Time domain modelling of concurrent insertion and capacitive storage using Laplace domain representations of impedance

Abstract The engineering of high rate electrochemical energy storage devices can benefit from analysis techniques that can accurately attribute charge storage to individual energy storage mechanisms. A new time-domain analysis for potentiostatic intermittent titration technique (PITT) experiments that uses Laplace domain representations of impedance is presented for the characterisation of charge storage in electrochemical systems where charge can be stored via a combination of Faradaic processes and electric double layer storage. The derivation of this model is presented, along with a proof that the model collapses into the single electric double layer storage model or the Faradaic charge storage model under limiting conditions (infinitely restricted diffusion or zero capacitive storage, respectively). The parameter space of the model is explored, along with an evaluation of when the simpler, single process charge storage models can be used in place of the more complex two-mode storage model. The model is validated with an electric double layer capacitor, for which reasonable agreement is seen between fitted capacitance and its stated value, and using experimental data obtained from amorphous TiO2 nanotube arrays hierarchically grown on Ti mesh electrodes. PITT measurements using the proposed model, and electrochemical impedance spectroscopy (EIS) yielded similar fitting parameters, with the exception of CEDL, which is too small to estimate with PITT, and Rct/RΩ, which becomes inseparable when CEDL is small.