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 monito r the growth of nickel hexacyanoferrate films and to study the mechanism of mass transfer during the system's redox reactions in supporting electrolyt es 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 prima rily achieved through the flux of potassium cations, the actual mechanism o f charge propagation in potassium salt electrolytes seems to be more comple x at higher potentials where a second redox reaction of nickel hexacyanofer rate occurs. Following careful analysis of microgravimetric: data, which in cludes determination df apparent molar masses and estimation of transport n umbers, we postulate the existence of the counterflux of either solvent mol ecules or anions. The involvement of anions seems to be the most pronounced in the presence of sulfates. The microgravimetric results have also been s upported with kinetic data such as effective diffusion coefficients (obtain ed using chronocoulometry) and with the Nernst plot analysis of voltammetri c mid-peak potentials (obtained using modified ultramicroelectrodes) giving insight into the ion-exchange properties of nickel hexacyanoferrate. In al l cases, the system's characteristics, which also include the dynamics of t he film growth, are different in the presence of sulfates when compared to chlorides or nitrates. (C) 2000 Elsevier Science Ltd. All rights reserved.