Evaluation of Alloy Anodes for Aluminum‐Air Batteries: IV . Electrochemical Impedance Analysis of Pure Aluminum in at 25°C

An electrochemical impedance analysis of pure aluminum in at 25°C is reported. Impedance spectra have been obtained at 30–80 mV intervals extending from the hydrogen evolution region at −1.96V (vs. , ) to the transpassive dissolution region at −1.35V. The impedance spectra are found to consist of two intersecting capacitive semicircles with a loop at intermediate frequencies. The low‐frequency capacitive arc and the loop become increasingly dominant with respect to the high‐frequency relaxation as the potential is shifted in the positive direction. The impedance spectra and the steady‐state current/voltage characteristics (including the partial anodic and cathodic curves) are accounted for by a model involving the stepwise addition of hydroxyl groups to surface aluminum atoms, culminating in the chemical dissolution of to form . This anodic process is coupled to hydrogen evolution via competition for bare surface sites. Comparison of the experimental and predicted impedance spectra indicate that the total concentration of reactive sites at the surface varies with potential in a manner that parallels the anodic partial current. This variation is attributed to the existence of a porous corrosion product film on the surface. The impedance analysis also indicates small values (<0.1) for the transfer coefficients for elementary charge transfer reactions; these are attributed to the highly asymmetric nature of the reaction coordinate for reactions involving reactive species or to strong repulsive interaction between adsorbed species, as embodied in the Temkin adsorption isotherm.