Facile Synthesis Mn 2 O 3 Nanorod Arrays: A Significant Material for Supercapacitor Electrode Application

Mn 2 O 3 is a significant candidate for various applications. In the present work, the Mn 2 O 3 nanorod arrays have been successfully prepared through facile sonochemical method with the aid of cetyl trimethyl ammonium bromide (CTAB) template. The crystalline phase and bonding properties have ben confirmed through X-ray diffraction analysis (XRD) and Fourier transform infrared (FTIR) spectroscopic analysis. The electrochemical properties were analysed through various techniques such as cyclic voltammetric and galvanostatic charge/discharge analysis. Interestingly, cyclic voltammetric (CV) curves confirms the electric double layer capacitor-based charge storage mechanism and it render the maximum specific capacitance of 647 Fg -1 at a scan rate 5 mVs -1 whereas the galvanostatic charge/discharge studies offer the specific capacitance of 656 Fg -1 at a current density of 1 Ag -1 . The Mn 2 O 3 nanorod arrays provide the maximum energy and power densities of 91.1 Wh Kg -1 and 14985 Wkg -1 respectively. In addition, the cyclic stability analysis exhibit only 12 % initial capacitance degradation over 3000 CV cycles at a scan rate of 100 mVs -1 . The hopeful outcomes demonstrate the significant of the Mn 2 O 3 nanorod arrays as electrode material for supercapacitor devices. results


Introduction
Developing innovative energy storage devices has become a significant challenge due to the rising of modern technological revolution and over-consumption of natural resources especially fossil fuels [1,2]. Supercapacitors have drawn tremendous interest, because of their attractive features such as rapid charge/discharge, extended cycle life and high-power density.
As a modern energy storage system, supercapacitors have played a significant role in various application areas such as hybrid electric vehicle, portable electronic gadgets and other micro energy storage systems [3,4]. Supercapacitors can store charges in two different ways such as electric double layer capacitor and pseudocapacitor mechanisms. The capacitance of electric double layer capacitors resulting from adsorption of ions at the electrode /electrolyte interface whereas redox reaction produce capacitance in pseudocapacitors which occur between electrode and electrolyte [5,6]. Carbon based substances (activated carbon, carbon aerogel, carbon nanotube and graphene oxide) are generating electric double layer capacitors while metal oxides and conducting polymers generating pseudocapacitance [7,8]. Till now, various types of materials have been extensively utilized as supercapacitor electrode materials.
Manganese oxide based materials such as MnO2, Mn2O3 and Mn3O4 are a suitable choice for successful supercapacitive electrode materials among these transition metal oxide substances owing to their various oxidation states, attractive structural features, low toxicity nature and excellent chemical and physical stability in different kinds of electrolytes [17].
Compared to the other traditional electrode substances, the Mn2O3 has received considerable interest in the area of energy storage devices owing to its interesting electrochemical features, superior theoretical capacitance value, high valency of manganese material, significant structural features, easy preparation process and environmental compatibility [18,19]. On the other hand, Mn2O3 has not been demonstrated as a good electrode substance for supercapacitors because of their uneven morphological features, low ionic conductivity, low porosity and poor cyclic stability. In addition, the volume of Mn2O3 has been expanded during the cyclic stability process, which restricted the performance of the supercapacitor devices [20,21]. Different techniques have been employed to overcome these disadvantages and alter Mn2O3 into significant supercapacitor electrode substance. Increased the specific surface area and tuning the morphological properties is the finest remedy to increase the utilization Mn2O3 into supercapacitor electrode materials.
Various synthetic methodologies have been utilized to alter the morphological properties of Mn2O3 materials. For example, the hydrothermal method was employed to prepare Mn2O3 nanocubics structure followed by annealing process, which render the specific capacitance of 191.1 Fg -1 , high rate capability and superior cycle life [22]. Xu et al [23] demonstrated Mn2O3 microsheet arrays on nickel foam using mild chemical reaction united with a heat treatment progression. The microsheet arrays offer the specific capacitance of 566.6 F g -1 , superior energy (19.65 Wh kg -1 ) and power (124 W kg -1 ) densities. Kharade et al [24]established the manganese oxide nanoflake structure via galvanostatic electrodeposition synthetic approach which exhibits the specific capacitance of 414 Fg -1 with 86 % cyclic retention.
Among the synthetic methodologies, the sonochemical approach is a useful, cost effective, less time-consuming technique for preparing nanosized materials with different morphologies and unique characteristics. It supports the large scale synthesis of various nanomaterials. The sonochemical method is based on the physical phenomena of acoustic cavitation ensuing from continuous bubbles generation, development and implosive collapse of bubbles in a liquid medium. Extreme heating, pressure and high cooling rates are produced during bubbles collapsing process. The transient, localized hot spots can initiative various chemical process and provide different type of nanostructures [25]. Till now, different kinds of nanostructures (nanorods [26], nanoflowers [27], nanoplates [28], nanofibers [29], nanocubes [30], nanofires [31] and nano petal [32]) have been prepared using the sonochemical synthetic approach.
In this work, the Mn2O3 nanorod arrays were demonstrated using sonochemical approach combined with calcination process. The cetyl trimethyl ammonium bromide (CTAB) template was used to restrict the aggregation during the synthesis process and it also tuned the morphological properties of the Mn2O3 materials. The x-ray diffraction analysis and Fourier transform infrared spectroscopic analysis confirms the formation of Mn2O3 materials. The nanorod arrays morphology was confirmed through high resolution scanning electron microscopy. The electrochemical properties of Mn2O3 materials were analysed via cyclic voltammetric and galvanostatic charge/discharge analysis. The CV curve provide the specific capacitance of 647 Fg -1 at 5 mVs -1 where GCD exhibit the specific capacitance of 656 Fg -1 at 1 Ag -1 with 89 % capacitance retention till 3000 cyclic voltametric cycles at a scan rate of 100 mVs -1 . These results confirmed that the freshly prepared Mn2O3 nanorod arrays is a good electrode material for supercapacitor device applications.

