Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

The behavior of ionic liquids (ILs) at charged interfaces is pivotal for their application in supercapacitors and electrochemical cells. Recently, we demonstrated for neat ILs that potential screening at polarized electrode interfaces shows a characteristic voltage dependence, as determined in situ by X-ray photoelectron spectroscopy. Herein, we use this fingerprint-type behavior to characterize the nature of the IL/electrode interfaces for IL mixtures of [C 8 C 1 Im][Tf 2 N] and [C 8 C 1 Im]Cl on Au and Pt electrodes. For Au, the IL/electrode interfaces are dominated by the Cl − anions, even down to a 0.1 mol% [C 8 C 1 Im]Cl content. In contrast, [Tf 2 N] − anions enrich at the IL/Pt electrode interfaces down to 10 mol% [C 8 C 1 Im][Tf 2 N]; only at lower concentrations does a transition to Cl − enrichment occur. These mixture studies demonstrate that even small concentrations of another IL or contamination, e.g. remaining from synthesis, can strongly influence the situation at charged IL interfaces. The interface of electrodes and IL mixtures has been studied by in situ XPS. We found that the concentration of counterions at the interface can strongly deviate from the bulk composition due to interactions between electrode and IL.