Manifestation of Mass-Transport Processes in the Electrochemical Impedance Spectroscopy Features of V~(2+)/V~(3+) and VO~(2+)/VO_2~+ Redox Couples

摘要

Effect of mass-transport processes on the electrochemical impedance spectroscopy (EIS) features of V~(2+)/V~(3+) and VO~(2+)/VO_2~+ redox reactions in acid electrolyte is illustrated on modified glassy-carbon disk electrode. The impedance features depend on the electrode potential, concentration of the redox species, concentration of the supporting electrolyte and constituents of the electrode. The results show that a combination of voltammetry and EIS is required to identify and explain the underlying physical processes that lead to the formation of specific EIS features. Optimum concentration of the vanadium redox species and supporting electrolyte is required to resolve the HF and LF semi-circles in the EIS features corresponding to the electron-transfer process and transport of vanadium ions, respectively. Nafion and Polytetrafluoroethylene (PTFE) binders are used to bind the carbon-black catalyst on the glassy carbon disk electrode surface. The binder-free electrode shows finite and semi-infinite linear transport of redox species under the dynamic and static conditions of the electrode, respectively. Addition of binder in the electrode offers extra resistance to the transport of vanadium redox species, which helps resolve EIS features of the transport of redox species through the porous thin-film electrode and that through the bulk of the electrolyte.