Date on Master's Thesis/Doctoral Dissertation
Chemical Engineering, PhD
Committee Co-Chair (if applicable)
Atmospheric plasma; non-stoichiometric oxides; mixed metal oxides; metastable phase; non-equilibrium phase; clean energy
Clean energy production and storage are two of the most significant challenges in the 21st century currently limited by the discovery and development of new and advanced materials. Complex oxides and alloys made using earth-abundant elements will play a crucial role in technology development moving forward, however, current preparation techniques are limited by their inability to produce complex oxides and alloys with precise composition control at fast timescales. A concept was proposed to produce mixed metal oxides with composition control through the oxidation of liquid precursors via plasma oxidation. It was hypothesized that the oxidation process can be completed in fast timescales owing to the rapid heating and cooling of the plasma process. Even though the rapid timescales for oxidation can be understood through fast heating processes during plasma exposure, the mechanisms responsible for composition control are not immediately obvious. So, fundamental experiments were carried out to elucidate the nucleation and growth steps responsible for metastable non-stoichiometric oxide formation. Interrupted oxidation experiments completed within twenty seconds revealed the following steps during plasma exposure of liquid droplets: the nucleation of monometallic oxide phases from an amorphous nutrient, solid-state reaction into intermediate mixed oxide phase, and formation of metastable phase. Evidence also suggests the fast kinetics of the oxidation process depends on the enormous heat released from the recombinative reactions among plasma species present in the plasma. The viability of a select set of plasma-synthesized oxides were tested in energy conversion and storage technologies. The technique was successfully used to synthesize W0.99Ir0.01O3-δ alloy which showed high oxygen evolution reaction (OER) activity and stability in acid with an overvoltage reduction in the excess of 500 mV compared to the same composition prepared via standard thermal oxidation route. The structural dilution of iridium with earth-abundant tungsten will enable the efficient use of scarce iridium resources. In alkaline media OER, charge-transfer type double perovskite (La0.9Ca0.1Co0.5Ni0.5O3-δ) prepared via the rapid plasma method shows excellent activity rivaling best performing complex oxide electrocatalysts. Most importantly, the obtained experimental data, combined with density functional theory calculations allows for relating the high OER activity to the strong hybridization of the transition metal 3d and oxygen 2p bands. Again, this technique has been used to fabricate manganese-enriched nickel-manganese-cobalt (NMC) oxides. The resulting NMC materials were tested as cathodes in lithium ion battery and show competitive results compared with NMCs prepared through other routes. This dissertation presents a concept utilizing plasma oxidation of liquid precursors for composition control of complex oxides and alloys. The presented concept could expedite the accelerated discovery and development of advanced materials for energy conversion and storage. Furthermore, the underlying nucleation and growth mechanistic aspects for forming non-stoichiometric oxide phases will add scientific knowledge to our understanding of the synthesis of materials far from equilibrium.
Ajayi, Babajide Patrick, "Plasma oxidation of liquid precursors for complex metal oxides." (2019). Electronic Theses and Dissertations. Paper 3145.