Microwave-Assisted Molten Salt Synthesis of Chevrel Phase Chalcogenides

The increase in energy demand has brought the challenge of developing efficient energy-conversion technologies. Currently, natural gas reforming is the most common method to generate hydrogen fuel, but this process is energy-consuming and also releases greenhouse gases into the atmosphere. Renewable hydrogen production has long been considered a clean energy carrier and a highly promising alternative to transition away from fossil fuels. Alternatively, electrochemical proton reduction is a sustainable and green method for producing hydrogen gas – a process that leaves virtually no carbon footprint when coupled to renewable sources of electricity. Chevrel phase (CP; M_xMo₆S₈; M=alkali, alkaline earth, transition, and post-transition metal), a class of molybdenum chalcogenides are especially attractive due to their tunable composition and lower coordination on their molybdenum moieties which has led to previously observed catalytic activity for oxygen evolution, hydrogen evolution, and CO₂ reduction reactions. Theoretical studies under various reaction conditions agree that proton adsorption interactions in the chevrel phase occur through the undercoordinated chalcogen-molybdenum bridging site. Hence, reducing the size of the CP particles will increase the surface-to-volume ratio exposing a more undercoordinated Mo-S bridging site for proton adsorption.<br/>Traditionally CPs were synthesized by high-temperature solid-state reactions which can take several days and require extremely high-energy input. Although some approaches have been made to reduce the preparation time of CPs, the methods either suffer from uncontrolled particle size, high level of agglomeration, presence of MoS₂ impurities, or high energy input. This work presents an efficient route to synthesize size-controlled ternary Chevrel phase chalcogenides (Cu₂Mo₆S₈) by a microwave-assisted molten salt approach. This methodology has significantly reduced the reaction times from days to minutes resulting in significant time and energy savings yielding highly crystalline, faceted morphology of CPs. The phase, morphology and surface chemistry of the resultant materials were characterized by X-diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive X-ray Spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The electrochemical reactions were done in a three-electrode cell with Ag/AgCl as the reference electrode, graphite as the counter electrode, and catalyst ink on Toray paper as the working electrode.