Supercapacitors are electrochemical energy storage devices characterized by rapid charge-discharge speeds, high power densities, and long cycle lifetimes compared to batteries. Supercapacitors have many promising applications as energy storage devices in electric vehicles, renewable energy systems, grid energy management, as well as stationary and portable electronics. A current limitation of supercapacitors is their low energy density compared to batteries, which hinders their application as stand-alone energy storage systems. Supercapacitor energy density can be improved using pseudocapacitive materials, including conducting polymers and metal oxides, that store charge by faradaic and double layer capacitance. This dissertation explores the use of nanomaterial designs and novel fabrication methods to increase energy density of pseudocapacitor electrodes and enable more widespread application of supercapacitor energy storage.
The present work uses both experimental and modeling approaches to develop and test high energy density conducting polymer and metal oxide pseudocapacitors. Pseudocapacitor electrode performance testing was conducted using two- and three-electrode measurement techniques, including cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy. Pseudocapacitor electrode material properties were characterized using electron microscopy and x-ray measurement techniques. Vertically-aligned carbon nanotubes (VACNTs) were used as high surface area, porous electrodes that are coated with pseudocapacitive materials for high energy density. Modeling work includes empirical modeling of EIS and chronoamperometry results, and the development of a simplified porous electrode model for pseudocapacitor design optimization.
For conducting polymer electrodes, a hybrid polypyrrole (PPY)-VACNT electrode design was developed in which VACNTs are uniformly coated with PPY by electrodeposition. The hybrid supercapacitor electrodes achieve a 5x increase in pseudocapacitance compared to uncoated CNT and pure PPY electrodes. Empirical modeling results confirm an increase in faradaic and double layer capacitance for the hybrid supercapacitor compared to uncoated VACNTs. For metal oxide pseudocapacitors, atomic layer deposition (ALD) was demonstrated as a novel fabrication method to achieve highly conformal and uniform pseudocapacitive coating for ruthenium oxide (RuOx) electrodes. Post-ALD electrochemical oxidation increases hydrated oxide content of the ALD films, resulting in a 170x increase in pseudocapacitance for the ALD RuOx-VACNT electrodes compared to uncoated CNTs. The ALD RuOx electrodes achieve an ultra-high capacitance of 644 F/g, with performance maintained over 10,000 charge-discharge cycles and at scan rates of up to 20 V/s. Lastly, a simplified porous electrode model was developed to predict charge storage in ohmically-limited pseudocapacitor electrodes. Preliminary results show that the model is an effective tool for identifying performance-limiting factors in supercapacitor electrodes and optimizing pseudocapacitor electrode design parameters. Experimental and modeling results presented in this work highlight the importance of a comprehensive design approach for supercapacitor electrodes that considers interrelationships between material properties, design parameters, and fabrication methods.