A case study using spectroscopy and computational modelling for Co speciation in a deep eutectic solvent

Abstract

Cobalt has a vital role in the manufacturing of reliable and sustainable clean energy technologies. However, the forecasted supply deficit for cobalt is likely to reach values of 150 kT by 2030. Therefore, it is paramount to consider end-of-life devices as secondary resources for cobalt. Electrorecovery of cobalt from leached solutions has attracted attention due to the sustainability of the recovery process over solvent extraction followed by chemical precipitation. Recently, we reported Co electrorecovery from two different cobalt sources (CoCl₂·6H₂O and CoSO₄·7H₂O) using ethylene glycol : choline chloride (EG : ChCl) in a 4.5 : 1 molar ratio, leading to higher purity and easier electrodeposition when sulfate was present as an additive. Here, Co²⁺ speciation is reported for the two EG : ChCl systems depending on the cobalt source using several spectroscopic techniques (e.g. NMR, EPR, FTIR) in combination with molecular dynamics simulations. Monodentate coordination of SO₄²⁻ to Co²⁺, forming the tetrahedral [CoCl₃(SO₄)]³⁻ was observed as the dominant structure in the system containing CoSO₄·7H₂O, whereas the system comprising CoCl₂·6H₂O shows a homoleptic tetrahedral [CoCl₄]²⁻ as the dominant structure. This resulted in knowledge being gained regarding Co²⁺ speciation and the correlation with electrochemistry will contribute to the science required for designing safe electrolytes for efficient electrorecovery.