Nathalie Job

Critical opalescence points to thermodynamic instability:relevance to small-angle X-ray scattering of resorcinol–formaldehyde gel formation at low pH

During the formation at low pH of resorcinol–formaldehyde gels with a structure in the micrometre range, small-angle X-ray scattering exhibits a non-monotonic intensity variation as a function of reaction time. The data are analyzed in terms of scattering by statistical fluctuations of polymer concentration, the amplitude of which is maximal close to the critical point for phase separation between polymer and solvent. The data do not carry any morphological information, but they unambiguously show that the driving force of the gel formation is a thermodynamic instability of the polymerizing solution.

Carbon xerogels as model materials: toward a relationship between pore texture and electrochemical behavior as anodes for lithium-ion batteries

The mechanisms of Li+ insertion in porous hard carbons used as anodes for Li-ion batteries are still a matter of debate, especially considering the divergence of electrochemical performances observed in the literature. Since these materials usually exhibit several levels of porosity, the pore texture versus electrochemical behavior relationship is difficult to establish. In this paper, we propose to use carbon xerogels, prepared from aqueous resorcinol–formaldehyde mixtures, as model materials for Li-ion battery anodes to study the influence of the pore texture on the overall electrochemical behavior. Indeed, carbon xerogels are described as microporous nodules linked together to form meso- or macroporous voids inside a 3D gel structure; the size of these voids can be tuned by changing the synthesis conditions without affecting other parameters such as the micropore volume. The materials are chosen so as to obtain identical average particle sizes, homogeneous coatings with similar thicknesses, and a comparable surface chemistry. The electrochemical behaviors of carbon xerogels as Li-ion anodes are correlated with the surface accessible to the electrolyte and are not dependent on the total specific surface area calculated by the BET method from nitrogen adsorption isotherms. The key parameter proposed to understand their behavior is the external surface area of the nodules, which corresponds to the surface of the meso/macropores.

A facile and fast electrochemical route to produce functional few-layer graphene sheets for lithium battery anode application

A simple approach for the production of polymer functionalized graphene nanosheets is reported. The resulting polyacrylonitrile chemisorbed on graphene sheets is made of 1 to 2 layers, with a large majority of graphene single-layers. This novel functionalized graphene exhibits good cycling stability as an anode in Li-ion batteries without a conductive additive or binder.

Immobilization of homogeneous catalysts in nanostructured carbon xerogels

A Rh diamine complex has been successfully immobilized by anchorage on the surface of carbon xerogels. The catalysts are active and fully recyclable for cyclohexene hydrogenation, with conversion higher than 80% kept in four catalytic runs. TEM analysis reveals the presence of Rh particles in the used catalysts, meaning that partial reduction of the metal complex takes place under reaction conditions. The XPS data show that in the used catalysts Rh is present as Rh(I) (anchored complex) and Rh(0) metallic particles (about 30%).

A Review on Recent Developments and Prospects for the Oxygen Reduction Reaction on Hollow Pt-alloy Nanoparticles

Due to their interesting electrocatalytic properties for the oxygen reduction reaction (ORR), hollow Pt-alloy nanoparticles (NPs) supported on high-surface-area carbon attract growing interest. However, the suitable synthesis methods and associated mechanisms of formation, the reasons for their enhanced specific activity for the ORR, and the nature of adequate alloying elements and carbon supports for this type of nanocatalysts remain open questions. This Review aims at shedding light on these topics with a special emphasis on hollow PtNi NPs supported onto Vulcan C (PtNi/C). We first show how hollow Pt-alloy/C NPs can be synthesized by a mechanism involving galvanic replacement and the nanoscale Kirkendall effect. Nickel, cobalt, copper, zinc, and iron (Ni, Co, Cu, Zn, and Fe, respectively) were tested for the formation of Pt-alloy/C hollow nanostructures. Our results indicate that metals with standard potential -0.4<E<0.4 V (vs. the normal hydrogen electrode) and propensity to spontaneously form metal borides in the presence of sodium borohydride are adequate sacrificial templates. As they lead to smaller hollow Pt-alloy/C NPs, mesoporous carbon supports are also best suited for this type of synthesis. A comparison of the electrocatalytic activity towards the ORR or the electrooxidation of a COads monolayer, methanol or ethanol of hollow and solid Pt-alloy/C NPs underlines the pivotal role of the structural disorder of the metal lattice, and is supported by ab initio calculations. As evidenced by accelerated stress tests simulating proton-exchange membrane fuel cell cathode operating conditions, the beneficial effect of structural disorder is maintained on the long term, thereby bringing promises for the synthesis of highly active and robust ORR electrocatalysts.

