Platinum-group-metal-free nanoelectrocatalysts for energy conversion and storage: effects and application of active sites poisoning

After a first introduction to the world of platinum-group-metal-free (PGM-free) electrocatalysts, concerning the state of the art at the end of 2021 on the nature of active sites, the ORR electrocatalysis, and the poisoning of both Pt-containing and Pt-free electrocatalysts, three different chapters are presented. The Ariadne's thread of the studies followed during the last three years is the contamination of PGM-free, not only of interest as one of the main possibility to probe the active sites responsible for the oxygen reduction reaction (ORR), but also as a useful methodology to understand the limits towards to push to the material. In addition, looking to the half-filled part of the glass, what is considered detrimental, if finetuned, can be also useful for practical applications, leading to new possibilities. As a material scientist, the attention was focused on the electrocatalytic material, synthesized by the Atanassov group following their methodology of the sacrificial support method (SSM). In SSM, an organic precursor and a metal-containing compound were ball-milled with monodisperse silica nanoparticles as sacrificial template agent. The resulting powder mix was then pyrolyzed at high temperature in inert atmosphere and then acid-washed to remove all the inorganic species. The resulting material, based on benzimidazole and iron nitrate, was named FeBZIM, and contained both the Fe-Nx coordination at the basis of PGM-free and metal-free N functionalities. Both are active towards ORR, with Fe-Nx considered the site for the most efficient oxygen conversion. The interest of my research was thus focused on the behavior of that specific class of PGM-free (SSM) on ORR, since they are the first to be ever commercialized (Pajarito Powder, LLC).Overall, in the following will be presented three chapters concerning the work done during this Ph.D. course based on the material science and electrochemistry for energy. In Chapter 2, the study of FeBZIM poisoning at pH 7 with the focus to microbial fuelcell (MFC) application, involved contaminants typical of wastewater at the mM range (chloride, perchlorate, nitrite, and nitrate). The identification of descriptors for poisoning based on nitrite, a known Fe-Nx contaminant from the literature, are first identified. Furthermore, the boundary conditions for an appropriate contamination are also depicted in terms of catalyst loading and Nafion content. In Chapter 3, the poisoning of the same PGM-free with sulfur-containing compounds (sulfide and sulfate) is tested at three different pH 1, 7, and 13, in parallel with a Pt-containing material. Differences and similarities of the two electrocatalysts are identified and, in addition, a spectroscopy analysis is implemented, to understand the effects of the contamination on the surface chemistry of FeBZIM. Comparisons with the nitrite case are made, testing the descriptors identified with the former contaminant. In this case, the possibility of a double effect on active sites and C matrix of FeBZIM is identified, underlying the importance in considering PGM-free as materials in a whole and not as Fe-Nx coordination only. Using the obtained results, an hypothesis on the nature of the overpotential of ORR in acidic pH is formulated. Finally, in Chapter 4 the application of the study of the boundary conditions for an appropriate contamination of nitrite is exploited to develop a sensor based on the inhibition of the ORR with PGM-free and standard laboratory equipment. The identification of the optimum parameters for the catalytic layer allowed to apply the methodology to the µM range of concentrations, obtaining results comparable to the state of the art based on nitrite oxidation. I hope you’ll enjoy the reading as much as I enjoyed the findings throughout the journey towards the improvement of my knowledge.