K-L plots from Fig. 3 are mostly linear, indicating the first-order kinetics with respect to molecular oxygen. K-L plots obtained from polarization curves for ORR on Pd(poly) in 0.1 M HClO4 solution recorded in anodic direction, Fig. 3a, are not parallel, while the ones obtained for cathodic direction, Fig. 3b are parallel. Non parallelism of K-L plots for the anodic direction means that LC Biotin the number of electrons exchanged changes with the potential. Indeed, n decreases from 4 electrons at lower potentials to 3.2 and 2.6 electrons with the increase of the potential to 0.575 V and to 0.6 V, respectively, pointing out to the change in reaction kinetics from 4e-reduction to 2e-reduction. The decrease of the number of electrons exchanged with the increase of the potential assumes zone of differentiation the reaction pathway most likely involves the production of H2O2 to some extent meaning that the ORR occurs partly through 2e-reduction. This might be due to the transition of PdOH to PdO, which takes place at these potentials according to our assumptions based on CV from Fig. 1a. On the other hand, the parallelism of K-L plots for cathodic direction means that the number of electrons exchanged and the reaction pathway do not change with the potential. Since the overall number of electrons is 4, ORR occurs through a 4e- reaction pathway in the whole potential region. 4e-reduction was also found in earlier studies of ORR on various Pd single crystals .