Oxygen reduction reaction Subject

A sheet of steel of thickness 0.50 mm is tinplated on both sides and subjected to a corrosive environment. During service, the tinplate becomes scratched, so that steel is exposed over 0.5% of the area of the sheet. Under these conditions it is estimated that the current consumed at the tinned surface by the oxygen-reduction reaction is 2 X 10 A m -. Will the sheet rust through within 5 years in the scratched condition The density of steel is 7.87Mg m . Assume that the steel corrodes to give Fe " ions. The atomic weight of iron is 55.9. 

The impedance polarization performance of LSM electrode is closely related to the mechanism and kinetics of the oxygen reduction reactions. 02 reduction at SOFC cathodes is the most heavily studied subject, and this subject is sufficiently broad and complex to warrant its own review. Interested readers should consult the recent excellent articles by Adler [1] and Fleig [55], Here, only the polarization performance and its influencing factors are discussed. 

The origin of PtsCo activity towards the oxygen reduction reactions has been the subject of numerous studies. Among the scenarios advanced are ... 

It is clear that the oxygen reduction reaction (ORR) is one of the most important electrochemical reactions since it has multiple applications. The potential fields range from the energy conversion to corrosion science. For this reason, it has been the subject of numerous works throughout the years. In the complete reduction of oxygen to water, there are four electrons exchanged. This high number of electrons... 

As stated in the introductory section, the main source of performance losses is the cathodic overpotential caused by the oxygen reduction reaction (ORR), which has therefore been the subject of several studies. As stated in the beginning of this section, the largest contribution to the performance loss is caused by the slow kinetics of the ORR. In order to reconsider the given example, at a current density ig = 1500 mA/cm the ohmic loss amounts just to 80 mV, whereas the performance loss due to the ORR is 400 mV. The reduction reaction by itself depends on factors such as the catalyst composition, particle sizes, pH-value and potential. 

The reaction has been subject of numerous experimental and theoretical studies mechanistic aspects based on theoretical considerations are described and summarized in Section 3.3.2. The detailed explanation of the reaction mechanism serves as the basis for the forthcoming discussion of the appropriate choice of catalysts and requirements due to the different intermediates. Experiments were either performed in the gas phase or in an electrochemical environment, whereas only the electrochemical experiments can be considered to bear a realistic resemblance of the fuel cell environment. Contrary to PEMFCs, where the oxygen reduction reaction is the rate-limiting step, in DMFCs the slow kinetics of the MOR represent the hmiting factor. 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

Underpotential deposited layers have a strong effect on the electro-catalytic properties of electrodes for surface-sensitive reactions such as organic oxidations, hydrogen evolution, oxygen reduction, etc. A review on this subject has recently been published by Adzic [131a, b]. 

An electrocatalytic reaction is an electrode reaction sensitive to the properties of the electrode surface. An electrocatalyst participates in promoting or suppressing an electrode reaction or reaction path without itself being transformed. For example, oxygen reduction electrode kinetics are enhanced by some five orders of magnitude from iron to platinum in alkaline solutions or from bare carbon to carbon electrodes modified with Fe phthalocyanines or phenylporphyrins. For a comprehensive discussion of the subject, the reader is referred to refs. (76, 95, and 132-136). 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

The transfer of a single electron between two chemical entities is the simplest of oxidation-reduction processes, but it is of central importance in vast areas of chemistry. Electron transfer processes constitute the fundamental steps in biological utilization of oxygen, in electrical conductivity, in oxidation reduction reactions of organic and inorganic substrates, in many catalytic processes, in the transduction of the sun s energy by plants and by synthetic solar cells, and so on. The breadth and complexity of the subject is evident from the five volume handbook Electron Transfer in Chemistry (V. Balzani, Ed.), published in 2001. The most fimdamental principles that govern the efficiencies, the yields or the rates of electron-transfer processes are independent of the nature of the substrates. The properties of the substrates do dictate the conditions for apphcability of those fimdamental... 

Despite the apparent simplicity of this reaction, the process by which the oxygen reduction occurs followed by incorporation of the ionic species into the electrolyte is the subject of some debate and is dependent on the mode of operation of the cathode material. Two typical cathode types are currently utilized in SOFCs -electronic conductors and mixed ionic-electronic conductors (MIECs). The cathode reactions, while nominally the same in both types of materials, occur at different locations, and hence, the active region varies, leading to differences in the operating regime and ultimately performance. In the case of a single phase electronic conductor. 

Obviously, the different types of interaction will lead to different kinetics and mechanisms, and different proportions of water to peroxide in the reaction product. Much has been written and speculated about these different interactions and the influence of metal d-orbital characteristics on the overall ORR. Reviews covering this aspect of the subject can be found in references such as [103] and [105]. On the whole, these conclusion can be drawn from the significant body of research on this topic (a) Fe and Co macrocyclic complexes appear to constitute the best catalysts for oxygen reduction (b) Ir complexes also appear to be quite active (c) the redox potential for the metal ion couple plays a major role in dictating the activity of the ORR, with optimal redox potentials existing for maximum activity (d) many of the non-noble metal catalysts lack the long-term stability needed for practical fuel cell applications and (e) heat treatment of supported non-noble metal catalysts tends to increase both their activity towards oxygen reduction and their stability, but optimum heat treatments achieve the best balance of the two. 

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