AN ANALYSIS OF THE PH-DEPENDENCE OF ENTHALPIES AND GIBBS ENERGIES OF ACTIVATION FOR O2 REDUCTION AT PT ELECTRODES IN ACID-SOLUTIONS

The symmetry factor in oxygen reduction at Pt electrodes in acid solutions in the high current density region is close to 1 2(=0.45 + 2.3 × 10-4T), and Tafel lines obtained at different temperatures intersect in extrapolation at the critical potential Ecr = 0.23 V vs she. Ecr is the same in solutions of different pH in the acid region. This is the potential at which Gibbs energy of activation is zero. At any constant potential, eg 0.8 V vs rhe, the activation energy decreases only very slowly with pH, ie by 2.8 kJ mol-1 pH. The activation energies at 0.8 V vs rhe obtained from Ecr agree with the energies previously reported from Arrhenius plots at the same potential. The energy of the unoccupied electron level in the ground state of the reacting complex in the rate-determining step is -4.72 eV and is pH independent in the acid region. © 1990.