UC Davis

The increased energy demands of the rising global population in a fossil fuel-based energy economy have caused unprecedented anthropogenic CO2 emissions, which presents a potentially irreversible threat to human societies. The imperative transition to carbon-neutral or carbon-negative energy solutions requires the interplay of advanced technologies that can provide electricity generated from renewable sources, including wind, solar, and hydropower, while also providing energy-dense liquid fuels for long-term storage. To this end, electrochemical processes such as CO2 reduction, N2 reduction and hydrogen evolution could provide chemical feedstocks and fuels currently obtained by large-scale emission processes without requiring massive changes to the present energy infrastructures. The economic viability of this approach is highly dependent on the design of catalysts from earth-abundant elements that exhibit high activity and selectivity along with chemical and catalytic stability over long periods of operation. In aqueous solutions, CO2 and N2 reduction must compete with the kinetically more facile hydrogen evolution reaction, which is one of the main challenges in achieving catalytic selectivity. Therefore, understanding hydrogen adsorption interactions on the catalyst surface can simultaneously assist in improving hydrogen evolution activity, as well as increasing selectivity for CO2 and N2 reduction reactions. To this end, compositionally diverse families of materials such as molybdenum chalcogenides (sulfides, selenides, and tellurides) offer an ideal platform to evaluate hydrogen adsorption interactions and develop composition-structure-function relationships. This dissertation details an integrative research approach that includes the (1) synthesis of molybdenum chalcogenides compositions through approaches that allow facile compositional changes as well as control of morphology and dimensionality, (2) characterization of their local and electronic structures, and (3) proposed structure-function relations that could assist in the design of molybdenum chalcogenides for hydrogen evolution, CO2 reduction and N2 reduction reactions.

Chapter 1 introduces metal chalcogenides as promising family of materials to evaluate the influence of composition, local chemical coordination, and electronic structure in energy conversion reactions. A discussion of the crystal structure, thermodynamic stability, and synthetic accessibility of binary pseudo-molecular Chevrel-Phases Mo6X8 (X = S, Se, Te) and ternary 1-dimensional Pseudo-Chevrel-Phases (K2Mo6X6; X=S, Se, Te) is included. Foundational information relevant to the generation of reactivity descriptors for electrocatalysis and the microwave-assisted solid state synthetic approach used in the following chapters is also provided.

Chapter 2 outlines the use of a rapid, microwave-assisted solid-state method to synthesize Chevrel-Phase sulfides, selenides, and tellurides. Hydrogen adsorption interactions are evaluated electrochemically under hydrogen evolution conditions as a function of chalcogen (S, Se, Te). Density functional theory and X-ray absorption spectroscopy are used to understand the interactions of the chalcogen with the adsorbed hydrogen and explain reactivity trends.

Chapter 3 expands the study of hydrogen adsorption interactions to Pseudo-Chevrel-Phases. Scanning electron microscopy, powder X-ray diffraction, and energy dispersive X-ray spectroscopy are used to elucidate the dimensional modifications in Pseudo-Chevrel-Phases as a function of chalcogen. Additional properties for this family of materials, such as charge transfer kinetics for proton reduction and specific capacitance are also discussed.

Chapter 4 summarizes the state of the art in experimental and computational evaluations of Chevrel Phases as electrocatalysts for hydrogen evolution, CO2 reduction, and nitrogen reduction reactions. The review examines composition-dependent physicochemical properties that have shown promising electrocatalytic activity for such small-molecule reduction reactions. Future directions to generate promising multinary chalcogenides for energy conversion reactions are also included.