Mass transport in thin films of [Fe(CN)6]4− exchanged Ni–Al layered double hydroxide monitored with an electrochemical quartz crystal microbalance

Changes in the resonant frequency of quartz crystals coated with thin films of a Ni–Al–Cl LDH exchanged with [Fe(CN)6]4− were monitored. Impedance measurements show no changes in the conductance spectra of the nickel LDH films, before and after adsorption of [Fe(CN)6]4−, or before and after potential scanning, indicating that the films were rigid and that the frequency shifts were due to mass changes. Adsorption of [Fe(CN)6]4− by a 7.5-μg Ni–Al–Cl films resulted in a frequency drop of −300 ± 10 Hz, corresponding to a mass gain of 260 ± 10 ng. This was attributed to the replacement of 85% of the chloride ions from the LDH interlayer spaces by much heavier [Fe(CN)6]4− ions. Electrochemical oxidation of [Fe(CN)6]4− in nickel LDH films was accompanied by a reversible decrease in mass. The mass per electron transferred (MPEs) obtained, −41, −23, and −14 g/mol, in potassium acetate, sodium acetate, and magnesium acetate, respectively, matched the molar mass of the electrolyte cations. This indicates that charge neutralization occurred by the simple loss or gain of electrolyte cations. Similarly, the MPEs obtained for a [Mo(CN)8]4− exchanged Ni–Al LDH and for [Fe(CN)6]4− exchanged Zn–Al LDH matched the molar mass of the electrolyte cations. Lost or gain of cations from the electrolyte appears to be a general charge balancing mechanism during the electrochemical reaction of metal cyanides anions in the LDH-modified electrodes.