RRDE experiments on noble-metal and noble-metal-free catalysts: Impact of loading on the activity and selectivity of oxygen reduction reaction in alkaline solution

Oxygen reduction reaction (ORR) is at the core of various applications, such as fuel cells, metal-air batteries and H2O2 electro-generation. Depending on the targeted application, catalysts that follow a direct four-electron (4e(-)) pathway or two-electron (2e(-)) pathway are envisaged. We have systematically investigated the impact of the electrocatalyst loading on the rotating ring-disk electrode (RRDE), on their activity and selectivity towards ORR in alkaline medium (0.1 M and 1M KOH). Four representative catalysts, including a noble metal catalyst (commercial 20 wt% Pt/C, ETEK), metal-free catalysts (Printex carbon black and graphene), and non-noble metal catalyst (Fe/N/C), were selected because they exhibit different behaviours for ORR following a direct four-electron (4e-) pathway, two-electron (2e-) pathway, or a series 2e(-) + 2e(-) pathway. The results confirmed that the catalyst loading influences the activity and the selectivity of the catalysts, with lower loadings favoring the H2O2 electrogeneration, and researchers should pay attention to it. Moreover, in order to make possible comparison between results coming from different research groups, we recommend reasonable loading ranges for each type of catalyst. (C) 2017 Elsevier B.V. All rights reserved.