Electrochemical behaviour of metal ions in ethaline-based solutions: Impact of electrode material

Deep eutectic solvents (DESs) have become increasingly important in material synthesis, particularly in electrodeposition processes. Ethaline – a DES composed of choline chloride and ethylene glycol – stands out due to its low viscosity compared to other DESs which is favourable for the electrodeposition process. Among the key factors influencing electrodeposition, the substrate material can also play a significant role in the electrochemical behaviour of metal ions and resulting deposit morphology at the early stages of film growth. This study investigates the electrochemical behaviour of various metal ions, including Co(II), Fe(II), Ni(II), In(III), Sn(II), Cu(I), and Cu(II), on four selected substrates (Pt, glassy carbon, Au, and Cu) in dried ethaline-based solutions. The research highlights the impact of different working electrode materials on deposition processes and examines the onset potentials for deposit dissolution, providing insights into predicting galvanic replacement reactions in the DES medium. The formation of the complexes of different composition between metal ions and ethaline components leads to the sequence of equilibrium potentials different from aqueous, non-complex electrolyte solutions. As a remarkable example, we demonstrate the occurrence of galvanic replacement reaction between Ni(II) ions and a more noble Sn substrate in ethaline. The findings underscore the importance of considering both the choice of substrate material and the speciation in ethaline-based DESs.