Spacers to improve performance and porosity of graphene based polymer electrolyte fuel cells

Graphene has been suggested as a potential support material to replace commercial carbon black due to its carbon corrosion resistance. However, graphene-based electrodes typically perform poorly in MEA testing due to restacking of the graphitic sheets. In this study we investigate the introduction of carbon black and their effects on the porosity and current density of graphene-based supports

Mitigated start-up of PEMFC in real automotive conditions: Local experimental investigation and development of a new accelerated stress test protocol

This study combines local electrochemical diagnostics with ex situ analysis to investigate degradation mechanism associated to start-up/shut-down (SU/SD) of PEMFC and mitigation strategies adopted in automotive stacks. Local degradation resulting from repeated SU/SD was analyzed with and without mitigation strategies by means of a macro-segmented cell setup provided with Reference Hydrogen Electrodes (RHEs) at both anode and cathode to measure local electrodes potential and current. Accelerated Stress Test (AST) for start-up with and without mitigation strategies are proposed and validated. A ten-fold acceleration of performance loss due to un-mitigated SU/SD has been calculated with respect to AST for catalyst support. Under mitigated SU/SD, instead, a strong degradation was observed as localized at cathode inlet region (i.e. -38% ECSA loss and -22 mV voltage loss after 200 cycles) due to local potentials transient reaching up to 1.5 V vs RHE. These localized stress conditions were additionally reproduced in a zero-gradient and a new AST protocol (named start-up AST) was proposed to mimic the potential profile observed upon SU/SD cycling. Representativeness of the start-up AST for real world degradation was confirmed up to 200 cycles. Platinum dissolution and diffusion/precipitation within the polymer electrolyte was shown to be the dominant mechanism affecting performance loss

Propriétés et applications des alliages intermétalliques B2-FeAl

Ordered FeAl intermetallic alloys are attractive materials for medium and high temperature industrial applications but their use has been restricted until now by their room temperature brittleness and their poor creep resistance. Powder metallurgy (P/M) techniques such as gas atomization and mechanical milling have been used to develop FeAl alloys with enhanced ductility and strength at both low and high temperatures. The improvement method combines ductilization by grain boundary strengthening, grain size reduction and oxide dispersion strengthening. These materials named FeA140 Grade 3 have been characterized and tested in the form of extruded bars. Microstructure and texture of as-extruded and heat treated samples have been studied by TEM and X-ray diffraction. Grains are 0,5 pm in size, resistant to recrystallization up to 1100°C and exhibit a strong wire texture parallel to the extrusion axis. The Y2O3 dispersoïds (20-30 nm in size) are cylindrical in shape and partially coherent with the matrix. The yield strength and the elongation of such alloys can reach 900 Mpa and 6,4% in air. Physical and mechanical properties of these materials are compared to some conventional engineering alloys in order to discuss the conceivable applications in aeronautical and automotive industries. Due to the high specific stiffness and strength of FeAl40 Grade 3 alloys, promising applications are the substitutions of steels and superalloys for the fabrication of moving parts in thermal and aeronautical engines, and especially parts submitted to critical vibrating modes

PEMFC performance decay during real-world automotive operation: Evincing degradation mechanisms and heterogeneity of ageing

A long-term dynamic load cycle is performed on state-of-the-art membrane electrode assemblies, aiming to evaluate the degradation mechanisms of Polymer Electrolyte Membrane Fuel Cell under real-world automotive operations. The load cycle, adapted from the stack protocol defined in H2020 ID-FAST European project, in- cludes load, pressure and temperatures cycling. Events that recover the temporary decay are included, specif- ically procedures classified in short-stops, cold-soaks, long-stops. Operando voltage and current distribution are measured through a segmented hardware, combined to local in-situ electrochemical characterization. Investi- gation is supported by scanning and transmission electron microscopy analysis, performed at different locations along-the-flow-field. Reversible degradation weights from few to 20 mV and changes local current distribution, mostly at air-inlet, since the dry-out of ionomer. Cycle efficiency decreases of 3%–9%: the largest irreversible performance losses are observed at air-inlet, while middle-region is the least impacted. Cathode catalyst layer and membrane are the most aged components: platinum active surface area drops in 200–400 h, because of electrochemical Ostwald ripening mechanism, and stabilizes around 62%–67% of initial value. Polymer mem- branes report ageing compatible with mechanical stress that causes localized thinning, increasing hydrogen crossover. Decay of ionomer in the catalyst layer is discussed, which would consistently explain alterations of mass transport resistance

Synthesis of germanium nano-crystals using a nanocluster source

International audienc

HREM study of precipitates shear in a Ni-Co based superalloy

SIGLEAvailable at INIST (FR), Document Supply Service, under shelf-number : RM 1378 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Structural properties of Ge nanocrystals synthesized by a PVD nanocluster source

International audienc

Effect of graphene flake size on functionalisation: quantifying reaction extent and imaging locus with single Pt atom tags

Here, the locus of functionalisation on graphene-related materials and the progress of the reaction is shown to depend strongly on the starting feedstock. Five characteristically different graphite sources were exfoliated and functionalized using a non-destructive chemical reduction method. These archetypical examples were compared via a model reaction, grafting dodecyl addends, evaluated with TGA-MS, XPS and Raman data. A general increase in grafting ratio (ranging from 1.1 wt% up to 25 wt%) and an improvement in grafting stoichiometry (C/R) were observed as flake radius decreased. Raman spectrum imaging of the functionalised natural flake graphite identified that grafting is directed towards flake edges. This behaviour was further corroborated, at atomistic resolution, by functionalising the graphene layers with bipyridine groups able to complex single platinum atoms. The distribution of these groups was then directly imaged using aberration-corrected HAADF-STEM. Platinum atoms were found to be homogeneously distributed across smaller graphenes; in contrast, a more heterogeneous distribution, with a predominance of edge grafting was observed for larger graphites. These observations show that grafting is directed towards flake edges, but not necessary at edge sites; the mechanism is attributed to the relative inaccessibility of the inner basal plane to reactive moieties, resulting in kinetically driven grafting nearer flake edges. This phenomenology may be relevant to a wide range of reactions on graphenes and other 2d materials