Simultaneous acquisition of impedance and gravimetric data in a cyclic potential scan for the characterization of nonstationary electrode/electrolyte interfaces

Simultaneous acquisition of electrochemical impedance spectroscopy and quartz crystal microbalance (EIS-EQCM) data in cyclic electrode potential scans was used to characterize nonstationary underpotential deposition (UPD) of atomic layers of Ag on Au and Cu on Pt. Both EIS and EQCM data sets complemented each other in the elucidation of interface models and the investigation of different aspects of the interfacial dynamics. EIS-EQCM provided an opportunity to monitor coadsorption and competitive adsorption of anions during the Ag and Cu UPD using (i) the electrode mass change, (ii) adsorption capacitances, and (iii) double-layer capacitances. Kinetic information is available in the EIS-EQCM through the charge transfer resistances and apparent rate coefficients. The latter expresses the rate of UPD into the partially covered electrode surface. The apparent rate coefficients for the Ag UPD were determined to vary from 0.15 to 0.45 cm/s which is between the standard constant rates k₀ of Ag bulk deposition on Ag reported previously for different Ag surfaces. Cu UPD on Pt and Ag UPD on Au contributed differently into a resonance resistance ?R(E) available from the EQCM data sets. Spontaneous surface alloying between Ag and Au during the Ag UPD continuously increased the ?R, while the Cu overlayer formation on Pt as well as experiments without Ag⁺ and Cu ²⁺ in the solution did not change this parameter significantly. The EIS-EQCM appeared to be a promising tool for an improved characterization and understanding of nonstationary electrochemical interfaces.