The Controllable Ratio of the Polyaniline-Needle-Shaped Manganese Dioxide for the High-Performance Supercapacitor Application

The nanohybrid development of metal oxide/conducting polymer as an energy storage material is an active research area, because of the device stability, conductive behavior, and easy fabrication. Herein, needle-like MnO2 was coupled with polyaniline fabricated through chemical polymerization followed by the hydrothermal process. The characterization results show that MnO2/polyaniline exhibited a needle-like morphology. Different characterization techniques such as X-ray diffraction patterns and scanning electron microscopy confirmed the formation of the MnO2/polyaniline nanohybrids. The electrochemical performance, including cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), specific capacitance (Csp), and cyclic stability, was examined using a three-electrode assembly cell. The optimized electrode displayed a Csp of 522.20 F g−1 at a current load of 1.0 A g−1 compared with the other electrodes. The developed synergism during MnO2/polyaniline fabrication provided enhanced conductive channels and stability during the charge–discharge process.


Introduction
The electronic device usage escalation, including wearable lightweight electronics, electronic papers, cell phones, and household applications, has forced the development of alternatives to fulfill daily life demands. This motivates research toward the development of lightweight and durable energy devices; among them are batteries and supercapacitors [1,2]. For any device to achieve cyclic stability, lightweight, excellent power density, fast charge-discharge ability, and cheap cost, it has to be assembled with a supercapacitor, which activates research on energy storage electrode materials [3,4]. Supercapacitor design could be traditional, with a high energy density or a modified high-energy storage for wide application [5,6]. For the demand of super energy storage and short-time period performance, an electrochemical supercapacitor is the best choice [1,2,6]. According to the design and energy storage mechanism, supercapacitors can have two different classifications. The first one is a double-layered capacitor followed by the adsorption and desorption of ions during the electrochemical process, while the second is a pseudo capacitor followed by the oxidation-reduction process as the charge-discharge method [6][7][8][9][10]. Moreover, the electrode configuration determines the supercapacitor's arrangement. For example, if both electrodes are identical, a symmetric capacitor is formed, whereas an asymmetric one is formed by different electrode types [4,9,[11][12][13][14]. Since the design of the electrode materials

Synthesis of Needle-like MnO 2 /Polyaniline Nanocomposites
The needle-like MnO 2 /polyaniline nanocomposites were prepared by in situ oxidative polymerization of aniline monomer, potassium persulfate, and needle-like MnO 2 that were prepared earlier. In a typical synthesis procedure, as-prepared MnO 2 was mixed with different moles of aniline monomer using a stirrer and sonication. The aniline adsorbed over the MnO 2 surface during the stirring and sonication process then freshly prepared potassium persulphate was added dropwise to start the polymerization of the polyaniline over the MnO 2 surfaces. The mixing of the oxidant led to a change of the solution's color to greenish black, and this was further stirred for 12 h. The collected precipitate was washed with excess water, acetone, and methanol to remove the acid, potassium persulphate, and oligomers of polyaniline. The composite materials were de-doped with the 1 M ammonia solution to neutralize the remaining acid, which converted the composites into emeraldine base form. The resultant was again doped with the 1 M HCl solution to make it a conductive material. Several samples were synthesized using different amounts of aniline and they are abbreviated as PP (Pure Polyaniline), PMO-1 (9 mL), PMO-2 (5 mL), and PMO-3 (2 mL).

Characterization
Scanning Electron Microscopy (SEM; JEOL JSM-6610 LV with Oxford Instruments X-Max N ) was conducted to examine the morphologies of the samples. The EDX was further used to analyze elements present in the nanocomposites using AZ tec-22164 software. X-ray diffraction (XRD) of the samples was performed to determine the crystallinity, phase, lattice parameters, and grain size. Vesta software was used to create crystal models for the MnO 2 . Thermogravimetric Analysis (TGA) was employed to examine the thermal properties of the PP, PMO-1, PMO-2, and PMO-3 samples. These studies were carried out using simultaneous DSC/TGA thermal analysis instruments with the SDT Q600 model. The cyclic voltammetry and galvanostatic charge-discharge methods were used to examine the electrochemical properties of PP, PMO-1, PMO-2, and PMO-3 on Nova Auto Lab (Metrohm).

