| Popis: |
The ORR is the key reaction in the development of catalysts for hydrogen fuel cells. In the development of new catalysts, a reliable assessment of their kinetic properties is crucial. However, the common half-cell RDE protocol delivers ORR activities that are linked to the chosen scan rate, limiting its value for fundamental and comparative studies. Moreover, surface oxygen has a significant contribution to the currently reported ORR activity values, severely limiting their meaningfulness. Here we present a deconvolution of the intrinsic ORR kinetics from the effect of surface oxygen on platinum. The deconvolution makes the obtained ORR activities for the first time indifferent to scan rate and scan direction in a wide potential range. This provides researchers with a new method to assess platinum catalysts, as it allows to separate the intrinsic ORR activity from the inevitable formation of surface oxygen species during fuel cell operation. We show that the surface oxygen affects the ORR dominantly via an electronic effect, substantiate a Tafel slope of around 120 mV/decade over the entire experimentally accessible potential range, calculate an ORR exchange current density of 13±4 µA/cm2, and an ORR activity of 7 mA/cm2 at 900 mV. Eventually, we broaden the scope of this analysis to effects of surface reconstruction, alloying, and supported Pt nanoparticles. We demonstrate that the common half-cell RDE protocol to analyze the ORR is insufficient in determining the intrinsic ORR parameters and provide insights into the discrepancy between the ORR activity in half cells and applied fuel cells. Using DFT calculations, we find that the binding energies of the ORR intermediates are weakened by the presence of highly coordinated oxygen atoms in the platinum surface. We show that the OER follows the same kinetics as the ORR leading to a single empirical description of both reactions like the HER/HOR kinetics. |
