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 (lambda) of Br-2 reduction in a redox-active IL 1-ethyl-1-methylpyrrolidinium polybromide (MEPBr2n+1) based on the Marcus-Hush-Chidsey model. Exceptionally fast mass transport of Br-2 in MEPBr2n+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, TiO2-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.

Automated summary

The relationship between the structure of the electric double layer (EDL) and its kinetics in ionic liquids (ILs) has been rarely studied, especially for electron transfer (ET) in the inner Helmholtz plane (IHP). In this study, the reorganization energy of Br-2 reduction in a redox-active IL was determined using the Marcus-Hush-Chidsey model. The results showed that the electron-transfer kinetics can influence the electrocatalytic reactions in the IHP of the IL, and the surface charges of the electrodes can switch the major ions at the IHP.