Kinetics of hydrogen underpotential deposition at polycrystalline platinum in acidic solutions

Kinetics of the hydrogen underpotential deposition (HUPD) reaction at smooth polycrystalline platinum has been studied in 0.1 M H2SO4 and HClO4 using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Impedance was measured at Pt electrodes (i) at lower frequencies (LF) up to 10 kHz in a classical set up with the potentiostat and the frequency response analyzer at a single Pt disk electrode in a hanging meniscus configuration or Pt wire and in a large frequency range up to 1 MHz with the FRA in a stand-alone mode and two arrangements of the electrodes, (ii) at small twin Pt electrodes, and (iii) at one hanging meniscus electrode and another of similar geometric size but of large surface area. Real surface area of the Pt electrodes was determined by the hydrogen adsorption method. All impedance spectra exhibited a CPE behavior. High frequency measurements permitted determination of the double layer capacitance, hydrogen adsorption pseudocapacitance and charge transfer resistance of the HUPD reaction. Deviations from the ideal capacitive response were larger at high than at low frequencies. The possible origins of this dispersion and the electrical equivalent circuits have been discussed.