Design and development of electrocatalysts and their microenvironments for applications in fuel cells : a focus on silver-based materials and the oxygen reduction reaction

Electrocatalysis plays a crucial role in a wide range of renewable and sustainable energy technologies, which are required to create a carbon-neutral or carbon-negative energy ecosystem, ultimately fostering the long-term prosperity of humankind. The oxygen reduction reaction (ORR) is key in electrochemical energy conversion and storage technologies such as fuel cells and metal-air batteries, which, for instance, have the potential to help decarbonize transportation and provide clean intermittent renewable energy storage. However, cheaper electrocatalyst materials with improved ORR activity, 4e---product selectivity, and stability are needed to deploy these promising technologies at a large scale. Anion exchange membrane fuel cells (AEMFCs) have emerged as a promising complementary alternative to the more mature proton exchange membrane fuel cells (PEMFCs) because the alkaline environment in AEMFCs allows for improved ORR kinetics and wider material stability compared to in the acidic conditions in PEMFCs. Moreover, diversifying ORR catalysts beyond conventional Pt-based materials is crucial for H2 FCs to achieve large scale deployment and thrive as a resilient and robust energy technology. Ag, which is two orders of magnitude cheaper than Pt, has emerged as a promising active, stable, and selective non-precious metal alkaline ORR catalyst. Moreover, Ag-bimetallics are an interesting class of materials for which density functional theory (DFT) modeling has predicted the possibility of intrinsically enhanced ORR kinetics. Ag-Cu, for instance, has already been shown to yield enhanced ORR active sites at certain surface compositions. Studying Ag-bimetallics in a well-controlled and systematic fashion is, therefore, crucial to developing material-property relationships that would aid in the design of optimal catalysts for the ORR and other important electrocatalytic reactions. In addition to catalyst material engineering, it is also important to study the electrolyte effects on electrocatalytic performance to design optimal electrochemical microenvironments. In this dissertation, I employ a wide range of complementary physical and electrochemical methods, in conjunction with DFT, to understand how to engineer high performing electrocatalysts. Specifically, I systematically synthesize, characterize, and test Ag-Pd and Ag-Mn alkaline ORR electrocatalysts, ultimately, establishing the fundamental material-property relationships attributed to the measured intrinsic catalyst performance as a function of composition and structure. The use of physical vapor deposition (PVD) is crucial for the systematic catalyst design and development in my work. Moreover, using PVD as a bridge between fundamental rotating disk electrode (RDE) and applied FC device studies, I systematically fabricate model ionomer-free Ag-Pd gas diffusion electrodes (GDEs) to investigate the performance of this material system in H2-O2 AEMFCs. Varying only the Ag:Pd alloy ratio, I find good agreement between the performance trends measured in the RDE and AEMFC configurations. In terms of electrocatalyst material engineering, in this work I develop Ag-based ORR electrocatalysts with tuned oxygen-adsorbate binding, affording state-of-the-art AEMFC performance. In addition, I also investigate the role of acid electrolyte anions on the ORR performance of Ag and Pd, as well as on the hydrogen and oxygen electrocatalysis performance of Pt. I find that performance varies as a function of electrolyte, that acid electrolyte anions effects are potential dependent, and that nitric acid affords improved electrolyte microenvironments conducive to improved performance compared to certain acids. By fundamentally understanding the interfacial processes responsible for the measured electrochemical performance, herein, I engineer high performing Ag-based ORR electrocatalysts and establish fundamental engineering principles to design optimal catalyst materials and catalyst--electrolyte microenvironments

Keywords: Chemical Engineering.
Advisor: Jaramillo Thomas Francisco
Bao Zhenan
Tarpeh William