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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.

The first generation of proton exchange membrane fuel cells uses costly and unsafe perfluorinated sulfonic acid polymers (PFSAs) as membranes and as ionomers impregnating electrodes to achieve the three-phase boundaries. PFSAs imply paramount issues for large-scale manufacture, use, commercialization, and recycling. Alternative nonfluorinated polymers should allow obtaining not only membranes but also adequate ionomer suspensions in convenient solvents for preparing efficient catalytic layers, which has not yet been achieved. Here, we propose a universal solution consisting of the transposition of the three-phase boundary at the molecular level by grafting directly at the surface of carbon-supported Pt nanoparticles a nonfluorinated proton-conducting polymer combining the catalytic activity of the former and the transport properties of the latter. The length of the polystyrenesulfonate polymer chain (as a model polymer) and the number of polymer feet per platinum nanoparticles have been optimized in order...

A copper-mediated fluorofunctionalization of alkenes and alkynes has been developed. This method provides ready access to trisubstituted difluoromethylated olefins (from dihydropyrans, glycal derivatives, or terminal alkynes) or to tetrasubstituted olefins (from disubstituted alkynes). The products were obtained in good to high yields with acceptable E/Z selectivities. Finally, a first direct route to difluoromethylated alkynes is reported, albeit with low yields.

Improving design and/or reducing noble metal content in electrocatalysts for fuel cell electrodes while maintaining and/or increasing proton exchange membrane fuel cell (PEMFC) performance in terms of durability and power density are crucial challenges for PEMFC mass market applications. The formation of platinum alloys with transition metals represents a promising way for improving the activity of Pt-based catalysts [1-3]. Pt based alloys have indeed often demonstrated higher electrocatalytic activity towards ORR than pure platinum. On the other hand, the structure and morphology of the catalyst [4] also play very important roles in the electrocatalytic efficiency for ORR. In this context, the present study aims at systematically comparing the catalytic activity and the selectivity towards the ORR of binary and ternary catalysts based on Pt, and Au in order to determine the effects of alloyed metals, in terms of kinetics and selectivity. For this purpose, monometallic Pt/C, binary PtxM1-x/C and ternary PtxAuyMz/C (with M a transition metal) nanocatalysts supported on carbon have been first synthesized by a wet chemical method allowing the synthesis of the whole range of alloys and comprehensively characterized. The morphologies, compositions and structures of the particles were characterized by physical methods (thermogravimetric analysis, transmission electron microscopy, X-ray diffraction, atomic absorption, X-ray photoelectron spectroscopy). Electrochemical active surface areas and surface compositions were estimated by cyclic voltammetry. Electrocatalytic activity and selectivity of catalysts were studied by rotating disc and rotating ring disc electrodes. Acknowledgement: The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement #325327 (SMARTCat project) and from the ADEME (French Environment and Energy Management Agency) for its financial support through the EXALAME project. [1] M.T. Paffet, J.G. Beery, S. Gottesfeld, J. Electrochem. Soc. 135 (1988) 1431–1436. [2] C. Wang, M. Chi, D. Li, D. van der Vliet, G. Wang, Q. Lin, J. F. Mitchell, K. L. More, N. M. Markovic, V. R. Stamenkovic, ACS Catal. 1 (2011) 1355–1359. [3] T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146 (1999) 3750–3756. [4] D. Alloyeau, C. Mottet, C. Ricolleau, Nanoalloys, London: Springer-Verlag; 2012.

A copper-mediated fluorofunctionalization of alkenes and alkynes has been developed. This method provides ready access to trisubstituted difluoromethylated olefins (from dihydropyrans, glycal derivatives, or terminal alkynes) or to tetrasubstituted olefins (from disubstituted alkynes). The products were obtained in good to high yields with acceptable E/Z selectivities. Finally, a first direct route to difluoromethylated alkynes is reported, albeit with low yields.

The Proton-exchange membrane fuel cell (PEMFC) will become a mature technology, and several car manufacturers in Asia, United State and Europe are communicating on the commercialization of Fuel Cell cars in the near future. The materials which will be used for the first generation of this technology of energy conversion are currently known: the solid electrolyte will consist in a perfluorinated membrane with attached pendant sidechains and sulfonic acid groups (–SO3H) at the sidechain terminals, which Nafion® is the mostly recognized representative and the electrodes will be composed of platinum-based nanoparticles (NPs) in the diameter range 1 - 10 nm dispersed on a high surface area electron conductive carbon black powder impregnated with proton conducting ionomer. Although such electrode formulation led to a significant increase of the catalyst efficiency, and therefore to a drastic decrease of Pt loading in electrodes, more efforts have still to be done to bring the state-of-the-art specific Pt power value (ca. 0.5 gPt kW−1) down to 0.2 gPt kW−1 for a large scale implementation of the PEMFCs for vehicle propulsion. In order to achieve this breakthrough, it is necessary to improve the platinum utilization efficiency. It was indeed estimated by Eikerling et al. that Pt activity in conventional catalytic layers reached at most 10 - 20 % of its full potential [[i]]. The limitation in the effectiveness factor of Pt utilization, determined as the ratio of the current delivered and the maximum theoretical current expected, is due to several reasons, particularly to mass transport and ohmic losses [[ii]]. So that, the performance improvement of the catalytic layers requires optimizing factors allowing maximizing the fraction of Pt surface atoms, and those allowing improving mass and charge transport properties of the layers. The optimizations of the catalytic support and of the catalytic layer architecture could greatly contribute to the enhancement of the effectiveness of the PEMFC catalytic layers, and thus enable further reduction of the Pt content, but classical architectures involve their impregnation by an ionomer in order to achieve the three-phase boundaries. A three-phase boundary corresponds to a confined spatial site where the reactant, the ionic conducting polymer and the electronic conductive substrate are present on the same platinum particle [[iii]], allowing the electrochemical reaction to occur with higher efficiency. However, Eikerling et al. also estimated that ca. 60% of unutilized Pt was that not located at the active three-phase boundary [1]. The grafting of ionic conducting polymer to the surface of platinum nanoparticles allows the creation of new catalyst layer architectures (Figure 1 and Figure 2) that promote both the ionic conduction to the solid electrolyte. The synthesis of polyfunctional catalyst, whose properties can be adjusted depending on synthesis parameters, allows optimize the electrical, chemical and mass transfer to electrodes and also reduce the overhead platinum. In this communication, we will report the synthesis of complex architecture of platinum supported on carbon, their physico-chemical characteristic, electrocatalytic properties (Figure 3) and fuel cell tests (Figure 4). [[i]] M. Eikerling, A. Korhyshev, A. Kulikovsky, The Fuel Cell Review 15 (20004/2005) FCR.IOP.org. [[ii]] H. A. Gasteiger, W. Gu, R. Makharia, M. F. Mathias, B. Sompalli, in: W. Vielstich, A. Lamm, H. A. Gasteiger (Eds.), Handbook of Fuel Cells – Fundamentals, Technology and Applications, Wiley, Chichester, Vol. 3, 2003, p. 593-610. [[iii]] R. O’Hayre, D. M. Barnett, F. B. Prinz, J. Electrochem. Soc. 152 (2005) A439-A444. Figure 1