Electrochemical Supercapacitive Performances
Half-cell was assembled to examine the electrochemical performance of PP, PMO-1, PMO-2, and PMO-3 electrode materials. Ag/AgCl was used as reference electrodes, and Pt was used as the counter electrode. The working electrode was prepared by adding active materials to PVDF (10%) and carbon black (10%) to make a slurry-like paste that was used as a coat for nickel foam. The coated nickel foam was further dried in an oven at 60 • C for 12 h. The specific capacitance of all the prepared electrodes of PP, PMO-1, PMO-2, and PMO-3 was calculated using the following equation [26,27]: where C is the specific capacitance, t is the discharge time, I is current, m is the mass loading of the materials, and V is the applied potential window. Figure 1 shows the XRD patterns for the PP, PMO-1, PMO-2, and PMO-3 recorded over 2θ range from 10 • to 80 • , in which PP displays broad peaks at 2θ = 19.26 • (150), respectively. These results show that all nanocomposites belong to the α-MnO 2 phase. The simulated XRD spectra of the α-phase of MnO 2 display a tetragonal crystal structure with a space group of I4/m [24]. A redshift combined with an intensity decrease at 25.04 • for all the PMO nanocomposites was noticed in the data. Surface coating of MnO 2 by polyaniline during the polymerization process could be the cause of the redshift [19]. Lattice parameters for the composites were determined to be a = b = 9.823 Å and c = 2.941 Å for PMO-1, a = b = 9.950 Å, and c = 2.880 Å PMO-2 and =b = 9.787 Å and c = 2.913 Å for case the of PMO-3. These lattice parameter results aligned with JCPDS No. 44-0141 (space group 14/m, a = b = 9.784 Å, c = 2.863 Å) for body-centered tetragonal α-MnO 2 [26]. Scherrer's formula was used to determine the crystalline size to be 1.73, 2.00, and 1.72 nm, respectively (Table 1). Figure 1 shows the XRD patterns for the PP, PMO-1, PMO-2, and PMO-3 recorded over 2θ range from 10° to 80°, in which PP displays broad peaks at 2θ = 19.26° and 25.52° along with other accompanying peaks which correspond to the characteristic diffraction pattern (200), (020), and (011) of the polyaniline. The other nanocomposites, PMO-1, PMO-2, and PMO-3, displayed diffraction peaks at 2θ = 12.31°, 17.66°, 28.54°, 37.07°, 38.31°, 40.63°, 41.56°, 46.88°, 49.23°, 55.85°, 59.69°, 64.81°, 69.3°, and 72.48°. These peaks corresponded to crystallographic planes (110), (200), (310), (121), (301), (411), (600), (260), (002), (640), (361), (330), (420), and (150), respectively. These results show that all nanocomposites belong to the α-MnO2 phase. The simulated XRD spectra of the α-phase of MnO2 display a tetragonal crystal structure with a space group of I4/m [24]. A redshift combined with an intensity decrease at 25.04° for all the PMO nanocomposites was noticed in the data. Surface coating of MnO2 by polyaniline during the polymerization process could be the cause of the redshift [19]. Lattice parameters for the composites were determined to be a = b = 9.823 Å and c = 2.941 Å for PMO-1, a = b = 9.950 Å, and c = 2.880 Å PMO-2 and =b = 9.787 Å and c = 2.913 Å for case the of PMO-3. These lattice parameter results aligned with JCPDS No. 44-0141 (space group 14/m, a = b = 9.784 Å, c = 2.863 Å) for body-centered tetragonal α-MnO2 [26]. Scherrer's formula was used to determine the crystalline size to be 1.73, 2.00, and 1.72 nm, respectively (Table 1).

