Impedance response of electrochemical interfaces. III. Fingerprints of couplings between interfacial electron transfer reaction and electrolyte-phase ion transport

Electron transfer (ET), electric double layer (EDL) charging, and ion transport (IT) are three elementary physicochemical processes in electrochemistry. These processes are coupled with each other in the way that the local reaction environment for the ET is shaped by EDL charging, which is nothing but IT in a nanoscale nonelectroneutral region. Herein, we investigate fingerprints of the coupling between these processes in electrochemical impedance spectroscopy. EDL charging and IT are described uniformly using the Poisson-Nernst-Planck theory, and interfacial ET is described using the Frumkin-Butler-Volmer theory. Different diffusion coefficients for cations and anions (D+ ≠ D-) are considered. Exact analytical expressions are obtained when the potential of zero charge (Epzc), the equilibrium potential of the reaction (Eeq), and electrode potential (EM) are equal. The analytical solution shows that a decoupling treatment is valid only for the case of D+ = D-. Using a new scheme of calculating impedance response at any electrode potential, we observe an inductive loop in the low frequency range, which is a clear impedance fingerprint of the coupling effects.