Oxygen-consuming cathodes

A fascinating option is offered by carbon-based oxygen-consuming cathodes, which can reduce oxygen into hydrogen peroxide (e.g. [50]). Thus, an oxidizing agent is produced at the cathode In combination... 

The standard potential of reaction (6), which takes place in acid solutions, is 1.23 V vs. standard hydrogen electrode (SHE). Reaction (7), in alkaline solutions, has a potential of 0.40 V vs. SHE. Both reactions are 1.23 V positive to the reversible potential of the hydrogen evolution reaction (HER) in the respective media. Therefore, the theoretical decomposition voltages of certain electrochemical processes can be reduced by 1.23 V if an oxygen-consuming cathode is substituted for the conventional hydrogen-evolving type. Examples of these processes [5] are ... 

This process contains two sub-reactions a metal-dissolving anode process and an oxygen-consuming cathode process. During the corrosion of iron, Fe, these processes... 

In contrast, the scope of improvements may be easily widened with the incorporation of oxygen-consuming gas-diffusion electrodes (GDE) in the construction of electrolysers as a substitute for the conventional hydrogen-evolving cathodes. 

The substitution of the traditional cathode with an oxygen-consuming GDE involves a new overall reaction, which may be written as... 

The substitution of conventional hydrogen-evolving cathodes with oxygen-consuming gas-diffusion electrodes (GDE), often referred to as oxygen-depolarised cathodes (ODC), also allows a substantial reduction in the energy consumption of the chlor-alkali process. 

Metals are subject to electrochemical corrosion in the presence of water Metal atoms lose electrons to become positively charged metal ions that go into solution. These then react with other chemical species in the soil ground-water to form solid corrosion products (e.g., metal oxides, hydroxides, sulfates). It is these solid corrosion products that often form a colored matrix with soil particles around the corroding object (Cronyn 1990). The initial formation of the metal ions takes place at a site on the metal known as the anode, whereas the electrons produced consumed by another reaction with an electron acceptor (the cathode). Due to the electrical conductivity of metals the location of the anode and cathode can be at different locations on the metal surface. In the presence of water and oxygen the cathodic reaction is... 

The construction of a cell permitting both FTIR measurements and electrochemical impedance measurements at buried polymer/metal interfaces has been described [266]. Ingress of water and electrolyte, oxidation (corrosion) of the aluminum metal layer, swelling of the polymer and delamination of the polymer were observed. A cell suitable for ATR measurements up to 80°C has been described [267]. The combination of a cell for ATR measurements with DBMS (see Sect. 5.8.1) has been developed [268]. It permits simultaneous detection of stable adsorbed species and relatively stable adsorbed reaction intermediates (via FTIR spectroscopy), quantitative determination of volatile species with DBMS and elucidation of overall reaction kinetics. An arrangement with a gas-fed electrode attached to the ATR element and operated at T = 60°C has been reported [269]. In this study, the establishment of mixed potentials at an oxygen consuming direct methanol fuel cell in the presence of methanol at the cathode was investigated. With infrared spec-... 

This decrease may not occur in the case of structures subjected to wetting/ drying cycles or in conditions where oxygen consumed at the surface of the passive steel is replaced. This may happen in hollow piles of offshore structures, as depicted in Figure 8.2. Similar conditions may arise in tuimels buried or submersed in chloride-containing environments. Rebars on the inside of hoUow (air-filled) structures may be effective cathodes with noble potentials. They increase the potential of rebars closer to the seawater side of the cross section, stimulating corrosion initiation at lower chloride contents than without additional cathodic effects. Subsequently they may increase the corrosion rate at the anodes by consuming the electrons produced. The final corrosion rate will be a function of the ratio between anodic and cathodic areas, which is influenced by the concrete resistivity. 

While the cell runs under alkaline anode with acidic cathode conditions, both OhT and are consumed at the anode and cathode at a rate of six for each molecule of methanol The maximum theoretical energy density (based on the reaction of 1M of methanol with ambient oxygen, consuming 6 equiv of H2SO4 and KOH) is 495 Wh kg , much lower than the theoretical value for the all-alkaline and all-acidic LFFC in which only methanol is consumed (6000Wh kg ) [26]. 

On the cathode hydrogen gas was formed (and maybe also oxygen consumed). In that case the pH is increased at the outer ring, resulting in the formation of the sphere of corrosion products. 

A similar set of equations may be applied for the cathode exhaust. Oxygen flow rate at the outlet, that is, unused oxygen, is equal to oxygen supplied at the inlet minus oxygen consumed in the fuel electrochemical reaction ... 

Iron atoms pass into solution in the water as Fe leaving behind two electrons each (the anodic reaction). These are conducted through the metal to a place where the oxygen reduction reaction can take place to consume the electrons (the cathodic reaction). This reaction generates OH ions which then combine with the Fe ions to form a hydrated iron oxide Fe(OH)2 (really FeO, H2O) but instead of forming on the surface where it might give some protection, it often forms as a precipitate in the water itself. The reaction can be summarised by... 

The anodic reaction consists of the passage of iron ions from the metallic lattice into solution, with the liberation of electrons, which are consumed at the cathode by reaction with water and oxygen. 

An electrolytic method for removing CaO from CaCl2 was suggested by Barletta, et.al.17 The salt is electrolyzed in a cell with a graphite consumable anode. Oxygen is removed at the anode where it reacts with carbon to form CO and C02. Calcium ions are reduced to metal at the cathode. Thus, the electrolytic reduction reaction should be... 

A fuel cell consists of an ion-conducting membrane (electrolyte) and two porous catalyst layers (electrodes) in contact with the membrane on either side. The hydrogen oxidation reaction at the anode of the fuel cell yields electrons, which are transported through an external circuit to reach the cathode. At the cathode, electrons are consumed in the oxygen reduction reaction. The circuit is completed by permeation of ions through the membrane. 

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