Three-dimensional step potential electrochemical spectroscopy (SPECS) simulations of porous pseudocapacitive electrodes

This study validates the step potential electrochemical spectroscopy (SPECS) method and refines the associated analysis for differentiating the contributions of electrical double layer (EDL) formation and Faradaic reactions to the total charge storage in three-dimensional porous pseudocapacitive electrodes. The modified Poisson–Nernst–Planck (MPNP) model coupled with the Frumkin–Butler–Volmer theory were used to numerically reproduce experimental data obtained from the SPECS method accounting for interfacial, transport, and electrochemical phenomena in porous electrodes consisting of monodisperse spherical nanoparticles ordered in face-centered cubic (FCC) packing. The fitting analysis of the SPECS method was modified for the Faradaic current. The new model can accurately predict the individual contributions of EDL formation and Faradaic reactions to the total current. Moreover, the contributions of EDL formation at the electrode surface or at the electrode/electrolyte interface within the porous electrode can be identified. Similarly, the Faradaic reactions due to surface-controlled or diffusion-controlled mechanisms can be distinguished. Furthermore, the capacitance associated with EDL formation obtained from SPECS was in good agreement with that obtained from cyclic voltammetry. Finally, cyclic voltammograms were reconstructed using the multiple potential step chronoamperometry (MUSCA) method, and the integral capacitance associated with each charge storage mechanism was calculated for a range of scan rates.