FIU Discovery

With the ever-advancing technology, there is an incessant need for reliable electrochemical energy storage (EES) components that can provide desired energy and power. At the forefront of EES systems are electrochemical capacitors (ECs), also known as supercapacitors that typically have higher power and superior cycle longevity but lower energy densities than their battery counterparts. One of the routes to achieve higher energy density for ECs is using the hybrid EC configuration, which typically utilizes a redox electrode coupled with a counter double-layer type electrode.

In this dissertation, both scale-up (coin-cell type) as well as scale-down (on-chip miniaturized) hybrid ECs were designed, constructed and evaluated. The first part of the dissertation comprised material identification, syntheses, and electrochemical analyses. Lithium titanate-anatase titanium oxide (Li₄Ti₅O₁₂-TiO₂) composites were synthesized via electrostatic spray deposition (ESD) and characterized in both half-cell and full-cell assembly against lithium and nanostructured carbon based counter electrodes, respectively. The second redox type material studied for hybrid electrochemical capacitors was ESD derived manganese oxide (MnO_x). The MnO_x electrodes exhibited a high gravimetric capacitance of 225F g^-1 in aqueous media. Further improvement in the rate handling of the MnO_x electrodes was achieved by using CNT additives. The MnO_x-CNT composites were tested in full-cell assembly against activated carbon counter electrodes and tested for different anode and cathode mass ratios in order to achieve the best energy-power tradeoff, which was the second major goal of the dissertation. The optimized hybrid capacitor was able to deliver a high specific energy density of 30.3 Wh kg^-1 and a maximal power density of 4kW kg^-1. The last part of the dissertation focused on a scale-down miniaturized hybrid microsupercapacitor; an interdigitated electrode design was adopted in order to shorten the ion-transport pathway, and MnO_x and reduced graphene oxide (rGO) were chosen as the redox and double layer components, respectively. The hybrid microsupercapacitor was able to deliver a high stack energy density of 1.02 mWh cm^-3 and a maximal stack power density of 3.44 W cm^-3, both of which are comparable with thin-film batteries and commercial supercapacitor in terms of volumetric energy and power densities.