Oxygen reduction at the platinum/Nafion(RTM) interface: Electrode kinetics and mass transport

Research in solid polymer electrolyte fuel cells is gaining momentum because of the prospects of attaining high energy efficiencies and power densities, essential for transportation and space applications. The most advanced solid polymer electrolytes for these fuel cells are the perfluorosulfonate ionomers (PFSIs) such as duPont's Nafion and the Dow PFSIs. The high oxygen solubility, chemical stability, proton conductivity and permselectivity exhibited by Nafion and the Dow PFSI's make them ideal candidates as electrolytes for fuel cells. Furthermore, the minimal anion adsorption on electrodes from fluorinated acids enhances oxygen reduction kinetics.; The primary objectives of this work were to determine the concentration and diffusion coefficient of oxygen in Nafion, and the electrode kinetic parameters for the reduction of oxygen at the Pt/Nafion interface under totally solid-state conditions (i.e. no contacting liquid electrolyte). Cyclic voltammetric and potentiostatic transient measurements were made at the Pt/Nafion interface. Slow sweep voltammograms yielded Tafel parameters for oxygen reduction. From the two-section Tafel plot, the calculated exchange current densities were found to be higher than those obtained at any other Pt/acid interface. From an analysis of the transients, the values of oxygen solubility and diffusion coefficient in Nafion were determined.; Electrochemical impedance spectroscopic (EIS) investigations were then used to study oxygen reduction under lower humidification conditions. EIS clearly permits the discrimination of electrode kinetics, mass transport of O{dollar}sb2{dollar} and the electrical characteristics of the membrane.; A temperature-dependence study in the range of 30{dollar}spcirc{dollar}C to 80{dollar}spcirc{dollar}C yielded the activation energy for oxygen reduction at the Pt/Nafion interface. The diffusion coefficient of oxygen in Nafion increases with temperature while its solubility decreases. The pressure-dependence of oxygen reduction kinetics shows that the reaction order of oxygen is unity. Further, it was shown that oxygen dissolution follows Henry's isotherm while the diffusion coefficient of oxygen is invariant with pressure.; The effect of temperature and pressure on oxygen reduction kinetics was also studied at porous gas-diffusion electrodes. Activation energies for oxygen reduction and reaction orders for oxygen were compared with the microelectrode investigations. There is virtually no difference between the low and high platinum surface area electrodes in terms of the mechanism of the oxygen reduction reaction. ftn{dollar}spcircler{dollar}Nafion is a registered trademark of E. I. du Pont de Nemours, Inc.