Impedance of a redox adsorption system with rapid and reversible adsorption kinetics in planar and porous electrodes

<p>This study presents the ac impedance analysis of an electrochemical system with adsorption of the redox species, undergoing a single-step surface redox reaction involving only the adsorbed forms, and governed by the Butler-Volmer type heterogeneous charge transfer kinetics. The adsorbed forms are in dynamic equilibrium with the dissolved forms of the redox species at the interface, and the system follows a Langmuir isotherm, with rapid and reversible adsorption-desorption kinetics. While modelling this system, two cases are considered: rapid adsorption kinetics coupled with either fast or slow diffusion kinetics. Accordingly, analytical expressions are derived for the ac impedance, as a function of the transient parameters, and small signal ac impedance, which is dependent only on the steady state/dc parameters. DC conditions for obtaining extremum points of faradaic impedance, including minimum charge transfer resistance and maximum adsorption-associated capacitance, are explored. Combined with charged double layer capacitance and solution resistance, equivalent circuit representations are presented for the adsorption systems, for planar and porous electrodes, and with ideally capacitive vs constant phase element (CPE) behaviour. Nyquist plots are theoretically generated. The current model provides insights into the impedance behaviour of an adsorption system that deviates from strong adsorption kinetics, and can prove to be helpful in designing efficient adsorption-controlled impedimetric sensing systems with optimised sensing resolution and sensitivity.</p>