Durability of carbon based electrocatalysts for PEMFC application. Influence of the graphitization level and fluorination treatment
Résumé
Proton exchange membrane fuel cells (PEMFC) are energy converters that can be used in nomad, automotive or stationary applications without emission of pollutants. The insufficient durability is strongly linked to the corrosion of the carbon support in the cathode electrocatalyst. It is particularly observed under high potential and especially during start/stop phases of the cell. It leads to the detachment and agglomeration of the catalyst nanoparticles, the decrease of the carbon hydrophobicity that adversely affects the water management and the collapse of the carbon structure, phenomena that increase mass-transport losses. In this study, firstly, we related the impact of the textures and the structure of different carbons on the durability of the resultant electrocatalysts. Graphitic carbons are more resistant to oxidation, whereas greater specific surface areas are more favorable to the dispersion of a large amount of catalyst nanoparticles per unit volume. Secondly, these model materials were modified by surface treatment in order to increase their durability by increasing their hydrophobicity through controlled fluorination. The objective was to limit the corrosion induced by the surface oxygen content and the electrolyte, by saturating dangling bonds with fluorine.The samples were texturally, morphologically and chemically characterized by XRD, TEM, nitrogen sorption, FTIR and TGA. The catalytic activity of these electrocatalysts towards the oxygen reduction reaction was determined by linear sweep voltammetry and start-up/shutdown protocols. The results and the impact of the fluorination are discussed and compared to those for a 40 wt% commercial state-of the-art electrocatalyst.