Electrochemical characterization of ion selectivity in electrodeposited nickel hexacyanoferrate thin films

The ion selectivity of electrodeposited nickel hexacyanoferrate (NiHCF) thin films was investigated using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). NiHCF thin films were prepared by cathodic deposition on Pt and Al substrates. EIS and CV curves were determined in 1 mol/L (KNO₃+C₅NO₃) and 1 mol/L (NaNO₃+CsNO₃) mixture solutions, which were sensitive to the concentration of Cs⁺ in the electrolytes. Experimental results show that all Nyquist impedance plots show depressed semicircles in the high-frequency range changing over into straight lines at lower frequencies. With increasing amounts of Cs⁺, the redox potentials in CV curves shift toward more positive values and the redox peaks broaden; the semicircle radius in corresponding EIS curves and the charge transfer resistance also increase. EIS combining CV is able to provide valuable insights into the ion selectivity of NiHCF thin films.