Synthesis of Niobium Pentoxide and its Reduced Graphene Oxide Nanocomposite for Enhanced Supercapacitor Applications

A simple hydrothermal synthetic route has been developed for the synthesis of perovskite lanthanum aluminate (Nb 2 O 5 ) and reduced graphene oxide composite (RGO). The synthesized materials have been characterized for its morphology using, XRD, SEM, techniques. The materials are used as electrode materials for supercapacitor applications. Electron microscopic images indicate RGO sheets are densely attached to surface of Nb 2 O 5 with perforated cage like morphology. A high specific capacitance of 751 Fg -1 at a scan rate of 2 mVs -1 was exhibited by RGO/ Nb 2 O 5 hybrid structure electrode when compared to pure RGO (535 Fg -1 ) and pure Nb 2 O 53 (175 Fg -1 ) electrodes at the same scan rate using cyclic voltammetry (CV). By using chronopotentiometry (CP), the RGO/ Nb 2 O 5 hybrid structure electrode out stretched specific capacitance of 572 Fg -1 at a current density of 0.5 Ag -1 and able to retain specific capacitance (C sp ) of about 427 Fg -1 even at very high current density of 10 Ag -1 .RGO/ Nb 2 O 5 composite also possesses an energy density of 179 Wh kg -1 with a maximum power density of 1500 W kg -1 . From the Electrochemical Impedance Spectroscopy (EIS), the RGO/ Nb 2 O 5 hybrid structure electrode has shown phase angle close to -90 o for frequency up to 0.01Hz, suggesting that the nanocomposite electrode material approached ideal capacitor behavior.


Introduction
The supercapacitor, also known as the electrochemical capacitor, is a new type of energy storage device that sits between conventional capacitors and batteries.Due to the high power density, fast charging and discharging time, long cycle life, and wide operating temperature range, etc. [1,2], supercapacitor has shown great potential in application as energy storage and supply green transportation, mobile power, wearable smart devices, military special equipment, long-distance telecommunication, and other new industrialization fields [3][4][5], which greatly promotes basic energy science research and enormous economic development.Depending on their energy storage principle, supercapacitors can be categorized as either faradaic pseudocapacitor or electric double layer capacitor.Due to their superior density and power density, electrochemical capacitors outperform conventional electrostatic capacitors and batteries in terms of • Email: editor@ijfmr.com

IJFMR240219431
Volume 6, Issue 2, March-April 2024 2 performance.The basic principle of electric double layer capacitors is that carbon-based materials store electrical energy at the electrode-electrolyte interface, whereas multivalent transition metal oxides and conducting polymers that can undergo redox reaction in electrolytes exhibit faradaic pseudocapacitance [6,7].Typically, supercapacitor assemblies consist of four components: the couple electrodes, electrolyte, a current collector, and a separator, with the electrode being the determining factor for the capacitor's primary performance.Therefore, low-resistance electrode materials are required to reduce energy loss during the charging and discharging processes [8,9].The electrode materials must simultaneously possess physicochemical stability, good performance of charge capacity, exceptional specific capacitance values, and outstanding anti-fatigue performance.Although carbon-based materials are common capacitive materials, conductive polymers and metal oxides [10,11] have been thoroughly investigated.However, these single materials do not meet the requirements for the development of new, superior energy storage materials, particularly the materials required for the development of highperformance supercapacitors [12,13].
Since its discovery, graphene has been studied extensively as a supercapacitor's carrier material due to its high specific surface area, high electrical conductivity, and good chemical and mechanical stability.Utilizing a single graphene material for energy storage is neither ineffective nor wasteful.Consequently, research into capacitive materials is crucial [14,15].It improves the performance of graphene by combining two-dimensional graphene with other materials, such as transition metal oxides, hydroxides, conductive polymers, sulphides, and polymer compounds, in order to obtain an excellent composite via chemical group doping and surface defect modification.It has been reported that RuO2, Co3O4, Ni(OH)2, CoS2, and other materials can be combined with graphene [16][17][18].Niobium pentachloride is a promising precursor among transition metal oxides and is easily converted into niobium pentoxide (Nb2O5).Nb2O5 has excellent acid and alkali corrosion resistance, a large number of active sites, excellent optical properties, and stable electrochemical properties, which have attracted increasing interest [19,20].This paper describes the facile synthesis of RGO-Nb2O5 nanocomposites with excellent capacitance and stability.The nanocomposite of Nb2O5 and RGO significantly increases specific capacitance.Using cyclic voltammetry and galvanostatic current charge-discharge, the electrochemical properties of the RGO-Nb2O5 nanocomposite were analyzed and compared to those of pure Nb2O5 and RGO.

