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

Abstract

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) electrocatalysts 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.

Converting carbon black into an efficient and multi-site ORR electrocatalyst: the importance of bottom-up construction parameters

“…While the former provides mechanism insights, by revealing the rate-determining step of the ORR, 22 the latter can be associated with the rate constant of the ORR, thus providing information on the electrocatalytic activity. 23 The Tafel plot slope for Pt/C is 77 mV dec −1 (Fig. 2c), which is consistent with previous reports.…”

Porous carbonaceous materials simultaneously dispersing N, Fe and Co as bifunctional catalysts for the ORR and OER: electrochemical performance in a prototype of a Zn–air battery

“…27 When dispersing transition metals in N-doped carbonaceous materials, N mediates the interactions between the metal species and the carbon matrix, thus allowing the formation of a highly functionalized surface and creating metalnitrogen-carbon matrix active sites. [28][29][30] Nitrogen can exist in…”

“…When dispersing transition metals in N-doped carbonaceous materials, N mediates the interactions between the metal species and the carbon matrix, thus allowing the formation of a highly functionalized surface and creating metal–nitrogen–carbon matrix active sites. 28–30 Nitrogen can exist in different forms: pyridinic-N, graphitic-N, pyrrolic-N, and oxidized-N or pyridinic N-oxide. 31 Each of these species exhibits different chemical and electronic properties, thus acting as different active sites for electrochemistry.…”

Molecular Design of Porous Organic Polymer-Derived Carbonaceous Electrocatalysts for Pinpointing Active Sites in Oxygen Reduction Reaction

“…Therefore, it is imperative to explore high-efficiency ORR catalysts to promote the development of these clean energy conversion systems. Up to now, the platinum group metal (PGM) materials, including platinum and platinum-based catalysts, have long been regarded as the most efficient ORR electrode materials [ 3 ]. However, the high cost and scarcity of platinum, together with several other drawbacks, including the time-dependent drift, methanol crossover, and CO deactivation, have impeded their commercial application for PEMFCs.…”