Electrode-electrolyte interactions in choline chloride ethylene glycol based solvents and their effect on the electrodeposition of iron

Although deep eutectic solvents (DES) and ethylene glycol (EG) electrolytes are gaining ground as media for the electrodeposition of metals, the influence of these electrolytes on the electrode and electrodeposits has not been elaborated before. In this work, we investigate how choline chloride: ethylene glycol based (ChCl:EG) electrolytes interact with the glassy carbon (GC) working surface at different potentials and, how these interactions change during the electrodeposition of iron from 1:2 and 1:4 ChCl:EG electrolytes. GC substrates are exposed to both electrolytes in absence of iron in order to study the pure electrolyte-substrate interactions. Linear sweep voltammetry was used to determine the potential range in which the different reactions occur, while time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to detect and identify the molecules adsorbed on the surface. In all systems, there is an accumulation of either choline or its derivatives on the electrode surface, with 1:4 ChCl:EG electrolytes showing different decomposition products from 1:2 ChCl:EG electrolytes. Choline accumulation and decomposition was also found on iron electrodeposits. The electrolyte composition has a major impact on the chemical speciation of iron, and on the deposit's adherence to the substrate. These are two crucial characteristics that define the efficiency of iron deposition from DES.