In-Situ Characterization of Electroreduced Tantalum and Niobium in Deep Eutectic Solvents

Abstract

Room temperature ionic solvents are expected to possess electrochemical properties suitable for tantalum and niobium redox analogous to high temperature molten salts, in which successful bulk metal electrodeposition is reported. However, with the combination of perplexing multivalent chemistry of niobium and tantalum and alien electrochemical properties of ionic solvents, when compared to better characterized aqueous environments, the fundamental understanding of the electrochemical behavior of niobium and tantalum in these promising, yet compositionally diverse, solvents is severely underpopulated. Herein we report a new toolbox of coordinating additives that form, in many traditional and ionic solvents, chemically-robust reduced niobium and tantalum complexes with distinct photometric and paramagnetic properties, that serve as metal redox indicators and intermediate valence traps during electroreduction. We showcase the utility of these indicator species for clarifying the electrochemical behavior of niobium and tantalum in chemically varied ionic solvent environments by comparing in-situ electrochemical UV-Vis and electron paramagnetic resonance (EPR) observations in a series of chemically dissimilar deep eutectic solvents.