Evaluation of the intrinsic kinetic activity of nanoparticle ensembles under steady-state conditions

We report theory and strategies for evaluating the intrinsic kinetic activity for oxygen reduction at Pt nanoparticle (NP) ensembles on a large glassy carbon electrode (GCE) under steady-state conditions. Pt NPs were synthesized using reverse microemulsions which facilitated the deposition of random ensembles of bare NPs with controlled NP mean size and coverage. Steady-state voltammograms (SSVs) for oxygen reduction were recorded for various NP ensembles with different NP size and coverage. The effects of NP coverage and mass-transport rate on SSV features were analyzed. For SSVs normalized with respect to their limiting current, more negative potentials are needed to reach the limiting current region and the i–E slope decreases as NP coverage decreases. For those normalized SSVs having unequal limiting currents, the kinetic rate relative to the mass-transport rate changes and plays a role in the decreasing steepness of the SSV. In contrast, normalized SSVs recorded under the same mass transport conditions and decreasing NP coverage are displaced negatively along the potential axis without a change in the i–E slope. Normalized SSVs recorded using the same mass transport conditions on electrodes with similar fractions of inactive area but different NP sizes were found to be similar. Tafel plots were constructed by processing the SSVs either directly through the use of the electroactive surface area AES or indirectly through a two-step procedure that uses the geometric surface area where an apparent potential-dependent kinetic current density jKapp(E) is first calculated. These two approaches are equivalent and the resulting kinetic current density jK(E) dependencies were shown to be equivalent. The direct method is applicable when AES can be determined whereas the indirect approach is useful when the measurement of AEAS is not possible, but information relating to the fraction of active or inactive area is available.