Heterogeneous electron transfer reorganization energy at the inner Helmholtz plane in a polybromide redox-active ionic liquid

In ionic liquids (ILs), the electric double layer (EDL) is where heterogeneous electron transfer (ET) occurs. Nevertheless, the relationship between the EDL structure and its kinetics has been rarely studied, especially for ET taking place in the inner Helmholtz plane (IHP). This is largely because of the lack of an appropriate model system for experiments. In this work, we determined the reorganization energy ( λ ) of Br 2 reduction in a redox-active IL 1-ethyl-1-methylpyrrolidinium polybromide (MEPBr 2 n +1 ) based on the Marcus-Hush-Chidsey model. Exceptionally fast mass transport of Br 2 in MEPBr 2 n +1 allows voltammograms to be obtained in which the current plateau is regulated by electron-transfer kinetics. This enables investigation of the microscopic environment in the IHP of the IL affecting electrocatalytic reactions through reorganization energy. As a demonstration, TiO 2 -modified Pt was employed to show pH-dependent reorganization energy, which suggests the switch of major ions at the IHP as a function of surface charges of electrodes. Ultrafast transport of Br 2 in a polybromide redox-active ionic liquid allows electron transfer-limited voltammograms of Br 2 reduction. The reorganization energy at the inner-Helmholtz plane can be determined based on the Marcus-Hush-Chidsey model.