Electrochemical impedance spectroscopy: Simultaneous detection of different diffusion behaviors as seen in finite element method simulations of mediator-type enzyme electrodes

Herein, a different interpretation of electrochemical impedance spectra with a double semicircle and a straight line obtained for systems involving diffusion processes is proposed. Simulations of electrochemical impedance spectra of mediator-type enzyme electrodes by using the finite element method revealed that under certain conditions, diffusion can be a combination of apparent semi-infinite and finite length, with the diffusion type differing depending on the position on the electrode or the diffusion direction. The Nyquist plot of a system with one charge transfer and the mixed-type diffusion shows a semicircle for the charge transfer, a second semicircle for finite length diffusion (short Warburg impedance), and a straight line for semi-infinite diffusion (Warburg impedance). In mediator-type enzyme electrodes with a free-diffusing enzyme and a mediator, such mixed-type diffusion can be observed at medium substrate concentrations when the reaction plane is close to the electrode at the edges and farther away in the middle of the electrode. These simulation results may help (bio-)electrochemists to more accurately interpret impedance spectra and gain better understanding of the phenomena occurring at mediator-type enzyme electrodes and other cases involving diffusion.