Fg −1 at current densities of 4 and 12 Ag −1 , respectively [64]. The variation of the specific capacitance with the current density in a MnO 2 /C/TiO 2 shell/core array electrode was studied. The specific capacitance was found to be 539.2 Fg −1 at 2 Ag −1 , 280.9 Fg −1 at 10 Ag −1 , and 223.7 Fg −1 at 20 Ag −1 , respectively [65].
The high rate capability in the HM3 sample is attributed to its nanoflake-like structure which results in a shorter ion diffusion path and increased electronic conductivity.

Stability
A supercapacitor requires not only high capacitance but also cycling stability, which is one of the critical parameters limiting its applicability. The cycling stability of the HM3 sample was investigated at room temperature over up to 1000 continuous CV cycles at a scan rate of 100 mVs −1 in 1 M Na 2 SO 4 aqueous electrolyte. The results are shown in Figure 7a. exhibited specific capacitances of 493 and 84 F·g −1 at current densities of 4 and 12 A·g −1 , respectively [64]. The variation of the specific capacitance with the current density in a MnO2/C/TiO2 shell/core array electrode was studied. The specific capacitance was found to be 539.2 F·g −1 at 2 A·g −1 , 280.9 F·g −1 at 10 A·g −1 , and 223.7 F·g −1 at 20 A·g −1 , respectively [65].
The high rate capability in the HM3 sample is attributed to its nanoflake-like structure which results in a shorter ion diffusion path and increased electronic conductivity.

Stability
A supercapacitor requires not only high capacitance but also cycling stability, which is one of the critical parameters limiting its applicability. The cycling stability of the HM3 sample was investigated at room temperature over up to 1000 continuous CV cycles at a scan rate of 100 mV·s −1 in 1 M Na2SO4 aqueous electrolyte. The results are shown in Figure 7a. The nanoflake MnO2 (HM3) electrode shows a high capacitance retention of about 95%. The capacitance decreases stepwise after 100 cycles. The capacitance loss may be due to the dissolution of Mn ions into the aqueous electrolyte [66,67]. A MnO2-based device reported in the literature showed a capacitance retention of 80% after 1000 GCD cycles [68]. Ke et. al. reported a capacitance retention of 93% after 1000 cycles for a hollow nest-like nanostructured MnO2 thin film [69]. The mixed nanorods and nanoplates in α-MnO2 electrode showed 83% capacitance retention after 100 cycles at 200 mA.g −1 current density [53].

Electrochemical Impedance Spectroscopy (EIS)
EIS was used to investigate the resistive and capacitive behavior of the thin films due to the electrochemical intercalation occurring at the interface between the electrode and electrolyte. The EIS was performed using the three-electrode system described earlier in 1 M Na2SO4 aqueous electrolyte over a frequency range from 0.1 Hz to 10 kHz at 10 mV potential. In general, the Nyquist plot for a supercapacitor consists of three parts, namely, a series resistance at the high-frequency region indicated by the intercept to the Z' axis (real impedance). It is a contact resistance between electrolyte and electrode surface. A semicircle at the mid-frequency region which represents the charge transfer reaction at the interface between the electrode and the electrolyte [70], and a straight line representing the Warburg resistance at the low-frequency region due to ion diffusion into the electrode material [71][72][73]. Figure 7b shows the Nyquist plots for all the MnO2 nanostructured samples (HM1 to HM4). The Nyquist plots for all the samples do not exhibit semicircles at the mid-frequency region. The absence of the semicircular regions implies the possibility of electrochemical reactions according to Equation The nanoflake MnO 2 (HM3) electrode shows a high capacitance retention of about 95%. The capacitance decreases stepwise after 100 cycles. The capacitance loss may be due to the dissolution of Mn ions into the aqueous electrolyte [66,67]. A MnO 2 -based device reported in the literature showed a capacitance retention of 80% after 1000 GCD cycles [68]. Ke et al. reported a capacitance retention of 93% after 1000 cycles for a hollow nest-like nanostructured MnO 2 thin film [69]. The mixed nanorods and nanoplates in α-MnO 2 electrode showed 83% capacitance retention after 100 cycles at 200 mA.g −1 current density [53].

