(571d) Mechanistic Understanding of Water-in-Salt Electrolytes to Electrodeposit Refractory Metals

Refractory metals possess unique properties including high melting points, high hardness at room temperature, chemical resistance, and high density. However, processing challenges from the high temperature stability require advanced manufacturing approaches to leverage the unique properties of refractory metals. The electrodeposition of refractory metals has received renewed attention in the past decade as a scalable approach to apply uniform coatings to enable a wide range of applications in aerospace, nuclear, catalysis, biomedical fields. This talk will focus on developing methods to electrodeposit rhenium (Re) and tungsten (W).

Both Re and W have traditionally been difficult to deposit by electrolysis from aqueous solutions due to the over potential for hydrogen evolution and complex electrochemical reactions. Electrodeposition of rhenium usually occurs from the perrhenate ion (ReO4^-) with an oxidation state of +7. The exact mechanism to reduce the perrhenate ion from +7 to metallic rhenium is still unknown. It is unlikely that the 7 electrons transfer in a single step, so reduction likely occurs through intermediate oxide species including ReO₂, ReO₃ and Re₂O₅. Combined with the hydrogen evolution reaction, Re coatings are generally deposited at low Faradaic efficiency, are brittle, and limited in the obtainable thickness (sub-micron). Similarly, electrodeposition of tungsten begins from the tungstate ion with an oxidation state of +6 with an unknown reduction mechanism.

Water-in-salt electrolytes have arisen as a promising approach to electrodeposit metals with reduction potentials that overlap the hydrogen evolution reaction (HER). In these electrolytes a high concentration of salts crowds out water molecules and disrupts the periodic tetrahedral network of hydrogen bonds. Ultimately, this results in reduced proton reduction.

Herein, we present on the synergistic effects of using water-in-salt electrolytes with complexing agents to further control the proton reduction reaction. A custom high-throughput cell was developed to rapidly screen electrolytes and explore time-dependent evolution of the electrodeposition processes. Solutions were analyzed with commonly used electrochemical techniques, Raman, and NMR spectroscopy. Specifically, we found that increasing the concentration of the complexing agent in water-in-salt electrolytes results in improved electrodeposition efficiency and morphology. This result is rationalized via surface interactions between the complexing agent and the substrate that preferentially enables reduction to the metal over proton reduction.