HYBRID NANOSTRUCTURED MATERIALS FOR SUPERCAPACITORS

Abstract

The recent development of modern electronic devices and the progressive research on renewable energy-based electrochemical energy conversion systems have fuelled the drive toward advanced high performance energy storage devices. Among the various types of energy storage devices, supercapacitors have been recognized as one of the most promising candidates for high-power applications due to their outstanding properties, including high power density, long cycle life, fast charge/discharge rate, and better safety. Basically, carbon materials are best known for their double layer capacitance behavior, which provides the high power density to the capacitors. Also, pseudocapacitors (metal oxides, polymers, metal sulfides and metal selenide) generally have high specific capacitance. However, the low specific capacitance of carbon-based materials and the poor cycling stability and low conductivity of the pseudocapacitive materials limits the effective utilization of these electroactive materials in the energy storage field. To enhance the energy density of supercapacitors, suitable pseudocapacitance materials have been under progressive research. The suitable pseudocapacitance materials having good electrical conductivity and high surface area is one of the key issues in the field of supercapacitor. Also, the nanostructured electrode-electrolyte interface is the heart of every supercapacitor, which determines energy storage capacity of the device. With the above motivation of enhanced electrochemical performance as well as to overcome those issues, the design of a hybrid nanostructure based on metal oxide, metal sulfide and metal selenides with a carbon matrix have been intensively studied in this thesis. The hierarchical hybrid nanostructures combining of EDLCs and pseudocapacitors have large surface area, good electrical conductivity, and short path for ion diffusion. All the electroactive materials were synthesized using hydrothermal method with various conditions. The physico-chemical properties of as-synthesized nanomaterials are investigated in detail and its effect on electrochemical performance and charge storage behavior are also explored.

Part-I of this dissertation covers the synthesis and electrochemical characterization of vanadium pentoxide nanobelts and high electrical conductivity graphene decorated vanadium pentoxide nanobelts. The various ratio of graphene to vanadium pentoxide was tailored and the corresponding charge storage behavior are studied in detail. Among the VxGy group of electro-active materials, the vanadium-rich composite V3G1 showed the maximum Cs value of around 288 F g-1 at the scan rate of 10 mV s-1 and excellent cyclic stability; the capacitance retention of about 82%, even after 5000 cycles in three electrode system.

To improve the surface area and the effective utilization of electrolyte ions to all the electroactive surfaces, a layered two dimensional materials were synthesized, characterized and evaluated as supercapacitor electrodes in part-II. Mainly focuses on the molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) based TMDCs and its corresponding composites with various carbon materials such as graphene and carbon fiber paper (CFP). When compared to TMDCs, transition metal oxides (TMOs) have good stability, high pseudocapacitance etc., However, the electrical conductivity of oxides are comparatively lower than the TMDCs.

The performance of the high surface area MoS2 sponge electrode material was tested by assembling a symmetric supercapacitor with an aqueous electrolyte. The symmetric cell exhibited a device and single electrode capacitance is 128 F g-1 and 510 F g-1 at a scan rate of 2 mV s-1, which is the highest value reported to date for this material. In addition, the symmetric supercapacitor revealed a high energy density of approximately 6.15 Wh kg-1 and good cyclic stability over 4000 cycles. The amorphous MoSx thin film coated carbon fiber paper (CFP/a-MoSx) as a binder-free three-dimensional (3-D) electrode was delivered a device capacitance value of 41.96 mF cm-2 at a scan rate of 1 mV s-1. More interestingly, the long term cycle test showed the substantial increase in capacitance retention of up to 600% for 4750 cycles is obtained. The increasing in specific capacitance trends indicates the electroactivation process, allowing more effective intercalation of cations between the layers (exfoliation of 2D materials).

The electrochemical energy-storage behavior of MoSe2 nanosheets and its carbon matrix was investigated for supercapacitor applications using symmetric cell configuration is discussed in the last section. The MoSe2 nanosheets electrode exhibited a maximum specific capacitance of 198.9 F g-1 and the symmetric device showed 49.7 F g-1 at a scan rate of 2 mV s-1 with capacitance retention of approximately 75% was observed even after 10,000 cycles at a high charge-discharge current density of 5 A g-1. The MoS2/rGO nanosheets electrode exhibited a specific capacitance of 211 F g-1 with excellent cycling stability (180% capacitance retention for 10,000 cycles), compared to its pristine MoSe2. The amorphous MoSex nanostructures (nanoneedles and nanoparticles) were grown on carbon fiber paper 3D substrate. There were two different pre-treatement methods (plasma cleaning and electro etching) were employed to induce the hydrophilicity of CFP. Interestingly, different pre-treatment methods induced the formation of different MoSex) nanostructure formation on the CFP. The amorphous MoSex) coated CFP electrodes were tested for the supercapacitor applications. The surface pre-treatment played an important role on the electrochemical performance. The overall enhanced electrochemical performance of the hybrid nanostructure electrode is mainly attributed to the improved electron and ion transfer mechanism involving synergistic effects of both the pseudocapacitance and the electric double layer charge-storage behavior. These results demonstrate that enhanced electrochemical performance of hybrid nanostructure electrode based on layered transition metal compounds with carbon matrix has the great potential application for next generation high-performance supercapacitor devices.