Toward an objective performance evaluation of commercial Pt/C electrocatalysts for oxygen reduction: Effect of catalyst loading

• The morphologies of catalyst films with different catalyst loadings were studied. • A low catalyst loading induced the coffee ring effect. • The coffee ring effect resulted in an underestimation of the performance parameters. • Catalyst overloading caused a higher oxygen transfer resistance and a lower ORR performance. • An optimal catalyst loading for commercial Pt/C was recommended. During the performance evaluation of electrocatalysts for the oxygen reduction reaction (ORR), the electrocatalysts are usually loaded on a rotation ring disk electrode (RRDE) to form a catalyst film (CF). To ensure accurate results, the CF must be as thin and even as possible to minimize the oxygen transport resistance within it. However, the catalyst loading used by recent publications varied significantly, preventing a direct comparison of the catalyst ORR performance between studies. In this study, a complementary metal-oxide-semiconductor (CMOS) camera and a Dektak XT profile meter are used to investigate the effect of Pt/C loading on the surface morphology, electrode coverage and average thickness, and thus the ORR performance of the CF. The results illustrate that the CF failed to completely cover the electrode surface at low catalyst loadings, which caused an underestimation of the limiting current density ( J L ), onset potential ( E o n ) and half-wave potential ( E 1 / 2 ) of the catalyst. It is also determined that the deeper the Pt nanoparticles are located in the CF, the more likely the ORR performance of CFs is to be underestimated due to the increased oxygen transfer resistance. As a result, as the catalyst loading increased, the high viscosity of the catalyst ink led to particle agglomeration and an over-thickness at the center of the CF, which increased the inner oxygen transport resistance of the CF. This may not only slow the ORR kinetics of the catalyst, but also cause the ORR performance of the catalyst to be underestimated. Based on these findings, this study revealed the optimal catalyst loading (16.2–24.2 μg Pt cm −2 ) and a guided accurate measurement of the ORR catalyst.