Results and discussion
. SEM images show surface morphologies of PP, PMO-1, PMO-2, and PMO-3 ( Figure  2). PP has a nanofiber-like morphology with a large number of interconnected tubules, whereas PMO-1, PMO-2, and PMO-3 display needle-like surface morphologies. The nanofiber structure has an average length larger than 2 μm (Figure 2a). The needle-like structure has diameters of 138.08 nm, 117.11 nm, and 98.41 nm for the PMO-1, PMO-2, and PMO-3, respectively (Figure 2b-d). Figure. 2    SEM images show surface morphologies of PP, PMO-1, PMO-2, and PMO-3 ( Figure 2). PP has a nanofiber-like morphology with a large number of interconnected tubules, whereas PMO-1, PMO-2, and PMO-3 display needle-like surface morphologies. The nanofiber structure has an average length larger than 2 µm (Figure 2a). The needle-like structure has diameters of 138.08 nm, 117.11 nm, and 98.41 nm for the PMO-1, PMO-2, and PMO-3, respectively (Figure 2b-d). Figure 2 shows a uniform needle-like morphology related to PMO-2. For PMO-3, MnO 2 agglomerated on the polyaniline surface because of its excessive amount. Figure 2a'-d' shows the EDS spectra of PMO-1, PMO-2, and PMO-3, in which large peaks can be seen for Mn, O, and C. The peak for Au is from the conducting tap for the SEM/EDS experiment. Small peaks for S, K, and Cl are also present in the spectrum. For the PP sample, the data show C, S, and Cl peaks only with high intensity compared with the other three samples. the spectrum. For the PP sample, the data show C, S, and Cl peaks only with high intensity compared with the other three samples.  Thermal stability was examined using TGA analysis. All samples were analyzed in the temperature range from 25 to 800 °C ( Figure 3). The y-axis represents the weight loss, and the x-axis refers to sample temperature. The initial weight loss below 150°C corresponds to the removal of physio-absorbed water molecules from the samples [18]. Weight loss in the temperature range of 150 to 300 °C is due to the removal of structural water from the α-MnO2 phase and deprotonation of PP through the loss of dopant HCl as well as the degradation of small, less stable oligomers in the composite materials [26,27]. For the temperature slightly before 600 °C, weight loss occurs due to the reduction of MnO2 to Mn2O3. This is related to the degradation and decomposition of PP with different polymerization degrees, resulting in the formation of different aliphatic and aromatic fragments at the end, such as p-phenylenediamine, ammonia, N-phenyl-1,4-, carbazole aniline, benzenediamine N-phenylaniline, acetylene pyridine-based heterocycle, and methane, as reported by Ansari et al [28]. A further reduction in oxygen due to the transition from Mn2O3 to Mn3O4 can occur in the temperature range of 700 °C to 850 °C according to the reaction 3Mn2O3(s) → 2Mn3O4(s) + ½O2 (g) [26]. Figure 4a shows weight loss for the pure polyaniline sample as a linear drop that occurred in two steps. The PMO-1, PMO-2, and PMO-3 nanocomposites have weight losses of 3.92%, 2.19%, and 6.74% that occurred around 390 °C to 570 °C. This was due to the continuous loss of oxygen as MnO2 converted to Mn2O3. Weight losses of 5.36%, 3.04%, and 6.39% occurred in the three samples at around 700 °C to 800 °C, which is related to the loss of oxygen during the phase transformation from Mn2O3 to Mn3O4. Comparing the TGA behavior of the PP, and PMO hybrids samples, the PMO samples are more thermally stable as their degradation curves are above that of PP. The thermal stability of the PMO hybrids is directly related to stabilizing Thermal stability was examined using TGA analysis. All samples were analyzed in the temperature range from 25 to 800 • C ( Figure 3). The y-axis represents the weight loss, and the x-axis refers to sample temperature. The initial weight loss below 150 • C corresponds to the removal of physio-absorbed water molecules from the samples [18]. Weight loss in the temperature range of 150 to 300 • C is due to the removal of structural water from the α-MnO 2 phase and deprotonation of PP through the loss of dopant HCl as well as the degradation of small, less stable oligomers in the composite materials [26,27]. For the temperature slightly before 600 • C, weight loss occurs due to the reduction of MnO 2 to Mn 2 O 3 . This is related to the degradation and decomposition of PP with different polymerization degrees, resulting in the formation of different aliphatic and aromatic fragments at the end, such as p-phenylenediamine, ammonia, N-phenyl-1,4-, carbazole aniline, benzenediamine N-phenylaniline, acetylene pyridine-based heterocycle, and methane, as reported by Ansari et al. [28]. A further reduction in oxygen due to the transition from Mn 2 O 3 to Mn 3 O 4 can occur in the temperature range of 700 • C to 850 • C according to the reaction 3Mn 2 O 3 (s) → 2Mn 3 O 4 (s) + 1 / 2 O 2 (g) [26]. Figure 4a shows weight loss for the pure polyaniline sample as a linear drop that occurred in two steps. The PMO-1, PMO-2, and PMO-3 nanocomposites have weight losses of 3.92%, 2.19%, and 6.74% that occurred around 390 • C to 570 • C. This was due to the continuous loss of oxygen as MnO 2 converted to Mn 2 O 3 . Weight losses of 5.36%, 3.04%, and 6.39% occurred in the three samples at around 700 • C to 800 • C, which is related to the loss of oxygen during the phase transformation from Mn 2 O 3 to Mn 3 O 4 . Comparing the TGA behavior of the PP, and PMO hybrids samples, the PMO samples are more thermally stable as their degradation curves are above that of PP. The thermal stability of the PMO hybrids is directly related to stabilizing effects of MnO 2 on PP. A similar stabilizing interaction was also reported for composites of PP with CNT and graphene [29].

