Identification of Processes Associated with Different Iron Sites in the Prussian Blue Structure by in Situ Electrochemical, Gravimetric, and Spectroscopic Techniques in the dc and ac Regimes

The physicochemical properties of Prussian blue films are strongly dependent on the ratio Fe3+/Fe2+ in the structure. This ratio can be modulated by electrochemical techniques such as cyclic voltammetry, and some information about physicochemical properties can be extracted from in situ auxiliary techniques. Prussian blue films have been studied by the use of in situ vis-NIR spectroscopy, quartz crystal microbalance, and electrochemical techniques (cyclic voltammetry and electrochemical impedance spectroscopy). By cyclic voltammetry + absorbance derivative curves + mass derivative curves, it has been possible to identify at least three different processes during the reduction of Prussian blue films to the Everitt's salt form. By numerical methods, it has been possible to deconvolute these curves and, thus, to obtain information separately from the three identified processes. These processes can be attributed to different Fe3+ sites in the PB structure with the participation of different counterions. By in situ electrochemical impedance + mass impedance + color impedance at a wavelength near 1000 nm, there has been observed a double loop plot with the color impedance where mass and electrochemical impedance show only one loop plot. Color impedance spectra contributions can be separated and identified separately by means of a nonlinear least-squares fitting procedure, and, thus, this double loop may be assigned to at least two different overlapped electrochemical processes. One of these electrochemical processes is related to Fe3+ sites (Fe(NC)(5)OH-) near the ferrocyanide vacancies in the Prussian blue structure, which are strongly affected by the pH of the outer solution.