Establishing Relationships Between Structure and Performance for Silicon Oxide Encapsulated Electrocatalysts
Marissa Beatty, Chemical Engineering, Esposito Group
Abstract: Using electrochemical devices like fuel cells and electrolyzers to convert excess energy into storable, chemical fuels is an attractive approach to buffer the variability from wind, solar, and hydroelectric energy sources with grid infrastructure. However, the development of highly active electrocatalysts that are required to efficiently drive the reactions within these devices is frequently constrained by issues like poor catalyst stability and selectivity when implemented at commercial scale. One approach towards mitigating these issues is through adhering a nanoscopic, semi-permeable oxide membrane onto the catalyst surface, creating a structure known as an oxide encapsulated electrocatalyst (OEC). These architectures have previously been shown to improve reaction selectivity, poisoning resistance and nanoparticle stability, however, little is understood about how metal oxide overlayers interact with – and affect – the catalyst surface, as well as alter the reactions occurring at the buried interface. Clear relationships that relate OEC structure to catalytic performance are therefore needed to accelerate the understanding and development of OECs. The aim of this dissertation is therefore to systematically investigate the design space of OEC architectures by using well-defined, model planar electrocatalysts to draw clear relationships between the structure, composition, and chemical/physical properties of OECs, and the resulting effects they have on electrocatalytic performance. Using planar Pt catalysts encapsulated by a thin, highly tunable carbon-modified silicon oxide (SiOxCy), properties like overlayer thickness, carbon concentration, and density can be specifically adjusted during fabrication, and ultimately is shown to influence the final reaction selectivity, transport properties, and stability of the resulting electrode. These relationships can be leveraged to control the electrocatalytic performance of complex alcohol oxidation reactions (AORs), where the addition of SiOxCy overlayers substantially increased AOR activity on Pt by promoting the formation of active surface intermediates. Operando characterization of OECS using ambient pressure X-ray photoelectron spectroscopy (APXPS) also demonstrate the dynamic interactions between the overlayer and electrolyte species, where local chemical and electronic conditions of the SiOx are affected by anionic species present in the electrolyte. As the overlayers themselves can introduce numerous perturbations in the system, the exact underlying mechanism that drives these phenomena remains ambiguous and will require further investigation. However, the studies presented in this thesis illustrate the unique opportunity that the application of OECs has towards the future customization of electrocatalyst structures, and would allow for more rational, targeted design for specific chemistries and applications.