Alloys, with alkaline metals

Both systems have an acid-based electtolyte (PEFC is sulfuric acid based), although the PAFC is a liquid electrolyte solution system and the PEFC electrolyte exists as a partially bound solution in a solid polymer matrix. Between the PEFC and PAFC, the anode HOR and cathode ORR are the same. A schematic of the materials and electrochemical reactions in the PAFC system is shown in Figure 7.20. Both systems use a noble metal catalyst or alloy with noble metals on the electrodes, and both suffer from poor ORR kinetics relative to alkaline-based systems. Ironically, since operation of the PEFC at 80°C results in catalyst poisoning from CO as well as water management issues that the PAFC avoids, developers seek higher temperature PEFC membranes that can operate at 120-200°C like the PAFC but maintain the high power density advantage of the PEFC. 

Rubidium can be liquid at room temperature. It is a soft, silvery-white metallic element of the alkali group and is the second most electropositive and alkaline element. It ignites spontaneously in air and reacts violently in water, setting fire to the liberated hydrogen. As with other alkali metals, it forms amalgams with mercury and it alloys with gold, cesium, sodium, and potassium. It colors a flame yellowish violet. Rubidium metal can be prepared by reducing rubidium chloride with calcium, and by a number of other methods. It must be kept under a dry mineral oil or in a vacuum or inert atmosphere. 

Rubidium metal alloys with the other alkaU metals, the alkaline-earth metals, antimony, bismuth, gold, and mercury. Rubidium forms double haUde salts with antimony, bismuth, cadmium, cobalt, copper, iron, lead, manganese, mercury, nickel, thorium, and 2iac. These complexes are generally water iasoluble and not hygroscopic. The soluble mbidium compounds are acetate, bromide, carbonate, chloride, chromate, fluoride, formate, hydroxide, iodide. 

Corrosion of industrial alloys in alkaline waters is not as common or as severe as attack associated with acidic conditions. Caustic solutions produce little corrosion on steel, stainless steel, cast iron, nickel, and nickel alloys under most cooling water conditions. Ammonia produces wastage and cracking mainly on copper and copper alloys. Most other alloys are not attacked at cooling water temperatures. This is at least in part explained by inherent alloy corrosion behavior and the interaction of specific ions on the metal surface. Further, many dissolved minerals have normal pH solubility and thus deposit at faster rates when pH increases. Precipitated minerals such as phosphates, carbonates, and silicates, for example, tend to reduce corrosion on many alloys. 

Among the alkali metals, Li, Na, K, Rb, and Cs and their alloys have been used as exohedral dopants for Cgo [25, 26], with one electron typically transferred per alkali metal dopant. Although the metal atom diffusion rates appear to be considerably lower, some success has also been achieved with the intercalation of alkaline earth dopants, such as Ca, Sr, and Ba [27, 28, 29], where two electrons per metal atom M are transferred to the Cgo molecules for low concentrations of metal atoms, and less than two electrons per alkaline earth ion for high metal atom concentrations. Since the alkaline earth ions are smaller than the corresponding alkali metals in the same row of the periodic table, the crystal structures formed with alkaline earth doping are often different from those for the alkali metal dopants. Except for the alkali metal and alkaline earth intercalation compounds, few intercalation compounds have been investigated for their physical properties. 

Alloys of aluminium with magnesium or magnesium and silicon are generally more resistant than other alloys to alkaline media. The corrosion rate in potassium and sodium hydroxide solutions decreases with increasing purity of the metal (Fig. 4.9), but with ammonium hydroxide the reverse occurs. 

Numerous proprietary electrolytes have been developed for the production of harder and brighter deposits. These include acid, neutral and alkaline solutions and cyanide-free formulations and the coatings produced may be essentially pure, where maximum electrical conductivity is required, or alloyed with various amounts of other precious or base metals, e.g. silver, copper, nickel, cobalt, indium, to develop special physical characteristics. 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

The alkaline EG synthesis method described in this chapter is highly efficient for preparing colloidal solutions of small and narrowly distributed unprotected noble metal or alloy nanoclusters with high metal concentration. 

The small metal particle size, large available surface area and homogeneous dispersion of the metal nanoclusters on the supports are key factors in improving the electrocatalytic activity and the anti-polarization ability of the Pt-based catalysts for fuel cells. The alkaline EG synthesis method proved to be of universal significance for preparing different electrocatalysts of supported metal and alloy nanoparticles with high metal loadings and excellent cell performances. 

The alkaline EG S5mthesis method is a very effective technology for the chemical preparation of unprotected metal and alloy nanoclusters stabilized by EG and simple ions. This method is characterized by two steps involving the formation of metal hydroxide or oxide colloidal particles and the reduction of them by EG in a basic condition. The strategy of separating the core formation from reduction processes provides a valid route to overcome the obstacle in producing stable unprotected metal nanoclusters in colloidal solutions with high metal concentrations. Noble metal and alloy nanoclusters such as Pt, Rh, Ru, Os, Pt/Rh and Pt/Ru nanoclusters with small particle... 

The mechanism of cathodic disintegration involves the formation of a metal alloy with a cation only deposited at high current densities and a large impressed electromotive force. Thus the disintegration of a lead cathode in alkaline solution is due to the formation of a lead sodium alloy which subsequently reacts with the water yielding a fine black dispersed lead suspension. This electrical dispersion... 

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