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The coordination polymer Zn3Na2[FeII(CN)6]2 has an open porous framework that is stable in acidic and neutral aqueous solutions and appears to be an attractive solid for investigation as a material for sodium ion-based batteries.

The hydration state and structural transformations of Ti films potentiostatically grown in 0.1 M NaOH are characterized utilizing EIS, Mott-Schottky and XPS techniques. Variations presented in the density of donors, the flatband potential and the fraction of species measured by XPS, in the region of characterization of these films are associated with changes in the ratio of different Ti sub-oxides within the passive film. Oxygen and hydroxyl vacancies and their respective formation rate constants are estimated from EIS spectra fits with a modified Point Defect Model. A consistent behavior is observed for the hydration state and structural transformations of the films as a function of potential. Films anodized at less positive potentials than 1.17 V vs SCE present higher hydroxyl vacancy diffusivities (e.g higher film hydration) and a heterogeneous mixture of titanium sub-oxides, whereas the opposite behavior is observed for films formed at more positive potentials than 1.17 V vs SCE, where the hydroxyl vacancy diffusivities decrease by one order of magnitude and the oxygen vacancy diffusivities increase three times. Thus, films anodized at more positive potentials present a higher dehydration, are more homogeneous and mainly composed of TiO 2 .Titanium anodization consists of the formation of an oxide layer on a Ti substrate by electrolytic polarization. The oxides formed through this methodology have become attractive due to the paramount performance exhibited by semiconductor TiO 2 films in photocatalytic and photoelectrochemical applications, biomaterials and corrosion resistance. 1-4 Indeed, the features and performance of these films associated with the aforementioned applications can be directly related to the electronic properties and crystal structure of these materials. For instance, the surface coloration, electrocatalytic activity, corrosion resistance, light refraction and absorption of the films rely upon the properties and crystal structure of the material. [5][6][7][8] This anodization could entail the transition through multiple intermediary oxidation states (TiO, Ti 2 O 3 , Ti 3 O 5 and/or Ti 4 O 7 ). 9-31 The proportion of these oxides within the anodic films varies depending on the conditions utilized to anodize, e.g. applied potential, time, electrolyte composition. This transition also affects the crystallinity and photoelectrochemical performance of TiO 2 photoanodes, as well as their corrosion resistance. 32,33 Molar fraction of chemical species vs E diagrams for the Ti-H 2 O system reveals that the formation of Ti sub-oxides could be connected to the hydration state of the oxide film, since the stability of the TiO 2 is displaced to more positive potentials when hydrated oxides are involved in the thermodynamic calculations. 34 Different characterization techniques have been used to study the formation of these intermediary oxides in different electrolytes. [10][11][12]15,18,19,21,[23][24][25]30,32,34 However, to date few studies have correlated the formation of these sub-oxides with their...

A simple model has been developed ͑within the framework of the point defect model, PDM͒ assuming that the impedance response of passivated metals is dominated by the barrier layer, which consists of a partly hydrated and highly defective oxide layer. Simulations were carried out for cation vacancy conducting barrier layers ͑with p-type electronic character͒ and O 2− vacancy conducting barrier layers ͑with n-type electronic character͒ including in both cases the possible contribution by transport of OH − vacancies. A very good agreement was obtained between calculated impedance diagrams and typical impedance spectra for passive films on nickel and titanium. For both types of films the impedance response is quite sensitive to variations in the formal rate constant k −3b involved in the generation of hydroxyl vacancies at the metal/film interface. Furthermore, in the case of films with p-type electronic character the shape of the Nyquist diagram is sensitive to variations in the ratio of the kinetics of hydroxyl vacancies generation to the kinetics of cation vacancies generation at the metal/film interface. . Passivity of metals and alloys is a phenomenon of great technological importance because it helps reduce the rapid deterioration of many construction materials. Thus, there are continuous efforts to improve understanding of the formation of passive films on metal surfaces.Starting with the work of Cabrera and Mott in the late 1940s, 1 several models have been proposed in the literature 2-11 to describe the mechanism of growth and dissolution of passive films on metal substrates in contact with different aqueous media. Many authors agree that passive films form as bilayer structures, consisting of defective oxide ͑the barrier layer͒ adjacent to the metal and an outer layer that forms from the reaction of metal cations with species in the solution.The point defect model ͑PDM͒ developed by Macdonald et al. [11][12][13][14][15][16][17] is perhaps the most well-known oxide film model nowadays, and has been used by different authors as a framework to model a wide range of passive film systems. It was originally developed to explain the growth, 11 breakdown, 12 and impedance characteristics 13 of passive films on Ni, Fe, and Fe-based alloys. It was later extended to consider the properties of barrier passive films under steady-state conditions 14,16 and finite-rate interfacial kinetics. 15,16 More recently, it was extended to account for the relaxation of film thickness. 17 The PDM establishes that the barrier layer can be viewed as a highly defective semiconductor in which the vacancies act as electronic dopants. It predicts that cation vacancies are formed at the film/solution interface, then diffuse and are annihilated at the metal/film interface, while oxygen vacancies that are formed at the metal/film interface diffuse to, and are consumed at, the film/solution interface.The incorporation of OH − ions into the passive films is a phenomenon of great practical importance because it influences the corrosion behavior...