Exchange Current Density as an Effective Descriptor of Poisoning of Active Sites in Platinum Group Metal-free Electrocatalysts for Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is of primary importance for the direct and clean conversion of energy in fuel cells, necessarily requiring an electrocatalyst to be exploited. At the state of the art, platinum group metal-free (PGM-free) electro-catalysts are the most promising alternative to carbon-supported Pt nanoparticles (Pt/C), which are more expensive and more performing but highly prone to deactivation in a contaminated working environment. The comparison of the two materials is at the level of fine-tuning, requiring specific activity descriptors, namely, turnover frequency (TOF) and site density (SD), to understand how to compare the performance of PGM-free electrocatalysts with Pt/C electrocatalysts. Specific probing molecules that bind with the active sites are required to evaluate the SD of PGM-free electrocatalysts. However, PGM-free electrocatalysts possess not a single active site like Pt/C, but a multitude of primary (metal-containing) and secondary (metal-free) sites arising from the pyrolysis synthesis process, eventually complicating SD evaluation. In this work, we propose a method for evaluating the direct interaction through the chemisorption of probing molecules over the PGM-free primary and secondary sites, the discrimination of which is of paramount importance in an effective SD evaluation. Based on the rotating disk electrode technique, the study investigates the electrochemistry of Fe-based PGM-free electrocatalysts poisoned with hydrogen sulfide at pH 1 in comparison with a Pt/C sample. In addition, X-ray photoelectron spectroscopy (XPS) is used to establish a relationship between the electrochemistry and surface chemistry of the poisoned material. The results identify the exchange current density as a meaningful tool that allows the discrimination of poisoning of specific active sites (metal-containing or metal-free). In addition, the understanding of the interaction phenomenon occurring between sites and probing molecules will be paramount for the selection of those contaminants capable of selectively interacting with the active sites of interest, paving the way to a more accurate SD evaluation.