Metal, uses as electrodes

Use your observations to decide which of the metals used as electrodes is the most effective reducing agent. Explain your reasoning. 

J. Tafel electrolyzed a soln. of 0-4 grm. of nitric acid and 20 c.c. of 50 per cent, sulphuric acid, using 10 sq. ems. of cathode surface and 2-4 amps, at 0°. The product of the reduction is largely dependent on the nature of the metal used as electrode. Some results are indicated in Table XXVII. With platinum, no ammonia or hydroxylamine was formed, and with palladium the reduction is extremely slow. Hie chief products of the reduction are hydroxylamine and ammonia. The largest proportion of the hydroxylamine is formed when mercury is used as cathode, and the conversion of the nitric acid into this can be carried out almost quantitatively. With lead electrodes, about 40 per cent, of the nitric acid is converted into hydroxylamine, and with copper electrodes only about 15 per cent. if the copper be in the form of a spongy mass, only about one per cent, of the acid is transformed into hydroxylamine, the remainder being reduced to ammonia. When... 

Electrochemical cells are made of two conducting electrodes, called the anode and the cathode. The oxidation reaction takes place at the anode, where electrons are released to flow through a wire to the cathode. At the cathode, reduction takes place. For the oxidation and reduction reactions to occur, the electrodes must be in a conducting solution called an electrolyte. The electrochemical cell voltage depends on the types of materials, usually conducting metals, used as electrodes, and the concentration of the electrolyte solution. (See Figure 6.5.)... 

In the lithium-ion approach, the metallic lithium anode is replaced by a lithium intercalation material. Then, tw O intercalation compound hosts, with high reversibility, are used as electrodes. The structures of the two electrode hosts are not significantly altered as the cell is cycled. Therefore the surface area of both elecftodes can be kept small and constant. In a practical cell, the surface area of the powders used to make up the elecftodes is nomrally in the 1 m /g range and does not increase with cycle number [4]. This means the safety problems of AA and larger size cells can be solved. 

Arsenic and antimony are metalloids. They have been known in the pure state since ancient times because they are easily obtained from their ores (Fig. 15.3). In the elemental state, they are used primarily in the semiconductor industry and in the lead alloys used as electrodes in storage batteries. Gallium arsenide is used in lasers, including the lasers used in CD players. Metallic bismuth, with its large, weakly bonded atoms, has a low melting point and is used in alloys that serve as fire detectors in sprinkler systems the alloy melts when a fire breaks out nearby, and the sprinkler system is activated. Like ice, solid bismuth is less dense than the liquid. As a result, molten bismuth does not shrink when it solidifies in molds, and so it is used to make low-temperature castings. 

In addition to metals, other substances that are solids and have at least some electronic conductivity can be used as reacting electrodes. During reaction, such a solid is converted to the solid phase of another substance (this is called a solid-state reaction), or soluble reaction products are formed. Reactions involving nomnetaUic solids occur primarily in batteries, where various oxides (MnOj, PbOj, NiOOH, Ag20, and others) and insoluble salts (PbS04, AgCl, and others) are widely used as electrode materials. These compounds are converted in an electrochemical reaction to the metal or to compounds of the metal in a different oxidation state. 

Another metal that has attracted interest for use as electrode material is rhodium, inspired by its high activity in the catalytic oxidation of CO in automotive catalysis. It is found that Rh is a far less active catalyst for the ethanol electro-oxidation reaction than Pt [de Souza et al., 2002 Leung et al., 1989]. Similar to ethanol oxidation on Pt, the main reactions products were CO2, acetaldehyde, and acetic acid. Rh, however, presents a significant better CO2 yield relative to the C2 compounds than Pt, indicating a... 

On the other hand, the only component present in the electrolysis cell that can be reduced is water, no matter what metal the electrode material is made from. So the formation of H2 gas by the reduction of water is unaffected when platinum, Cu or most other common metals are used as electrodes. 

Degner [194] and Couper et al. [75] have recently critisized the technology as it unavoidably produces, after the separation of the products, aqueous solutions containing stoichiometric amounts of salts of metals used as anodes. Solutions to this problem are possible as demonstrated in the case of Mg and Zn [177] electrodes. Al and Mg can easily be precipitated as hydroxides, recovered by filtration and dehydrated to the corresponding oxides, whereas Zn is recycled electrochemically. 

