Effect of aggregate size and film quality on the electrochemical properties of non-noble metal catalysts in rotating ring disk electrode measurements

Non-noble metal catalysts (NNMCs) are promising candidates to replace the high-cost Pt-based catalysts required to catalyze the oxygen reduction reaction (ORR) in the cathode of proton exchange membrane fuel cells (PEMFCs), and their electrochemical properties (i.e., ORR-activity and H2O vs. H2O2 selectivity) are commonly studied using rotating ring disk electrode (RRDE) measurements. However, such RRDE measurements are often performed using high NNMC-loadings that can lead to underestimated H2O2-yields. These effects can be curtailed by using lower loadings entailing the preparation of thin films whose homogeneity reportedly impacts the RRDE-results observed for Pt-based catalyst, but of unknown effects in the NNMC field. To shed light on this matter, in this study we investigated the effect of film thickness and quality on the electrochemical behavior of three NNMCs featuring different aggregate sizes ranging from approximate to 5 mu m to approximate to 100 nm. The RRDE-results displayed a significant enhancement in the ORR limiting current of the NNMC with a smaller particle size that we tied to its improved mass transport properties. Moreover, improving the thin film quality led to a approximate to 2-fold enhancement of the H2O2-yield (vs. a similarly loaded, yet non-homogeneous NNMC-film) that demonstrates the importance of this parameter for the ORR-selectivity trends inferred from these RRDE measurements.