Electrochemical impedance spectroscopy of electrodes modified with thin films of NiAlCl layered double hydroxides

Abstract

The impedance spectra of NiAlCl LDH-modified electrodes after oxidation at different dc potentials were recorded. The data were fitted to a simple Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The film charge transfer resistance decreased dramatically at the onset of nickel oxidation, from more than 10 6 Ω for the unoxidized films, to a minimum of about 60 Ω in a film oxidized to Ni 2.5+, before increasing slightly for more extensively oxidized films. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by larger capacitive loops at lower frequency where the impedance was dominated by mass transport. Effective diffusion coefficients estimated from the low frequency impedance varied between 1 and 5×10 −9 cm 2 s −1, in agreement with previous reports for proton diffusion in nickel hydroxide films. The resistive component of the mass transport impedance decreased with increase in the nickel oxidation state. The addition of [Mo(CN) 8] 4− caused an increase of 140 mV in the dc potential required to affect the changes in the NiAlCl film impedance spectra associated with oxidation of the nickel sites. The impedance of a redox inactive ZnAlCl film was unaffected by the use of dc conditioning potentials.