M-N-C electrocatalysts supported on engineered carbon materials for the reduction of Oxygen

Fuel cells represent a valid alternative to traditional energy sources that contain the use of fossil fuels, since H2 and O2 are respectively used as fuel and oxidant in a reaction that mainly produce H2O. The reaction of oxygen reduction at the cathode, due to the tetra-electron mechanism, has a slow and sluggish kinetics and therefore requires large quantities of catalyst. For a commercialization of these devices the traditional catalysts based on platinum supported on mesoporous carbon must be replaced with cheaper materials. Carbonaceous supports with a discrete porous structure, doped with nitrogen and with transition metals such as Fe, have proven to be able to catalyse the reduction of O2 with performances that can approach Pt-based materials. Although numerous studies have been conducted on the subject, there is still a strong debate on nature of active sites in these systems and, in particular, on which the experimental conditions of synthesis that allow to control their formation. One of the fixed points of these studies, which normally still combine spectroscopic techniques with electrochemical techniques, is that the primary centres of oxygen reduction to water are the FeN4 sites, however the conditions that lead to the formation of these sites and the quantification require one in-depth study. In this PhD project different M-N-C based materials have been studied to evaluate the influence of the porosity of the carbonaceous support on the formation of the Fe-N and C-N sites and consequently on the electrocatalytic activity. Also the effect of the precursors of Fe and N was studied showing that Fe(phen)3Cl2 is a good choice. Later Bimetallic systems were studied, by adding Sn in the mixture and finally the study of C-N group was achieved by grafting of organic molecule on the surface. One big part of the project was to show that gas diffusion electrode setup are perfectly suitable for these materials and allow to complete the information obtained with typical rotating electrode analysis. All materials were studied by deeply combining physico-chemical and electrochemical characterization techniques.