High temperature electrochemical studies on nickel: glycerol and nickel electro-oxidation




Borsboom-Hanson, Tory

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In this dissertation electrochemical nickel oxide formation in alkaline solution and the electro-oxidation of glycerol on polycrystalline electrodes are studied as a function of temperature. This is done using electrochemical impedance spectroscopy (EIS), Tafel analysis, cyclic voltammetry, chronoamperometry, and chronopotentiometry among other techniques. Additionally, in order to facilitate the study of aqueous alkaline systems beyond the normal boiling point of water, an electrochemical cell was designed utilizing a self-pressurizing autoclave. This allowed for the study of aqueous alkaline systems up to 140 °C. Product analysis of glycerol electro-oxidation on nickel was performed at various temperatures using HPLC. A reaction pathway for the organic products was determined. At su ciently high temperatures a polymer was discovered. This polymer product was characterized by DLS, DSC, CP-MAS NMR, and ATR-IR and determined to likely be a pseudo-polysaccharide. DSC analysis suggests that the polymer exists as three distinct structures, and DLS analysis suggests that the polymer exists in three di erent size distributions. The lack of a glass transition temperature in the DSC spectrum indicates that it is likely thoroughly cross-linked. The aging process of alpha-Ni(OH)2 to beta-Ni(OH)2 was studied as a function of temperature using cyclic voltammetry and dynamic EIS. This lead to the observation that beta-Ni(OH)2 does not appear to form on the oxide surface at 100 °C and above. A methodology was developed for preferentially stabilizing either beta-NiOOH or gamma-NiOOH on the electrode surface. This methodology was used to determine that beta-NiOOH is the better oxygen evolution catalyst of the two oxide phases. The reversible potential of Ni(OH)2 oxidation was observed to have a shift of -1:14 mV K-1, and this data was used in a thermodynamic analysis to identify the nickel species involved in the reaction. Based on data from the literature the oxidation of NiO or Ni(OH)2 to NiO2 appears to best match the observed data. Mechanistic analysis was performed for glycerol on nickel in alkaline solution using a combination of Tafel analysis, cyclic voltammetry, AC voltammetry, and EIS. This study indicates that glycerol oxidation behaves differently on gamma-NiOOH and beta-NiOOH, perhaps explaining the discrepancy between various pieces of data found in the literature. Tafel analysis led to the observation that there appear to be two di erent glycerol oxidation regimes. Below 80 °C, alpha = 0.5, indicating that the rate determining step is an electron transfer step with no pre-equilibrium electron transfers. At 80 °C and above alpha = 1, indicating that the rate determining step has no electron transfer and one pre-equilibrium electron transfer. This was determined to be caused by the transition of the underlying nickel oxide phase from gamma-NiOOH to beta-NiOOH because the change is retained upon cooling. Additionally, EIS showed two semicircles which indicates the presence of one kinetically signifficant adsorbed intermediate. These observations were incorporated into a detailed proposed reaction mechanism.



science, chemistry, electrochemistry, nickel, glycerol, oxidation