The Effect of the Voltage Scan Rate on the Determination of the Oxygen Reduction Activity of Pt/C Fuel Cell Catalyst

Abstract

The oxygen reduction reaction (ORR) is one of the most important chemical reactions. Besides others, it plays a crucial role for the development of a sustainable energy scenario, as it is the decisive reaction in proton exchangemembrane fuel cell (PEMFC) [1, 2]. Improving the catalysis of the ORR can lead to a breakthrough in electrochemical energy conversion; therefore, the fundamental understanding of the reaction processes as well as the rapid assessment of the kinetic activity of different catalyst materials is of utmost important [3]. So far, it is well established that the surface coverage of electrosorbed oxygenated species like H2O, OH, and O determines platinum ORR activity, although the true nature of the potential dependent oxygenated species has yet to be resolved [4, 5]. Under the assumption that the Pt surface is almost fully covered at low overpotentials also referred as kinetic region, where Θad is the surface coverage of any adsorbate species, the ORR kinetic current becomes directly proportional to the pre-exponential factor (1-Θad) of the rate expression [6]. This know-how has helped to interpret effects introduced for instance by the particle size of Pt catalysts [7–9] or the development of catalysts with enhanced activities like Ptalloys with lower Θad compared to pure Pt [5, 6, 10–13]. The advancement in ORR fundamental understanding and performance evaluation has benefited to a great extent from informative half-cell kinetic survey studies of applied catalysts, as for instance performed with the thin-film rotating disk electrode technique (TF-RDE) [2, 14–16]. Such electrochemical model studies enable the determination of true kinetic current densities even of porous materials without complex interference with mass-transport or catalyst utilization effects, which are additional important factors for the final performance in electrochemical reactors [17]. In order to perform these ORR half-cell measurements accurately, certain practical guidelines have been shown to be essential. These include especially the cleanliness of the experimental setup [18], catalyst film preparation on the electrode [14], appropriate potentiostat sampling mode, evaluation of kinetic data [15], IR compensation [19], correction for true reversible hydrogen electrode (RHE) potential, and background currents, later especially at high scan rates [15]. Interestingly, although already reported in literature for polycrystalline and high surface area Pt catalysts [2, 16, 20], the effect of the voltage scan rate on the activity determination is often underestimated or not clearly separated from the effect of impurities. In the present work, the impact of the voltage scan rate on the activity determination of a high surface area Pt catalyst is systematically studied. We take special care in order to work extra clean and avoid the effect of impurities. We confirm that the relatively slow platinum surface oxidation process significantly influences the kinetic evaluation [20, 21], which is one reason behind varying ORR-specific activities in literature. Moreover, we describe the scan rate-dependent influence of chloride ions, as well as discuss the relationship to surface coverage and the implications of the results for practical applications. * Nejc Hodnik n.hodnik@mpie.de