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Date of Award
Chemical Engineering and Materials Science
In this dissertation, a multifaceted approach involving detailed kinetic studies, an arsenal of characterization techniques, and atomistic simulations were combined to allow for interpretation of macroscopic reactivity and stability trends of heterogeneous electrocatalysts. Specifically, atomic scale insights were developed to understand the key factors that govern electrochemical transformations of molecular oxygen via its reduction and evolution reactions (ORR/OER). These oxygen-based electrochemical reactions were chosen as probe reactions because they are central for sustainable energy conversion and storage technologies in regenerative H2-fuel cells and Li-O2 batteries. Currently, these reactions are catalyzed by cost-prohibitive Pt and Ir-based catalysts, thus limiting the widespread adoption of these technologies.
Non-precious metal containing non-stoichiometric mixed metal oxides of the general form An+1BnO3n+1 (A = alkaline earth/rare earth metal; B = transition metal; n = 1, 2, 3, …∞) remain a high interest class of electrocatalytic materials for catalyzing these reactions. These oxides are compositionally versatile and can accommodate >90% of the metals in the periodic table, allowing for practically limitless opportunities to tune their catalytic performance. However, lack of effective design strategies that can link the initial oxide composition with their resulting catalytic activity and stability has hampered their development. To overcome these limitations, local surface electronic structure of the active centers in these oxides were probed both experimentally and theoretically and correlated to their resulting electrochemical activity and stability towards ORR/OER.
To begin with, the effect of different 3d transition metals in these oxides, on their ORR performance was studied. It was found that the strength of metal–oxygen bonds in the surface of the oxide, as described by the oxide surface reducibility was crucial in determining their electrocatalytic performance. It was found that LaMnO3 provides the optimal metal–oxygen bond strength, consequently leading to enhanced ORR performance. Further, the differences in the metal–oxygen bond strength in these oxides was exploited to effectively tailor the electronic structure of infinitesimal amounts of 4d/5d metal cations. This was shown to switch catalytically inert Rh and supported Rh oxides into highly active cationic centers in LaNi1-xRhxO3 (0.01≤x≤0.02) for ORR. On the other hand, the surfaces of these oxides were found to be dynamic in nature during OER. Consequently, a link between the initial oxide composition and the dynamic factors that control the catalytic activity toward OER was developed. Finally, a fundamental framework to investigate electrocatalysis at solid-solid interfaces between an oxide electrocatalyst and the solid discharge products in Li-O2 batteries was also developed.
The rational design strategies developed in this dissertation clearly outlines the impact of investigating the surface electronic structure of heterogenous catalysts and correlating it with their catalytic performance. Although, the insights developed here were specifically for oxygen electrocatalysis on non-stoichiometric mixed metal oxides, the principles used here can be extended to other catalytic systems, as well as other targeted reaction chemistries. This leads to a bottom-up approach of catalyst design, rather than a trial and error one.
Samira, Samji, "Towards Molecular Level Insights Into Oxygen Electrocatalysis On Non-Stoichiometric Mixed Metal Oxide Electrocatalysts" (2021). Wayne State University Dissertations. 3520.