Doped Cobalt Oxide Catalysts for Aqueous, Electrochemical Oxygen Evolution and Alcohol Oxidation
Creators: name: Michaud, Samuel
Publisher: name: My University
Year: 2022
Subjects: subject: Understanding doped metal oxides for electrochemical oxidative transformations.
subject: Chemistry
subject: Science

description: As global energy production transitions to more renewable yet intermittent sources such as solar and wind, long term storage of this energy is a critical challenge that needs to be overcome. Hydrogen gas produced through electrochemical water splitting is one promising method for energy storage, however the viability generating large quantities of hydrogen gas is limited by the sluggish kinetics at the anode making improving the anodic activity vital in the advancement of renewable hydrogen production. Cobalt based materials show promise as active catalysts for anodic reactions under alkaline conditions. Spinel Co3O4 is one such cobalt material that is both active and stable for elongated times for the oxygen evolution reaction (OER). The defined crystalline system of this catalyst makes it an ideal candidate for systematic material alteration, where bulk replacement of atoms in the cobalt oxide lattice with transition metals allows for activity trends to be developed across a range dopant level giving insight into how these metals alter anodic activity. By systematically performing these alterations we can further the understanding of how different dopant ions affect OER activity and additionally understand what makes an active OER catalyst. In Chapter 1, I discuss the literature on transition metal based OER catalyst with a focus on the factors which affect catalytic activity in these systems. I then discuss how transition metal doping is used previously in the literature to improve upon the OER activity of cobalt oxide based materials. The remainder of the chapter discusses how by transitioning away from the OER at the anode to alcohol oxidation, the overall anodic reactivity can be increased. In Chapter 2 I discuss the doping of various transition metals alters the activity for the OER, with a specific focus on the activity trends observed in iron doped Co3-xFexO4 materials, highlighting a Co2.75Fe0.25O4 catalyst which operates at 360±1 mV overpotential however further iron doping is shown to decrease OER activity with CoFe2O4 operating at 460±1 mV overpotential, suggesting a highly complex relationship between Fe content and electrochemical activity, which goes against our previously published hypothesis. In Chapter 3 I discuss a CoV2O4 catalyst for the OER which shows remarkable OER activity, with a BET normalized activity of 368±73 uA/cm2, 300 times higher than Co3O4. However, through postmortem material analysis the activity is found to not be due to vanadium ions but rather due to the formation of a vanadium free cobalt oxide material. This study highlights the critical importance of post-mortem material analysis in electrocatalytic measurements. In Chapter 4 I discuss how a Co2NiO4 catalyst can be used to increase the anodic activity by oxidizing an alcohol substrate rather than water operating at 94±4 mA/cm2 current density. Additionally, I report how through changing the alcohol and the applied potential at the electrode, the proportion of carboxylic acid and aldehyde products is changed, with carboxylic acid charge efficiency increasing with increased alcohol size, and decreasing with increased applied potential. In Chapter 5 I highlight the major conclusions of my work, and the directions I believe the projects discussed in this Thesis should take. This dissertation highlights how transition metal doping can increase anodic activity and highlights the critical importance of in-depth systematic studies for improving catalyst systems. The work also highlights key activity trends in anodic activity and selectivity in the OER and AOR systems.