Synthesis 2.1.1 Synthesis of Graphene oxide
A modest modification to the hummer's approach was used to synthesis GO.It was decided to make a 9:3 mixture of H2SO4 and H3PO4.A second graphite/KMnO4 mixture with a mass ratio of 2:6 was made.
To avoid a vigorous reaction, the acid mixture was slowly poured into the beaker containing the graphite-KMnO4 mixture in an ice bath at a temperature of 5 to 10 °C.The solution was kept at a constant temperature of 60 °C in a water bath.The heated, somewhat dark solution was withdrawn from the water bath and mixed with 400 mL distilled cold water to end the reaction.After that, 5 mL of H2O2 was added dropwise, resulting in a yellowish-colored solution.The addition of H2O2 was intended to terminate the oxidation reaction by reacting directly with the excess potassium permanganate.The reaction product was rinsed four times with HCl aqueous solution and distilled water until it reached the appropriate pH. [19]

Conversion of GO-RGO
Graphene oxide (GO) was synthesized from natural graphite flakes using a modified Hummers method.
To prepare a suspension of GO, 0.3 mg/mL of graphene oxide was dispersed in deionized (DI) water.The reduction of GO was then carried out by adding green tea extract (GTE) to the GO suspension in a batch reactor-closed system.Typically, GTE was added at a weight ratio of GTE/GO=1, with 30 mL of GO suspension.The reduction reaction took place in the reactor for 8 hours at a temperature of 90 °C.Throughout the reduction process, the GO-GTE mixture was continuously stirred at a speed of 200 rpm.
After the reaction, the suspension containing the final product was filtered using a nylon membrane with a pore size of 0.22 µm.The collected material was washed 3-5 times with DI water.The resulting product was named Reduced Graphene Oxide (RGO) [20].

Synthesis of Niobium pentoxide
Niobium pentoxide can be synthesized through a precipitation method.Initially, a niobium-containing precursor, such as niobium chloride or niobium oxalate, is dissolved in a suitable solvent, typically water or an organic solvent.The solution is then subjected to precipitation by adding a precipitating agent, such as ammonium hydroxide or sodium hydroxide, to form niobium hydroxide.The obtained niobium hydroxide precipitate is further washed with water to remove any impurities and then dried.Subsequently, the dried niobium hydroxide is subjected to calcination at a high temperature, typically around 600-800 °C, in a furnace.This thermal treatment leads to the conversion of niobium hydroxide into niobium pentoxide.The resulting niobium pentoxide is then cooled, collected, and stored for further use.

Synthesis of RGO-Nb2O5
To synthesize RGO-Nb2O5, start by preparing graphene oxide (GO) using the modified Hummers method.Next, exfoliate the GO by sonicating it in water or a suitable solvent to obtain reduced graphene oxide (RGO) sheets [21].Then, mix the RGO sheets and Nb2O5 nanoparticles in a solvent, such as ethanol or water, and sonicate the mixture to ensure a homogeneous dispersion.Subsequently, perform a hydrothermal treatment under controlled temperature and pressure conditions to promote the growth and deposition of Nb2O5 onto the RGO surface.This process allows for the formation of RGO-Nb2O5 composite material.Finally, wash the obtained composite, followed by drying and calcination to remove any residual solvents or organic species, resulting in the final RGO-Nb2O5 composite material ready for characterization and subsequent applications.

