Résumé
Proton exchange membrane fuel cells (PEMFC), are energy converters being developed for transport as well as for stationary and portable applications. Their distinguishing features include lower temperature/pressure ranges without emission of pollutants (gas or particles). They are a leading candidate to replace the aging alkaline fuel cell technology, which was used in the space shuttle [1]. The PEMFC technology is now mature and starts to be deployed on the field, but some drawbacks must be overcome ; in particular, the durability must be increased. 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. This study evaluates the effect of fluorination on textures and structures of different carbons and gives first insights into their durability when used as platinum electrocatalyst substrate for proton exchange membrane fuel cell (PEMFC) cathodes. Different types of carbonaceous materials such as carbon nanotubes (graphitized but with a low specific surface area), carbon blacks (currently used in the best commercial electrocatalysts, with either a large specific surface area and medium degree of organization for the one and a large degree of graphitization but a low specific surface area for the second)) and carbon aerogels (controlled texture but low degree of organization) are chosen. These textures induce a trade-off in terms of properties: 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. These model materials were modified by surface treatment in order to increase their durability by increasing their hydrophobicity through controlled fluorination [2]. The objective is to limit the corrosion induced by the surface oxygen content and the electrolyte, by saturating dangling bonds with fluorine atoms. Fluorination is carried out using dynamic process (under a flow of pure molecular fluorine F2(g) [3]. Fluorination conditions were drastically controlled in order to obtain a composition i.e F/C molar ratio not greater than 0.2, to avoid decomposition of carbon into gaseous species such as CF4 and C2F6(g) [4]. All fluorinated samples were texturally, morphologically and chemically characterized by XRD, TEM, nitrogen sorption, FTIR, TGA and solid state NMR (13C and 19F nuclei). The catalytic activity of these electrocatalysts towards the oxygen reduction reaction was determined by linear sweep voltammetry (rotating disk electrode technique). Accelerated stress tests, load cycle (0.6-1 V) and start-up/shutdown (1-1.5 V) protocols, conducted at 80°C in a four-electrode cell, were performed to investigate the robustness of the bare and fluorinated Pt electrocatalysts. The results and the impact of the fluorination are discussed and compared to those for a 40 wt% commercial state-of the-art electrocatalyst. 1.Loyselle, P.P., Kevin., Teledyne Energy Systems, Inc., Proton Exchange Member (PEM) Fuel Cell Engineering Model Powerplant. Test Report: Initial Benchmark Tests in the Original Orientation". NASA. Glenn Research Center. Retrieved 15 September 2011. 2.Berthon-Fabry, S., et al., First Insight into Fluorinated Pt/Carbon Aerogels as More Corrosion-Resistant Electrocatalysts for Proton Exchange Membrane Fuel Cell Cathodes. ELECTROCATALYSIS, 2015. 6(6): p. 521-533. 3.Ahmad, Y., et al., Structure control at the nanoscale in fluorinated graphitized carbon blacks through the fluorination route. Journal of Fluorine Chemistry, 2014. 168: p. 163-172. 4.Ahmad, Y., et al., The synthesis of multilayer graphene materials by the fluorination of carbon nanodiscs/nanocones. Carbon, 2012. 50(10): p. 3897-3908.