Comparative Analysis of Rotating Electrode and Gas Diffusion Electrode Methods for Assessing Activity and Stability of Fe-N-C Based Catalysts in ORR

Rotating (ring) disk electrode (R(R)DE) voltammetry is considered a simple means of benchmarking the oxygen reduction reaction (ORR) activity of platinum-free electrocatalysts for proton exchange membrane fuel cells or hydrogen peroxide electrocatalysts. However, the R(R)DE methodology has shown high variability across laboratories and reproducible ORR activities can be obtained when a strict experimental protocol is followed. Despite this, objections in the literature have been raised regarding the usefulness of screening measurements on RRDEs in identifying a good catalyst that maintains the same performance when switching from the RRDE to a gas diffusion electrode (GDE). As a result, new experimental approaches have been proposed in the literature to better evaluate a catalyst under conditions similar to those of a fuel cell or an electrolyzer. Our study, along with others, points out that even with a new electrochemical set-up, the dominant factors in the screening experimental protocol include the ink formulation, electrocatalyst film quality, and electrochemical procedures. In this study, a platinum-free Fe-N-C type catalyst (Fe2XC72) is considered a benchmark electrocatalyst for ORR. The activity and selectivity performances of the catalyst are evaluated and compared on an RRDE, a half-cell with a GDE electrode, and an H-cell with a GDE electrode but with a larger surface area. The impact of various experimental parameters, including catalyst loading and pH, on the electrocatalytic activity and selectivity, are evaluated and the different techniques, although not completely comparable, manage on individual aspects to produce similar if not overlapping results. Furthermore, explicit experimental procedures and measurement protocols are reviewed and revised.