CINaM - Centre Interdisciplinaire de Nanoscience de Marseille

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Accueil du site > Séminaires > A venir ...

A venir ...

 
Jeudi 22 Novembre 2018
Raphael Chattot
Université Grenoble Alpes, LEPMI, Grenoble, France
Promoting Surfaces Distortion in Nanomaterials for Fuel Cell Electrocatalysis
The electrochemical activation of oxygen is the cornerstone of electrochemical conversion and storage devices, such as fuel cells, metal-air batteries, and electrolysers. It is well established that Pt is the only metal that can catalyse efficiently the oxygen reduction reaction (ORR) in acidic electrolyte, the reaction limiting the performance of low temperature proton-exchange membrane fuel cells (PEMFCs). However, due to the high cost and scarcity of Pt, research efforts recently focused on enhancing simultaneously its intrinsic activity (specific activity i.e. the current produced per cm2 of Pt) and its mass activity (the current produced per gram of Pt). Studies on Pt or PtNi single crystals have established that the ORR is a structure sensitive reaction, which is best electrocatalyzed on (111) facets in acidic electrolyte. Combining alloying and ensemble effects recently led to 20-30-fold enhancement of the specific activity for the ORR on PtNi/C nanooctahedra relative to Pt/C nanoparticles. However, due to the highly oxidizing conditions of the PEMFC cathode (high electrochemical potential, presence of oxygen, acidic pH), the stability of these “dream” catalysts was found poor in PEMFC cathode operating conditions, thus compromising their utilization in real devices [1]. Strikingly, it also turned out recently that alloyed but structurally-disordered nanocatalysts, such as hollow PtNi/C nanoparticles, porous PtNi/C nanoparticles, PtNi aerogels or PtNi nanosponges also feature highly desirable and sustainable ORR activity (x 10-12 in specific activity relative to pure Pt/C). Even more striking, the ORR kinetics depends on the concentration of structural defects: the higher the structural disorder in a given nanocatalyst, the best is its intrinsic activity for the ORR but also other oxidation reactions [2]. This talk will address our recent insights about the quantification and the role played by structural defects in heterogeneous electrocatalysis from the beaker cell to the fuel cell device. Our proposal is based on Rietveld measurements of wide angle high energy X-rays scattering measurements and high resolution electron microscopy for a broad range of nanocatalysts combined with density functional theory calculations [3]. REFERENCES 1. C. Cui, L. Gan, M. Heggen, S. Rudi, and P. Strasser, “Compositional segregation in shaped Pt alloy nanoparticles and their structural behaviour during electrocatalysis,” Nat. Mater., vol. 12, no. 8, pp. 765–771, 2013. 2. R. Chattot, T. Asset, P. Bordet, J. Drnec, L. Dubau, and F. Maillard, “Beyond strain and ligand effects: Microstrain-induced enhancement of the oxygen reduction reaction kinetics on various PtNi/C nanostructures,” ACS Catal., vol. 7, pp. 398–408, 2017. 3. R. Chattot et al., “Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis,” Nat. Mater., vol. 17, no. September, pp. 827–833, 2018.

Jeudi 10 Janvier 2019
Prof. Wouter Maes
UHasselt, Institute for Materials Research (IMO-IMOMEC), Design & Synthesis of Organic Semiconductors (DSOS), Agoralaan 1, 3590 Diepenbeek, Belgium
On the ‘True’ Structure of Push-Pull Type Low Bandgap Polymers for Organic Electronics
Donor-acceptor or push-pull type conjugated polymers have become a dominating class of active materials in the field of organic electronics. Their adjustable light-harvesting, charge transfer and charge transport characteristics have been beneficially applied in organic photovoltaics, photodetectors and thin-film transistors. The conventional synthetic approach toward these push-pull polymers is based on Suzuki or (mostly) Stille cross-coupling of complementary functionalized heterocyclic precursors. In the ideal world, this should give rise to a perfect alternation of the employed building blocks throughout the polymer backbone and this alternation of electron rich (donor/push) and electron deficient (acceptor/pull) moieties leads to a substantial decrease of the bandgap. In recent years, however, it has become increasingly clear that the ‘real’ structure of the resulting alternating copolymers is often quite different from the projected one [1]. Structural imperfections can for instance result from homocoupling of two identical building blocks. Furthermore, the end groups of these donor-acceptor copolymers are often also not those expected or targeted. In this contribution, recent results from our group will be presented, providing insights on the impact of homocoupling ‘defects’ on the device characteristics of organic solar cells [2-4]. Additionally, different types of end groups were identified via MALDI-TOF mass spectrometry. References [1] Pirotte G, Verstappen P, Vanderzande D, Maes W, Advanced Electronic Materials, 1700481 (2018); [2] Vangerven T, Verstappen P, Drijkoningen J, Dierckx W, Himmelberger S, Salleo A, Vanderzande D, Maes W, Manca JV, Chemistry of Materials, 27, 3726 (2015); [3] Vangerven T, Verstappen P, Patil N, D’Haen J, Cardinaletti I, Benduhn J, Van den Brande N, Defour M, Lemaur V, Beljonne D, Lazzaroni R, Champagne B, Vandewal K, Andreasen JW, Adriaensens P, Breiby DW, Van Mele B, Vanderzande D, Maes W, Manca J, Chemistry of Materials, 28, 9088 (2016); [4] Pirotte G, Kesters J, Cardeynaels T, Verstappen P, D’Haen J, Lutsen L, Champagne B, Vanderzande D, Maes W, Macromolecular Rapid Communications, 39, 1800086 (2018).

Jeudi 17 Janvier 2019
Henrik Grönbeck
Department of Physics and Competence Centre for Catalysis, Chalmers University of Technology, Sweden
Operando Computational Catalysis
A key focus in heterogeneous catalysis is to understand the dominant reaction paths and isolate the character of the active site. This is challenging because of the dynamic character of the catalyst, which may undergo structural and phase changes as a response to the reaction conditions. This stresses the importance to perform physical and chemical characterization of catalysts during operando conditions. This applies also to computational work, aiming at establishing links between elementary steps and catalyst activity. In this presentation, I will discuss our recent efforts to understand CO and methane oxidation over palladium and platinum using first principles calculations exemplifying different aspects of kinetic simulations based on theoretical data [1-4]. Special attention will be given the attempt to perform explicit simulations of reaction kinetics over metal nanoparticles. These simulations reveal that kinetic couplings between different sites on the particles largely determine the overall catalytic activity. Our results show that it is rather the site-assembly than a special site that determines the activity. [1] M. Jørgensen, H. Grönbeck, ACS Catalysis 6, 6730 (2016). [2] M. Van den Bossche and H. Grönbeck, J. Am. Chem. Soc. 137, 12035 (2015). [3] M. Jørgensen, H. Grönbeck, ACS Catalysis 7, 5054 (2017). [4] M. Jørgensen, H. Grönbeck, Angew. Chem. Int Ed. 57, 5086 (2018).