Cathodic oxygen reduction processes

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

During the processes described by equations 1 and 2, dissolution from the coating layer gives rise to the deposition of a hydroxide layer impeding the cathodic oxygen reduction reaction. [8] This reaction involves the rapid hydrolysis of water via equations 3 and 4. 

Under most common DMFC operation conditions, the oxygen reduction process is under interfacial kinetics control and the methanol oxidation process is determined by the methanol flux across the membrane,. /crossover, (Eqs 59-61). The increase in cathode overpotential is therefore be given by... 

Because of the ready access of atmospheric oxygen to a metal surface exposed to a thin liquid layer, the most common cathode reaction (see Chapter 1, Volume 4) for metals exposed to a thin aqueous phase is the oxygen-reduction process (see Chapter 1, Volume 4) ... 

These IMPs of technical alloys, which are covered by electric conducting oxides, act as local cathodes for the oxygen reduction process in the tail (Fig. 36a-c). 

Snyder J, McCue I, Livi K, Erlebacher J (2012) Structure/processing/properties relationships in nanoporous nanoparticles as applied to catalysis of the cathodic oxygen reduction reaction. J Am Chem Soc 134 8633-8645... 

Air-cathode composition showing the structure and functions of each component, (b) Three-phase diagram for the oxygen-reduction process in the aqueous electrolyte. 

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

The hydrogen evolution reaction (h.e.r.) and the oxygen reduction reaction (equations 1.11 and 1.12) are the two most important cathodic processes in the corrosion of metals, and this is due to the fact that hydrogen ions and water molecules are invariably present in aqueous solution, and since most aqueous solutions are in contact with the atmosphere, dissolved oxygen molecules will normally be present. 

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