Constant phase element affected by ion transport in nanoporous electrodes

Abstract

• A constant phase element (CPE) depends on the concentration of the electrolyte and ionic size at nanoporous electrodes. • Ion depletion in nanopores and the ion transport toward nanopores causes the non-ideality of the capacitance. • These findings are not observed at flat electrodes. A constant phase element (CPE) is an equivalent electrical circuit component that describes the behavior of a double layer formed at the electrode and electrolyte interface, which is a kind of imperfect capacitors. When the CPE concept is applied to model the behavior of the electrical double layer at nanoporous electrodes, the physical interpretation of the CPE becomes more complicated because of the morphological effect of the nanoporous electrode. Here we show that the CPE depends on the concentration of the electrolyte and ionic size at nanoporous electrodes. At low electrolyte concentration, the depletion of ions in the nanopore and the ion transport toward nanopores increase the dependence of capacitance on frequency. Moreover, larger ions have more restricted transport into the nanopore than smaller ions, leading to non-ideality of capacitance, which is significantly different from that of flat electrodes. The results suggest insight into the CPE at nanoporous electrodes.