Water as a catalyst for ion transport across the electrical double layer in ionic liquids

Abstract

It was previously shown experimentally that the ionic liquid (IL) system based on 1-ethyl-3-methylimidazolium dicyanamide $([{\mathrm{C}}_{2}\mathrm{mim}][\mathrm{dca}])$ outperforms $N$-butyl-$N$-methylpyrrolidinium dicyanamide $([{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}])$ as an electrolyte in a rechargeable zinc battery. Specifically, $[{\mathrm{C}}_{2}\mathrm{mim}][\mathrm{dca}]$ requires a lower overpotential to initiate electrodeposition of zinc, enables a higher zinc deposition peak current density, and supports a greater number of charge-discharge cycles. In the present work, a set of molecular dynamics simulations of two IL systems (i.e., IL + 9 mol % Zn + 3 wt % water) reveals that these differences could be related to the overscreening and crowding phenomena that exist in these IL systems. Furthermore, the work reveals the most likely mechanism by which water acts as an electrochemical catalyst in transferring metal cations to a negatively charged electrode. This transfer is a necessary step for subsequent metal deposition, as required for a rechargeable battery system, for example. In particular, the results of the simulations reveal three primary findings. First, it is shown that water activates the transfer of zinc onto the negative electrodes by temporarily disrupting zinc's [dca] coordination shell while zinc is located within the interfacial region; second, it is shown that the transfer of zinc is obviously greater in the $[{\mathrm{C}}_{2}\mathrm{mim}][\mathrm{dca}]$ system than in the $[{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}]$ system, which is consistent with experimental measurements; and third, when the effects of crowding are largely eliminated in the $[{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}]$ system (i.e., when the layering instability and the anion layer collapse in the anodic interfacial region are avoided by the use of a smaller electrode charge), the zinc transfer dynamics are completely changed and the $[{\mathrm{C}}_{4}\mathrm{mpyr}][\mathrm{dca}]$ system starts providing a more favorable environment for zinc transfers. These findings present a significant advance in the understanding of the electrode-electrolyte interface in ILs.