Electrochemical Performance in the Half Cell
Aiming to understand the electrochemical supercapacitive behavior and the energy storage performance of all the samples (PP, PMO-1, PMO-2, and PMO-3) that could provide a pathway to the practical application of the electrode materials, the galvanostatic charge-discharge and cyclic voltammetry performance of the electrode were examined in a three-electrode assembly cell within a 3 M KOH electrolytic solution. The CV test was performed in a potential window of 0.0 to 0.5 V at varying scan rates of 10 to 100 mV s −1 ( Figure 5). The fabricated MnO 2 /polyaniline nanohybrids exhibited exceptional electro-chemical performance, which was expected from the SEM results as they show a PMO branch-like network structure. Branch-like structure leads to a shortening of the charges' diffusion path and a deep redox reaction which simultaneously enhances the overall electrochemical performance of the materials [30][31][32].
Nanomaterials 2023, 13, x FOR PEER REVIEW 6 of 12 effects of MnO2 on PP. A similar stabilizing interaction was also reported for composites of PP with CNT and graphene [29].   effects of MnO2 on PP. A similar stabilizing interaction was also reported for composites of PP with CNT and graphene [29].    The elongated charge-discharge duration, large integrated area, and improved specific capacitance of the PMO-2 nanohybrid electrode can be attributed to the synergy that develops due to the presence of optimized amounts of metal and polymer in composite electrode materials [35,36]. This plays an important role in providing a large number of active sites and facilitating excessive ion diffusion during the electrochemical procedure. The specific capacitance of individual electrodes was also measured at different current densities using GCD ( Figure 5 and Figure 6a), and the results are shown in Figure 5b-d. The potential range reached the specific level due to its lower conductivity. Some electrode materials had a longer charging time to reach their potential range at lower current density. This type of behavior is generally observed in some transition metal compounds and pseudocapacitors in 3-electrode measurement. If we could test the same materials in a practical system, this erroneous behavior would not appear. The resulting specific capacitance values of the PP, PMO-1, PMO-2, and PMO-3 electrodes were extracted from the discharge curve and equation. At current densities of 1, 2, 3, 5, 7, and 10 Ag −1 , the specific capacitance of the optimized PMO-2 nanohybrid composite was around 522. The comparative cyclic voltammetry measurement for the nanocomposite electrode material was performed at a scan rate of 50 mV/s, as illustrated in Figure 4a. The results showed that the PMO-2 integrated area is larger than that of PP, PMO-1, and PMO-3, which suggests a synergistic effect between the MnO 2 and polyaniline in the optimized composite electrode (PMO-2). The redox peak in the CV curves (Figure 4a) shows in PMO-1, PMO-2 and PMO-3, but in the case of PP, the CV is nearly rectangular in shape. The CV measurement of all electrode materials, PMO-1 (Figure 4b), PMO-2 (Figure 4c), and PMO-3 (Figure 4d), was taken at different scan rates, which shifted the redox peak as it increased. The shift in redox peak with increasing scan rates could be explained by highly reversible and fast charge transfer during the electrochemical test, which implies lower polarization of the electrode materials and fast electron and ion transport rates during the electrochemical processes. Based on an electroanalytical point of view, the small potential difference in anodic and cathodic peaks may be attributed to the reversible electrochemical reaction, which can be explained by the above reaction equation [33,34]. The reversibility and integrated capacitive area of the electrode during the faradic reaction were homogeneously maintained (Figure 4b-d), even at a higher scan rate, which supports the existence of increased ion diffusion during the electrochemical procedure. CV curves show that nanostructures conduct electrons and ions at higher scan rates due to a significant increment in the current at higher scan rates [30]. Moreover, the behavior of PMO-1 CV is almost same as that of PP due to the lower ratio of manganese oxide present in this compound. The increased in the amount of aniline monomer in PMO-2 and PMO-3 is mention in the synthesis section, while the increased percentage of MnO 2 in all electrodes calculated by EDS is mentioned in Figure 2.
