Cathodic ReductiOn of Oxygen on Platinum Electrode of a Fuel Cell with Mixed Solution of Trifluoromethanesulfonic Acid and Phosphoric Acid

Abstract

The solubility of oxygen is usually high in fluorinated organic solvents such as trifluoromethanesulfonic acid (TFMSA). The kinetic of the oxygen reduction in TFMSA and 85%phosphoric acid solutions\ is of interest from the stand point of searching for electrolytes alternative to the phosphoric acidspecifically used in a practical acid fuel cell. The main aims of the present work are to compare the behavior of oxygen reduction in the TFMSA and 85% phosphoric acid solutions on a rotating platinum disk electrode (Pt-R. D. E. ). The species generated in the mixed solutions have been analyzed with a laser Raman spectroscopy.
The band at 1042 cm^-1 assigned to S-O in CF₃S0₃^- and the band at 777 cm^-1 assigned to C-S in both CF₃S0₃^- and CF₃S0₃H were observed in the laser Raman spectra. The intensity ratio of the band at 1042 cm^-1 to that of 777 cm^-1 (I₁₀₄₂/I₇₇₇) was dependent on the degree of electrolytic dissociation of TFMSA. From the plots of the ratio against the mole fraction of TFMSA (X), the TFMSA may be nearly completely dissociated in the range of X<<0.45. However in X >0.45, a part of the TFMSA remained in the form of molecule in spite of the interaction between a part of the proton produced by the electrolytic dissociation of TFMSA and oxygen of P=0 bond in molecular H₃PO₄.
In the Pt-R. D. E. method, the diffusion-limiting current density (i_d) was proportional to the Levich′s B value, where B is evaluated from the reciprocal of the slope of 1/i_d vs.1/ω^1/2 plots. As X increased, the B value increased due to the enhancement of oxygen solubility and diffusion coefficient. Simultaneously, in the range of X>>O.2, the exchange current density (i₀) in the mixed solutions was larger than that in 85% H₃PO₄. The addition of TFMSA in 85% H₃PO₄ is effective to increase io and the cell voltage of this acid fuel cell.