Quartz crystal microbalance monitoring of mass transport during redox processes of cyanometallate modified electrodes: complex charge transport in nickel hexacyanoferrate films

The electrochemical quartz crystal microbalance has been employed to monitor the growth of nickel hexacyanoferrate films and to study the mechanism of mass transfer during the system's redox reactions in supporting electrolytes containing the same concentrations of potassium ions and various anions, sulfates, chlorides or nitrates. Although the results are consistent with the general view that charge compensation during electron transfer is primarily achieved through the flux of potassium cations, the actual mechanism of charge propagation in potassium salt electrolytes seems to be more complex at higher potentials where a second redox reaction of nickel hexacyanoferrate occurs. Following careful analysis of microgravimetric: data, which includes determination df apparent molar masses and estimation of transport numbers, we postulate the existence of the counterflux of either solvent molecules or anions. The involvement of anions seems to be the most pronounced in the presence of sulfates. The microgravimetric results have also been supported with kinetic data such as effective diffusion coefficients (obtained using chronocoulometry) and with the Nernst plot analysis of voltammetric mid-peak potentials (obtained using modified ultramicroelectrodes) giving insight into the ion-exchange properties of nickel hexacyanoferrate. In all cases, the system's characteristics, which also include the dynamics of the film growth, are different in the presence of sulfates when compared to chlorides or nitrates. (C) 2000 Elsevier Science Ltd. All rights reserved.