Examples Pure metal powders may be compressed into solid discs or cylinders which can be used as electrodes. Likewise, the analyzing alloys can also be used. 

The compound cerium oxide (either Ce Oj or CeO ) is used to coat the inside of ovens because it was discovered that food cannot stick to oven walls that are coated with cerium oxide. Cerium compounds are used as electrodes in high-intensity lamps and film projectors used by the motion picture industry. Cerium is also used in the manufacturing and polishing of high-refraction lenses for cameras and telescopes and in the manufacture of incandescent lantern mantles. It additionally acts as a chemical reagent, a misch metal, and a chemical catalyst. Cerium halides are an important component of the textile and photographic industries, as an additive to other metals, and in automobile catalytic converters. Cerium is also used as an alloy to make special steel for jet engines, solid-state instruments, and rocket propellants. 

The structure of a SPE cell is shown in Fig. 2.3. The basic unit of a SPE electrolyzer is an electrode membrane electrode (EME) structure that consists of the polymer membrane coated on either side with layers (typically several microns thick) of suitable catalyst materials acting as electrodes [43,49,50], with an electrolyzer module consisting of several such cells connected in series. The polymer membrane is highly acidic and hence acid resistant materials must be used in the structure fabrication noble metals like Pt, Ir, Rh, Ru or their oxides or alloys are generally used as electrode materials. Generally Pt and other noble metal alloys are used as cathodes, and Ir, Ir02, Rh, Pt, Rh-Pt, Pt-Ru etc. are used as anodes [43,46]. The EME is pressed from either side by porous, gas permeable plates that provide support to the EME and ensure... 

Unalloyed tungsten has several major applications. An important use is in the electric lamp filaments for light bulbs. Also, it is used as electrodes in arcwelding, in heating elements for high-temperature furnaces, in electron and television tubes, in glass-to- metal seals, and in solar energy devices. 

Nickel presents no interest for the reduction of O2, but this metal is used as electrode material for the oxidation of OH to O2, which proceeds over surface oxides of different composition, depending on the potential [31, 45]. 

Thus, a more appropriate question to ask is Is it possible to measure the absolute potential of the hydrogen reaction, /H+(abs) Actually it is possible. Remembering the definition of a standard hydrogen electrode potential (see Section 6.3.4), this was defined as the potential obtained when a metal comes in contact with a solution containing H+ under thermodynamically reversible conditions at unit activity, and H2 at 1 atm, at 298 K. As to the identity of the metal base, it can in principle be any metal at which it is possible to observe the reaction H2 H+ + e taking place at equilibrium. In practice, the metals used as substrates can only be noble metals because most other metals enter into equilibria with their own species in solution. Usually platinum is the metal chosen. 

One important matter is the availability of electrons in the oxide, which for a metal would not be a factor because there are always plenty. Pure perovskites are nonconductors and before they can be used as electrodes, it is necessary to add to them other substances that increase their conductance. BaO is one substance used. The process is... 

In recent years, mercury film ultramicroelectrodes have received wider application in electroanalytical practice [51,54,55]. Such electrodes are especially useful in analytical determinations, since they combine the features of ultramicroelectrodes (Chap. 12) with those of mercury film electrodes. In this case, the mercury can be deposited on carbon fibers, but many prefer a metallic support which is wetted by mercury. The solubility of the supporting metal in mercury should be low. Iridium [54] and silver [55], as well as platinum and nickel, have been used as supporting metals. Surprisingly, even gold fibers wetted by mercury have been very successfully used as electrodes in microchromatography and capillary electrophoresis detectors (Chap. 27). 

Another method for prepn of small solid particles is electric atomization, which is achieved by passing an electric arc between electrodes of pure metal in distilled water contained in a vessel made of practically insol material. This method produces a stable colloidal soln of the metal used as the electrodes (Ref 3)... 

In addition to metal, other materials such as ITO [134,273-275,727,869] or graphite [291] were used as electrodes on microfluidic chips. 

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