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Show moreDeep eutectic solvents (DES), on account of their low-cost, non-flammability and electrochemical stability, are attracting attention for their potential use in applications such as electrodeposition and energy storage. In these applications, knowledge of the transport and electrochemical kinetics properties of DES is critically important. To date, attempts to measure the kinetics parameters of the Cu2+ + e Cu1+ reaction in ethaline DES have yielded a cathodic charge transfer coefficient (α) in the range of 0.2–0.3 suggesting an unexpected asymmetric polarization behavior. In the present work, we pursued a comprehensive study of the kinetics and transport properties of the aforementioned reaction. Using steady-state and transient polarization measurements on RDE and microelectrodes combined with diffusion-reaction modeling, we demonstrate that the Cu2+/Cu1+ transition exhibits a charge transfer coefficient in the range of 0.49–0.54 and an exchange current density in the range 1.72–1.88 mA/cm2. Recommendations are provided for avoiding pitfalls in kinetics analysis of highly resistive DES electrolytes. These recommendations include the application of accurate correction and the use of electrode configurations where the secondary current distribution is relatively uniform.
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Show moreWe report on the effect of the substrate on electrochemical deposition of Cu from deep eutectic solvent ethaline. We investigated the polarization behavior during electrodeposition of Cu on Pt and glassy carbon (GC) from both Cu2+ and Cu+ containing ethaline using cyclic voltammetry (CV). Formation of bulk Cu deposits on both substrates underwent nucleation and growth processes; however, the nucleation was considerably sluggish on GC compared to Pt. While experiments in Cu+ solutions indicated that coalescence of Cu islands on Pt is a slow process and that its surface may not be fully covered by Cu, such determination of Cu coverage could not be made on GC. Cu dissolution is also slower from GC than from Pt. It was observed that CV of Cu deposition on GC is influenced by the surface preparation method. Since ethaline has high chloride concentration, a parallel study in aqueous 3 M NaCl solution was conducted in order to examine the influence of the chloride medium on the electrodeposition process. This revealed that electrodeposition in both media occurred in the same manner but with different charge and mass transfer rates caused by the differences in viscosity and chloride concentrations of the two solutions.
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Show moreThe Ag/AgCl reference electrode is commonly used in choline chloride based deep eutectic solvents. However, we found it undergoes significant potential shifts in electrochemical tests which previous reports largely ignored. In this work, we studied the degradation mechanism leading to its instability. Results show that due to the high Cl− concentration in ethaline, the AgCl film easily dissolves and forms AgCl2 − species causing a potential shift. Therefore, we suggest a [Fe(CN)6]3−/[Fe(CN)6]4− reference electrode based on the reversibility and low diffusivity of [Fe(CN)6]3−/[Fe(CN)6]4− redox couple in ethaline, which was demonstrated to be reliable and stable over weeks of operation.
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Show moreDeep eutectic solvents (DES) are environmentally-friendly electrolytes that are gaining interest for electrodeposition and energy storage applications. In these applications, metal electrodeposits with smooth, non-dendritic morphology are desired and thus effective strategies for suppressing roughness evolution are critically needed. A commonly employed and rather effective strategy for suppressing roughness evolution in metal electrodeposition is the use of electrolyte additives; however, the availability of such additives in DES electrolytes is limited and so is the understanding of the mechanisms through which additives suppress roughness amplification in DES media. In the present contribution, we demonstrate that polyethylenimine (PEI) is an effective electrolyte additive that suppresses roughness evolution during Cu electrodeposition in ethaline DES. PEI, due to its adsorption–deactivation properties, exhibits a unique hysteresis response during voltammetric studies of Cu electrodeposition – this response is analyzed using a mathematical model incorporating the relevant PEI transport, surface adsorption and deactivation processes. The model provides guidelines for selection of optimal conditions (e.g., PEI concentration) for effective suppression of roughness amplification in Cu electrodeposition.
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Show moreDeep eutectic solvent (DES) based electrolytes are gaining attention for electrochemical applications. As such, knowledge of the charge transfer kinetics in DES and its dependence on electrolyte composition and temperature is important. Using Cu2+ + e ↔ Cu1+ as a model system, we demonstrate that metal redox reactions in chloride-containing DES media suffer from sluggish charge transfer kinetics. The exchange current density, i0, displayed a peculiar inverse relationship with the bulk chloride concentration, [Cl−]b. The i0 decreased from 7.9 to 3.0 mA/cm2 when [Cl−]b increased from 2.8 to 4.6 M at 30°C. Such dependence is shown to originate from Cl− complexation with reactant and product species. Furthermore, i0 increased with an increase in Cun+ bulk concentration in the 50–200 mM range. Also, increase in temperature elevated i0. The charge transfer coefficient αc, however, remained constant (~0.5) and was unaffected by Cun+ or Cl− concentrations or by temperature. To explain how charge transfer kinetics depend on the various system parameters, a model incorporating complexation phenomena was developed and its predictions were compared to experiments.
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