The Oxygen Electrode in Molten Carbonates

The reversibility of reaction 2 was established by thorough studies [21,22] of the dependence of the open-circuit potential upon the concentration [21] of COj , upon the partial pressures [22] pco and po and upon the temperature [22]. In all cases, the electrode potential displayed the dependence predicted thermodynamically for the given carbonate melt. 

Curve a pQ2 = 0.19siim, curve b po = 0A5a.tm, curve c pQ = 0A3a.tm 

The three curves (Po =. 19 atm, 0.45 atm, 0.13 atm) in Fig. 87 consist of two portions. The portion between 0 and about —0.05 V was attributed [36] to reaction 2. When the limiting current of reaction 2 is reached, part of the oxygen reacts [36] according to Eq. 22. 

As was to be expected on the basis of the stoichiometry of reaction 2, the limiting current determined [36] as a function of PcoJPo2 found to have a maximum at PcoJPo2 limiting current of curve b in Fig. 87 is the largest one for this reason. The currents above the limiting current of reaction 2 increase with the oxygen pressure. 

Similar i—U curves with two segments were obtained [37,38] in mixtures of 50% Li2C03 and 50% Na2C03 at different temperatures on platinum or palladium foil. The shape of the curves depended strongly on the length of the heat pretreatment of the electrodes at various temperatures in air. The initial segment virtually disappeared after palladium had been heated for 30 to 60 minutes at 700 °C in air. It was suggested that the two portions are due to the involvement of two different types of oxides in reaction 2 on platinum and palladium and not to the successive occurrence of reaction 2 and reaction 22. 

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