Electrochemical Impedance Spectroscopy (EIS)
EIS was used to investigate the resistive and capacitive behavior of the thin films due to the electrochemical intercalation occurring at the interface between the electrode and electrolyte. The EIS was performed using the three-electrode system described earlier in 1 M Na 2 SO 4 aqueous electrolyte over a frequency range from 0.1 Hz to 10 kHz at 10 mV potential. In general, the Nyquist plot for a supercapacitor consists of three parts, namely, a series resistance at the high-frequency region indicated by the intercept to the Z' axis (real impedance). It is a contact resistance between electrolyte and electrode surface. A semicircle at the mid-frequency region which represents the charge transfer reaction at the interface between the electrode and the electrolyte [70], and a straight line representing the Warburg resistance at the low-frequency region due to ion diffusion into the electrode material [71][72][73]. Figure 7b shows the Nyquist plots for all the MnO 2 nanostructured samples (HM1 to HM4). The Nyquist plots for all the samples do not exhibit semicircles at the mid-frequency region. The absence of the semicircular regions implies the possibility of electrochemical reactions according to Equation (3). The EIS data were fitted with the help of ZSimpWin based on the equivalent electrical circuit. The obtained series resistances, capacitance and Warburg resistance for MnO 2 (HM1, HM2, HM3 and HM4) samples are mentioned in Table 4. The HM3 sample shows a smaller series resistance (R s ) compared to other samples. All the MnO 2 samples show a short arc and a 45 • diagonal line with respect to the real impedance (Z') axis, indicating capacitive behavior [74,75]. To study the detailed charge storage behavior of the MnO 2 samples, we analyzed the kinetics based on the CV measurements. In general, the relationship between the current (i) and scan rate (ϑ) is given by: where i is the current at specific voltages; ϑ is the scan rate; a and b are constants. The total current exhibited by the material includes contributions from (1) the surface-controlled capacitive process (ion adsorption/desorption) and (2) diffusion-controlled charge storage (fast faradic reaction of redox species). A value of b = 1 in Equation (8) indicates capacitive-dominant behavior, whereas b = 0.5 represents a diffusion-controlled process [76,77]. From Figure 8a, b = 0.64, which means that the HM3 sample exhibits both capacitive and diffusion-controlled behavior [78]. The capacitive effect was investigated by measuring how the current changed with respect to the scan rate at a specific potential. The b values of HM3 for the cathodic and anodic curves at 0 to 1.0 V vs the SCE potential are shown in Figure 8d. The values of b at different potentials in the cathodic and anodic scans range between 0.5 to 1 and show both diffusion and capacitive-controlled behavior.
The specific contribution of the capacitive and diffusion-controlled mechanisms to the total current can be quantified using: where i(V) is the current response at a specific voltage; ϑ is the scan rate; K 1 ϑ the capacitive current, and K 2 ϑ 1/2 the diffusion-controlled current. K 1 and K 2 are determined by plotting i(V)/ϑ 1/2 vs ϑ 1/2 to separate out the charges from the capacitive and diffusion-controlled processes [78]. Figure 8b shows a CV plot of the capacitive contribution (highlighted area) to the total charge storage for the HM3 sample at 100 mVs −1 scan rate in 1 M aq. Na 2 SO 4 . It was observed that about 97%, 88%, 48%, and 94% of the charge storage contribution in the HM1, HM2, HM3, and HM4 samples, respectively, are due to the capacitive process. Here, we show the capacitive and diffusion-controlled contributions in the HM3 sample under different scan rates from 20 to 100 mVs −1 in Figure 8c, and also in the HM1, HM2, and HM4 samples under different scan rates from 20 to 100 mVs −1 in Figure S1. It seems that the capacitive process is unaffected by the scan rate whereas the diffusion-controlled contribution decreases with increasing scan rate for all the MnO 2 samples. The redox intercalation contribution decreases with increasing scan rate because there is insufficient time for ion diffusion into the lattice at a high scan rate. The capacitive and diffusion-controlled contributions of all the MnO 2 samples at different scan rates are listed in Table S1. The high specific capacitance obtained for the HM3 sample compared to the other samples (HM1, HM2, and HM4) is attributed to the larger contribution from the diffusion-controlled process in HM3, which agrees with the GCD results described earlier. It seems that the capacitive process is unaffected by the scan rate whereas the diffusioncontrolled contribution decreases with increasing scan rate for all the MnO2 samples. The redox intercalation contribution decreases with increasing scan rate because there is insufficient time for ion diffusion into the lattice at a high scan rate. The capacitive and diffusion-controlled contributions of all the MnO2 samples at different scan rates are listed in Table S1. The high specific capacitance obtained for the HM3 sample compared to the other samples (HM1, HM2, and HM4) is attributed to the larger contribution from the diffusion-controlled process in HM3, which agrees with the GCD results described earlier.
In addition to the comprehensive comparison of the electrochemical behavior of the hydrothermally deposited MnO2 thin films, a radar chart is shown in Figure 9. Each vertex of the chart corresponds to a supercapacitive parameter, and the integral area under the curve implies the electrochemical performance of the samples. From Figure 9, it can be seen that the integral area of the HM3 sample is notably larger compared to the other samples. This confirms that the HM3 sample with a nanoflake structure exhibits a large SSA, a large potential window, a small minimum series resistance, and a large diffusion-controlled contribution, which contribute to its high specific capacitance. In addition to the comprehensive comparison of the electrochemical behavior of the hydrothermally deposited MnO 2 thin films, a radar chart is shown in Figure 9. Each vertex of the chart corresponds to a supercapacitive parameter, and the integral area under the curve implies the electrochemical performance of the samples. From Figure 9, it can be seen that the integral area of the HM3 sample is notably larger compared to the other samples. This confirms that the HM3 sample with a nanoflake structure exhibits a large SSA, a large potential window, a small minimum series resistance, and a large diffusion-controlled contribution, which contribute to its high specific capacitance.

Conclusions
We successfully synthesized nanostructured MnO2 thin films directly on SS substrate via the hydrothermal method. The concentration variation of the KMnO4-tailored morphology of MnO2 from plates to flakes is due to the rearrangement of the nanoparticles with the increase in the number of

Conclusions
We successfully synthesized nanostructured MnO 2 thin films directly on SS substrate via the hydrothermal method. The concentration variation of the KMnO 4 -tailored morphology of MnO 2 from plates to flakes is due to the rearrangement of the nanoparticles with the increase in the number of Mn ions. A 3 mM concentration of KMnO 4 produced a nanoflake morphology with a high SSA, resulting in the maximum specific capacitance. From the charge storage kinetics, we can conclude that the diffusion-controlled current is dominant at low scan rates while the capacitive-controlled current is dominant at higher scan rates. The diffusion-controlled current is highest in the nanoflake sample because of its high porosity and electrical conductivity, which increase the contact time between the material and electrolyte. All these aspects of the nanoflake sample allow it to exhibit an acceptable level of supercapacitive performance. The performance of hydrothermally deposited MnO 2 thin films was compared based on the six supercapacitor parameters.