Evaluating the Stability and Catalytic Reactivity of Molybdenum Chalcogenides: Towards the Design of Earth-Abundant Hydrogen Evolution and Carbon Dioxide Reduction Electrocatalysts
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Evaluating the Stability and Catalytic Reactivity of Molybdenum Chalcogenides: Towards the Design of Earth-Abundant Hydrogen Evolution and Carbon Dioxide Reduction Electrocatalysts


Sustainable terawatt-scale energy solutions for a constantly evolving energy landscape necessitate the rapid development of transformative materials that facilitate energy conversion reactions that can power industrial, commercial, and even residential sectors without further accelerating anthropogenic climate change. Transitioning from a highly pollutive fossil-fuel based energy economy to a carbon-neutral or even carbon-negative one could require massive infrastructural changes in energy production and distribution at both the national and global scales if this transition were to be achieved by complete removal of the liquid fuels currently afforded by crude oil refining. However, drop-in replacements for these fossil fuels that include short-chain hydrocarbons and oxygenates like methane, ethylene, methanol, and ethanol—maintain desirable energy densities and are synthesizable via electrochemical methods that can be readily coupled to renewable sources of electricity—namely, intermittent wind and solar electricity. The conversion of vastly abundant yet intermittent solar energy into easily storable fuels such as hydrocarbons and alcohols represents one of the defining challenges of our time. Hence, the onus of proof is ours as researchers to verify the existence of rational design principles for novel catalyst materials that can facilitate chemical transformations of abundant feedstocks such as atmospheric CO2 and water to viable fuels using sunlight as the only energy source. Such catalysts will not only provide a means of storing solar power but will also precipitate a paradigm shift within the chemical industry from reliance on petroleum feedstocks to the largescale utilization of CO2 as the primary carbon source. The design of novel catalytic materials is subject to a stringent set of criteria-the materials need to be constituted from inexpensive and earth-abundant elements, exhibit high activity and selectivity in terms of chemical transformations, and be stable over long periods of operation. This dissertation details an iterative research effort that includes the synthesis andstability analysis of new catalyst compositions, characterization of their local and electronic structures, as well evaluation of their catalytic reactivity for foundational energy conversion reactions like H+ reduction, CO reduction (COR), and CO2 reduction (CO2R).The development of precisely tailored catalyst materials is predicated on a fundamental understanding of their composition-structure-function relationships. Hence, efforts here were focused on investigating dimensionally controlled binary and ternary chalcogenide materials wherein compositional and structural variations give rise to tunable charge transport dynamics, interfacial reaction kinetics, as well as bulk and surface energetics. These studies span synthetic materials chemistry, synchrotron X-ray spectroscopy analysis of electronic and atomistic structure, CO2 reduction and hydrogen evolution electrocatalysis, as well as computational modeling, all in order to develop foundational materials design principles, where the evolution of electrochemical reactivity is evaluated in order to gain fundamental insights into energy conversion and storage. Chapter 1 provides a review of the state-of-the art in CO2 reduction and hydrogen evolution electrocatalysis, including fundamental principles that will be discussed in this work related to the interplay between physicochemical properties of catalyst materials and their surface reactivity. Also included in this chapter is a review of gaps in the current understanding of design principles for multinary chalcogenide-based electrocatalysts, as well as the route toward establishing such design principles that is highlighted in this dissertation. Foundational information relevant for synthetic and characterization techniques discussed through out the dissertation is also provided. Chapter 2 outlines the development of rapid, microwave-assisted solid-state synthetic methods for promising binary and ternary chalcogenides in the pseudo-molecular Chevrel-phase sulfide, selenide, and telluride electrocatalyst families of composition MyMo6X8 (M = metal intercalant; X = S, Se, Te; y = 0-4) which enable significant compositional and structural flexibility which is critical for developing the aforementioned composition-structure-function relationship. Our focus is on establishing synthetic accessibility based upon the thermodynamic stability of the target phases compared to other binary and ternary chalcogenide phases with competitive stability at intermediate (300-700°C) and high (>700°C) reaction temperatures. Chapter 3 elucidates the structural and electronic structure features of promising binary and ternary Chevrel-phase sulfide electrocatalysts using synchrotron-based X-ray spectroscopic methods. These studies highlight potential electronic signatures that can be used as proxies for electrochemical CO2R, COR, and HER reactivity, and offer glimpses into a new avenue of operando interfacial spectroscopic evaluation for these materials of interest. Observed spectral features in the S K-edge X-ray Absorption Near-Edge Structure (XANES) indicate that the increased population of frontier S 3p orbitals upon ternary intercalation into binary Chevrel-phase frameworks leads to a reactive Mo-S active site that is more conducive to mediating requisite proton-coupled electron-transfer steps in the CO2 and CO reduction pathways to liquid methanol. Additionally, the effect of composition and structure on thermodynamic stability is discussed through the lens of synergistically incorporating insights from experiment and theory to accelerate the discovery of new energy materials. Chapter 4 highlights an investigation into the effects of dimensional modification in the ternary molybdenum sulfide composition space wherein chalcogen-deficient 1-dimensional pseudo-Chevrel-phase (PCP) sulfide nanorods of composition (M2Mo6S6) are directly synthesized for the first time through solid-state methodologies. A nucleation mechanism is proposed based on computational surface modeling, and a series of alkali metal-intercalated PCP phases are evaluated electrochemically to elucidate interfacial charge-transport dynamics.

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