Electrochemical analysis of the kinetics and mechanism of the oxygen reduction reaction on Au nanoparticles

Au nanoparticles have been chemically synthesized and supported in carbon Vulcan XC-72. Using the Au nanoparticles as catalyst, the kinetics and mechanism of the oxygen reduction reaction (ORR), in 0.5 M H2SO4, were analyzed. From rotating disk electrode (RDE) studies, it was found that the number of electrons (n) involved in the ORR depends on the applied potential (E). For E ∼0.38 V, n was ∼2, at more negative potentials the value of n increases almost to 4. The kinetic parameters obtained from RDE studies were: −b = 0.110 V dec−1 and α = 0.54. The impedance spectra of Au nanoparticles show one time constant for E > 0.38 V vs. NHE, the time constant was associated to the reduction process from O2 to H2O2 as the main intermediate. For E < 0.38 V vs. NHE a second time constant appears, and it was associated to the reduction of H2O2 to H2O. The values of charge transfer resistance (Rct1) obtained from the impedance spectra allows determining the kinetic parameters of the ORR on Au, the Tafel slope (−b = 0.111 V dec−1) and charge transfer coefficient (α = 0.53) were calculated. The kinetic values b and α obtained from RDE and EIS measurements are in good agreement with each other.