Going beyond traditional descriptors based on the surface electronic structure for electron transfer kinetics, we explore an alternative path that leverages physical chemistry of liquids, namely the water structure at the outer-Helmholtz layer (OHL), to control the electrochemical reaction kinetics. Water structure making/breaking spectator cations that are redox-inactive have been employed to perturb the solvation structure of redox metal centers through non-covalent interactions associated with redox centers, spectator ions, solvent molecules, and metal surfaces. The interfacial water features were characterized by surface enhanced infrared absorption spectroscopy and kinetic parameters (e.g. exchange current density and reorganization energy) were extracted through Marcus-Hush-Chidsey formalism. We will examine the cation-dependent, pKa-dependent or pH-dependent exchange current density and reorganization energy of reactions including ferro/ferricyanide redox, hydrogen evolution and oxidation and oxygen reduction. The origin to these trends will be discussed in the context of local water-ion structures and solvation environments at the interface and the influence on electron transfer and proton-coupled electron transfer barriers. Such work provides molecular insights into potential use of chemical physics of electrolytes to control the electrochemical kinetics.

Role of interfacial water structure on electrochemical reaction kinetics

ACS Spring 2022

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