Understanding Intercalation Kinetics and Structural Changes of Chevrel Phase Sulfides as a Function of Stoichiometric Control of Cu Intercalation in Aqueous Environment via Electrochemical Methods

New materials with versatile applications that can meet growing global energy demands while simultaneously aligning to the global call to shift away from carbon-intensive energy systems are highly desirable. Finely controlling stoichiometry in multinary materials will afford frameworks with tunable electro/photoelectrochemical properties (e.g. interfacial charge transfer kinetics, adsorption thermodynamics, small-molecule conversion selectivity), tunable charge storage and transfer properties (e.g. charge transfer resistance, electrochemical capacitance, conductivity), and tunable semiconductor properties (e.g. carrier concentration/lifetime, mid-gap state formation/population, conduction/valence band positions).<br/>Chevrel-phase (CP) M_xMo₆T₈(M=alkali/transition metal; x=0-4; T=S, Se, or Te) materials have shown promise as intercalant hosts for multivalent transition metals, owing to their unique intercalation capacity which in turn affects their structural and electronic properties.<br/>Furthermore, powder X-ray diffraction (PXRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) can be used to further understand crystal structure, electron localization, structural disorder, and elucidate electron transfer within the CP’s interesting cluster framework. <b>I have focused efforts in understanding these effects by evaluating stoichiometric control of Cu intercalation into the binary CP sulfide-phase (Mo₆S₈) host framework via electrochemical methods in aqueous environment.</b> To date, I have successfully identified the electrochemical potentials that correspond to the intercalation/deintercalation of stoichiometric amounts of Cu in the CP host (C_xMo₆S₈; x=1-3). Additionally, PXRD, EDS, XPS, and XAS confirm successful incorporation and subsequent structural and electronic changes to the CP-sulfide host as a function of Cu intercalation.<br/>In this work CP cluster frameworks comprised of a transition metal (Mo) octahedron enclosed in a chalcogen (S) cage are used as a host material. These individual cluster units are aligned in an extended structure via the sharing of terminal chalcogen and as a result, produce cavities that are ideal for the hosting of cationic transition metal (CTM) species via intercalation/deintercalation. By controlling the stoichiometric amounts of these CTM’s (in this case Cu) intercalated into the structure via the varying of experimental parameters (e.g., electrochemical potentials, scan rates, electrolyte concentrations, etc.), we can take advantage of electronic and structural changes in the host materials.<br/>To identify the electrochemical potentials at which Cu ions were intercalated/deintercalated into the CP host, cyclic voltammetry was used. These potential extremes that defined our cyclic voltammograms’ potential windows were based on pourbaix diagrams of Cu to 1) operate within a potential window with an ideal oxidation state (i.e. Cu^2+/1+ vs Cu) and 2) to avoid competing reactions, especially metal plating and hydrogen evolution reactions (HER). Next linear sweep voltammetry was used to control the stoichiometric amounts of Cu incorporated into the CP cavities by varying potential and scan rates.<br/>Confirmation of the presence and stoichiometric amount of the metal in the CP host was achieved firstly via bulk analysis methods such as PXRD and EDS. After confirmation of C_xMo₆S₈; x=1-3 via bulk analysis, more sensitive characterization (i.e. XPS and XAS) was used to further understand crystal structure, electron localization, and structural disorder of the intercalated CP host. The XAS data, specifically the X-ray absorption near edge structure (XANES), was especially helpful in monitoring host changes as it allowed for the observation of the effect and amount of intercalated Cu in the CP-sulfide host, due to increased electron density donation as a function of Cu content and subsequent depression of the S-k pre-edge feature.