Alkaline cathode reaction

Cathode Reaction. There are many different types of manganese dioxide (18), having varying activity in batteries. The only type suitable for alkaline batteries is y-Mn02, the mineral form of which is nsutite. The chemical composition of has been described (19) by the general formula... 

In neutral solutions the application of cathodic polarisation prevents crack initiation and this could be taken to indicate that hydrogen embrittlement is not the operative mechanism, since the discharge and entry of hydrogen might be expected to fracture the specimen more readily. The beneficial effect of cathodic polarisation has been interpreted , however, to result from more rapid film repair in the alkaline catholyte generated by the cathode reaction. The film serves as a barrier to rapid hydrogen entry. Consistent with this is the observation that in an environment of low pH (e.g. 10 N HCl) where film formation would not be expected, cathodic polarisation has no effect upon crack propagation. 

Essentially, the pH is controlled to suppress the hydrogen evolution cathodic reaction- The Pourbaix Diagram for iron indicates that high pH values as well as low values may lead to corrosion. The construction of these diagrams for higher than ambient temperatures - shows how the area of the alkaline zones increases considerably under boiler conditions, so that the risk of corrosion is correspondingly higher. Many feed systems contain copper alloys. 

The hydrogen evolution reaction (h.e.r.) is of particular importance in corrosion for a number of reasons. Firstly, the reduction of the HjO ion in acid solutions or the H2O molecule in neutral and alkaline solution is a common cathodic reaction for the corrosion of metals in acid, neutral and alkaline solutions the fact that iron will corrode in neutral water free from dissolved... 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

In alkaline solutions, nickel electrodes are quite common, particularly when conducting cathodic reactions (hydrogen generation, the reduction of certain organic materials). They resist corrosion under these conditions. With certain precautions (taking care not to exceed the limits of potential), nickel electrodes in alkaline solutions are useful, too, for certain anodic reactions such as the oxidation of hydrogen and methanol. 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

For forced-convection studies, the cathodic reaction of copper deposition has been largely supplanted by the cathodic reduction of ferricyanide at a nickel or platinum surface. An alkaline-supported equimolar mixture of ferri- and ferrocyanide is normally used. If the anolyte and the catholyte in the electrochemical cell are not separated by a diaphragm, oxidation of ferrocyanide at the anode compensates for cathodic depletion of ferricyanide.3... 

Termination of the plateau at a sufficiently high overpotential. The potential at which a consecutive electrode reaction sets in (e.g., hydrogen evolution in cathodic reactions) is determined by the composition of the electrolyte (specifically, the pH) and by the nature and state of the electrode surface (hydrogen overpotential). The reduction of ferricyanide in alkaline solution on nickel also provides a better-defined plateau in this respect than the deposition of copper in acid solution. 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with nonprecious, relatively inexpensive metals. To date, most low cost catalyst development work has been directed towards Raney nickel powders for anodes and silver-based powders for cathodes. The essential characteristics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). 

As the name indicates alkaline electrolyzers use high pH electrolytes like aqueous sodium hydroxide or potassium hydroxide. This is the oldest, most developed and most widely used method of water electrolysis. Hydrogen evolution takes place at the cathode, and oxygen evolution takes place at the anode. The cathodic reaction can be represented by the following steps [26,27]... 

An alternative to the Leclanche cell is the alkaline manganese cell, in which the electrolyte is a strongly alkaline paste and the anode and cathode reactions in their respective compartments are... 

As to the cathodic reaction in discharge, it is the reduction of MnOz, that has attracted considerable discussion. Thus, if the Mn02 undergoescomplete discharge in alkaline solution, the reaction can be written ... 

According to this model, the first stage in the treatment of nitrophenols aqueous wastes was the release of the nitro group from the aromatic ring. As a consequence, phenols or quinones were formed. These organic compounds were oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Also the cathodic reaction steps were considered in the global process when the electrochemical cell was undivided at the cathode, the reduction of the nitro to the amine group and the transformation of nitrate into ammonia were observed. In alkaline media, aminophe-nols were polymerised and transformed into a dark brown solid. 

The example of the iron/zinc couple refers to an acid solution, if a neutral or alkaline (basic) solution is considered the cathodic reaction would be ... 

Mn02 has a number of uses in chemical processes as an oxidizing agent, and it is also used in dry cell and alkaline batteries. In both cases the anode is made of zinc. The anode reaction (oxidation) and cathode reaction (reduction) are as follows for an alkaline cell ... 

Kalu and Oloman [75] studied the simultaneous synthesis of alkaline hydrogen peroxide and sodium chlorate in a bench-scale flow-by single-cell electrochemical reactor. A schematic of the electrode conditions is shown in Fig. 18. Graphite felt was used as the cathode to synthesize peroxide from 0.5 -2.0 M NaOH chlorate was the product at a dimensionally stable anode (DSA). The anodic and cathodic reactions were as follows ... 

Figure 13 shows schematically the current- and partial current-potential behavior of p-GaP ((a) and (b)) and n-GaP ((c) and (d)) in alkaline Fe(CN) solutions. In Fig. 13 (a) and (c), the partial current density at rest-potential or under open-circuit, and hence the etch rate, is limited by the cathodic partial reaction rate. This is the case for (111) GaP (for which the cathodic reaction is under kinetic control) and for (ITT) GaP at low Fe(CN) concentrations (for which the cathodic reaction is under diffusion control). In Fig. 13 (b) and (d), the partial current density at rest-potential or under open-circuit is limited by the anodic partial reactioi rate, which is limited by the OH diffusion rate (see Sec. 2.1) this is the case for (111) GaP at... 

The electrolysis is carried out in an undivided cell using copper electrodes and an aqueous solution of chlorides of alkali or alkaline earth metals. So copper (I) chloride, which is practically insoluble in pure water, will be formed and solved as a complex. At the cathode reaction (3) takes place... 

Combining both anode and cathode reactions, the overall cell reaction of the alkaline Zn/Mn02 battery can be described as follows ... 

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