Compressing some sol-gel materials reduces their stiffness : a textural analysis

The mechanical behaviour of two series of silica and of resorcinol xerogels is analyzed by mercury porosimetry. The data are expressed as pressure-density curves, which enables textural information to be obtained. In particular, it is shown that some of the analyzed samples exhibit a marked lowering of their mechanical stiffness upon compression. This observation is analyzed in terms of the collapse of the samples porosity and of the heterogeneity of the microstructure.

Synthesis and characterization of highly loaded Pt/carbon xerogel catalysts prepared by the strong electrostatic adsorption method

In order to decrease the mass transport limitations reported in classical PEMFC electrodes, Pt/carbon xerogel catalysts have great potential to replace Pt/carbon black catalysts. These nanostructured materials with well defined pore texture allow for better gas/water diffusion and better contact between the platinum particles and the ionomer (Nafion®). Pt/carbon xerogel catalysts with high metal content (~ 25 wt.%) and high metal dispersion (nanoparticles ca. 2 nm in size) were prepared via the ‘Strong Electrostatic Adsorption’ method; the impregnation-drying-reduction step with H2PtCl6 was repeated until the desired metal loading was achieved. However, both physico-chemical and electrochemical characterization show that the use of H2PtCl6 leads to Pt catalysts poisoned with chlorine, especially if the reduction temperature is lower than 450 °C. This induces a dramatic decrease of the Pt utilization ratio in the final PEMFC catalytic layer.

Development of Novel Solid Materials for High Power Li Polymer Batteries (SOMABAT). Recyclability of Components

SOMABAT aims to develop more environmental friendly, safer and better performing high power Li polymer battery by the development of novel breakthrough recyclable solid materials to be used as anode, cathode and solid polymer electrolyte, new alternatives to recycle the different components of the battery and life cycle analysis. This challenge is being achieved by using new low-cost synthesis and processing methods in which it is possible to tailor the different properties of the materials. Development of different novel synthetic and recyclable materials based carbon based hybrid materials, novel LiFePO4 and LiFeMnPO4 based nanocomposite cathode with a conductive polymers or carbons, and highly conductive polymer electrolyte membranes based on fluorinated matrices with nanosized particles and others based on a series of polyphosphates and polyphosphonates polymers respond to the very ambitious challenge of adequate energy density, lifetime and safety. An assessment and test of the potential recyclability and revalorisation of the battery components developed and life-cycle assessment of the cell will allow the development of a more environmental friendly Li-polymer battery in which a 50 % weight of the battery will be recyclable and a reduction of the final cost of the battery up to 150 €/kWh is achievable. The consortium is made up of experts in the field and is complementary in terms of R&D expertise and geographic distribution.

Functionalization of carbon xerogels for the preparation of Pd/C catalysts by grafting of Pd complex

Abstract A mesoporous carbon xerogel was functionalized by treatments with nitric acid in order to introduce oxygenated functions at its surface. The carbon supports were characterized by Boehms titrations, XPS and nitrogen adsorption, and the results were compared with those obtained with a microporous activated carbon. It was shown that carbons with high oxygen content were obtained. The oxygenated functions were then used as anchors for the grafting of a palladium complex. High Pd surface concentrations are observed by XPS but it appears that the Pd particles are not homogeneous in size and repartition.

Metal catalysts supported on texture-tailored carbon xerogels

Abstract Carbon materials issued from evaporative drying and pyrolysis of resoreinol-formaldehyde gels were used as metal catalyst supports. These carbons, whose pore texture and composition are controllable, lead to high metal dispersion. Moreover, pore texture control is a great advantage: diffusional limitations can be lowered, and even eliminated, by choosing an appropriate pore texture range.