working electrode preparation.
The flower petals like RGO-Nb2O5 composite, pure RGO and pure Nb2O5 were used in a standard technique for fabricating the working electrode for capacitance measurements.The composition of the working electrodes was 85 wt% RGO-Nb2O5, 10 wt% acetylene black, and 5 % polytetrafluoroethylene (PTFE) in 1 -methyl-2-pyrrolidinone (NMP).A small amount of IPA was added to the mixture to fully mix and grind it into a homogeneous slurry.Then, 10 pieces of carbon paper with an average area of 1.0 cm 1.0 cm were uniformly coated with the prepared slurry.The solid electrodes were then dried using a vacuum oven equipped with a pressure gauge for 8 hours at 60 °C.The average amount of active  The specific capacitance ( ) was calculated using the following equation. ( Where represents the scan rate (mVs -1 ), W represents the active mass of the material (g), and V represents the potential window (V).All of the capacitance values determined for each electrode are displayed in Table 1.The CV behaviour of the composite were conducted by variation of different scan rate in the range of 2 -100 mVs -1 can be observed in the Figure 4. data at lower scan rate is 751 Fg -1 is may due to the more interaction between the active material and electrolyte, this goes on decreasing as increases in the scan rate of CV studies which implies that at higher scan rate produces of 397 Fg -1 .The significant improvement in the capacitance behaviour of the composite material is may due to the redox nature of the Nb were aligned on the surface of RGO, which enhances the capacitance through improving catalytic activity.The comparison of the electrode materials was carried out shown in the Figure 5, its corresponding values are tabulated in Table 1.electrodes, which produces corresponding values 473, 123 and 73 Fg -1 respectively.It is due to the RGO-Nb2O5 composites shows higher the integral area and higher the discharge time was observed in the Figure 6 than that of the individuals.For composite, GCD studies were continued at different current densities from 0.5 to10 mA cm -2 (Figure 7), which produces of 575 Fg -1 (0.5 mA cm -2 ) and 427 F/g (10 mA cm -2 ).The higher the value at lower current density is due to the higher the rate of electron diffusion with the electrode materials and electrolyte, at higher current density decreases in the value because of the rapid kinetics in the chemical reactions.A considerable retention of the composite electrode was satisfactory, which shows the 74.65% of Cs retention at 20 times of current density (10mA cm -2 ) and best values were obtained than the pure RGO and pure Nb2O5 electrodes were shown in Figure 8.On the basis of charge/discharge curves, specific capacitance of electrodes was calculated using the equation ( 2) depicted in Table 1. ( where I represents the current (A), t represents the discharge time (s), V represents the potential window (V), and W represents the mass of the electroactive materials (g).Further the all electrode materials were evaluated to check its energy density and power density, the RGO-Nb2O5 composites show an appreciable value than individuals.using equations ( 3) and ( 4), the energy density (ED) and power density (PD) of electrodes were calculated. ( Where E is the energy density (Whkg -1 ), V is the potential window (V), C is the specific capacitance (Fg -1 ), t is the discharge time (s), and P is the power density (Wkg -1 ).In Table 2, the calculated ED and PD of all electrode systems are tabulated.The composite active material electrode complete discharges and produces an ED of 179 Wh kg -1 and PD 1500 W/kg at 0.5 Ag -1 .As the increase in the applied current density the PD also increases and RGO-Nb2O5 composites are produced 30 kW kg -1 , which is characteristic best values on compare with the pure RGO and pure Nb2O5.

Galvanostatic charge discharge studies (GCD)
EIS testing has been carried out on the electrodes between 0.01 and 10 5 Hz.The Nyquist plot and values were calculated using the equivalent circuit shown in Figure 10.The electrodes provided values of and that were estimated.The Nyquist plots high frequency region showed low resistance, which is useful for charge-discharge cycle analysis and power density.It is due to electrodes are possesses good capacitive and low diffusion resistances, a low frequency area Nyquist plot that is roughly parallel to the imaginary axis was found in the composites, this produces the supporting evidence to the CV and GCD analysis of the composites.

Conclusions
Using a simple hydrothermal technique, Nb2O5 nanoparticles are firmly anchored on nitrogen-doped RGO and used as an electrode material for its electrochemical performance in supercapacitor applications.In comparison to pure Nb2O5 and RGO, the electrochemical performance of Nb2O5 anchored on RGO was superior.The RGO-Nb2O5 nanocomposite demonstrated a high energy density of 179 Wh kg -1 at a power density of 1500 W kg -1 and was able to maintain an energy density of 133 Wh kg -1 even at a power density of 30000 W kg -1 .Additional work on the fabrication of an asymmetric device using RGO-Nb2O5 nanocomposite could pave the way for future energy storage research utilizing Niobium-related materials.

2 . 3
materials in 10 electrodes is used to calculate the loading mass in each electrode.A working electrode, counter electrode (Pt wire), and reference electrode (calomel) were used in a three-electrode setup for the electrochemical performance, which was carried out in 6.0 M KOH.It has been used to conduct electrochemical tests on the CHI-660E workstation.[21]Material characterization Bruker D2 Phaser XRD tools were used to obtain X-ray diffraction (XRD) patterns.A scanning electron microscope was used to analyze the surface morphology (SEM) (JEOL JSM 840A) 2.3.1 SEM Analysis The SEM image of the pure Nb2O5 particles shows irregular shapes petals of flower on the surface of the layers of reduced graphene oxide (RGO) were represented in figure.1A &B.The merged particles of the pure Nb2O5 particles indicated in the figure. 1 C.

Figure 8 :
Figure 8: Variation of capacitance as a function of various current densities for RGO-Nb2O5 composites, pure RGO and pure Nb2O5 electrodes