Hydrothermal Synthesis and Electrochemical Examination of Nanostructured Cobalt Sulfide for High Performance Energy Storage Devices

With increasing worlds energy and power demand, there is urgency needed in developing high performance and stable materials for energy applications. Supercapacitors hold great potential in future energy storage devices due to their high-energy performance, ability to have high power density, fast charge-discharge capability and long cyclability. Researchers in recent years have been shown significant progress for the improvement of supercapacitor performance and development of cost effective performing materials for energy storage applications. In this work, we have reported the synthesis of nanostructured cobalt oxide (Co3O4) converted into cobalt sulfide (Co8S9) that prepared using a facile hydrothermal method. We have analyzed the obtained nanostructured cobalt sulfide (Co8S9) electrochemically and structurally. The crystallinity and phase purity of the synthesized (Co8S9) nanostructures were evaluated using X-ray diffraction. Morphology and particle size of the synthesized (Co8S9) on nickel foam has been analyzed using scanning electron microscopy (SEM). Electrochemical investigations have carried out systematically on fabricated electrodes. The cyclic voltammograms (CV) of cobalt sulfide electrode showed redox peaks suggesting typical pseudocapacitive behavior. The electrochemical properties of cobalt oxide have been improved significantly after converting to cobalt sulfide, showing specific capacitances of 983 and 7358 mF/cm2 at 2 mA/cm2, respectively. A supercapacitor device fabricated and the performance of the device was examined at room temperature and elevated temperatures using cyclic voltammetry and galvanostatic charge-discharge. Results showed excellent flexibility and cyclic stability with the maximum specific capacitance of 1,000 mF/cm2. Charge storage capacity was increased when the temperature was increased, suggesting improvement in the electrochemical properties of the device at elevated temperatures. Our results indicate that Co8S9 under harsh conditions could be appropriate material for high performance energy storage devices

SULFURIZATION OF NANOSTRUCTURED COBALT OXIDE FOR ENERGY STORAGE APPLICATIONS

Development of energy storage devices with high energy performance, power density, fast charge-discharge capability and long cyclability is needed to meet the increasing demand for energy, power and environmental protection in our daily life. Supercapacitors have great potential in future energy storage devices with magnificent properties. Recently, researchers have shown great progress for the improvement of supercapacitor performance by fabrication of nanostructured transition metal chalcogenides materials. One of the main objectives of this thesis is to synthesize nanostructured cobalt oxide and then converte them to cobalt sulfide using a facile hydrothermal method. The synthesized cobalt oxide and cobalt sulfide were structurally and electrochemically characterized. The structural characterizations were performed using X-ray diffraction and scanning electron microscopy. The electrochemical properties were studied using a standard three-electrode cell containing a platinum wire as a counter electrode, saturated calomel electrode as a reference electrode, and synthesized materials as a working electrode. The energy storage capacity was investigated using cyclic voltammetry (CV) and galvanostatic charge-discharge techniques. Cobalt oxide and cobalt sulfide showed specific capacitances of 983 and 7358 mF/cm2 at 2 mA/cm2, 5 respectively. The electrochemical properties of cobalt oxide have been improved significantly after converting to cobalt sulfide. Moreover, the effect of temperature on the electrochemical properties of the supercapacitor device fabricated using cobalt sulfide was studied. It was observed that the charge storage capacity of the device increased with increase in the temperature, which could be due to decrease in series resistance of the device. Our results suggest that cobalt sulfide could be used as an advanced material for energy storage applications

Nanostructured cobalt oxide and cobalt sulfide for flexible, high performance and durable supercapacitors

Transition metal oxides and sulfides have great potential for energy storage devices due to their large theoretical energy storage capacities. A facile technique was used for the synthesis of nanostructured and phase pure cobalt oxide (Co3O4) and subsequently converting it to cobalt sulfide (Co9S8). The effect of sulfurization on energy storage capacity of the cobalt oxide was explored. Microstructural characterizations using X-ray diffraction and scanning electron microscopic reveal formation of phase pure and nanostructured Co3O4 and Co9S8. It was observed that the areal capacitance of Co3O4 (983 mF/cm2) improved significantly after converting to Co9S8 (7358 mF/cm2). The CV curves of the Co9S8 electrode on bending showed outstanding stability with no change in energy storage properties. New insights into the better performance of Co9S8 over Co3O4 based on electrochemical investigations are presented. The performance of the Co9S8 as an electrode material for energy storage applications was further investigated by fabricating a supercapacitor device. The supercapacitor device showed outstanding stability up to 5000 cycles of charge-discharge study. The performance of the supercapacitor was observed to be improving with temperature. The supercapacitor displayed ~100% enhancement in energy storage property on increasing temperature from 10 to 70 °C. Our results suggest that hydrothermally grown Co9S8 on nickel foam can be utilized for high capacity, flexible and binder free electrode for energy storage applications

Highly Efficient and Durable Electrocatalyst Based on Nanowires of Cobalt Sulfide for Overall Water Splitting

Water electrolysis to generate hydrogen and oxygen at low overpotential is one of the main requirements for clean and renewable energy technology. Currently, precious-metal-based catalysts such as Pt, IrO2, and RuO2 are being used for water electrolysis limiting its wide range of applicability due to their high cost and rare-earth abundance. In this research, we have used nanowires of cobalt sulfide as an efficient electrocatalyst for overall water splitting. Directly grown nanowires of cobalt sulfide on nickel foam provided superior electrocatalytic activities favoring electron transfer. High surface area and porosity of the nanowires allowed easy escape of the generated oxygen and hydrogen. Cobalt sulfide nanowires required an overpotential of 299 and 217 mV to achieve a current density of 10 mA/cm2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Furthermore, nanowires of cobalt sulfide required a low cell voltage of 1.66 V to achieve a current density of 10 mA/cm2 which is among the best-reported value. A facile preparation method, outstanding bi-functionality, and electrochemical stability of cobalt sulfide as both HER and OER electrocatalyst, suggest that cobalt sulfide could be a promising material for commercial applications in water electrolysis