To understand the exact charge storage mechanism and potential specific capacitance storage of the fabricated composite electrode materials, the charge-discharge analysis was performed. The GCD profiles of all fabricated electrode materials, PP, PMO-1, PMO-2, and PMO-3, were determined at fixed and different current densities. The corresponding specific capacitance was calculated from the above charge-discharge equation. The GCD SC values of the activated electrode materials were estimated at different current densities ranging from 1 to 10 Ag −1 . Figure 5a shows a comparative GCD for PP, PMO-1, PMO-2, and PMO-3 at fixed current density, and indicates that PMO-2 has longer chargingdischarging behavior compared to other samples, leading to a good specific capacitance. The comparative CV and GCD graphs indicate that the optimized PMO-2 nanohybrid is an appropriate choice for the fabrication of supercapacitor electrode material. At the fixed current density of 1 Ag −1 , the optimized PMO-2 nanohybrid electrode exhibited a value of 522.2 F g −1 with a long discharge time, which is 3.2 times higher than that of PP (162.1 F g −1 ), and 3.2 times higher than that of PMO−1 (198.3 F g −1 ) and PMO−3 (419.8 F g −1 ).
The elongated charge-discharge duration, large integrated area, and improved specific capacitance of the PMO-2 nanohybrid electrode can be attributed to the synergy that develops due to the presence of optimized amounts of metal and polymer in composite electrode materials [35,36]. This plays an important role in providing a large number of active sites and facilitating excessive ion diffusion during the electrochemical procedure. The specific capacitance of individual electrodes was also measured at different current densities using GCD (Figures 5 and 6a), and the results are shown in Figure 5b-d. The potential range reached the specific level due to its lower conductivity. Some electrode materials had a longer charging time to reach their potential range at lower current density. This type of behavior is generally observed in some transition metal compounds and pseudocapacitors in 3-electrode measurement. If we could test the same materials in a practical system, this erroneous behavior would not appear. The resulting specific capacitance values of the PP, PMO-1, PMO-2, and PMO-3 electrodes were extracted from the discharge curve and equation. At current densities of 1, 2, 3, 5, 7, and 10 Ag −1 , the specific capacitance of the optimized PMO-2 nanohybrid composite was around 522.  (Figure 5d). These results confirm that the PMO-2 nanohybrid has an excellent capacitance performance compared with the other electrodes fabricated in the present work. Table 2 displays the previously reported MnO 2 /polyaniline nanohybrid supercapacitor from the literature compared to the optimized PMO-2 in this work. The measured supercapacitive values and rate capability (%) of the active electrode materials were plotted against varying current densities, as illustrated in Figure 6a and b. The optimized PMO-2 nanohybrid composite delivered a higher capacitance, demonstrating a good rate capability.
