Redox switching of Prussian blue thin films investigated by ac-electrogravimetry

In this paper, the redox switching behaviour of Prussian blue (PB) in KCl environment at pH 2.5 and 5.4 was investigated by using ac-electrogravimetry in a potential range around the conversion of PB to ES (Everitt Salt). This technique allows the electrochemical impedance and a mass-potential transfer function to be simultaneously measured. From the impedance the charge-potential transfer function is calculated. A model of the PB film, is presented, it takes into account the porous structure of the material, the insertion of two cations on the pore wall, and the electronic charge transfer from the electrode. A fitting procedure was employed to obtain the kinetic parameters. Attractive information extracted from the impedance concerns the porosity of the PB film which is related to the zeolitic nature of the film. The resistivity of the pore depends obviously on the electrolyte conductance and changes with respect to the potential with a minimum, in the vicinity of the PB <-> ES conversion potential. Ac-electrogravimetry and the charge-potential transfer function gave essentially information on the kinetics of the ionic transfer. Thanks to the model of electroactive porous films and the digital fitting procedure, for the first time, the two cations K+ and H3O+ are identified with well separated kinetics for each cation. Several attractive informations were reached. First, it confirms that ionic transfer at the film/electrolyte interface limits the kinetics of the film reduction or oxidation as electronic transfer at the electrode/film interface is faster. Moreover, the rates of the electronic and ionic transfer kinetics vary with the potential. They are faster around the potential where the PB <-> ES conversion occurs. It was confirmed that charge compensation occurs only by K+ and H3O+ movements whatever the pH of the solution. However, H3O+ transfer kinetics is faster than K+ transfer, whatever the KCl concentration, and the role of H3O+ is more important for large KCl concentrations where the process is more limited by the kinetics of the hydronium ions. In addition, in these situations, even if ionic transfer is also accelerated the global charge transfer is nevertheless more controlled by electronic transfer. (C) 2012 Elsevier Ltd. All rights reserved.