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A recent physics-based model for liquid and gaseous water transport in the cathode catalyst layer (CCL) is incorporated into our 1d ＋ 1d model for the PEM fuel cell impedance. The model includes parametric dependencies of the CCL oxygen diffusivity and proton conductivity on the liquid saturation. Fitting of the 1d ＋ 1d model to experimental impedance spectra of a PEM fuel cell reveals two intriguing effects. Contrary to common belief, the liquid water saturation in the CCL is nearly independent of cell current density due to the growing liquid pressure gradient that drives liquid water removal from the CCL. Further, the "dry" oxygen diffusivity of the catalyst layer increases with cell current density. Apparently, at small current density, electrochemical conversion proceeds primarily in narrow pores, where the Knudsen oxygen diffusivity is low. With growing current density, larger and better connected pores with higher oxygen diffusivity dominate in the current conversion, leading to increase in effective oxygen diffusivity observed in impedance spectroscopy data. • Two-phase water transport in the cathode catalyst layer (CCL) is incorporated into our model for PEMFC impedance. • The new model is fitted to experimental local impedance spectra. • Successful fitting of a low-frequency inductive loop due to liquid saturation transients in the CCL is demonstrated. • The fitting parameters show formation of a strong liquid pressure gradient in the CCL. • The effect of "dry" oxygen diffusivity growth with the cell current density is explained. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a two-consecutive steps anodic dissolution mechanism is simulated at the atomic level by a Monte Carlo algorithm on a 3D lattice. In contrast with a previous work restricted to the 2D flat case, it is shown that the steady state and transient behaviors are controlled not only by the adsorbed intermediate coverage degree but also by the area of the rough interface build up by the dissolution of interfacial atoms. It is established that the complex impedance behavior results from a combination of the individual frequency responses of these two quantities. The emphasis is put on the specific properties of the impedance induced by the surface roughness relaxation for which a kinetic description is proposed and compared to the Monte Carlo data. It is clearly concluded that, by taking into consideration the surface re-arrangement, the number of frequency domains in the impedance may be larger than predicted by the derivation of a kinetic model on a flat interface. [Copyright &y& Elsevier]

Abstract: Much has been argued about the inductances in electrochemical impedance spectroscopy (EIS), which are recurrently ascribed to electrochemical or non-electrochemical events and even experimental artifacts. In the present paper, computational microscopic simulations of metallic dissolution based on the microscopic Monte Carlo algorithm allowed the effect of surface adsorbate relaxation to be isolated from other possible intervening features such as surface relaxation. Results have unambiguously shown that surface adsorbate relaxation can effectively yield inductive loops as predicted by the classical macroscopic description of the metal–electrolyte interface by kinetic equations. [Copyright &y& Elsevier]

Abstract: Electrochemical impedance spectroscopy was used to study the mechanism by which copper oxides are reduced in alkaline solutions. For the reductions of CuO and Cu2O, a capacitive loop and also an inductive loop under certain conditions were observed in the complex plane. The electrochemical impedance for CuO reduction was not greatly dependent on the solution alkalinity and the kind of alkali hydroxide. However, the electrochemical impedance for Cu2O reduction was considerably affected by the kind and concentration of alkali hydroxide. The diameter of the capacitive loop, i.e., the charge-transfer resistance (R ct), was increased with increase in solution alkalinity. It should also be noted that R ct was increased in the order of KOH6M) and LiOH (>0.2M) suggested the existence of an intermediate species (probably CuOH). The specific inhibitory effect of Li+ ions on the reduction of Cu2O might be explained by a possible stabilization of CuOH by Li+ ions. [Copyright &y& Elsevier]

• TMAZ-HAZ region showed elongated and refined grains due to the plastic deformation process. • 2D microhardness mapping exhibited higher values in the SZ region. • Passive film was completely reconstructed in the SZ at 6.0 and 11.5 mol L–1 HNO 3. • Fe and Cr contamination was deleterious to the TiO 2 film stability. • XPS measurements showed the presence of not native metal species in the passive film. Mechanical and microstructural characteristics of Ti-6Al-4V/stainless steel (SS) dissimilar friction stir welds have been well described in the literature; however, little is known about the electrochemical properties of such welds in terms of the passive film stability of TiO 2 grown in oxidizing media. To clarify this issue, potentiodynamic polarization, cyclic voltammetry, and electrochemical impedance measurements were carried out in the Ti-6Al-4V alloy to analyze the effects of Fe and Cr contamination from the underlying SS on the passive film stability of TiO 2 in concentrated (6.0 and 11.5 mol L–1) HNO 3 solutions. Lower transpassive potentials were observed for samples in the stirred zone (SZ) than those of the base metal (BM) due to Fe and Cr contamination. Anodic charges obtained during cyclic voltammetry using 6.0 mol L–1 HNO 3 showed that the grown passive film was completely dissolved and regrown after consecutive scans, except for the BM sample. According to the results of X-ray photoelectron spectroscopy (XPS) analyses an Al depleted layer was obtained for the latter condition, which confirms the stability of TiO 2 film. The increasing values of charge transfer resistance obtained through electrochemical impedance measurements at distinct potentials in the passive region also supported the stability of the TiO 2 film grown in 6.0 mol L–1 HNO 3 solution. The formation of a medium frequency inductive loop and low frequency constant phase element are related to the dissolution of the oxide film and adsorption of NO 3 species into the formed passive film, respectively, as evidenced by XPS analyses. [ABSTRACT FROM AUTHOR]