Separating Convective from Diffusive Mass Transport Mechanisms in Ionic Liquids by Redox Pro-fluorescence Microscopy

The study of electrochemical reactivity requires analyticaltechniquescapable of probing the diffusion of reactants and products to andfrom electrified interfaces. Information on diffusion coefficientsis often obtained indirectly by modeling current transients and cyclicvoltammetry data, but such measurements lack spatial resolution andare accurate only if mass transport by convection is negligible. Detectingand accounting for adventitious convection in viscous and wet solvents,such as ionic liquids, is technically challenging. We have developeda direct, spatiotemporally resolved optical tracking of diffusionfronts which can detect and resolve convective disturbances to lineardiffusion. By tracking the movement of an electrode-generated fluorophore,we demonstrate that parasitic gas evolving reactions lead to 10-foldoverestimates of macroscopic diffusion coefficients. A hypothesisis put forward linking large barriers to inner-sphere redox reactions,such as hydrogen gas evolution, to the formation of cation-rich overscreeningand crowding double layer structures in imidazolium-based ionic liquids.

Automated summary

The study develops a direct, spatiotemporally resolved optical tracking method to detect and resolve convective disturbances in electrochemical reactivity. It is found that parasitic gas evolving reactions lead to overestimates of macroscopic diffusion coefficients by 10 times. A hypothesis is proposed linking large barriers to inner-sphere redox reactions to the formation of cation-rich overscreening and crowding double layer structures in imidazolium-based ionic liquids.