The Development of the Controlled-Atmosphere Flame Fusion Methodology for the Growth of Nickel Single Crystals and the Interplay between the Oxidation of Nickel and the Hydrogen Oxidation Reaction in Alkaline Media

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Esau, Derek M.
Electrochemistry , Physical Chemistry , Single Crystal Growth , Inductive Heating
Nickel (Ni) and Ni-based materials have become promising alternatives to the platinum group metals that are currently used in sustainable energy technologies. To improve the fundamental knowledge of Ni electrochemistry, Ni single crystal electrodes that have highly ordered surface arrangements of atoms were used. The development of the controlled atmosphere flame fusion methodology as part of this thesis work has allowed for bench top growth of poly-oriented spherical single crystals (POSSCs) of non-noble metals. Ni POSSCs were oriented, cut, and polished to produce hemispherical monocrystalline electrodes (Ni(hkl)), namely, Ni(111), Ni(100) and Ni(110). The surfaces were then characterised using cyclic voltammetry (CV) in solutions outgassed with nitrogen gas (N2(g)) in 0.10 M aqueous NaOH solution at a potential scan rate (s) of 50.0 mV s-1 and temperature (T) of 295 K. Each Ni(hkl) had a unique electrochemical response for the formation and reduction of α‒Ni(OH)2 and β‒NiOOH. Due to the importance of inductive heating for the pre-treatment of Ni(hkl), the technique was described in detail, including the relevant theory of electromagnetism, heat conduction and radiation emission with temperature. This was followed by a discussion of practical, experimental considerations to best utilize inductive heating and examples of the electrochemical response of a Ni(111) with varying degrees of surface oxidation. Lastly, the influence of s on the formation and reduction of α‒Ni(OH)2, and the hydrogen oxidation reaction (HOR) in 0.10 aqueous NaOH at T = 298 K was studied. The CV response in electrolyte that was outgassed with N2(g) and saturated with hydrogen gas (H2(sat.)) with s = 1.00, 2.00, 5.00, 10.0, 20.0, 50.0 and 100 mV s-1 in the potential range of ‒0.20 V ≤ E ≤ 0.50 V was studied. In solution outgassed with N2(g), the analysis suggests that Ni is likely partially covered by α‒Ni(OH)2 with sections of metallic Ni at the peak. In H2(sat.) electrolyte, the HOR occurs after the formation α‒Ni(OH)2 and allowed us to propose the “bifunctional mechanism” for the HOR. This work has provided valuable experimental methodology to produce Ni(hkl) cheaply and effectively, but also valuable fundamental information about the electrochemistry of monocrystalline Ni electrodes.
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