The stability of the electrode material is a major issue of concern and one of the most important parameters to consider when investigating potential energy storage applications. In the present case, the consecutive charge-discharge cycle test was conducted, and the results are depicted in Figure 6b. Compared to PP, the optimized PMO-2 nanohybrid composite exhibited excellent performance in the cyclic stability test over 4000 cycles run at a fixed current load. The stability results also show that the specific capacitance was also maintained over all cycles, and retention did not drop quickly, which supports the excellent properties of the fabricated optimized PMO-2 nanohybrid electrode material. However, compared with the PMO-2 nanohybrids, the retention percentage of PP decreased much faster after starting the charge-discharge cycles. This implies that the optimized PMO-2 Nanomaterials 2023, 13, 101 9 of 12 nanohybrid electrode has good rate capability, long life cycle stability, and superior specific capacitive properties, suggesting that it has great potential for energy storage applications. Moreover, the two major parameters used for evaluating the practicability of the supercapacitors, namely the energy and power density, were recorded during the electrochemical measurements and further calculated using equations 2 and 3 [37][38][39]. These values related to PMO-2 nanohybrid electrodes were calculated at various current densities, as shown in Figure 6c (Ragone plot). The Ragone plot shows that the energy density decreased when the power density increased, which is in accordance with the calculated specific capacitance value of the electrodes. This behavior can be justified as follows: at a high current load, the charging and discharging procedures are fast, and the electrolyte ions do not have sufficient time to penetrate available pores on the electrode surface. In the present case, the optimized PMO-2 nanohybrid electrodes exhibited the highest energy density of 14.6 Wh/kg at a power density of 224.6 W/kg, demonstrating the excellent high-power performance of the prepared electrode. Moreover, the PMO-2 nanohybrid electrode achieved the highest power density of 1603.3 W/kg, corresponding to an energy density of 4.8 Wh/kg.

Conclusions
The MnO2/polyaniline nanohybrids were developed through a simple hydrothermal process combined with the chemical polymerization method. The needle-like MnO2 coupled with polyaniline was further assembled into a three-electrode assembly cell as a working electrode, and its electrochemical supercapacitive performance was assessed using the CV and GCD techniques in an aqueous electrolyte. The optimized electrode (PMO-2) displayed a Csp of 522.20 F g −1 at a current load of 1.0 A g −1 and exhibited good cyclic stability under various charge-discharge processes. The enhanced performance of the PMO-2 electrode is credited to the combined properties of the individual components of  Moreover, the two major parameters used for evaluating the practicability of the supercapacitors, namely the energy and power density, were recorded during the electrochemical measurements and further calculated using equations 2 and 3 [37][38][39]. These values related to PMO-2 nanohybrid electrodes were calculated at various current densities, as shown in Figure 6c (Ragone plot). The Ragone plot shows that the energy density decreased when the power density increased, which is in accordance with the calculated specific capacitance value of the electrodes. This behavior can be justified as follows: at a high current load, the charging and discharging procedures are fast, and the electrolyte ions do not have sufficient time to penetrate available pores on the electrode surface. In the present case, the optimized PMO-2 nanohybrid electrodes exhibited the highest energy density of 14.6 Wh/kg at a power density of 224.6 W/kg, demonstrating the excellent high-power performance of the prepared electrode. Moreover, the PMO-2 nanohybrid electrode achieved the highest power density of 1603.3 W/kg, corresponding to an energy density of 4.8 Wh/kg.

Conclusions
The MnO 2 /polyaniline nanohybrids were developed through a simple hydrothermal process combined with the chemical polymerization method. The needle-like MnO 2 coupled with polyaniline was further assembled into a three-electrode assembly cell as a working electrode, and its electrochemical supercapacitive performance was assessed using the CV and GCD techniques in an aqueous electrolyte. The optimized electrode (PMO-2) displayed a C sp of 522.20 F g −1 at a current load of 1.0 A g −1 and exhibited good cyclic stability under various charge-discharge processes. The enhanced performance of the PMO-2 electrode is credited to the combined properties of the individual components of the MnO 2 , conduction polyaniline, and their nanohybrid, which provides enhanced conductive channels and stability due to the synergism developed during the fabrication of the MnO 2 /polyaniline. The excellent performance of the PMO-2 electrode introduces it as a good choice for developing energy devices to serve various applications.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.