Abstract
Ionic Liquids are a promising alternative to water electrolytes for the electrodeposition of metals. These solvents have a much larger electrochemical window than water that expands the potential of electrodeposition. However, mass transport in Ionic Liquids is slow. The slow mass transport dramatically affects the rate of reactions at the solid–liquid interface, hampering the exploitation of Ionic Liquids in high-throughput electrodeposition processes. In this paper, we clarify the origin of such poor mass transport in the diffusion–advection (convection) regime. To determine the extent and the dynamics of the convection boundary layers, we performed Rotating Disk Electrode (RDE) experiments on model reactions along with the finite element simulation. Both the experiments and the finite element modelling showed the occurrence of peaks in the RDE curves even at relatively high rotation rates (up to 2000 rpm). The peak in the RDE is the fingerprint of partial diffusion control that happens for the relative extent of the diffusion and convection boundary layers. In looking for a close match between the experiments and the simulations, we found that the ohmic drop plays a critical role and must be considered in the calculation to find the best match with the experimental data. In the end, we have shown that the combined approach consisting of RDE experiments and finite elements modelling providing a tool to unravel of the structure of the diffusion and convection boundary layers both in dynamic and stationary conditions.
Highlights
Ionic Liquids are a promising alternative to water electrolytes for the electrodeposition of metals
Water-based electrolytes with the same concentration of the electroactive species and under the same experimental did not show any peak[47] (ESI Fig. S1). This behaviour results from the lower diffusion coefficient of ferrocene/ferrocinium (1.75 10–11 m2/s) in the IL compared to the one of ferrocynide/ferricyanide (6.9 1 0–10 m2/s) in water and to the large viscosity (9.25 10–5 m2/s) of BMImBF4
To better understand the effect of the limitation of ILs viscosity on the mass transport rate at the solid–liquid interface, we have developed a hybrid experimental/numerical approach that consists of performing Rotating Disk Electrode (RDE) measurements coupled with numerical modelling
Summary
Ionic Liquids are a promising alternative to water electrolytes for the electrodeposition of metals. The diffusion coefficients in ILs are smaller that in water based electrolytes This aspect has significant implications in the processes where the rate is controlled by interfacial mass transport of the interface process (e.g. electrochemistry, absorption and heterogeneous catalysis). No analytical expression that can adequately describe the dynamics of the voltammetry profile under mixed control of the charge flow (charge transfer and transport) is available at this time[37] Still, this regime can be described by the discretisation of the convection problem employing finite element analysis. The comparison between experimental and simulated voltammetry, under such mixed control conditions, yield much more information about the system than the analysis of just the limiting current regime Both experiments and simulations in ILs have been carried out using an exemplary IL, B MImBF4 (1-Butyl3-methylimidazolium tetrafluoroborate), as the electrolyte. Ferrocyanide has been considered in the simulation instead of ferrocene, for the limited solubility of the latter[42]
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