The Kinetics of Oxygen Reduction in Molten Phosphoric Acid at High Temperatures

The solubility and diffusivity of oxygen in concentrated (98%) phosphoric acid, and the kinetics of its reduction on platinum within the 25~176 temperature interval have been investigated utilizing glass insulated Pt microelectrodes with radii of 12.7 and 250 ~m. The enthalpy of solution of oxygen in phosphoric acid was found to be 2.2 -+ 0.3 kcal mol-', and the entropy of solution is - 12.6 -+ 0.8 cal K-' mol -'. Fundamental entropy calculations indicate that the large negative entropy change upon solution is due to the loss of three degrees of freedom in translation (free translation becoming hindered) and two degrees of freedom in rotation (only libration occurs in solution). A hydrogen bonded model for the state of oxygen in phosphoric acid is found to be consistent with the experimental values obtained for enthalpy of solution, entropy of solution, and heat of activation for diffusion, 5.9 -+ 0.4 kcal mol-'. The electrochemical transfer coefficient for the oxygen reduction reaction was found to remain constant with temperature, at 0.63 +- 0.05. The exchange current density at 25~ in 98% H3PO4 is 8 x 10-I2A cm -2, increasing with temperature with an apparent heat of activation of 17.3 kcal mol-'. The growing interest in practical fuel cells leads to the need to investigate the kinetics of reactions under conditions relevant to their function. Of these reactions, the. most important is certainly that of oxygen reduction. In the fuel cell which is of greatest practical interest at this time (the phosphoric acid cell), the conditions of measurement are extremely taxing, 190~ and 2% water in phosphoric acid. On the other hand, determination of the kinetics of oxygen reduction under these conditions is very important, because inter alia of the importance of understanding the oxygen reduction rate in media such as phosphoric acid, compared with those in media which appear to give higher rates, for example, trifluoromethane sulfonic acid. Because of the great difficulty of measurements under practical conditions, vital parameters concerning the oxygen reduction reaction under fuel cell conditions are not known. Thus, previous workers who have approached this area (1-5) have made determinations in solutions which are less concentrated than those of practical use, and at lower temperatures. Even here, the data in the literature are highly discrepant. In order to discern the reasons for the differences of results in media used for oxygen reduction, it is necessary to know not only the exchange current density, Tafel parameters, and other relevant factors of electrode kinetic interest, but particularly the heat of activation for the reaction itself. Thus, kinetic measurements over a wide range of current densities are essential in arriving at a reasonable experimental base for theoretical considerations.