Controlling Composition and Stoichiometry of Co-Intercalated Chevrel Phase Sulfides via Electrochemical and Solid-State Synthesis Pathways

Carbon dioxide levels (CO₂) in the atmosphere continue to rise exponentially as fossil fuels remain a primary energy source for many industrial processes. Consequently, alternative energy technologies, such as solar and wind, must be harnessed to convert CO₂ into liquid fuels through the development of electrocatalytic materials that exhibit high selectivity and efficiency for CO₂ reduction (CO₂R). One such class of materials that have shown promise for CO₂R are Chevrel phase sulfides (CPs), due to their ability to host a wide variety of metal cations that can alter the structural and electronic properties of the crystal framework and in turn CO₂ binding motifs. CPs with the general formula M_aMo₆S₈ (M = transition, alkali, alkaline, or post transition metals) are excellent materials to investigate ion (de)insertion mechanisms due to the channels and cavities formed by the extended array of Mo₆S₈ clusters. While electrochemical insertion of a single mono or multivalent metal cation into CPs has been thoroughly investigated, co-insertion of two or more metal cations remains underexplored, despite the potential to further stabilize binding intermediates for CO₂R. Alternate electrochemical synthesis pathways can be exploited to not only co-insert two or more metal cations into the CPs framework for CO₂R studies but to further understand the thermodynamics and kinetics of chemical and electrochemical processes occurring during co-insertion. Furthermore, insertion methods, such as open circuit potential (OCP) mechanisms, can be used to investigate the synergistic movement of metal cations throughout the channels formed in the extended Mo₆S₈ solid and their effect on the electronic properties of the crystal structure that could prove beneficial for understanding product selectivity in CO₂R and other catalytic reactions. Aside from electrochemical co-insertion in aqueous electrolytes, medium temperature solid-state synthesis can also be utilized as a synthesis pathway to CPs materials with multiple metal cations inserted. In this work, control of composition and stoichiometry of co-inserted transition metal CPs was achieved through OCP and cyclic voltammetry (CV) electrochemical synthesis techniques, as well as medium temperature solid state synthesis methods. Materials were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), and x-ray absorption near edge structure spectroscopy (XANES).