Low-frequency Inductive Loop Research Articles

Interpretation of Low-Frequency Inductive Loops in PEM Fuel Cell Impedance Data

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Exfoliation corrosion of aluminum alloy AA7075 examined by electrochemical impedance spectroscopy

AbstractA typical aluminum alloy, AA7075, was immersed in the EXCO solution, and its corrosion properties during different immersion time were measured repetitively using electrochemical impedance spectroscopy technique (EIS). The EIS data a were simulated using equivalent circuit with ZView program. The results show that once the exfoliation occurs, the low frequency inductive loop in the Nyquist plot associated with the relaxation phenomenon of reaction intermediates disappears, and the Nyquist plane is mainly composed of two capacitive arcs in the high frequency range and low frequency range respectively. The former originates from the original corroded surface, while the latter from the newly formed interface by exfoliation corrosion (EXCO). With the increased immersion time, the high frequency capacitance arc decreases gradually, while the low frequency capacitance arc increases gradually. From the beginning of immersion up to 9 hours, charge transfer resistance gradually decreases, illustrating the acceleration of the corrosion rate, whereas the proton concentration decreases steeply, indicating the cathodic process is pre‐dominant. Then the corrosion rate decreases gradually corresponding to the exhausting of proton ions. The results also show that the exfoliation corrosion is developed from pitting corrosion through intergranular corrosion to general corrosion at the end.

Electrochemical Oxidation of CO on Pt and Ni Point Electrodes in Contact with an Yttria-Stabilized Zirconia Electrolyte: II. Steady-State and Impedance Measurements

Electrochemical measurements were performed with Pt and Ni electrodes forming an approximate (macroscopic) point contact with a solid electrolyte, for mixtures of in the temperature range 827 to 950°C and 775 to 924°C, respectively. Steady-state current and impedance spectra vs. overpotential under the different experimental conditions were compared with the theoretical models derived in Part I of this paper. A low-frequency inductive loop in the fourth quadrant of the impedance spectra appearing under certain experimental conditions is consistent with a model for a reaction mechanism involving two adsorbed intermediates. At high anodic overpotentials, the data for Ni electrodes are consistent with the formation of an insulating layer of nickel oxide (NiO) in the electrode/electrolyte interface. Irrespective of the experimental conditions, no effects of diffusion limitations were seen in the impedance spectra. The electrode metal plays a significant role in the overall reaction. For Ni, the results indicate a lower specific electrocatalytic activity for than for the reaction. © 2002 The Electrochemical Society. All rights reserved.

AC impedance study of Ni–YSZ cermet anodes in methane-fuelled internal reforming YSZ fuel cells

The impedance characteristics of the Ni–YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic overpotential in methane-fuelled internal reforming YSZ fuel cells. AC impedance measurements carried out under open-circuit conditions in the temperature range 800–900 °C showed the presence of at least two rate-limiting processes which were influenced by temperature and gas phase composition. The apparent polarization resistance, R app, of the Ni–YSZ cermet anode/YSZ interface was found to increase with increasing methane partial pressure and decreasing H 2O partial pressure. The apparent capacitance, C app, calculated from AC impedance data under open-circuit conditions was found to decrease with increasing methane partial pressure. AC impedance measurements carried out under closed-circuit conditions at 900 °C and four different P H 2O showed the appearance of a low-frequency inductive loop at high anodic overpotentials, in addition to the features observed under open-circuit conditions. The effect of gas phase composition on R app was similar to the one observed under open-circuit conditions. The present results are compared with results presented in literature and discussed in the frame of the mechanism of the electrochemical and catalytic reactions taking place over the anode.

Polarization behavior of Ni-YSZ cermet anodes in YSZ fuel cells running on methane under internal reforming conditions

The polarization characteristics of the Ni-YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic overpotential in methane fuelled internal reforming YSZ fuel cells. AC impedance measurements carried out under open circuit conditions in the temperature range 800 to 900°C showed the presence of at least two rate-limiting processes that were influenced by temperature and gas phase composition. The apparent polarization resistance, Rapp, of the Ni-YSZ cermet anode/YSZ interface was found to increase with increasing methane partial pressure and decreasing H2O partial pressure. The activation energy related to the apparent polarization conductance R app −1 varied between 0.7 and 1.1 eV, mainly due to the variation of PH 2 O. The order of magnitude of the apparent capacitance, Capp, values calculated from AC impedance data under open circuit conditions indicate that the observed capacitance corresponds to the double layer capacitance of the Ni/YSZ interface and/or a pseudo-capacitance related to adsorption of species on the Ni-YSZ cermet electrode accompanied by charge transfer. Capp was found to decrease with increasing methane partial pressure. AC impedance measurements carried out under closed circuit conditions at 900 °C and four different PH 2 O showed the appearance of a low-frequency inductive loop at high anodic over-potentials, in addition to the features observed under open circuit conditions. The effect of gas phase composition on Rapp was similar to the one observed under open circuit conditions. In addition an almost exponential decrease of Rapp with increasing anodic overpotential was observed. The present results are compared with results presented in literature and discussed in the frame of the mechanism of the electrochemical and catalytic reactions taking place over the anode.

On the kinetics of lead electrodeposition in fluorosilicate electrolyte Part I: Inhibiting effect of sodium lignin sulphonate

The kinetics of lead electrodeposition in Betts type electrolytes was investigated by electrode impedance measurements. In the absence of additives, the complex plane impedance plots exhibit two separate capacitive features and a, low-frequency inductive loop. The sizes and proper frequencies of the capacitive loops are influenced by the current density, the electrode rotation speed and the presence of sodium lignin sulphonate (LS), a colloidal organic additive used in practice as levelling agent. The low-frequency inductive loop, connected to the active area where lead is deposited, disappears with the LS-containing electrolyte. A reaction model is proposed in which the charge transfer reaction is coupled to the formation of adsorbed complexed species able to inhibit lead deposition before diffusing towards the bulk solution. The inhibiting effect of LS additive is discussed in terms of changes in the kinetic parameters of reactions.

Oxygen transfer properties of ion-implanted yttria-stabilized zirconia

The influence of surface modification by ion implantation on oxygen transfer in yttria-stabilized zirconia (YSZ) has been studied. Implantation of 15 keV 56Fe in YSZ with a maximum dose of 8×10 16 atoms cm −2 yields reproducible surface layers of approximately 20 nm deep with a maximum Fe cation fraction of 0.5 at the surface. After annealing these layers are stable up to 700–800°C. The exchange current densities for the Fe-implanted layers, measured using porous gold electrodes, are a factor of 10–50 larger than observed for not-implanted YSZ. 18O isotope exchange experiments show that for Fe-implanted samples the surface oxygen exchange rate is at least a factor 30 larger than for normal YSZ. The electrode kinetics has been studied for normal and implanted YSZ using current-overvoltage measurements and impedance measurements under bias. An electrode reaction model for the transfer of oxygen has been developed. This model is able to explain the low frequency inductive loop in the impedance diagram which is observed at high cathodic and anodic polarizations for implanted as well as normal YSZ.