Room temperature ionic liquids (RT-ILs) are promising electrolytes for electrocatalysis. Since the electrical double layer (EDL) is where heterogeneous electron transfer occurs, understanding the effects of the electrode-electrolyte interface structure on electrocatalysis in RT-ILs is important. Ultrafast mass transport of redox species in N-methyl-N-ethyl-pyrrolidinium polybromide (MEPBr_2n+1) enabled evaluation of the reorganization energy (λ) for the heterogeneous electron transfer, which reflects the solvation structure in the inner Helmholtz plane (IHP). λ was achieved by fitting the electron transfer rate-limited voltammogram at an ultramicroelectrode (UME) to the Marcus-Hush-Chidsey model for heterogeneous electron transfer kinetics.^[1] However, it is time-consuming or even impossible to prepare electrode materials, including alloys of numerous compositions, in the form of UME for each experiment. Herein, we report a method to evaluate the λ of MEPBr_2n+1 using scanning electrochemical cell microscopy (SECCM), which allows high throughput electrochemical measurements on a single electrode. Furthermore, investigating λ on a high-entropy alloy materials library (HEA-ML) composed of Pt, Pd, Ru, Ir, and Ag suggests a negative correlation between λ and the work function. Given that the potential of zero charge correlates with the work function of electrodes, this can be attributed to the surface-charge sensitive ionic structure in the IHP of MEPBr_2n+1, modulating the solvation energy of the redox-active species in the IHP.

Moonjoo Kim

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