Solid-state planar micro-supercapacitors: materials interfaces, state-of-art electrode design and self-powered device
The demand for miniaturization of energy storage systems has accelerated the development of on-chip micro-power devices for integration into portable electronic devices. Micro-supercapacitors can suitably cater to the need by functioning as efficient miniaturized energy storage devices with high power density, fast charge-discharge rates and long cyclic lifetime. The electrode material and its design are two dominant factors affecting the performance of a micro-supercapacitor. The thesis work focuses on fabrication of solid-state micro-supercapacitors using electrode design and material as the parameters to enhance the performance. The interfaces of materials were designed to attain pseudocapacitive properties for high electrochemical performance. Further, the strength of the electric field was improved through the rational design of electrodes utilized for the fabrication of micro-supercapacitors. Solid-state micro-supercapacitor is presented with a planar sharp edge concentric circular geometry of gold electrode coated on a glass substrate and pseudocapacitive material for the charge storage. A few metal oxides and metal dichalcogenides such as ruthenium dioxide, manganese oxide (MnO), molybdenum disulphide (MoS2) are known to exhibit capacitive response similar to the carbon materials through a phenomena called pseudocapacitance. Thesis work uses key pseudocapacitive materials like iron (III) oxide (Fe2O3), MnO, and MoS2 in a matrix of electrically conducting carbon foam (CF) for solid-state micro-supercapacitor. Fe2O3 nanoparticles were directly synthesized on the walls of a three-dimensional (3D) CF matrix for high surface (209 m2/g) and enhanced pseudocapacitance. A systematic optimization was performed to achieve an optimal ratio of CF and Fe2O3 for a maximum enhancement of ~48% in charge storage capacity in a three-electrode electrochemical measurement. Polyvinyl alcohol–phosphoric acid was used solid and transparent electrolyte. The modified electrode design with ~23% larger perimeter and higher electric field at the tip of spikes (~68%) resulted in a high capacitance (~235%) compared with the conventional interdigitated electrodes. The device exhibited an excellent areal energy density (1.73 µWh/cm2) and cyclic stability (99.5% retention after 10000 cycles). Asymmetric electrode configuration is used for a large potential window and hence, an increase in the energy density. All-pseudocapacitive asymmetric solid-state micro-supercapacitor was fabricated using MnO, which exhibits a potential window of -0.4 to 0.4 V in combination with Fe2O3 (a potential window of -1 to 0 V) along with CF matrix, where 3D CF-MnO and CF-Fe2O3 were used as anode and cathode materials, respectively. An elaborated optimization, provided a larger voltage window of 1.4 V with a high areal energy density of 5 µWh/cm2, which is 189% higher than that of CF-Fe2O3 for the planar electrode of micro-supercapacitor. Metal dichalcogenides, recently have drawn a lot of attention due to the layered structure with high surface area and attributed to the charge transfer through Faradic reactions on the surface. MoS2 was combined with the CF matrix, where a significant contribution of charge storage was attributed to a diffusion-controlled mechanism (intercalated pseudocapacitance), elaborated using the Dunn’s method. In another electrode design, a novel zig-zag edge of the planar interdigitated electrodes was used to fabricate micro-supercapacitor. The design provided higher perimeter (~50% enhancement) and higher electric field (~57%) at each edge compared to the planar interdigitate electrodes. The solid-state micro-supercapacitors with novel zig-zag design exhibited a ~241% enhancement in capacitance compared with the planar edge electrode. The obtained result in modified design is higher than previously reported geometries introduced for better performance. Moreover, overall equivalent series resistance was lower by ~95% in the zig-zag edge electrodes than in planar edge electrodes, thus improving power capability. Furthermore, semiconducting MoS2, having an energy bandgap of 1.9 eV was utilized for the fabrication of an optically chargeable micro-supercapacitor under 600 nm illumination. The optical interaction in optically chargeable micro-supercapacitor is further evaluated in a narrow bandgap material for infrared (IR) interaction. A novel pseudocapacitive, titanium sesquioxide (Ti2O3), a Mott-insulator with a bandgap of ~ 0.1 eV, was used as an electroactive material for device fabrication. In Ti2O3, the transition from semiconducting state to metallic state occurs after heating at 142 °C. Hence, pristine and annealed Ti2O3 (after ex-situ heating at 200 °C, 300 °C and 400 °C) was used in a symmetric configuration for the fabrication of planar micro-supercapacitor. Ti2O3-based solid-state micro-supercapacitors, annealed at 300 °C, showed a maximum enhancement in areal capacitance (~560%) compared to the pristine Ti2O3. The semiconducting to metallic states transition has a significant impact on the observed changes in the electrochemical response. A band merging is observed after annealing at a certain temperature, as obtained from the density functional theoretical calculation using temperature-dependent x-ray diffraction data of Ti2O3. The theoretical observation is in agreement with the obtained changes in electrochemical results observed because of annealing. The total charge storage due to different mechanisms in the Ti2O3 electrodes was quantitatively separated using Dunn’s method. Further, the optical interaction of Ti2O3 with IR exposure was exploited for the optically chargeable capability. The thesis work opens wider avenues to discover smart micro-supercapacitor designs for much-improved performance with added self-powering capability through combined optical and electrochemical interactions.