Nanostructured Materials for Energy Storage and pH Ultramicroelectrodes
AdvisorWipf, David O.
Emerson, Joseph P.
Mlsna, Todd E.
This dissertation presents the synthesis and characterization of new types of nanostructured materials for use in high-performance aqueous rechargeable batteries and supercapacitors. In the first chapter, nanostructured nickel cobalt sulfide (Ni4.5Co4.5S8) was prepared through pulse-electrodeposition method. In addition, iron oxide nanosheets were prepared from graphite-coated iron carbide/α-Fe in a two-step annealing/electrochemical cycling process. A full-cell battery with supercapacitor-like power behavior was assembled with Ni4.5Co4.5S8 and iron oxide nanosheets as the positive and negative electrodes, respectively. The full-cell device delivers a specific energy of 89 Wh kg−1 at 1.1 kW kg−1 with a rate performance of 61 Wh kg−1 at a very high specific power of 38.5 kW kg−1. In the second chapter, we propose a route towards developing asymmetric supercapacitor devices having high volumetric energy densities though the modification of commercially available current collectors (CCs): nickel foam (NF) and carbon fiber cloth (CFC). A soft templating/solvothermal treatment route was employed to generate NiO/NiOOH nanosheets on NF current collectors (as positive electrode). CFCs were also modified via an electrochemical oxidation/reduction route to generate an exfoliated core-shell structure followed by electropolymerization of pyrrole into the shell structure (as negative electrode). Combining the individual materials resulted in a full-device asymmetric supercapacitor that delivers volumetric energy densities in the range of 1.67-2.65 mWh cm−3 with corresponding power densities in the range of 5.9-273.6 mW cm−3. Such performance is comparable to lithium thin film (0.3-10 mWh cm−3) and better than some commercial supercapacitors (< 1 mWh cm−3). In the third chapter, we established a simple, precise, and reproducible method to construct carbon fiber ultramicroelectrodes (CF-UMEs) with tip radius r < 1 μm. CF-UMEs were successfully used as SECM-tips to examine the “crystal structure orientation-OER electrocatalytic activity” relationship of iridium/iridium oxide catalysts. In addition, CF-UMEs were used as a substrate electrode for the electrodeposition of pH-sensitive iridium oxide. The pH response of these micrometer-sized pH electrodes has a rapid response (< 5 s) over the pH range of 2-12 with a super-Nernstian slope of 65.3 mV/pH. The prepared pH-UMEs were successfully employed as a potentiometric SECM-tip to image the pH changes at different substrates.