Magnesium alloy electrodes

Magnesium-air air cells with NaCl-electrolyte were developed and investigated. The current-voltage and the discharge characteristics of the cells with were studied. Air gas-diffusion electrodes suitable for operation in NaCl-electrolytes were designed. Various carbon-based catalysts for the electrochemical reduction were tested in these air electrodes. Magnesium alloys suitable for use as anodes in Mg-air cells were found. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

The subject of surface films on electrodes in non-aqueous solutions is mostly important for the field of batteries. The performance of both Li and Li-ion batteries depends strongly on passivation phenomena that relate to surface film formation on both the anodes and the cathodes. Lithium and lithiated carbon anodes reduce all the solvents and salt anions in electrolyte solutions relevant to Li batteries. The products of these surface reactions always contain insoluble Li salts that precipitate on the electrodes as surface films. All charge transfer processes of Li, Li-C, and Li alloy anodes in Li batteries involve the critical step of Li-ion migration through the surface films. Thereby, the composition, structure, morphology, and electrical properties of surface films on Li, Li-C, and Li alloy electrodes were smdied very intensively over the years. In contrast, reversible magnesium electrodes can function only in surface film-free conditions. ... 

This reaction stops in alkaline electrolytes because of the formation of an insoluble film of magnesium hydroxide on the electrode surface which prevents further reaction. Acid tends to dissolve the film. An important consequence of the film on magnesium elecfrodes (see also Chap. 9) is that there is a delayed response to an increase in the load because of the need to disrupt the film to create new bare surfaces for reaction. Pure magnesium anodes usually do not give good cell performance, and several magnesium alloys have been developed for use as anodes tailored to provide the desired characteristics. 

When an Mg alloy does not fulfill the chemical composition specified for a sacrificial magnesium anode, features as inductive loops at lower frequencies appear in the Nyquist representation of the measured impedance. As the magnesium alloy is polarized further away from its E, in the anodic direction, the Nyquist representation of the impedance exhibits inductive loop behavior (Fig. 2.18). This fact leads to the consideration of an inductor component in the corresponding electrical equivalent circuit. This inductive loop can be associated with the adsorption and desorption phenomena occurring on the surface of the sample and leading to the process of formation of the corrosion product layer on the surface of the electrode (Guadarrama-Mu-oz et al., 2006). 

Current-potential curves of the O2 reduction were measured [68] in concentrated KOH on alloy electrodes prepared from silver and small amounts of elements that form oxides of low electron affinity. The reactivity of a silver — 1.7w/o (weight percent) magnesium alloy was better than that of pure silver. In contrast, no improvement was found for foils of silver — Iw/o thorium, silver — Iw/o radium, and silver — 1 w/o barium. 

Lead hydride is the least well characterized of the Group IVB hydrides. It is formed along with hydrogen, on the electrolysis of dilute sulphuric add with lead electrodes, and by the dissolution of lead-magnesium alloy in dilute add. The hydride of lead formed in small quantities is assumed to be PbH4 (b.p.--13°C)m. 

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