Nanostructured materials for energy storage and pH ultramicroelectrodes

• Main Author: Khani, Hadi
• Other Authors: David O. Wipf
• Format: Others
• Language: en
• Published: MSSTATE 2017
• Subjects: Chemistry
• Online Access: http://sun.library.msstate.edu/ETD-db/theses/available/etd-02162017-154540/

<p>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 (Ni_4.5Co_4.5S8) 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 Ni_4.5Co_4.5S8 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). </p> <p>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. </p>