2.1.Materials
All reagents were analytical grade and employed without further purification.

Synthesis of Mn2O3 material
In a typical synthesis, 0.01 m of cetyltrimethylammonium bromide (CTAB) was dissolved in a 100 mL of DI to form a homogeneous solution. After that, O.5 M of manganese acetate tetrahydrate precursor was dissolved in 100 ml of De-ionized water and it was added to CTAB solution. In this mixture, 10 ml of 2 M NaOH was added and stirred for 10 min. to occur precipitation process. Then, it was transferred to the ultrasonicator and sonicate for 0, 20 and 40 min for Mn2O3-1, Mn2O3-2 and Mn2O3-3 materials. The final products were collected by centrifugation and washed with deionized water and ethanol respectively. The resultant samples were dried in an oven at 70 °C for 12 h and it was annealed at 300 °C for 3h to get the final product.

2.3.Material characterization and electrode preparation
The BRUKER D8 Advance instrument with Cu-Kα radiation ((λ=0.154060 nm) was used to analyse the x-ray diffraction analysis whereas the FTIR analyses were done in

X-ray diffraction analysis
X-ray diffraction patterns of the freshly prepared Mn2O3-1, Mn2O3-2 and Mn2O3-3 materials were recorded to identify crystal phase features as presented in fig. 1 a- respectively. All the x-ray diffraction patterns provide similar spectrum whereas the intensity only varies. The absence of impurity peaks indicates that the present CTAB assisted sonochemical synthetic approach generates highly pure Mn2O3 materials. The result confirms the formation of highly pure Mn2O3 materials and it related to the previous literature also [33].

3.3.Morphological analysis
The morphological features of the Mn2O3 materials plays a significant role in the electrochemical characteristics and it analysed using HR-SEM analysis (fig 3 a-f). Figure 3 a and b shows the lower and higher magnification HR-SEM images of the Mn2O3-1 material which shows definite shaped nanoparticle structure with the size of 120 ±10 nm. The small amount of nanorods also formed in between the nanoparticles. Interestingly, small such nanoparticles with a size of 12 ±5 nm were grown on the outer region of both nanoparticles and nanorods which may lead to rising the specific surface area for electrochemical process.

