Impedance Analysis of Ion Transportation in Hierarchical Porous Core-Shell Carbons with Transmission Line Model

Abstract

Electrochemical Impedance Spectroscopy (EIS) with Transmission Line Model (TLM) was utilized to study the electrochemical capacitive performance in a three dimensional hierarchical carbon network with hollow core and mesoporous shell (HCMS) structure in KOH electrolyte. In order to systematically study the performance-structure relationship, the shell thickness (S) of the HCMS structure was stepwise increased from S=0 to 100 nm while other geometric parameters were kept constant. Nyquist plots indicated the hierarchical carbons gradually depart from the characteristics of an ideal capacitor as the shell thickness increased. Ion transportation through the HCMS hierarchical structure, RC time constants and capacitance distribution across the carbon electrodes were derived from the 6-loop TLM. The stepwise increased shell thickness (S) of the HCMS structure affects both the electrochemical and physical properties of the carbon electrodes, for example, the electrode resistance, diffusion resistance, capacitance, and rate capability. TLM demonstrated that major capacitance were contributed from the second to fourth loop, which can be associated to the mesoporous shell region in the HCMS texture; while the 5th loop is associated to the diffusion of ions into micropores. Though the carbon with thicker shells can provide large specific capacitance, the rate capability (determined by RC time constants) worsens, no direct stepwise relationship was observed for increased shell thickness. © The Electrochemical Society.