Conductive Polymer Grafting Platinum Nanoparticles as Efficient Catalysts for the Oxygen Reduction Reaction: Influence of the Polymer Structure

International audience; Platinum nanoparticles supported on a carbon support (Pt-NPs/C) were synthesized by a polyol method and modified by grafting of different non-fluorinated and fluorinated proton conducting polymers. In the case of fluorinated polymers, the sulfonyl functions were attached either directly or through spacers (-O-PSA and -S-PSA) to the tetrafuorovynilic groups. Results in three-electrode electrochemical cell showed that the nature and structure of the grafted proton conducting polymer influenced mass transport in the catalytic film towards the oxygen reduction active sites, the limiting current density in the catalytic film decreasing from ca. 97 mA cm-2 for Pt-NPs/C to ca. 80 mA cm-2 for Pt-NPs-(PSSA)/C and less than 60 mA cm-2 for Pt-NPs-(PTFV-O-PSA)/C and Pt-NPs-(PTFV-S-PSA)/C. This influence was directly linked to the hydrophobic character of the polymers. The importance of the spacer on the electrochemically active surface area (ECSA), kinetic current density (jk) and mass activity (MA) at 0.9 V was pointed out. The jk at 0.9 V vs. RHE increased from 2.8 mA cm-2 to 3.6 mA cm-2 for the nanocomposite catalysts without spacer and with a -O-PSA spacer, respectively. However, the best performance was obtained with Pt-NPs-(PSSA)/C with jk = 8.6 mA cm-2 (Pt-NPs/C leading to 4.6 mA cm-2). Fuel cell tests also showed the influence of the grafted polymer on the water management in cathodes. Maximum power density of ca. 1 W cm-2 at ca.2.1 A cm-2 was obtained with a Pt-NPS-(Nafion)/C cathode and a Pt-NPs-(PSSA)/C cathode without Nafion, and ca. 0.85 W cm-2 with a Pt-NPs-(PTFV-O-PSA)/C cathode. Durability under fuel cell working conditions revealed that the presence of the grafted conducting polymers in the cathode catalytic layer led to comparable electrical performances, but to better stabilities of the fuel cell performances than in the case of a classical Pt-NPs-(25 wt%Nafion)/C cathode: the potential losses at 38 °C were two and four times lower with a Pt-NPs-(PTFV-O-PSA)/C (16 µV h-1) cathode than with a Pt-NPs-(PSSA)/C (40 µV h-1) and a Pt-NPs-(Nafion)/C (80 µV h-1) cathodes, respectively. At 60 °C, the potential loss with a Pt-NPs-(PTFV-O-PSA)/C cathode remained twice lower than with a Pt-NPs-(Nafion)/C cathode.