3.4.Electrochemical analysis
The electrochemical properties of freshly prepared Mn2O3 materials were analysed using cyclic voltametric and galvanostatic charge/discharge analysis. Cyclic voltammetric techniques is a powerful tool to analyse redox properties, charge storage mechanism, specific capacitance and rate capability of the electrode materials. Figure 4 a-c shows the cyclic voltammetric curves of the Mn2O3-1, Mn2O3-2 and Mn2O3-3 materials respectively. CV analyses were recorded within in a potential limit from 0 to 1 V at six different sweep rates such as 5, 10, 25, 50, 70, 100 mVs -1 . Additionally, 1 M Na2SO4 was used as an electrolyte for entire electrochemical analysis. All the CV curves exhibit rectangular shape, which confirms the electrochemical double layer capacitor behaviour with rapid charge transport properties [36]. The electrochemical performance of the CV curves increased with rising of scan rate from 5 to 100 mVs -1 which is due to the rapid ionic and electronic transport across the surface of the electrode. The shape profile of the CV curves are remain same at higher scan rates confirming the high stability and fast charge transport feature of the electrodes materials [22]. Generally, the area under the CV curve of electrode material is directly proportional to the specific capacitance feature. In the present endeavour, the Mn2O3-3 electrode material possesses high area under the CV curve when compared to Mn2O3-1 and Mn2O3-2 electrodes, confirming superior specific capacitive feature of the Mn2O3-3 electrodes.
The specific capacitance of the CV curve is calculated using the equ. 1. [37] = ∫ 2 * * * (1) Where Cs is the specific capacitance (Fg -1 ), ꭍidV is the integrated area of CV curves in the cyclic voltammogram, S is the scan rate (mV s −1 ), M is the active mass of the electrode material (g), and V is the applied potential window (V). The specific capacitance values of Mn2O3-1, Mn2O3-2 and Mn2O3-3 electrodes are were calculated to be 272, 524 and 647 Fg -1 respectively at a scan rate of 5 mVs -1 . The rate capability of all Mn2O3 electrode materials were studied using sweep rate vs specific capacitance graph as shown in fig. 4 d. Due to time limitation progression, the scan rate decreases with rising of scan rate from 5 to 100 mVs -1 [38].
The galvanostatic charge/discharge studies of all Mn2O3 electrodes were demonstrated at various current densities such as 1, 2, 3, 10 and 30 Ag -1 and it shown in fig. 5 a-c. It is perfectly visible that all the charge discharge curves are exhibiting triangular shape, which confirms the electric double layer-based charge storage mechanism and it is more reliable with CV results. The charge curves can be seen to be symmetrical to their respective discharge equivalents, further suggesting their outstanding reversibility and ideal electrochemical capacitive features of the Mn2O3 electrodes [39]. The charge/discharge curve of Mn2O3 -3 electrode possesses the larger time then remaining electrodes such as (Mn2O3 -1 and Mn2O3 -2) confirms the high specific capacitance behaviour of the Mn2O3 -3 electrode. The specific capacitance has been estimated from the galvanostatic charge/discharge studies by the equ.2.
Where Cs is the specific capacitance (Fg -1 ), I is the current density (Ag -1 ), ∆t is the discharge time (s), m is the mass of the active materials and ∆V is the potential window. The Mn2O3-1, The specific capacitance vs current density graph is shown in fig. 5 d. The resultant specific capacitance values were found to be decrease when rising of current density from 1 to 30 Ag -1 and this outcome is well agreed with the CV results. At lower current densities, the larger time would allow the effective participation of all electrode materials in electrochemical process but it is not probable for high current densities. Consequently, the specific capacitance is lower at high current densities [42].

Mn2O3-2 and
In supercapacitors, the energy and power density factors are very significant. The energy and power densities can be obtained by charge discharge curves and it calculated using and 88 % (Mn2O3-3) of initial capacitance till 3000 continuous cycles. This result establishing the good cyclic stability behaviour of the Mn2O3-3 electrode materials and signifying the potential to be utilized for supercapacitor electrode materials.

Conclusion
In summary, the Mn2O3 nanorod arrays were successfully synthesised using sonochemical method with the aid CTAB template. The Mn2O3 materials were calcined at 300 °C to get the final product. The crystalline behaviour and bonding nature of the prepared materials were characterized using XRD and FTIR techniques, which are confirmed the formation of Mn2O3 materials. The HR-SEM was used to characterize the surface morphological features and it confirms the nanorod arrays structure. The CV and GCD analysis were carried out to examine the electrochemical properties. The CV curves provide the maximum specific capacitance of 647 Fg -1 at a scan rate of 5 mVs -1 while the GCD curves exhibit 656 Fg -1 at a current density of 1 Ag -1 respectively. In addition, the energy and power density feature of were also calculated and it attained the 91.1 Wh Kg -1 and 14985 Wkg -1 respectively. The cyclic stability studies show only 12 % reduction and withstand 88% of initial capacitance till 3000 CV cycles at scan rate of 100 mVs -1 . Such significant outcomes of electric double layer capacitive Mn2O3 nanorod arrays is the potential electrode materials for supercapacitor application.

Conflicts of interest
There are no conflicts to declare.