On the impedance of potassium nickel(II) hexacyanoferrate(II) composite electrodes—the generalization of the Randles model referring to inhomogeneous electrode materials

Composite electrodes of graphite, paraffin and potassium nickel(II) hexacyanoferrate(II) (Ni hcf) were studied by cyclic voltammetry and impedance spectroscopy at the formal potentials for the electrolytes LiCl, NaCl and KNO3. All Nyquist impedance plots exhibit depressed semicircles in the high frequency range changing over into straight lines with slopes smaller than 45° at lower frequencies. The experimental Nyquist plots can be well described theoretically with a new model based on the well known Randles model. In the new model, the double layer capacity as well as the Warburg impedance for semi-infinite diffusion are replaced by constant-phase elements taking into consideration an inhomogeneous electrode surface and diffusion of the cations in an electrode with inhomogeneities. The model enables the determination of the rate constants of the ion transfer (of the order of 10−6 m s−1) between the solution and the solid, the double layer capacities (of the order of 0.3 μF) of Ni hcf particles, the diffusion coefficients of the cations in the Ni hcf (of the order of 10−8 cm2 s−1) as well as the distribution coefficients referring to the inhomogeneities at the electrode surface and in the bulk of the electrode.