Fabrication and Characterization of Metal Oxide Based Supercapacitors
Creators: name: Yogesh, Kumar
Publisher: name: Zenodo
Year: 2021
Subjects: Supercapacitor
Batteries
Electrochemical
Hydrothermal Synthesis
Energy Storage

description: <strong>Abstract</strong> Energy storage is equally important as energy production. The modern human society demands lightweight, flexible, inexpensive and environment friendly energy storage systems. Batteries are the major energy storage devices, but slow charge-discharge rate, short life cycles and bulkiness of battery limit its applications in portable and wearable devices. Lately, supercapacitors have been receiving a great attention as alternative energy storage devices because of their distinctive features such as high power density, light weight, fast charging-discharging rate, secure operation and long life span. Supercapacitors, also called electrochemical capacitors, are already being used in various applications such as hybrid vehicles, power back up, military services and portable electronics like laptops, mobile phones, wrist watches, wearable devices, roll-up displays, electronic papers, etc. The materials utilized in the supercapacitors play a prominent role, because the performance of supercapacitors depends on its properties. Specific capacitance of a supercapacitor depends on the surface area and the pore size distribution of the electrode material used for its fabrication. Compared with the transition metal oxides and conducting polymers, carbon and its different types provide larger surface area. However, this high surface area of carbon is not completely accessible for the electrolyte. In this context, metal oxide nanostructures are considered quite attractive candidates in energy storage applications due to their unique properties. Metal oxide nanostructures based energy storage devices have been shown to exhibit superior electrochemical performance due to their high surface to volume ratio and high mechanical flexibilities. The supercapacitor performance depends on morphology and oxidation state of metal oxide. Metal oxides such as RuO<sub>2</sub>, MnO<sub>2</sub>, TiO<sub>2</sub>, NiO, CoO, CuO, and composite materials are potential candidates for supercapacitor applications. RuO<sub>2</sub> and MnO<sub>2</sub> are the prominent electrode materials due to higher energy density with higher theoretical capacitance of about 1450 and 1270 F/g, respectively. RuO<sub>2</sub> limits its utilization being scarce, extremely expensive (5000/- @1g) and toxic to some extent. At the same time MnO<sub>2</sub> has its own benefits like cheap material cost, plentiful availability in the earth’s crust, and environmental friendliness. However, the conductivity of MnO<sub>2</sub> is much lower ranging from 10<sup>-7 </sup>to10<sup>-3</sup> S/cm. The main advantage of MnO<sub>2 </sub>is that it shows much higher specific capacitance with aqueous electrolytes as compared to other gel or solid electrolytes. But associated with this advantage is the problem that MnO<sub>2</sub> is soluble in water and cell becomes dead after a few charging and discharging cycles. The previous reported studies suggest that MnO<sub>2</sub> electrode supercapacitors (when material is synthesised by hydrothermal method without a surfactant) show low cyclic stability and specific capacitance. Triethanolamine (TEA) is a good surfactant for synthesis of metal oxides but has not been used for synthesis of MnO<sub>2</sub>. It is possible to synthesise MnO­<sub>2</sub> nanostructures with desirable crystalline structures and morphology using TEA as surfactant, which won’t dissolve in water over high number of charge-discharge cycles. The aim of this study is to synthesize the metal oxide based manganese nanostructure material to explore its applicability as the electrode in supercapacitor. For this, b-MnO<sub>2</sub> nanostructures have been synthesized via TEA assisted hydrothermal method at different reaction temperatures. Silver doped MnO<sub>2</sub> nanocomposite and Carbon-MnO<sub>2 </sub>composite electrode materials have also been synthesized by hydrothermal method expecting enhanced electrochemical performance of the cell. The structural and crystallite size study of the materials have been carried out using X- ray diffraction (XRD). The morphological studies have been carried out by using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). X-ray Photoelectron Spectroscopy (XPS) analysis is performed for material purity, quantity and binding energy of those elements that are present within the top 1-12 nm of the sample surface. Fourier Transform Infrared Spectroscopy (FTIR) investigation has been undertaken to identify the functional groups in the material. Supercapacitors have been fabricated using the electrodes which are prepared with the various synthesized β-MnO<sub>2</sub> and its composite materials. The electrochemical performance has been tested with three different analytical tools including cyclic voltammetry, galvanostatic charge discharge and electrochemical impedance spectroscopy. XRD data of MnO<sub>2</sub> samples are in agreement with JCPDS card no. 81-2261, which suggests the formation of β-MnO<sub>2</sub> tetragonal planes with lattice parameters a = b = 4.3985 Å and c = 2.8701 Å. The intensity of the diffraction peaks is found to increase as the synthesis temperature is increased from 60℃ to 120. XPS analysis of β-MnO<sub>2 </sub>sample synthesised at 120℃ reveals the surface oxidation state of manganese with Mn<sup>4+</sup> 2P<sub>3/2</sub> and Mn<sup>4+</sup> 2P<sub>1/2 </sub>peaks located at 642.1 eV and 652.5 eV. SEM micrographs suggested that the temperature of synthesis affects the surface morphology and nanorods structure of MnO<sub>2</sub> and that the high surface area and porous β-MnO<sub>2 </sub>nanorods are developed by hydrothermal synthesis at the reaction temperature near 100℃. The porosity of the nanostructures assists in better ion transportation in the electrode and improves the contact area for electrolyte ions to perform rapid surface redox reactions. Selected Area Electron Diffraction (SAED) pattern of β-MnO<sub>2</sub> showed crystal planes at (110), (101), (210), (220), and (301) with interplanar spacing 3.109 Å, 2.403 Å, 1.959 Å, 1.555 Å, and 1.300 Å, respectively in agreement with the results obtained from XRD. FTIR exhibited the MnO<sub>2</sub> vibrational bands at 707.1 and 529.2 cm<sup>-1</sup>. As expected, the β-MnO<sub>2 </sub>nanorods analysed as supercapacitor electrode in 1M Na<sub>2</sub>SO<sub>4 </sub>liquid electrolyte exhibited high specific capacitance of 89.63, 128.05, 461.59 and 288.72 F/g at a scan rate of 10 mV/s (on a wide potential window of 0-0.8 V) for the sample synthesized at temperatures 60℃, 80℃, 100℃ and 120℃, respectively. The best performance is observed for electrode of the MnO<sub>2</sub> synthesised at 100℃, which can be attributed to large surface area (due to larger size nanorods) available for charge-storage and for redox-reactions (pseudocapacitive type of charge-storage). The energy densities recorded are 7.11, 10.55, 38.89 and 30.67 Wh/Kg at a power density of 0.4 kW/Kg. The morphological analysis of Ag doped MnO<sub>2 </sub>performed by SEM revealed a particle size of about 50 nm. The highest specific capacitance is found to be 523.91 F/g at the scan rate 10 mV/s (on a potential window of 0-0.8 V). Doping of Ag in MnO<sub>2</sub> increases the specific capacitance of the supercapacitor as compared to pure MnO<sub>2</sub> nanorods. Carbon-MnO<sub>2</sub> supercapacitor has given specific capacitance value (560 F/g at scan rate 10 mV/s on a potential window of 0-0.8 V) which is even higher than AgMnO<sub>2</sub> electrode based symmetric supercapacitor. The specific capacitance increases as carbon has higher surface area leading to increased adsorption of ions on the surface of electrode. The thesis also suggests various extended research options like different electrolytes, other electrode materials and fabrication of asymmetric supercapacitors aiming at further enhanced and desirable characteristics.