Temperature oxygen overvoltage

In an alkali-chlorine cell a saturated (about 6 N) solution of sodium chloride is electrolyzed at ordinary temperatures, between a steel cathode (hydrogen overvoltage 0.2) and a graphite anode (oxygen overvoltage 0.6 volt chlorine overvoltage negligible). The nature of the electrode process. Explained ... 

For example, in the anodic oxidation of sulfates to per-oxysulfates (Experiment 48), platinum, a metal with a high oxygen overvoltage, is used as the anode, the anode is made very small and therefore the cmrent density is very high, and the temperature is kept low. In this way the evolution... 

Perchlorates are made commercially by the electrolytic oxidation of chlorates. Sodium perchlorate is made first as a rule, and other perchlorates are prepared from it by double decomposition. The conditions for a good yield of perchlorate are concentrated neutral chlorate solution, temperature below 50°, current density 0.1 amp/cm or more, and anode of platinum. Platinum is better than graphite, probably because of its higher oxygen overvoltage. 

Further, the value of b over a range of temperature has been found to be 2 X 2.3ftr/F for oxygen as well as for hydrogen evolution it follows, therefore, that the factor a is approximately 0.5 in each case. The anodic overvoltage at smooth platinum is said to be independent of the concentration of the sulfuric acid used as electrolyte in sodium hydroxide, however, the value apparently decreases with increasing hydroxyl ion concentration. In very dilute alkaline solutions a is about 0.5, but it apparently increases as the concentration of the electrolyte is increased. 

The data summarized in this paper have established that the oxide pyrochlores under discussion substantially reduce the activation energy overvoltages associated with oxygen electrocatalysis. Specifically, it is found that these catalysts, in aqueous alkaline media near ambient temperature, are superior to any other oxygen evolution catalyst and are equal in performance to the best known oxygen reduction catalysts. As bidirectional oxygen electrocatalysts, they appear to be unmatched. 

An optimum catalysis of the oxygen eleetrode reaction depends on a critical compromise of adsorption energies of the intermediates. It is, therefore, not surprising that no electrocatalyst has been found where this reaction occurs reversibly at ambient temperatures. In both directions rather large overvoltages are required to reduce O2 to H2O or to oxidize H2O to O2. 

The temperature increase would also favour the dissociation of the water decomposition intermediates which require thermal activation of the dissociation. For example the dissociation HO2 —H + O2 is characterised by =17.5 kJ and A Z =27.3 kJ. To cite yet another circumstance, it is known from electrochemistry that the temperature increase by one degree would decrease the overvoltage in oxygen and l drogen release by 3 to 4 mV. For example, the temperature rise in the electrolyser from room temperature to 70-80 C would lead to a 40% decrease in overvoltage. 

PEM fuel cell characteristics are generally described with polarization curves. The thermodynamic equilibrium potential of the hydrogen/oxygen reaction is reduced by various overvoltage terms that depend on mass transport, kinetic, and ohmic phenomena within cell. In other words, the output voltage of a single cell is attributable to different current, temperature, and pressure dependant factors [1]. 

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